Bone and the cAMP Signaling Pathway: Emerging Therapeutics

This chapter will review the evidence that stimulation of cAMP signaling through inhibition of cyclic nucleotide phosphodiesterases (PDEs) may provide a novel means to build bone and thus form the basis of a new therapeutic treatment for osteoporosis. The maintenance of normal bone mass depends on a balance between osteoblastic bone formation and osteoclastic bone destruction [8, 10, 24]. Osteoclasts are highly motile cells, and bone resorption involves active motility of osteoclasts on bone surfaces to be resorbed [13]. Hence, agents that inhibit osteoclast motility should inhibit bone resorption. Rho GTPases, small GTP-binding proteins that belong to the Ras superfamily, represent a group of proteins that play a pivotal role in regulating cell motility and migration [69]. RhoA, a major isoform of Rho, promotes stress fiber formation in cells through activation of a downstream kinase effector termed ROCK (Rho-associated kinase). Once activated by RhoA, ROCK promotes phosphorylation of myosin light chain (MLC), producing increased contractility and stress fiber formation, necessary for motility [25]. In osteoclasts, RhoA is activated by engagement of the αV/β3 integrin receptor and the hyaluronan receptor, CD44, by osteopontin, and this activation of RhoA in osteoclasts is critical for its motility and function [13–15, 58]. RhoA can be directly phosphorylated and inactivated by cAMP-dependent protein kinase (PKA) [25, 45]. Hence, agents that stimulate the cAMP signaling pathway should inactivate RhoA, inhibit osteoclast motility, and inhibit bone resorption. Inhibitors of cyclic nucleotide phosphodiesterases (PDEs), the enzymes that normally degrade cAMP, would be excellent candidates for increasing cAMP and inhibiting motility in osteoclasts. PDEs are encoded by 21 different genes grouped into 11 different gene families based on sequence similarity, mode of regulation, and specificity for cAMP or cGMP as substrate [17]. With alternative splicing as well as the existence of multiple transcription initiation sites, at least 100 different forms of PDE have been cloned and many are expressed in a cell and tissue selective manner. As depicted in Fig. 16.1, PDEs play a central role in controlling cAMP signaling in that they regulate the steady-state levels of cAMP as well as the temporal and spatial components of cAMP within the cell, thus affecting a host of cell processes through both posttranslational modification of proteins, as well as changes in gene transcription. Pharmacological inhibitors of PDEs are under intense development and are rapidly becoming available for clinical use. Moreover, recent studies have shown that inhibitors of the fourth gene family of PDE (PDE4) profoundly inhibit the migration of several types of cells, including fibroblasts [25, 92], endothelial cells [60], leukocytes [21, 34], and several types of cancer cells [23, 57, 62, 80]. Little work has been done on modulation of RhoA activity in osteoclasts, by PDE inhibitors and the cAMP signaling pathway, but studies have demonstrated effects of PDE4 inhibitors to increase bone density. Although the mechanism(s) by which PDE4 inhibitors achieve this is still not known, we hypothesize that they may act in part through inhibition of RhoA, to inhibit osteoclast motility and function, thus inhibiting bone resorption and increasing bone density. Additionally, PDE inhibitors appear to have direct effects on osteoblasts, as well, to stimulate bone formation [37–39, 44, 53, 93, 97, 100]. This chapter will examine current evidence that PDE inhibitors may be beneficial as therapeutic agents to build bone density and treat osteoporosis.