alpha 2-Macroglobulin (alpha 2M) 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 2M 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 2M on the PDGF-BB-induced proliferation of mouse Swiss 3T3 and rat lung fibroblasts. Nondenaturing polyacrylamide gel electrophoresis showed that plasmin converted alpha 2M to its electrophoretically "fast" form at a 2:1 molar ratio and that 125I-PDGF-BB bound both alpha 2M and alpha 2M-plasmin. PDGF-BB-induced growth was not affected by native alpha 2M (0.3 microM) or plasmin (0.6 microM). The combination of plasmin and alpha 2M (2:1 molar ratio) inhibited PDGF-BB-induced cell proliferation 80-90%. Complexes of PDGF-BB.alpha 2M purified by gel filtration chromatography retained growth promoting activity, but the PDGF-BB.alpha 2M-plasmin complex did not. Preincubation of fibroblasts (37 degrees C for 24 h) with alpha 2M-plasmin did not change 125I-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 2M-plasmin (0-4 degrees C for 4 h) increased 125I-PDGF-BB binding 2-fold, and this increase was blocked by a receptor-associated protein antagonist of the alpha 2M-receptor/low density lipoprotein receptor-related protein. The receptor-associated protein antagonist blocked 125I-alpha 2M-methylamine binding, inhibited PDGF-BB-alpha 2M-plasmin uptake from fibroblast-cultured supernatants, and abolished the inhibitory effect of alpha 2M-plasmin on PDGF-stimulated growth. These data suggest that inhibition of PDGF-stimulated proliferation by alpha 2M-plasmin is mediated in part by clearance of PDGF-BB-alpha 2M-plasmin through the lipoprotein receptor-related protein.