Hypoxia-inducible factor (HIF)-independent expression mechanism and novel function of HIF prolyl hydroxylase-3 in renal cell carcinoma

Abstract

We previously found that hypoxia-inducible factor (HIF) prolyl hydroxylase-3 (PHD3) was frequently overexpressed in renal cell carcinomas (RCCs), unlike in normal tissues, and therefore, we studied the mechanism and role of PHD3 expression in RCC. The von Hippel-Lindau (VHL)-gene-mutant RCC cell lines SMKT-R2 and SMKT-R3 and wild-type VHL cell lines Caki-1 and ACHN were used. Associations of the expression of PHD3 with HIF-α proteins and signal transduction pathways were evaluated under normoxic conditions. The effect of PHD3 on cell proliferation was also examined by small interference RNA and cDNA transfection. Moreover, the prognostic impact of PHD3 expression in clear cell RCC (CCRCC) was evaluated using primary cancer tissues. In SMKT-R2 and SMKT-R3, HIF-α proteins were expressed and PHD3 was highly expressed. On the other hand, ACHN had low expression of HIF-α proteins and PHD3. However, Caki-1 had high expression of PHD3 even though there was no distinct expression of HIF-α proteins. PHD3 expression was inhibited by blockade of Akt and mammalian target of rapamycin (mTOR), but not by HIF-1α and HIF-2α double knockdown. In addition, PHD3 knockdown resulted in the promotion of cell proliferation in SMKT-R2, SMKT-R3 and Caki-1. On the other hand, forced expression of PHD3 reduced cell proliferation in ACHN. In immunohistochemistry, PHD3 expression was a significant factor for better recurrence-free survival in patients with CCRCC. PHD3 expression can be induced by the phosphatidylinositol-3 kinase/Akt/mTOR pathway in RCC independently of HIF proteins. Furthermore, PHD3 has an antiproliferative function independent of HIF protein status in RCC, indicating a novel expression mechanism and function of PHD3.