Abstract
Exercise reduces the risk of breast cancer development, and improves survival in breast cancer patients. However, the underlying mechanisms of this protective effect remain to be fully elucidated. It is unclear whether exercise can attenuate or modify the pro-tumour effects of obesity and related conditions, such as hyperlipidaemia, on breast cancer growth. The main aims of this study were (1) to develop a relevant, tumour-bearing mouse exercise model and (2) to determine the effect of exercise and hyperlipidaemia on the breast tumour microenvironment. We hypothesise that exercise attenuates the negative effect of hyperlipidaemia through ‘normalisation’ of the tumour microenvironment.
Hyperlipidaemic ApoE-/- and wild-type (WT) C57BL/6 mice with orthotopic EO771 breast tumours were randomly assigned to intermittent or continuous voluntary wheel running or sedentary control. When tumours reached maximum size, mice were sacrificed and the serum, organs and tumours removed for analysis of tumour cell proliferation, immune infiltrate, circulating inflammatory factors, perfusion, microvessel density, hypoxia, HIF-1α protein level and GLUT-1 protein expression. This was done by immunohistochemistry, immunofluorescence, ELISA and Western blotting.
Although exercise and hyperlipidaemia did not significantly impact tumour growth rate, exercising mice had significantly reduced body weights. Tumour-bearing mice showed a significant increase in serum monocyte chemoattractant protein 1 (MCP-1) compared to non-tumour-bearing mice (p<0.05) and this was further increased in mice bearing internal tumours (p<0.001). Serum MCP-1 was drastically reduced in exercising mice bearing internal tumours. In addition, analysis of immunofluorescent images revealed that CD3+CD8+ cytotoxic T lymphocytes as a percentage of total T cells was unchanged by either exercise or hyperlipidaemia. The percentage of CD3+FoxP3+ regulatory T cells was significantly reduced in sedentary ApoE-/- compared to sedentary WT mice (p<0.05), and this reduction tended to be attenuated in exercising mice. Furthermore, intratumoral Treg cell percentage inversely correlated with individual running distance in WT mice (p<0.05). Analysis of CD31+ vessels revealed that sedentary ApoE-/- mice had a significant reduction in microvessel density compared to sedentary WT mice (p<0.05). Hypoxia was also significantly reduced in ApoE-/- compared to WT mice (p<0.05), but perfusion was not significantly altered. Further studies are necessary to clarify and confirm these results, as this study was limited both by a short exercise and tumour-bearing time period.
This study identifies a number of key considerations in the design of future preclinical exercise studies in tumour-bearing mice. In addition, our results provide evidence for the potential value of MCP-1 as an exercise-regulated, prognostic biomarker in mouse models, and indicate that hyperlipidaemia normalises the microenvironment of the tumour.