Ethanol Exposure to Ethanol-Oxidizing HEPG2 Cells Induces Intracellular Protein Aggregation

Aggresomes are collections of intracellular protein aggregates. In liver cells of patients with alcoholic hepatitis, aggresomes appear histologically as cellular inclusions known as Mallory-Denk (M-D) bodies. The proteasome is a multicatalytic intracellular protease that catalyzes the degradation of both normal (native) and abnormal (misfolded and/or damaged) proteins. The enzyme minimizes intracellular protein aggregate formation by rapidly degrading abnormal proteins before they form aggregates. When proteasome activity is blocked, either by specific inhibitors or by intracellular oxidants (e.g., peroxynitrite, acetaldehyde), aggresome formation is enhanced. Here, we sought to verify whether inhibition of proteasome activity by ethanol exposure enhances protein aggregate formation in VL-17A cells, which are recombinant, ethanol-oxidizing HepG2 cells that express both alcohol dehydrogenase (ADH) and cytochrome P450 2E1 (CYP2E1).

We exposed ethanol-non-oxidizing HepG2 cells (^ADH-/CYP2E1-) or ethanol-oxidizing VL-17A (^ADH+/CYP2E1+) to varying levels of ethanol for 24 h or 72 h. After these treatments, we stained cells for aggresomes (detected microscopically) and quantified their numbers and sizes. We also conducted flow cytometric analyses to confirm our microscopic findings. Additionally, aggresome content in liver cells of patients with alcohol-induced hepatitis was quantified.

After we exposed VL-17A cells to increasing doses of ethanol for 24 h or 72 h, 20S proteasome activity declined in response to rising ethanol concentrations. After 24 h of ethanol exposure, aggresome numbers in VL-17A cells were 1.8-fold higher than their untreated controls at all ethanol concentrations employed. After 72 h of ethanol exposure, mean aggresome numbers were 2.5-fold higher than unexposed control cells. The mean aggregate size in all ethanol-exposed VL-17A cells was significantly higher than in unexposed control cells but was unaffected by the duration of ethanol exposure. Co-exposure of cells to EtOH and rapamycin, the latter an autophagy activator, completely prevented EtOH-induced aggresome formation. In the livers of patients with alcohol-induced hepatitis (AH), the staining intensity of aggresomes was 2.2-fold higher than in the livers of patients without alcohol use disorder (AUD).

We conclude that ethanol-induced proteasome inhibition in ethanol-metabolizing VL-17A hepatoma cells causes accumulation of protein aggregates. Notably, autophagy activation removes such aggregates. The significance of these findings is discussed.