Role of ornithine decarboxylase in diallyl sulfide inhibition of colonic radiation injury in the mouse.

Diallyl sulfide (DAS), a major flavor component of garlic (Allium sativum) that has previously been shown to inhibit colon carcinogenesis in experimental animals, was investigated for its ability to reduce acute colonic mucosal injury following gamma-ray exposure. Female C57BL/6J mice received either vehicle or DAS (200 mg/kg) by gavage 3 h prior to a single, whole body dose of radiation from a 60Co source. After 24 h, animals were killed and their colons were excised, fixed, and sectioned. DAS significantly inhibited nuclear aberration formation (a measure of nuclear damage) over a radiation dose range of 0.5 to 10 Gy. The degree of protection was related to the dose of DAS and the compound was ineffective if given after irradiation. Following 6 Gy, both DNA synthesis in vivo (measured by [3H]thymidine incorporation into DNA) and the activity of ornithine decarboxylase (an important regulator of DNA synthesis) were elevated for more than 14 days. The induction of both these parameters was significantly suppressed by administering DAS prior to radiation exposure. To determine the role of polyamine synthesis in affecting the severity of radiation damage in the large intestine, difluoromethylornithine, an ornithine decarboxylase inhibitor, was administered in the drinking water of the animals 24 h prior to and following radiation treatment. Difluoromethylornithine abolished the ability of DAS to reduce colonic nuclear damage caused by radiation exposure. Thus DAS protects against colonic radiation injury via a polyamine-dependent pathway.

Heterogenicity of ornithine decarboxylase during mouse colon carcinogenesis and in human colon tumors.

Ornithine decarboxylase (ODC) was separated, using diethylamino-ethyl ion-exchange chromatography, into multiple peaks of activity. We investigated the isoforms of ODC during 1,2-dimethylhydrazine-induced colon carcinogenesis and in human colon tumors. ODC in both mouse and human normal-appearing colonic mucosa was consistently separated into two active peaks by diethylaminoethyl-Sepharose CL-6B column chromatography. The major peak (Peak I) contained about 75% of the mouse and 72% of the human colonic mucosal ODC activity. During and after 10 weekly injections of 1,2-dimethylhydrazine (20 mg/kg, i.p.), colonic ODC activity was significantly enhanced with induction of both peaks but with a more significant increase in Peak II. ODC activity in both 1,2-dimethylhydrazine-induced and human colon tumors was significantly higher compared with the normal colon mucosa. The chromatographic profile of tumors showed the predominance of the second peak. Furthermore, the chromatographic profile of ODC after alkaline phosphatase treatment yielded an elution of only one peak coincident with the Peak I and the disappearance of Peak II. The second peak of ODC (the phosphorylated form) may be a specific isoform associated with colon tumorigenesis and tumor growth.

Substrate specificity in the stimulation of intestinal ornithine decarboxylase activity by refeeding after starvation.

Jejunal loops in 3-day-fasted rats were perfused with hexoses and amino acids to test for their ability to stimulate intestinal ornithine decarboxylase activity. Intraluminal L- and D-glucose, galactose and 3-O-methylglucose were potent stimulants, while D-fructose and L-leucine were not. Intravenously infused D-glucose was also without effect. Induction of ornithine decarboxylase therefore appears to involve a receptor-mediated event which is probably located at the luminal cell surface.

Effects of gastric inhibitory polypeptide in the response to prolonged parenteral or enteral alimentation in rats.

To examine the effects of long-term elevation of plasma gastric inhibitory polypeptide (GIP), the responses to parenteral (PA) or enteral (EA) alimentation were studied in conscious rats with duodenal and venous cannulae. A weight-maintaining liquid diet (84% as glucose, 16% as amino acids) was infused at a constant rate for 6 days by either route, and daily blood samples were taken. A subset of animals receiving PA also received porcine GIP with the infusate (PA plus GIP; plateau plasma immunoreactive GIP, IRGIP, 610 +/- 120 pg/ml). With PA, plasma IRGIP did not change from basal levels, whereas with EA IRGIP rose to virtual plateau levels (mean 530 +/- 110 pg/ml). In the steady state, plasma immunoreactive insulin (IRI) was significantly lower with EA (mean, 153 +/- 5 microU/ml) than with PA (mean, 226 +/- 15 microU/ml), which in turn was lower than with PA plus GIP (mean, 375 +/- 23 microU/ml, P less than 0.001 by ANOVA). A similar ranking of plasma glucose levels occurred in the steady state, with means of 113 +/- 7 (EA), 126 +/- 3 (PA), and 184 +/- 9 (PA plus GIP) mg/dl (P less than 0.001 by ANOVA). To assess the response to transient hyperglycemia in the steady state, an intravenous glucose bolus was given to each group on the fifth day. Peak plasma IRI levels did not differ among the three groups; however, the glucose disappearance rate was significantly slower with PA plus GIP compared with either EA or PA.(ABSTRACT TRUNCATED AT 250 WORDS)

The assessment of recovery of the intestine after acute radiation injury.

