Clear cell renal cell carcinoma (ccRCC), which accounts for approximately 85% of all renal cancers, is resistant to a variety of cancer therapies and is highly lethal. A hallmark of ccRCC is the inactivation of the von Hippel Lindau (VHL) tumor suppressor gene, which is inactivated either by mutation or hypermethylation in up to 90% of ccRCC. While pVHL functions as an E3 ubiquitin ligase adaptor protein that promotes the degradation of hypoxia inducible (HIF?) transcription factors, other key pVHL substrates are only now emerging and their functional roles in renal cancer remains undefined. Here we identify a zinc-finger/homeodomain protein ZHX2 as a potential novel pVHL substrate by using a newly developed genome-wide in vitro expression strategy coupled with GST-binding screening. Our preliminary data suggests that pVHL interacts with and degrades ZHX2 in a hydroxylase activity-dependent manner, similar to regulation of the HIF? proteins. Functionally, knockdown of ZHX2 blocks cell proliferation, soft agar growth and xenograft tumor growth of pVHL-deficient renal cancer cells. Importantly, ZHX2 expression levels are strongly elevated in ccRCC tumors compared to normal patients. Mechanistically, ZHX2 interacts with the RelA/p65 subunit of NF-kB and positively regulates pVHL-loss induced RelA/p65 nuclear localization. Integrated analyses of ChIP-Seq and microarray also reveal that ZHX2 regulates both NF-kB-dependent and independent pathways in ccRCC. Therefore, we hypothesize that ZHX2 promotes renal oncogenesis through both NF-kB dependent and independent pathways following VHL inactivation. This is the first study directed at a pro-oncogenic function for ZHX2, with the focus on its key role downstream of the loss of pVHL in renal cancer.
In Specific Aim 1, we will study the mechanism by which ZHX2 is regulated by pVHL in a hydroxylation-dependent manner.
In Specific Aim 2, we will determine the functional significance of deregulated ZHX2 in pVHL-deficient renal cancer.
In Specific Aim 3, we will determine the mechanisms by which ZHX2 regulates NF-kB-dependent and -independent signaling and renal tumorigenesis upon pVHL loss. Successful completion of this proposal would provide significant new molecular insight into oncogenic mechanisms associated with the great majority of ccRCC as well as the potential for new therapies for this typically lethal disease.
Loss of expression of the key E3 ubiquitin ligase component pVHL is a driving oncogenic event in the great majority of clear cell renal carcinoma. pVHL is known to downregulate the key transcription factor HIF, but reports exist that HIF-independent functions may be critical for the development of renal cancer upon loss of pVHL. Here we provide evidence for a novel substrate of pVHL, ZHX2, and show that ZHX2 functions as a critical oncoprotein in pVHL-deficient renal cancer potentially via NF-kB dependent and independent signaling pathways. The studies, if successful, will provide critical insight into the development of pVHL deficient renal cancer and also provide a foundation for new therapeutic approaches for this lethal disease.