<p><b>BACKGROUND</b></p><p>Skeletal muscle loss is a well-recognized consequence of cancer, and chemotherapy exacerbates myotoxicity through multiple mechanisms. Retaining muscle mass improves tumour response to therapies and tolerance to chemotherapy; hence, interventions to mitigate myotoxicity warrant investigations.

This study aimed to investigate the protective effects of dietary eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) provided in the form of fish oil on chemotherapy-induced myotoxicity using next-generation RNA sequencing (NGS).</p><p><b>METHODS</b></p><p>Fischer 344 rats were fed either a standard diet (STD) for the entire study or switched to a diet containing fish oil (2.3 g/100 g of diet) when chemotherapy (irinotecan + 5-fluorouracil) was initiated. All rats received the Ward colon tumour and tumours were allowed to grow for 2 weeks.

Comparisons were made between (1) tumour-bearing rats on a standard diet (TUMOUR STD), (2) tumour-bearing rats that received chemotherapy on a standard diet (CHEMO STD) and (3) tumour-bearing rats that received chemotherapy on a fish oil diet (CHEMO FO). After two cycles of chemotherapy, NGS was performed on gastrocnemius muscle.

Differential expression of genes was performed using DEseq2, with a fold-change cut-off ≥ 1.5 and p value < 0.05. Ingenuity pathway analysis (IPA) was used for functional annotation, canonical pathways and upstream regulators analysis.</p><p><b>RESULTS</b></p><p>Transcriptomic analysis revealed distinct alterations in skeletal muscle gene expression profiles.

In the CHEMO STD versus TUMOUR STD comparison, 272 genes showed differential expression. Of these, 55% were upregulated and 45% were downregulated.

Two cycles of chemotherapy altered genes in the pathways of proliferation of muscle cells (p < 10 -7), connective tissue disorder (p < 10 -6), apoptosis (p < 10 -3) and neurodevelopmental disorders (p < 10 -3). In the CHEMO FO versus CHEMO STD comparison, 274 genes were differentially expressed (73% upregulated and 27% downregulated).

Dietary fish oil exclusively altered immune-related functions, notably downregulating several genes in the leukocyte extravasation pathway (-logp value 5.92, z score -2.83). Upstream regulatory molecules after chemotherapy were predicted to inhibit transcription factors involved in myogenic regeneration, while those fed a diet containing fish oil showed inhibition of inflammation-related cytokines.</p><p><b>CONCLUSION</b></p><p>Chemotherapy negatively impacts processes involved in muscle homeostasis, including muscle cell proliferation and regeneration.

The provision of fish oil primarily showed protective effects from pro-inflammatory mediators by downregulating genes involved in the leukocyte extravasation pathway. Our findings provide novel insights into the molecular mechanisms underlying chemotherapy-induced myotoxicity and the potential therapeutic benefits of dietary EPA + DHA to restore muscle homeostasis in cancer.</p>