Deciphering the Role of Protein Kinase D1 (PKD1) in Cellular Proliferation Ilige Youssef1,2 and Jean-Marc Ricort1,2,3

Published OnlineFirst July 16, 2019; DOI: 10.1158/1541-7786.MCR-19-0125

Review Molecular Cancer Research Deciphering the Role of Protein Kinase D1 (PKD1) in Cellular Proliferation Ilige Youssef1,2 and Jean-Marc Ricort1,2,3

Abstract

Protein kinase D1 (PKD1) is a serine/threonine kinase that context-dependent and poorly understood. In this review, belongs to the calcium/calmodulin-dependent kinase family, we present and discuss the current landscape of studies and is involved in multiple mechanisms implicated in tumor investigating the role of PKD1 in the proliferation of both progression such as cell motility, invasion, proliferation, pro- cancerous and normal cells. Indeed, as a potential thera- tein transport, and apoptosis. While it is expressed in most peutic target, deciphering whether PKD1 exerts a pro- or tissues in the normal state, PKD1 expression may increase or antiproliferative effect, and under what conditions, is of decrease during tumorigenesis, and its role in proliferation is paramount importance.

Introduction several biological processes such as cell proliferation, migration, invasion, apoptosis, angiogenesis, cardiac contraction, PKD1, also called PKCm, is a serine/threonine kinase that and immune regulation (5). In this context, its dysregulation belongs to the PKD family, a subgroup of the calcium/calmodu- (over- or underexpression) was shown to be associated to lin-dependent kinase (CAMK) family (1). PKD1 is a 912 amino diverse pathologies such as inflammation, cardiac hypertrophy, acid residue protein with an apparent molecular weight of and cancer (6). However, it remains largely unknown what 115 kDa that contains a carboxy-terminus catalytic domain regulates PKD1 gene (prkd1)expressionintumors.PKD1gene and a regulatory domain at the amino-terminus. The latter promoter was shown to be either activated by the oncogenic regulates the catalytic activity of PKD1 by maintaining the KRas–NFkB pathway increasing the expression of PKD1 in proteininaninactivestatethroughanautoinhibitorymech- pancreatic cancer cells (7) or inhibited by b-catenin in prostate anism exerted toward the catalytic domain (2). PKD1 can be cancer (8). It was also shown to bethetargetofepigenetic activated by a wide variety of extracellular stimuli including methylation decreasing PKD1 expression in some breast tumor growth factors, vasoactive peptides, chemokines, neuropep- cells (9–11). These different molecular mechanisms lead to tides, phorbol esters, and others. To date, the best characterized tumor tissue–specific PKD1 mRNA expression profiles. Accord- signaling pathway responsible for the activation of PKD1 ing to TCGA data, PKD1 mRNAs are mostly expressed in involves the activation of phospholipases Cb or g (PLC or b prostate cancer and melanoma (Fig. 1). Also, the data relative PLCg;ref.3).Theseproteinssynthesize inositol-triphosphate to 11 studies [Breast Invasive Carcinoma, Colorectal Adeno- (IP ) and diaglycerol (DAG), which allows the activation of 3 carcinoma, Head and Neck Squamous Cell Carcinoma, Kidney several protein kinase C (PKC) isoforms and their recruitment Renal Clear Cell Carcinoma, Kidney Renal Papillary Cell Car- close to PKD1. Once nearby, PKCs phosphorylate PKD1 onto cinoma, Lung Adenocarcinoma, Lung Squamous Cell Carcino- two serine residues (738 and 742, or 744 and 748, human or ma, Pancreatic Adenocarcinoma, Prostate Adenocarcinoma, murine numbering, respectively) localized in its activation Skin Cutaneous Melanoma, Stomach Adenocarcinoma (TCGA, loop leading to the stimulation of the catalytic domain and PanCancer Atlas)], has shown PKD1 mRNA levels to be upre- its autophosphorylation onto its serine 910 (or 916 for murine gulatedby1.8%(pancreas)to18%(lungadenocarcinoma) PKD1) residue (4). Activated PKD1 thus translocates into with a mean value of 7.8% (additional files 1 to 8: Supple- different cellular compartments modulating its targets. The mentary Figs. S1–S8). Moreover, analysis of the prkd1 gene wide diversity of its substrates makes PKD1 a main actor in reveals that only 4% of the tumors analyzed (206 patients over 5,615) carry a mutation or a copy number alteration in the 11 1 fi Centre National de la Recherche Scienti que, CNRS UMR_8113, Laboratoire de abovementioned TCGA studies (Fig. 2). Taken together, these Biologie et Pharmacologie Appliquee, Cachan, France. 2Ecole Normale Superieure Paris-Saclay, Universite Paris-Saclay, Cachan, France. 3Centre de results suggest, at least for tumors with the lowest upregulated Recherche des Cordeliers, INSERM, Sorbonne Universite, USPC, Universite Paris values, that a dysregulated PKD1 activity may certainly play a Descartes, Universite Paris Diderot, Paris, France. more significant role in tumor progression than its gene over- fi Note: Supplementary data for this article are available at Molecular Cancer expression or ampli cation. Research Online (http://mcr.aacrjournals.org/). Although PKD1 seems to play an essential role in oncogenesis and is activated by a large variety of stimuli, especially by many Corresponding Author: Jean-Marc Ricort, Ecole Normale Superieure Paris- growth factors, it has a complex relationship with cell prolifera- Saclay, 61 avenue du President Wilson, Cachan 94230, France. Phone: 331- fi 4427-5582; E-mail: [email protected] tion both in normal and cancer cells. In fact, the speci city of the role of PKD1 with regards to cell proliferation depends not only Mol Cancer Res 2019;17:1961–74 on the tissue type but also on the phenotype (normal vs. tumor) doi: 10.1158/1541-7786.MCR-19-0125 because PKD1 has been described to be either proproliferative or 2019 American Association for Cancer Research. antiproliferative (Table 1). Moreover, this complexity drastically

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15 Figure 1. 10 Prkd1 RNA expression overview. RNA- seq data in 9 cancer types are 5 reported as median FPKM (number Fragments Per Kilobase of exon per 0 Million reads), generated by the The Kilobase of exon per Million reads)

