Effects of dietary protein and tumor necrosis factor on components of the insulin-like growth factor-I pathway in the colon and small intestine in protein-depleted rats

Abstract

Intestinal cell growth is markedly affected by nutrient intake and the presence of cytokines. Since insulin-like growth factor-I (IGF-I) is an important hormone regulator of intestinal proliferation, this study examined the effects of dietary protein content and tumor necrosis factor (TNF) on mRNA levels of IGF-I, IGF-I receptor, IGF binding protein-3 (IGFBP-3), and IGFBP-4 and on the histology of the colon, jejunum, and ileum in protein-malnourished rats. After 2 weeks of feeding a 2% casein diet, rats continued on the 2% casein diet or were refed with a 20% casein diet and received daily intraperitoneal injections of either TNF (50 μg/kg) or saline for 4 days. The abundance of mRNA in the intestine was determined by RNA dot-blot analysis, and morphology measurements were performed by light microscopy. Simultaneous refeeding with the 20% casein diet and administration of TNF led to a modest increase in IGF-I and IGFBP-4 mRNA abundance in the colon. However, in the jejunum and ileum, refeeding had no effect but TNF caused a decrease in IGF-I and IGFBP-3 mRNA levels in malnourished rats. Refeeding with the 20% casein diet resulted in relatively modest histologic changes, which were greater in the colon versus the small intestine. The decreased magnitude of histologic changes in the order of the colon, ileum, and jejunum may reflect a response to a gradient of amino acid availability from intraluminal nutrients. These data demonstrate that TNF has distinct effects on colon and small intestine mRNA, but these mild changes had only a slight impact in the colon and did not translate into identifiable histologic changes in the small intestine. Combined protein restrictio and TNF administration had only a modest effect on intestinal mRNA levels and mucosal histology.