P-Rex in Cancer

Author Manuscript Published OnlineFirst on June 10, 2013; DOI: 10.1158/1078-0432.CCR-12-1662 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited.

Molecular Pathways: P-Rex in cancer.

Atanasio Pandiella and Juan Carlos Montero

Instituto de Biología Molecular y Celular del Cáncer. Centro de Investigación del Cáncer.

CSIC-Universidad de Salamanca, Spain.

Running title: P-Rex in cancer.

Keywords: P-Rex, Rac, receptor tyrosine kinase, signalling.

Conflict of Interest: The authors declare no conflicts of interest.

Financial support: This work was supported by grants from the Ministry of Science and Innovation of Spain (BFU2009-07728/BMC) to AP; and Instituto de Salud Carlos III (CP12/03073 to JCM). Our Cancer Research Institute, and the work carried out at our laboratory receive support from the European Community through the regional development funding program (FEDER), and from the Fundación Ramón Areces.

Correspondence:

Atanasio Pandiella Instituto de Biología Molecular y Celular del Cáncer-CIC. Campus Miguel de Unamuno 37007-Salamanca, Spain Phone and fax: +34 923 294815 e-mail: [email protected]

Downloaded from clincancerres.aacrjournals.org on September 28, 2021. © 2013 American Association for Cancer Research. Author Manuscript Published OnlineFirst on June 10, 2013; DOI: 10.1158/1078-0432.CCR-12-1662 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited.

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ABSTRACT

P-Rex proteins are Rho/Rac guanine nucleotide exchange factors which participate in

the regulation of several cancer-related cellular functions such as proliferation, motility and

invasion. Expectedly, significant part of these actions of P-Rex proteins must be related to

their Rac regulatory properties. In addition P-Rex proteins control signaling by the PI3K route

by interacting with PTEN and mTOR. The interaction with PTEN inhibits its phosphatase

activity, leading to AKT activation. The interaction with mTOR may be important in nutrient-

stimulated Rac activation and migration. In humans, several studies have implicated P-Rex

proteins in the pathophysiology of various neoplasias. Thus, overexpression of P-Rex proteins

has been linked to poor patient outcome in breast cancer, and may facilitate metastatic

dissemination of prostate cancer cells. In addition, whole genome sequencing described P-

Rex2 as a significantly mutated gene in melanoma. Furthermore, expression in melanocytes

of mutated forms of P-Rex2 found in melanoma patients demonstrated the protumorigenic

role of these P-Rex mutations in melanoma genesis. These evidences open interesting

opportunities for P-Rex targeting in cancer. Moreover, the implication of P-Rex partner

proteins such as Rac, mTOR or PTEN in cancer has opened the possibility of acting on P-Rex

to restrict protumorigenic signaling through these pathways.

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BACKGROUND

P-Rex proteins have recently been involved in the pathophysiology of several

neoplasias, opening the possibility of their targeting with therapeutic purposes (1). P-Rex

proteins belong to the large family of guanine nucleotide exchange factors (GEFs) which act

on the Rho/Rac family of GTPases (2, 3). Rho/Rac proteins play important roles in several

physiological processes, and have also been implicated in neoplastic transformation and

metastatic dissemination (4). Expectedly, substantial part of the roles of P-Rex proteins in

cellular physiology must be due to their Rac regulatory properties. In addition, P-Rex proteins

may also control signaling by the phosphoinositide 3-kinase (PI3K) route (5, 6), and carry out

Rac-independent actions (7). In this paper, we will review the functions of P-Rex proteins,

and the potential relevance of their targeting in cancer.

P-Rex genes and protein isoforms.

The first member of the P-Rex family, P-Rex1, was identified in a search for

phosphatidylinositol-(3,4,5)-trisphospahate (PIP3)-activated Rac GEFs (8). The isolated

protein was named P-Rex1 for PIP3-dependent Rac exchanger. Up to six different P-Rex

proteins derive from two genes, PREX1 and PREX2 (9). Transcription from the PREX1 gene

may generate three different P-Rex1 protein isoforms of 186 (isoform 1), 110 (isoform 2), and

175 kDa (isoform 3) (Figure 1A). The PREX2 gene generates three isoforms of 182 (also

known as P-Rex2a), 171, and 111 kDa (also known as P-Rex2b) (9).