Median FPKM Fragments Per (number Cancer Genome Atlas (TCGA). RNA cancer tissue category is calculated on the basis of mRNA expression levels across all 9 cancer tissues and include: cancer tissue enriched, cancer group enriched, cancer tissue enhanced, expressed in all, mixed and not Head and Breast Lung Stomach Colorectal Renal Prostate Pancreac detected. Table presents the different Melanoma neck invasive cancer cancer cancer cancer cancer cancer representative values obtained for cancer carcinoma Max 4.9 2.6 2.3 12.4 2 3.1 9.9 22.6 5.7 each tumor. 3rd quarle 2.5 1.3 1.1 6.8 0.9 1.6 5.5 13 3.2 Median 1.5 0.7 0.6 4.9 0.4 1.1 3.9 9.5 2.3 1st quarle 0.8 0.4 0.3 2.7 0.2 0.6 2.6 6.5 1.5 Min 0.1 0 0 0.1 0 0 0.2 1.2 0.1

increases when, for a given cell type, some controversial data exist. pharmacologic target in oncology. In this context, it becomes However, increasing data described PKD1 activity as affecting obviously and urgently crucial to have a clear knowledge of its role tumor behavior both in vitro and in vivo through its ability to in cell proliferation. Therefore, this review aims to list the major regulate cell proliferation making PKD1 a putative pertinent data existing to date concerning the pro- or antiproliferative

9 Mulple alteraons

8 Deep deleon

7 Ampliﬁcaon

6 Fusion

5 Mutaon

Alteraon frequency (%) 3

Figure 2. Prkd1 gene mutation and copy number alterations. Prkd1 genomic alteration types obtained by querying 5,609 patients in 11 TCGA PanCan studies. For each study, results are expressed as the frequency of alteration of the prkd1 gene compared with the total number of cancers analyzed in the study.

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Table 1. Role of PKD1 in normal and cancer cell proliferation Anti () or pro (þ) Tissue Cell type—cell line/Species proliferative effect Comments References Breast Breast cancer cell lines (human) Relative expression and invasion/metastasis (9) MCF-7 (human) þ PKD1-overexpressing cells (91, 92) MDA-MB-415 (human) þ (92) MCF-7-ADR (human) - drug resistant cells þ (93) Endothelium HUVEC (human) þ (16) Cell migration (19) EPCs (human) þ Cell migration and tube formation (17) Aortic ring (mouse) þ Microvessel sprouting (18) Zebraﬁsh (in vivo) þ (22) Fibroblast Swiss 3T3 (mouse) þ (24, 25) NIH 3T3 (mouse) þ PKD1-overexpressing cells (30) Head and neck HNSCC cell lines (human) and tissue sections þ (109) squamous cells Kidney M1 (mouse) þ (65, 66) Lung Idiopathic pulmonary ﬁbrosis (human) þ (69) A549, H520 (71) Pancreas Acinar cells (rat) þ NFkB-dependent pancreatitis (45, 46) Rat (in vivo) þ (46) Acinar cells (mouse) þ Acinar-to-ductal metaplasia, ADM (47) Human pancreatic adenocarcinoma þ PKD1 expression (7, 52) Panc-1 and Panc-28 cells þ WT and PKD1-overexpressing or -depleted cells (21, 51, 53) Colo357, PancTul, Panc89 þ Survival and telomerase activity (52) Prostate ALVA-41, LNCaP, C4-2, DU-145 PKD1-overexpressing or -depleted cells (75, 77, 78, 81) LNCaP þ Prosurvival (84) LNCaP, PC3 þ (85–88) PC3 þ PKD1-overexpressing cells (88) Skin Primary keratinocytes (murine) þ (30, 32) þ PKD1-overexpressing or -depleted cells (33, 34, 36) Mouse (in vivo) þ DMBA-induced tumors (31) þ Wound healing (35) Human basal cell carcinoma, psoriasis þ (37) Melanoma (human) þ (40) Stomach SNU gastric cell lines (human) Cell migration and invasion (11) AGS (human) PKD1-overexpressing cells (54) Small Intestine IEC-18 (rat) þ (58, 59, 61) PKD1 transgenic mouse (in vivo) þ (58, 59, 61) Colon SW480 (human) PKD1-overexpressing cells (55)

effects of PKD1 and tries to bring elements of discussion to VEGFR2, through a PLCg/PKCa-dependent signaling path- explain, when necessary, potentially contradictory results. way (16). VEGF-stimulated ERK1/2 phosphorylation and DNA synthesis in HUVEC (16), and VEGF-induced microvessels PKD1 Stimulates Angiogenesis, A Key sprouting from mouse aortic rings (18) were markedly inhibited by PKD1 knockdown and PKD1 kinase–negative mutant expres- Determinant in Cancer Development sion, respectively, making PKD1 a proproliferative protein in Angiogenesis is the process by which new blood vessels are endothelial cells. As previously mentioned, histone deacetylases formed and is of pivotal importance in processes such as wound (HDAC) help control gene expression by regulating the acetyla- healing and embryonic vascular development (12). It also plays a tion state of the chromatin. VEGF stimulates HDAC7 and HDAC5 fundamental role in tumor growth and metastasis (13). It pro- phosphorylation and their nucleocytoplasmic shuttling through a vides tumors with oxygen and nutrients, crucial for their devel- PKD1-dependent signaling pathway (17–19). Once phosphory- opment and also helps in discarding tumor metabolites (13). lated and in the cytosolic compartment, HDAC7 is localized away Inhibition of angiogenesis has thus been regarded as a valuable from its substrates promoting gene expression leading to cell new approach to cancer therapy (14). A number of stimulators proliferation. Moreover, PKD1 was shown to regulate the VEGF- and inhibitors regulate angiogenesis (15). In the case of PKD1, a induced expression of metalloproteinases such as MT1- consensus exists with regard to its proangiogenic and thus pro- MMP (13), whose gene expression is implicated in angiogenesis proliferative role whatever the cell model [endothelial progenitor in vivo (20). Thus, it is very interesting to note that PKD1 has a cells (EPC) or cell lines] or the species (human or even zebraﬁsh). particular role with regard to VEGF because, on one hand, PKD1 is VEGF, a major component of angiogenesis under both physio- an actor of the VEGF signaling pathway and, on the other hand, logic and pathologic conditions, induces the phosphorylation PKD1 regulates VEGF secretion as shown in pancreatic cancer of PKD1 in human umbilical vein endothelial cells (HUVEC), cells (21). Thus, in a tumor context, PKD1 could allow the bovine aortic endothelial cells (BAEC) (16), and EPCs (17). This activation of a self-sustained loop allowing the formation of new occurs within minutes upon binding of VEGF to its receptor, vessels promoting the progression of the tumor (22).

