The role of VHL in clear-cell renal cell carcinoma and its relation to targeted therapy

Abstract

The basic biology underlying the development of clear-cell renal cell carcinoma (ccRCC) is critically dependent on the von Hippel-Lindau gene (VHL), whose protein product is important in the cell's normal response to hypoxia. Aberrations in VHL's function, either through mutation or promoter hypermethylation, lead to accumulation of the transcriptional regulatory molecule, hypoxia-inducible factor alpha (HIFalpha). HIFalpha can then dimerize with HIFbeta and translocate to the nucleus, where it will transcriptionally upregulate a series of hypoxia-responsive genes, including vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and others. Binding of these ligands to their cognate receptors activates a series of kinase- dependent signaling pathways, including the RAF-MEK-ERK and phosphatidylinositol-3 kinase-AKT-mTOR pathways. Targeted agents developed and now approved for use in advanced ccRCC include humanized monoclonal antibodies against VEGF, small-molecule tyrosine kinase inhibitors, and inhibitors of mTOR. Understanding the biology of ccRCC is critical in understanding the current therapy for the disease and in developing novel therapeutics in the future. This review will provide an overview of the genetics of ccRCC, with an emphasis on how this has informed the development of the targeted therapeutics for this disease.

Figure 1. VHL and HIFα

In the presence of normal tissue oxygenation levels (depicted to the left in figure), prolyl hydroxylases hydroxylate HIFα. Once hydroxylated, an E3 ligase complex binds HIFα in a process that is VHL dependent. This leads to ubiquitination (Ub) of HIFα, marking it for degradation by the proteasomal machinery of the cell. In the right of the figure is depicted the disruption of this normal regulatory process when VHL function is aberrant. In the absence of functional VHL, the E3 ligase complex cannot bind HIFα, irrespective of its hydroxylation status. This leads to accumulation of HIFα in the cell cytoplasm, allowing it to dimerize with the constitutively present HIFβ, translocate to the nucleus, bind to hypoxia response elements, and regulate gene transcription. HIFα, hypoxia-inducible factor alpha; HIFβ, hypoxia-inducible factor beta; HRE, hypoxia response element; Ub, ubiquitin; VHL, von Hippel–Lindau.

Figure 2. Hypoxia responsive genes

Binding of the HIFα–HIFβ heterodimer to the HRE in the promoter region results in transcriptional upregulation of genes important to the cell’s response to hypoxia. These include vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), transforming growth factor alpha (TGFα), carbonic anhydrase IX (CA-IX), erythropoietin (EPO), glucose transporter 1 (GLUT-1), and BCL2/adenovirus E1B 19 kDa-interacting protein 3 (BNIP3). Binding of ligands, such as VEGF, to their respective cell surface receptors, such as VEGFR, leads to tyrosine phosphorylation of the receptor and subsequent downstream signaling through several kinase-dependent pathways, such as the PI3K–Akt–mTOR pathway and the RAS–RAF–MEK–ERK pathway. Note that mTOR leads to upregulation of basal levels of HIFα. Also shown within these pathways is a subset of the known therapeutic target sites for the monoclonal antibody to VEGF (bevacizumab), the tyrosine kinase inhibitors (sorafenib and sunitinib), and the mTOR inhibitors (temsirolimus and everolimus). AKT, protein kinase B; HIFα, hypoxia-inducible factor alpha; HIFβ, hypoxia-inducible factor beta; HRE, hypoxia response element; mTOR, mammalian target of rapamycin; PI3K, phosphatidylinositol-3 kinase.