Rosmarinic Acid Enhances Doxorubicin Activity in MDA-MB-231 Cells: Associations with Intracellular Oxidative Stress, Mitochondrial Dysfunction, and Apoptosis

Abstract

Background/Objectives: Rosmarinic acid (RA) is a naturally occurring polyphenolic compound with promising anticancer activity; however, its potential to enhance the therapeutic efficacy of doxorubicin (DOX) against triple-negative breast cancer (TNBC) has not been comprehensively characterised. This study investigated the pharmacological interaction and associated cellular responses of RA combined with DOX in MDA-MB-231 breast cancer cells, while including HaCaT human keratinocytes as a non-malignant, non-mammary reference model for comparative cytotoxicity assessment. Methods: Cell viability was assessed using the MTT assay, and pharmacological interactions were evaluated using Chou–Talalay combination index (CI) and dose reduction index (DRI) analyses. Intracellular oxidative activity was evaluated using DCFH-DA fluorescence analyses together with N-acetyl-L-cysteine (NAC) modulation experiments. Apoptosis, cell-cycle distribution, mitochondrial membrane potential (JC-1), Caspase-9 immunocytochemistry, nuclear morphology (NucBlue staining), apoptosis- and proliferation-related gene expression (RT-qPCR; BAX, BCL2, CASP3, CASP9, TP53, CDKN1A, MKI67, and PCNA), and bioinformatic pathway analyses were performed to characterise cellular and molecular responses associated with the combined treatment. Results: After 48 h, RA exhibited IC50 values of 188.4 ± 4.2 µM in MDA-MB-231 cells and 146.6 ± 6.8 µM in HaCaT cells, while DOX showed IC50 values of 1.2 ± 0.08 µM and 2.6 ± 0.15 µM, respectively. The Chou–Talalay analysis demonstrated synergistic interactions in MDA-MB-231 cells, with CI values of 0.86, 0.72, and 0.64 at the effect levels of IC25, IC50, and IC75, respectively; the model-derived DOX DRI value at Fa = 0.50 was 1.8. Combination treatment markedly increased intracellular DCFH-DA fluorescence, whereas NAC pretreatment attenuated this signal and partially restored cell viability, supporting a contributory role of intracellular oxidative stress in treatment-associated cytotoxicity. Increased intracellular oxidative activity coincided with mitochondrial membrane depolarisation, increased apoptotic cell death, accumulation of cells in the sub-G1 phase, enhanced Caspase-9 immunoreactivity, and pronounced apoptotic nuclear alterations. RT-qPCR analysis demonstrated significant upregulation of BAX, CASP3, CASP9, and TP53, together with downregulation of BCL2, MKI67, and PCNA, resulting in a marked reduction in the BCL2/BAX mRNA expression ratio. A modest but non-significant increase in CDKN1A expression was also observed. Bioinformatic analyses further identified predicted associations with mitochondrial apoptosis and p53-associated signalling pathways. Conclusions: The RA + DOX combination showed synergistic cytotoxic activity in MDA-MB-231 cells, accompanied by increased intracellular oxidative activity, mitochondrial membrane depolarisation, apoptosis, cell-cycle perturbation, and changes in proliferation-related gene expression. These findings indicate associated cellular responses but do not establish a ROS-dependent mitochondrial apoptotic mechanism. The findings provide an exploratory in vitro basis for further investigation of the RA + DOX combination in additional TNBC models, non-malignant mammary epithelial cells, and appropriate in vivo systems.