Estrogen receptor α is a novel target of the Von Hippel-Lindau protein and is responsible for the proliferation of VHL-deficient cells under hypoxic conditions

Abstract

The Von Hippel-Lindau gene (VHL) is frequently deleted or mutated in human renal cell carcinoma (RCC) at the early stage. According to the well-established theory, pVHL acts as a tumor suppressor through its E3 ligase activity, which targets hypoxia-inducing factor-1 alpha (HIF-1 alpha). However, the elevated expression of HIF-1 alpha did not promote cell proliferation, indicating that there would be another target, which could promote cell proliferation at the early cancer stage of RCC. In this study, we show that estrogen receptor-alpha (ER-alpha) is a novel proteasomal degradation target of the pVHL E3 ligase. Indeed, the overexpression of VHL suppresses exo- and endogenous ER-alpha expression, whereas si-pVHL can increase ER-alpha expression. The negative regulation of pVHL on ER-alpha expression is achieved by its E3 ligase activity. Thus, pVHL can promote the ER-alpha ubiquitinylation. In addition, we revealed that ER-alpha and HIF-1 alpha are competitive substrates of pVHL. Thus, under normal conditions, ER-alpha overexpression can increase the transcription factor activity of HIF-1 alpha. Under the hypoxic condition, where HIF-1 alpha is not a suitable target of pVHL, ER-alpha is more rapidly degraded by pVHL. However, in VHL-deficient cells, the expression of ER-alpha and HIF-1 alpha is retained, so that the hypoxic condition did not suppress cell proliferation obviously compared with cells that are expressing pVHL. Thus, blocking of ER-alpha using its inhibitor could suppress the proliferation of VHL-deficient cells as effectively as hypoxia-induced growth suppression. Considering our results, blocking of ER-alpha signaling in VHL-deficient cancer cells would be beneficial for cancer suppression. Indeed, we showed the anti-proliferative effect of Faslodex in VHL-deficient cells.