Abstract
Free fatty acids (FFAs) are important substrates for mitochondrial oxidative metabolism and ATP synthesis but also cause serious stress to various tissues, contributing to the development of metabolic diseases. CD36 is a major mediator of cellular FFA uptake. Inside the cell, saturated FFAs are able to induce the production of cytosolic and mitochondrial reactive oxygen species (ROS), which can be prevented by co-exposure to unsaturated FFAs. There are close connections between oxidative stress and organellar Ca(2+) homeostasis. Highly oxidative conditions induced by palmitate trigger aberrant endoplasmic reticulum (ER) Ca(2+) release and thereby deplete ER Ca(2+) stores. The resulting ER Ca(2+) deficiency impairs chaperones of the protein folding machinery, leading to the accumulation of misfolded proteins. This ER stress may further aggravate oxidative stress by augmenting ER ROS production. Secondary to ER Ca(2+) release, cytosolic and mitochondrial matrix Ca(2+) concentrations can also be altered. In addition, plasmalemmal ion channels operated by ER Ca(2+) depletion mediate persistent Ca(2+) influx, further impairing cytosolic and mitochondrial Ca(2+) homeostasis. Mitochondrial Ca(2+) overload causes superoxide production and functional impairment, culminating in apoptosis. This vicious cycle of lipotoxicity occurs in multiple tissues, resulting in β-cell failure and insulin resistance in target tissues, and further aggravates diabetic complications.