Browsing by Subject "Glucose"

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    ChREBP: insights into the mechanism of action by glucose.
    (2010-06) Davies, Michael Neal
    Carbohydrate Response Element Binding Protein (ChREBP) is a glucose-responsive transcription factor that activates genes involved in de novo lipogenesis in mammals. The current model for glucose activation of ChREBP proposes that increased glucose metabolism triggers a cytoplasmic to nuclear translocation of ChREBP that is critical for activation. However, we find that ChREBP actively shuttles between the cytoplasm and nucleus in both low and high glucose in the glucose-sensitive β cell-derived line, 832/13. Glucose stimulates a three-fold increase in the rate of ChREBP nuclear entry, but trapping ChREBP in the nucleus by mutagenesis or with a nuclear export inhibitor does not lead to constitutive activation. In fact, mutational studies targeting the nuclear export signal of ChREBP also identified a distinct function essential for glucose-dependent transcriptional activation. From this, we conclude that an additional event independent of nuclear translocation is required for activation. Glucose regulation of ChREBP has been mapped to its conserved N-terminal region of 300 amino acids, designated the MondoA Conserved Region (MCR). Within the MCR, five domains (MCRs 1-5) have a particularly high level of conservation and are likely to be important for glucose regulation. We carried out a large-scale deletion and substitution mutational analysis of the MCR domain of ChREBP. This analysis revealed that MCRs 1-4 function in a concerted fashion to repress ChREBP activity in basal (non-stimulatory) conditions. Deletion of the entire MCR 1-4 segment or combining four specific point mutations (Quad mutant) throughout this region lead to a highly active, glucose-independent form of ChREBP. However, deletion of any individual MCR domain and the majority of point mutations throughout MCRs 1-4 rendered ChREBP inactive. These observations suggest that MCRs 1-4 interact with a factor required for activation and that this interaction occurs after repression is relieved. This possibility is supported by the observation that the MCR 1-4 region can compete for activity with wild type ChREBP and the derepressed Quad mutant in both basal and stimulatory conditions. Thus, the MCR domains act in a complex and coordinated manner to regulate ChREBP activity in response to glucose.
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    Satiety, glycemic, and gastrointestinal effects of novel fibers
    (2012-06) Klosterbuer, Abby Susan
    Epidemiological studies have shown that dietary fiber consumption is inversely associated with body weight, and some research suggests that foods high in fiber increase satiety and reduce energy intake. The mechanism for this relationship is unknown, but may be related to changes in glucose, insulin, or gut hormone concentrations. Fiber may also benefit health by improving laxation, altering the gut microbiota, and increasing production of short chain fatty acids (SCFA). The following work describes two review articles, as well as an intervention study designed to help examine these effects. The first review focuses on the benefits of dietary fiber in clinical nutrition. This allowed for evaluation of the physiological effects of different types and combinations of fiber in subjects on a controlled diet. In general, blends of fibers with varying physicochemical properties provided greater benefits and were better tolerated than single fiber sources. Next, a systematic review of the effects of fiber intake on gut hormone concentrations examined the evidence for this relationship. Considerable variation was found in study design, population, fiber type and dose, which made comparisons difficult. Few studies reported a significant effect of fiber on gut hormone levels, and data suggest caloric load may have a more significant influence. Lastly, a randomized, double-blind, crossover study examined the effects of three novel fibers with varying physicochemical properties on satiety, stool characteristics, and the role of gut hormones, glucose, and insulin in appetite regulation. On Day 1 of the study, healthy men and women consumed either a low-fiber control breakfast or 1 of 4 iii breakfasts containing 25 g fiber from soluble corn fiber (SCF) or resistant starch (RS), alone or in combination with pullulan (SCF+P and RS+P). Subjects rated satiety using visual analog scales (VAS), and blood samples were collected at various time points for 3 hours following breakfast. The fiber treatments did not influence satiety or energy intake compared to control. The RS+P treatment significantly reduced glucose, insulin, and GLP-1 concentrations. To examine the effects of chronic fiber intake, subjects consumed the fiber treatments at home for 6 additional days, with a 3 week washout between periods. Stool samples were collected on Day 7 and tolerance was assessed following fiber intake on Day 1 and Day 6. Fiber did not alter stool weight or stool consistency. SCF reduced pH and increased total SCFA production compared to control, while RS+P increased the percentage of butyrate. Overall, fiber was well tolerated, although treatments containing pullulan tended to cause minor increases in symptoms. Both SCF treatments resulted in a significant shift in the microbial community. Results from these studies confirm that different fibers vary in their physiological effects, and consuming fiber from a variety of sources may be most beneficial. Although increased satiety and improved bowel function are commonly reported benefits of fiber intake, it is clear that not all fibers exert these effects. In addition, the relationship between fiber and potential biomarkers of satiety remains unclear. Thus, it is important to evaluate the effects of different fibers in human studies to better guide recommendations for their use.