Hamid Abid, Wani Nissar Ahmad, Rana Satyavati, Vaiphei Kim, Mahmood Akhtar, Kaur Jyotdeep
Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India.
FEBS J. 2007 Dec;274(24):6317-28. doi: 10.1111/j.1742-4658.2007.06150.x. Epub 2007 Nov 12.
Folate plays a critical role in maintaining normal metabolic, energy, differentiation and growth status of all mammalian cells. The intestinal folate uptake is tightly and diversely regulated, and disturbances in folate homeostasis are observed in alcoholism, attributable, in part, to intestinal malabsorption of folate. The aim of this study was to delineate the regulatory mechanisms of folate transport in intestinal absorptive epithelia in order to obtain insights into folate malabsorption in a rat model of alcoholism. The rats were fed 1 g.kg(-1) body weight of ethanol daily for 3 months. A reduced uptake of [(3)H]folic acid in intestinal brush border membrane was observed over the course of ethanol administration for 3 months. Folate transport exhibited saturable kinetics and the decreased intestinal brush border membrane folate transport in chronic alcoholism was associated with an increased K(m) value and a low V(max) value. Importantly, the lower intestinal [(3)H]folic acid uptake in ethanol-fed rats was observed in all cell fractions corresponding to villus tip, mid-villus and crypt base. RT-PCR analysis for reduced folate carrier, the major folate transporter, revealed that reduced folate carrier mRNA levels were decreased in jejunal tissue derived from ethanol-fed rats. Parallel changes were observed in reduced folate carrier protein levels in brush border membrane along the entire crypt-villus axis. In addition, immunohistochemical staining for reduced folate carrier protein showed that, in alcoholic conditions, deranged reduced folate carrier localization was observed along the entire crypt-villus axis, with a more prominent effect in differentiating crypt base stem cells. These changes in functional activity of the membrane transport system were not caused by a general loss of intestinal architecture, and hence can be attributed to the specific effect of ethanol ingestion on the folate transport system. The low folate uptake activity observed in ethanol-fed rats was found to be associated with decreased serum and red blood cell folate levels, which might explain the observed jejunal genomic hypomethylation. These findings offer possible mechanistic insights into folate malabsorption during alcoholism.
叶酸在维持所有哺乳动物细胞的正常代谢、能量、分化和生长状态方面起着关键作用。肠道对叶酸的摄取受到严格且多样的调节，在酒精中毒中可观察到叶酸稳态的紊乱，部分原因是肠道对叶酸的吸收不良。本研究的目的是阐明肠道吸收上皮细胞中叶酸转运的调节机制，以便深入了解酒精中毒大鼠模型中的叶酸吸收不良情况。给大鼠每日喂食1 g·kg⁻¹体重的乙醇，持续3个月。在给予乙醇3个月的过程中，观察到肠道刷状缘膜中[³H]叶酸的摄取减少。叶酸转运呈现出饱和动力学，慢性酒精中毒时肠道刷状缘膜叶酸转运的减少与K(m)值升高和V(max)值降低有关。重要的是，在对应于绒毛顶端、绒毛中部和隐窝底部的所有细胞组分中，均观察到乙醇喂养大鼠的肠道[³H]叶酸摄取较低。对主要叶酸转运体——还原型叶酸载体进行RT-PCR分析发现，乙醇喂养大鼠空肠组织中还原型叶酸载体mRNA水平降低。沿整个隐窝-绒毛轴的刷状缘膜中还原型叶酸载体蛋白水平也观察到平行变化。此外，还原型叶酸载体蛋白的免疫组织化学染色显示，在酒精中毒条件下，沿整个隐窝-绒毛轴观察到还原型叶酸载体定位紊乱，对分化中的隐窝底部干细胞影响更为显著。膜转运系统功能活性的这些变化并非由肠道结构的普遍破坏引起，因此可归因于乙醇摄入对叶酸转运系统的特定影响。发现乙醇喂养大鼠中观察到的低叶酸摄取活性与血清和红细胞叶酸水平降低有关，这可能解释了观察到的空肠基因组低甲基化现象。这些发现为酒精中毒期间叶酸吸收不良提供了可能的机制性见解。
