Endothelial Cells in Culture Synthesize a Potent Bone Cell Active Mitogen*

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Abstract
Bovine aortic endothelial cells (BAEC) are known to synthesize in vitro multiple factors which act on a variety of cells in culture. The fact that vascularization appears to be required for endochondral and intramembranous ossification promoted us to examine whether BAEC produce a bone cell active mitogen. Conditioned media collected from exponential and confluent BAEC cultures were concentrated by ultrafiltration and assayed for growth-stimulating activity on bone cell populations isolated from 1-day-old-rat calvaria by sequential enzymatic digestions. As assessed by the incorporation of [3H]thymidine into DNA, it was found that BAEC synthesized a potent bone cell active mitogen. Bovine and rat fibroblasts, as well as rabbit articular chondrocytes, were not affected by the factor which was produced by exponential and confluent cultures, regardless of whether BAEC were cultured in the presence or absence of fetal bovine serum. Employing gel filtration chromatography, Bio-Gel P60, the mitogenic activity eluted as a major peak. It was flanked on either side by a minor one. Apparent mol wt were calculated to be 43,000 38,000, and 30,000, respectively. Upon heat treatment 56 C and for 30 min, the mitogenic activity was fully retained. No effect of the endothelial derived-growth factor on the synthesis of collagen was found. Our data suggest that endothelial cells by virtue of their ability to synthesize bone cell active mitogen(s), may represent an important element in the genesis of bone formation.