Loss of retinoblastoma tumor suppressor protein makes human breast cancer cells more sensitive to anti-metabolite 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 and Methotrexate investigated in MCF-7 breast cancer cells, according to their RB status. Experimental Design: RB protein (pRB) expression was prospectively evaluated by immuno-cytochemistry in 518 consecutive patients and its predictive value determined according to the adjuvant therapeutic treatments. MCF-7 breast cancer cells silenced for RB1 expression, were treated with 5-Fluorouracil and Methotrexate, at the same concentrations and time exposures as determined in the interstitium of breast cancers of patients treated with adjuvant chemotherapy. Results: Standard systemic chemotherapy (Cyclophosphamide, Methotrexate and 5-Fluorouracil) greatly improved the clinical outcome of patients whose cancer lacked pRB but not of patients either expressing pRB or with inactivated, hyper-phosphorylated pRB. Absence of pRB was associated with a worse prognosis in patients treated with endocrine therapy alone. 5-Fluorouracil and Methotrexate significantly reduced the growth rate of RB1-silenced but not of control MCF-7 cells. This was likely due to the absence of a DNA damage checkpoint in RB1-silenced but not in control cells, 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 MTX and predicts a good clinical outcome for patients treated with adjuvant chemotherapy. We suggest that patients with RB-negative breast cancers should be treated by systemic chemotherapy.