The Constitutive Lipid Droplet Protein Plin2 Regulates Autophagy in Liver

Excess triglyceride (TG) accumulation in the liver underlies fatty liver disease, a highly prevalent ailment. TG occurs in the liver sequestered in lipid droplets, the major lipid storage organelle. Lipid droplets are home to the lipid droplet proteins, the most abundant of which are the perilipins (PLINs), encoded by 5 different genes, <i>Plin1</i> to <i>Plin5</i>. Of the corresponding gene products, PLIN2 is the only constitutive and ubiquitously expressed lipid droplet protein that has been used as a protein marker for lipid droplets. We and others reported that <i>plin2^−/−</i> mice have an ∼60% reduction in TG content, and are protected against fatty liver disease. Here we show that PLIN2 overexpression protects lipid droplets against macroautophagy/autophagy, whereas PLIN2 deficiency enhances autophagy and depletes hepatic TG. The enhanced autophagy in <i>plin2^−/−</i> mice protects against severe ER stress-induced hepatosteatosis and hepatocyte apoptosis. In contrast, hepatic TG depletion resulting from other genetic and pharmacological manipulations has no effect on autophagy. Importantly, PLIN2 deficiency lowers cellular TG content in wild-type mouse embryonic fibroblasts (MEFs) via enhanced autophagy, but does not affect cellular TG content in <i>atg7^−/−</i> MEFs that are devoid of autophagic function. Conversely, adenovirus-sh<i>Atg7</i>-mediated hepatic <i>Atg7</i> knockdown <i>per se</i> does not alter the hepatic TG level, suggesting a more complex regulation <i>in vivo</i>. In sum, PLIN2 guards its own house, the lipid droplet. PLIN2 overexpression protects against autophagy, and its downregulation stimulates TG catabolism via autophagy.