PARATHYROID-HORMONE PREVENTS 1,25(OH)(2)D-3 INDUCED DOWN-REGULATION OF THE VITAMIN-D-RECEPTOR IN GROWTH-PLATE CHONDROCYTES IN-VITRO

1,25(OH)(2)D-3 has an antiproliferative effect on growth plate chondro cytes when given in high doses, whereas low doses stimulate chondrocyt e proliferation. In the present in vitro study we investigated the eff ects of parathyroid hormone (PTH) when given concomitantly with 1,25(O H)(2)D-3 on cell proliferation and vitamin D receptor (VDR) regulation Freshly isolated rat tibial chondrocytes were grown in monolayer cult ures or in agarose stabilized suspension cultures (10% charcoal-treate d FCS). VDR expression was determined by RT PCR generating a 297 bp fr agment and by binding assays (Scatchard analysis) with [H-3]-1,25(OH)( 2)D-3. Cell proliferation was measured by [H-3]-thymidine incorporatio n, growth curves in monolayer cultures and by colony formation in agar ose-stabilized suspension cultures. Optimal concentration of 1,25(OH)( 2)D-3 (10(-12) M) and of PTH fragments [bPTH(1-34) or hPTH(28-48), 10( -10) M] showed additive effects on DNA synthesis of and colony formati on by growth plate chondrocytes. This may be explained in part by an u p-regulation of VDR by PTH: PTH increased both mRNA expression of VDR and binding capacity. 1,25(OH)(2)D-3 (10(-12) mu M) induced an up-regu lation of the VDR within 24 hours followed by a down-regulation after incubation for more than 24 hours. PTH fragments added concomitantly p revented the downregulation seen with 1,25(OH)(2)D-3. These findings p rovide evidence that PTH is a growth promoting hormone that also modul ates the effects of 1,25(OH)(2)D-3 by regulating the VDR status of 1,2 5(OH)(2)D-3 target cells.