J.A. Gallagher

Despite its acute inhibitory effect on bone formation in vitro, PTH has been shown to have an anabolic effect on bone in vivo and to stimulate cell proliferation in osteoblastic cell lines and organ cultures. We have examined the effects of PTH on cells derived from human trabecular bone and compared these effects with those on human skin fibroblasts. Human bone cells have the capacity to synthesize type I collagen and osteocalcin, and to respond to 1,25-dihydroxyvitamin D3 with an increase in the synthesis of osteocalcin and alkaline phosphatase. PTH stimulated adenylate cyclase activity at both low and high cell density. However, the same concentrations of hormone stimulated the proliferation of these cells only when they were cultured at a high cell density. The effect of PTH was bone cell specific in that no proliferative effect of PTH was detected in cultures of human skin fibroblasts obtained from the same donor and cultured under the same conditions. The effect of PTH on DNA synthesis by human bone cells may be important in the generation of a long term anabolic response to PTH.

The effects of six natural vitamin D metabolites of potential biological and therapeutic interest, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), 25-hydroxyvitamin D3 (25-OH-D3), 24R,25-dihydroxyvitamin D3 (24R,25-(OH)2D3), 1,24R,25-trihydroxyvitamin D3 (1,24R,25-(OH)3D3), 25S,26-dihydroxyvitamin D3 (25S,26-(OH)2D3) and 1,25S,26-trihydroxyvitamin D3 (1,25S,26-(OH)3D3) on cell replication and expression of the osteoblastic phenotype in terms of osteocalcin production were examined in cultured human bone cells. At a dose of 5 X 10(-12) mol/1, 1,25-(OH)2D3 stimulated cell proliferation, whereas at higher doses (5 X 10(-9)-5 X 10(-6) mol/1) cell growth was inhibited in a dose-dependent manner. The same pattern of effects was seen for the other metabolites in a rank order of potency: 1,25-(OH)2D3 greater than 1,25S,26-(OH)3D3 = 1,24R,25-(OH)3D3 greater than 25S,26-(OH)2D3 = 24R,25-(OH)2D3 = 25-OH-D3. Synthesis of osteocalcin was induced by 1,25-(OH)2D3 in doses similar to those required to inhibit cell proliferation. Biphasic responses were observed for some of the metabolites in terms of osteocalcin synthesis, inhibitory effects becoming apparent at 5 X 10(-6) mol/1. The cells did not secrete osteocalcin spontaneously. These results indicate that vitamin D metabolites may regulate growth and expression of differentiated functions of normal human osteoblasts.