Protein Kinase Cα-dependent Phosphorylation of the mRNA-stabilizing Factor HuR: Implications for Posttranscriptional Regulation of Cyclooxygenase-2

Abstract

In this study, we investigated the molecular mechanisms underlying the ATP analogue adenosine-5′-O-(3-thio)triphosphate–induced nucleocytoplasmic shuttling of the mRNA stabilizing factor HuR in human (h) mesangial cells (MC). Using synthetic protein kinase C (PKC) inhibitors and small interfering RNA approaches, we demonstrated that knockdown of PKCα efficiently blocked the ATP-dependent nuclear HuR export to the cytoplasm. The functional importance of PKCα in HuR shuttling is highlighted by the high cytosolic HuR content detected in hMC stably overexpressing PKCα compared with mock-transfected cells. The ATP-induced recruitment of HuR to the cytoplasm is preceded by a direct interaction of PKCα with nuclear HuR and accompanied by increased Ser phosphorylation as demonstrated by coimmunoprecipitation experiments. Mapping of putative PKC target sites identified serines 158 and 221 as being indispensable for HuR phosphorylation by PKCα. RNA pull-down assay and RNA electrophoretic mobility shift assay demonstrated that the HuR shuttling by ATP is accompanied by an increased HuR binding to cyclooxygenase (COX)-2 mRNA. Physiologically, the ATP-dependent increase in RNA binding is linked with an augmentation in COX-2 mRNA stability and subsequent increase in prostaglandin E₂ synthesis. Regulation of HuR via PKCα-dependent phosphorylation emphasizes the importance of posttranslational modification for stimulus-dependent HuR shuttling.