Hemichannels formed by connexin 43 play an important role in the release of prostaglandin E(2) by osteocytes in response to mechanical strain

Osteocytes embedded in the matrix of bone are mechanosensory cells that translate strain into signals and regulate bone remodeling. Our previous studies using osteocyte-like MLO-Y4 cells have shown that fluid flow shear stress (FFSS) increases connexin (Cx) 43 protein expression, prostaglandin E(2) (PGE(2)) release, and intercellular coupling, and PGE(2) is an essential mediator between FFSS and gap junctions. However, the role of Cx43 in the release of PGE(2) in response to FFSS is unknown. Here, the FFSS-loaded MLO-Y4 cells with no or few intercellular channels released significantly more PGE(2) per cell than those cells at higher densities. Antisense Cx43 oligonucleotides and 18 beta-glycyrrhetinic acid, a specific gap junction and hemichannel blocker, significantly reduced PGE(2) release by FFSS at all cell densities tested, especially cells at the lowest density without gap junctions. FFSS, fluid flow-conditioned medium, and PGE(2) increased the activity of dye uptake. Moreover, FFSS induced Cx43 to migrate to the surface of the cell; this surface expressed Cx43 developed resistance to Triton-X-100 solublization. Our results suggest that hemichannels formed by Cx43, instead of intercellular channels, are likely to play a predominant role in the release of intracellular PGE(2) in response to FFSS.