Rosmarinic Acid Potentiates Cisplatin-Induced Antitumour Activity Through ROS-Associated Apoptotic Signalling in Two-...

Triple-negative breast cancer (TNBC) remains a highly aggressive malignancy with limited therapeutic options and frequent resistance to platinum-based chemotherapy. Rosmarinic acid (RA), a naturally occurring polyphenol, has attracted considerable interest as a potential chemosensitising agent. This study investigated the anticancer activity and the underlying mechanisms of RA combined with cisplatin (CDDP) in 4T1 breast cancer cells while assessing the cytotoxic responses of non-cancerous HaCaT keratinocytes as a preliminary indicator of differential treatment sensitivity. Cytotoxicity was assessed using the MTT assay, followed by calculation of the Combination Index (CI), Drug Reduction Index (DRI), and Selectivity Index (SI). The generation of intracellular reactive oxygen species (ROS) was evaluated by DCFH-DA fluorescence imaging, and the functional contribution of oxidative stress was examined using N-acetyl-L-cysteine (NAC) rescue experiments. Apoptosis was analysed by Annexin V/PI flow cytometry, NucBlue nuclear staining, and Calcein-AM/propidium iodide (PI) Live/Dead fluorescence imaging. Three-dimensional (3D) tumour spheroids were used to assess treatment-induced alterations in spheroid morphology, morphometric parameters, viability based on adenosine triphosphate (ATP), and Live/Dead staining. The expression of genes related to apoptosis was determined by RT-qPCR, and potential molecular mechanisms were explored using the construction of protein-protein interaction (PPI) networks together with Gene Ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment analyses. The combination of RA + CDDP exhibited strong synergistic cytotoxicity in 4T1 cells while demonstrating comparatively lower toxicity toward HaCaT keratinocytes. Combination treatment markedly increased intracellular ROS generation, whereas NAC significantly reduced ROS accumulation and partially restored cell viability, indicating that oxidative stress is a major but not exclusive mediator of cytotoxicity. Combined treatment significantly enhanced apoptotic cell death, increased chromatin condensation and membrane damage, upregulated the expression of Bax, Casp9, Cycs, and Trp53, and downregulated Bcl2, consistent with transcriptional regulation of intrinsic apoptotic signalling. In 3D tumour spheroids, the combination markedly reduced spheroid size, disrupted structural integrity, decreased ATP-based viability, and substantially increased tumour cell death compared to monotherapy. Bioinformatic analyses identified central genes related to apoptosis and cell survival and predicted significant enrichment of PI3K/Akt, p53, MAPK, and apoptosis signalling pathways. RA significantly potentiates the antitumor efficacy of CDDP through synergistic induction of ROS-associated apoptotic signalling while showing a more favourable cytotoxic response in 4T1 breast cancer cells than in non-cancerous HaCaT keratinocytes. The integrated findings from two-dimensional (2D) and 3D models, NAC rescue experiments, molecular analyses, and bioinformatics collectively support the potential of RA as a promising chemosensitising adjuvant for CDDP-based breast cancer therapy and warrant further validation in preclinical in vivo models.