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PKD1 Promotes Fibroblast Proliferation mutant inhibited it (34). Altogether, these results clearly established PKD1 as a proproliferative and antidifferentiating protein The tumor microenvironment (TME) of cancer cells has been in mouse keratinocytes. Nevertheless, one study reveals that these found to be a key determinant in tumor progression and metastasis proproliferative and antidifferentiative PKD1 functions would and has thus been gaining increasing interest in cancer research. only be revealed in particular situations. In fact, mice carrying Fibroblasts, a major component of the TME, are responsible for the conditional and specific disruption of PKD1 in keratinocytes synthesis, deposition, and remodeling of the extracellular matrix (K14-Cre-PKD1-cKO) displayed no alteration in epidermal pro- and are a source of paracrine growth factors that regulate the liferation and differentiation suggesting that PKD1 would be growth of cancer cells (23). PKD1 is portrayed as a proproliferative dispensable for skin development and homeostasis under normal protein in fibroblasts. Overexpression of PKD1 in murine Swiss- conditions (35). However, PKD1-cKO–deficient mice displayed 3T3 cells enhances the proliferative response to G-protein–coupled strongly impaired wound healing and reepithelialization and receptor agonists and to phorbol esters. In fact, treatment with became mostly totally refractory to DMBA/TPA–induced tumor angiotensin, bombesin or phorbol 12,13-dibutyrate (PDBu) led to formation (35). These results are of crucial importance because PKD1 phosphorylation onto its 744, 748, and 916 serine residues they underline the role of PKD1 in adaptive responses such as skin (murine numbering). Furthermore, PKD1 overexpression poten- carcinogenesis and are finally in total accordance with data tiated neuropeptide-induced mitogenesis (24) probably through showing that PKD1 was activated by UVB and that its overexpres- an increased duration of the ERK signaling pathway characterized sion protected keratinocytes from UVB-induced apoptosis (36). by a significant increase in the phosphorylation of FAK and RSK, In fact, through the stimulation of prosurvival signaling path- and by the accumulation of the early gene c-Fos (25). ways, PKD1 could thus allow the proliferation of mutated cells leading to cancer formation. PKD1 as a Proproliferative and Prosurvival When considering normal human skin cells, the status of PKD1 Factor in Skin Carcinogenesis and a expression remains more elusive. In fact, PKD1 was initially described to be expressed throughout the superbasal layers with Putative Target in Melanoma a predominant expression in the stratum basalis, in accordance Skin cancer is characterized by an abnormal growth of skin with its proproliferative role as previously described in mice (37). cells. Depending of the skin cell type involved, two major cate- However, further results indicated that PKD1 was not detected in gories of cancers were defined: basal and squamous cell skin human keratinocytes, these cells being more dependent on the cancer and melanoma (26). Basal and squamous cell cancers are two other isoforms of the PKD family, PKD2 (prodifferentiative), the most common types and are mainly caused by UV exposure, and PKD3 (proproliferative), for their proliferation (38). As thus usually developing on body parts exposed to sunlight (27). hypothesized by the authors, such discrepancies could be the On the other hand, melanoma develops from melanocytes and consequence of a certain lack of specificity of the antibodies used has less well-defined origins. It can also be caused by UV light but, by Ristich and colleagues (37) because sc-935 antibody has been unlike basal and squamous cell cancers, can also develop on body reported by others to cross-react with PKD2 in Western blot parts unexposed to sunlight (28). Melanoma is more likely to analyses (39). This hypothesis does not however provide an form metastasis in other tissues making it usually more aggressive explanation as to why the prodifferentiative PKD2 would be the than basal and squamous cell carcinomas (29). most abundantly expressed in actively dividing cells. In hyper- PKD1 expression was first described in 1999 in mouse epider- plastic human skin disorders, such as melanoma (40), basal cell mis and positively correlated with cell proliferation (30). Kera- carcinoma, and psoriasis (37), PKD1 was found to be upregu- tinocytes proliferation was decreased after treatment with PKD1 lated. Taken together, these data suggest that, despite the fact that pharmacologic inhibitor, Goedecke 6976 (Go6976),€ and PKD1 seems not to be of primary importance in normal skin enhanced in PKD1-overexpressing cells (30). Moreover, carcino- homeostasis (35), it has, whatever the species models, an evident ma (but not papilloma) from a two-stage carcinogenesis induced proproliferative role in the context of skin carcinogenesis, wound mouse model, using first 7,12-dimethylbenz[a]anthracene healing, and other skin hyperproliferative diseases like psoriasis. (DMBA) as initiator, and then tetradecanoylphorbol-13-acetate Consequently, these data also suggest why targeting PKD1, by (TPA) as promoter, expressed high levels of PKD1 (30) and were relatively selective inhibitors, has to be considered as an option strongly impaired for their development after peracetylated for the treatment and prevention of epidermal tumorigenesis, and EGCG (AcEGCG)-induced PKD1 inhibition (31). According to for other hyperproliferative diseases such as psoriasis. To this goal, these data, murine PKD1 was shown to be mostly expressed in the we showed that inhibition of PKD1 in melanoma cells (i) basal, proliferative, layer of the epidermis and, although present, decreased their colony-forming capacities probably through the less expressed in suprabasal layers (32). Consistently, overexpres- regulation of the ERK, JNK, and NFkB signaling pathways, and (ii) sing PKD1 in primary keratinocytes stimulates keratin 5 (propro- induced the relocation of b-catenin from nucleus to plasma liferative marker), but inhibits involucrin (prodifferentiative membrane, and the subsequent expression decrease of some marker) promoter activities, respectively (32). Moreover, genetic proproliferative target genes such as cyclin D1 (40). PKD1 depletion not only inhibited cell proliferation but also strongly potentiated the calcium-induced expression of late, intermediate, and early differentiation markers of mouse kerati- PKD1 Positively Affects Pancreatic nocytes such as loricrin, involucrin, and keratin 10 (33). Finally, Inflammation and Adenocarcinoma further evidence of a proproliferative role of PKD1 came from adenovirus-transfected primary mouse epidermal keratinocytes Proliferation showing that a constitutively active PKD1 mutant significantly Although PKD1 was shown to have a main role in insulin increased DNA synthesis. In contrast, a dominant-negative PKD1 secretion of pancreatic islets (41), our review will only focus on

