The Hypoxic Inducible Stress Response as a Target for Cancer Drug Discovery

Abstract

All solid tumors experience some degree of hypoxia. The response to the stress of hypoxia is mediated in large part by the hypoxia inducible factor-1 (HIF-1) transcription factor that increases the expression of a variety of genes that allow the tumor to survive and grow in the hostile hypoxic environment. Increased tumor HIF-1 has been correlated with increased angiogenesis, aggressive tumor growth, and poor patient prognosis. This has lead to the current interest in HIF-1 as a cancer drug target. HIF-1 activity in tumors depends on the availability of the HIF-1alpha subunit, the levels of which increase under hypoxic conditions or through the activation of oncogenes and/or inactivation of tumor-suppressor genes. HIF-1alpha level and HIF-1 activity are regulated by multiple pathways involving transcription, translation, post-translational modification, and the interaction with other transcription factors. A number of agents have been reported to block HIF-1 activity, acting on all of these mechanisms. Not all of the agents have been shown to block tumor HIF-1 activity in vivo. Some have shown marked HIF-1 inhibition and anti-tumor activity. There are agents already, or soon to be, tested in the clinic as anti-tumor inhibitors of HIF-1. The challenges will be to determine whether the effects of these agents that are seen is due to HIF-1 inhibition and to identify which patients are most likely to benefit from treatment with HIF-1 inhibitors.