Dietary Lycopene Mitigates Reproductive Impairment in Heat-Stressed Rongchang Boars: Roles of Antioxidant, Anti-Inflammatory and Nrf2 Pathway

Heat stress (HS) severely impairs boar reproductive function by inducing oxidative stress and inflammatory responses, while lycopene (LYC), as a potent antioxidant, exerts a potential protective effect on the male reproductive system. This study aimed to clarify the mechanism underlying LYC-mediated alleviation of HS-induced decline in semen quality in Rongchang boars, identify the most affected tissues, and explore its regulatory role in the Nrf2 (Nuclear factor E2-related factor 2) pathway. A total of 18 Rongchang boars with an initial body weight of 15.81 ± 1.07 kg were randomly assigned to three groups (6 boars per group): the control group (CON, 26 ± 1 °C), the heat stress group (HS, exposed to 35 ± 1 °C for 8 h daily), and the heat stress + 100 mg/kg lycopene group (HS + LYC). After 28 days of adaptive feeding and 14 days of HS treatment, samples were collected for semen quality analysis, testicular histological analysis, antioxidant index detection, transcriptome analysis, Nrf2 pathway detection, and inflammatory index detection. The results showed that HS significantly increased the sperm abnormality rate (<i>p</i> < 0.05), damaged the testicular structure, and induced oxidative stress in serum, lung, liver, left ventricle, testis, and epididymis (caput epididymis, corpus epididymis, cauda epididymis), with varying degrees of oxidative stress observed in these samples. Among these tissues, the testis and cauda epididymis exhibited the most significant responses to HS and LYC, with the comprehensive impact magnitudes of 317% and 514%, respectively. Enrichment analysis of differentially expressed genes (DEGs) in these two tissues revealed that the pathways mediating oxidative stress response displayed distinct tissue specificity, and all of them were closely associated with the Nrf2 antioxidant signaling pathway. HS significantly downregulated the mRNA expressions of <i>Nrf2</i>, <i>Quinone Oxidoreductase (NQO1)</i>, <i>Heme Oxygenase 1 (HMOX1)</i> and <i>Glutamate-Cysteine Ligase Catalytic Subunit (GCLC)</i> genes as well as the protein level of Nrf2 in the testis and cauda epididymis, increased the protein level of Keap1, and significantly elevated the levels of interleukin-6 (IL-6), interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in these two tissues (<i>p</i> < 0.05). Compared with the HS group, dietary supplementation of LYC significantly improved sperm motility and the proportion of rapidly progressive sperm, reduced the proportion of immotile sperm and sperm abnormality rate (<i>p</i> < 0.05), alleviated testicular damage and oxidative stress in various tissues, upregulated the mRNA expressions of <i>Nrf2</i> and <i>HMOX1</i> genes in the testis as well as the mRNA expressions of <i>Nrf2</i>, <i>NQO1</i>, <i>HMOX1</i> and <i>GCLC</i> genes in cauda epididymis (<i>p</i> < 0.05), significantly increased the Nrf2 protein level and decreased the Keap1 protein level in these two tissues, and simultaneously decreased the levels of the aforementioned inflammatory factors (<i>p</i> < 0.05). In conclusion, dietary supplementation with 100 mg/kg LYC can alleviate HS-induced decline in semen quality and testicular damage by regulating the oxidative status and inflammatory level of relevant tissues (e.g., testis and cauda epididymis) in boars, and this protective effect may be associated with the regulation of the Nrf2 signaling pathway.