Role of transforming growth factor-alpha in von Hippel-Lindau (VHL)(-/-) clear cell renal carcinoma cell proliferation: A possible mechanism couplingVHL tumor suppressor inactivation and tumorigenesis

Mutations of the VHL tumor suppressor gene occur in patients with VHL disea se and in the majority of sporadic clear cell renal carcinomas (VHL-/- RCC) , Loss of VHL protein function is associated with constitutive expression o f mRNAs encoding hypoxia-inducible proteins, such as Vascular endothelial g rowth factor. Overproduction of angiogenic factors might explain why VHL-/- RCC tumors are so highly vascularized, but whether this overproduction is sufficient for oncogenesis still remains unknown. In this report, we examin ed the activity of transforming growth factor-alpha (TGF-alpha), another VH L-regulated growth factor. We show that TGF-alpha mRNA and protein are hypo xia-inducible in VHL-/- RCC cells expressing reintroduced VHL, In addition to its overexpression by VHL-/- RCC cells, TGF-alpha can also act as a spec ific growth-stimulatory factor for VHL-/- RCC cells expressing reintroduced wild-type VHL, as well as primary renal proximal tubule epithelial cells, the likely site of origin of RCC, This role is in contrast to those of othe r growth factors overexpressed by VHL-/- RCC. cells, such as vascular endot helial growth factor and TGF-beta1, which do not stimulate RCC cell prolife ration. A TGF-cr-specific antisense oligodeoxynucleotide blocked TGF-alpha production in VHL-/- RCC cells, which led to the dependence of those cells on exogenous growth factors to sustain growth in culture. Growth of VHL-/- RCC cells was also significantly reduced by a drug that specifically inhibi ts the epidermal growth factor receptor, the receptor through which TGF-alp ha stimulates proliferation. These results suggest that the generation of a TGF-alpha autocrine loop as a consequence of VHL inactivation in renal pro ximal tubule epithelial cells may provide the uncontrolled growth stimulus necessary for the initiation of tumorigenesis.