Pathological, immunohistochemical and molecular studies on rutin's protective effects against water hardness‐induced cytotoxicity in Nile tilapia, <scp> <i>Oreochromis niloticus</i> </scp>

Elevated water hardness (HW) level could negatively impact the health of various kinds of fish, especially Nile tilapia. The current work aimed to explore the cytoprotective role of rutin (RUT), a natural antioxidant, against HW-induced cell injury in Nile tilapia, and to find the possible mechanistic pathways underlying this protection. Sixty monosex fish were allocated into four different experimental groups (n = 15): control, RUT (1 g/kg), HW-exposed group (300 ppm) and HW + RUT group. The daily exposure of fish to elevated HW for 90 days significantly altered the haematological profile, increased malondialdehyde (MDA) and nitric oxide (NO) levels and reduced the antioxidant enzyme activity. Additionally, it elevated the serum levels of both liver and kidney function biomarkers, alongside pronounced histopathological alterations in gills, the liver and kidneys. Mechanistically, the HW markedly increased the expression of either gene or protein levels of iNOS, casp-3, CYP1A1, MT-1 and Hsp70 in both the liver and kidneys, alongside strong proliferating cell nuclear antigen (PCNA) immunoexpression in the gills. In contrast, dietary RUT effectively ameliorated the toxicological alterations induced by elevated HW, contributing to the enhancement of immune and antioxidant defence mechanisms, which therefore reduced the aforementioned cell signalling pathways. In conclusion, the potent antioxidant, anti-apoptotic and immunostimulant properties of RUT can contribute to reducing HW-induced toxicity in Nile tilapia.