The acquisition of tamoxifen resistance is a major therapeutic problem in breast cancer. We developed a tamoxifen-resistant MCF-7 (TRM-7) cell line to elucidate the molecular mechanisms and factors associated with acquisition of such resistance. We showed that phosphorylation of STAT3 at tyrosine 705 (Y705) and RANTES expression are increased in response to tamoxifen in human breast cancer cells. On the basis of these results, we hypothesize that upregulated STAT3 phosphorylation and RANTES may be correlated with the development of drug resistance. Here, we showed that STAT3 and RANTES contribute to the maintenance of drug resistance. STAT3 phosphorylation is constitutively retained via a RANTES autocrine loop, which in turn upregulates antiapoptotic signals in TRM-7 cells. STAT3-RANTES autocrine signaling affected expression of anti-apoptotic BCL-2 family genes and prevented TRM-7 cells from undergoing programmed cell death by inhibiting PARP and caspase-9 cleavage. Subsequently, blockade of STAT3 and RANTES in TRM-7 cells resulted in reduction of antiapoptotic signals, which was rescued by exogenous RANTES treatment; drug resistance was also restored. Taken together, our results suggested that STAT3-RANTES autocrine signaling is essential for maintenance of drug resistance and inhibition of programmed cell death. These mechanisms of STAT3-RANTES autocrine signaling suggest a novel strategy for management of patients with tamoxifen-resistant tumors. Mol Cancer Res; 11(1); 31-42. Ó2012 AACR.
IntroductionBreast cancer is the second most common cancer worldwide after lung cancer, the fifth most common cause of cancer death, and the leading cause of cancer death in women. The global burden of breast cancer exceeds that of all other cancers, and its incidence is increasing (1). In women younger than 50 years with breast cancer, chemotherapy increases the 15-year survival rate by 10%,