B121: Molecular Mediators of Mammographic Density

Alastair Ironside1,Jenny Gomm1,Linda Haywood1,Jun Wang1,Afonso Guerra1,Claude Chelala1,Louise Jones1

1Barts Cancer Institute, London, UK

Presenting date: Tuesday 3 November
Presenting time: 12.20-13.10

Background

Mammographic density (MD) is a major risk factor for the development of breast cancer though little is known about the biological mechanisms mediating it. Tamoxifen prevents breast cancer in a sub-set of high-risk women in a mechanism that appears to be dependent on reduction of MD. Animal model studies suggest that tamoxifen remodels the mammary stroma to a tumour-inhibitory phenotype. This study aims to analyse the effect of tamoxifen on breast fibroblast function and identify potential pro-tumourigenic pathways contributing to density-associated risk.

Method

Primary human breast fibroblasts from normal, high risk or breast cancer patients were treated with hydroxytamoxifen (100nm-5?M). Fibroblast function was analysed by measuring: proliferation; expression of stromal proteins fibronectin (FN), LOX and collagen 1; effects on TGF-? signalling via SMAD phosphorylation and upregulation of the myofibroblast marker SMA.  Genome wide analysis was performed using RNA-Seq.

Results

Fibroblasts from 25 patients were treated with tamoxifen. All patients showed reduced proliferation with treatment. In 62% of patients tamoxifen treatment resulted in reduced expression of FN.  TGF-?-mediated upregulation of SMA and FN were consistently inhibited by tamoxifen, as was fibroblast contraction of collagen gels. RNA-Seq analysis revealed modulation of a number of metabolic pathways by tamoxifen, including significant upregulation of DHCR7, part of the microsomal anti-oestrogen binding site (AEBS), a complex which is also known to carry out cholesterol epoxide hydrolase (ChEH) enzyme activity.

Conclusion

These data indicate that tamoxifen can directly remodel the stromal microenvironment in a subset of patients, generating a less ‘reactive' stroma. The binding of tamoxifen to the AEBS inhibits ChEH activity - a mechanism which has been proposed to contribute to its anti-tumourigenic effects. Increased expression of DHCR7 has also been implicated as a suppressor of Hedgehog signalling another pro-tumourigenic pathway. Thus, tamoxifen impacts on multiple pathways to create a tumour inhibitory microenvironment.