Tumor Extrinsic Factors Mediate Primary T-DM1 Resistance in HER2-Positive Breast Cancer Cells

SIMPLE SUMMARY: In this investigation, we employed an unconventional approach to explore the mechanisms of the primary resistance of human epidermal growth factor 2 (HER2)-positive breast cancer cells to ado-trastuzumab emtansine (also known as T-DM1). Specifically, we used Matrigel matrix as a model of the tumor microenvironment and examined its effect on the sensitivity of HER2-positive breast cancer cells to T-DM1. We found that epidermal growth factor receptor (EGFR) is activated in HER2-positive, T-DM1-sensitive JIMT1 and SKBR-3 cells on the Matrigel matrix. This leads to phosphorylation and degradation of HER2 in these cells, resulting in the loss of or reduced sensitivity to T-DM1. The discovery of extrinsic factors contributing to the primary resistance of HER2-positive breast cancer cells to T-DM1 provides an opportunity to develop a novel therapeutic strategy to overcome T-DM1 resistance. ABSTRACT: To explore if the tumor microenvironment contributes to the primary resistance of HER2-positive breast cancer cells to T-DM1, we examined whether Matrigel, a basement membrane matrix that provides a three-dimensional (3D) cell culture condition, caused the primary resistance of HER2-positive, T-DM1-sensitive breast cancer cells (JIMT1 and SKBR-3 cells) to T-DM1. This is different from the conventional approach such that the cells are exposed with escalated doses of drug to establish a drug-resistant cell line. We found that these cells were able to grow and form spheroids on the Matrigel in the presence of T-DM1. We further explored the molecular mechanisms that enables these cells to be primarily resistant to T-DM1 and found that EGFR was activated in the spheroids, leading to an increased HER2 tyrosine phosphorylation. This in turn enhances cell growth signaling downstream of EGFR/HER2 in the spheroids. HER2 tyrosine phosphorylation promotes receptor internalization and degradation in the spheroids, which limits T-DM1 access to HER2 on the cell surface of spheroids. Blocking EGFR activity by erlotinib reduces HER2 tyrosine phosphorylation and enhances HER2 cell surface expression. This enables T-DM1 to gain access to HER2 on the cell surface, resumes cell sensitivity to T-DM1, and exhibits synergistic activity with T-DM1 to inhibit the formation of spheroids on Matrigel. The discovery described in this manuscript reveals a novel approach to investigate the primary resistance of HER2-positive breast cancer cells and provides an opportunity to develop a therapeutic strategy to overcome primary resistance to T-DM1 by combing T-DM1 therapy with kinase inhibitors of EGFR.