A low calcium environment enhances AP-1 transcription factor-mediated gene expression 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 of a low calcium environment on osteoblast-like cells cultured with 10% fetal bovine serum and ascorbic acid. Under the culture condition, nodules with characteristics of normal bone appeared by day 30 regardless of the calcium conditions. However, the low calcium environment enhanced the mRNA expressions of c-fos, c-jun and osteocalcin, a specific marker of the osteoblast phenotype. And the exposure to the low calcium medium inhibited the formation of bone nodules. We further studied the differential expressions of c-fos and c-jun in relation to their responses to serum as a function of phenotypic development in the low calcium environment. Both c-fos and c-jun expressions were highly activated by treatment with epidermal growth factor (EGF), but the magnitude of activation was significantly larger under the low calcium condition than the normal condition at each stage. In addition, DNA-binding activities of activating protein-1 (AP-1), Fos/Jun family dimers, were also accelerated by EGF treatment in the low calcium environment. Our findings suggested that osteocalcin, a bone formation marker, c-fos and c-jun genes, and family protein products (AP-1) interacted to restore the normal cell function which deteriorated in the low calcium environment.