The role of vitamin D signalling in the interaction between mesenchymal mammary tumour cells and macrophages

The transition of epithelial-like tumour cells to those exhibiting mesenchymal characteristics (Epithelial-to-mesenchymal Transition; EMT) is an integral process in breast cancer metastasis. EMT can be promoted by Transforming growth factor-beta (TGF-β) which can be found at high levels in the tumour stroma. Tumour-associated macrophages (TAMs) can also induce EMT in breast cancer cells, which is one way that they promote breast cancer metastasis. Vitamin D signalling has been implicated in EMT suppression and plays a role in modulating macrophage differentiation and stimulating their anti-inflammatory functions. This project had two major aims. First, we aimed to create and verify a unique fluorescent reporter gene construct designed to evaluate the dynamics of EMT in real-time and at the single-cell level. While some components of this reporter system were successfully validated, work to complete the final reporter construct is ongoing. The second and main aspect of this project focused on exploring the ability of 1,25-dihydroxyvitamin D3 (1,25D3) to modulate the interaction between mesenchymal mammary tumour cells and TAMs. Unexpectedly, in short-term treatment (48 hours) studies of 4T1 murine mammary tumour cells, we observed that 1,25D3 and TGF-β signalling work together to increase expression of the mesenchymal markers, Snai1, Fn1, and Col1a1. 1,25D3 and TGF-β also synergistically activate transcription of the gene encoding the 1,25D3-catabolizing enzyme, Cyp24a1. The ability of 1,25D3 and TGF-β to enhance expression of these genes was diminished in a long-term treatment (14 days) of 4T1 cells, and this effect was accompanied by a decrease in cell proliferation. 1,25D3 may also cooperate with cytokines produced by normal macrophages and macrophages considered to be TAM-like. Conditioned media experiments revealed that in the presence of factors from normal macrophages, 1,25D3 enhanced expression of Fn1, and in the presence of factors from TAM-like macrophages, 1,25D3 enhanced expression of Fn1 and Cyp24a1. Rather than mitigating the interaction as hypothesized, 1,25D3 may exacerbate the tumour-promoting effects of the EMT-TAM relationship. Also, signalling pathways involved in the EMT-TAM relationship may synergize with 1,25D3 to upregulate Cyp24a1 expression. These findings are important for understanding the potential of vitamin D compounds to be used in the treatment of breast cancer.

Hormonal factors and insulin-like growth factor-1 in North American postmenopausal women

Background: Mechanisms underlying the effect of estrogen exposure on breast cancer risk remain unclear. Insulin-like growth factor-1 (IGF-1) levels have been positively associated with breast cancer and are a potential mechanism. Objectives: The objectives of this thesis are: 1) to explore whether the reproductive risk factors and the lifetime cumulative number of menstrual cycles (LCMC), as measures for long-term estrogen exposure, are associated with IGF-1 levels, and 2) to examine the effect of an aromatase inhibitor (AI) on IGF-1 levels, and the potential interaction with BMI. Methods: A cross sectional study and a randomized controlled trial nested with the MAP.3 chemoprevention trial were used to address objective 1 and 2, respectively. 567 postmenopausal women were selected. Anthropometric measurements, lifestyle factors, reproductive characteristics and serum IGF-1 concentrations were collected at baseline and one year. Objective 1. The LCMC was computed as a composite measure of the reproductive characteristics. Multivariable linear regression models were used to assess the association between IGF-1 levels and LCMC and the hormonal risk factors, while adjusting for potential covariates. Objective 2. Changes in IGF-1 were compared between the exemestane and placebo, and effect modification by BMI was tested with an interaction term. Results: Objective 1. Women aged 55 years or older at menopause had 16.26 ng/mL (95% CI: 1.76, 30.75) higher IGF-1 compared to women aged less than 50 years at menopause. Women in the highest category of menstrual cycles (≥500 cycles) had an average 19.00 ng/mL (95%CI: 5.86, 32.14) higher concentration of IGF-1 compared to women in the lowest category (<350). Exogenous hormones had no effect on postmenopausal IGF-1 levels. Objective 2. Exemestane significantly increased IGF-1 levels by 18% (95% CI: 14%-22%); while, placebo had no effect on IGF-1. The changes in IGF-1 were significantly different between the treatment arms (P<0.0001) and no significant interaction was observed between treatment and BMI on IGF-1 changes (P=0.1327). Conclusion: Objective 1. Larger number of menstrual cycles and a later age at menopause are positively associated with IGF-1. IGF-1 may be one mechanism by which prolonged estrogen exposure increases cancer risk. Objective 2. We conclude that the reduced cancer risk observed with AI therapy likely occurs in an IGF-1 independent mechanism. Further studies exploring the clinical consequences of increased IGF-1 on AI therapy are needed.