Regulation of cAMP levels in osteocytes by mechano-sensitive focal adhesion kinase and phosphodiesterase 8A.

Osteocytes are the primary mechano-sensitive cell type in bone. Mechanical loading is sensed across the dendritic projections of osteocytes leading to transient reductions in focal adhesion kinase (FAK) activity. We performed tyrosine-focused phospho-proteomic profiling in osteocyte-like Ocy454 cells to identify FAK substrates. Gsα, parathyroid hormone receptor (PTH1R), and phosphodiesterase 8A (PDE8A), three proteins associated with cAMP signaling, were found as potential FAK targets. FAK pharmacologic inhibition or gene deletion increased basal and G-protein-coupled-receptor (GPCR) ligand-stimulated cAMP levels and downstream signaling events. PDE8A inhibition mimicked FAK inhibition at the levels of increased cAMP, protein kinase A (PKA) activity, and expression of cAMP-regulated target genes. Co-immunoprecipitation assays revealed an intracellular association between FAK and PDE8A. Thus, mechanically regulated FAK can modulate intracellular cAMP levels via effects on PDE8A. These data suggest a signal transduction mechanism that mediates crosstalk between mechanical and cAMP-linked hormonal signaling in osteocytes.