Selvaraj_2025_Cells_14_

Increased hepatic triacylglycerol (TG) storage in lipid droplets (LDs) is a hallmark of metabolic dysfunction-associated steatotic liver disease (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). Human carboxylesterase 1 (CES1) regulates TG storage and secretion in hepatocytes, but the mechanism remains to be elucidated. We performed studies in rat hepatoma McArdle RH7777 cells stably transfected with CES1 cDNA and in Ces1d-deficient mice using a variety of biochemical, pharmacological and cell biology approaches including the assessment of gene expression, confocal immunofluorescence microscopy, lipid synthesis measurements and quantitative mass spectrometry. CES1-expressing cells accrued more TG compared to cells lacking CES1 when incubated with oleic acid. CES1 increased the expression of Srebf1c, Nr1h3 and Nr1h2 encoding transcription factors (SREBP1c and LXRalpha and LXRbeta, respectively) that regulate the expression of lipogenic genes. Additionally, CES1 increased the expression of Acsl1 encoding an enzyme catalyzing fatty acid activation and the expression of Dgat1 and Dgat2 encoding enzymes catalyzing TG synthesis. Treatment of CES1-expressing cells with PPARgamma antagonist (GW9662), LXR antagonist (GSK2033) or CYP27A1 inhibitor Felodipine prevented CES1-mediated fatty acid esterification into TG. Ces1d-deficient mice fed high-fat diet (HFD) presented with decreased expression of Nr1h3, Nr1h2, Srebf1c and reduced hepatic TG content. Felodipine and GSK2033 treatment eliminated the differential effects on TG concentration between wild-type and Ces1d-deficient hepatocytes. The results suggest that CES1/Ces1d activates PPARgamma, LXR and SREBP1c pathways, thereby increasing TG synthesis and LD storage by augmenting fatty acid esterification.