P-Rex proteins contain an N-terminal Dbl-homology (DH) domain, the typical catalytic

domain of the GEFs which act on the Rho/Rac family of GTPases (Figure 1A). P-Rex

proteins also include a pleckstrin homology (PH) domain, as well as two pairs of DEP and

PDZ domains. The C-terminal region includes a domain homologous to inositol

polyphosphate 4-phosphatase (IP4P) (8, 10), which is absent in P-Rex2b (11).

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Physiological relevance of P-Rex proteins.

P-Rex1 is mainly expressed in peripheral blood leukocytes and brain, and is also

expressed in several other tissues (8, 11-14). P-Rex2 is widely expressed, and is strongly

present in skeletal muscle, small intestine, heart, lung and placenta, being absent from

peripheral blood leukocytes (10, 11, 13).

P-Rex1-/- mice are viable and apparently healthy, but have mild neutrophilia (15). In the

neutrophils of these mice the G protein coupled receptor (GPCR)-dependent Rac activation

and recruitment to inflammatory sites was impaired (15, 16). In leukocytes, P-Rex1 has been

involved in integrin-mediated rolling and intravascular crawling (17). In lung microvascular

endothelial cells, P-Rex1 participated in TNF-α-stimulated Rac activation and ROS

production (18). Removal of P-Rex1 significantly reduced neutrophilic transendothelial

migration and leukocyte sequestration in TNF-α-challenged mouse lungs. The P-Rex1

knockout mice were also refractory to lung vascular hyper-permeability and edema in a LPS-

induced sepsis model. P-Rex2b also appears to be relevant in the regulation of Rac1 activation

and cell migration of endothelial cells (19).

P-Rex proteins are widely expressed in the nervous system (13), and have been involved

in neuronal migration (14), differentiation (7), and memory-related functions (20).

Transfection of P-Rex1 inhibited neurite sprouting in PC12 cells (7). Moreover, expression of

an N-terminal truncated form of P-Rex had the same action as full length P-Rex1, suggesting

that the neurite-regulating functions of P-Rex1 may be partially independent of its Rac

regulatory properties (7). On the other hand, expression of a mutant lacking the DH domain,

which acts as a dominant negative form with respect to the GEF activity of P-Rex1, decreased

neuronal migration induced by neurotrophic factors (14). With respect to P-Rex2, mice

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lacking the PREX2 gene are viable and fertile, but develop a mild motor coordination defect

and have alterations of the Purkinje cell dendrite morphology (13).

Together, these studies demonstrate the non-redundant functions of P-Rex proteins in

animal physiology. This is relevant if one considers the large complement of Rac GEF factors

reported which are also widely expressed (21).

Regulation of P-Rex proteins.

The activation of P-Rex is directly and synergistically produced by the binding of PIP3

to the PH domain, and the Gβγ subunits to the DH domain (8, 22, 23) (Figure 1B). These

bindings favor translocation of P-Rex from the cytosol to the plasma membrane, where P-Rex

GEF activity is higher (8, 23). Deletion studies indicated that the DH/PH domains are

sufficient to activate P-Rex by PIP3 (22). The other domains (two DEP, two PDZ and IP4P)

keep the catalytic activity of full-length P-Rex1 low (22), suggesting that interactions between

the DH/PH domains and the other domains may autoinhibit P-Rex activity.

Phosphorylation of P-Rex represents an important mechanism of regulation of its

function. Several reports have indicated that P-Rex1 is phosphorylated at multiple sites (9, 24,

25), and the phosphorylation of these sites regulates P-Rex1 activity in an opposite fashion. In

breast cancer cells, phosphorylation of Ser313 and Ser319, located in the PH domain, inhibits P-

Rex1 activity under resting conditions and mutation to non-phosphorylatable residues

augments Rac activity (9). Stimulation of ErbB receptors, which increases Rac activity in

those cells, was accompanied by dephosphorylation of Ser313 and Ser319 (9). Interestingly, β-

adrenergic receptor activation increases the phosphorylation of P-Rex1, likely through PKA,

and such a phosphorylation diminishes P-Rex1 GEF activity (24). Moreover,

dephosphorylation of P-Rex1 by λ-phosphatase (24) or protein phosphatase 1α (PP1α) (25)

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increases the activity of P-Rex1. In aggregate, these reports support the concept that

phosphorylation of P-Rex1 at certain residues inhibits its GEF activity towards Rac.

