MRP4 deficiency exacerbates diet-induced MASLD through dysregulation of hepatic lipid handling.

Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) is characterized by impaired hepatic lipid homeostasis. Multidrug resistance-associated protein 4 (MRP4/ABCC4) is a sinusoidal efflux transporter of endogenous signaling metabolites. We previously showed that Mrp4 knockout (Mrp4-/-) mice develop prolonged hepatic steatosis after partial hepatectomy compared with wild-type (WT) controls. Whether MRP4 contributes to hepatic lipid homeostasis and MASLD remains unclear. To address this question, WT and Mrp4-/- mice were fed standard chow or high-fat, high-sucrose (HFHS) diet for 24 weeks. Liver pathology and lipid homeostasis were assessed by liver-to-body weight ratios, serum biochemical measurements, histopathology, and transcriptional analyses. Complementary studies employed human hepatic cell models with siRNA-mediated MRP4 silencing or pharmacological inhibition under free fatty acid (FFA)-induced metabolic stress. Under basal (chow) conditions, Mrp4-/- mice showed no changes in serum biochemistry or liver histopathology compared with WT controls, despite transcriptional perturbations. In contrast, HFHS-fed Mrp4-/- mice developed exacerbated hepatic steatosis, with increased liver-to-body weight ratio, hepatic triglycerides, serum ALT, macrovesicular steatosis, and fibrosis. These changes were accompanied by dysregulation of genes involved in lipid synthesis, oxidation and export. Similarly, MRP4 deficiency in human hepatocytes altered lipid regulatory gene expression, with stronger effects after FFA exposure. Long-term pharmacological inhibition of MRP4 with MK571 increased lipid accumulation under basal conditions but did not further increase FFA-induced lipid accumulation. These findings extend the functional scope of MRP4, highlighting its role in hepatic metabolic adaptation. MRP4 deficiency is associated with transcriptional alterations under basal conditions and greater susceptibility to hepatic steatosis under sustained nutrient stress.