IGFBP-3 activates TGF-beta receptors and directly inhibits growth in human intestinal smooth muscle cells.
We have shown that human intestinal smooth muscle cells produce IGF-I and IGF binding protein-3 (IGFBP-3). Endogenous IGF-I acts in autocrine fashion to stimulate growth of these cells. IGFBP-3 inhibits the binding of IGF-I to its receptor and thereby inhibits IGF-I-stimulated growth. In several carcinoma cell lines and some normal cells, IGFBP-3 regulates growth independently of IGF-I. Two mechanisms for this effect have been identified: IGFBP-3 can directly activate transforming growth factor-beta (TGF-beta) receptors or it can undergo direct nuclear translocation. The aim of the present study was to determine whether IGFBP-3 acts independently of IGF-I and to characterize the mechanisms mediating this effect in human intestinal smooth muscle cells. The direct effects of IGFBP-3 were determined in the presence of an IGF-I receptor antagonist to eliminate its IGF-I-dependent effects. Affinity labeling of TGF-beta receptors (TGF-betaRI, TGF-betaRII, and TGF-betaRV) with 125I-labeled TGF-beta1 showed that IGFBP-3 displaced binding to TGF-betaRII and TGF-betaRV in a concentration-dependent fashion. IGFBP-3 stimulated TGF-betaRII-dependent serine phosphorylation (activation) of both TGF-betaRI and of its primary substrate, Smad2(Ser465/467). IGFBP-3 also caused IGF-I-independent inhibition of basal [3H]thymidine incorporation. The effects of IGFBP-3 on Smad2 phosphorylation and on smooth muscle cell proliferation were independent of TGF-beta1 and were abolished by transfection of Smad2 siRNA. Immunoneutralization of IGFBP-3 increased basal [3H]thymidine incorporation, implying that endogenous IGFBP-3 inhibits proliferation. We conclude that endogenous IGFBP-3 directly inhibits proliferation of human intestinal smooth muscle cells by activation of TGF-betaRI and Smad2, an effect that is independent of its effect on IGF-I-stimulated growth.
Kuemmerle
John F
JF
Department of Medicine, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia 23298-0711, USA. jkuemmerle@hsc.vcu.edu
Murthy
Karnam S
KS
Bowers
Jennifer G
JG
D030201:Activin Receptors, Type I / Q000378:metabolism*; D019898:Autocrine Communication / Q000502:physiology; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000378:metabolism* / Q000494:pharmacology; D007422:Intestines / Q000166:cytology*; D007457:Iodine Radioisotopes; D032389:Myocytes, Smooth Muscle / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases; D034741:RNA, Small Interfering; D000077293:Receptor, Transforming Growth Factor-beta Type I; D000077294:Receptor, Transforming Growth Factor-beta Type II; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism*; D051899:Smad2 Protein; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism
D004268:DNA-Binding Proteins; D018972:Insulin-Like Growth Factor Binding Protein 3; D007457:Iodine Radioisotopes; D034741:RNA, Small Interfering; D018125:Receptors, Transforming Growth Factor beta; C497782:SMAD2 protein, human; D051899:Smad2 Protein; D015534:Trans-Activators; D017346:Protein Serine-Threonine Kinases; D030201:Activin Receptors, Type I; D000077293:Receptor, Transforming Growth Factor-beta Type I; D000077294:Receptor, Transforming Growth Factor-beta Type II