Identification of integrin receptors on cultured human bone cells

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Abstract
The interactions of bone cells with their surrounding extracellular microenvironment may be mediated by integrins, a family of heterodimeric glycoproteins consisting of α and β subunits that noncovalently interact to form cell-substratum adhesion receptors. We previously described the integrins on calvarial bone cells in rats with use of polyclonal antibodies against some integrin subunits. In the present study, we expanded this initial characterization by employing a more complete panel of monoclonal antibodies to identify integrins on human bone cells. Minced fragments of trabecular bone obtained during total knee arthroplasty were grown in culture until bone cells became confluent. The cells then were dissociated, plated again, grown to confluence, and assayed for alkaline phosphatase activity, response of cyclic adenosine monophosphate to stimulation with parathyroid hormone, and osteocalcin content. The percentage of the cells that adhered to various substrates was measured; 60–70% adhered to type-I collagen, fibronectin, vitronectin, and poly-D-lysine; 40–50% adhered to type-IV collagen, laminin, and gelatin; and only 10% adhered to fibrinogen. Flow cytometric analysis with anti-integrin monoclonal antibodies and sodium dodecyl sulfate-polyacryamide gel electrophoresis analysis of immunoprecipitates of the human bone cells revealed high levels of α1β1, α3β1, α5β1, and αVβ5 integrins and much lower levels of α2β1, α4β1, αVβ1, and αVβ3 integrins. This description of the integrin repertoire of cultured human bone cells represents the first step toward an understanding of the role played by integrins in the growth, maintenance, and repair of bone.