Is HIF-1 a Valid Therapeutic Target?

Hypoxia, a reduction in the normal level of tissue oxygen tension, occurs during several pathophysiological processes including tumorigenesis. It may occur during the initial avascular phase or develop in established tumors as a result of new blood vessel formation which is ineffective and provides poor blood flow. Although hypoxia generates a situation unfavorable for cell growth, cancer cells may undergo a series of genetic and metabolic changes that allow them to survive and even proliferate. Tumor hypoxia plays a major role in the induction of the “angiogenic switch” during tumor development. Targeting the critical steps in tumor angiogenesis, including hypoxia-induced pathways, has become a focus for many basic research scientists and clinical investigators. Hypoxia is associated with poor treatment outcome regardless of the modality, suggesting that it should be considered in the development of optimum treatment strategies. One mechanism through which tumor cells respond to reduced oxygen levels is via activation of hypoxia-inducible transcription factors (HIF) (1). HIF-1 is a heterodimer consisting of the hypoxic response factor HIF-1α and the constitutively expressed receptor nuclear activator ARNT or HIF-1β (2). In the presence of oxygen, HIF-1α is bound to the tumor suppressor von Hippel–Lindau (VHL) protein that determines the ubiquitination and consequent proteosomal degradation of the transcription factor (3). In the absence of oxygen, HIF-1α degradation is impaired with consequent binding to hypoxia-response genes, such as the proangiogenic vascular endothelial growth factor (VEGF) (4). To date, three members of the HIFα family have been cloned: HIF-1α HIF-2α and HIF-3α. Whether these components are redundant with overlapping functions or whether they have distinct biological properties remains to be revealed. Of the HIFα subunits, the function of HIF-1α has been the most extensively characterized. A growing body of evidence suggests that HIF-1α is an important contributor to tumor progression and metastasis. Remaining questions related to the role of HIFs in general, and HIF-1α in particular, is whether they are in all cases a positive regulator of tumor growth (5,6) and whether HIF-1α is the only member of the HIFα family capable of promoting tumor growth (7,8). In this issue of the Journal, Yeo et al. (9) describe a novel and important biological activity of the vascular agent YC-1. Previously the authors reported that YC-1 downregulates the expression of two hypoxia-related genes, erythropoietin (EPO) and VEGF in vitro, and inhibits the expression of HIF-1α at the post-translational level by an undefined mechanism (10). In the current study, the authors provide evidence that YC-1 targets HIF-1α and tumor angiogenesis in vivo, induces both delayed tumor growth and tumor regression in the hepatoma model Hep3B, and slows tumor growth in a number of other relevant tumor models. The observed effects on tumor growth are quite dramatic. These data are consistent with the notion that inhibition of HIF-1α is responsible for both decreased angiogenesis and inhibited tumor growth. HIF-1α may be an important and approachable therapeutic target, but whether YC-1 is the most appropriate agent to do so is unclear. YC-1 belongs to a class of agents called soluble guanylyl cyclase (sGC) stimulators (11). Soluble guanylyl cyclase is the receptor for the ubiquitous biological messenger nitric oxide (NO) and is involved in many signal transduction pathways, most notably, in regulating vascular tone and platelet function. A number of compounds in this class, including YC-1, have been identified that demonstrate antihypertensive and antiplatelet activity along with penile erectile stimulation in preclinical models. The precise mechanism underlying the decrease of HIF-1α protein expression by YC-1 is unclear, yet Yeo et al. (9) suggest that the antiangiogenic effect of YC-1 is not related to the activation of sGC. This may need further investigation, given the important role of NO and other elements of this pathway in endothelial cell biology. Various experimental observations support the existence of a biological link between NO and angiogenesis (12). NO substantially contributes to the survival of capillary endothelium by triggering cell growth and differentiation via endothelial NO synthase activation and cyclic GMP-dependent gene transcription. Elevation of NO activity induces HIF-1α stabilization and correlates with angiogenesis and tumor aggressiveness. In turn, HIF-1α activates NO synthase, further promoting angiogenesis and vasodilation (13). Given the findings of Yeo et al. (9) with YC-1, it is surprising that this agent has been shown to release NO from endothelial cells (14). Taken together, these observations argue strongly for a careful mechanistic evaluation of YC-1 action so that the antitumor effect can be isolated from confounding effects. The clinical development of YC-1 or other sGC activators as antiangiogenesis drugs may be hampered by their secondary effects on the vascular compartment. The data presented by Yeo et al. (9), demonstrate that relatively high drug concentrations (>5–10 μM) are required to achieve significant in vitro inhibition of HIF-1α and VEGF. The biologically effective antitumor dose might therefore induce changes in vascular tone and platelet aggregation. Whether the mouse is a good model to evaluate these effects is questionable. Elucidation of the mechanism underlying YC-1 induced downregulation of HIF-1α protein expression should also lead to the development of compounds that lack the side effects on vascular smooth muscle cells and platelets. A challenge for the clinical development of potential angiogenesis inhibitors such as YC-1 lies in determining the biological effective dose and in assessing tumor response. Establishment of validated surrogate markers will be crucial in the clinical testing of these agents (15). Noninvasive imaging techniques hold great promise but are still at early stages of development. Imidazole and non–imidazole-containing imaging agents have been developed with the purpose of imaging areas of hypoxia by positron emission tomography (PET) and single photon emission computed tomography (SPECT) (16,17). SPECT imaging studies to assess the extent of apoptosis occurring in vivo, such as quantitative analysis of 99mTc-annexin V uptake in human tumors and its relationship with therapy outcome, are under development. There is confidence that novel radiopharmaceuticals resulting in higher tumor-to-background ratios will assist the drug development of agents targeting tumor hypoxia and angiogenesis such as YC-1. Understanding this difficult area becomes more challenging with reports that suggest that in response to hypoxia cancer cells may activate both proangiogenic (VEGF) and apoptotic (NIP3) pathways (18). How this balance is regulated and whether it depends on the tumor type or the microenvironment remains to be determined. As noted by Harris (1), there is still a question of whether hypoxia generates an aggressive tumor phenotype or whether an aggressive tumor phenotype generates hypoxia. The implications for therapy would obviously differ. There seems little question that hypoxia inducible pathways and HIF-1α are therapeutic targets worthy of attention. Important oncogenic signaling pathways regulate HIF-1α independently of hypoxia, adding to the potential importance of this factor. Strategies under evaluation include the use of an antisense construct (19) and a cell-based high throughput screen for the identification of small-molecule inhibitors of the HIF-1 pathway (20). YC-1 may not be the first clinical agent in this class, but the studies reported here should refocus our attention on this important target.