Molecular Mechanisms and Molecular Subtype-Specific Responses to Paclitaxel in Breast Cancer Cells

Paclitaxel (PTX), a taxane-derived chemotherapeutic agent, is frequently used in the treatment of breast cancer (BC). Its anticancer effects are primarily associated with microtubule stabilization, disruption of cell-cycle progression, and triggering of apoptotic cell death. In the present study, we investigated the effects of PTX on the expression of genes involved in cancer-related pathways, energy metabolism, and drug resistance in four molecularly distinct BC cell lines: MCF-7, BT-474, SK-BR-3, and MDA-MB-231. The half-maximal inhibitory concentrations (IC₅₀) of PTX in BC cell lines and the non-tumorigenic hTERT-HME1 breast epithelial cell line were determined by the MTT assay to assess cell cytotoxicity. BC cells were exposed to nine different concentrations of PTX for 24, 48, and 72 h to evaluate concentration- and time-dependent effects. Following treatment, total RNA was isolated and converted into cDNA, and RT-qPCR analysis was performed to investigate PTX-mediated alterations in the expression of genes associated with cancer-related pathways. The impact of PTX on the cell-cycle phase distribution and apoptotic cell death was evaluated by flow cytometry. Treatment with PTX for 48 h at concentrations of 12.60 nM in MCF-7, 5.09 nM in BT-474, 16.09 nM in SK-BR-3, and 36.66 nM in MDA-MB-231 cells reduced cell viability and increased apoptosis. PTX treatment also altered the expression of genes involved in apoptosis, cell-cycle regulation, angiogenesis, epithelial-mesenchymal transition, hypoxia-related signaling, energy metabolism, telomere maintenance, and therapy resistance. Collectively, these findings demonstrate that PTX elicits heterogeneous molecular and cellular responses across molecularly distinct BC cell lines, particularly in cell viability, apoptosis, metabolic regulation, and treatment response. These in vitro findings suggest potential molecular mechanisms that could explain why some cells are more sensitive to PTX than others, but further experimental and clinical validation is needed to confirm this.