HIF-1Α: a Potential Treatment Target in Chronic Lymphocytic Leukemia
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Editorials HIF-1α: a potential treatment target in chronic lymphocytic leukemia Martina Seiffert Molecular Genetics, German Cancer Research Center (DKFZ), Heidelberg, Germany E-mail: MARTINA SEIFFERT - [email protected] doi:10.3324/haematol.2019.246330 n 2019, William Kaelin Jr., Peter J. Ratcliffe and Gregg in CLL change the binding of protein substrates including L. Semenza were jointly awarded the Nobel Prize in HIF-1α and are therefore involved in the increased stabili- IPhysiology or Medicine for their work in elucidating ty of HIF-1α in patients with these mutations. how the transcription factor Hypoxia-inducible factor 1 HIF-1α has been suggested to be transcriptionally regu- (HIF-1) senses oxygen availability and adapts cellular lated in CLL cells via signals delivered by the microenvi- response accordingly.1 HIF-1 is a heterodimeric protein ronment. In oxygenated blood, circulating CLL cells were complex that consists of two proteins: a constitutively shown to be primed for hypoxia and to undergo a rapid expressed HIF-1β subunit and an inducible HIF-1α sub- induction of HIF-1α activity when entering lymphoid tis- 6 unit. During hypoxia, HIF-1α hydroxylation is reduced, sues. In these tissues, CLL cells are in constant interaction preventing its proteasomal degradation, and the stabi- with accessory cells which deliver essential signals for sur- lized HIF-1 complex comprised of HIF1α and HIF-1β is vival and proliferation of CLL cells and contribute to transported to the nucleus where it regulates the expres- resistance to drug-induced apoptosis.7 The contact of CLL sion of several hundred genes to counter the lack of oxy- cells with stromal cells induces HIF-1α expression via gen. induction of phosphatidylinositol 3-kinase (PI3K) and ERK In solid cancer, hypoxia is widely involved in tumor mitogen-activated protein kinase (MAPK) signaling. In biology. Overexpression of HIF-1α is associated with addition, hypoxia in lymphoid tissues was shown to mod- aggressive cancer cell behavior and is correlated with poor ify central metabolic pathways in CLL cells, including overall patient survival. Tumor cells react to low oxygen increased adenosine generation and signaling via the A2A 8 levels by inducing HIF-1α expression, which results in an adenosine receptor. Inhibiting the A2A receptor has been activation of many crucial cancer hallmarks, such as angio- shown to counteract the effects of hypoxia in CLL and genesis, glucose metabolism, cell proliferation/viability, accessory cells, and to increase their susceptibility to phar- invasion and metastasis.2 Even though hypoxia was ini- macological agents. tially identified as a driver of HIF-1α expression, it has In line with this, Griggio et al. observed increased become clear in recent years that its overexpression in expression of HIF-1α in CLL cells upon co-culture with cancer can be also driven by genetic alterations, such as stromal cells to a level that is comparable to hypoxic con- 3 gain-of-function mutations in oncogenes or loss-of-func- ditions. They further showed that HIF-1α transcript lev- tion mutations in tumor-suppressor genes. els of primary CLL cells positively correlate with CLL cell In this issue of Haematologica, Griggio et al. report that survival in culture. This is likely related to the observation disrupting mutations in the tumor-suppressor gene TP53 that HIF-1α critically regulates the interaction of CLL cells are associated with higher HIF-1α levels and activity in with their microenvironment. It has previously been 3 patients with chronic lymphocytic leukemia (CLL). They shown that HIF-1α induces an upregulation of chemokine further show that elevated HIF-1α levels are due to an receptors, such as CXCR4, and cell adhesion molecules increased transcription as well as a decreased protein that control the interaction of leukemic cells with bone 9 degradation of HIF-1α. Evidence for both regulatory marrow (BM) and lymphoid microenvironments. In line mechanisms have already been described in CLL and are with this, inactivation of HIF-1α was shown to impair summarized in Figure 1. chemotaxis and cell adhesion to stroma, reduce BM and A constitutively higher expression and activity of HIF- spleen colonization in xenograft and allograft CLL mouse 1α in CLL cells compared to normal B cells was suggested models, and prolong survival of these mice. In support of to be due to reduced levels of the von Hippel-Lindau gene a modulation of CLL cell motility, HIF-1α transcript levels product (pVHL), which is responsible for HIF-1α degrada- were shown to correlate with the expression of CXCR4 tion. Downregulation of pVHL was attributed to the and other target genes in CLL patient samples. inhibitory activity of miR-155, a highly abundant Further evidence for stroma-induced activation of HIF- microRNA in CLL which is suggested to contribute to 1α