IGFBP-3 activates TGF-β receptors and directly inhibits growth in human intestinal smooth muscle cells

Abstract

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-β (TGF-β) 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-β receptors (TGF-βRI, TGF-βRII, and TGF-βRV) with 125I-labeled TGF-β1 showed that IGFBP-3 displaced binding to TGF-βRII and TGF-βRV in a concentration-dependent fashion. IGFBP-3 stimulated TGF-βRII-dependent serine phosphorylation (activation) of both TGF-βRI 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-β1 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-βRI and Smad2, an effect that is independent of its effect on IGF-I-stimulated growth.

Expression of albumin, IGF-1, IGFBP-3 in tumor tissues and adjacent non-tumor tissues of hepatocellular carcinoma patients with cirrhosis

“…At the intracellular level this may either involve modulation of the bcl-2/bax ratio [22] or the p53 protein [23] . Kuemmerle et al, reported that endogenous IGFBP-3 directly inhibits proliferation of human intestinal smooth muscle cells by activation of TGF-betaRI and Smad 2, an effect which is independent of its effect on IGF-1 stimulated growth [24] . Interestingly, IGFBP-3 has been localized in the nucleus, implying a more direct transcriptional regulatory role, but it remains largely unknown how extracellular IGFBP-3 enters the cell [25] .…”

Signaling cross-talk between IGF-binding protein-3 and transforming growth factor-β in mesenchymal chondroprogenitor cell growth

“…IGFBP-3 has been also shown to enhance Smad-2 and Smad-3 phosphorylation induced by TGF-activation of T RII, and to activate the PAI-1 promoter (Fanayan et al 2000(Fanayan et al , 2002. Furthermore affinity labeling studies of T Rs have showed that IGFBP-3 displaced binding of iodinated TGF-to T RII and stimulated Smad-2 phosphorylation (Kuemmerle et al 2004). IGFBP-3 has been also shown to bind the latent TGF-binding protein-1, although the functional significance of this binding remains unclear (Gui & Murphy 2003).…”

9-cis retinoic acid induces insulin-like growth factor binding protein-3 through DR-8 Retinoic acid responsive elements

“…5,7,8 Besides its IGF-I-dependent function, IGFBP-3, like IGFBP-1 and IGFBP-5, has IGF-I-independent anti-proliferative and pro-apoptotic effects. 9 The bioactivity of IGF system is modulated by retinoids, which not only decreases directly the level of IGF-1 10 but also increases IGFBP-3. 11,12 IGFBP-3, in turn, mediates a wide variety of growth suppression signals, including retinoic acid.…”