Continuous PTH: A Tale of Two Pathways

Parathyroid hormone (PTH) is a key factor in bone metabolism, able to stimulate both osteoblasts to cause bone formation and osteoclasts to cause bone resorption. The studies presented herein set forth to examine both the formative and resorptive pathways in light of a newly discovered inhibitory factor, serum amyloid a3 (Saa3). Saa3 is particularly germane to the PTH signaling pathway as it is secreted by cells of the osteoclast lineage in response to PTH stimulation of prostaglandin. Saa3 then enacts its inhibitory effects by acting back onto osteoblasts to inhibit PTH’s ability to stimulate cAMP/PKA signaling. In the absence of Saa3 we have observed that PTH is able to stimulate the phosphorylation of β-catenin at two novel sites, serine 552 and serine 675. These sites represent a cAMP/PKA dependent mechanism by which β-catenin signaling may be stimulated outside the dogma of canonical wnt signaling. Our data suggest that PTH phosphorylation of these sites increases β-catenin signaling as measured by TCF/LEF activity and downstream gene expression, and that the inclusion of Saa3 into these conditions completely ablates this pathway. PTH is also known to stimulate receptor activator of nuclear kappa-b ligand (RANKL) expression by osteoblasts via cAMP/PKA. As a potent stimulator of osteoclastogenesis we were then interested to examine the effects of the cAMP inhibitor, Saa3, upon PTH stimulation of RANKL. Interestingly, we discovered that PTH stimulation of RANKL was not only unaffected by inhibition of the cAMP/PKA pathway, but that antagonists of the Ca2+/PKC pathway were able to prevent PTH stimulation of RANKL even in the presence of a robust cAMP response to PTH. We then conclude that the anabolic portions of the PTH pathway are mediated through cAMP/PKA and the catabolic portions are mediated through Ca2+/PKC. This dichotomy explains how the catabolic effects of PTH are more visible and pronounced as they go unhindered by the inherent Saa3 negative feedback loop which renders the anabolic effects invisible in basal conditions.