| Summary: | 碩士 === 中國醫藥大學 === 癌症生物學研究所碩士班 === 100 === HER2/neu gene, overexpressed in approximately 30% of breast cancers, plays an important role in tumorigenesis of breast cancer, and is associated with increased disease recurrence and worse prognosis. Lapatinib, a dual EGFR/HER2 tyrosine kinase inhibitor, received FDA approval for advanced or metastatic breast cancers in combination with chemotherapy. However, the rapid recurrence and progression were frequently developed after several months of treatment with lapatinib, and the underlying molecular mechanisms of acquired resistance to lapatinib remain unclear. Activation of NF-kB transcription factor has been reported to confer chemoresistance and tumor progression through induction of anti-apoptotic and metastatic gene expressions. In this study, we observed that long-term treatment with lapatinib, but not other EGFR TKIs, specifically induces Ser536 phosphorylation and nuclear translocation of p65 through activation of Src/IKK signaling pathway in both HER2-positive and –negative breast cancers. IkBa as well as other target genes were induced by the lapatinib-activated NF-kB, but the newly synthesized IκBα did not negatively feedback to inhibit NF-kB due to phosphorylation at Y42 by Src. Our data further showed that lapatinib-treated breast cancer cells were more sensitive to NF-kB inhibition by p65 shRNA than their parental cells. Furthermore, targeting NF-kB activity by IKK inhibitors or proteasome inhibitors also can effectively suppress the viability of lapatinib-resistant HER2-positive breast cancer cells. These findings indicate that lapatinib treatment may render breast cancer cells addicted to oncogenic NF-kB activity, and targeting NF-kB may circumvent the acquired lapatinib resistance. The switch of oncogenic addiction to NF-kB by lapatinib was also found in triple-negative breast cancer (TNBC) cells, further suggesting the lapatinib-activated NF-kB as a potential and inducible Achilles'' heel of such breast cancer cells. Indeed, our data showed that treatment with lapatinib or targeting NF-kB by IKK inhibitors or proteasome inhibitors alone did not showed significant anti-tumor activity in TNBC cells. However, combination of lapatinib and proteasome inhibitors can obviously suppress the growth rate of TNBC cells both in vitro and in vivo. Taken together, our findings revealed that activation of NF-kB by lapatinib is mediated by Src/IKK signaling pathway but independent of HER2 inhibition and contributes to lapatinib resistance. Targeting NF-kB by proteasome inhibitors may be a promising therapeutic strategy for circumventing the lapatinib resistance in HER2-positive breast cancer cells as well as an effective treatment in combination with lapatinib for TNBC cells.
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