The Rab11 Effector Protein FIP1 Regulates Adiponectin Trafficking and Secretion

Adiponectin is an adipokine secreted by white adipocytes involved in regulating insulin sensitivity in peripheral tissues. Secretion of adiponectin in adipocytes relies on the endosomal system, however, the intracellular machinery involved in mediating adiponectin release is unknown. We have previously reported that intracellular adiponectin partially compartmentalizes with rab 5 and rab11, markers for the early/sorting and recycling compartments respectively. Here we have examined the role of several rab11 downstream effector proteins (rab11 FIPs) in regulating adiponectin trafficking and secretion. Overexpression of wild type rab11 FIP1, FIP3 and FIP5 decreased the amount of secreted adiponectin expressed in HEK293 cells, whereas overexpression of rab11 FIP2 or FIP4 had no effect. Furthermore shRNA-mediated depletion of FIP1 enhanced adiponectin release whereas knock down of FIP5 decreased adiponectin secretion. Knock down of FIP3 had no effect. In 3T3L1 adipocytes, endogenous FIP1 co-distributed intracellularly with endogenous adiponectin and FIP1 depletion enhanced adiponectin release without altering insulin-mediated trafficking of the glucose transporter Glut4. While adiponectin receptors internalized with transferrin receptors, there were no differences in transferrin receptor recycling between wild type and FIP1 depleted adipocytes. Consistent with its inhibitory role, FIP1 expression was decreased during adipocyte differentiation, by treatment with thiazolidinediones, and with increased BMI in humans. In contrast, FIP1 expression increased upon exposure of adipocytes to TNFα. In all, our findings identify FIP1 as a novel protein involved in the regulation of adiponectin trafficking and release.

脂联素（Adiponectin）是由白色脂肪细胞（white adipocytes）分泌的一种脂肪细胞因子（adipokine），参与调控外周组织的胰岛素敏感性。脂肪细胞中脂联素的分泌依赖于内体系统（endosomal system），但介导脂联素释放的细胞内机制至今尚不明确。我们此前曾报道，细胞内的脂联素可分别与早期/分选内体（early/sorting compartment）和循环内体（recycling compartment）的标志物Rab5及Rab11发生部分共定位。本研究探究了数种Rab11下游效应蛋白（rab11 FIPs）在调控脂联素转运与分泌中的作用。在HEK293细胞（HEK293 cells）中，过表达野生型Rab11 FIP1、FIP3与FIP5可降低分泌型脂联素的表达水平，而过表达Rab11 FIP2或FIP4则无显著影响。进一步实验显示，短发夹RNA（shRNA）介导的FIP1敲低可增强脂联素释放，而FIP5敲低则会抑制脂联素分泌，FIP3敲低无明显效应。在3T3-L1脂肪细胞（3T3L1 adipocytes）中，内源性FIP1与内源性脂联素在细胞内共分布；且FIP1敲低可增强脂联素释放，但不会改变胰岛素介导的葡萄糖转运蛋白Glut4（glucose transporter Glut4）的转运过程。尽管脂联素受体可与转铁蛋白受体（transferrin receptors）一同发生内吞，但野生型与FIP1敲低的脂肪细胞中，转铁蛋白受体的循环效率并无差异。与FIP1的抑制性功能一致，在人体中，脂肪细胞分化过程中、经噻唑烷二酮类（thiazolidinediones）药物处理后，以及身体质量指数（BMI）升高时，FIP1的表达均会下调；与之相反，脂肪细胞经肿瘤坏死因子α（TNFα）处理后，FIP1的表达会上调。综上，本研究结果证实FIP1是一种新型的调控脂联素转运与释放的蛋白。