β 2-microglobulin enhances insulin-like growth factor I receptor levels and synthesis in bone cell cultures

Beta 2-Microglobulin (beta 2m), a component of the major histocompatibility complex in chordates, has growth factor-like activity for cultured rat cells. We presently describe interactions between beta 2m and insulin-like growth factor I (IGF I) in osteoblast-enriched cultures. beta 2m increased DNA synthesis and was synergistic with IGF I. Affinity labeling revealed that beta 2m enhanced IGF I receptor number, and Northern analysis and radioimmunoassay showed that beta 2m increased steady state IGF I transcripts and medium IGF I polypeptide levels. These results indicate that the growth-promoting activity of beta 2m is mediated at least in part by regulating local IGF I binding and synthesis by skeletal cells.