The effect of microgravity on 1,25-dihydroxyvitamin d3 signalling in osteoblasts

Microgravity encountered during space flight induces bone loss, as seen in both humans and rats. This type of bone loss is mainly caused by decreased bone formation due to reduced osteoblast proliferation and differentiation. Yet, the molecular alterations induced by microgravity during osteoblast differentiation are still enigmatic. Therefore, the effect of microgravity on the intracellular signalling pathway of 1,25-dihydroxyvita-min D3 was investigated during the Odissea Mission. The ligand 1,25-dihydroxyvitamin D3 interacts with the vitamin D receptor (VDR) and this complex binds to vitamin D response elements (VDRE) in the promoter region of target genes to stimulate or suppress gene transcription. To investigate the interaction of liganded VDR with VDRE, the mouse osteoblastic cell line, MC3T3, was stable transfected with a construct containing multiple VDREs of the rat osteocalcin promoter fused to growth hormone as reporter gene. Treatment of these transfectants with 1,25-dihydroxyvitamin D3 resulted in a time- and dose-dependent release of growth hormone in the culture medium. Space flight cultures responded to 1,25-dihydroxyvitamin D3 treatment with increased growth hormone production that was comparable with the induction observed in ground cultures. No 1g centrifuge was available during the space flight. These data indicate that microgravity for 5 days did not alter the interaction of VDR with the osteocalcin VDRE or the subsequent gene transcription.