Characterization of the progestin receptors in the human TE85 and murine MC3T3-E1 osteoblast-like cell lines

Progestins are believed to exert positive effects on bone density through r eceptors located in osteoblasts. In the present studies, the binding charac teristics and regulation of the progestin receptors in two osteoblast-like cell lines were compared with those in human breast lines. Human TE85 and m urine MC3T3-E1 osteoblast-like cells contain a single, high-affinity proges tin binding site whose affinity and concentration are lower than in human b reast cells. The osteoblastic progestin binding sites showed the expected s teroid specificity and associated with the cell nuclei when occupied by lig and. The progestin receptors in osteoblastic cells also had sedimentation c oefficients similar to those receptors in breast cells. The regulation of t he progestin receptor in the osteoblast-like cells was explored by treating them with estradiol. In contrast to the large, rapid change seen in the br east cells, the progestin receptor levels in the MC3T3-E1 cells showed only a small, delayed up-regulation with estradiol treatment. The progestin rec eptor number in the TE85 cells was unaffected by estradiol. Down-regulation of the progestin receptors was explored by treating the cells with the pro gestin, norethindrone (NET). NET administration produced a rapid drop in pr ogestin binding sites in the breast cells and a smaller, more gradual decli ne in MC3T3-E1 progestin binding. While the maximal decrease in receptor nu mber occurred within 24 h in the breast cells, the receptor number was stil l continuing to fall after 72 h in the MC3T3-E1 cells. The data presented h ere demonstrate that both human and murine osteoblastlike cells contain a f unctional progestin receptor whose binding characteristics and regulation a re similar, but not identical, to those receptors in other progestin target tissues such as the breast.