Search

Magnesium is involved in a wide range of biochemical reactions that are crucial to cell proliferation, differentiation, angiogenesis, and apoptosis. Changes in magnesium availability have been shown to influence biological responses of immuno-inflammatory cells. Equally plausible seems to be an involvement of magnesium in the multistep and interconnected processes that lead to tumor formation and development; however, the "how" and "when" of such an involvement remain to be defined. Here, we reviewed in vitro and in vivo data that indicated a role for magnesium in many biological and clinical aspects of cancer (from neoplastic transformation to tumor growth and progression or pharmacologic treatment). In adopting this approach we went through a full circle from molecular aspects to observational or epidemiological studies that could reconcile in a unifying picture the otherwise fragmentary or puzzling data currently available on the role of magnesium in cancer.

Magnesium (Mg) is required for cellular proliferation; however, the differences in subcellular regulation of Mg between proliferating and differentiated cells has not been determined. We used electron probe microanalysis (EPMA) to investigate the subcellular distribution of Mg in HL60 cells (a promyelocytic leukemia cell line) before and after retinoic acid (RA)-induced differentiation. Most intracellular Mg is bound to ATP and the Mg-ATP complex regulates several metabolic enzymes. We also compared alterations in Mg content following differentiation with the changes in ATP and ADP levels. Using atomic absorption spectrophotometry, we observed a significant decrease (-20%) in cellular Mg content in RA-differentiated HL60 cells. To investigate which intracellular compartments were involved in these changes, we analyzed subcellular elemental composition in freeze-dried cryosections of rapidly frozen undifferentiated and differentiated HL60 cells by EPMA. Following differentiation of HL60 cells, we observed an 18% decrease in Mg content in both the cytoplasm (regions of the cell excluding mitochondria and nuclei) and mitochondria. There was also a significant (40%) decrease in cytoplasmic Ca content after RA-induced differentiation. Nuclear Mg concentration was not significantly different between differentiated and undifferentiated HL60 cells, although differentiation was accompanied by a 30% decrease in the nuclear K/Na ratio. After differentiation, cellular ATP and ADP content decreased by 31 and 40%, respectively. We conclude that during exit from the cell cycle, Mg redistributes within cells and that the decrease in cytoplasmic and mitochondrial Mg is accompanied by a decrease in ATP and ADP content., (Copyright 1998 Academic Press.)

Rat spleen lymphocytes (RSL) incubated at 37 degrees C in Mg-free medium (O-trans conditions) exibited Mg2+ efflux with apparent velocity of 0.2 nmol/mg protein/min. After 30 min, this process accounted for the mobilization of about 15% of cell total Mg2+. Half of the Mg2+ efflux depended on extracellular Na+ and was stimulated by cAMP. IFN-alpha significantly enhanced Mg2+ efflux under O-trans conditions as well as in the presence of physiological extracellular Mg2+. Pretreatment of RSL with indomethacin completely abolished IFN-alpha-induced Mg2+ efflux, suggesting a crucial role for cyclooxygenase-dependent arachidonate metabolism. On the other hand, pretreatment of RSL with the PKA inhibitor (Rp)8-Br-cAMPS prevented IFN-alpha stimulation of Mg2+ efflux, indicating the involvement of cAMP. Consistently, both IFN-alpha and exogenous PGE1 increased cAMP from 50 to 125 pmol/mg protein. Altogether these results show that IFN-alpha stimulates Mg2+ efflux by activating arachidonate metabolism and synthesis of prostaglandins. By influencing adenylcyclase activity, PGEs can eventually promote cAMP-dependent Mg2+ efflux, possibly through the activity of a Na-Mg antiport. In RSL, therefore, magnesium movements can be under the control of IFN-alpha and, perhaps, of other cytokines, suggesting the involvement of Mg2+ in cell response to receptor-mediated stimuli.

