G. Klaus et al., PARATHYROID-HORMONE PREVENTS 1,25(OH)(2)D-3 INDUCED DOWN-REGULATION OF THE VITAMIN-D-RECEPTOR IN GROWTH-PLATE CHONDROCYTES IN-VITRO, Kidney international, 52(1), 1997, pp. 45-51
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.