An investigation into the apoptosis-inducing potential of different combinations of monoterpenoids and flavonoids on cancer cell lines

Abstract

For the past decades there has been considerable revival of interest in the use of natural products in chemotherapy. Although several phytochemical compounds have been investigated for their apoptosis-inducing potential, these have very often been studied separately. This project aimed at investigating the cytotoxic and apoptosis inducing potential of combinations of phytochemicals on a range of human cancer cell lines and the effects compared to those induced by single compounds. The cytotoxicity of different structural isomers and enantiomers was also studied. Since the activity of phytochemicals sometimes depends on their solubility in aqueous media and/or their interaction with phospholipid membranes, an additional objective of this project was to test the cytotoxic effect of liposome delivery systems. Liposomes were prepared from synthetic phospholipids via the freeze-thaw extrusion method and loaded with the most cytotoxic phytochemicals. A standard colorimetric assay (MTT) was used to compile growth-inhibition curves for different concentration and exposure times to the phytochemical, from which the median growth inhibition (GI50) values were determined. When tested individually, thymoquinone and (-)-a-pinene were found to be the most cytotoxic phytochemicals, especially on human promyelocytic (HL-60) and erythromyeloblastic (K562) leukaemia cell lines. Phytochemical combinations, such as those involving thymoquinone and flavonoids, showed synergistic effects on the prostate adenocarcinoma (PC-3) cancer cell line especially after 24 hour treatment. Other combinations involving S-(-)-limonene with either (-)-linalool or apigenin showed synergistic effects on K562, with increased exposure time. Results for the liposomal study showed that specific liposomal formulations such as liposomal-quercetin and liposomal-genistein resulted in increased cytotoxicity against K562 and PC-3 respectively, compared to other liposomal formulations such as liposomal (-)-a-pinene and liposomal-thymoquinone. Morphological studies showed that the mode of cell death induced by thymoquinone and (-)-a-pinene was by apoptosis, which was enhanced by increasing concentrations and exposure times.