Extracellular ATP causes 40% stimulation of Mg2+ efflux from Ehrlich ascites tumor cells (EATC) incubated under 0-trans conditions. ATP also causes a threefold increase of arachidonic acid (AA) metabolite release from [3H]AA-preloaded EATC, indicating that, under these experimental conditions, it induces phospholipase A2 (PLA2) activation. ATP-induced Mg2+ efflux can be prevented by cyclooxygenase inhibition with indomethacin or lysine acetylsalicylate, but not by 5-lipooxygenase inhibition with BWA4C. Mg2+ efflux is also directly stimulated by exogenous AA in a concentration-dependent manner. This phenomenon involves PKA as it is virtually abolished by the specific inhibitor 8-bromoadenosine-3',5'-cyclic monophosphothioate. While stimulating Mg2+ efflux, exogenous AA also increases cAMP content of EATC and this effect can be prevented by cyclooxygenase inhibition. Measurements in mag-fura-2-loaded EATC reveal that stimulation of Mg2+ efflux does not correlate with significant fluctuations of [Mg2+]i. This suggests that Mg2+ efflux is compensated for by mobilization of bound Mg2+, leaving [Mg2+]i unaltered. Altogether these data indicate that Mg2+ efflux can be modulated by extracellular stimuli capable of activating PLA2. This modulation is triggered by cyclooxygenase products which, by activating adenylcyclase, determine an elevation of cAMP. This intracellular messenger upregulates Na-dependent Mg2+ efflux.

Magnesium efflux from intact Ehrlich ascites tumor cells (EATC) has been characterized under 0-trans conditions. It is shown that a magnesium-extruding mechanism operates in these cells which brings about magnesium efflux up to 20% of the cell total content. EATC magnesium efflux is independent from extracellular Ca2+ and its apparent velocity is not affected by [Mg2+]o up to 160 microM. This extrusion, however, is strictly dependent on extracellular Na+ and it is inhibited by ouabain (1 mM), amiloride (1 mM), imipramine (0.5 mM) and quinidine (1 mM). It is not affected by HMA (5-N,N-hexamethylene) amiloride) (0.5 microM) and vanadate (0.1 mM). Bumetanide (0.5 mM) enhances magnesium extrusion in these cells. EATC magnesium extrusion can be significantly stimulated by db cAMP, forskolin, and IBX (3-isobutyl-1-methyl-xanthine). The intracellular magnesium distribution, studied also by means of the ionophore A23187, is regulated by energy metabolism and cell ATP content. Altogether our data demonstrate that EATC exhibit a magnesium extrusion sustained by Na+ gradient across the plasma membrane and carried out by a Na+/Mg2+ antiport. In these cells magnesium homoeostasis results, regulated efficiently by cell ATP and cAMP content.

A fraction of enriched plasma membranes from bovine parathyroid cells has been prepared by differential centrifugation. Biochemical characterization shows that this fraction has a specific activity enrichment of 7.2-fold in ouabain-sensitive Na+-K+ ATPase, and 3.5-fold in 5'-nucleotidase. Less than 4% of the total mitochondria and lysosomes are present within the plasma membranes, while microsomal contamination accounts for 14% of total specific activity. Parathyroid hormone radioimmunoassay also reveals the presence of some secretory granules within the plasma membrane fraction. The characteristic morphological aspect of the unusual surface membrane is shown by freeze-fracture electron microscopy. In the enriched pellets, vesicles identified as having a plasma membrane origin have variable sizes, and 50% show an inside-out conformation. Even though the plasma membrane fraction described herein is not absolutely free from contamination by other subcellular components, this protocol represents the first attempt to purify surface membrane from parathyroid tissue and provide the starting material for understanding, at a molecular level, the properties of extracellular Ca2+ regulation and its coupling with secretion of parathyroid hormone.

The effect of Mg2+ addition to intact Ehrlich ascites tumor cells (EATC) has been investigated. A decrease of glucose 6-phosphate (G6P) content and an increase of fructose 1,6-diphosphate (FDP) content are detected in glucose utilizing EATC incubated with increasing Mg2+ concentrations (from 0 to 5.0 mM). The strong enhancement of FDP/G6P ratio is taken as evidence for in vivo stimulation of phosphofructokinase 1 (PFK) (ATP:D-fructose-6-phosphate 1-phosphotransferase; EC 2.7.1.11). A similar effect can be observed when glucose is replaced by fructose as the glycolytic substrate. Stimulation of PFK is paralleled by substantial depletion of ATP. Cytochalasin B prevents the observed phenomena. Cell total Mg increases by about 15% when EATC are incubated with 5 mM Mg2+. The overall data show that extracellular Mg2+ may modulate glycolytic flux in EATC in vivo. Implications and significance of these phenomena in the regulation of cancer cell metabolic features are discussed.