Fasting upregulates the monocarboxylate transporter Mct1 at the rat blood-brain barrier through Ppar δ activation

Fasting is increasingly recognized as a potential therapeutic intervention in the field of neurology and psychiatry. However, its impact upon the blood-brain barrier (BBB), which strictly regulates the cerebral uptake of a wide range of endogenous compounds and drugs, is unknown. In this study we used a combination of in vivo and in vitro experiments to assess the response of the brain endothelium in rats that were fed ad libitum or fasted for one to three days. Fasting did not change the expression of the main drug efflux ATP-binding cassette transporters or P-glycoprotein activity at the BBB but modulated a restrictive set of solute carrier transporters. These included the monocarboxylate transporter Mct1, which is pivotal for the cerebral uptake of ketone bodies and thus for brain adaptation to fasting. Transcriptional profiling of freshly isolated brain endothelial cells showed that Ppar δ, a major lipid sensor, was selectively activated in response to fasting. In primary cultured rat brain endothelial cells, pharmacological agonists and free fatty acids selectively activated Ppar δ, resulting in the upregulation of Mct1 expression at the transcriptional and protein level. This was confirmed in vivo, by showing that dosing rats with a specific Ppar δ antagonist blocked the upregulation of Mct1 expression and activity induced by fasting. Altogether, our study describes for the first time a selective adaptive response of the brain vasculature to fasting and opens new perspectives for the metabolic manipulation of the BBB in the healthy or diseased brain.禁食在神经学与精神病学领域正日益被视为一种具有潜力的治疗干预手段。然而，其对血脑屏障（blood-brain barrier, BBB）的影响尚不明确——该屏障可严格调控多种内源性化合物与药物的脑内摄取过程。本研究结合体内与体外实验，评估了自由进食或禁食1至3天的大鼠脑内皮细胞的响应情况。研究结果显示，禁食并未改变血脑屏障处主要药物外排型ATP结合盒转运体（ATP-binding cassette transporters）的表达水平，亦未影响P-糖蛋白（P-glycoprotein）的活性，但可调控一组受限的溶质载体转运体（solute carrier transporters）。其中包括单羧酸转运蛋白Mct1（monocarboxylate transporter Mct1），该蛋白对于酮体的脑内摄取、进而实现大脑对禁食的代谢适应至关重要。对新鲜分离的大鼠脑内皮细胞进行转录组分析（transcriptional profiling）后发现，过氧化物酶体增殖物激活受体δ（PPARδ）作为主要脂质传感器，可响应禁食刺激而被选择性激活。在原代培养的大鼠脑内皮细胞中，药理学激动剂与游离脂肪酸（free fatty acids）可选择性激活PPARδ，进而在转录与蛋白水平上调Mct1的表达。该结论在体内实验中得到验证：给大鼠注射特异性PPARδ拮抗剂（antagonist）后，可阻断禁食诱导的Mct1表达与活性上调。综上，本研究首次阐明了脑血管系统对禁食的选择性适应性响应，为健康或病变大脑中血脑屏障的代谢调控开辟了全新视角。