The general goal of this project is to increase our understanding of the transcriptional mechanisms regulating carbohydrate- responsive genes. Glucose homeostasis depends on the strict control of numerous enzymatic and absorptive pathways. As a model, we have been studying the small intestinal high-affinity Na+/glucose cotransporter (SGLT1), the activity of which shows diurnal periodicity and induction by carbohydrate. We have shown that SGLT1 expression exhibits diurnal periodicity in both mRNA abundance and transcription. The rat SGLT1 promoter contains an HNF-1 (hepatocyte nuclear factor 1) element that forms complexes with intestinal epithelial cell nuclear extracts differing in the presence of the HNF-1beta isoform depending on the time of day nuclei were isolated. The presence of homologous HNF-1 sites in the carbohydrate-response elements in the promoters of two other carbohydrate-responsive genes--liver pyruvate kinase and sucrase- isomaltase, which we have also shown to exhibit diurnal periodicity--indicates that HNF-1 may mediate both the diurnal transcription program as well as induction by dietary carbohydrate. Periodicity in the intestinal SGLT1 mRNA levels in rhesus monkeys suggests that a similar mechanism is also present in humans. It is proposed that rhythmic SGLT1 transcription is controlled by the diurnal activation (or synthesis) of transcription factors interacting with the SGLT1 promoter. Identifying the factors responsible and defining their functions are important steps toward delineating these homeostatic mechanisms. To these ends, two hypotheses will be tested: (1) """"""""Circadian"""""""" elements in the SGLT1 promoter drive its diurnal transcription; and (2) Carbohydrate induces SGLT1 and other carbohydrate-responsive genes by modulating their diurnal expression program. Because a wide range of proteins involved in carbohydrate metabolism--digestive enzymes, transporters, glycolytic enzymes, and gluconeogenic enzymes--are carbohydrate- responsive, knowledge of the shared and distinctive pathways regulating the expression of members of this class will lead to a better understanding of changes in carbohydrate tolerance. Underscoring the importance of elucidating this regulation, mutations have recently been identified in both HNF-1alpha and beta dimerization partners that are etiologic in one or more forms of diabetes in humans. Finally, it is anticipated that elucidation of the mechanisms regulating SGLT1 may reveal targets amenable to pharmaceutical control of carbohydrate absorption in conditions such as obesity and diabetes mellitus.
