Autophagy Facilitates the Development of Breast Cancer Resistance to the Anti-HER2 Monoclonal Antibody Trastuzumab

Abstract

Autophagy has been emerging as a novel cytoprotective mechanism to increase tumor cell survival under conditions of metabolic stress and hypoxia as well as to escape chemotherapy-induced cell death. To elucidate whether autophagy might also protect cancer cells from the growth inhibitory effects of targeted therapies, we evaluated the autophagic status of preclinical breast cancer models exhibiting auto-acquired resistance to the anti-HER2 monoclonal antibody trastuzumab (Tzb). We first examined the basal autophagic levels in Tzb-naive SKBR3 cells and in two pools of Tzb-conditioned SKBR3 cells (TzbR), which optimally grow in the presence of Tzb doses as high as 200 µg/ml Tzb. Fluorescence microscopic analyses revealed that the number of punctate LC3 structures -a hallmark of autophagy- was drastically higher in Tzb-refractory cells than in Tzb-sensitive SKBR3 parental cells. Immunoblotting analyses confirmed that the lipidation product of the autophagic conversion of LC3 was accumulated to high levels in TzbR cells. High levels of the LC3 lipidated form in Tzb-refractory cells were accompanied by decreased p62/sequestosome-1 protein expression, a phenomenon characterizing the occurrence of increased autophagic flux. Moreover, increased autophagy was actively used to survive Tzb therapy as TzbR pools were exquisitely sensitive to chemical inhibitors of autophagosomal formation/function. Knockdown of LC3 expression via siRNA similarly resulted in reduced TzbR cell proliferation and supra-additively interacted with Tzb to re-sensitize TzbR cells. Sub-groups of Tzb-naive SKBR3 parental cells accumulated LC3 punctate structures and decreased p62 expression after treatment with high-dose Tzb, likely promoting their own resistance. This is the first report showing that HER2-overexpressing breast cancer cells chronically exposed to Tzb exhibit a bona fide up-regulation of the autophagic activity that efficiently works to protect breast cancer cells from the growth-inhibitory effects of Tzb. Therapeutic targeting autophagosome formation/function might represent a novel molecular avenue to reduce the emergence of Tzb resistance in HER2-dependent breast carcinomas.

Aurora kinase A inhibition-induced autophagy triggers drug resistance in breast cancer cells

“…These results indicated that autophagy caused by targeting AURKA contributed to breast cancer cells resistance to VX-680. Similar studies, consistent with these findings, show that activated autophagy can protect tumor cells from targeted therapies, such as trastuzumab in breast cancer, 44 the imatinib mesylate in Philadelphia chromosome-positive cells, 45 and proteasome inhibitors in prostate cancer. 46 Thus, our data, along with these reports, supported that autophagy might contribute to a drug-resistance mechanism.…”

The Prolyl Isomerase Pin1 Induces LC-3 Expression and Mediates Tamoxifen Resistance in Breast Cancer

“…The induction of autophagy is closely related to the cell survival triggered by ErbB2 gene-amplified human breast cancer cells in response to the anti-ErbB2 monoclonal antibody, trastuzumab (30). Knockdown of autophagy genes in combination with tamoxifen in tamoxifen-resistant ErbB2-overexpressing MCF7 cells, reduced cell viability with increased mitochondria-mediated apoptosis (29).…”

Autophagy protects breast cancer cells from epirubicin-induced apoptosis and facilitates epirubicin-resistance development

“…17,18 A series of studies reported that chemotherapy induced-autophagy promotes the survival of some kinds of cancer cell lines. [19][20][21][22][23] These results suggest that autophagy might be involved in the development of chemotherapy resistance.…”