Phosphorylation of P-Rex1 at other sites activates its Rac GEF function. In breast

cancer cells, stimulation of ErbB receptors caused phosphorylation of Ser605 and Ser1169 (9).

Mutagenesis experiments demonstrated that phosphorylation of Ser1169 facilitated P-Rex1

GEF function. Therefore, stimulation of ErbB receptors activates a

phosphorylation/dephosphorylation cycle of P-Rex1 which results in dephosphorylation of

inhibitory residues (Ser313, Ser319) and phosphorylation of stimulatory residues (Ser605,

Ser1169) to control de GEF activity of P-Rex1.

The PI3K/mTOR route and P-Rex proteins.

Of particular relevance is the interplay among the phosphoinositide 3-kinase (PI3K)

route and P-Rex proteins. P-Rex1 was initially described as a Rac GEF up-modulated by the

βγ subunits of heterotrimeric G proteins and by PIP3, which is generated by PI3K (8). P-Rex

proteins interact with mTOR and PTEN, two important players in PI3K signal transduction (5,

6). The interaction of P-Rex1 and P-Rex2 with mTOR occurs through the DEP domains of the

P-Rex proteins, and regulates nutrient-stimulated Rac activation and cell migration (6).

P-Rex may participate in the up-regulation of the activity of the PI3K pathway by

antagonizing PTEN (5). This latter protein dephosphorylates PIP3, an important lipid second

messenger implicated in the regulation of cellular proliferation and survival (26). Inactivation

of PTEN leads to increased levels of PIP3, and therefore promotes cellular responses, such as

augmented cell proliferation or decreased cell death which are hallmarks of oncogenic

progression. In breast cancer cells, Fine et al. (5) found that PTEN was able to bind P-Rex2a.

Such interaction decreased the phosphatase activity of PTEN, and resulted in augmented

phosphorylation of AKT at activating residues. Interestingly, this action on PTEN was

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independent on the GEF activity of P-Rex2a. In MCF10A cells which express endogenous

active PTEN, forced expression of P-Rex2a augmented AKT phosphorylation at serine 473,

and favored the formation of multiacinar structures (5), a feature that in these cells has been

associated to a pretumorigenic status (27). In ovarian cancer cells expression of P-Rex1 has

been shown to promote activation of AKT1 and P-Rex1 knockdown inhibited AKT1(28).

CLINICAL-TRANSLATIONAL ADVANCES

Several circumstances indicate that P-Rex proteins may play a role in the

pathophysiology of various neoplastic diseases. On one hand, P-Rex expression levels have

been shown to correlate with patient outcome in several neoplasias. Moreover, molecular

alterations of P-Rex have been described in melanoma, and some of these alterations

promote tumorogenesis. On the other hand, P-Rex proteins have been shown to participate

in the regulation of several prooncogenic properties such as migration, cell proliferation, or

invasion. These clinicopathological and biological findings open the possibility of targeting

P-Rex proteins with therapeutic purposes. Moreover, the implication of P-Rex partner

proteins such as Rac, mTOR or PTEN in cancer offers the possibility of acting on P-Rex to

restrict protumorigenic signaling through these pathways.

Expression and molecular alterations of P-Rex proteins in cancer.

Studies in breast (9, 29) and prostate cancer (30) demonstrated a link between P-Rex

expression and patient outcome. Sosa et al. found that >50% of breast tumors stained for P-

Rex1 (29). Normal breast tissue and the stroma did not stain for P-Rex1. P-Rex1 staining was

higher in patients with metastatic disease, suggesting a role of P-Rex1 in metastatic

dissemination. Montero et al analyzed P-Rex expression in breast tumors by quantitative

Western blotting (9). Expression of P-Rex was found to be variable among different patient

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samples. Kaplan-Meier analyses confirmed that patients whose breast tumors expressed high

levels of P-Rex1 had a shorter disease-free survival.

