Aortic endothelial cell proteoheparan sulfate. II. Modulation by extracellular matrix

The effects of extracellular matrix components on proteoheparan sulfate biosynthesis was studied for bovine aortic endothelial cells in tissue culture. When the cells were maintained on a variety of different purified components of the extracellular matrix, the cells expressed the same three species of proteoheparan sulfates as the cells cultured on tissue culture plastic (HS I, HS II, and HS III). However, the amounts of the three species recovered from the tissue culture medium were found to be dependent on the substrate on which the cells are grown as well as on other factors. In comparison with plastic, much less HS I was found in the medium of cells maintained on substrates containing diverse matrix molecules, whereas the amounts of HS II and HS III essentially remained the same. In contrast, when bovine aortic organ cultures were analyzed under pulsatile flow, marked differences in the profile of proteoheparan sulfate biosynthesis were observed: HS I was found exclusively associated with the plasma membrane of the endothelial cells; HS II was localized only to the subendothelial matrix; and HS III represented the only proteoheparan sulfate species in the medium. This distribution is consistent with polarized secretion and deposition into the subcellular matrix of HS III and retention of HS I in the plasma membrane in the organ culture situation, a biosynthetic phenotype which can only be approximated at best by maintaining the endothelial cells on a substrate other than plastic. When aortic media (devoid of endothelial cells) was placed in organ culture, no HS III could be detected, which suggested that the vascular endothelial cell is the major cell type responsible for its synthesis in organ culture. Thus, the extracellular matrix, depending upon its composition and organization, may play an important role in stabilizing cell polarity and thereby contribute to maintenance of the differentiated phenotype appropriate for the endothelial cell.