Melatonin Sensitizes Human Myometrial Cells to Oxytocin in a Protein Kinase Cα/Extracellularly Regulated Kinase-Dependent Manner

Context: Studies have shown that labor occurs primarily in the night/morning hours. Recently, we identified the human myometrium as a target for melatonin (MEL), the neuroendocrine output signal coding for circadian night. Objective: The purpose of this study was to determine the signaling pathway underlying the effects of MEL on contractility and the contractile machinery in immortalized human myometrial cells. Design: To ascertain the signaling pathway of MEL leading to its effects on myometrial contractility in vitro, we performed gel retraction assays with cells exposed to iodo-MEL (I-MEL) with or without oxytocin and the Rho kinase inhibitor Y27632. I-MEL effects on inositol trisphosphate (IP3)/diacylglycerol (DAG)/protein kinase C (PKC) signaling were also investigated. Additionally, we assayed for caldesmon phosphorylation and ERK1/2 activation. Results: I-MEL was found to activate PKCα via the phospholipase C/IP3/DAG signaling pathway, which was confirmed by PKC enzyme assay. I-MEL did not affect myosin light chain phosphatase activity, and its effects on contractility were insensitive to Rho kinase inhibition. I-MEL did increase phosphorylation of ERK1/2 and caldesmon, which was inhibited by the MAPK kinase inhibitor PD98059 or the PKC inhibitor C1. Conclusions: MEL sensitizes myometrial cells to subsequent procontractile signals in vitro through activation of the phospholipase C/IP3/DAG signaling pathway, resulting in specific activation of PKCα and ERK1/2, thereby phosphorylating caldesmon, which increases actin availability for myosin binding and cross-bridging. In vivo, this sensitization would provide a mechanism for the increased nocturnal uterine contractility and labor that has been observed in late-term human pregnancy.