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the exocrine function of the organ because tumors arise mainly the final tumor volume of orthotopic implanted Panc-1 cells (51). from these structures. Among pancreatic cancers, the most com- These results therefore highlighted the role of PKD1 not only in mon is the pancreatic ductal adenocarcinoma (PDAC) represent- the genesis, but also in the maintenance of pancreatic tumors. ing 90% of all cancers and considered among the most lethal These effects could be the consequence of the regulation of cancers with a very low 5-year survival rate of about 3%–5%. It is angiogenesis. In fact, PKD1 expression stimulates the secretion characterized by an early metastatic state associated with a rapidly of proangiogenic factors such as VEGF and CXCL-8 and enhances succeeding chemoresistance (42). the association between pancreatic cancer cells and endothelial The expression status of PKD1 is often incorrectly formulated cells on Matrigel, whereas the PKD1 inhibitor CRT0066101 due to sentences that, using shortcuts, become inaccurate in some reduces angiogenesis in orthotopic PDAC tumor explant publications. Indeed, stating that PKD1 is not expressed in the in vivo (21). normal exocrine pancreatic cells is incorrect because PKD1 was Taken all together, these data define PKD1 as a clear and detected in untreated rat pancreatic acini and very rapidly phos- prominent proproliferative factor in PDAC making this protein phorylated (detectable effect after 30 seconds) and activated by a putative target for the development of new therapeutic strategies cholecystokinin (CCK) through a PKC-d–dependent signaling against pancreatic cancer. pathway (43). PKD1 plays a major role in rat pancreatic acini modulating experimental pancreatitis. Pharmacologic inhibitors of PKD1 attenuated early pancreatitis events (44), but also sig- Pro- or Antiproliferative Role of PKD1 in the nificantly attenuated pancreatic injury when used as posttreat- Gastrointestinal Tract, Depending on the ment (45). These results are of crucial importance because, by promoting pancreatitis-associated necrosis (46), PKD1 would Localization promote a proinflammatory state, especially characterized by the Despite the scarcity of studies, it seems that PKD1 plays secretion of IL6 and MCP-1 proteins (45), which could give rise to different roles depending on which part of the tract is considered. pancreatic lesions, known as risk factors for cancer development. Compared with normal tissues, PKD1 expression was shown to be This is consistent with data showing that PKD1 is upregulated in markedly downregulated in gastric (11, 48, 54) and colorectal mice pancreatic acinar cells that undergo acinar-to-ductal meta- human cancer cells (55, 56), with a more pronounced decrease in plasia (ADM; ref. 47). ADM occurs after inflammation or injury higher grade tumors (54, 55) suggesting an antioncogenic role of and is reversible unless in a persistent proproliferative context this protein in these tissues. In fact, overexpression of PKD1 in where it can progress to neoplasia and cancer (48, 49). During human gastric adenocarcinoma cells (AGS cells; ref. 54) or in ADM, PKD1 was shown to act downstream of TGFa and K-Ras, human SW480 colorectal cancer cells (55) inhibited cell prolif- and upstream of the Notch pathway, to promote the formation of eration, clonogenicity, and motility, and delayed tumor growth in ductal structures (47). a xenograft mouse model. Such effect could be dependent on On the other hand, PKD1 is very moderately expressed in PKD1-induced nuclear exclusion of b-catenin and the subsequent normal mouse and human pancreatic tissue contrary to PKD3 decrease of its transcriptional activity (55) toward several proto- that represents the major, if not the single, isoform (50). However, oncogenic genes like cyclin D1 or c-Myc (57). Although these transformed acinar cells and human PDAC show a strong increase results were both provided upon PKD1 overexpression, poten- in PKD1 expression compared with normal tissue (7, 51, 52). tially generating false, very low-level affinity interactions, and in a PKD1 shortened the doubling time of PKD1-transfected Colo357 cell model, SW480 cells, chosen upon its particular PKD1 and cells by 20% probably through an enhanced expression and b-catenin expression and localization levels, they suggest that activity of hTERT (52) and dose-dependently increased DNA PKD1 may negatively regulate cell proliferation through a b-cate- synthesis and cell proliferation of inducible PKD1-expressing nin–dependent mechanism in normal colorectal cells. Therefore, Panc-1 cells (53). The latter result is of main importance because loss of PKD1 expression during steps of tumorigenesis would it makes a clear proportional link between PKD1 expression levels release this break, promoting cell proliferation. Thus, downregu- and cell proliferation rates. By strengthening the duration of ERK lation of PKD1 expression levels appears as a key determinant for signaling and inhibiting G protein–coupled receptors (GPCR)- gastric and colorectal tumorigenesis process and could be the induced c-Jun phosphorylation, increased PKD1 levels stimulate consequence of an epigenetic inactivation occurring on the PKD1 cell-cycle progression allowing the accumulation of immediate promoter as demonstrated in gastric cancer cells (11). However, gene products such as c-Fos, whereas inhibition of c-Jun phos- this mechanism does not seem to be generalizable to other parts phorylation leads to the attenuation of the JNK signaling switch- of the gastrointestinal tract because authors mentioned that DNA ing its proapoptotic action to a proproliferative one (53). Another methyltransferase inhibitors were unsuccessful to reexpress PKD1 hallmark of PDAC is a highly increased NFkB signaling, linked to in colorectal cancer cells (56). an increased proliferation of tumor cells. Oncogenic K-Ras In contrast, PKD1 was shown to have a proproliferative role in induces canonical NFkB signaling and upregulates PKD1 expres- intestinal cells both in vitro and in vivo suggesting a potential sion and activity (7). Moreover, overexpression of PKD1 increased prooncogenic role in the intestine. Selective knockdown of endog- anchorage-independent growth of PDAC cells (21), whereas its enous PKD1 inhibited DNA synthesis and cell proliferation pharmacologic inhibition by CRT0066101 (21), or the expres- induced by angiotensin II and vasopressin in IEC-18 rat intestinal sion of a PKD1 kinase–dead mutant (53), or its molecular cell line (58). Moreover, overexpression of PKD1 in small intes- silencing (21) decreased the number of colonies formed in a tine of transgenic mice increased the proliferation rate and the semi-solid medium. The prooncogenic role of PKD1 was further number of intestinal cells per crypt in vivo (58). PKD1 overexpres- demonstrated in vivo because orally given PKD1 inhibitor sion would enhance the proliferation induced by GPCR agonist– CRT0066101 significantly reduced the volume of established dependent signaling pathways through the phosphorylation and tumors in subcutaneous Panc-1 xenograft models, or inhibited the subsequent nuclear export of class IIa histone deacetylase

