Parathyroid hormone modulates protein kinase in giant cell tumors of human bone

The physiological effects of parathyroid hormone (PTH) in bone are mediated at least in part by cyclic AMP. The biochemical events subsequent to this step have not been well characterized in this tissue. Giant cell tumors of bone (GT) increase cyclic AMP in response to PTH. This response can be inhibited by an analogue of bovine PTH, [Nle8, Nle18, Tyr34] bPTH-(3-34) amide (PTH-Inh). Cyclic AMP content and cyclic AMP-dependent protein kinase (cAMP-PK) were assayed in fresh tumors and cells in culture incubated with 1 microgram/ml of bPTH and/or PTH-Inh. PTH fully activated cAMP-PK in GT, and PTH-Inh completely inhibited PTH-stimulated increases in cyclic AMP content and cAMP-PK activity. When endogenous protein substrates were sought for cAMP-PK, three phosphoproteins of 55,000, 43,000, and 38,000 mol wt maximally increased their phosphorylation by 30% after 12-min incubation with bPTH. Dephosphorylation of proteins of 200,000 and 120,000 mol wt was also observed. These data are consistent with the hypothesis that PTH action in bone is mediated by the phosphorylation and dephosphorylation of specific substrates.