The effect of heparin on osteoblast differentiation and activity in primary cultures of bovine aortic smooth muscle cells.

Recent studies have suggested that aortic smooth muscle cells undergo a phenotypic transition into osteoblast-like cells and mineralize when cultured in the presence of beta-glycerophosphate. Since we had previously demonstrated that heparin could inhibit osteoblast differentiation and mineralization in primary cultures of murine calvaria cells, we were interested in determining if heparin would have a similar effect when primary aortic smooth muscle cells were cultured in the presence of beta-glycerophosphate. The effect of heparin and low molecular weight heparin (LMWH) on osteoblast differentiation and activity was therefore examined in primary cultures of bovine aortic smooth muscle cells (BASMC) over a 14-day period. Here, we report that BASMC differentiate into osteoblast-like cells when cultured in the presence of beta-glycerophosphate. Moreover, we report that heparin not only inhibits this process but that it also inhibits the ability of BASMC to mineralize as well. Importantly, these effects were found not to be dependent upon heparins' anticoagulant activity since unfractionated heparin and heparins with low anti-thrombin III affinities inhibited the mineralization process equally well. Sulfation, however, was found to be a major determinant of heparins ability to inhibit BASMC mineralization since neither dermatan sulfate nor N-desulfated heparin were able to demonstrate an effect. We conclude that BASMC cultures can undergo a phenotypic transition into mature osteoblasts and that both the differentiation process and their ability to mineralize are inhibited by heparin.