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(such as HDAC4, HDAC5, and HDAC7; ref. 59). In fact, HDACs to stimulate the phosphorylation of PKD1 through a mineralo- regulate gene expression by interacting with and repressing var- corticoid receptor- (MR) and EGFR-dependent mechanism (66). ious transcription factors (60). More recently, PKD1 overexpres- PKD1 knockdown inhibited aldosterone-stimulated proliferation sion was also shown to promote angiotensin II–stimulated cell demonstrating a proproliferative role of PKD1 in this cell proliferation by inducing b-catenin translocation to the nucle- line (65). PKD1 may promote aldosterone-induced cell prolifer- us (61). These intriguing results were in total contradiction with ation by maintaining a sustained activation of ERK1/2 and previous ones found in human colon cancer cells (55), highlight- inducing its translocation to the nucleus (65). Taken together, ing the apparent complexity of the molecular mechanisms reg- these findings highlight the proproliferative role of PKD1 in renal ulated by PKD1. However, it is essential to notice that these collecting duct cells. However, it is important to notice that contradictory results were (i) not conducted in the same species among 17 tissues analyzed from TCGA data, high PKD1 mRNA (murine vs. human) suggesting that PKD1 "species-specific roles" expression is a good prognostic factor in kidney tumors (Table 2). cannot be excluded; (ii) performed either in tumor models, for Despite their apparent contradiction, these results mainly high- colon and stomach, or in normal cells, for the intestine, each light, as previously mentioned in the introduction, that a direct expressing specific cellular contexts that could be determinant to correlation between tumor mRNA expression levels and PKD1 define the role of PKD1. Among the suspected proteins, the two activity cannot be assumed and that the relevant and interesting other members of the PKD family are of interest and most prkd1 gene expression analysis cannot be freed from that of PKD1 particularly PKD2. Indeed, PKD2 is often described to display activity. opposite functions to PKD1 and their respective expression is often inversely regulated as demonstrated in colon (56) and gastric (54) cancers. Therefore, one may suppose that depending PKD1 and the Lung: a Pro- or on the relative expression level of PKD2, the apparent role of Antiproliferative Role Yet to be PKD1 could be consequently modulated. Therefore, whenever Determined possible, this point should be taken into consideration and Lung cancers are the leading cause of cancer mortality world- further studies are still needed for a better understanding of the wide (67). Among the two subtypes, non–small cell lung cancers role of PKD1 in the whole gastrointestinal tract. (NSCLC) are the most common (85%), whereas small-cell lung cancers (SCLC) are usually more likely to spread and become life PKD1 Stimulates Renal Duct Cells threatening (68). Because of a few studies about PKD1 in the lung, Proliferation through a Sustained ERK1/2 it remains difficult to have a clear idea about the pro- or antiproliferative role of this protein in this tissue. Activation PKD1 was shown to be highly expressed and phosphorylated in Kidney cancer is among the tumors with the fastest growth rate bronchiolar and regenerative alveolar epithelia from patients with and is the deadliest type of urinary tract cancer (62). Aldosterone idiopathic pulmonary fibrosis (69). This pathology is character- is a mineralocorticoid hormone that regulates ion fluxes among ized by lung fibroblast activation and proliferation suggesting a nephron epithelium. In addition to its well-characterized role as proproliferative role of PKD1 in this tissue (70). In contrast, PKD1 an ion transport modulator (63), aldosterone was also shown to mRNA expression was shown to be downregulated in NSCLC stimulate the proliferation of human RCC (renal cell carcinoma) compared with normal tissues especially when patients with cell lines (64) as well as the murine M1 cortical collecting duct cell NSCLC displayed venous invasion or lymph node metastasis, line (M1-CCD; ref. 65). In M1-CCD cells, aldosterone was shown suggesting that PKD1 would negatively regulate NSCLC tumor

Table 2. PKD1 mRNA expression levels and survival analysis Number of 5-year 5-year tumors survival survival PKD1 as Cancer analyzed Alive Dead P high low prognostic Correlation Melanoma 102 73 29 0.024 0%a 53%a No Glioma 153 30 123 0.21 10%a 8%a No Thyroid 501 485 16 0.00029 81% 95% Yes High expression is unfavorable Lung 1,294 600 394 0.089 47% 44% No Liver 365 235 130 0.0011 26% 57% No Pancreas 176 84 92 0.071 33% 21% No Head and neck 499 281 218 0.19 43% 52% No Stomach 354 208 146 0.00095 22% 45% No Colorectal 597 473 124 0.023 55% 65% No Urothelial 406 227 179 0.39 38% 44% No Kidney 877 651 226 0.000024 75% 61% Yes High expression is favorable Prostate 494 484 10 0.16 98% 97% No Testis 134 130 4 0.20 100% 96% No Breast 1,075 923 152 0.053 80% 88% No Cervical 291 220 71 0.32 64% 67% No Ovarian 373 143 230 0.053 28% 44% No Endometrial 541 450 91 0.067 73% 82% No Log-rank P value for Kaplan–Meier plot showing results from analysis of correlation between mRNA expression level and patient survival. aFor melanoma and glioma, 3-year survival is shown.

