Due to increased use of mammographic screening, up to 25% of newly diagnosed breast cancers are actually at an early pre-invasive stage called ductal carcinoma in situ (DCIS). It is currently very difficult to predict the risk of invasive recurrence in patients diagnosed with DCIS and therefore the development of prognostic biomarkers are needed to individualise patient treatment. Myoepithelial cells represent a distinguishing barrier between DCIS and invasive carcinoma and have been implicated in tumour suppression. In a search for protease inhibitors that are abundant in these cells surrounding normal breast ducts, we identified the cysteine protease inhibitor stefin A.
To study the functional role of myoepithelial stefin A in early breast cancer, a 3D co-culture model was developed whereby normal immortalised myoepithelial cells (N1ME) reverted the invasive potential of the MDA-MB-231 breast cancer cell line to a DCIS-like state. The suppressive function of myoepithelial cells was reliant on stefin A as reduced expression of this protease inhibitor in N1ME cells reversed this effect and allowed MDA-MB-231 invasion. Addition of a cathepsin B inhibitor (CA-074) to these cultures rescued the DCIS-like non-invasive phenotype. A role for cathepsin inhibition in early stage tumorigenesis was also observed in vivo. Treatment of the MMTV-PyMT spontaneous mammary tumour mouse model with CA-074, reduced the development of invasive carcinoma.
Further, analysis of samples from 130 patients revealed that stefin A was lowest in the myoepithelial cells surrounding higher grade DCIS lesions, those that are associated with an increased risk of invasive carcinoma recurrence. Our studies suggest that stefin A expression in myoepithelial cells protects from invasion. Future validation could lead to myoepithelial stefin A being used as a prognostic marker to distinguish patients who are at low risk of developing invasive breast cancer and therefore could be spared unnecessary therapeutic intervention. This unique 3D co-culture model will now be used to screen for other myoepithelial-derived tumour suppressors that could be incorporated into a prognostic panel.