Department of Biochemistry , University of Virginia Health Sciences Center , Charlottesville 22908 , USA .
J Biol Chem 270 : 30741-8 ( 1995)
Abstract
Cellular response to platelet-derived growth factor AA ( PDGF-AA ) is mediated exclusively by the PDGF alpha-receptor .
Vascular smooth muscle cells ( VSMCs ) in culture typically express very low levels of alpha-receptor .
In this study , we demonstrate that the proteinase inhibitor and cytokine carrier alpha 2-macroglobulin ( alpha 2M ) increases rat VSMC PDGF alpha-receptor expression .
PDGF alpha-receptor mRNA levels increased 3-fold by 6 h and were sustained at that level through 24 h in VSMCs treated with 280 nM methylamine-modified alpha 2M ( alpha 2M-MA ) , a form of activated alpha 2M .
PDGF beta-receptor mRNA levels were unchanged in the same time period .
In 125I-PDGF-AA binding experiments , treatment of VSMCs with alpha 2M-MA increased the maximum binding capacity ( Bmax ) from 1.9 to 9.2 fmol/mg of cell protein without affecting binding affinity ( KD approximately 80 pM ) .
alpha 2M-MA also increased the VSMC response to PDGF-AA as determined by tyrosine phosphorylation of a 170-kDa band , corresponding in mass to the PDGF alpha-receptor .
The native form of alpha 2M was comparable to alpha M-MA in its ability to increase PDGF-AA binding to VSMCs and tyrosine phosphorylation of the 170-kDa band .
Recombinant and proteolytic alpha M derivatives were used to demonstrate that alpha 2M increases PDGF alpha-receptor expression by binding VSMC-secreted cytokine(s ) and interrupting an autocrine loop that ordinarily suppresses alpha-receptor expression in these cells .
Transforming growth factor-beta-neutralizing antibody mimicked the activity of alpha 2M , increasing the binding capacity of VSMCs for PDGF-AA .
This study demonstrates that VSMC PDGF alpha-receptor expression and responsiveness to PDGF-AA are regulated by autocrine transforming growth factor-beta activity , potentially other autocrine growth factors , and alpha 2M .