BINDING OF PLATELET-DERIVED GROWTH FACTOR-BB AND TRANSFORMING GROWTH FACTOR-BETA(1) TO ALPHA(2)-MACROGLOBULIN IN-VITRO AND IN-VIVO - COMPARISON OF RECEPTOR-RECOGNIZED AND NON-RECOGNIZED ALPHA(2)-MACROGLOBULIN CONFORMATIONS

Alpha2-Macroglobulin (alpha2M) undergoes a major conformational change when reacting with proteinases or primary amines. This conformational change has been referred to as the 'slow' to 'fast' transformation ba sed on the increase in alpha2M mobility shown by non-denaturing PAGE. Previous studies demonstrated that many cytokines, including transform ing growth factor beta1 (TGF-beta1) and interleukin-1beta, bind prefer entially or exclusively to alpha2M which has undergone conformational change. In this study, we demonstrate that platelet-derived growth fac tor-BB (PDGF-BB) also binds preferentially to conformationally transfo rmed alpha2M (alpha2M-methylamine, alpha2M-trypsin) in vitro. Purified I-125-PDGF-BB-alpha2M-methylamine complex cleared rapidly from the ci rculation of mice via the alpha2M receptor/low-density-lipoprotein-rec eptor-related protein (alpha2M-R/LRP). In order to determine whether P DGF-BB or TGF-beta1 binds to native alpha2M, we defined the native con formation by lack of interaction with alpha2M-R/LRP instead of electro phoretic mobility. I-125-PDGF-BB was incubated with 4.3 muM native alp ha2M and 0.47 muM alpha2M-methylamine. The I-125-PDGF-BB distributed e venly between slow-form and fast-form alpha2M without shifting the ele ctrophoretic mobility of either species. When the mixed preparation wa s injected intravenously in mice, I-125-PDGF-BB-fast-form-alpha2M clea red rapidly and selectively from the circulation; I-125-PDGF-BB which was bound to slow-form alpha2M was stable in the blood (apparently not recognized by alpha2M-R/LRP). Therefore, while conformationally trans formed alpha2M binds PDGF-BB preferentially in vitro, non-alpha2M-R/LR P-recognized alpha2M binds PDGF-BB as well. Binding of I-125-PDGF-BB a nd I-125-TGF-beta1 to alpha2M was demonstrated in vivo by injecting th e free growth factors intravenously into mice. Plasma samples which we re subjected to non-denaturing PAGE and autoradiography demonstrated b inding of both growth factors exclusively to the slow-form of alpha2M. Therefore, under normal physiological conditions, native alpha2M (non -alpha2M-R/LRP-recognized) is the primary form of the proteinase inhib itor functioning as a carrier of PDGF-BB and TGF-beta1 in the blood.