Hazelgrove KB, Flynn RS, Qiao LY, Grider JR, Kuemmerle JF. Endogenous IGF-I and ␣v3 integrin ligands regulate increased smooth muscle growth in TNBS-induced colitis. Am J Physiol Gastrointest Liver Physiol 296: G1230 -G1237, 2009. First published April 9, 2009 doi:10.1152/ajpgi.90508.2008.-Endogenous insulinlike growth factor-I (IGF-I) regulates intestinal smooth muscle growth by concomitantly stimulating proliferation and inhibiting apoptosis. IGF-I-stimulated growth is augmented by the ␣v3 integrin ligands vitronectin and fibronectin. IGF-I expression in smooth muscle is increased in both TNBS-induced colitis and Crohn's disease. We hypothesized that intestinal inflammation increased vitronectin and fibronectin expression by smooth muscle and, along with IGF-I upregulation, increased intestinal muscle growth. Intestinal smooth muscle cells were examined 7 days following the induction of TNBS-induced colitis. Although ␣ v3 integrin expression was not altered by TNBSinduced colitis, vitronectin and fibronectin levels were increased by 80 Ϯ 10% and 90 Ϯ 15%, above control levels, respectively. Basal IGF-I receptor phosphorylation in inflamed muscle from TNBStreated rats was increased by 86 Ϯ 8% over vehicle-treated controls. Basal ERK1/2, p70S6 kinase, and GSK-3 phosphorylation in muscle cells of TNBS-treated rats were also increased by 140 -180%. TNBS treatment increased basal muscle cell proliferation by 130 Ϯ 15% and decreased apoptosis by 20 Ϯ 2% compared with that in vehicletreated controls. The changes in proliferation and apoptosis were reversed by an IGF-I receptor tyrosine kinase inhibitor or an ␣v3 integrin antagonist. The results suggest that smooth muscle hyperplasia in TNBS-induced colitis partly results from the upregulation of endogenous IGF-I and ligands of ␣ v3 integrin that mediate increased smooth muscle cell proliferation and decreased apoptosis. This paper has identified one mechanism regulating smooth muscle hyperplasia, a feature of stricture formation that occurs in the chronically inflamed intestine of TNBS-induced colitis and potentially Crohn's disease.