Cholesterol and fatty acids regulate cysteine ubiquitination of ACAT2 through competitive oxidation

Ubiquitin linkage to cysteine is an unconventional modification targeting protein for degradation. However, the physiological regulation of cysteine ubiquitination is still mysterious. Here we found that ACAT2, a cellular enzyme converting cholesterol and fatty acid to cholesteryl esters, was ubiquitinated on cysteine 277 (C277) for degradation when lipid level was low. gp78/Insigs catalyzed K48-linked polyubiquitination on this C277. High concentration of cholesterol and fatty acid, however, induced cellular reactive oxygen species (ROS) that oxidized C277, rendering ACAT2 stabilization and subsequently elevated cholesteryl esters. Furthermore, ACAT2 knockout mice were more susceptible to HFD-associated insulin resistance. By contrast, expression of constitutively stable form of ACAT2 (C277A) resulted in higher insulin sensitivity. Together, these data indicate that lipid-induced stabilization of ACAT2 ameliorates lipotoxicity from excessive cholesterol and fatty acid. This unconventional cysteine ubiquitination of ACAT2 constitutes an important mechanism sensing lipid overload-induced ROS and fine-tuning lipid homeostasis.