Mechanisms of TNF-alpha action in post-menopausal breast cancer
2017-02-28T04:56:34Z (GMT) by
Approximately one in nine women in Australia are diagnosed with breast cancer throughout their lifetime. Major risk factors, including increased age and adiposity, coincide with prolonged exposure to circulating oestrogens in the body. As a consequence, two thirds of post-menopausal women with breast cancer will have an oestrogen receptor positive (ER+) tumour, responsive to oestrogen for growth and proliferative advantage. Aromatase is the key enzyme responsible for conversion of androgens to oestrogens, and to date, inhibition of oestrogen production via aromatase inhibitors are the frontline treatment in targetting post-menopausal breast cancers. Despite this, inhibition of aromatase throughout the body results in adverse side effects. Understanding the transcriptional control of the CYP19A1 gene that encodes aromatase is therefore a critical avenue to the development of breast specific targeted therapeutics. CYP19A1 has a number of upstream promoters that drive transcription in a tissue-specific manner to restrict expression to oestrogen-producing tissues. Serum levels of the inflammatory factor Tumour Necrosis Factor-α (TNFα) also increases with age and adiposity and is closely correlated to breast cancer risk. TNFα is one major regulator of the breast adipose-specific promoter I.4 (PI.4). However, the signalling mechanisms that drive TNFα production in breast cancer, and the pathways by which it stimulates aromatase expression, are not well understood. The overall aim of this thesis was to determine how TNFα acts upon breast adipose fibroblasts (BAFs) to stimulate oestrogen production, and how in turn TNFα is regulated in tumour epithelial cells. Using primary human BAFs as a model, this thesis demonstrates that TNFα stimulates expression of the Early Growth Response (EGR) transcription factors via activation of the NFκB and MAPK signalling pathways. These transcription factors then stimulate activity of PI.4, via a defined short distal promoter region. However, this does not appear to be the result of a direct interaction between EGR factors and promoter I.4; rather, intermediate factors induced by the EGRs appear to bind to the aromatase promoter to mediate transcription. Increased PI.4 activity then leads to increased aromatase activity and therefore oestrogen production from BAFs surrounding an ER+ breast tumour. Studies on a panel of breast cancer cell lines show that this oestrogen may then stimulate TNFα expression and secretion from ER+ breast cancer cells. Thus, a positive feedback loop is established that maintains TNF ix production and ultimately drives oestrogen signalling to promote further ER+ breast cancer growth. Excess local production of oestrogen within a breast tumour microenvironment is the major driver or ER+ tumour development, and identifying the factors and mechanisms which contribute to oestrogen biosynthesis is critical to restricting this process. The data presented in this thesis establishes TNFα as a critical, tumour-derived signalling factor which drives oestrogen biosynthesis in BAFs surrounding the tumour. These findings enhance our understanding of how currently used drug-based treatments inhibit breast cancer growth, and potentially open up avenues for novel ER+ breast cancer therapeutics based on the action of TNFα.