The effect of Mg2+ upon 6-phosphofructokinase activity in Ehrlich ascites tumor cells in vivo

Cell (patho)physiology of magnesium

There is an unsettled debate about the role of magnesium as a ‘chronic regulator’ of biological functions, as opposed to the well-known role for calcium as an ‘acute regulator’. New and old findings appear to delineate an increasingly complex and important role for magnesium in many cellular functions. This review summarizes the available evidence for a link between the regulation of intracellular magnesium availability and the control of cell growth, energy metabolism and death, both in healthy and diseased conditions. A comprehensive view is precluded by technical difficulties in tracing magnesium within a multicompartment and dynamic environment like the cell; nevertheless, the last few years has witnessed encouraging progress towards a better characterization of magnesium transport and its storage or mobilization inside the cell. The latest findings pave the road towards a new and deeper appreciation of magnesium homoeostasis and its role in the regulation of essential cell functions.

Magnesium restriction induces granulocytic differentiation and expression of p27Kip1 in human leukemic HL-60 cells

When cultured in Mg restricted medium, human leukemic HL-60 cells develop morphological and functional granulocytic differentiation. In 0.03 mM Mg, cells display the distinctive features of differentiation, without appreciable inhibition of proliferation. In 0.01 mM Mg, cells show terminal differentiation, accompanied by clear inhibition of proliferation. Such cells accumulate in the G0/G1 phase and subsequently die via apoptosis, similar to HL-60 cells that have been induced to differentiate by DMSO. These phenotypic changes are associated with a marked increase in the expression level of the cyclin dependent kinase inhibitor p27Kip1. Cyclin E expression is also slightly increased in Mg restricted cells, whereas no changes are observed in the expression level of cyclin D1. We also show that during differentiation cell total Mg decreases, whereas [Mg2+]i increases in both Mg-depleted and DMSO-treated cells. These data suggest that the maturation process is paralleled by a redistribution of intracellular Mg, leading to a shift from the bound to the free form. These changes could modulate the kinetics of Mg-dependent enzyme(s) that are involved in the control of the differentiation pathway. We propose that this model may represent an useful tool for the study of the mechanisms of cell differentiation and related events, such as aging and death.

Intracellular free-magnesium levels in vascular smooth muscle and striated muscle cells of the spontaneously hypertensive rat

In humans with essential hypertension and in spontaneously hypertensive rats (SHR), insulin resistance may be present even in lean individuals. As the basis for this abnormality is unknown, we have used a newly developed fluorophore to measure intracellular free-Mg2+ concentrations in cultured aortic vascular smooth muscle cells and striated muscle cells from SHR and normotensive Wistar Kyoto (WKY) rats. Intracellular free-Mg2+ levels were lower in both striated muscle cells (SHR, 0.423 +/- 0.077 mmol.L-1 v WKY, 0.559 +/- 0.068 mmol.L-1; P less than .001) and vascular smooth muscle cells (SHR, 0.406 +/- 0.067 mmol.L-1 v WKY, 0.625 +/- 0.077 mmol.L-1; P less than .001) from hypertensive animals. This widespread, intrinsic defect in the regulation of intracellular Mg2+ may explain the increased vascular resistance and reduced insulin sensitivity present in hypertension.

The insulin receptor--single function and dual effect

Net effects of insulin on glucose entry, metabolism and other cellular processes have been well documented over the past 30-40 years. Although it is known that insulin binds to a specific cell membrane receptor protein which undergoes autophosphorylation and tyrosine kinase activation, the individual reactions following receptor activation that cause the metabolic changes remain unknown. It is well documented that the isolated insulin receptor has a high degree of basal autophosphorylation capacity and externally directed tyrosine kinase. There is also evidence that some in vivo autophosphorylation can take place in the total absence of insulin. If receptor activity does exist in the absence of insulin, then receptor function needs to be reanalyzed. It will be proposed here that the insulin binding membrane protein functions mainly to inhibit glucose transport under low physiological levels of insulin. Evidence of basal receptor enzymatic activity in the absence of insulin supports this theory. Under metabolically sufficient conditions, enough insulin receptors are functionally active to interact with the glucose transport system in an inhibitory manner, providing membrane control of internal glucose metabolism. Insulin acts by aggregating this inhibitory system. If inhibitory insulin receptors are aggregated following insulin elevation, their inhibitory action is prevented and glucose transport increases. This increase in transport will be in direct proportion to the temporal inhibitory level of the receptor and to the area of the cell membrane cleared of their inhibitory effect. When insulin receptor protein is confined to small areas of the cell membrane through aggregation, any potential inhibitory function is negated and glucose entry increases dramatically. This is the classical insulin effect. Both of these concepts were suggested 37 years earlier. Randle & Smith (1957, Biochem. Biophys. Acta 25, 442; 1958, Biochem. J. 70, 490) proposed that the internal supply of energy rich compounds limited glucose entry and that the effect of insulin was to inhibit this process which was inhibiting glucose entry. The present report provides a mechanism for this.

The effect of magnesium on glycolysis of permeabilized Ehrlich ascites tumor cells

We have previously observed that extracellular Mg2+ influences the phosphofructokinase (PFK) activity of intact Ehrlich Ascites tumour cells (EATC). In this study we have investigated the mechanism by which Mg2+ modulates this key glycolytic enzyme in EATC made permeable to the cation by either digitonin or dextran sulphate. Results showed that when Mg2+ is freely permeable to the cytosol, the in vivo PFK activity, calculated as FDP/G6P ratio, is not increased as it is in intact cells. We also observed that in permeabilized cells Mg2+ determines the increase of glucose 6 phosphate (G6P), fructose 1,6 bisphosphate (FDP) and lactate production. We hypothesize that extracellular Mg2+ regulates PFK and glycolysis in these neoplastic cells not by entering the cytosol but by a specific interaction with the plasma membrane.