PREX1 is located on chromosome 20q13.13, a region commonly amplified in breast

tumors (31) and some breast cancer cell lines (32). The PREX2 gene is located on

chromosome 8q13, a region frequently amplified in breast, prostate or colorectal cancer (33-

35). Analysis of the COSMIC database indicates that mutations in PREX2 are present in 3%

of tumors, a frequency slightly higher than mutations in PREX1 (2% of tumors). Mutations in

PREX1 and PREX2 have a tendency to cluster in the GEF exchange region. Additional

mutations in PREX1 accumulate in the IP4P region. Skin and lung cancers represent the

tumors which most frequently bear PREX mutations. Around 10% of lung cancers contain

PREX1 or PREX2 mutations. Mutations of PREX1 (0.99%) or PREX2 (1.17%) in breast

cancer are scarce, and are absent in hematopoietic cancers.

In skin cancer, 23% of tumors carry PREX2 mutations, and 3.6% of tumors bear PREX1

mutations. Massive sequencing of melanoma genomes identified PREX2 as a significantly

mutated gene, accounting for a 14% frequency in a cohort of 107 human melanomas (36).

These mutations were distributed along the entire length of P-Rex2, and included non-

synonymous PREX2 mutations as well as truncations (36). Biological experiments in which

some of the P-Rex2 mutations found in patients were reproduced in TERT-immortalized

human melanocytes demonstrated the role of this GEF in melanoma genesis, and

demonstrated that PREX2 mutations generate altered proteins with oncogenic activity (36).

Prometastatic properties of P-Rex.

In prostate cancer cell lines and human samples expression of P-Rex1 strongly

correlated with the metastatic phenotype (30). Qin et al (30) reported that P-Rex1 acted

through Rac to facilitate migration and invasion of prostate cancer cells upon activation of the

epidermal growth factor receptor. Moreover, overexpression of P-Rex1 in a non-metastatic

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cell line increased its motility and invasion properties. In their prostate cancer models, P-Rex1

did not affect prostate tumor growth, but caused spontaneous lymph node metastases.

Similarly, in ovarian cancer cells knockdown of P-Rex1 inhibited insulin-like growth factor-1

(IGF-1)-induced activation of cell migration or invasion (28). In breast cancer cells, activation

of ErbB or chemokine receptors facilitated cellular migration in a P-Rex dependent manner

(9, 29).

In the skin, Lindsay et al. showed that P-Rex1 was required for efficient melanoblast

migration and melanoma dissemination (37). Using an animal model of melanoma, which

included mutant N-Ras as well as deletion of the INK4a tumor suppressor, the authors

described decreased melanoma metastases when these mice were crossed with P-Rex1-/- mice

(37). Interestingly, no differences in tumor latency or burden in the primary melanomas were

observed when P-Rex+/+ mice and P-Rex-/- mice were compared. In line with these data

Campbell et at (38) reported that P-Rex1 favored the invasiveness of WM852 melanoma

cells. Therefore, targeting of P-Rex1 in prostate, ovarian, breast or skin cancer may be

beneficial in fighting disease dissemination.

Role of P-Rex proteins in proliferation.

While the above reports support a prometastatic role of P-Rex proteins, their

intermediate action in cell proliferation is less clear. In breast cancer cells, knockdown of P-

Rex1 reduced cell proliferation both under resting conditions and after ligand-induced

stimulation of ErbB receptors (9, 29). However, the lack of effect of P-Rex knockdown on the

proliferation of prostate cancer cells (30) or tumor burden in melanoma (37) contrasts with the

data obtained in breast cancer, and indicates intertumoral differences in the protumorigenic

actions of P-Rex proteins.

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P-Rex proteins in prooncogenic signaling

The role of P-Rex proteins as mediators of signaling by the cell surface receptors,

especially those involved in the pathophysiology of several neoplasias, merits consideration.

