Mutual regulation of hypoxic and retinoic acid related signalling in tubular proximal cells

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) and all-trans retinoic acid (ATRA) afford protection in several experimental models of kidney disease. HIF-1 alpha protein is degraded under normoxia but stabilized by hypoxia, which activates its transcription factor function. ATRA activates another set of transcription factors, the retinoic acid receptors (RAR) alpha, beta and gamma, which mediate its effects on target genes. ATRA also up-regulates the expression of RAR alpha, beta and gamma at the transcriptional level. Here we demonstrate the presence of mutual regulation of hypoxic and retinoic acid related signalling in tubular proximal cells. In human proximal tubular HK-2 cells we have found that: (i) ATRA treatment induces HIF-1 alpha under normoxic conditions and also synergizes with hypoxia leading to the over-expression of HIF-1 alpha and vascular endothelial growth factor-A, a HIF-1 alpha-regulated renal protector. ATRA-induced HIF-1 alpha expression involved stabilization of HIF-1 alpha mRNA but not of HIF-1 alpha protein. (ii) Expression of HIF-1 alpha is an absolute requirement for the transcriptional up-regulation of RAR beta by ATRA. Transfection with HIF-1 alpha siRNA abolished the induction by ATRA of the expression of both RAR beta mRNA and protein while treatment with HIF-1 alpha inhibitor YC-1 results in the abolishment of ATRA-induced activity of a retinoic acid-response element (RARE) construct from the RAR beta promoter. (iii) Hypoxia up-regulates RAR beta through HIF-1 alpha since this effect was inhibited by HIF-1 alpha knockdown. In contrast to ATRA-induced RAR beta up-regulation, induction of RAR beta expression by ATRA did not involve transcriptional up-regulation as hypoxia did not increase the expression of RAR beta mRNA or the activity of the RARE construct. These results suggest the presence of crosstalk between hypoxia/HIF-1 alpha and ATRA/RAR beta that may be physiologically and pharmacologically relevant. (C) 2011 Elsevier Ltd. All rights reserved.