Effect of Chronic Heat Stress on the Expression of Inflammatory and Oxidative Stress-Related Genes in Liver and Muscle Tissues of Lambs

Climate-induced heat stress poses a major challenge to small ruminant productivity in arid and semi-arid regions, affecting growth, metabolism, and immune function. This study examined tissue-specific molecular responses to chronic heat stress in lambs by evaluating the expression of key genes associated with inflammation, oxidative stress, proteostasis, and muscle function in the liver and skeletal muscle. Twenty-four lambs were reared under either thermoneutral or heat-stressed field conditions for 42 days. In the liver, chronic heat exposure led to the upregulation of antioxidant genes (SOD1, FOXO3) and pro-inflammatory IL-6, while TNF-α and PPARγ were significantly downregulated. In muscle, a different profile emerged: heat shock proteins (HSP70, HSP90) and the apoptotic marker CASP3 were strongly upregulated, MYOD was suppressed, and ACTB3 remained stable. These results suggest impaired muscle regeneration, enhanced proteotoxic stress, and tissue-specific shifts in redox and inflammatory balance. Composite gene expression ratios—such as SOD1/IL-6 and FOXO3/TNF-α—were elevated under heat stress and negatively correlated with rectal temperature, indicating a potential role as molecular indicators of thermotolerance. Principal component analysis further distinguished control and heat-stressed animals based on transcriptional profiles. These findings highlight the coordinated yet divergent molecular strategies employed by liver and muscle tissues under prolonged thermal stress. The study provides foundational insight into gene-level responses associated with heat resilience and offers molecular targets for future selection or intervention strategies in climate-adapted sheep production.