Erica spiculifolia Extract Potentiates Cisplatin Cytotoxicity by Reactivating p53 and Caspase-3-Dependent Apoptosis in Colorectal Carcinoma

Resistance to apoptosis represents a major limitation of platinum-based chemotherapy in colorectal carcinoma, frequently arising from impaired p53 signaling and inefficient execution of programmed cell death. In this study, we investigated the anticancer activity of <i>Erica spiculifolia</i> extract (ESE) and its ability to synergistically enhance cisplatin cytotoxicity in HT-29 colorectal carcinoma cells. Cell viability was assessed using the MTT assay, followed by formal combination analysis based on the Chou-Talalay methodology. Combination experiments employed a non-constant ratio regimen in which a fixed ESE concentration (45 µg/mL) was combined with serial cisplatin dilutions (45.0-2.8 µg/mL) to define interaction behavior across multiple effect levels. Quantitative analysis revealed a strong superadditive effect, with Combination Index values well below 1 and markedly elevated Dose Reduction Indices for cisplatin, indicating substantial dose-sparing across effect levels. To elucidate the molecular basis of this synergism, apoptosis-related protein expression was profiled using a membrane-based immunoassay. Combined ESE and cisplatin treatment induced full-scale p53 reactivation, including restoration of phosphorylated p53 isoforms associated with DNA damage-dependent apoptotic signaling. Acridine orange/propidium iodide staining confirmed a pronounced increase in early and late apoptotic/necrotic cells following combination treatment. UHPLC-HRMS analysis identified kaempferol 3-O-glucoside (8830.19 ± 11.01 ng/mg dw) and myricitrin (3074 ± 3.12 ng/mg) as predominant flavonols, followed by naringenin 7-O-glucoside (5958.96 ± 9.98 ng/mg), while chlorogenic, cinnamic, quinic, and gallic acids were the main phenolic acids detected. These constituents may contribute to HT-29 cell sensitization to cisplatin.