FAT/CD36介导的肾小管细胞脂肪酸异常转运: 肾间质纤维化的潜在新机制

Patients with chronic kidney disease (CKD) have an atherogenic lipid profile which is typified by high circulating levels of very low density lipoprotein (VLDL)/ triglycerides (TG), Non-Esterified Fatty Acids (NEFAs) and low high density lipoprotein (HDL). Albumin normally carries >99% of plasma NEFA. In nephrosis the FA load per albumin molecule is markedly increased, with a molar ratio of approximately 6 (FA:albumin) compared with a ratio of ≤1 in health. Serum NEFAs bound to albumin are filtered through the glomeruli and reabsorbed by the proximal tubular cells and mediate tubular damage in proteinuric kidney disease by unknown mechanisms...CD36 belongs to the scavenger receptor family and acts as a NEFA receptor. It is detected primarily on renal tubular epithelial cells and is markedly up-regulated in animal models and CKD patients. Our preliminary studies demonstrated that hyperlipidaemia and inflammatorystress ('Metabolic Inflammatory Stress, MIS') causes interstitial fibrosis in the kidneys of wild-type mice with the appearance of large lipid droplets in tubular cells and infiltration of inflammatory cells. Knockout of the CD36 gene reduced lipid content and fibrosis, suggesting that CD36 may play an important role in tubular injury...We propose that MIS up-regulates tubular CD36 expression on the tubular apical cell membrane, enhancing NEFA transport from the urine into the cell ('first hit') and triggering the release of inflammatory cytokines and inflammatory cell infiltration. This in turn increases CD36 on the apical surface of tubule cells, enhancing NEFA uptake and transfer to the endoplasmic reticulum (ER) and mitochondrial membrane with excess NEFA accumulation in these organelles ('second hit'). The combined effect of these two 'hits' is enhanced ER and oxidative stress leading to apoptosis and fibrosis. We will examine this 'two-hit hypothesis' in cultured tubular cells, in wild-type and CD36 knockout mice and in biopsy samples from patients with CKD.

肾小管损害及间质纤维化是导致慢性肾功能衰竭的最主要原因。脂肪酸在肾小管的异常跨膜转运是肾小管损害及间质纤维化不可忽视的重要因素。研究发现：脂肪酸转运酶（FAT/CD36）是脂肪酸在细胞跨膜转运中的主导蛋白，尤其在疾病（炎症或代谢紊乱）状态下会异常大量转运脂质到细胞内积聚。由于该蛋白分布的广泛性和高度的可调节性，我们推测高脂或炎症等病理因素可通过上调肾小管细胞FAT/CD36表达和促进它在细胞内异常趋膜转移，从而增加脂肪酸在肾小管细胞内异常转运和积聚，最终导致肾小管损害和间质纤维化。.本项目在我们前期工作基础上（NSFC重点项目30530360），应用肾小管细胞模型和相关的模式动物，系统地探讨高脂或炎症等病理因素如何干扰FAT/CD36，使脂肪酸异常转运到肾小管和细胞亚器，导致内质网应激和氧化应激，并进一步加重炎症反应，诱导肾小管细胞凋亡、坏死及肾间质纤维化。

肾小管损害及间质纤维化是导致慢性肾功能衰竭的最主要原因。脂肪酸在肾小管的异常跨膜转运是肾小管损害及间质纤维化的重要因素。研究发现：脂肪酸转运酶（FAT/CD36）是脂肪酸在细胞跨膜转运中的主导蛋白，尤其在疾病（炎症或代谢紊乱）状态下会异常大量转运脂质到细胞内积聚。由于该蛋白分布的广泛性和高度的可调节性，我们提出“脂肪酸二次异常转运的假说”：高脂或炎症等病理因素敏感地、特异地上调肾小管上皮细胞FAT/CD36基因的表达，并促其向细胞膜的转移，导致的FFA从尿中向肾小管细胞内的异常转运 （‘第一次异常转运’），造成脂肪酸在细胞内异常积聚；过多的脂质积聚进一步增加FAT/CD36向内质网（ER）和线粒体膜的转移，增加脂肪酸向ER和线粒体膜的转运（‘第二次异常转运’）。通过以上两次异常转运，过多的FFA聚集在ER内和线粒体内，产生内质网应激和氧化应激，进一步加重炎症反应，并诱导肾小管细胞凋亡、坏死及肾间质纤维化发生。.课题组应用肾小管细胞模型和相关的模式动物，系统地探讨高脂或炎症等病理因素如何干扰FAT/CD36，使脂肪酸异常转运到肾小管和细胞亚器，导致内质网应激和氧化应激，并进一步加重炎症反应，诱导肾小管细胞凋亡、坏死及肾间质纤维化。通过4年的研究，我们证实了①高脂或慢性炎症可以诱导肾小管细胞FAT/CD36基因和蛋白异常高表达。②高脂或慢性炎症可以影响FAT/CD36在肾小管细胞内蛋白翻译水平、翻译后修饰。③高脂或慢性炎症促进肾小管细胞FAT/CD36蛋白棕榈酰化修饰与趋膜转移，从而导致FAT/CD36介导的脂肪酸可以大量异常转运到肾小管上皮细胞，并进一步到细胞ER和线粒体聚集，导致内质网应激和氧化应激，从而加重炎症反应，引起肾小管细胞凋亡、坏死，最后导致肾间质纤维化发生。我们的研究为今后深入探讨FAT/CD36介导的脂肪酸在肾脏的转运机制及慢性肾脏病（CKD）的发病机制提供了理论依据。对针对脂肪酸转运靶点的新药研发、临床采取措施，早期干预、改善CKD预后均有非常重要的意义。