Objective: Trop-1/Ep-CAM modulates growth and survival of transformed cells, and it is highly expressed in most carcinomas including breast cancer. Only membranous staining is typically considered in evaluating Trop-1/epithelial cell adhesion molecule (Ep-CAM) expression in tumor cells. However, there is evidence of retention of Trop-1/Ep-CAM, as functionally incompetent molecules, in intra-cytoplasmic vesicles. Hence, we investigated whether cytoplasmic immunostaining may have an independent clinical significance with respect to membranous staining.
Methods: Membranous and cytoplasmic Trop-1/Ep-CAM expression was immunohistochemically investigated in 642 unilateral breast cancers from patients with a 99-month median follow-up. Multiple correspondence analysis was used to investigate the association between Trop-1/Ep-CAM and other biological variables. The impact of Trop-1/Ep-CAM expression on the patient's outcome was evaluated as event-free survival by the Kaplan-Meier method and proportional hazard Cox model.
Results: While tumors with intermediate/strong membranous staining were mostly associated with concomitant cytoplasmic Trop-1/Ep-CAM expression (97%), tumors with weak-to-nil membranous staining showed intermediate/high cytoplasmic expression in 23% of cases. Cytoplasmic overexpression was associated with a favorable outcome, especially in node-positive patients, regardless of the adjuvant therapy received.
Conclusion: Trop-1/Ep-CAM expression may have different clinical implications according to its subcellular localization.