PICOMOLAR NORETHINDRONE IN-VITRO STIMULATES THE CELL-PROLIFERATION AND ACTIVITY OF A HUMAN OSTEOSARCOMA CELL-LINE AND INCREASES BONE-COLLAGEN SYNTHESIS WITHOUT AN EFFECT ON BONE-RESORPTION

To determine how progestins increase bone formation in vivo, the effec ts of the synthetic progestin norethindrone (NET), on aspects of bone formation in vitro were determined. NET at picomolar concentrations in vitro stimulated the proliferation of human TE85 osteosarcoma cells a s assessed by the increase in [H-3]thymidine incorporation into DNA an d in cell number and also stimulated the release of osteocalcin in bot h the presence and absence of 10 nM 1,25-(OH)(2)D-3. NET increased cel lular alkaline phosphatase specific activity (an index of osteoblastic differentiation), but at much higher concentrations, that is, nanomol ar. These findings suggest that low concentrations of NET act directly on human TE85 osteosarcoma cells to stimulate their proliferation, di fferentiation, and cell activity. Furthermore, mitogenic doses of NET stimulated bone collagen synthesis both in a chicken calvarial organ c ulture assay (assessed by the incorporation and hydroxylation of [H-3] proline) and in a human TE85 osteosarcoma cell culture assay (determin ed by the incorporation of [H-3]proline into collagenase-digestible pr oteins). In contrast, NET at 10(-6)-10(-12) M had no apparent effect o n the rate of basal or PTH-stimulated release of Ca-45 from prelabeled mouse calvariae in vitro. In summary, this study has demonstrated for the first time that picomolar NET acted directly on human TE85 osteos arcoma cells to increase (1) cell proliferation and differentiation, ( 2) osteoblastic activity (i.e., osteocalcin synthesis), and (3) bone c ollagen synthesis in vitro. The same doses of NET in vitro did not red uce the bone resorption rate under our assay conditions. In conclusion , these in vitro osteogenic activities of NET suggest that it may be a useful therapeutic agent to increase bone formation in patients with low bone mass.