RECIPROCAL MODULATION BY SEX STEROID AND CALCIOTROPHIC HORMONES OF SKELETAL CELL-PROLIFERATION

We have demonstrated previously that 17-beta-estradiol (E2) stimulates cell proliferation in skeletal tissues, as measured by increased DNA synthesis and creatine kinase (CK) specific activity, and that calciotrophic hormones modulate E2 activity in rat osteoblastic sacroma cells (ROS 17/2.8). Moreover, E2 failed to stimulate DNA synthesis in vitamin D-depleted female rat bone in the absence of prior i.p. injections of 1,25(OH)2D3. We have, therefore, studied the effects of pretreatment of cells by one hormone on their response to challenge by a second hormone. We now report reciprocal interactions of sex steroids and other hormones modulating bone formation on cell proliferation parameters in primary bone and cartilage cell cultures; these interactions can selectively augment or diminish cell responsiveness to a given hormone. Pretreatment of rat epiphyseal cartilage cell cultures with 1,25(OH)2D3, 24,25(OH)2D3 or parathyroid hormone (PTH) for 5 days, followed by E2 treatment for 24h, resulted in increased DNA synthesis compared to cultures pretreated with vehicle. Prostaglandin (PGE2) pretreatment blocked further response to E2. In the reciprocal case, rat epiphyseal cartilage cells, pretreated with E2, showed an increased response to PTH, a loss of the response to PGE2 or 24,25(OH)2D3 and an inhibition of CK activity and DNA synthesis by 1,25(OH)2D3, similar to the characteristic inhibitory action of 1,25(OH)2D3 in osteoblasts. By contrast, rat epiphyseal cartilage cells pretreated with testosterone showed no changes in response to PTH, 24,25(OH)2D3 or PGE2 and a decreased response to E2, but were stimulated by 1,25(OH)2D3. Rat embryo calvaria cell cultures behaved similarly to epiphyseal cartilage cultures except that 24,25(OH)2D3 pretreatment did not increase the response to E2. Reciprocally, pretreatment with E2 before exposure to calciotrophic hormones did not change the responses of rat embryo calvaria cell cultures to 1,25(OH)2D3 or 24,25(OH)2D3. These findings suggest that the mutual interactions between calciotrophic hormones and E2, demonstrated here in vitro, could selectively affect the responses of bone and cartilage cells to E2 by several mechanisms. These possibilities include increased E2 receptors and E2-stimulated differentiation of cartilage cells to more E2 responsive cells showing some characteristics of osteoblasts.