Epigenetic activation of mu opioid receptor (MOR) gene via increased expression and function of mitogen- and stress-activated protein kinase 1 (MSK1)

55455 This article has not been copyedited and formatted. The final version may differ from this version. This article has not been copyedited and formatted. The final version may differ from this version. ABSTRACT Since the discovery of mu opioid receptor (MOR) gene two decades ago, various regulatory factors have been shown to interact with the MOR promoter and modulate transcript levels. However, the majority of early transcriptional studies on MOR gene have not addressed how intracellular signaling pathways mediate extracellular modulators. In this study, we demonstrate that MOR epigenetic regulation requires multiple coordinated signals converged at the MOR promoter, involving mitogen-activated protein kinase (MAPK) activation and mitogen-and stress-activated protein kinase (MSK1)-similar ranges of intracellular signaling pathways that are activated by opioid agonists. Inhibiting p38 MAPK or ERK 1/2 MAPK (upstream activators of MSK1) reduced MOR expression levels, and accordingly, the functional role of MSK1, but not MSK2, was demonstrated using genetic approaches. However, for maximal MSK1 effect, an open chromatin configuration was required, because in-vitro CpG methylation of the MOR promoter abolished MSK1 activity. Finally, endogenous MSK1 levels concomitantly increased to regulate MOR gene expression during neuronal differentiation of P19 cells, suggesting a conserved role of this kinase in the epigenic activation of MOR in neurons. Taken together, our findings indicate that the expression of MOR gene requires the activity of intracellular signaling pathways that have been implicated in the behavioral outcomes of opioid drugs, which suggests that an auto-regulatory mechanism may function in opioid systems.