Inhibition of Mxi1 suppresses HIF-2alpha-dependent renal cancer tumorigenesis.

In clear cell renal cancers, the primary molecular defect is inactivation of the von Hippel-Lindau (VHL) gene. Loss of pVHL, the VHL gene product, leads to constitutive activation of hypoxia-inducible factor (HIF) signaling. While downregulation of HIF suppresses tumor formation by pVHL-defective renal carcinoma cells, the relative contribution of individual HIF regulated genes to HIF-dependent tumorigenesis remains under investigation. Mxi1, a c-Myc antagonist, is a HIF target gene that inhibits mitochondrial biogenesis, reprograms cellular energy metabolism, and protects cells from c-Myc-dependent apoptosis in vitro. In the present study we show that Mxi1 is overexpressed in primary human clear cell kidney cancers. Inhibition of Mxi1 in pVHL-defective kidney cancer cells using shRNA alters their cell cycle parameters, inhibits their ability to invade matrigel, and suppresses their ability to form tumors in vivo. Compared to Mxi1-proficient tumors, Mxi1-deficient tumors display reduced cellular proliferation. These results establish Mxi1 as an important downstream target of HIF that contributes to pVHL-deficient renal cancer tumorigenesis.