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

Expression of insulin-like growth factor system components in colorectal tissue and its relation with serum IGF levels

CONTEXT

The insulin-like growth factor (IGF)-system has been implicated in colorectal tumor carcinogenesis. Although both tumor expression levels and serum concentrations of IGF-system components are related to colorectal cancer risk, it is unknown whether IGF levels in tissue and serum are correlated.

OBJECTIVE

The objective of this study was to determine expression levels of various IGF-system components in different locations of the colorectum, and to investigate whether normal tissue IGF expression levels are correlated with serum IGF-I and IGF-II concentrations.

DESIGN

Biopsies from macroscopically normal mucosa at four locations in the colorectum (ascending, transverse, sigmoid colon, and rectum) and a fasting serum sample were obtained from 48 asymptomatic patients at increased risk of colorectal cancer. Expression levels of IGF-I, IGF-II, IGF-IR, IGF-IIR, and IGFBP-3 messenger RNA (mRNA) in tissue were quantitatively evaluated using real-time RT-PCR. Expression of IGF-IR protein in the ascending colon and rectum tissue specimens was assessed semi-quantitatively by immunohistochemistry. Serum IGF-I and IGF-II concentrations were determined using immunometric assays.

RESULTS

With the exception of IGF-IIR, mRNA levels of all the IGF-system components investigated, as well as IGF-IR protein expression, were significantly higher in the rectum compared with the ascending colon (p<or=0.001). Serum IGF-I and IGF-II concentrations did not correlate with any of the parameters studied in colorectal tissues.

CONCLUSIONS

Our results indicate that in humans IGF-system components are differentially expressed in the colorectum. Moreover, our findings suggest that local and circulating components of the IGF-system are differentially regulated. However, due to large intra-individual variation in mRNA expression, we cannot formally exclude undetected but existing routes of co-regulation.

Effects of protein malnutrition on IL-6-mediated signaling in the liver and the systemic acute-phase response in rats

This study examines the effects of malnutrition on IL-6 signaling pathways of rats fed 2% vs. 20% casein diets for 14 days. Effects of malnutrition on the abundance and IL-6-stimulated phosphorylation of signaling proteins in the JAK-STAT and MAP kinase pathways were examined in the liver. Changes of the acute-phase response as reflected by serum α1-acid glycoprotein (AG), TNF-α (TNF), and IL-1β (IL-1) were compared in the two dietary groups at 0, 4, 8, 16, and 24 h after IL-6 administration. Under basal conditions, the abundance of the IL-6 receptor, gp130, JAK1, STAT1, and STAT3 proteins and levels of phosphorylation of ERK1/2 and p38 were significantly increased in the liver in the 2% casein group compared with the 20% casein group. With IL-6 stimulation, the increased phosphorylation per unit of protein of these signaling proteins was not different in the liver between the two groups. Before IL-6 stimulation, serum levels of TNF, IL-1, IL-6, and AG were significantly higher in the 2% casein group than in the 20% casein group. After bolus injection of IL-6, changes in IL-1 and AG were similar in the two dietary groups, although a slight decline in AG level was noted after 8 h of IL-6 administration in the 2% protein group. These data demonstrate that protein malnutrition produces changes in inflammation-related proteins characteristic of a low-grade systemic inflammatory response and, thus, can serve as an inflammatory stimulus. The capacity for response to IL-6 is preserved, suggesting adaptive preservation of acute-phase responsiveness during malnutrition.

Transfer of iodine through the milk in protein-restricted lactating rats

Iodine supply is important to avoid neonatal hypothyroidism. This study evaluated whether protein restriction during lactation affects iodine transfer to the pups through the milk. We studied lactating rats fed an 8% protein-restricted diet (PR), a control 23% protein diet (C), and an energy-restricted diet group (ER). On days 4, 12 and 21, mothers were separated from their pups for 4 h, injected with (131)I IP, and put together with their pups. The animals were killed 2 h later. PR pups had a significant decrease in iodine uptake in the gastric content and duodenal mucosa on the 4th day. On the contrary, at 12 and 21 days radioiodine was increased in the gastric content and in the duodenal mucosa. ER pups had an increase in iodine uptake in the gastric content and in the duodenal mucosa only at the end of lactation. The thyroid iodine uptake in PR pups was significantly decreased on the 4th day and significantly increased on the 21st day compared to control. When injected IP with an equivalent amount of (131)I, the PR pups had a decrease in thyroid iodine uptake on the 4th and 12th day, while ER pups had no significant changes. So, these data suggest that protein restriction during lactation was associated with lower iodine secretion into the milk in the beginning of lactation. However, at the end of lactation, an adaptation process seems to occur leading to a higher transfer of iodine through the milk that compensates the impairment of thyroid iodine uptake in these pups.

Hepatic production and intestinal uptake of IGF-I: response to infection

The role of the liver and gut in contributing to the infection-induced fall in circulating insulin-like growth factor I (IGF-I) was examined in chronically catheterized conscious dogs. Two weeks before study, catheters and Doppler flow probes were implanted to assess hepatic and gut balance of IGF-I. To control nutrient intake, dogs were placed on total parenteral nutrition (TPN) as their sole caloric source. After dogs received TPN for 5 days, net hepatic and intestine IGF-I balances were assessed. A hypermetabolic infected state was then induced by the intraperitoneal implantation of a fibrin clot containing Escherichia coli. TPN was continued, and organ IGF-I balance was assessed 24 and 48 h after induction of infection. Arterial IGF-I levels were significantly decreased following infection (111 +/- 18, 62 +/- 10, and 63 +/- 8 ng/ml before and 24 and 48 h after, respectively). Net hepatic IGF-I output decreased markedly (221 +/- 73, to 73 +/- 41 and 41 +/- 17 ng. kg-1. min-1 before and 24 and 48 h after, respectively). The infection-induced decrease in hepatic IGF-I output could not be explained by concomitant alterations in plasma cortisol or insulin levels. The gut demonstrated a net uptake of IGF-I before infection (178 +/- 29 ng. kg-1. min-1). However, after infection, intestinal IGF-I uptake was completely suppressed (-10 +/- 15 and -8 +/- 36 ng. kg-1. min-1). In summary, infection decreases net hepatic IGF-I release 65-80% and completely suppresses net IGF-I uptake by the intestine. As a consequence of these reciprocal changes in IGF-I balance across the liver and intestine, splanchnic production of IGF-I was unchanged by infection. These data suggest that changes in the clearance and/or production of IGF-I by extrasplanchnic tissues contribute to the infection-induced decrease in circulating IGF-I levels.