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Research Article

Insulin-like Growth Factor Binding Protein-5 Stimulates Growth of Human Intestinal Muscle Cells by Activation of Gi3

¹Virginia Commonwealth University ²Medical College of Virginia/Virginia Commonwealth University ³Med Col Virginia, VCU

Submitted 4 August 2009 ; revision received 21 September 2009 ; accepted in final form 24 September 2009

ABSTRACT

In human intestinal smooth muscle cells, endogenous Insulin-like Growth Factor-I (IGF-I) regulates growth and IGF binding protein-5 (IGFBP-5) expression. The effects of IGF-I are facilitated by IGFBP-5. We previously showed that IGFBP-5 acts independently of IGF-I in human intestinal muscle to stimulate proliferation and up-regulate IGF-I production by activation of Erk1/2 and p38 MAP kinase. Thus a positive feedback loop exists between IGF-I and IGFBP-5 whereby both stimulate muscle growth and production of the other factor. In Crohn's disease, IGF-I and IGFBP-5 expression are increased and contribute to stricture formation through this effect on muscle growth. To determine the signaling pathways coupling IGFBP-5 to MAP kinase activation and growth, smooth muscle cells were isolated from muscularis propria of human intestine and placed into primary culture. Erk1/2 and p38 MAP kinase activation and Type I collagen production were measured by immunoblot. Proliferation was measured by [³H]thymidine incorporation. Activation of specific G proteins was measured by ELISA. AG1024, an IGF-I receptor tyrosine kinase inhibitor, was used to isolate the IGF-I-independent effects of IGFBP-5. IGFBP-5-induced phosphorylation of Erk1/2 and p38 MAP kinase, and proliferation were abolished by pertussis toxin implying the participation of Gi. IGFBP-5 specifically activated Gi3 but not other G proteins. Transfection of cells with an inhibitory Gi minigene, but not an inhibitory Gβ minigene, inhibited MAP kinase activation, proliferation and collagen production. Our results indicate endogenous IGFBP-5 activates Gi3 and regulates smooth muscle growth and collagen production via the -subunit of Gi3, independently of IGF-I, in human intestinal muscle cells.

IGFBP-5; smooth muscle cell; heterotrimeric G protein; p38 MAP kinase; Erk1/2