Ontogenic changes in proglucagon mRNA in BB diabetes prone and normal rats weaned onto a chow diet

Regulation of rat intestinal GLUT2 mRNA abundance by luminal and systemic factors

Fructose in the lumen of the small intestine is transported across the brush border membrane by GLUT5, then across the basolateral membrane by GLUT2, which also transports glucose. Diets containing high fructose (HF) specifically enhance intestinal GLUT5 expression in neonatal rats, but there is little information concerning the dietary regulation of GLUT2 expression during early development. In this study, we perfused for 1-4 h 100 mM fructose, glucose (HG), alpha-methylglucose, or mannitol solutions into the jejunum of anaesthetized 20-day-old rat pups. GLUT2 mRNA abundance increased only in HF- and HG-perfused intestines, an effect inhibited by actinomycin D but not by cycloheximide. Bypassed (Thiry-Vella) intestinal loops were constructed, then pups were fed either HF or low-carbohydrate diets for 5 days. GLUT2 mRNA abundance increased significantly in both bypassed and anastomosed intestines of Thiry-Vella pups fed HF. In contrast, GLUT5 mRNA abundance increased only in the anastomosed segment. In sham-operated pups, GLUT2 and GLUT5 mRNA abundance increased in both intestinal regions that corresponded to the bypassed and anastomosed regions of Thiry-Vella pups. SGLT1 mRNA abundance was independent of diet and intestinal region in both Thiry-Vella and sham-operated pups. Unlike GLUT5 expression, which is regulated at the level of transcription only by luminal fructose, GLUT2 mRNA expression is transcriptionally regulated by luminal fructose and glucose as well as by systemic factors released during their absorption.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

The metabolic availability of vitamin A is decreased at the onset of diabetes in BB rats

Streptozotocin (STZ)-induced diabetic rats have been associated with an impaired metabolic availability of vitamin A (retinol). This study was undertaken to investigate whether Biobreeding (BB) rats, in which diabetes mellitus resembling human type I diabetes develops spontaneously, respond the same way at the onset of diabetes. Weaning diabetes-prone (BBdp) and normal (BBn) BB rats consumed NIH-07 nonpurified diet ad libitum until 120 d of age. Plasma and hepatic concentrations of retinol and its carriers, retinol-binding protein (RBP) and transthyretin (TTR) were lower in diabetic BB (BBd) rats than in BBn rats. In parallel with RBP, the abundance of mRNA was lower in the liver of BBd rats. Furthermore, the status of zinc, an important factor for the synthesis of RBP, was also disturbed in BBd rats, as indicated by lower circulatory levels and greater urinary excretion. To determine whether the biochemical evidence of vitamin A deficiency in BBd rats could be reversed, BBdp rats were fed a diet supplemented with vitamin A either alone or in combination with zinc. None of these treatments increased plasma vitamin A concentration. The hepatic abundance of RBP mRNA was significantly greater, whereas circulatory RBP concentrations were unaffected by vitamin A plus zinc supplementation. Overall, these results suggest that impaired metabolic availability of vitamin A, possibly caused by its decreased transport from hepatic stores, is another metabolic derangement associated with type I diabetes.

Ontogeny of intestinal nutrient transport

Children born prematurely lack the ability to digest and to absorb nutrients at rates compatible with their nutritional needs. As a result, total parenteral nutrition may need to be given. While this nutritional support may be life-saving, the baby who receives this therapy is exposed to the risks of possible sepsis, catheter dysfunction, and liver disease. The rodent model of postnatal development provides a useful framework to investigate some of the cellular features of human intestinal development. The up-regulation of intestinal gene expression and precocious development of intestinal nutrient absorption can be achieved by providing growth factor(s) or by modifying the composition of the maternal diet during pregnancy and nursing or the weaning diet of the infant. Accelerating the digestive and absorptive functions of the intestine would thereby allow for the maintenance of infant nutrition through oral food intake, and might possibly eliminate the need for, and risks of, total parenteral nutrition. Accordingly, this review was undertaken to focus on the adaptive processes available to the intestine, to identify what might be the signals for and mechanisms of the modified nutrient absorption, and to speculate on approaches that need to be studied as means to possibly accelerate the adaptive processes in ways which would be beneficial to the newborn young.Key words: absorption, adaptation, diet, peptides.

Proglucagon and glucose transporter mRNA is altered by diet and disease susceptibility in 30-day-old biobreeding (BB) diabetes-prone and normal rats

Diet is an important factor influencing the development of diabetes in the Biobreeding (BB) rat. Changes in gut development and absorption of nutrients in the diabetes-prone rat (Bbdp) and the subsequent effect on pancreatic function may play a role in the ultimate development of the disease. BBdp and normal (BBn) dams were fed one of three diets, chow or semipurified diets containing either soy or casein as the protein source. Pups were weaned at 21 d onto the same diet as their respective dam and killed at 30 d. Chow-fed BBn animals weighed significantly more than casein- or soy-fed BBn animals. Chow-fed BBdp had significantly greater small intestine and colon weight when expressed on a per body weight basis than BBn. With all three diets, BBdp animals had significantly lower colonic proglucagon mRNA abundance than did BBn animals. Adjusting for total colonic RNA content resulted in only casein- and soy-fed BBdp animals demonstrating significantly lower colonic proglucagon mRNA abundance than did BBn animals. BBn animals had higher levels of sodium-dependent D-glucose cotransporter (SGLT)-1 and sodium-independent glucose transporter (GLUT) 5 mRNA than did BBdp. Within BBn animals, casein and soy produced significantly lower levels of SGLT-1 and GLUT5 mRNA than did chow. This study demonstrates that BBn and BBdp animals respond differently to chow versus semipurified weaning diets. These differences may explain the differences in growth and intestinal development among the BBdp and BBn strains weaned onto different diets.