Trastuzumab Induces Apoptosis and Cell Cycle Arrest in Triple-Negative Breast Cancer, Suggesting Repurposing Potential.

Background: Breast cancer remains the most common invasive cancer in women worldwide. Triple-negative breast cancer (TNBC) is an aggressive subtype with limited treatment options. Trastuzumab (Tz) is typically used to treat HER2-positive breast cancers, but its potential in TNBC is unclear.

Objectives: To investigate the effects of trastuzumab on cell viability, apoptosis, cell cycle progression, and gene expression in TNBC cell lines compared with HER2-positive and normal cell lines.

Design: This is an in vitro experimental pre-clinical study using cultured cancer cell lines.

Methods: MDA-MB-231 and 4T1 (TNBC), MCF-7 (HER2-positive), and HSF (normal) cell lines were treated with 20 μg/mL trastuzumab for 24 hours. Cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, apoptosis by flow cytometry, cell cycle progression by DNA content analysis, and gene expression by qPCR.

Results: Trastuzumab significantly reduced cell viability and induced apoptosis in TNBC cell lines, comparable to effects in HER2-positive MCF-7 cells. Cell cycle analysis revealed G2/M phase arrest in TNBC cells. Gene expression analysis showed upregulation of ERBB2, NOTCH1, EGFR, PIK3CA, and PTEN in MDA-MB-231 cells, while 4T1 cells exhibited downregulation of most genes except NOTCH1.

Conclusion: This study provides initial evidence for trastuzumab's potential therapeutic effects in TNBC, despite low HER2 expression. The observed cytotoxicity, apoptosis induction, and cell cycle modulation in TNBC cells warrant further investigation into trastuzumab's mechanisms of action in HER2-negative contexts and its potential repurposing for TNBC treatment.