Lipid droplets disrupt mechanosensing in human hepatocytes.

Hepatocellular carcinoma (HCC) is the fourth-leading cause of cancer death in the world. Although most cases occur in stiff, cirrhotic livers, and stiffness is a significant risk factor, HCC can also arise in noncirrhotic livers in the setting of nonalcoholic fatty liver disease (NAFLD). We hypothesized that lipid droplets in NAFLD might apply mechanical forces to the nucleus, functioning as mechanical stressors akin to stiffness. We investigated the effect of lipid droplets on cellular mechanosensing and found that primary human hepatocytes loaded with the fatty acids oleate and linoleate exhibited decreased stiffness-induced cell spreading and disrupted focal adhesions and stress fibers. The presence of large lipid droplets in hepatocytes resulted in increased nuclear localization of the mechano-sensor Yes-associated protein (YAP). In cirrhotic livers from patients with NAFLD, hepatocytes filled with large lipid droplets showed significantly higher nuclear localization of YAP as compared with cells with small lipid droplets. This work suggests that lipid droplets induce a mechanical signal that disrupts the ability of the hepatocyte to sense its underlying matrix stiffness and that the presence of lipid droplets can induce intracellular mechanical stresses. This work examines the impact of lipid loading on mechanosensing by human hepatocytes. In cirrhotic livers, the presence of large (although not small) lipid droplets increased nuclear localization of the mechanotransducer YAP. In primary hepatocytes in culture, lipid droplets led to decreased stiffness-induced cell spreading and disrupted focal adhesions and stress fibers; the presence of large lipid droplets resulted in increased YAP nuclear localization. Collectively, the data suggest that lipid droplets induce intracellular mechanical stress.