Effects of acute heat stress on liver damage, apoptosis and inflammation of pikeperch (Sander lucioperca)

Pikeperch (Sander lucioperca) is a sub-cold water fish species with high aquaculture potential. Its culture is seriously affected by increasing summer temperatures in recent years. Aim to investigate the effects of heat stress on apoptosis, oxidative stress, and the immune response in pikeperch. the fish were heat stressed at 30 °C, 32 °C and 34 °C for 2h respectively, followed by a 48h recovery period. The results showed that as temperature increased, the contents of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in the liver increased significantly. Meanwhile, acute heat stress results in progressive deleterious alterations in liver tissue, especially vascular rupture, blood infiltration, and severe vacuolation at 34 °C. TUNEL staining revealed that the apoptosis level increased significantly with the rising temperature. Acute heat stress significantly induced the mRNA expression of apoptosis-related genes, including tumor suppressor (p53), B-cell lymphoma-2 (bcl-2), bcl-2-associated X (bax), apoptotic protease activating factor-1 (apaf-1), cysteinyl aspartate specific proteinase (caspase-3 and caspase-9), and the expression of p53 was also positively correlated with bax expression and the bax/bcl-2 ratio. Additionally, caspase-3 and caspase-9 activity increased significantly at 34 °C compared with the control group (23 °C). Innate immune genes, including tumor necrosis factor (tnf-α), interleukins (il-7, il-8, il-10 and il-1β), complement 3 (c3) were activated under acute heat stress, and H2O2 content was positively correlated with the expressions of tnf-α and il-1β. After the temperature reached again 23 °C, most measured indexes in heat-stressed groups didn't return to stress-free levels, and liver tissue also didn't return to its normal state in the histopathology. It was found that p53-mediated mitochondrial apoptosis pathway was triggered in pikeperch under acute heat stress, and there may be a vicious cycle between oxidative stress and inflammation. In summary, the present study is helpful to elucidate how acute heat stress mediates liver injury of pikeperch through mitochondrial pathway, inflammation and oxidative stress.