Endogenous PKIγ limits the duration of the anti-apoptotic effects of pth and β-adrenergic agonists in osteoblasts

PKIγ knockdown substantially extended the anti-apoptotic effects of PTH and β-adrenergic agonists, whereas PKIγ overexpression decreased these effects. Therefore, inhibition of PKIγ activity may provide a useful co-therapy in combination with intermittent PTH or β-adrenergic agonists for bone loss in conditions such as osteoporosis. Introduction: PTH has both catabolic and anabolic effects on bone, which are primarily caused by cAMP/protein kinase A (PKA) signaling and regulation of gene expression. We previously showed that protein kinase inhibitor-γ (PKIγ) is required for efficient termination of cAMP/PKA signaling and gene expression after stimulation with PTH or β-adrenergic agonists. Inhibition of osteoblast apoptosis is thought to be an important, but transient, mechanism partly responsible for the anabolic effects of intermittent PTH. Therefore, we hypothesized that endogenous PKIγ also terminates the anti-apoptotic effect of PTH. Materials and Methods: PKIγ knockdown by antisense transfection or siRNA was used to examine the ability of endogenous PKIγ to modulate the anti-apoptotic effects of PTH and β-adrenergic agonists in ROS 17/2.8 cells. Results: Knockdown of PKIγ substantially extended the anti-apoptotic effects of PTH, whether apoptosis was induced by etoposide or dexamethasone. In contrast, overexpression of PKIγ decreased the anti-apoptotic effect of PTH pretreatment. This study is also the first demonstration that β-adrenergic agonists mimic the anti-apoptotic effects of PTH in osteoblasts. Moreover, PKIγ knockdown also substantially extended this anti-apoptotic effect of β-adrenergic agonists. Taken together, these results show that endogenous PKIγ limits the duration of the anti-apoptotic effects of cAMP/PKA signaling in osteoblasts. Conclusions: Because significant individual variability exists in the anabolic responses to PTH therapy in current clinical treatment of osteoporosis, inhibition of PKIγ activity may provide a useful co-therapy in combination with intermittent PTH or β-adrenergic agonists for bone loss in conditions such as osteoporosis. However, the potential use of such a co-therapy would depend on it not adversely affecting bone formation or other organ systems.