Retinoic acid increases hypoxia-inducible factor-1α through intracrine prostaglandin E(2) signaling in human renal proximal tubular cells HK-2.

We have previously shown in HK-2 cells that ATRA (all-trans-retinoic acid) up-regulates HIF-1α (hypoxia-inducible factor-1α) in normoxia, which results in increased production of renal protector VEGF-A (vascular endothelial growth factor-A). Here we investigated the role of COXs (cyclooxygenases) in these effects and we found that, i) ATRA increased the expression of COX-1 and COX-2 mRNA and protein and the intracellular levels (but not the extracellular ones) of PGE(2). Furthermore, inhibitors of COX isoenzymes blocked ATRA-induced increase in intracellular PGE(2), HIF-1α up-regulation and increased VEGF-A production. Immunofluorescence analysis found intracellular staining for EP1-4 receptors (PGE(2) receptors). These results indicated that COX activity is critical for ATRA-induced HIF-1α up-regulation and suggested that intracellular PGE(2) could mediate the effects of ATRA; ii) Treatment with PGE(2) analog 16,16-dimethyl-PGE(2) resulted in up-regulation of HIF-1α and antagonists of EP1-4 receptors inhibited 16,16-dimethyl-PGE(2)- and ATRA-induced HIF-1α up-regulation. These results confirmed that PGE(2) mediates the effects of ATRA on HIF-1α expression; iii) Prostaglandin uptake transporter inhibitor bromocresol green blocked the increase in HIF-1α expression induced by PGE(2) or by PGE(2)-increasing cytokine interleukin-1β, but not by ATRA. Therefore only intracellular PGE(2) is able to increase HIF-1α expression. In conclusion, intracellular PGE(2) increases HIF-1α expression and mediates ATRA-induced HIF-1α up-regulation. Copyright Â