Il-6 And Il-8 Stimulation Of Breast Cancer Stem Cells Contributes To Trastuzumab Resistance Following Pten Deletion

Abstract Recent studies strongly implicated breast cancer stem cells (CSC) in therapeutic resistance and recurrence. Despite the benefits of HER2 targeted therapeutics including trastuzumab for the treatment of HER2 amplified breast cancer, one third of tumors do not respond and resistance develops in patient with chronic exposure. Although the mechanisms mediating HER2 resistance have not been completely elucidated, there is evidence that PTEN deletion plays a role. We have previously demonstrated that HER2 regulates the breast cancer stem cell population through regulation of Akt signaling resulting in Wnt activation. In order to investigate the mechanisms of trastuzumab resistance, we overexpressed HER2 and/or knocked down PTEN in luminal MCF7 and basal SUM159 breast cancer cells. In addition, we knocked down PTEN in two HER2 amplified breast cancer cell lines BT474 and SKBR3. In each case, PTEN knock down increased the CSC population as determined by the Aldefluor assay as well as markedly increasing the production of the cytokines IL-6 and IL-8. Cytokine production was partially mediated through Akt activation of NF-κB. Short term trastuzumab treatment decreased the CSC population in HER2 amplified breast cancer cell lines, an effect that was completely abrogated by PTEN knockdown. Long term trastuzumab treatment of HER2 amplified and PTEN deleted cell lines generated resistant populations that secreted significantly elevated levels of IL-6 and IL-8. In vivo, intrinsically or experimentally amplified HER2 cells with PTEN knockdown were highly tumorigenic and produced metastasis in multiple organs. We previously demonstrated that the Akt inhibitor perifosine is able to reduce CSCs in PTEN-deleted tumors. In order to examine the role of IL-6 and IL-8 in mediating trastuzumab resistance, we treated mice bearing HER2 overexpressing PTEN deleted xenografts with perifosine alone or in combination with an anti-IL-6 receptor and/or anti-IL-8 antibodies. Tumor growth was substantially suppressed by the addition of IL-6 and IL-8 targeting antibodies. These studies suggest that resistance to trastuzumab in PTEN deleted cells may be partially mediated by the activation of cytokine loops involving IL-6 and IL-8. The use of agents which block these cytokine loops may provide a novel approach to overcome trastuzumab resistance. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3337. doi:10.1158/1538-7445.AM2011-3337