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 plast ic. Under the same conditions, SMC proliferation is stimulated 1.6-fol d. Purified matrix molecules, such as collagen type IV (COLL) or fibro nectin (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 o f 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 gro wn on plastic (in the absence of heparin). Concomitant with matrix-sti mulated proliferation, we observed a marked reduction in PC containing alpha vascular smooth muscle actin (alphaVSMA), as seen by immunofluo rescence using affinity-purified antibodies (173/61 5 positive pericyt es on DOC matrix (28%) vs. 221/285 (77%) positive on glass). SMC respo nd similarly. Whereas alphaVSMA protein is markedly altered when PC an d SMC are cultured on EC matrix, similar reductions in mRNA are not ob served. However, Northern blotting does reveal that PC contain 17-30 t imes the steady-state levels of alphaVSMA mRNA compared to SMC. When S MC and PC cultures on plastic are treated with heparin, the steady-sta te levels of vascular smooth muscle actin mRNA increase 5 and 1.5 fold , respectively. Similarly, heparin treatment of PC grown on plastic in duces a 1.8 fold increase in nonmuscle actin mRNA. These heparin-induc ed alterations in isoactin mRNA levels are not seen when PC are cultur ed on EC matrix. We also observed reductions in alphaVSMA and beta act in mRNA levels when PC are plated on FN, where they maintain a ratio o f 13:1 (alpha:beta). Similar ratios are found in SMC present in rat an d bovine aortae in vivo. These steady-state isoactin mRNA ratios are s lightly 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 an d contractile phenotype during normal vascular morphogenesis, angiogen esis, or during the microvascular remodelling that accompanies hyperte nsive onset.