Epidermal growth factor receptor transactivation by intracellular prostaglandin E2-activated prostaglandin E2 receptors. Role in retinoic acid receptor-β up-regulation.

The pharmacological modulation of renoprotective factor vascular endothelial growth factor-A (VEGF-A) in the proximal tubule has therapeutic interest. In human proximal tubular HK-2 cells, treatment with all-trans retinoic acid or prostaglandin E2 (PGE2) triggers the production of VEGF-A. The pathway involves an initial increase in intracellular PGE2, followed by activation of EP receptors (PGE2 receptors, most likely an intracellular subset) and increase in retinoic acid receptor-β (RARβ) expression. RARβ then up-regulates transcription factor hypoxia-inducible factor-1α (HIF-1α), which increases the transcription and production of VEGF-A. Here we studied the role in this pathway of epidermal growth factor receptor (EGFR) transactivation by EP receptors. We found that EGFR inhibitor AG1478 prevented the increase in VEGF-A production induced by PGE2- and all-trans retinoic acid. This effect was due to the inhibition of the transcriptional up-regulation of RARβ, which resulted in loss of the RARβ-dependent transcriptional up-regulation of HIF-1α. PGE2 and all-trans retinoic acid also increased EGFR phosphorylation and this effect was sensitive to antagonists of EP receptors. The role of intracellular PGE2 was indicated by two facts; i) PGE2-induced EGFR phosphorylation was substantially prevented by inhibitor of prostaglandin uptake transporter bromocresol green and ii) all-trans retinoic acid treatment, which enhanced intracellular but not extracellular PGE2, had lower effect on EGFR phosphorylation upon pre-treatment with cyclooxygenase inhibitor diclofenac. Thus, EGFR transactivation by intracellular PGE2-activated EP receptors results in the sequential activation of RARβ and HIF-1α leading to increased production of VEGF-A and it may be a target for the therapeutic modulation of HIF-1α/VEGF-A.