Activated alpha 2-macroglobulin promotes mitogenesis in rat vascular smooth muscle cells by a mechanism that is independent of growth-factor-carrier activity.
Vascular smooth muscle cell (vSMC) proliferation is important in atherosclerosis. We previously demonstrated that methylamine-activated alpha 2-macroglobulin (alpha 2M) and transforming growth factor beta 1 (TGF-beta 1) cause a synergistic proliferative response in quiescent rat aortic vSMCs [Stouffer, G. A., La-Marre, J., Gonias, S. L. & Owens, G. K. (1993) J. Biol. Chem. 268, 18,340-18,344]. The first goal of this study was to determine whether the synergy is due to the ability of alpha 2M-methylamine (alpha 2M-MeNH2) to bind TGF-beta 1 and target the growth factor to vSMCs that express the alpha 2M receptor. Receptor-recognized alpha 2M derivatives without TGF-beta 1-binding activity, including ternary alpha 2M-trypsin, an 18-kDa proteolytic fragment of the alpha 2M subunit, and the corresponding recombinant receptor-binding fragment (rRBF) increased vSMC [3H]thymidine incorporation and cell number in a manner similar to alpha 2M-MeNH2. In combination with TGF-beta 1, each alpha 2M derivative caused a synergistic vSMC proliferative response. vSMCs responded comparably when treated with alpha 2M-MeNH2 and TGF-beta 1 simultaneously or in sequence. Furthermore, alpha 2M-MeNH2-TGF-beta 1 complexes increased [3H]thymidine incorporation no more than alpha 2M-MeNH2 alone. These results indicate that TGF-beta 1 binding to alpha 2M is not responsible for the synergistic mitogenic activity. Additional studies were undertaken to determine whether activated alpha 2M independently induces a signal-transduction response in vSMCs. alpha 2M-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-alpha 2M-MeNH2 and 125I-rRBF to vSMC; 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 alpha 2M. This receptor may mediate the mitogenic activity of alpha 2M in vSMC culture.
Webb
D J
DJ
Department of Pathology, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
Hussaini
I M
IM
Weaver
A M
AM
Atkins
T L
TL
Chu
C T
CT
Pizzo
S V
SV
Owens
G K
GK
Gonias
S L
SL
D000818:Animals; D002352:Carrier Proteins / Q000494:pharmacology; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D006023:Glycoproteins / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D026563:LDL-Receptor Related Protein-Associated Protein; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008744:Methylamines / Q000494:pharmacology; D008934:Mitogens / Q000494:pharmacology; D009131:Muscle, Smooth, Vascular / Q000166:cytology* / Q000378:metabolism; D037342:Pertussis Toxin; D011485:Protein Binding; D051381:Rats; D017207:Rats, Sprague-Dawley; D011971:Receptors, Immunologic / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction; D016212:Transforming Growth Factor beta / Q000378:metabolism* / Q000494:pharmacology; D014357:Trypsin / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D000511:alpha-Macroglobulins / Q000378:metabolism / Q000494:pharmacology*
D002352:Carrier Proteins; D006023:Glycoproteins; D026563:LDL-Receptor Related Protein-Associated Protein; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008744:Methylamines; D008934:Mitogens; D011971:Receptors, Immunologic; D011994:Recombinant Proteins; D016212:Transforming Growth Factor beta; D010566:Virulence Factors, Bordetella; D000511:alpha-Macroglobulins; D015544:Inositol 1,4,5-Trisphosphate; C027451:methylamine; D037342:Pertussis Toxin; D014357:Trypsin