Several aspects of intestinal function and morphology are affected by acute radiation damage, including changes in the activity of proliferative cells in the crypts, immune cell populations, and the transport of various substrates. This study was designed to compare the time course of the recovery of intestinal proliferation, transport, and leukocyte population following radiation injury. Rats received a single dose of 6 Gy to the abdomen from a 137Cs source and were studied 3, 7, and 14 days later. No changes in the passive uptake of L-glucose or D-leucine were observed in the jejunum. Active transport of D-glucose and maximal water uptake were reduced at 3 days but had returned to normal by 7 days, whereas L-leucine uptake required more than 7 days to return to control levels. Mucosal permeability, assessed by an in vivo potential difference technique, remained increased 7 days after irradiation. Ornithine decarboxylase, an indicator of DNA synthetic activity, was elevated following radiation treatment and remained so even after 14 days. By comparison, myeloperoxidase activity, used as a quantitative monitor of granulocyte numbers, was still reduced after 7 days. These data indicate that while certain parameters of gut function may return to normal soon after radiation injury, the recovery of other factors is more prolonged. Thus the return of transport function to normal values post irradiation may be viewed as an adaptive change rather than simply the recovery of the tissue.

Suppression of insulin receptor binding by prolonged enteral or parenteral nutrient infusion in the rat: role of gastric inhibitory polypeptide.

In the rat, prolonged enteral or parenteral alimentation with a high-carbohydrate diet results in hyperinsulinemia, which is substantially greater with the parenteral route. Supplementing the parenteral infusate with porcine gastric inhibitory polypeptide (GIP) to approximate plasma immunoreactive GIP levels achieved with enteral feeding further increases steady-state plasma insulin and glucose concentrations, suggesting insulin resistance. We examined the effects of sustained hyperinsulinemia elicited by continuous nutrient infusion on insulin binding to isolated rat adipocytes and the modification of this response by GIP. Compared with a baseline group, both enterally and parenterally alimented groups showed decreased insulin receptor binding affinity. However, despite substantially different steady-state plasma insulin levels, insulin binding was similar with either infusion route. Factors other than plasma insulin concentration alone therefore contribute to insulin receptor down-regulation during prolonged enteral alimentation. Supplementing the parenteral infusate with exogenous GIP resulted in a further reduction in insulin receptor affinity. Thus, adaptation to continuous nutrient infusion is characterized by insulin receptor down-regulation regardless of the route of nutrient delivery. An additional suppression of insulin receptor binding may in part be responsible for the insulin resistance elicited by prolonged exogenous GIP administration.

Basic and clinical investigations of dietary calcium in the prevention of colorectal cancer.

Long-standing investigations into the role of diet in colon cancer have generally supported the notion that some aspect of dietary fats acts to promote cancer at this site. Understanding of the chemical behavior of lipids in the colon led to a hypothesis suggesting that depletion of calcium could partly explain the tumor-promoting effects of dietary fat. Calcium levels may control critical intracellular events in the course of proliferation. Lack of availability or loss of calcium may result in abnormalities in the regulation of colonic proliferation. Basic and clinical studies suggest that calcium supplementation reduces colonic proliferation implying a potential reduction in cancer risk. The current evidence supporting calcium as a cancer chemoprevention agent is reviewed.

Dietary factors and colorectal cancer.

Epidemiologic, laboratory, and clinical research studies have provided strong evidence for an environmental cause for colorectal cancer. Notably, the first investigations into the role of diet in cancer causation focused on the macronutrients in the food supply, namely fat and fiber, as risk factors and preventive agents respectively. Recent studies on colorectal cancer inhibition have assigned more importance to micronutrients, including vitamins, anticarcinogens derived from fruits and vegetables, and minerals, as factors in the prevention of colon cancer. Much further research is needed before definitive dietary recommendations can be made.