HUMAN OSTEOBLASTS EXPRESS A REPERTOIRE OF CADHERINS, WHICH ARE CRITICAL FOR BMP-2-INDUCED OSTEOGENIC DIFFERENTIATION

Direct cell-cell interactions are fundamental for tissue development a nd differentiation, We have studied the expression and function of cad herins in human osteoblasts during in vitro differentiation, Using rev erse transcription-polymerase chain reaction and mRNA hybridization, w e found that human trabecular bone osteoblasts (HOBs), osteoprogenitor marrow stromal cells (BMCs), and the osteogenic sarcoma lines, SaOS-2 and MG-63, expressed mRNA for cadherin-ll (C11) and N-cadherin (N-cad ). HOBs and BMCs also expressed low levels of cadherin-4 (C4) mRNA, C1 1 was the most abundant cadherin protein present in human osteoblasts, and its expression was unaffected by bone morphogenetic protein-2 (BM P-2) treatment of either BMCs or HOBs, Likewise, N-cad mRNA did not ch ange during BMP-2 incubation, Conversely, C4 protein, undetectable in transformed cell lines, was down-regulated by BMP-2 treatment of norma l cells, Both C11 and C4 were localized to sites of cell-cell contact in both HOBs and BMCs, colocalized with beta-catenin, and bands corres ponding to cadherins were coimmunoprecipitated by a beta-catenin antib ody, findings indicative of functional cadherins, A decapeptide contai ning the HAV motif of human N-cad partially inhibited Ca2+-dependent c ell-cell adhesion and completely prevented BMP-2-induced stimulation o f alkaline phosphatase activity by BMCs, Thus, human osteoblasts and t heir progenitor cells express a repertoire of multiple cadherins, Cadh erin-mediated cell-to-cell adhesion is critical for normal human osteo blast differentiation.