GLUCOSE TRANSPORT REGULATION IN SMALL INTESTINE

高亲和力的Na+/葡萄糖共转运蛋白（SGLT1）在肠道中的表达具有昼夜节律性，并受膳食碳水化合物的诱导。SGLT1负责膳食中葡萄糖和半乳糖的吸收。由于SGLT1活性的每日变化是由摄食计划（无论是自由摄食还是强制摄食）确定的，并且在没有食物的情况下持续存在，因此这种变化被描述为预期的。我们提供的证据表明这种预期节律有遗传基础。在12小时的光周期和自由喂养的大鼠中，研究了SGLT1表达的正常日周期性。SGLT1 mRNA水平变化高达10倍，最大丰度发生在光周期黑暗期开始附近，最小丰度发生在光周期开始附近。SGLT1的转录也发生了变化，在1000 - 1100小时的密度估计速率（相对于-微管蛋白）是1600 - 1700小时的6.4 - 1.0倍（n=4; p< 0.007，双尾t检验）。我们克隆了大鼠SGLT1近端启动子区域，并鉴定了一个大鼠和人之间保守的肝细胞核因子1 （HNF-1）元件。由该元素产生的探针与小肠核提取物形成不同的复合物，这取决于源动物何时被杀死。血清学测试表明，HNF-1a存在于所有复合物中，而HNF-1b存在于1600和2200 h，但不存在于0400和1000 h。我们提出HNF-1二聚体伴侣的交换有助于SGLT1转录的昼夜变化。HNF-1的这种扩展作用暗示，除了在空间模式形成中发挥典型作用外，该同型蛋白还在时间模式形成中发挥作用。由于我们也观察到恒河猴中SGLT1 mRNA水平的差异（与大鼠相差约半天），我们认为灵长类动物中也存在类似的机制。这项研究已被《生物化学杂志》接受发表。一项RO1拨款正在等待延长这些研究（NIDDK）。

Intestinal expression of the high-affinity Na+/glucose cotransporter (SGLT1), which is responsible for the absorption of dietary glucose and galactose, exhibits both circadian periodicity in its activity and induction by dietary carbohydrate. Because the daily variation in SGLT1 activity is established by the feeding schedule (whether ad libitum or imposed) and persists in the absence of food, this variation has been described as anticipatory. We provide evidence indicating a genetic basis for this anticipatory rhythm. The normal daily periodicity in SGLT1 expression has been examined in rats maintained in a 12-h photoperiod and allowed free access to chow. SGLT1 mRNA levels varied up to 10-fold, with the maximum abundance occurring near the beginning of the dark phase of the photoperiod and the minimum near the onset of light. SGLT1 transcription also changes, with the rate estimated densitometrically (relative to -tubulin) 6.4 q 1.0 times greater between 1000 h and 1100 h than between 1600 h and 1700 h (n=4; p<.007, 2-tailed t-Test). We cloned the rat SGLT1 proximal promoter region and identified an element for hepatocyte nuclear factor 1 (HNF-1) conserved between rat and human. A probe generated from this element formed different complexes with small intestinal nuclear extracts, depending on when the source animal was killed. Serological tests indicated that HNF-1a was present in all complexes, while HNF-1b was present at 1600 h and 2200 h but not at 0400 h or 1000 h. We propose that exchange of HNF-1 dimerization partners contributes to circadian changes in SGLT1 transcription. This expanded role for HNF-1 implicates this homeoprotein in temporal pattern formation in addition to its canonical role in spatial pattern formation. because we have also observed differential SGLT1 mRNA levels in rhesus monkeys (off-set by approximately one-half day from rats), we suggest that a similar mechanism is present in primates. This study has been accepted for publication by the Journal of Biological Chemistry. An RO1 grant is pending to extend these studies (NIDDK).