Combination Treatment Of Breast Cancer Cells With Sulindac Derivative K-80003 And MEK Inhibitor GDC-0973

Retinol X receptor-a(RXRa)is a unique member of the nuclear receptor superfamily.Numerous studies have shown that RXRa is implicated in the regulation of almost all important biological processes including cell proliferation,differentiation,and apoptosis.Altered expression and/or function of RXRa are associated with the development of various diseases and cancers,demonstrating the importance of targeting RXRa for treating human malignancies including breast cancer.Previously,our group discovered that the anti-inflammatory drug sulindac can inhibit the interaction of tRXRa,a protealytically-cleaved form of RXRa,with p85a,resulting in apoptosis of tumor cells and inhibition of tumor growth.Sulindac,however,has many side effects because of it can also inhibit the activity of COX-1 and COX-2.Fortunately,we were able to identify a sulindac derivative,K-80003,which shows improved tRXRa binding and lack of COX binding activity.K-80003 is a new promising anti-cancer drug candidate.In evaluating the biological effects of K-80003 and its underlying molecular mechanism of action,we surprisingly found that the compound could activate p-ERK in breast cancer cells.As ERK activation represents an important event for cancer cell survival and drug resistance,we hypothesized that inhibition of K-80003-activied p-ERK by MEK inhibitor could potentiate the anti-cancer effect of K-80003.We therefore evaluated the effect of GDC-0973,a MEK inhibitor,on sensitizing the effect of K-80003 in and MCF-7 breast cancer cells and MMTV-PyMT transgenic mammary mouse model through the analysis of Western Blot,flow cytometry,MTT assay,immunofluorescence staining,H&E staining and immunohistochemical experiment.Our data demonstrated that the K-80003 combination with GDC-0973 showed a strong synergistic effect on inhibiting the growth of breast cancer cells in vitro and in animals.We also showed that the sensitizing effect of GDC-0973 was due to its inhibition of K-80003-activated p-ERK.Together,our results reveal a potential resistant mechanism of breast cancer cells to K-80003 treatment and identify an important strategy to overcome the resistance of breast cancer cells to K-80003 by using MEK inhibitor.Our data thus provide a molecular basis for future development of improved K-80003 analogs for breast cancer therapy.