Distinct OA:PA Ratios Generate Discrete Hepatocellular Phenotypes in HepG2 Cells.

Metabolic dysfunction-associated steatohepatitis (MASH) is characterized by hepatic steatosis, metabolic stress, and inflammation. Although free fatty acid (FFA)-based in vitro models are widely used, how OA:PA ratios shape hepatocellular phenotypes remains incompletely characterized. HepG2 cells were treated with oleic acid (OA), palmitic acid (PA), or four OA:PA mixtures (O1P1, O2P1, O3P2, O1P2) for 24 or 48 h. OA and OA:PA mixtures were evaluated at 500 μM total FFA (O2P1: 333.3 μM OA + 166.7 μM PA; O3P2: 300 μM OA + 200 μM PA), whereas PA alone was evaluated at 200 μM. Cell viability, lipid accumulation, ultrastructure, oxidative response, secreted cytokines, and molecular responses were evaluated. Distinct OA:PA ratios generated discrete hepatocellular states rather than a linear steatosis-to-lipotoxicity continuum. O3P2 produced a steatosis-dominant phenotype with pronounced lipid accumulation and relatively limited stress signaling. In contrast, O2P1 combined substantial lipid accumulation with elevated DCF fluorescence, AMP-activated protein kinase (AMPK) phosphorylation, nuclear factor kappa B (NF-κB)-associated inflammatory signaling, and interleukin-8 (IL-8) secretion, representing a pronounced stress-responsive phenotype with selected MASH-relevant features. Selected responses were also evaluated in SNU-449 cells, revealing cell-line-specific patterns. Collectively, these findings support a phenotype-oriented framework for OA:PA ratio selection in hepatocyte-based in vitro MASH modeling.