We have been examining the regulation of the high-affinity Na+/glucose cotransporter isoform, SGLT1. This transporter mediates all or most of the glucose and galactose uptake from the small intestine. Glucose transport activity in the rat intestine exhibits both circadian variation and induction by dietary carbohydrate. While studying the influence of carbohydrate intake on SGLT1 expression in the rat, we observed that normal daily fluctuations in SGLT1 mRNA levels were much greater than those induced by carbohydrate. Moreover, this """"""""circadian"""""""" fluctuation in SGLT1 mRNA levels appears to be a general phenomenon since it was also observed in three rhesus monkeys. To delineate the molecular mechanisms regulating SGLT1 expression, a better understanding of the process responsible for its circadian variation is necessary. We have found that periodic activation of SGLT1 transcription contributes to the fluctuations in mRNA levels. Therefore, we have cloned 1.5 kb of 5'-flanking sequence from the rat SGLT1 gene to map the elements responsible and to identify the transcription factors involved. This promoter is currently being analyzed by DNaseI footprinting and by electrophoretic gel mobility shift assays to localize the """"""""circadian elements."""""""" DNaseI hypersensitivity analysis is also being employed to examine the SGLT1 gene outside of the proximal promoter region. Promoter element(s) so identified will be used as probes to isolate the transcription factor(s) involved for further characterization and complementary DNA cloning. These data currently form the preliminary experiments in a pending NIH grant application (NIDDK). A manuscript is in preparation.
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