Background: n-3 PUFA derived from marine sources, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), exhibit potential for breast cancer prevention. In contrast, higher dietary intakes of n-6 PUFA, such as linoleic acid (LA), have been implicated in promoting mammary tumourigenesis. However, there is a need for further exploration into how n-3 PUFA influence breast cancer development in comparison to different amounts and sources of LA. Objective: The purpose of this study was to compare the effects of n-3 PUFA-enriched diets versus n-6 PUFA diets differing in LA content, including corn oil (50% LA) and safflower oil (70% LA), on mammary tumour development in a HER2+ breast cancer model. Methods: Using the HER2+ breast cancer MMTV-neu(ndl)YD5 transgenic mouse model, this study determined the effects of: (1) 10% w/w corn oil (CO, n-6 PUFA, n = 14), (2) 10% w/w safflower oil (SO, n-6 PUFA, n = 14), (3) 3% w/w menhaden oil + 7% w/w CO (3% FO 7% CO, n-3 PUFA, n = 12), and (4) 3% w/w menhaden oil + 7% w/w SO (3% FO 7% SO, n-3 PUFA, n = 14) on puberty onset, tumour incidence, tumour volume, and tumour number in utero until 20 weeks of age. Results: Mice fed the n-3 PUFA-enriched diets showed a lower trajectory of tumour development compared to the n-6 PUFA diets, although the differences for palpated tumour volume and number over time reached significance only between the 10% CO and 3% FO 7% CO groups. This suggests that high LA content in CO may represent a threshold for promoting tumour growth whereby further LA content marginally influences additional tumour development. Exposure to the CO n-6 PUFA diet further resulted in earlier onset of puberty compared to the n-3 PUFA-enriched diet containing CO. To investigate the underlying mechanisms, a qPCR analysis of mammary glands and tumour tissue revealed that the n-3 PUFA diets downregulated the expression of pro-tumourigenic immune markers, including CD206 and F4/80 in the mammary glands and the cannabinoid receptor CB2 in tumours, compared to the n-6 PUFA diets. Conclusions: These findings indicate that the presence of dietary n-3 PUFA plays a key role in modulating mammary tumour development, which may be further influenced by the underlying n-6 PUFA background. The associated changes in immune markers suggest that n-3 PUFA exert anticancer effects in part by shifting the tumour immune microenvironment toward an anti-tumour phenotype and modulating cannabinoid receptor signalling. Collectively, this work informs future human studies investigating the role of dietary fat composition in breast cancer risk.

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