HIP/RPL29 down-regulation accompanies terminal chondrocyte differentiation

HIP is a heparin/heparan sulfate (Hp/HS) binding protein identical to ribosomal protein L29 that displays diverse biological functions. There is strong evidence that abnormal expression and quantitative deficiencies of essential molecules such as extracellular matrix (ECM) proteins, transcription factors, and ribosomal proteins can seriously impair embryonic development. As observed for HS-bearing molecules, high levels of HIP/RPL29 are found in proliferating chondrocytic precursors and chondrocytes of developing growth plate. Here, we demonstrate both in vitro and in developing mouse embryos that HIP/RPL29 is down-regulated in terminally differentiated chondrocytes corresponding to the late hypertrophic zone of the growth plate. Because cartilage serves as a template for endochondral bone formation, we hypothesize that the presence of HIP/RPL29 during early chondrogenesis is essential for normal skeletal growth and patterning. In particular, we believe that HIP/RPL29 expression is required to maintain proliferation of chondrocytes and avoid skeletal shortening. Increasing evidence suggests that multifunctional ribosomal proteins of eukaryotic cells are important regulators of cell growth and differentiation, not simply structural parts of translational machinery. To investigate the role of HIP/RPL29 normal expression during cartilage formation, we designed a ribozyme-mediated knock-down approach to partially down-regulate HIP/RPL29 expression in the multipotent mouse embryonic skin fibroblast cell line C3H/10T (1/2). This technology permitted us to avoid the insufficient expression associated with more severe consequences, such as lethality, and provided advantages similar to those obtained with mutations generating hypomorphic phenotypes. Our results show that partial reduction of HIP/RPL29 levels accelerates differentiation of C3H/10T(1/2) into cartilage-like cells. In conclusion, our data indicate that HIP/RPL29 constitutes an important novel regulator of chondrocytic growth and differentiation.