INHIBITORY EFFECTS OF 1,25(OH)(2)D-3 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 mineraliz ation. In this study, we examined the effects of 1,25(OH)(2)D-3 on CK activities in MC3T3-E1 osteoblast-like cells. Different concentrations (ranging from 10(-7) to 10(-11)M) of 1,25(OH)(2)D-3 were included in a culture medium. After incubation for various lengths of time, MC3T3- E1 cells were homogenized and segregated into cytosolic (c) and micros omal (m) fractions. To measure CK activity, each fraction was used as an enzyme source to phosphorylate casein. MC3T3-E1 cells showed the hi ghest cCK activity after incubation for 21 days, and showed the highes t mCK activity after incubation for 14 days. 1,25(OH)(2)D-3 inhibited mCK activity at the early stage of culture, but inhibited cCK activity at the late stage of culture. In contrast, 1,25(OH)(2)D-3 had a sligh t 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 t he function of osteoblasts, and 1,25(OH)(2)D-3 regulates intracellular and extracellular casein kinase activities related to the function of osteoblasts.