Hypoxia Inducible Factor Activates the Transforming Growth Factor-α/Epidermal Growth Factor Receptor Growth Stimulatory Pathway in VHL^-/- Renal Cell Carcinoma Cells

Bi-allelic-inactivating mutations of the VHL tumor suppressor gene are found in the majority of clear cell renal cell carcinomas (VHL^-/- RCC). VHL^-/- RCC cells overproduce hypoxia-inducible genes as a consequence of constitutive, oxygen-independent activation of hypoxia inducible factor (HIF). While HIF activation explains the highly vascularized nature of VHL loss lesions, the relative role of HIF in oncogenesis and loss of growth control remains unknown. Here, we report that HIF plays a central role in promoting unregulated growth of VHL^-/- RCC cells by activating the transforming growth factor-α (TGF-α)/epidermal growth factor receptor (EGF-R) pathway. Dominant-negative HIF and enzymatic inhibition of EGF-R were equally efficient at abolishing EGF-R activation and serum-independent growth of VHL^-/- RCC cells. TGF-α is the only known EGF-R ligand that has a VHL-dependent expression profile and its overexpression by VHL^-/- RCC cells is a direct consequence of HIF activation. In contrast to TGF-α, other HIF targets, including vascular endothelial growth factor (VEGF), were unable to stimulate serum-independent growth of VHL^-/- RCC cells. VHL^-/- RCC cells expressing reintroduced type 2C mutants of VHL, and which retain the ability to degrade HIF, fail to overproduce TGF-α and proliferate in serum-free media. These data link HIF with the overproduction of a bona fide renal cell mitogen leading to activation of a pathway involved in growth of renal cancer cells. Moreover, our results suggest that HIF might be involved in oncogenesis to a much higher extent than previously appreciated.