in CLL comes from a comparative transcriptome analy- 4 increased HIF-1α stability in CLL cells. As pVHL expres- sis of primary CLL cells co-cultured or not with protective sion levels were lower in CLL patients with disrupted BM stromal cells which revealed that oxidative phospho- TP53 compared to wild-type TP53 cases, Griggio et al. sug- rylation, mitochondrial function and hypoxic signaling are gest that the observed accumulation of HIF-1α protein in the most significantly deregulated functions in non-pro- TP53-disrupted CLL might be partially explained by an tected CLL cells.10 These relevant transcriptomic changes 11 increased stability of HIF-1α. were compared to the Connectivity Map database, It has recently been suggested that the mutant forms of which contains transcriptomic responses of cell lines to 5 FBXW7 contribute to HIF-1α stability in CLL cells. treatment with small molecules. This comparison identi- FBXW7 is known to target proto-oncoproteins including fied drugs that act by repressing HIF-1α and disturbing HIF-1α for proteasomal degradation. In silico modeling intracellular redox homeostasis as potential compounds predicted that mutations in FBXW7 that recurrently occur blocking stroma-mediated support of CLL cells. 856 haematologica | 2020; 105(4) Editorials Figure 1. Schematic summary of mechanisms regulating HIF-1α in chronic lymphocytic leukemia (CLL). Contact of CLL cells with stromal cells induces transcrip- tional upregulation of HIF-1α via PI3K- or MAPK-signaling. Stability of HIF-1α is tightly connected to von Hippel-Lindau (VHL) tumor suppressor protein which triggers proteasomal degration of HIF-1α. In CLL, high levels of miR-155, disrupted p53, and mutant FBXW7 were connected to reduced HIF-1α degradation. Whether direct binding of p53 to HIF-1α is involved in the stability or transcriptional activity of HIF-1α in CLL remains unclear. Under hypoxic conditions, the tumor suppressor protein advantage in the hypoxic lymphoid microenvironment is p53 plays an important role in sensing stress and inducing still not clear. apoptosis in cells. It was suggested that hypoxia in tumors Based on its role and function in cancer, HIF-1 is a prom- represents a selection pressure for loss of functional p53 to ising target for the treatment of many tumor entities, and 12 avoid cell death. p53 directly interacts with HIF-1α as an a multitude of small molecules of diverse chemical com- unfolded protein, but the biological implications of this position and promising biological potency have been iden- interaction have not been completely clarified.13 Recent tified as inhibiting HIF-1 activity.15 Most of these sub- work showed that hypoxia-induced HIF-1α leads to TP53 stances are, however, only indirect inhibitors of HIF-1 and expression, where the resulting p53 protein has a reduced exhibit off-target effects by affecting other pathways as capacity to modulate transcription of target genes but is well, such as cell signaling, cell division, and DNA replica- abundantly available for protein-protein interactions.14 tion. To date, no HIF-1 inhibitor has been clinically Interestingly, both wild-type and mutant p53 proteins approved. were shown to bind and chaperone HIF-1α to stabilize its The potential of targeting HIF-1α in CLL has previously binding to DNA response elements and therefore impact been tested in immunocompromised mice that were on HIF-1α transcriptional activity. Whether direct interac- transplanted with the CLL cell line MEC-1, as well as in tions of p53 with HIF-1α play a role in CLL, or whether the syngeneic Em-TCL1 adoptive transfer mouse model of CLL cells with disrupted TP53 merely have a selective CLL.9 Camptothecin-11 (EZN-2208), a cytotoxic topoiso- haematologica | 2020; 105(4) 857 Editorials merase I inhibitor that also inhibits HIF-1α, was used for References treatment of these mice which affected homing and reten- 1. The Nobel Prize in Physiology or Medicine 2019. NobelPrize.org. tion of CLL cells in protective microenvironments, as Nobel Media AB 2020. Wed. 12 Feb 2020. <https://www.nobel- shown by reduced spleen weight and decreased coloniza- prize.org/prizes/medicine/2019/press-release/> tion of spleen and/or BM with CLL cells, and prolonged 2. Semenza GL. Targeting HIF-1 for cancer therapy. Nat Rev Cancer. survival of mice. As knockdown of HIF-1 did not cause 2003;3(10):721-732. α 3. Griggio V, Vitale C, Todaro M, et al. HIF-1α is overexpressed in apoptosis of CLL cells in vitro or in vivo, the authors sug- leukemic cells from TP53-disrupted patients and is a promising ther- gested that the pro-apoptotic effect of EZN-2208 towards apeutic target in chronic lymphocytic leukemia. Haematologica. CLL cells in vivo may be caused by mechanisms that do 2019;105(4):1042-1054. 4. Ghosh AK, Shanafelt TD, Cimmino A, et al. Aberrant regulation of not depend on HIF-1α inhibition. pVHL levels by microRNA promotes the HIF/VEGF axis in CLL B To further explore the potential of HIF-1α targeting in cells.