Emetine Promotes von Hippel-Lindau-Independent Degradation of Hypoxia-Inducible Factor-2α in Clear Cell Renal Carcinoma

Inactivating mutations of the von Hippel-Lindau (VHL) tumor suppressor gene are associated with inherited VHL syndrome, which is characterized by susceptibility to a variety of neoplasms, including central nervous system hemangioblastoma and clear cell renal cell carcinoma (CCRCC). Mutations in the VHL gene are also found in the majority of sporadic clear cell renal carcinoma, the most common malignant neoplasm of the human kidney. Inactivation of VHL ubiquitin ligase is associated with normoxic stabilization of hypoxia-inducible factor-1 alpha and 2-alpha (HIF-1 alpha and HIF-2 alpha), transcriptional regulators of tumor angiogenesis, invasion, survival, and glucose utilization. HIF-2 alpha has been particularly implicated in the development of CCRCC. Although several inhibitors of HIF-1 alpha have been described, these drugs typically have a minimal affect on HIF-2 alpha. 786-O is a VHL-deficient CCRCC cell line that constitutively expresses only HIF-2 alpha and is therefore suitable for the screening of novel HIF-2 alpha inhibitors. Using this cell line, we have identified emetine as a specific inhibitor of HIF-2 alpha protein stability and transcriptional activity. Without altering HIF-2 alpha mRNA level, emetine rapidly and dramatically down-regulated HIF-2 alpha protein expression in 786-O cells. HIF-2 alpha down-regulation was accompanied by HIF-2 alpha ubiquitination and was reversed by proteasome inhibition. Emetine-induced HIF-2 alpha down-regulation was confirmed in three additional VHL-renal cancer cell lines, was insensitive to the prolyl hydroxylase inhibitor dimethyloxaloyl glycine, and did not require neural precursor cell expressed developmentally down-regulated-8, suggesting that emetine accesses a previously undescribed cullin-independent proteasome degradation pathway for HIF-2 alpha. These data support the use of emetine or structurally related compounds as useful leads for the identification of novel HIF-2 alpha inhibitors.