Signal transduction pathways involved in fluid flow-induced PGE₂production by cultured osteocytes

Abstract

To maintain its structural competence, the skeleton adapts to changes in its mechanical environment. Osteocytes are generally considered the bone mechanosensory cells that translate mechanical signals into biochemical, bone metabolism-regulating stimuli necessary for the adaptive process. Prostaglandins are an important part of this mechanobiochemical signaling. We investigated the signal transduction pathways in osteocytes through which mechanical stress generates an acute release of prostaglandin E2(PGE2). Isolated chicken osteocytes were subjected to 10 min of pulsating fluid flow (PFF; 0.7 ± 0.03 Pa at 5 Hz), and PGE2release was measured. Blockers of Ca2+ entry into the cell or Ca2+ release from internal stores markedly inhibited the PFF-induced PGE2 release, as did disruption of the actin cytoskeleton by cytochalasin B. Specific inhibitors of Ca2+-activated phospholipase C, protein kinase C, and phospholipase A2 also decreased PFF-induced PGE2 release. These results are consistent with the hypothesis that PFF raises intracellular Ca2+ by an enhanced entry through mechanosensitive ion channels in combination with Ca2+- and inositol trisphosphate (the product of phospholipase C)-induced Ca2+ release from intracellular stores. Ca2+ and protein kinase C then stimulate phospholipase A2activity, arachidonic acid production, and ultimately PGE2 release.

Osteopontin Gene Regulation by Oscillatory Fluid Flow via Intracellular Calcium Mobilization and Activation of Mitogen-activated Protein Kinase in MC3T3–E1 Osteoblasts

“…2APB, a novel IP 3 blocker that is specific to the IP 3 channel (44), resulted in a total block of the Ca 2ϩ response. This finding is consistent with our previous observations using neomycin sulfate (57) and published results from other laboratories supporting the involvement of this second messenger in fluid flow responses in various cell types and flow regimes (13,14,31,32).…”

“…during the gait cycle) the pressure gradients, and consequently the direction of fluid flow, are reversed resulting in a flow-time history experienced by the cells that is oscillatory in nature. In vitro experiments have shown fluid flow to have a number of effects on bone cells including Ca 2ϩ i mobilization (12), production of nitric oxide and prostaglandin E 2 (8,13), and regulation of the expression of genes for OPN, Cyclooxygenase-2, and c-Fos (9,14,15). However, it is important to note that only one study to date utilized a reversing flow profile and found significantly different results when contrasted with nonreversing flow (16).…”

Biomechanical forces promote blood development through prostaglandin E2and the cAMP–PKA signaling axis

“…Mechanosensation in hemogenic endothelium and other cells of the AGM involves release of second messengers into the cytosol, including intracellular calcium, that directly stimulate enzyme functions required for prostaglandin synthesis. In osteocytes, calcium flux triggers rapid PGE 2 synthesis and secretion through a biochemical cascade that includes PLC, DAG, PKC, and PLA 2 (Ajubi et al, 1999). Prostaglandin production has also been .…”

From streaming‐potentials to shear stress: 25 years of bone cell mechanotransduction

“…13,25,49,68 Unlike the effect of fluid flow on ATP release, these responses require alterations in gene expression, namely an increase in the expression of cyclooxygenase-2 14,34 and endothelial nitric oxide synthase. 69,70 Interestingly, the induction of prostaglandin synthesis in response to fluid flow appears to be dependent upon the release of ATP.…”