Initial work suggested a potential role of P-Rex1 in signaling by GPCRs. This idea was

confirmed by showing the intermediate action of P-Rex1 in migration upon activation of the

chemokine receptor CXCR4 (29).

In addition to the intermediate role of P-Rex in GPCR signaling, evidence is

accumulating suggesting that P-Rex proteins may play a relevant role in signaling by different

classes of cell surface receptors. This idea is supported by the fact that the RTKs ErbB (9),

PDGFRβ (38), TrkA (7) and IGF-1R (39) utilize P-Rex. That P-Rex proteins may act as a

general intermediate in signaling by RTKs is a relevant concept, especially because of the

important roles of these receptors in oncology and physiology. It will be necessary to analyze

whether RTKs other than those already reported use P-Rex as part of their signal transduction

machinery. These studies may uncover novel roles of P-Rex proteins in mediating

physiological actions of RTKs which may be relevant in tumor growth. An example of the

latter is the role of P-Rex proteins in glucose metabolism. P-Rex1 has been reported to

regulate the membrane exposure of the glucose transporter GLUT4 in adipocytes in response

to IGF-1R activation (39). This action indicates that P-Rex proteins may control supply of

metabolic energy to cells by up-regulating glucose transport into their cytosol. Obviously, this

effect of P-Rex could be of relevance for the fitting of the tumoral tissues, but such

physiological action may even overweight its oncological relevance in case P-Rex proteins

are demonstrated to play important roles in insulin receptor-mediated glucose transport.

Mechanistically, it will be important to define whether RTKs and other cell surface

receptors activate P-Rex by the same mechanism used by the ErbB receptors, i.e. by

activating the P-Rex phosphorylation/dephosphorylation cycle. It will also be relevant to

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identify the kinases and phosphatases involved in controlling that cycle, and the signaling

cascades which mediate activation of those kinases and phosphatases. Finally, elucidation of

additional P-Rex-interacting proteins could help in understanding the protein networks in

which P-Rex proteins participate.

Targeting P-Rex in cancer.

Given the ample number of Rac GEFs (21), we may consider unreasonable to target P-

Rex proteins. However, clinicopathological as well as preclinical data support roles of P-Rex

proteins in human cancer, opening therapeutic options for the targeting of P-Rex proteins in

cancer. In fact, P-Rex expression has been correlated with patient outcome in breast and

prostate cancers, and reduction of P-Rex expression has been shown to restrict proliferation

and/or metastatic dissemination (9, 29, 30). Therefore, targeting P-Rex proteins may be

therapeutically relevant in neoplasias in which those proteins play an important role in

regulating their growth or dissemination.

Of particular relevance is the role of P-Rex proteins in melanoma pathophysiology. The

fact that mutated forms of P-Rex2 have been detected in melanoma and are tumorigenic (36),

raises the possibility of targeting P-Rex2 in melanomas bearing such mutations. Moreover,

Lindsay et al demonstrated that P-Rex1 was required for efficient melanoblast migration and

melanoma dissemination (37). Interestingly, expression of P-Rex1 in melanoma cell lines has

been reported to be controlled by the ERK route (40). This is relevant given the high

frequency of activation of the ERK pathway in melanomas due to upstream mutations in

BRAF (36). It is possible that BRAF inhibitors used in the clinic, such as vemurafenib (41),

or other BRAF/ERK pathway inhibitors may reduce P-Rex1 levels and therefore contribute to

preventing P-Rex1-facilitated melanoma dissemination.

Besides controlling P-Rex proteins by down regulating their cellular levels, another way

of acting on these proteins may be the use of strategies which affect their functionality. Such

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strategies may include inhibition of the Rac-GEF activity of P-Rex proteins or their

interaction with mTOR or PTEN. Inhibition of the Rac-GEF activity of P-Rex proteins could

be possible by using compounds that prevent interaction of the PH domain of P-Rex proteins

with PIP3, or that directly inhibit the GEF activity of the DH domain. Interference with the

GEF activity should reduce the Rac-mediated actions of P-Rex proteins, especially those

related to cellular motility and invasion. Reduction of the interaction of P-Rex proteins with

mTOR and PTEN could be used to restrict P-Rex-dependent PI3K signaling. Development of

compounds which may inhibit interaction of the DEP domain of P-Rex proteins with mTOR

are expected to reduce P-Rex/mTOR-dependent Rac activation and migration. Similarly,

preventing the binding of the PH domain of P-Rex proteins to PTEN may allow the latter to

inhibit PI3K signaling.

