Regulation of the expression of microvillar lactase in small intestine

乳糖酶基因表达的调控机制进行了研究。1.乳糖酶基因沿着绒毛-隐窝轴的表达：从绒毛-隐窝柱的6个连续组分中提取乳糖酶mRNA进行斑点杂交，结果表明，与蔗糖酶mRNA在下部绒毛中表达最高不同，乳糖酶mRNA直到上部绒毛才达到最高水平，这表明乳糖酶基因在分化中的吸收细胞中以及在成熟的吸收细胞中持续表达。一项短期的强制喂食实验表明，蔗糖诱导乳糖酶mRNA水平，而是一种不可代谢的糖蔗糖诱导的乳糖酶mRNA水平的增加沿着整个绒毛高度观察到，这种增强作用可被放线菌素D注射液完全抑制，提示糖类代谢产物可能促进乳糖酶基因的转录。 ...更多信息 肠上皮细胞中的乳糖酶：免疫电镜显示乳糖酶不仅存在于微绒毛中，而且存在于侧膜中，提示乳糖酶通过侧膜向微绒毛的细胞内转运是另一条途径。4.饮食诱导乳糖酶翻译后修饰的变化：喂食中链甘油三酯（MCT）导致肠腔中胆汁酸浓度降低，导致微绒毛中蔗糖酶降解延迟。喂食高长链甘油三酯（LCT）饲料的动物显示乳糖酶活性低于喂食高MCT饲料的大鼠，这是伴随着每单位重量的免疫反应性乳糖酶的乳糖酶活性的降低，表明LCT可能加速乳糖酶的失活和/或降解。5.乳糖酶表达的昼夜节律：蔗糖酶mRNA有明显的日变化，乳糖酶mRNA无明显的日变化，微绒毛膜总乳糖酶活性有明显的日变化，微绒毛膜总蛋白含量也有明显的日变化，这表明乳糖酶活性的昼夜节律是由微绒毛蛋白质周转的变化引起的。少

Regulatory mechanisms of lactase expression were investigated.1.Gene expression of lactase along the villus-crypt axis :Dot blot hybridization of lactase mRNA extracted from six consecutive fractions of villus-crypt columns revealed that, unlike sucrase mRNA which was expressed maximally in the lower villus, maximal lactase mRNA level was not attained until the upper villus, suggesting that lactase gene is continuously expressed in the differentiating as well as in the mature absorptive cells.2.Dietary regulation of lactase gene expression :A short-term force-feeding experiment showed that sucrese evoked lactase mRNA levels, but a non-metabolizable sugar (alpha-methylglucoside) was unable to enhance it.The sucrose-induced increase in lactase mRNA levels was observed along the entire height of villi, and this enhancement was completely inhibited by actinomycin D injection, suggesting that lactase gene transcription might be enhanced by metabolite (s) of carbohydrates.3.Localization of l … More actase in enterocytes :Immunoelectron microscopy revealed that lactase was detected not only in the microvilli, but also in the lateral membranes, suggesting that an alternative route for intracellular transport of lactase via the lateral membranes to the microvilli.4.Diet-induced changes in posttranslational modifications of lactase :Feeding medium-chain triglycerides (MCT) causes a decrease in bile acid concentrations in the intestinal lumen, resulting in a retarded degradation of sucrase in the microvilli.The animals fed the high long-chain triglyceride (LCT) diet showed a lower lactase activity than the rats fed the high-MCT diet, which was accompanied by the reduction in the lactase activity per unit weight of immunoreactive lactase, suggesting that LCT might accelerate inactivation and/or degradation of lactase.5.Diurnal rhythm of lactase expression :Sucrase mRNA exhibited diurnal variations, but lactase mRNA did not show diurnal variations.Total lactase activity in the microvillar membranes showed a diurnal variation together with a parallel variation of total microvillar proteins, suggesting that diurnal rhythm of lactase activity is caused by a change in turnover of microvillar proteins. Less