Targeting obesity-related breast cancer progression: the impact of metformin on tumour growth and lipid metabolism

Modifiable lifestyle factors, such as obesity, are reported to account for nearly a quarter of breast cancer (BC) cases. Obesity is characterised by the pathological expansion of adipose tissue, leading to both systemic and local alterations, including increased availability of free fatty acids (FAs). Metformin, an anti-diabetic drug, has been implicated to improve outcomes for BC patients with underlying metabolic disturbances. Yet the pleiotropic nature of the drug means delineating the mechanism of action behind this effect has been challenging. Therefore, this study aimed to gain a greater understanding of the obesity-associated drivers of BC progression and the mechanisms through which metformin confers a therapeutic benefit within an obese context. In vitro assays demonstrated that elevated FA levels induced lipid accumulation in human BC cells and reduced both glycolytic activity and basal mitochondrial respiratory rates. Upon metformin treatment, these cells exhibited impaired metabolic plasticity, as the elevated FA environment prevented glycolytic upregulation to compensate for inhibited mitochondrial respiration. This did not result in reduced viability or migratory changes but does suggest tumour cells in obese contexts may be metabolically compromised. To model these effects pre-clinically, a murine diet-induced obesity model was used, which reflected the increased adipose tissue volume and elevated circulating FA levels reported in patients with obesity. Using the EO771 mammary tumour cell line and the novel EO771-HER2 cell line, both anti-PD1 and anti-HER2 therapies (respectively) were shown to be less effective in mice fed a high fat diet. However, when combined with metformin, both anti-PD1 and anti-HER2 therapy were augmented, improving survival rates in these obese mice. This effect was independent of adiposity and circulating FA levels but was associated with altered intratumoral FA levels. Thus, these findings support the hypothesis that the tumour cell-intrinsic metabolic effects of metformin are central to its anti-cancer benefit, potentially changing the tumour metabolic environment to support immune cell function. To investigate this further, a metformin-resistant cell line was generated using the metformin-insensitive complex I variant, NDI1. Exploring the wider effects of metformin and obesity on the tumour microenvironment highlighted that treatment with metformin shifted metabolically activated macrophages to a more pro-inflammatory phenotype; as shown by reduced expression of the M2-like marker CD206 and increased MHCII expression. Although this did not enhance phagocytic capacity, it may reflect a more immune activated tumour microenvironment that could underlie the observed enhancement in immunotherapeutic responses. This study provides supportive evidence for the use of metformin to augment immunotherapeutic efficacy in obesity-associated BC, primarily through tumour-targeted metabolic modulation and consequent immune modulation.