Mechanisms of the resistance to the mitochondrial permeability transition in tumour cells

Magnesium and its transporters in cancer: a novel paradigm in tumour development

The relationship between magnesium and cancer is not as simple as could be assumed from the well-established requirement of magnesium for cell proliferation. Basic and pre-clinical studies indicate that magnesium deficiency can have both anti- and pro-tumour effects. In the present review, we briefly outline the new findings on the role of magnesium in angiogenesis and metastatization, and focus on the relationship between tumour cell proliferation and metabolic reprogramming, discussing how magnesium and its transporters are involved in these processes. The role of magnesium in cancer is also critically examined with regard to mitochondrial function, apoptosis and resistance to treatment. Finally, we bring together the latest experimental evidence indicating that alteration in the expression and/or activity of magnesium channels is a frequent finding in cancer cells and human tumour tissues examined to date, and we discuss the potential implications for developing novel diagnostic and therapeutic strategies.

Effects of polyamines on mitochondrial Ca2+ transport

Mammalian mitochondria are able to enhance Ca(2+) accumulation in the presence of polyamines by activating the saturable systems of Ca(2+) inward transport and buffering extramitochondrial Ca(2+) concentrations to levels similar to those in the cytosol of resting cells. This effect renders them responsive to regulate free Ca(2+) concentrations in the physioloical range. The mechanism involved is due to a rise in the affinity of the Ca(2+) transport system, induced by polyamines, most probably exhibiting allosteric behaviour. The regulatory site of this mechanism is the so-called S(1) binding site of polyamines, which operates in physiological conditions and is located in the energy well between the two peaks present in the energy profile of mitochondrial spermine transport. Spermine is bidirectionally transported across teh inner membrane by cycling, in which influx and efflux are driven by electrical and pH gradients, respectively. Most probably, polyamine affects the Ca(2+) transport system when it acts from the outside-that is, in the direction of its uniporter channel, in order to reach the S(1) site. Important physiological functions are related to activation of Ca(2+) transport systems by polyamines and their interactions with the S(1) site. These functions include a rise in the metabolic rate for energy supply and modulation of mitochondrial permeability transition induction, with consequent effects on the triggering of the apoptotic pathway.

Glycyrrhetinic acid-induced permeability transition in rat liver mitochondria

Glycyrrhetinic acid, a hydrolysis product of one of the main constituents of licorice, the triterpene glycoside of glycyrrhizic acid, when added to rat liver mitochondria at micromolar concentrations induces swelling, loss of membrane potential, pyridine nucleotide oxidation, and release of cytochrome c and apoptosis inducing factor. These changes are Ca(2+) dependent and are prevented by cyclosporin A, bongkrekic acid, and N-ethylmaleimide. All these observations indicate that glycyrrhetinic acid is a potent inducer of mitochondrial permeability transition and can trigger the pro-apoptotic pathway.

Aroclor 1254 inhibits the mitochondrial permeability transition and release of cytochrome c: a possible mechanism for its in vivo toxicity

The mitochondrial permeability transition (MPT) occurs in several forms of necrotic cell death induced by various insults, including oxidative stress, ischemia/reperfusion injury Ca(2+)-ionophore toxicity, and apoptosis. In fact, the release of an apoptogenic factor such as cytochrome c is often associated with the opening of the transition pore. The present study shows that Aroclor 1254, a mixture of polychlorinated biphenyls that was banned in the U.S. in 1977 but is still present in the environment, inhibits the MPT in a dose-dependent manner in a concentration range of 1 to 25 nmol/mg protein. The compound prevents key phenomena associated with the MPT, including colloid-osmotic swelling, the collapse of membrane potential, nonspecific bidirectional traffic of solutes through the transition pore, and the oxidation of pyridine nucleotides. In contrast, Aroclor 1254 does not inhibit uptake of Ca(2+) or P(i). The effects of Aroclor 1254 are evident both in sucrose-based media and in saline and are observed when the compound is added before the opening of the pore. Aroclor 1254 prevents MPT induction provoked by a variety of agents, including phosphate, menadione, tert-butylhydroperoxide, and atractyloside. Aroclor 1254 also inhibits the specific release of cytochrome c, a correlate of MPT induction. These effects reveal a possible toxicological mechanism of action of this compound. The possibility that its effect on mitochondrial function is linked to its action as a tumor promoter is discussed.

Magnesium. An update on physiological, clinical and analytical aspects

There is an increased interest in the role of magnesium ions in clinical medicine, nutrition and physiology. The characteristics of the binding of magnesium and calcium ions to various components, macromolecules and biological membranes are described. Magnesium affects many cellular functions, including transport of potassium and calcium ions, and modulates signal transduction, energy metabolism and cell proliferation. The mechanism of cellular uptake and efflux of magnesium, its intracellular transport, intestinal absorption, renal excretion and the effect of hormones on these are reviewed. Magnesium deficiency is not uncommon among the general population: its intake has decreased over the years especially in the western world. The magnesium supplementation or intravenous infusion may be beneficial in various diseased states. Of special interest is the magnesium status in alcoholism, eclampsia, hypertension, atherosclerosis, cardiac diseases, diabetes, and asthma. The development of instrumentation for the assay of ionized magnesium is reviewed, as are the analytical procedures for total magnesium in blood and free magnesium in the cytosol. The improved procedures for the assay of different magnesium states are useful in understanding the role of magnesium in health and disease.