Expression and Role of c-myc in Chondrocytes Undergoing Endochondral Ossification

Abstract

To analyze the relationship between c-myc gene expression and chondrocyte proliferation and maturation during endochondral ossification, Day 18-19 chick embryo sterna were pulse-labeled with [3H]thymidine, and serial sections were processed for autoradiography and in situ hybridization. Proliferating chondrocytes, located in four distinct areas of the developing sternum, all contained high levels of c-myc transcripts, whereas postmitotic chondrocytes (such as hypertrophic chondrocytes) contained undetectable amounts. These findings were confirmed by Northern blot analysis and by the observation that antisense cmyc oligomer treatment inhibited proliferation in cultured chondrocytes. Constitutive overexpression of cmyc by retroviral vectors in immature chondrocyte cultures (c-myc cultures) maintained the cells in a proliferative state and blocked their maturation into hypertrophic chondrocytes. The lack of maturation in the c-myc cultures was corroborated by analysis of type X collagen gene regulation. Control immature cultures contained strong repressor activity for the type X collagen gene promoter, as revealed by transfection assays; repressor activity was lost upon maturation and activation of type X collagen synthesis. In the c-myc cultures, however, repressor activity persisted. Thus, c-myc participates in the normal changes in proliferation accompanying chondrocyte maturation in vivo and in culture. The decreases in c-myc expression and cell proliferation appear to be required for completion of maturation.