Defective translocation of PKCepsilon in EtOH-induced inhibition of Mg2+ accumulation in rat hepatocytes.

BACKGROUND: Rats chronically fed ethanol for 3 weeks presented a marked decreased in total hepatic Mg(2+) content and required approximately 12 days to restore Mg(2+) homeostasis upon ethanol withdrawal. This study was aimed at investigating the mechanisms responsible for the EtOH-induced delay. METHODS: Hepatocytes from rats fed ethanol for 3 weeks (Lieber-De Carli diet-chronic model), rats re-fed a control diet for varying periods of time following ethanol withdrawal, and age-matched control rats fed a liquid or a pellet diet were used. As acute models, hepatocytes from control animals or HepG2 cells were exposed to varying doses of ethanol in vitro for 8 minutes. RESULTS: Hepatocytes from ethanol-fed rats presented a marked inhibition of Mg(2+) accumulation and a defective translocation of PKCepsilon to the cell membrane. Upon ethanol withdrawal, 12 days were necessary for PKCepsilon translocation and Mg(2+) accumulation to return to normal levels. Exposure of control hepatocytes or HepG2 cells to a dose of ethanol as low as 0.01% for 8 minutes was already sufficient to inhibit Mg(2+) accumulation and PKCepsilon translocation for more than 60 minutes. Also in this model, recovery of Mg(2+) accumulation was associated with restoration of PKCepsilon translocation. The use of specific antisense in HepG2 cells confirmed the involvement of PKCepsilon in modulating Mg(2+) accumulation. CONCLUSIONS: Translocation of PKCepsilon isoform to the hepatocyte membrane is essential for Mg(2+) accumulation to occur. Both acute and chronic ethanol administrations inhibit Mg(2+) accumulation by specifically altering PKCepsilon translocation to the cell membrane.

Delayed restoration of Mg2+ content and transport in liver cells following ethanol withdrawal.

Liver cells from rats chronically fed a Lieber-De Carli diet for 3 wk presented a marked decreased in tissue Mg(2+) content and an inability to extrude Mg(2+) into the extracellular compartment upon stimulation with catecholamine, isoproterenol, or cell-permeant cAMP analogs. This defect in Mg(2+) extrusion was observed in both intact cells and purified liver plasma membrane vesicles. Inhibition of adrenergic or cAMP-mediated Mg(2+) extrusion was also observed in freshly isolated hepatocytes from control rats incubated acutely in vitro with varying doses of ethanol (EtOH) for 8 min. In this model, however, the defect in Mg(2+) extrusion was observed in intact cells but not in plasma membrane vesicles. In the chronic model, upon removal of EtOH from the diet hepatic Mg(2+) content and extrusion required approximately 10 days to return to normal level both in isolated cells and plasma membrane vesicles. In hepatocytes acutely treated with EtOH for 8 min, more than 60 min were necessary for Mg(2+) content and extrusion to recover and return to the level observed in EtOH-untreated cells. Taken together, these data suggest that in the acute model the defect in Mg(2+) extrusion is the result of a limited refilling of the cellular compartment(s) from which Mg(2+) is mobilized upon adrenergic stimulation rather than a mere defect in adrenergic cellular signaling. The chronic EtOH model, instead, presents a transient but selective defect of the Mg(2+) extrusion mechanisms in addition to the limited refilling of the cellular compartments.

Effect of thyroid hormone on Mg(2+) homeostasis and extrusion in cardiac cells.

The present study investigated the effect of alteration in thyroid hormone level on Mg(2+) homeostasis in cardiac ventricular myocytes. Hyperthyroid conditions increased cardiac myocytes total Mg(2+) content by ~14% as compared to cells from eu-thyroid animals. The excess Mg(2+) was localized predominantly within cytoplasm and mitochondria, and was mobilized into the extracellular compartment by addition of isoproterenol (ISO) or cAMP but not phenylephrine (PHE). Hypothyroid conditions, instead, decreased cardiac myocytes total Mg(2+) content by ~10% as compared to cells from eu-thyroid animals. Also in this case, cytoplasm and mitochondria were the two cellular pools predominantly affected. Under hypothyroid conditions, administration of ISO or cAMP resulted in a decreased Mg(2+) extrusion as compared to that observed in cardiac cells from eu-thyroid animals. Similar changes in cellular Mg(2+) content and transport were observed in cardiac ventricular myocytes isolated from hyper- and hypo-thyroid animals, as well as in cultures of H9C2 cells rendered hyper- or hypo-thyroid under in vitro conditions. Supplementation of thyroid hormone to hypothyroid animals restored Mg(2+) level and transport to levels comparable to those observed in eu-thyroid animals. Taken together, these results indicate that changes in thyroid hormone level have a major effect on Mg(2+) homeostasis and compartmentation in cardiac cells. The enlarged Mg(2+) mobilization via beta- but not alpha(1)-adrenergic receptor stimulation further suggests that beta- and alpha(1)-adrenergic receptors target selectively different Mg(2+) compartments within the cardiac myocyte. These results provide a new rationale to interpret changes in cardiac function under hyper- or hypo-thyroid conditions.

