Fatty Acids and Albumin are Transported by Distinct Mechanisms in the Proximal Tubule

Under physiologic conditions, proximal tubules depend on basolateral fatty acid (FA) uptake for metabolism. In pathophysiologic conditions due to glomerular filtration barrier disruption, albumin-bound FA undergo filtration and proximal tubule reabsorption, which leads to lipotoxicity and tubular atrophy. Apical proximal tubule albumin uptake is accomplished by the megalin/cubilin complex and receptor-mediated endocytosis, whereas apical proximal tubule FA uptake is primarily mediated by apical fatty acid transport protein-2 (FATP2). However, a commonly proposed (but untested) alternative model is that intact albumin-FA complex is co-transported by megalin/cubilin-mediated endocytosis, similar to apolipoproteins. Microperfused mouse proximal tubules demonstrated divergent one- vs. two-phase albumin and FA uptake kinetics, with significantly faster albumin compared to FA uptake. LLC-PK1, HPCT and OK proximal tubule cell lines all expressed megalin, cubilin and FATP2 mRNA, though in varying amounts. LLC-PK1 cells showed similar one-phase kinetics of dual fluorescently-labeled albumin and FA uptake, whereas HPCT cells demonstrated one-phase albumin and two-phase FA uptake kinetics, with significantly faster albumin compared to FA uptake (similar to perfused proximal tubules). FATP2 inhibition blocked FA uptake, but had no effect on albumin uptake in LLC-PK1 and HPCT cells. Megalin and cubilin deletion in OK cells inhibited albumin uptake, but had no effect on FA uptake. We conclude that apical proximal tubule albumin and FA are transported by distinct mechanisms, implying that FAs dissociate from albumin within the proximal tubule lumen prior to uptake.

FATP2; cubilin; fluorescence; kinetics; megalin.