Hypoxia-inducible factor-1α (HIF-1α) and all-trans retinoic acid (ATRA) afford protection in several experimental models of kidney disease. HIF-1α 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) α, β and γ, which mediate its effects on target genes. ATRA also up-regulates the expression of RAR α, β and γ 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-1α under normoxic conditions and also synergizes with hypoxia leading to the over-expression of HIF-1α and vascular endothelial growth factor-A, a HIF-1α-regulated renal protector. ATRA-induced HIF-1α expression involved stabilization of HIF-1α mRNA but not of HIF-1α protein. (ii) Expression of HIF-1α is an absolute requirement for the transcriptional up-regulation of RARβ by ATRA. Transfection with HIF-1α siRNA abolished the induction by ATRA of the expression of both RARβ mRNA and protein while treatment with HIF-1α inhibitor YC-1 results in the abolishment of ATRA-induced activity of a retinoic acid-response element (RARE) construct from the RARβ promoter. (iii) Hypoxia up-regulates RARβ through HIF-1α since this effect was inhibited by HIF-1α knockdown. In contrast to ATRA-induced RARβ up-regulation, induction of RARβ expression by ATRA did not involve transcriptional up-regulation as hypoxia did not increase the expression of RARβ mRNA or the activity of the RARE construct. These results suggest the presence of crosstalk between hypoxia/HIF-1α and ATRA/RARβ that may be physiologically and pharmacologically relevant.