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development (71). Consistent with this, although PKD1 was b-catenin–mediated proliferation function (78), or through the shown to induce a prolonged activation of the ERK1/2 signaling secretion of matrix metalloproteinase-2 and -9 (75). However, pathway in Swiss-3T3 cells (see Chapter Fibroblast), PKD1 med- despite many studies, the precise function of PKD1 with regards to iates the inhibition of PMA-induced ERK phosphorylation in cell proliferation and the mechanisms it controls remain some- A549 cells. Thus, pharmacologic PKD1 inhibition or its down- what unclear. For instance, inhibition of PKD1 expression dras- regulation resulted in enhanced PMA-induced S6K1 and ERK tically decreased ERK phosphorylation in DU145 cells (75) phosphorylation and A549 cell proliferation, whereas constitu- although this protein is described as mediating proproliferative tively active PKD1 results in S6K1 and ERK inhibition (71). signaling pathways (80). Moreover, the molecular mechanisms Interestingly, the antiproliferative role of PKD1 may also be by which PKD1 would inhibit cell proliferation have been dependent upon its ability to maintain a low-proliferative epi- mostly demonstrated in PKD1-overexpressing prostate cancer thelial phenotype of lung cells. In fact, PKD1 was also described to cells (75, 77, 78, 81). This technical approach is not necessarily directly bind to E-cadherin leading to its membrane redistribution the better way to proceed because a recent study showed that and activation independently of DAG or PKC in A549 cells (72). according to TCGA data [Prostate Adenocarcinoma (TCGA, Pan- Because PKD1 positively regulates E-cadherin transcription, this Cancer Atlas)], mRNA PKD1 expression levels in prostate cancer interaction/activation generates a positive feedback loop favoring are upregulated in about 5% tumors suggesting that PKD1 hyper- the maintenance of a low proliferative epithelial phenotype. activity may play a more important role in tumor progression than Conversely, and in accordance with these results, knocking overexpression (82). Moreover, LNCaP cells already express very down PKD1 induces the loss of expression of E-cadherin pro- high amounts of PKD1 making the relevance of such a model moting the epithelial-to-mesenchymal transition and the acqui- questionable insofar as overexpression can only lead to nonspe- sition of migratory capacities (72). cific and nonrelevant interactions. Furthermore, the comparison However, it is important to note that in the study by Ni and of results obtained in different cell lines is also complicated colleagues (71), PKD1 protein expression levels were not deter- and very hazardous. Indeed, the most commonly used cell lines mined in NSCLC tissue specimens and even if mRNA levels (i.e., LNCaP, C4-2, and PC3) display different sensitivities to decreased, no one can conclude that protein levels will automat- androgens. However, PKD1 has a particular relationship toward ically follow the same profile. However, if these results are also androgen receptor, AR. Indeed, PKD1 would inhibit AR-mediated later confirmed at the protein level, future studies would have to transcriptional activity (observed in PKD1 and AR-overexpressing carefully consider these data to study the role of PKD1 in lung. cells; ref. 81) while androgens would inhibit PKD1 expression Indeed, the use of the adenocarcinoma-derived human alveolar through the expression of a repressor, FRS2 (83). Consistent with basal epithelial cell model, A549 cells, cannot then represent a the latter results, incubation of cells in an androgen-depleted good model enough for NSCLC investigations insofar as this medium increased PKD1 expression (83) indicating that AR tumor cell line expresses large amounts of PKD1 (73) unlike what expression and androgen sensitivity status of the cell lines must has been found in human tissues. be considered in a serious way and that the extrapolation of results between different cell models cannot be done as easily as this. Therefore, even if these results should not be questioned, one must Are the Proproliferative and Prosurvival be aware of their limits. In fact, it is interesting to note that Roles of PKD1 Hormone-dependent in contradictory results exist even in studies conducted by the same Prostate Cancer Cells? team in which inhibition of PKD1 expression was shown to have either an effect (77) or not (78) on the growth of LNCaP cells. Prostate cancer is the second leading cause of cancer-related Although PKD1 was described as an antiproliferative protein in death and the most commonly diagnosed cancer in males in the prostate cancer cells, many PKD1-targeting pharmacologic inhib- United States (74). Despite advances in the screening methods, itory compounds were assayed in these cell lines (84–88). Despite effective treatments of advanced androgen-independent tumors the fact that these molecules are not totally specific toward PKD1, are still to be found. they all induced cell growth arrest that can be reversed through Scores of studies analyzed the roles of PKD1 in prostate infection with adenovirus carrying PKD1 gene (87, 88) suggesting cancer cells. PKD1 was shown to be downregulated in metastatic a proproliferative role of PKD1 that appears discrepant with data androgen-independent prostate cancers compared with their presented in the previous paragraph. However, due to the drastic respective primary tumor (75, 76). Moreover, PKD1 is highly effects induced by these molecules on cell growth, PKD1 appears expressed in low proliferative and low metastatic androgen- more likely as a prosurvival factor as also suggested in LNCaP sensitive LNCaP cells and downregulated in the castration- where PKD1 was demonstrated to protect cells from PMA- resistant LNCaP-derivative cell line, C4-2 cells (androgen-hyper- induced apoptosis by promoting ERK and NFkB activities (84). sensitive), or in the highly metastatic androgen-insensitive DU145 Although an antiproliferative effect is compatible with a prosur- and PC3 cells (75–77). These results suggested an association vival role, these results indicate that more data are needed to better between the downregulation of PKD1 and the progression and understand the role of PKD1 in prostate cancer cells taking into aggressiveness of prostate cancer. Knockdown of PKD1 using account each particular cellular environment. shRNA enhanced cell growth (77), whereas its overexpression inhibited cell proliferation (77, 78). Moreover, curcumin was also thought to inhibit prostate cancer cell proliferation through a PKD1 Expression Is a Poor Prognostic PKD1-dependent mechanism (79). All these results indicated that PKD1 can be considered as an antiproliferative protein in Factor in Breast Tumors prostate cancer cells. Such an effect could be mediated through the Breast cancer is the most common cancer, and the second binding of PKD1 with b-catenin and the subsequent inhibition of leading cause of cancer-related death in women (89). Despite

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Table 3. Inhibition of PKD1 by various compounds

Name IC50 (mmol/L) Experimental approach to determine IC50 References 1-NA-PP1 0.155 0.022 In vitro kinase assay (87) 22.5 1.5 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (87) 23.3 5.7 Cell viability in PC3 cells (87) 1-NM-PP1 0.139 0.033 In vitro kinase assay (87) 2,6-naphthyridines From 0.0004 to >40 Time-resolved ﬂuorescence resonance transfer (TR-FRET) assay (110) 3,5-diarylazoles From 0.0037 to 6.9 Time-resolved ﬂuorescence resonance transfer (TR-FRET) assay (111) BPKDi 0.001 In vitro kinase assay (112) CID755673 From 15.510 2.550 to 46.700 27.650 Colorectal cancer cell proliferation determined by WST-1 assay (5 cell (56) lines studied) 0.182 0.027 In vitro kinase assay (85, 86, 113) 319.8 Cell viability in PC3 cells (85) 0.5 0.03 IMAP-based FP or TR-FRET kinase assays (86) 0.64 0.03 IMAP-FP PKD1 (113) 11.8 4.0 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (113) CID797718 7.0 0.83 IMAP-based FP or TR-FRET kinase assays (86) 2.13 0.21 In vitro kinase assay (86) 13.7 0.42 IMAP-FP PKD1 (113) 2.34 0.16 In vitro kinase assay (113) CRT0066101 From 0.770 0.250 to 1.560 0.340 Colorectal cancer cell proliferation determined by WST-1 assay (5 cell (56) lines studied) 1.000 Panc-1 cell proliferation (BrdU) (51) 0.001 In vitro kinase assay (51) CRT5 0.001 In vitro kinase assay (114) 17 Cell viability in HUVEC cells (114) Go6976€ 0.020 In vitro kinase assay (115) IKK-16 0.154 0.008 In vitro kinase assay (87) K252a 0.007 In vitro kinase assay (115) kb-NB142-70 0.026 0.006 In vitro kinase assay (116) 77.970 Cell viability in endothelial cells (56) From 2.820 0.67 to 8.35 4.31 Colorectal cancer cell proliferation determined by WST-1 assay (5 cell (56) lines studied) 2.2 0.6 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (85, 113) 0.0283 0.0023 In vitro kinase assay (85, 113) 8.025 Cell viability in PC3 cells (85) 0.71 0.02 IMAP-FP PKD1 (113) kb-NB165-09 0.0825 0.005 In vitro kinase assay (85, 113) 3.1 0.5 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (85, 113) 49.98 Cell viability in PC3 cells (85) kb-NB165-31 0.114 0.024 In vitro kinase assay (85, 113) 8.6 2.0 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (85, 113) 31.91 Cell viability in PC3 cells (85) kb-NB165-92 0.111 0.006 In vitro kinase assay (85, 113) 2.6 0.7 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (85, 113) 78.259 Cell viability in PC3 cells (85) kb-NB184-02 0.193 0.027 In vitro kinase assay (85, 113) 18.6 2.0 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (85, 113) 33.84 Cell viability in PC3 cells (85) SD-208 0.106 0.006 In vitro kinase assay (88) 17.0 1.5 Cellular inhibition of PKD1 autophosphorylation (in LNCaP cells) (88) 17.0 5.7 Cell viability in PC3 cells (88) VCC251801 0.028 0.002 In vitro kinase assay (116) 10.120 Cell viability in endothelial cells (116)

