Reciprocal modulation by sex steroid and calciotrophic hormones of skeletal cell proliferation

Abstract

We have demonstrated previously that 17β-estradiol (E 2) stimulates cell proliferation in skeletal tissues, as measured by increased DNA synthesis and creatine kinase (CK) specific activity, and that calciotrophic hormones modulate E 2 activity in rat osteoblastic sarcoma cells (ROS 17/2.8). Moreover, E 2 failed to stimulate DNA synthesis in vitamin D-depleted female rat bone in the absence of prior i.p. injections of 1,25(OH) 2D 3. 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) 2D 3, 24,25(OH) 2D 3 or parathyroid hormone (PTH) for 5 days, followed by E 2 treatment for 24 h, resulted in increased DNA synthesis compared to cultures pretreated with vehicle. Prostaglandin (PGE 2) pretreatment blocked further response to E 2. In the reciprocal case, rat epiphyseal cartilage cells, pretreated with E 2, showed an increased response to PTH, a loss of the response to PGE 2 or 24,25(OH) 2D 3 and an inhibition of CK activity and DNA synthesis by 1,25(OH) 2D 3, similar to the characteristic inhibitory action of 1,25(OH) 2D 3 in osteoblasts. By contrast, rat epiphyseal cartilage cells pretreated with testosterone showed no changes in response to PTH, 24,25(OH) 2D 3 or PGE 2 and a decreased response to E 2, but were stimulated by 1,25(OH) 2D 3. Rat embryo calvaria cell cultures behaved similarly to epiphyseal cartilage cultures except that 24,25(OH) 2D 3 pretreatment did not increase the response to E 2. Reciprocally, pretreatment with E 2 before exposure to calciotrophic hormones did not change the responses of rat embryo calvaria cell cultures to 1,25(OH) 2D 3 or 24,25(OH) 2D 3. These findings suggest that the mutual interactions between calciotrophic hormones and E 2, demonstrated here in vitro, could selectively affect the responses of bone and cartilage cells to E 2 by several mechanisms. These possibilities include increased E 2 receptors and E 2-stimulated differentiation of cartilage cells to more E 2 responsive cells showing some characteristics of osteoblasts.