THE INFLUENCE OF TYPE-I COLLAGEN ON THE DEVELOPMENT AND MAINTENANCE OF THE OSTEOBLAST PHENOTYPE IN PRIMARY AND PASSAGED RAT CALVARIAL OSTEOBLASTS - MODIFICATION OF EXPRESSION OF GENES SUPPORTING CELL-GROWTH, ADHESION, AND EXTRACELLULAR-MATRIX MINERALIZATION

Osteoblasts derived from Day 21 fetal rat calvaria grown on films of c ollagen type I exhibit an earlier and enhanced expression of the diffe rentiated phenotype, compared to cells cultured on plastic. The tempor al expression of genes characterizing three distinct periods of growth and differentiation are dramatically modified. During the initial pro liferation period, expression of genes normally expressed at high leve ls on plastic (fibronectin, beta 1 integrin, and actin) was decreased from 50 to 70% in cells grown on collagen. Genes normally expressed at maximal levels in the postproliferative period (osteonectin, osteocal cin, and osteopontin) were up-regulated severalfold very early. Alkali ne phosphatase enzyme activity was elevated 2- to 3-fold during the pr oliferation period, while mRNA levels remained low, suggesting post-tr anscriptional modifications. The most dramatic consequence of culture of cells on collagen is the accelerated and uniform mineralization of the matrix in contrast to the focal mineralization confined to bone no dules in cultures on plastic. Type I collagen supports maintenance of osteoblast phenotypic properties of passaged cells in the absence of g lucocorticoid supplementation required for differentiation of osteobla sts subcultivated on plastic. Treatment of proliferating rat osteoblas ts on plastic with 1,25(OH)(2)D-3 blocks osteoblast differentiation an d matrix mineralization. Although differentiation-related genes (alkal ine phosphatase and osteocalcin) were up-regulated by vitamin D, cultu re on the collagen matrix could not overcome the inhibition of mineral ization. Taken together, these studies define the critical role of typ e I collagen in mediating the signaling cascade for expression of a ma ture osteoblast phenotype and mineralization of the extracellular matr ix in a physiological manner. (C) 1995 Academic Press, Inc.