By now you’re probably noticing a running theme at OncoBites: cancer is complicated. Cancers have many moving parts, and despite our best efforts it can be difficult to know what factors drive them. Even when we have identified causal mutations in genes, it can be difficult to figure out how these mutations contribute to cancer development. Many proteins have multiple functions, and even when we think we understand how a mutation contributes to cancer, we may later find that the story is more complicated than we previously knew.
One reason why it has been so difficult to figure out the role of mutations in cancer is that we have an incomplete understanding of the normal function of many proteins. In fact, we often discover how proteins function in normal biology by first understanding what happens when they are mutated in cancer. This has turned out to be the case with mutations in Von Hippel-Lindau (VHL) in clear cell renal cell carcinoma (ccRCC), the most common form of kidney cancer. The VHL gene was initially discovered in studies of a familial tumor syndrome in which one copy of the VHL gene is lost. Patients with Von Hippel-Lindau syndrome are prone to developing many kinds of tumors, including ccRCC. Loss of function mutations in VHL occurs in 90% of ccRCC cases. Since the loss of VHL is so prevalent in ccRCC and is sufficient to promote tumor formation in individuals with Von Hippel-Lindau syndrome, VHL must play an important role in preventing cancer development.
Two papers published in Science in 2001 identified the VHL protein as a regulator of the stability of other proteins.1,2 The VHL protein is a ubiquitin ligase, an enzyme that adds a small protein tag to other proteins. These proteins, the substrates of VHL, are then marked for destruction by the cellular protein destruction machinery. VHL thus regulates the levels of its substrates by promoting their destruction. The Hypoxia-Inducible Factors (HIFs) are such substrates of VHL. The HIF proteins are upregulated in ccRCC and other cancers and help tumors to survive and thrive in low oxygen environments by inducing the growth of blood vessels to provide the tumor with nutrients. Under normal oxygen conditions, when the HIF proteins are not needed, they are hydroxylated, modified by the addition of an oxygen group. When hydroxylated, HIFs interact with VHL and are ubiquitinated, which marks them for destruction. Hydroxylation, ubiquitination, and destruction of HIF proteins ensure that the overall amount of these proteins are maintained at low levels. Under low oxygen conditions, the HIFs do not get hydroxylated and are not marked for destruction by ubiquitination by VHL. Instead, the proteins accumulate and move into the nucleus to induce the expression of their target genes. In ccRCC and other tumors lacking VHL function, HIF proteins accumulate in this way and promote blood vessel growth. Drugs that inhibit blood vessel growth are the first line of defense against ccRCC, but VHL-deficient tumors typically become resistant to these drugs. Loss of VHL is critically important in these tumors, but it is not clear whether regulation of the HIF proteins is the only function of VHL important in these tumors. If HIF isn’t the only important VHL substrate in ccRCC, perhaps targeting other substrates of VHL could be effective in treating these tumors.
This month in Science, Zhang and colleagues identified Zinc fingers and homeoboxes 2 (ZHX2) as a new substrate of VHL that promotes tumor growth. The authors found that like the HIF proteins, ZHX2 is a transcription factor whose protein levels are regulated through hydroxylation, ubiquitination by VHL, and degradation. Likewise, turning off ZHX2 in ccRCC cells led to decreased cellular proliferation and decreased expression of gene pathways implicated in cancer development following VHL loss. These findings suggest that VHL loss contributes to ccRCC development through regulation of both HIF proteins and ZHX2. If this is the case, ZHX2 could also be a potential drug target, opening up new possibilities for treating ccRCC.
If cancer is complicated, it’s because biology is complicated. Historically, we have learned about normal biology by studying disease. The identification of genes that are altered in cancer has led to the discovery of critical biological processes, and in turn, we apply those insights of basic biology to inform further research to improve human health.
- Ivan, M. et al. HIFalpha targeted for VHL-mediated destruction by proline hydroxylation: implications for O2 sensing. Science 292, 464–8 (2001).
- Jaakkola, P. et al. Targeting of HIF-alpha to the von Hippel-Lindau ubiquitylation complex by O2-regulated prolyl hydroxylation. Science 292, 468–72 (2001).
- Zhang, J. et al. VHL substrate transcription factor ZHX2 as an oncogenic driver in clear cell renal cell carcinoma. Science 361, 290–295 (2018).
Networks by Jurgen Appelo on Flickr (Featured Image)
Metastatic renal carcinoma by Yale Rosen