A low calcium environment enhances AP-1 transcription factor-mediated geneexpression in the development of osteoblastic MC3T3-E1 cells

Animals fed a low calcium diet develop hypocalcemia and osteoporotic bone. Earlier we conjectured. that a low calcium environment might be one of the factors causing abnormalities in hard tissues. Osteoblastic MC3T3-E1 cells (El cells) undergo a process of proliferation and differentiation and then produce small mineralized nodules. In this study, we examined the effects o f a low calcium environment on osteoblast-like cells cultured with 10% feta l bovine serum and ascorbic acid. Under the culture condition, nodules with characteristics of normal bone appeared by day 30 regardless of the calciu m conditions. However, the low calcium environment enhanced the mRNA expres sions of c-fos, c-jun and osteocalcin, a specific marker of the osteoblast phenotype. And the exposure to the low calcium medium inhibited the formati on of bone nodules. We further studied the differential expressions of c-fo s and c-jun in relation to their responses to serum as a function of phenot ypic development in the low calcium environment. Both c-fos and c-jun expre ssions were highly activated by treatment with epidermal growth factor (EGF ), but the magnitude of activation was significantly larger under the low c alcium condition than the normal condition at each stage. In addition, DNA- binding activities of activating protein-1 (AP-1), Fos/Jun family dimers, w ere also accelerated by EGF treatment in the low calcium environment. Our f indings suggested that osteocalcin, a bone formation marker, c-fos and c-ju n genes, and family protein products (AP-1) interacted to restore the norma l cell function which deteriorated in the low calcium environment.