INHIBITION OF PLATELET-DERIVED GROWTH FACTOR-BB-INDUCED FIBROBLAST PROLIFERATION BY PLASMIN-ACTIVATED ALPHA(2)-MACROGLOBULIN IS MEDIATED VIA AN ALPHA(2)-MACROGLOBULIN RECEPTOR LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN-DEPENDENT MECHANISM

alpha(2)-Macroglobulin (alpha(2)M) is a potentially important regulator of platelet-derived growth factor-BB (PDGF-BB)stimulated cell growth due to our previous observation that PDGF-BB binds to alpha(2)M noncovalently (Bonner, J. C., Goodell, A. L., Lasky, J. A., and Hoffman, M. R. (1992) J. Biol. Chem. 267, 12837-12844). We examined the in vitro effect of native and plasmin activated (receptor-recognized) alpha(2)M on the PDGF-BB-induced proliferation of mouse Swiss 3T3 and rat lung fibroblasts. Nondenaturing polyacrylamide gel electrophoresis showed that plasmin converted alpha(2)M to its electrophoretically ''fast'' form at a 2:1 molar ratio and that I-125-PDGF-BB bound both alpha(2)M and alpha(2)M-plasmin. PDGF-BB-induced growth was not affected by native alpha(2)M (0.3 mu M) or plasmin (0.6 mu M). The combination of plasmin and alpha(2)M (2:1 molar ratio) inhibited PDGF-BB induced cell proliferation 80-90%. Complexes of PDGF-BB .alpha(2)M purified by gel filtra tion chromatography retained growth promoting activity, but the PDGF-BB .alpha(2)M-plasmin complex did not, Preincubation of fibroblasts (37 degrees C for 24 h) with alpha(2)M-plasmin did not change I-125-PDGF BB binding or affect gene expression of the 6.5 kilobase PDGF-alpha receptor or 5.2-kilobase PDGF-beta receptor mRNA However, preincubation with alpha(2)M-plasmin (0-4 degrees C for 4 h) increased I-125-PDGF-BB binding a fold, and this increase was blocked by a receptor-associated protein antagonist of the alpha(2)M-receptor/low density lipoprotein receptor-related protein, The receptor-associated protein antagonist blocked I-125-alpha(2)M-methylamine binding, inhibited PDGF-BB-alpha(2)M-plasmin uptake from fibroblast-cultured supernatants, and abolished the inhibitory effect of alpha(2)M-plasmin on PDGF-stimulated growth, These data suggest that inhibition of PDGF-stimulated proliferation by alpha(2)M-plasmin is mediated in part by clearance of PDGF-BB-alpha(2)M-plasmin through the lipoprotein receptor-related protein.