Avicins, a novel plant-derived metabolite lowers energy metabolism in tumor cells by targeting the outer mitochondrial membrane

Abstract

Avicins are pro-apoptotic, anti-inflammatory molecules with antioxidant effects both in vitro and in vivo. Based on their ability to perturb mitochondrial functions and initiate apoptosis in tumor cells, we chose to study the bioenergetic effects of avicins on tumor cell mitochondria. Avicin-treated Jurkat cells, showed a decrease in the levels of cellular ATP as well as the rate of oxygen consumption. These effects on cellular metabolism appear to be a result of avicin’s actions on the outer mitochondrial membrane (OMM). We speculate that avicins might initially inhibit the exchange of metabolites across the OMM leading to its subsequent permeabilization to cytochrome c. This speculation is supported by biophysical studies using lipid bilayers, which suggest that upstream of these effects, avicins target and close the voltage dependent anion channel (VDAC). Closure of VDAC would lead to an overall lowering of the cell energy metabolism, subsequently pushing these cells towards the apoptotic pathway by permeabilization of the OMM and release of cyt-c. Avicins therefore not only represent a novel pharmacological tool for treatment of cancers, but also highlight the influence ancient plant metabolites could have on human health.

Introduction

Avicins, represent a new class of plant stress metabolites, which are pro-apoptotic (Haridas et al., 2001a, Mujoo et al., 2001), anti-inflammatory (Hanausek et al., 2001, Haridas et al., 2001b) and activate antioxidant stress defense (Haridas et al., 2004) in human cells. Our previous finding that avicins induce apoptosis in tumor cells by direct perturbation of the mitochondria (Haridas et al., 2001a), which in turn are the site of most of the cellular energy generation, prompted us to study the effects of avicins on cellular bioenergetics.

Mitochondria have been known to play a key role in cell survival and form one of the important targets for apoptosis inducing agents. Most mitochondria-reactive inducers of apoptosis cause release of cytochrome c by permeabilizing the OMM. The permeabilization of OMM could be direct or as a result of mitochondrial swelling and subsequent rupture. More recently other components of the OMM, such as the voltage dependant anion channel (VDAC) have been implicated in the intrinsic apoptotic pathway (Szabo and Zoratti, 1993, Crompton et al., 1998). VDAC is a channel-forming protein in the mitochondrial outer membrane responsible for metabolic flux (including nucleotide phosphates) through that membrane (Colombini, 1979). Although still quite controversial (Vander Heiden et al., 2000), closure of the VDAC and the subsequent decrease in metabolic flux can precede, and cause, the permeabilization of the OMM to relatively small proteins, leading to a metabolic slowdown and eventually apoptosis.

The findings reported in this manuscript suggest that avicins could be targeting mitochondria to initiate the apoptotic cascade in more than one way. In addition to the previously reported ability of avicins to permeabilize the OMM and release cyt-c (Lemeshko et al., 2006), we now demonstrate using biophysical studies, that avicins target and close the VDAC in lipid bilayers. These findings in conjunction with the observation that avicins reduce the rate of respiration and levels of ATP in Jurkat cells, we hypothesize that avicin treatment could result in a hypometabolic state which could subsequently culminate in the induction of apoptosis via OMM permeabilization. Therefore it appears that in addition to elimination of damaged cells (Haridas et al., 2001a), avicin-induced adaptations to stress include a reduction in the mitochondrial energy metabolism, which could be diverted towards maintenance of somatic homeostasis.