Involvement of ERK1/2 and p38 in Mg2+ accumulation in liver cells.

Activation of PKC signaling induces Mg(2+) accumulation in liver cells. To test the hypothesis that PKC induces Mg(2+) accumulation via MAPKs activation, hepatocytes were incubated in the presence of PD98059 and SB202190 as specific inhibitors of ERK1/2 and p38, respectively, and stimulated for Mg(2+) accumulation by addition of PMA or OAG. Accumulation of Mg(2+) within the cells was measured by atomic absorbance spectrophotometry in the acid extract of cell pellet. The presence of either inhibitor completely abolished Mg(2+) accumulation irrespective of the dose of agonist utilized while having no discernible effect on beta -adrenoceptor mediated Mg(2+) extrusion. A partial inhibition on alpha (1)-adrenoceptor mediated Mg(2+) extrusion was observed only in cells treated with PD98059. To confirm the inhibitory effect of PD98509 and SB202190, total and basolateral liver plasma membrane vesicles were purified in the presence of either MAPK inhibitor during the isolation procedure. Consistent with the data obtained in intact cells, liver plasma membrane vesicles purified in the presence of PD98509 or SB202190 lost the ability to accumulate Mg(2+)in exchange for intra-vesicular entrapped Na(+) while retaining the ability to extrude entrapped Mg(2+) in exchange for extra-vesicular Na(+). These data indicate that ERK1/2 and p38 are involved in mediating Mg(2+) accumulation in liver cells following activation of PKC signaling. The absence of a detectable effect of either inhibitor on beta -adrenoceptor induced, Na(+)-dependent Mg(2+) extrusion in intact cells and in purified plasma membrane vesicles further support the hypothesis that Mg(2+) extrusion and accumulation occur through distinct and differently regulated transport mechanisms.

Role of glucose in modulating Mg2+ homeostasis in liver cells from starved rats.

Alpha1- and beta-adrenoceptor stimulation elicits Mg2+ extrusion from liver cells in conjunction with hepatic glucose output (T. Fagan and A. Romani. Am J Physiol Gastrointest Liver Physiol 279: G943-G950, 2000.). To characterize the role of intrahepatic glucose on Mg2+ transport, male Sprague-Dawley rats were starved overnight before being anesthetized and used as organ donors. Perfused livers or collagenase-dispersed hepatocytes were stimulated by alpha1 (phenylephrine)- or beta (isoproterenol)-adrenergic agonists. Mg2+ extrusion was assessed by atomic absorbance spectrophotometry. In both experimental models, the administration of pharmacological doses of adrenergic agonists did not elicit Mg2+ extrusion. The determination of cellular Mg2+ indicated an approximately 9% decrease in total hepatic Mg2+ content in liver cells after overnight fasting, whereas the ATP level was unchanged. Hepatocytes from starved rats accumulated approximately four times more Mg2+ than liver cells from fed animals. This enlarged Mg2+ accumulation depended in part on extracellular glucose, since it was markedly reduced in the absence of extracellular glucose or in the presence of the glucose transport inhibitor phloretin. The residual Mg2+ accumulation observed in the absence of extracellular glucose was completely abolished by imipramine or removal of extracellular Na+. Taken together, these data indicate 1) that hepatic glucose mobilization is essential for Mg2+ extrusion by adrenergic agonist and 2) that starved hepatocytes accumulate Mg2+ via two distinct pathways, one of which is associated with glucose transport, whereas the second can be tentatively identified as an imipramine-inhibited Na+-dependent pathway.