To understand the mechanisms leading to trastuzumab resistance in HER2-overexpressing breast tumors we created trastuzumab insensitive cell lines (SKBR3/100-8 and BT474/100-2). The cell lines maintain HER2 receptor overexpression, and show increase in EGFR. Upon trastuzumab treatment, SKBR3/100-8 and BT474/100-2 cell lines displayed increased growth rate and invasiveness. The trastuzumab resistance in SKBR3/100-8 and BT474/100-2 was accompanied with activation of the Wnt/β-catenin signaling pathway. Further investigation found that Wnt3 overexpression played a key role toward the development of trastuzumab resistance. The expression of Wnt3 in trastuzumab resistant cells increased nuclear expression of β-catenin and transactivated expression of EGFR. The increased Wnt3 in the trastuzumab resistant cells also promoted a parental EMT-like transition (epithelial to mesenchymal transition), increased N-cadherin, Twist, SLUG and decreased E-cadherin. Knockdown of Wnt3 by siRNA restored cytoplasmic expression of β-catenin, and decreased EGFR expression in trastuzumab resistant cells. Furthermore the EMT markers were decreased, E-cadherin was increased and the cell invasiveness was inhibited in response to the Wnt3 down-regulation. Conversely, SKBR3 cells which had been stably transfected with full-length Wnt3 exhibited EMT-like transition. The Wnt3 transfectants, SKBR3/Wnt3-7 and SKBR3/Wnt3-9, showed a significant decrease in E-cadherin and increase in N-cadherin, Twist and SLUG. The cells were less sensitive to trastuzumab compared to parental SKBR3 and vector transfected cells. In summary, our data suggests that Wnt3 overexpression activates Wnt/β-catenin signaling pathway that leads to transactivation of EGFR and promotes EMT-like transition. This could be an important mechanism leading to trastuzumab resistance in HER2 overexpressing breast cancer cells.

Clinically, HER2 proto-oncogene amplification is found in about 25–30% of human breast cancers, where it is correlated to a poor prognosis. Constitutive STAT3 activation is found in about 50–60% of the breast tumors and associated with tumorigenesis and drug resistance. In this study, we showed that STAT3 was phosphorylated in HER2-overexpressing, ER-positive human breast tumors and, furthermore, phosphorylated STAT3 promoted the stem-like cell phenotype. We examined the dysregulation of the stem cell markers (Oct-4, Sox-2 and CD44) and the tumorsphere formation in HER2-overexpressing human breast cancer cell lines. We demonstrated that the STAT3 inhibitor, Stattic, treatment abolished the cancer stem cell phenotype in HER2-positive breast cancers. Combined treatment of Herceptin and Stattic showed the synergistic effect on the cancer cell growth in vitro. In addition, when the STAT3 gene was knocked down, the expression of the stem cell markers Oct-4, Sox-2 and CD44 were downregulated and tumorsphere formation was abolished. HER2-elicited STAT3 signaling may provide a potential model for drug resistance induced by stem-like cell characteristics. This mechanism may be responsible for acquiring resistance to Herceptin in the treatment of HER2-overexpressing breast tumors. Based on our findings, targeting pSTAT3 could overcome Herceptin-induced resistance in HER2-overexpressing breast tumors.

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