improvement in early detection and treatment of breast tumors, pared with normal cells where it is undetectable (91). Consis- advanced metastatic breast cancer remains life threatening. Accu- tently, a large-scale analysis performed in 152 malignant breast mulating evidence show a potential role of PKD1 in breast tumor tissues showed that patients with poor prognosis overexpressed progression. However, the link between PKD1 expression levels, PKD1, while those with good prognosis had signiﬁcantly lower PKD1 activation/activity, and tumor aggressiveness remains PKD1 expression levels (91) suggesting that PKD1 expression was unclear. In fact, PKD1 expression was shown to be high in normal positively linked to disease progression. Although PKD1 expres- breast tissues and reduced in more than 95% of invasive breast sion was shown to be regulated through epigenetic modiﬁcations cancer tissue and triple-negative tumors (9, 90). Conversely, its such as DNA methylation of its promoter sequence (9), its expression is markedly increased in breast cancer cell lines com- expression status should be more carefully studied in tissues and

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Pancreas PKD1 Endothelium Skin Notch Kidney Prostate ERK1/2 β-cat JNK NFkB Breast +++ Fibroblast Intesne ERK1/2 P HDAC

β-cat

FAK RSK ↓c-Jun

↑c-Fos ↑c-Fos NF-κB HDAC ↑c-Fos ² NUCLEUS

Figure 3. Proproliferative signaling pathways regulated by PKD1. The schematic representation illustrates how PKD1 may have proproliferative actions in various tissues. Activated PKD1 can increase the duration of the ERK1/2 signaling pathway, leading in some tissues to a significant increase in the phosphorylation of FAK and RSK, and the accumulation of the early-gene c-Fos. PKD1 has also been described to regulate JNK and NFkB activities. It can decrease the duration of the JNK signaling thus diminishing c-Jun activity, and stimulate NFkB signaling. PKD1 can also induce the translocation of b-catenin from the plasma membrane into the nucleus where b-catenin has a proliferative role through the induction of expression of proproliferative genes. PKD1 was also shown to act upstream of and activate the Notch pathway. Moreover, phosphorylation of HDACs (HDAC 4, 5 and 7) through a PKD1-dependent mechanism induces their nucleocytoplasmic shuttling. Once in the cytosolic compartment, HDACs were away from their target genes thus promoting gene expression and cell proliferation. b-cat, b-catenin; ERK, extracellular signal-regulated kinase; FAK, focal adhesion kinase; HDAC, histone deacetylase; JNK, c-Jun N-terminal kinase; PKD1, protein kinase D1; RSK, ribosomal S6 kinase. cell lines taking also into account the expression levels of other models in vivo (93). Altogether, these results strongly define PKD1 determinant markers of breast cancers' progression such as ERa, as a proproliferative and protumorigenic factor in breast cancer Her2, and progesterone receptor (PR). cells. Overall, PKD1 was shown to positively regulate cell proliferation. In fact, PKD1 overexpression strongly and specifically PKD1 Ectopic Expression Stimulates increased MCF-7 cell growth by promoting G0–G1 to S-phase transition of the cell cycle through a MEK/ERK-dependent sig- Pre-T-Cell Proliferation and Is a Potential naling pathway (92). Moreover, PKD1 overexpression improved Molecular Target in EBV-associated B-Cell anchorage- and growth factor–independent proliferation in vitro Lymphoma and promoted tumor growth in vivo (92). It also increased ERa expression further demonstrating the link that exists between As proactive members of the tumor microenvironment, these two proteins (91). Furthermore, PKD1 overexpression immune cells are main actors in tumor progression because increased MCF-7 cells' sensitivity to estradiol, their independence they can either positively or negatively regulate tumor growth toward estrogen for proliferation, and their partial resistance to depending on their nature, activity, and reciprocal interac- the antiestrogen, ICI 182,780 (91). Interestingly, this new cell tions. In the perspective of developing antitumor strategies, behavior looks like the one of prostatic C4-2 cells which, contrary it is therefore relevant to know whether PKD1 regulates to their parental cells, LNCaP, display an androgen-independent immune cell proliferation to anticipate, as much as possible, and -hypersensitive phenotype for proliferation associated, in this the potential consequences of targeting PKD1 in tumors. case, with a loss of PKD1 expression (see Chapter Prostate). More Studies have shown that PKD1 is not expressed in murine recently, our data were further confirmed in the drug-resistant T- and B-lymphocytes, nor in malignant B cells, nor in thymus – model of MCF-7-ADR cells expressing high levels of PKD1 as well and spleen (94 96), PKD2 being the major PKD isoform as cancer stemness markers compared with parental MCF-7 expressed. However, ectopic expression of a constitutively cells (93). In fact, knockdown of PKD1 by siRNA- or miRNA- active form of PKD1 induced pre-T-cell proliferation (97) targeting PKD1 in MCF-7-ADR cells was shown to reduce the illustrating again the proproliferative role of PKD1 when number of tumorspheres, to increase doxorubicin-induced apo- expressed and raising questions about its putative function ptosis in vitro, as well as to suppress tumor formation in xenograft in hematopoietic malignancies. Nevertheless, PKD1 was

