Inhibitory Effects of 1,25(OH)2D3 on Membrane-Associated and Cytosolic Casein Kinase Activity in MC3T3-E1 Osteoblast-like Cells

The secretion of phosphorylated matrix proteins is high in osteoblasts. Phosphorylation of these proteins may be catalyzed by casein kinases (CK), and CK may play an important role in the site of bone mineralization. In this study, we examined the effects of 1,25(OH)2D3 on CK activities in MC3T3-E1 osteoblast-like cells. Different concentrations (ranging from 10−7 to 10−11M) of 1,25(OH)2D3 were included in a culture medium. After incubation for various lengths of time, MC3T3-E1 cells were homogenized and segregated into cytosolic (c) and microsomal (m) fractions. To measure CK activity, each fraction was used as an enzyme source to phosphorylate casein. MC3T3-E1 cells showed the highest cCK activity after incubation for 21 days, and showed the highest mCK activity after incubation for 14 days. 1,25(OH)2D3 inhibited mCK activity at the early stage of culture, but inhibited cCK activity at the late stage of culture. In contrast, 1,25(OH)2D3 had a slight stimulatory effect on CK activity in the culture medium of MC3T3-E1 cells. Our data suggest that cCK and mCK may play different roles in the function of osteoblasts, and 1,25(OH)2D3 regulates intracellular and extracellular casein kinase activities related to the function of osteoblasts.