Activated α2‐Macroglobulin Promotes Mitogenesis in Rat Vascular Smooth Muscle Cells by a Mechanism that is Independent of Growth‐Factor‐Carrier Activity

Abstract

Vascular smooth muscle cell (vSMC) proliferation is important in atherosclerosis. We previously demonstrated that methylamine‐activated α2‐macroglobulin (α2M) and transforming growth factor β1 (TGF‐β1) cause a synergistic proliferative response in quiescent rat aortic vSMCs [Stouffer, G. A., LaMarre, J., Gonias, S. L. & Owens, G. K. (1993) J. Biol. Chem. 268, 18340–18344], The first goal of this study was to determine whether the synergy is due to the ability of α2M‐methylamine (α2M‐MeNH2) to bind TGF‐β1 and target the growth factor to vSMCs that express the α22M receptor. Receptor‐recognized α2M derivatives without TGF‐β1‐binding activity, including ternary α1M‐trypsin, an 18‐kDa proteolytic fragment of the α2M subunit, and the corresponding recombinant receptor‐binding fragment (rRBF) increased vSMC [3H]thymidine incorporation and cell number in a manner similar to α2M‐MeNH2. In combination with TGF‐β1, each α2M derivative caused a synergistic vSMC proliferative response. vSMCs responded comparably when treated with α2M‐MeNH2 and TGF‐β1 simultaneously or in sequence. Furthermore, α1M‐MeNH2–TGF‐β1 complexes increased [3H]thymidine incorporation no more than α2M‐MeNH2 alone. These results indicate that TGF‐β1 binding to α2M is not responsible for the synergistic mitogenic activity. Additional studies were undertaken to determine whether activated α2M independently induces a signal‐transduction response in vSMCs. α2M‐MeNH2 and rRBF caused a rapid, transient increase in vSMC inositol 1,4,5‐trisphosphate. This response was pertussis‐toxin insensitive. Receptor‐associated protein (RAP; 170 nmol/L) inhibited 91–95% of the specific binding of 125I‐α2M‐MeNH2 and 125I‐rRBF to vSMCs; however, RAP did not affect the inositol 1,4,5‐trisphosphate response or the mitogenic response. These studies suggest that vSMCs express a receptor, other than low‐density‐lipoprotein‐receptor‐related protein, that transduces a signal in response to activated α1M. This receptor may mediate the mitogenic activity of α2M in vSMC culture. Copyright © 1995, Wiley Blackwell. All rights reserved