Saponins as cytotoxic agents: a review

The saponins: polar isoprenoids with important and diverse biological activities

Saponins are polar molecules that consist of a triterpene or steroid aglycone with one or more sugar chains. They are one of the most numerous and diverse groups of plant natural products. These molecules have important ecological and agronomic functions, contributing to pest and pathogen resistance and to food quality in crop plants. They also have a wide range of commercial applications in the food, cosmetics and pharmaceutical sectors. Although primarily found in plants, saponins are produced by certain other organisms, including starfish and sea cucumbers. The under explored biodiversity of this class of natural products is likely to prove to be a vital resource for discovery of high-value compounds. This review will focus on the biological activity of some of the best-studied examples of saponins, on the relationship between structure and function, and on prospects for synthesis of ‘‘designer’’ saponins.

Dietary spinach saponin-enriched lipophilic fraction inhibits platelet aggregation and blood coagulation

In this study, we investigated the effect of spinach saponin-enriched lipophilic fraction (SSEF) on collagen (10 μg/mL)-stimulated platelet aggregation in vivo. Dietary SSEF dose-dependently inhibited collagen-induced platelet aggregation by decreasing thromboxane A₂ production and intracellular Ca²⁺ agonist activity as an aggregation-inducing autacoidal molecule. In addition, SSEF significantly increased the formation of cyclic AMP and cyclic GMP, intracellular Ca²⁺ antagonists that are aggregation-inhibiting molecules in collagen-stimulated platelets. These results suggest that SSEF is a potent inhibitor of collagen-stimulated platelet aggregation in vivo. Prothrombin time and activated partial thromboplastin time, indicators of blood coagulation, were potently prolonged by dietary SSEF in vivo. These findings suggest that SSEF prolongs the interval time between the conversion of fibrinogen to fibrin. Dietary SSEF also inhibited 0.4 M sucrose-induced hemolysis. Accordingly, our data demonstrate that SSEF might be a useful tool for inhibiting platelet activation and blood coagulation in thrombotic diseases.

Unraveling the response of plant cells to cytotoxic saponins: role of metallothionein and nitric oxide

A wide range of pharmacological properties are ascribed to natural saponins, in addition to their biological activities against herbivores, plant soil-borne pathogens and pests. As for animal cells, the cytotoxicity and the chemopreventive role of saponins are mediated by a complex network of signal transduction pathways which include reactive oxygen species (ROS) and nitric oxide (NO). The involvement of other relevant components of the saponin-related signaling routes, such as the Tumor Necrosis Factor(TNF)-α, the interleukin(IL)-6 and the Nuclear Transcription Factor-kB (NF-κB), has been highlighted in animal cells. By contrast, information concerning the response of plant cells to saponins and the related signal transduction pathways is almost missing. To date, there are only a few common features which link plant and animal cells in their response to saponins, such as the early burst in ROS and NO production and the induction of metallothioneins (MTs), small cysteine-rich, metal-binding proteins. This aspect is discussed in the present paper in view of the recent hypothesis that MTs and NO are part of a novel signal transduction pathway participating in the cell response to oxidative stress.