EXPRESSION AND ROLE OF C-MYC IN CHONDROCYTES UNDERGOING ENDOCHONDRAL OSSIFICATION

To analyze the relationship between c-myc gene expression and chondroc yte proliferation and maturation during endochondral ossification, Day 18-19 chick embryo sterna were pulse-labeled with [H-3]thymidine, and serial sections were processed for autoradiography and in situ hybrid ization. Proliferating chondrocytes, located in four distinct areas of the developing sternum, all contained high levels of c-myc transcript s, whereas postmitotic chondrocytes (such as hypertrophic chondrocytes ) contained undetectable amounts. These findings were confirmed by Nor thern blot analysis and by the observation that antisense c-myc oligom er treatment inhibited proliferation in cultured chondrocytes. Constit utive overexpression of c-myc by retroviral vectors in immature chondr ocyte cultures (c-myc cultures) maintained the cells in a proliferativ e state and blocked their maturation into hypertrophic chondrocytes. T he lack of maturation in the c-myc cultures was corroborated by analys is of type X collagen gene regulation. Control immature cultures conta ined strong repressor activity for the type X collagen gene promoter, as revealed by transfection assays; repressor activity was lost upon m aturation and activation of type X collagen synthesis. In the c-myc cu ltures, however, repressor activity persisted. Thus, c-myc participate s in the normal changes in proliferation accompanying chondrocyte matu ration in vivo and in culture. The decreases in c-myc expression and c ell proliferation appear to be required for completion of maturation.