Mitogen-activated protein kinase (MAPK) activated by prostaglandin E₂ phosphorylates connexin 43 and closes osteocytic hemichannels in response to continuous flow shear stress

Cx43 hemichannels serve as a portal for the release of prostaglandins, a critical process in mediating biological responses of mechanical loading on bone formation and remodeling. We have previously observed that fluid flow shear stress (FFSS) opens hemichannels; however, sustained FFSS results in hemichannel closure, as continuous opening of hemichannels is detrimental to cell viability and bone remodeling. However, the mechanism that regulates the closure of the hemichannels is unknown. Here, we show that activation of p44/42 ERK upon continuous FFSS leads to Cx43 phosphorylation at Ser²⁷⁹-Ser²⁸², sites known to be phosphorylated sites by p44/42 MAPK. Incubation of osteocytic MLO-Y4 cells with conditioned media (CM) collected after continuous FFSS increased MAPK-dependent phosphorylation of Cx43. CM treatment inhibited hemichannel opening and this inhibition was reversed when cells were pretreated with the MAPK pathway inhibitor. We found that prostaglandin E₂ (PGE₂) accumulates in the CM in a timedependent manner. Treatment with PGE₂ increased phosphop44/42 ERK levels and also Cx43 phosphorylation at Ser²⁷⁹-Ser²⁸² sites. Depletion of PGE₂ from CM, and pre-treatment with a p44/42 ERK pathway-specific inhibitor, resulted in a complete inhibition of ERK-dependent Cx43 phosphorylation and attenuated the inhibition of hemichannels by CM and PGE₂. Consistently, the opening of hemichannels by FFSS was blocked by PGE₂ and CM and this blockage was reversed by U0126 and the CM depleted of PGE₂. A similar observation was also obtained in isolated primary osteocytes. Together, results from this study suggest that extracellular PGE₂ accumulated after continuous FFSS is responsible for activation of p44/42 ERK signaling and subsequently, direct Cx43 phosphorylation by activated ERK leads to hemichannel closure.