Loss of Retinoblastoma Tumor Suppressor Protein Makes Human Breast Cancer Cells More Sensitive to Antimetabolite Exposure

Purpose: The RB tumor-suppressor activity may influence the therapeutic response in human breast cancers. The effect of adjuvant therapy on clinical outcome of breast cancer patients was analyzed, and the sensitivity to 5-fluorouracil (5-FU) and methotrexate was investigated in MCF-7 and HCT-116 human cancer cells, according to their RB status. Experimental Design: RB protein (pRB) expression was prospectively evaluated by immunocytochemistry in 518 consecutive patients and its predictive value was determined according to the adjuvant therapeutic treatments. MCF-7 and HCT-116 human cancer cells silenced for RB1 expression were treated with 5-FU and methotrexate, at the same concentrations and time exposures as determined in the interstitium of breast cancers of patients treated with adjuvant chemotherapy. Results: Multivariate analysis of disease-free survival, including all the established clinical and histopathologic prognostic variables, indicated that the absence of pRB expression was the only predictive factor of good clinical outcome in patients treated with standard systemic chemotherapy (cyclophosphamide, methotrexate, and 5-FU) but not in patients treated with endocrine therapy alone. 5-FU and methotrexate significantly reduced the growth rate of RB1 -silenced but not of control MCF-7 and HCT-116 cells. This was likely due to the absence of a DNA damage checkpoint with accumulation of DNA double-strand breaks in RB1 -silenced but not in control cells. Conclusions: The absence of pRB expression renders human breast cancer cells more sensitive to 5-FU and methotrexate and predicts a good clinical outcome for patients treated with adjuvant chemotherapy. We suggest that patients with RB-negative breast cancers should be treated with systemic chemotherapy.