Regulation of STRA13 by the von Hippel-Lindau tumor suppressor protein, hypoxia, and the UBC9/ubiquitin proteasome degradation pathway.

In this study, we focus on different modes of regulation of STRA13, a human ortholog of the mouse basic helix-loop-helix transcriptional factor, previously identified by us as a new von Hippel-Lindau tumor suppressor gene (VHL) target. The gene was overexpressed in VHL-deficient cell lines and tumors, specifically clear cell renal carcinomas and hemangioblastomas. Introduction of wild type VHL transgene into clear cell renal carcinoma restored low level expression of STRA13. Overexpression was also detected in many common malignancies with an intact VHL gene, suggesting the existence of another, VHL-independent pathway of STRA13 regulation. Similar to many other von Hippel-Lindau tumor-suppressor protein (pVHL) targets, the expression of STRA13 on the mRNA level was hypoxia-sensitive, indicating oxygen-dependent regulation of the gene, presumably through the pVHL/hypoxia-inducible factor 1 (HIF-1) pathway. The yeast two-hybrid screening revealed interaction of the STRA13 protein with the human ubiquitin-conjugating enzyme (UBC9) protein, the specificity of which was confirmed in mammalian cells. By adding the proteasome inhibitor acetyl-leucinyl-leucinyl-norleucinal, we demonstrated that the 26 S proteasome pathway regulates the stability of pSTRA13. Co-expression of STRA13 and UBC9 led to an increase of the pSTRA13 ubiquitination and subsequent degradation. These data established that UBC9/STRA13 association in cells is of physiological importance, presenting direct proof of UBC9 involvement in the ubiquitin-dependent degradation of pSTRA13. Hypoxia treatment of mammalian cells transiently expressing STRA13 protein showed that stability of pSTRA13 is not affected by hypoxia or VHL. Thus, STRA13, a new pVHL target, is regulated in cells on multiple levels. We propose that STRA13 may play a critical role in carcinogenesis, since it is a potent transcriptional regulator, abundant in a variety of common tumors.