We would like to thank Dr Y. invaded cells per high powered field (HPF, 40). Although MCF7 is typically known for being a non-invasive cell line, HER2 overexpression leading to STAT3 activation resulted in an increase of the invasiveness of the cell line. Open in a separate window Open in a separate window Figure 4. HER2 overexpressed cells display enhanced cell invasiveness have shown that the expression of Sox-2 and Oct-4 are important indicators for cancer progression to metastasis and drug resistance (24). This supports the notion that HER2/ER overexpression activates STAT3 which leads to an increase in cancer stem cell markers, causing overexpression of HER2 and cells become GSK 0660 resistant to chemotherapy. Upon treatment with Stattic, we observed a significant reduction in the stem cell marker expression. In addition, when we knocked down the STAT3 gene, CD44+ subpopulation was reduced, suggesting the pivotal role of STAT3 in the cancer stem cell transition in HER2 amplified environment. More importantly, we found that MCF7-HER2 cells that were treated with Stattic, GSK 0660 were more sensitive to Herceptin than MCF7 cells that were only treated with Herceptin or Stattic. This is a reflection of previous studies indicating that inhibited STAT3 has been correlated with increased apoptosis in cancer cells, increased chemosensitivity, suppressed tumor growth, reduced invasiveness, and decreased angiogenesis (25C27). Although STAT3 may play a vital role in early embryogenesis, its presence in the vast majority of adult cells is largely expendable, thus make it an attractive target for certain cancer therapies (28,29). Our study, as well as others, suggests that STAT3 plays a crucial role in metastasis and therapeutic resistance in solid tumors. For example, STAT3 has been considered a fundamental component of resistant tumor growth in breast cancer (12,13), head and neck squamous cell carcinoma (27) and lung cancer (30,31) due to induction of an invasive EMT-like phenotype. There have been numerous studies linking the EMT phenomenon to the expression of stem cell-like characteristics to the point where they seem to overlap (32C34). Thus, given that EMT is associated with lasting tumor aggressiveness, invasion, and angiogenesis, it is considered a prime suspect for driving GSK 0660 cancer stem-like cells. Most of the reports arguing for EMT and cancer stem cell correlation focus on the idea that an EMT phenotype drives a cancer stem cell microenvironment that is characterized as CD44hi/CD24low in breast cancer, which is associated with therapeutic resistance, tumor GSK 0660 invasion and poor prognosis (35,36). As we observed in the current study, HER2 overexpression leading to STAT3 activation resulted in upregulation of CD44 expression. Furthermore, a recent study by Oliveras-Ferraros concluded that a mesenchymal CD44+/CD24? microenvironment in HER2 overexpressed breast cancer was linked to resistance to Herceptin treatment (20). We conclude that HER2 overexpression in ER-positive breast cancer results in STAT3 activation, further causing stem cell-like characteristics and resistance to Herceptin. We have found a model for commonly incurred Herceptin resistance. After an extended period of time constitutively activated STAT3, from HER2 and ER expression, may induce more and more resistant stem cell-like characteristics. While we used the specific STAT3 activation inhibitor Stattic in our study, Rabbit polyclonal to ADI1 other STAT3 activation inhibitors and shRNA should be explored for further effectiveness with Herceptin treatment. Acknowledgments We.