PRETREATMENT WITH 1,25(OH)(2) VITAMIN-D OR 24,25(OH)(2) VITAMIN-D INCREASES SYNERGISTICALLY RESPONSIVENESS TO SEX STEROIDS IN SKELETAL-DERIVED CELLS

We demonstrated previously that vitamin D metabolites modulate the response of bone and cartilage cells to 17 beta-estradiol (E(2)) and dihydrotestosterone (DHT) both in cell cultures and in vivo. In the present study, we investigated to what extent pretreatment with 1,25(OH)(2)D-3 or 24,25(OH)(2)D-3 would reduce the minimal effective dose of E(2), DHT or progesterone (P) required for stimulation of DNA synthesis and creatine kinase specific activity in cultured osteoblast-like ROS 17/2.8 cells and in rat embryo epiphyseal cartilage cells, and to what extent such pretreatment would increase the maximal response. We measured responses to sex steroids after pretreatment of the cells for 5 days with 0.02% ethanol vehicle or with the vitamin D metabolites 1,25(OH)(2)D-3 (0.12 nM), or 24,25(OH)(2)D-3 (1.2 nM) singly or in combination. Pretreatment of ROS 17/2.8 cells with 1,25(OH)(2)D-3, but not 24,25(OH)(2)D-3, increased synergistically their response to E(2) but not to P, and did not affect their lack of response to DHT. Pretreatment of epiphyseal cartilage cells with either 1,25(OH)(2)D-3 or 24,25(OH)(2)D-3 increased synergistically their DNA synthetic response to all three steroids, but their CK response only to E(2) or DHT. The minimal dose for causing a significant response to E(2) in ROS 17/2.8 cells or to either E(2) or DHT in epiphyseal cartilage cells was reduced 10-fold after pretreatment with vitamin D metabolites. After pretreatment, the maximal response was more than doubled in ROS 17/2.8 cells; epiphyseal cartilage cells showed a similar 10-fold decrease in the dose required for maximal response to E(2) or DHT; the improvement in the response to P was significant only for DNA synthesis. We conclude that pretreatment with the appropriate vitamin D metabolite(s) both reduces by an order of magnitude, or more, the amount of sex steroids needed to stimulate skeletal derived cells and increases synergistically the maximal response of the cells.