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Youssef and Ricort

MMP MMP Lung MMP

Stomach PKD1

Prostate ERK

Akt ↓S6K E-cad E-cad PKD1 E-cad β-cat

β-cat AR ² NUCLEUS

Figure 4. Antiproliferative signaling pathways regulated by PKD1. The schematic representation illustrates how PKD1 may have antiproliferative actions in various tissues. PKD1 can inhibit downstream ERK and Akt pathways decreasing, in some tissues, S6K activity. PKD1 can inhibit AR transcriptional activity. It can also induce the membrane redistribution of E-cadherin and b-catenin, leading to the inhibition of the transcriptional activity of the latter. Furthermore, PKD1 can induce the expression of E-cadherin and metalloproteinases (MMP-2 and -9). b-cat, b-catenin; ERK, extracellular signal-regulated kinase; MMP, matrix metalloproteinases; PKD1, protein kinase D1; S6K, ribosomal S6 kinase.

shown not to be expressed in non-Hodgkin and Hodgkin and different strategies were developed in the attempt to inhibit lymphoma (98) and, although it regulates migration, PKD1 its activity. Among them, different pharmacologic inhibitors were has no proproliferative role in multiple myeloma (99). None- developed and characterized as to their effectiveness and speci- theless, Epstein–Barr virus (EBV) latent membrane protein-1 ficity toward PKD1 both in vitro and in vivo (Table 3). Their (LMP1) induces PKD1 expression in B-cell lymphoma and inhibitory characteristics (i.e., IC50) vary from one study to protects them from apoptosis. This contributes to the LMP-1– another because the experimental approaches used by authors induced drug resistance and progression of the pathology and to determine them were not normalized. Three main techniques makes PKD1 a potential molecular target in EBV-associated B- were commonly used such as the measurement of the phosphor- cell lymphoma (100). ylation of a PKD1 substrate, mainly syntide 2 in an in vitro kinase Furthermore, cancer development is associated with a local assay, the quantification of the cellular inhibition of the autopho- inflammatory response that generally surrounds the tumor. PKD1 sphorylation of the PKD1 serine 910 (human numbering) residue has a dual role considering inflammation because it was shown analyzed by Western immunoblotting (cellular inhibition of either to promote or inhibit inflammation through, among PKD1 autophosphorylation) and the analysis of their effects on others, the secretion of chemokines by mast cells (101–105). cell viability. Despite very variable characteristics, these com- However, the use of distinct inflammation-inducing agents and pounds were found to be effective in blocking proliferation and methods to analyze the inflammatory response makes it impos- other cellular functions such as invasion and migration of differ- sible to conclude, until now, about the role of PKD1 in this ent cell models making them promising inhibitors for cancer physiopathologic response. treatment (85). Unfortunately, they were also described to be too rapidly metabolized, which limited their efficacy in vivo. However, among them, CRT0066101 was shown to inhibit growth of Attempts to Develop Pharmacologic pancreatic, colorectal, bladder and triple-negative breast cancer in vivo Agents Altering PKD1 Activity cells xenografts [review in ref. (106); refs. 107, 108]. It thus appeared as a relatively good therapeutic candidate because it Because PKD1 was mainly described as a proproliferative blocked cell-cycle progression at the G1 phase and increased protein in several cancer cells both in vitro and in vivo, it emerged apoptosis by inhibiting the phosphorylation of "classical" pro- as an interesting putative therapeutic target to fight against tumors proliferative proteins such as Myc, MAPK1/3, Akt, Yap, and

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Cdc2 (107). Studies have also suggested that the overexpression activation process, a direct relationship between PKD1 activation þ and activation of PKD1 observed in CD34 skin stem cells and and its catalytic activity is not always observed. Consistently, the skin tumors are potential targets for the treatment of skin carci- development of robust techniques allowing the direct and precise nogenesis (31). However, to our knowledge, no PKD1 inhibitors measurement of PKD1 activity would be a major breakthrough in have been used in clinical trials and further studies are absolutely the field of PKD1 studies. Moreover, a very recent study showed necessary to notably increase the specificity of such compounds that a direct correlation cannot be automatically made between toward not only the two other members of the PKD family, PKD2 the relative PKD1 expression levels in normal and tumor tissues and PKD3, but also toward other protein kinases to only interfere and its role in tumorigenesis (109). In fact, PKD1 is significantly with PKD1-regulated (or -dysregulated) signaling pathways. In downregulated in head and neck localized tumors and metastases fact, many of these compounds cannot be considered as specific compared with normal tissues due to epigenetic modifications, inhibitors of PKD1 and should be used with great caution in suggesting an antiproliferative role of this protein. However, its experiments concluding about the specific role of PKD1 in several expression has been positively correlated with both the subcuta- cell functions. To this end, molecular extinction of PKD1 protein neous head and neck squamous cell carcinoma xenografts growth expression, when possible, remains a reliable technical approach and a sustained bombesin-induced ERK1/2 activation demon- to strengthen and comfort results obtained with pharmacologic strating a proproliferative role of this protein (109). In addition, compounds. despite numerous studies concerning PKD1, most of the data come from experimental studies and very few information come from cohort ones. Thus, obtaining in vivo data on large scales is Concluding Remarks also an important point to be considered to better understand the Whatever the cell type, the tissue, and its normal versus cancer role of this protein in the different tissues. status considered, it remains clear that PKD1 plays a crucial role in growth-dependent signaling pathways. However, due to its Disclosure of Potential Conflicts of Interest potential opposite functions, pro- or antiproliferative, illustrated No potential conflicts of interest were disclosed. in Figs. 3 and 4, respectively, the development and the putative use of PKD1-targeting inhibitors as therapeutic tools may be Acknowledgments considered with major caution. Some contradictory data exist The authors thank Dr. Sylvie Babajko for critical reading of the manu- but are sometimes the consequences of studies in which the script. This work was supported by the Centre National de la Recherche fi analysis of the PKD1 phosphorylation level was too rapidly Scienti que (CNRS) and the Ecole Normale Superieure Paris-Saclay (ENS Paris-Saclay). correlated to the activity of the protein. But many results clearly indicated that such a transposition cannot be made directly. Indeed, although the phosphorylation of PKD1 onto its serine Received February 1, 2019; revised June 5, 2019; accepted July 11, 2019; S738/742 and S910 residues seems to be important for its published first July 16, 2019.

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1974 Mol Cancer Res; 17(10) October 2019 Molecular Cancer Research

Deciphering the Role of Protein Kinase D1 (PKD1) in Cellular Proliferation

Ilige Youssef and Jean-Marc Ricort

Mol Cancer Res 2019;17:1961-1974. Published OnlineFirst July 16, 2019.

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