This revised application focuses on the roles and mechanisms of Insulin-like growth factor (IGF-I) and IGF binding proteins (IGFBPs) in intestinal adaptation. Systemic growth hormone (GH) and IGF-I promote intestinal adaptation in an endocrine manner. IGF-I mediates many of the actions of GH in bowel. Clinical trials of GH and IGF-I as therapy for short bowel syndrome and other bowel diseases are in progress. Yet, there is virtually no direct information about the intracellular mechanisms of IGF-I action in bowel. Adaptive changes in growth of small bowel mucosa correlate with levels of locally expressed IGF-I indicating paracrine or autocrine effects of IGF-I in bowel. GH increases local expression of IGF-I within small bowel. The role of locally expressed IGF-I in adaptive growth of the small bowel mucosa is not defined. This information is critical for understanding the relative benefits of therapy with systemic IGF-I compared with GH in patients with bowel disease. As well as IGF-I, mucosal mesenchymal cells express three IGFBPs, IGFBP3, IGFBP4 and IGFBP5. Local expression of IGFBPs is altered during adaptive growth of bowel mucosa. The actions of locally expressed IGF-I on mucosal growth likely depend on whether IGF-I is secreted into the extracellular fluid, whether secreted IGF-I associates with secreted IGFBPs and whether IGF-I is sequestered onto the cell surface or extracellular matrix via interactions with IGFBPs.
Specific aim 1 will use SMP- IGF-I transgenic mice to test the hypothesis that mesenchymal cell derived IGF-I alters growth and function of bowel in vivo. Small bowel of SMP- IGF-I and WT mice will be compared for mucosal mass, crypt cell proliferation, apoptosis and brush border enzyme activities to define the role of locally expressed, mesenchymal cell derived IGF-I in regulating mucosal growth and function.
Specific aim 2 will use IRS-1 null mice and IRS-1 null/SMP- IGF-I crossbreeds tot test the hypothesis that IRS-1 mediates cell specific, trophic actions of IGF-I on bowel.
Specific aim 3 will test the hypothesis that differences in expression of IGFs/IGFBPs mediate phenotypic differences in two intestinal fibroblast subtypes and/or their distinct effects on growth and differentiation of intestinal epithelium. For these studies the co-culture system with two phenotypically distinct intestinal fibroblast subtypes that preferentially mediate proliferation (A1:F1 cells) or differentiation (F1:G9 cells) of intestinal endoderm.
Specific aim 4 will derive transgenic mice with alpha-SM-actin promoter (SMP) mediated over-expression of des-IGF-I, an analog of IGF-I with low affinity for IGFBPs, to test the hypothesis that IGFBPs modulate the cell specific autocrine/paracrine actions of IGF-I in bowel in vivo.
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