In addition to the above mentioned strategies which directly target P-Rex proteins,

indirect ways to reduce P-Rex signaling may be contemplated. These strategies rely on the use

of drugs that act on signaling proteins upstream or downstream of P-Rex, such as inhibitors of

RTKs, GPCRs, Rac, or dual PI3K/mTOR inhibitors. It is expected that increasing the

knowledge about the regulatory mechanisms responsible for controlling P-Rex functioning

may allow the development of additional strategies. As an example, augmenting the

phosphorylation of P-Rex1 at inhibitory residues may also be exploited to reduce its

functionality. This would require unveiling the kinases and phosphatases responsible for

maintaining P-Rex in its “off” state (Figure 1B).

In conclusion, the ample range of physiological cancer-related functions of P-Rex,

together with the accumulating evidences that link P-Rex proteins to several types of

neoplasias, open possibilities for the targeting of P-Rex proteins with therapeutic purposes.

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FIGURE LEGEND

Figure 1. A, Schematic representation of the different P-Rex protein isoforms. The

different domains and the number of residues of the different proteins are shown. The

properties of different regions in P-Rex, as well as proteins interacting with such regions are

described. B, Regulation of P-Rex function. Under resting conditions, P-Rex proteins are

located in the cytosol. In these conditions, phosphorylation at inhibitory residues (labeled in

red), restricts P-Rex activity. Phosphorylation at these inhibitory residues may be mediated by

kinases such as PKA. Upon stimulation of GPCR or receptor tyrosine kinases, P-Rex may

translocate to the plasma membrane, an effect facilitated by the interaction between the PH

domain of P-Rex and PIP3. Interaction between Gβγ subunits of G proteins and the GEF

domain of P-Rex may also favor membrane translocation of P-Rex. Receptor activation also

causes dephosphorylation of inhibitory residues, together with phosphorylation of other

activatory residues (labeled in green). This results in up-regulation of P-Rex GEF function,

causing release of GDP from Rac, and loading of Rac with GTP. In addition, P-Rex proteins

may interact and regulate the function of several key components of the PI3K pathway, such

as mTOR or PTEN. The interaction of mTOR with P-Rex proteins occurs through the DEP

domains of the latter, and regulates nutrient-stimulated Rac activation and cell migration. The

interaction of P-Rex proteins with PTEN inhibits the phosphatase activity of the latter,

increasing the amount of active AKT. The composite actions of P-Rex on various signaling

pathways contribute to cancer-related functions of P-Rex, such as cell proliferation,

migration, or invasion.

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Figure 1:

A Rac-GEF activity PIP3 binding Gaf binding PTEN binding Interaction with PP1alpha

Interaction with mTOR

DH PH DEP PDZ InsPx 4-phosphatase aa 1659 P-Rex1 isoform 1

aa 956 P-Rex1 isoform 2

aa 1561 P-Rex1 isoform 3

aa 1606 P-Rex2 isoform 1 (P-Rex2a)

aa 1504 P-Rex2 isoform 2

aa 979 P-Rex2 isoform 3 (P-Rex2b)

B Ligand

GPCR RTK Extracellular

PIP3 PIP2 Intracellular P P PTEN p85 Gα Akt p110 f (active) a P mTOR P PI3K

P-Rex Kinases Rac Rac ON Phosphatases GDP GTP Kinases (PP1α) (PKA) PP Metastasis Invasion Migration P Motility Proliferation P-Rex Adhesion Off

Molecular Pathways: P-Rex in cancer.

Atanasio Pandiella and Juan Carlos Montero

Clin Cancer Res Published OnlineFirst June 10, 2013.

Updated version Access the most recent version of this article at: doi:10.1158/1078-0432.CCR-12-1662

Author Author manuscripts have been peer reviewed and accepted for publication but have not yet been Manuscript edited.

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