Lactose induced redox-dependent senescence and activated Nrf2 pathway

Lactose is a disaccharide found in milk and thus a part of our daily food intake. Upon ingestion, it is hydrolyzed to glucose and galactose by the enzyme lactase and absorbed in the small intestine. People who suffer from lactose intolerance are unable to completely digest it due to deficiency of lactase, leading to intestinal problems such as diarrhoea, and bloating. Various studies have focused on treating these symptoms. However, the effects of lactose that diffuses passively into cells, on cellular senescence have largely remained unknown. Thus, the present study investigated the effects and mechanisms of lactose on senescence both in vitro and in vivo. The study was conducted in MRC-5 cells. The cellular senescence was estimated by determining the expression of SA-β-gal and p16^ink4a. The cell viability of MRC-5 cells was determined by the CCK-8 Assay. Activity of intracellular reactive oxygen species was estimated by measuring the levels of superoxide dismutase (SOD), glutathione (GHS), and reactive oxygen species (ROS). The mechanism of lactose on cellular senescence was explored by western blotting. We also studied the effect of lactose on the lifespan of Caenorhabditis elegans. Increased activities of SA-β-gal and p16^ink4a revealed the ability of lactose to induce senescence in MRC-5 cells. The elevated intracellular ROS level and decreased GSH and SOD levels in these cells were indicative of cellular oxidative stress induced by lactose. Furthermore, western blotting analysis of Nrf2 and mRNA expression of its downstream genes suggested the Nrf2/ARE pathway was involved in the oxidative stress induced by lactose. These results were further validated by the shortened lifespan of C. elegans after lactose supplement. Moreover, the lactose-induced senescence could be alleviated by an antioxidant, N-Acetyl-L-cysteine (NAC), both in vitro and in vivo. The present study observed a positive correlation between lactose and cellular oxidative stress, suggesting the latter to be an underlying mechanism of lactose-induced senescence.