EXPRESSION OF CELL-CYCLE REGULATORY FACTORS IN DIFFERENTIATING OSTEOBLASTS - POSTPROLIFERATIVE UP-REGULATION OF CYCLIN-B AND CYCLIN-E

The representation of cyclins and cyclin-dependent kinases (cdks) was analyzed during progressive development of the bone cell phenotype in cultures of normal diploid rat calvarial osteoblasts, Three developmen tal stages were examined: (a) proliferation; (b) monolayer confluency; and (c) mineralization of the bone extracellular matrix. We demonstra te that the presence of cyclins and cdks is not restricted to the prol iferation period. Consistent with their role in cell cycle progression , cdc2 and cdk2 decrease postproliferatively. However, cdk4 and cyclin s A, B, and D1 persist in confluent cells, Cyclin E is significantly u p-regulated during the extracellular matrix mineralization development al period. Examination of the cytoplasmic levels of these cell cycle r egulatory proteins indicates a marked increase in cyclin B in the late differentiation stage. The elevation of nuclear cyclin E and cytoplas mic cyclin B is not observed in osteoblasts maintained under culture c onditions that do not support differentiation. Furthermore, treatment with transforming growth factor beta for 48 h during the proliferation period renders the cells incompetent for differentiation and abrogate s the postproliferative up-regulation of cyclins B and E. Density-indu ced growth inhibition of ROS 17/2.8 osteosarcoma cells is not accompan ied by up-regulation of nuclear cyclin E and cytoplasmic cyclin B when compared to the proliferation period. This observation is consistent with abrogation of both growth control and differentiation regulatory mechanisms in tumor cells. These results suggest that cell cycle regul atory proteins function not only during proliferation but may also pla y a role in normal diploid osteoblast differentiation.