Anti-progestin therapy targets hallmarks of breast cancer risk through epithelial-stromal remodelling

Author and Presenter:
Bruno M. Simões, Ph.D.

Background

The luminal progenitor (LP) population in the normal breast is under the control of paracrine progesterone signalling and likely represents the cell of origin of estrogen receptor negative (ER-) BC. Exogenous progestins, as contraception or menopausal hormone replacement therapy (HRT), increase the risk of ER- and ER+ BC. We sought to examine the effect of antagonism of progesterone signalling on the tissue composition and cellular hierarchy of normal human breast and thus the potential for progesterone receptor antagonism in breast cancer prevention.

Methods

BC-APPS1 is a single-arm phase 2 pilot study in which premenopausal women at increased familial BC risk underwent vacuum-assisted biopsy (VAB) in the luteal phase of the menstrual cycle before commencing a 12-week course of the selective progesterone receptor modulator (SPRM) ulipristal acetate (UA; 5mg daily). VAB was repeated in the final week of therapy. The primary endpoint of BC-APPS1 was the change in epithelial Ki67 assessed by immunohistochemistry. Secondary endpoints included 2D mixed luminal/basal and 3D (mammosphere) colony formation assays, LP fraction by FACS (CD49f+/EPCAM+ cells), immunofluorescence (IF) staining of LP marker SOX9, single-cell and bulk RNAseq, epithelium/stromal laser capture microdissection-based proteomics, and tissue stiffness analyses.

Results

Twenty-six women were recruited to BC-APPS1, and 24 underwent paired biopsies. The trial met its primary endpoint with a significant reduction in the percentage of Ki67+ cells between baseline and 12 weeks. FACS analysis demonstrated a significant reduction in the LP proportion with UA treatment, and the SOX9+Ki67+ population reduced significantly with UA therapy. In functional analyses, both mammosphere and mixed luminal/basal colony formation were reduced with UA treatment. Single-cell RNAseq and epithelium/stromal laser capture microdissection-based proteomics identified stromal components and remodelling as key pathways perturbed by UA treatment. Collagen VI (Col6) was the top protein down-regulated in the lobular epithelium by anti-progestin treatment, and Col6 expression correlated with SOX9-positive cells. Collagen organization (PSR staining), tissue stiffness (AFM), and fibro-glandular volume (FGV on MRI images) were significantly reduced with UA treatment. Anti-progestin treatment of normal breast microstructures in collagen gels (VitroGel®, The Well Bioscience) blocked stiffness-induced increases in progesterone signalling and progenitor cell activity.

Conclusions

Anti-progestin therapy reduces both the proliferation of luminal progenitor cells and the stiffness of normal breast tissue through stromal remodelling, making it an attractive candidate for BC prevention. Reduction in FGV on MRI suggests that mammographic density should be investigated as a potential biomarker of activity.

Product Used

VitroGel® COL

Application

The Multiple Ways to Study 3D Breast Cancer Models with VitroGel®

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Anti-Progestin Therapy Targets Hallmarks of Breast Cancer Risk