PERICYTE GROWTH AND CONTRACTILE PHENOTYPE - MODULATION BY ENDOTHELIAL-SYNTHESIZED MATRIX AND COMPARISON WITH AORTIC SMOOTH-MUSCLE

We compared the effects of endothelial-synthesized matrix and purified matrix molecules on pericyte (PC) and aortic smooth muscle cell (SMC) growth, heparin sensitivity, and contractile phenotype in vitro. When PC are plated on endothelial-synthesized (EC) matrix, cell number is, on average, 3.1-fold higher than identical populations grown on plastic. Under the same conditions, SMC proliferation is stimulated 1.6-fold. Purified matrix molecules, such as collagen type IV (COLL) or fibronectin (FN), both major components of the EC matrix, stimulate PC/SMC growth 1.2-1.7-fold. Heparin (100 mug/ml), which inhibits the growth of early passage SMC by 60%, inhibits PC growth approximately 50%, when cells were plated on plastic. However, PC plated on EC matrix in the presence of heparin (100 mug/ml) grow as well as parallel cultures grown on plastic (in the absence of heparin). Concomitant with matrix-stimulated proliferation, we observed a marked reduction in PC containing alpha vascular smooth muscle actin (alphaVSMA), as seen by immunofluorescence using affinity-purified antibodies (173/61 5 positive pericytes on DOC matrix (28%) vs. 221/285 (77%) positive on glass). SMC respond similarly. Whereas alphaVSMA protein is markedly altered when PC and SMC are cultured on EC matrix, similar reductions in mRNA are not observed. However, Northern blotting does reveal that PC contain 17-30 times the steady-state levels of alphaVSMA mRNA compared to SMC. When SMC and PC cultures on plastic are treated with heparin, the steady-state levels of vascular smooth muscle actin mRNA increase 5 and 1.5 fold, respectively. Similarly, heparin treatment of PC grown on plastic induces a 1.8 fold increase in nonmuscle actin mRNA. These heparin-induced alterations in isoactin mRNA levels are not seen when PC are cultured on EC matrix. We also observed reductions in alphaVSMA and beta actin mRNA levels when PC are plated on FN, where they maintain a ratio of 13:1 (alpha:beta). Similar ratios are found in SMC present in rat and bovine aortae in vivo. These steady-state isoactin mRNA ratios are slightly different from those seen in cultured PC (8-10:1; alpha:beta). These results suggest that selective synthesis and remodelling of the endothelial basal lamina may signal alterations in pericyte growth and contractile phenotype during normal vascular morphogenesis, angiogenesis, or during the microvascular remodelling that accompanies hypertensive onset.