Cytotoxic triterpene and steroidal glycosides from the seeds of Digitalis purpurea and the synergistic cytotoxicity of steroidal glycosides and etoposide in SBC-3 cells

Abstract

The phytochemical investigations of the seeds of Digitalis purpurea have revealed their richness in cardenolide and pregnane glycosides exhibiting potent cytotoxicity; further chemical examinations of the D. purpurea seeds have achieved the isolation of six triterpene glycosides (1–6), six spirostanol glycosides (7–12), and three furostanol glycosides (13–15), including seven previously unidentified compounds (1–3, 10–12, and 14). Here, the structures of 1–3, 10–12, and 14 were determined via extensive spectroscopic analyses, including two-dimensional (2D) NMR; hydrolysis, followed by chromatographic and spectroscopic analyses; and X-ray crystallographic analysis. The cytotoxic activities of the isolated compounds (1–15) against SBC-3 small cell lung carcinoma and TIG-3 normal human diploid fibroblast cells were evaluated. Triterpene glycoside 3 and spirostanol glycoside 9 exhibited considerable cytotoxicity with IC50 values of 1.0 and 1.7 µM, respectively; they induced apoptotic cell death, which was accompanied by the activation of caspase-3 in SBC-3 cells. Spirostanol glycoside 7 exhibited cytotoxicity toward the SBC-3 cells (IC50 1.3 μM). Additionally, 7 at 0.1 and 1.0 µM synergistically enhanced the cytotoxicity of etoposide against SBC-3 cells; compound 7 induced the release of DAMPs; the release of HMGB1, the secretion of ATP, and the exposure of CALR in the SBC-3 cells. Furthermore, the combination of 7 and etoposide resulted in increasing the extracellular release of DAMPs. These data indicated that 7, as well as its combination with etoposide, might potentially cause immunogenic cell death.