microRNA-25 targets PKCζ and protects osteoblastic cells from dexamethasone via activating AMPK signaling

AMP-activated protein kinase (AMPK) activation could protect osteoblasts from dexamethasone (Dex). This study aims to provoke AMPK activation via microRNA downregulation of its negative regulator protein kinase C sigma (PKC sigma). Results show that microRNA-25-5p (miR-25-5p) targets PKC sigma's 3' untranslated regions (UTRs). Forced-expression of miR-25 downregulated PKC sigma and activated AMPK in human osteoblastic cells (OB-6 and hFOB1.19 lines), which thereafter protected cells from Dex. Reversely, expression of antagomiR-25, the miR-25 inhibitor, upregulated PKC. and inhibited AMPK activation, exacerbating Dex damages. Notably, PKC sigma shRNA knockdown similarly activated AMPK and protected osteoblastic cells from Dex. AMPK activation was required for miR-25-induced osteoblastic cell protection. AMPKa shRNA or dominant negative mutation almost completely blocked miR-25-induced cytoprotection against Dex. Further studies showed that miR-25 expression increased NADPH activity and suppressed Dex-induced oxidative stress in osteoblastic cells. Such effects by miR-25 were abolished with AMPKa knockdown or mutation. Significantly, miR-25-5p level was increased in patients' necrotic femoral head tissues, which was correlated with PKC sigma downregulation and AMPK hyper-activation. These results suggest that miR-25-5p targets PKC sigma and protects osteoblastic cells from Dex possibly via activating AMPK signaling.