Heat stress (HS) poses a great challenge to the poultry industry by inducing oxidative damage to the liver, endangering the health and production of broilers. As an important type of seaweed polyphenols, phlorotannin has been shown to have antioxidant properties. The present study evaluated the protective effects of dietary phlorotannin on HS-induced liver injury in broilers based on oxidative damage parameters. A total of 108 twenty-one days old male Arbor Acres plus (AA+) broilers were randomly divided into three groups: TN group (thermoneutral, 24 ± 1 °C, fed with basal diet), HS group (HS, 33 ± 1 °C for 8 h/day, fed with basal diet), and HS + phlorotannin group (HS + 600 mg/kg phlorotannin). Each group has six replicate cages with six birds per cage. The feeding experiment lasted 21 days. At the termination of the feeding experiment (42 days old), samples were collected for analysis of morphological and biochemical features. The results showed that HS decreased the liver index, serum albumin (ALB) content, hepatic antioxidant enzymes activities of catalase (CAT), total superoxide dismutase (T-SOD), glutathione S-transferase (GST), and glutathione peroxidase (GSH-Px) (p < 0.05), while increasing the hepatic histopathology score, apoptosis rate, and malondialdehyde (MDA) content (p < 0.05) in 42-day-old broilers. Compared with the HS group, dietary phlorotannin improved the activities of antioxidant enzymes (GST and GSH-Px) but decreased the histopathology score and apoptosis rate in the liver (p < 0.05). Moreover, HS down-regulated hepatic mRNA expression of CAT1, NQO1, HO-1, and SLC7A11 (p < 0.05), while up-regulated hepatic mRNA expression of Keap1, MafG, IκBα, NF-κB P65, IFN-γ, TFR1, ACSL4, Bax, and Caspase-9 (p < 0.05). Compared with HS group, dietary phlorotannin up-regulated hepatic mRNA expression of Nrf2, CAT1, MafF, GSTT1, NQO1, HO-1, GCLC, GPX1, TNF-α, Fpn1, and SLC7A11 (p < 0.05), while down-regulated hepatic mRNA expression of IκBα, Bax, Caspase-9, and TFR1 (p < 0.05). In conclusion, dietary supplementation of 600 mg/kg phlorotannin could alleviate HS-induced liver injury via regulating oxidative status, apoptosis, and ferroptosis in broilers; these roles of phlorotannin might be associated with the regulation of the Nrf2 signaling pathway.
