Microgravity Reduces the Differentiation of Human Osteoblastic MG‐63 Cells

Spaceflight leads to osteopenia in both humans and animals, principally as a result of decreased bone formation, which might be the consequence of impaired osteoblast differentiation. The effect of microgravity on osteoblast differentiation in vitro was investigated using the human osteosarcoma cell line MG‐63. Genes related to matrix formation and maturation were quantified both at the protein and mRNA level in untreated and hormone‐treated (dihydroxyvitamin D3 [1,25(OH)2D3], 10−7 M; transforming growth factor β2 (TGF‐β2), 10 ng/ml) cells cultured for 9 days under microgravity conditions aboard the Foton 10 satellite and compared with ground and inflight unit‐gravity cultures. The expression of alkaline phosphatase (ALP) activity following treatment at microgravity increased only by a factor of 1.8 compared with the 3.8‐fold increase at unit‐gravity (p < 0.01), whereas no alteration was detected in the production of collagen type I between unit‐ and microgravity. In addition, gene expression for collagen Iα1, ALP, and osteocalcin following treatment at microgravity was reduced to 51, 62, and 19%, respectively, of unit‐gravity levels (p < 0.02). The lack of correlation between collagen type I gene and protein expression induced by microgravity is most likely related to the different kinetics of gene and protein expression observed at unit‐gravity: following treatment with 1,25(OH)2D3 and TGF‐β2, collagen Iα1 mRNA increased gradually during 72 h, but collagen type I production was already maximal after treatment for 48 h. In conclusion, microgravity decreases the activity of osteoblasts in vitro; in particular the differentiation of osteoblasts in response to systemic hormones and growth factors is reduced by microgravity.