Abstract Body: Fat storage-inducing transmembrane protein 2 (FITM2), an endoplasmic reticulum (ER) transmembrane protein, is crucial for cytoplasmic lipid droplet (LD) formation. Recently, we reported that FITM2 in the liver plays a key role in lipidation of apolipoprotein B100 (apoB100), with deficiency leading to defective triglyceride (TG) loading onto nascent VLDL. Notably, the number of secreted VLDL particles was unaffected, but their density increased, with a higher cholesteryl ester (CE)/TG ratio. Since CE/TG ratio positively correlate with the atherogenicity of apoB100-containing lipoproteins, and FITM2-deficient mice exhibited a greater CE/TG ratio compared to controls, we hypothesized that mice lacking FITM2 in their livers would secrete more atherogenic apoB-containing lipoproteins, resulting in greater atherosclerotic plaques compared to their controls. To test this, mice (N ≥ 8/group) with liver-specific FITM2 deficiency and their controls were fed a Western Diet for 12 weeks and injected with AAV8-PCSK9 to induce hypercholesterolemia. FITM2-deficient mice had ~60% larger plaque areas (0.257 vs. 0.160 mm^2, P = 0.002) and ~80% larger inflammatory macrophage areas (0.108 vs. 0.059 mm^2, P = 0.0005) compared to controls, despite significantly lower total plasma cholesterol (868 vs. 1075 mg/dl, P = 0.0035) but a higher CE/TG ratio in VLDL. FITM2 deficiency also led to profound macrosteatosis, hepatocyte ballooning, and inflammatory cell accumulation, mimicking a Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) phenotype (5.5/8 vs. 3/8 MASLD score, P = 0.0075).Since MASLD is itself an atherosclerotic risk factor, we are investigating whether the observed plaques result from (1) the intrinsic atherogenicity of apoB-containing lipoproteins, (2) the inflammatory state caused by MASLD, or (3) both. To address the above questions, we have put the same type of mice as above on a low fat/high cholesterol diet, to mitigate the effects of a high fat diet on the liver. Results from these ongoing experiments will be available at the time of this meeting. Nonetheless, the results already suggest that FITM2 is of pivotal importance for the proper lipid handling in liver and defective function of it could lead to a MASLD-like phenotype, secretion of more atherogenic apoB100 containing lipoproteins, and enhanced atherosclerosis.