Trastuzumab-resistant breast cancer cells-derived tumor xenograft models exhibit distinct sensitivity to lapatinib treatment in vivo.

BACKGROUND: Resistance to HER2-targeted therapies, including the monoclonal antibody trastuzumab and tyrosine kinase inhibitor lapatinib, frequently occurs and currently represents a significant clinical challenge in the management of HER2-positive breast cancer. We previously showed that the trastuzumab-resistant SKBR3-pool2 and BT474-HR20 sublines were refractory to lapatinib in vitro as compared to the parental SKBR3 and BT474 cells, respectively. The in vivo efficacy of lapatinib against trastuzumab-resistant breast cancer remained unclear. RESULTS: In tumor xenograft models, both SKBR3-pool2- and BT474-HR20-derived tumors retained their resistance phenotype to trastuzumab; however, those tumors responded differently to the treatment with lapatinib. While lapatinib markedly suppressed growth of SKBR3-pool2-derived tumors, it slightly attenuated BT474-HR20 tumor growth. Immunohistochemistry analyses revealed that lapatinib neither affected the expression of HER3, nor altered the levels of phosphorylated HER3 and FOXO3a in vivo. Interestingly, lapatinib treatment significantly increased the levels of phosphorylated Akt and upregulated the expression of insulin receptor substrate-1 (IRS1) in the tumors-derived from BT474-HR20, but not SKBR3-pool2 cells. CONCLUSIONS: Our data indicated that SKBR3-pool2-derived tumors were highly sensitive to lapatinib treatment, whereas BT474-HR20 tumors exhibited resistance to lapatinib. It seemed that the inefficacy of lapatinib against BT474-HR20 tumors in vivo was attributed to lapatinib-induced upregulation of IRS1 and activation of Akt. Thus, the tumor xenograft models-derived from SKBR3-pool2 and BT474-HR20 cells serve as an excellent in vivo system to test the efficacy of other HER2-targeted therapies and novel agents to overcome trastuzumab resistance against HER2-positive breast cancer.

NSD1 mediates antagonism between SWI/SNF and polycomb complexes and is required for transcriptional activation upon EZH2 inhibition.

Disruption of antagonism between SWI/SNF chromatin remodelers and polycomb repressor complexes drives the formation of numerous cancer types. Recently, an inhibitor of the polycomb protein EZH2 was approved for the treatment of a sarcoma mutant in the SWI/SNF subunit SMARCB1, but resistance occurs. Here, we performed CRISPR screens in SMARCB1-mutant rhabdoid tumor cells to identify genetic contributors to SWI/SNF-polycomb antagonism and potential resistance mechanisms. We found that loss of the H3K36 methyltransferase NSD1 caused resistance to EZH2 inhibition. We show that NSD1 antagonizes polycomb via cooperation with SWI/SNF and identify co-occurrence of NSD1 inactivation in SWI/SNF-defective cancers, indicating in vivo relevance. We demonstrate that H3K36me2 itself has an essential role in the activation of polycomb target genes as inhibition of the H3K36me2 demethylase KDM2A restores the efficacy of EZH2 inhibition in SWI/SNF-deficient cells lacking NSD1. Together our data expand the mechanistic understanding of SWI/SNF and polycomb interplay and identify NSD1 as the key for coordinating this transcriptional control.