Nutrient excess underpins the development of nonalcoholic fatty liver disease (NAFLD). The ensuing metabolic derangement is characterised by increased cellular respiration, oxidative stress and mitochondrial impairment. We have previously recapitulated these events in an in vitro cellular steatosis model. Here, we examined the distinct patterns of protein expression involved using a proteomics approach.
Human hepatoblastoma C3A cells were treated with a combination of energy substrates; lactate (L), pyruvate (P), octanoate (O) and ammonia (N). Proteins extracts were trypsinized and analyzed on a capillary HPLC OrbitrapXL mass spectrometer. Proteins were quantified using a label-free intensity based approach. Functional enrichment analysis was performed using ToppCluster via Gene Ontology (GO) database.
Of the 1327 proteins identified, 104 were differentially expressed between LPON and untreated cells (defined as: ≥2 peptides; fold change ≥1.5; p-value <0.05). Seventy of these were upregulated with LPON. Functional enrichment analysis revealed enhanced protein biosynthesis accompanied by downregulation of histones H2A type 1-A, H1.2, H1.5 and H1.0I in LPON cells. Lipid binding annotations were also enriched as well as proteins involved in cholesterol synthesis, uptake and efflux. Increased expression of aldo-keto reductase family 1, member C1 and C3 suggests enhanced sterol metabolism and increased ROS-mediated lipid peroxidation.
The surge of energy substrates diverts free fatty acid metabolism towards pathways that can mitigate lipotoxicity. The histones depletion may represent an adaptation to increased protein synthesis. However, this can also expose DNA to oxidative stress thus should be explored further in the context of NAFLD progression.