Stimulatory effect of glucose 6-phosphate on the non-mitochondrial Ca2+ uptake in permeabilized hepatocytes and Ca2+ release by inositol trisphosphate

Acute effect of ethanol on hepatic reticular G6Pase and Ca2+ pool

BACKGROUND

Hydrolysis of glucose 6-phosphate (G6P) via glucose 6-phosphatase (G6Pase) enlarges the reticular Ca(2+) pool of the hepatocyte. Exposure of liver cells to ethanol (EtOH) impairs reticular Ca(2+) homeostasis. The present study investigated the effect of acute EtOH administration on G6P-supported Ca(2+) accumulation in liver cells.

METHODS

Total microsomes were isolated from rat livers acutely perfused with varying doses of EtOH (0.01, 0.1, or 1% v/v) for 8 minutes. Calcium uptake was assessed by (45) Ca redistribution. Inorganic phosphate (Pi) formation was measured as an indicator of G6Pase hydrolytic activity.

RESULTS

G6P-supported Ca(2+) uptake decreased in a manner directly proportional to the dose of EtOH infused in the liver, whereas Ca(2+) uptake via SERCA pumps was decreased by ~25% only at the highest dose of alcohol administered. The reduced accumulation of Ca(2+) within the microsomes resulted in a smaller inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release. Kinetic assessment of IP(3) and passive Ca(2+) release indicated a faster mobilization in microsomes from EtOH-treated livers, suggesting alcohol-induced alteration of Ca(2+) releasing mechanisms. Pretreatment of livers with chloromethiazole (CMZ) or dithiothreitol (DTT), but not 4-methyl-pyrazole prevented the inhibitory effect of EtOH on G6Pase activity and Ca(2+) homeostasis.

CONCLUSIONS

Liver G6Pase activity and IP(3) -mediated Ca(2+) release are rapidly inhibited following acute (8 minutes) exposure to EtOH, thus compromising the ability of the endoplasmic reticulum to dynamically modulate Ca(2+) homeostasis in the hepatocyte. The protective effect of CMZ and DTT suggests that the inhibitory effect of EtOH is mediated through its metabolism via reticular cyP4502E1 and consequent free radicals formation.

Biphasic effect of extra-reticular Mg2+ on hepatic G6P transport and hydrolysis

Magnesium ions (Mg(2+)) play a key role in regulating hepatic cellular functions and enzymatic activities. In the present study, we report a concentration-dependent effect of cytosolic Mg(2+) on G6P and pyrophosphate (PPi) transport and hydrolysis in digitonin-permeabilized rat hepatocytes. The stimulatory effect of Mg(2+) on G6P is specific but biphasic, with a maximal effect at a concentration of 0.25 mM, whereas the effect on PPi increases in a dose-dependent manner. Both effects can be abolished by addition of EDTA to the system. Addition of taurocholate, histone-2A, alamethicin or A23187 to the incubation system results in a marked decrease in the Mg(2+) concentration present within the endoplasmic reticulum lumen. Under these conditions, the stimulatory effect of extra-reticular Mg(2+) on G6P transport and hydrolysis is abolished. Taken together, these data suggest that cytosolic Mg(2+) stimulates G6P transport by acting at the level of the substrate binding site of the G6Pase enzymatic complex or the surrounding phospholipid environment. The effect, which is lost when G6P has readily access to the ER lumen, requires physiological endoplasmic reticulum Mg(2+) content.

Glucose-6-phosphatase in the insulin secreting cell line INS-1

The glucose-6-phosphatase system of the glucose sensitive insulin secreting rat insulinoma cells (INS-1) was investigated. INS-1 cells contain easily detectable levels of glucose-6-phosphatase enzyme protein (assessed by Western blotting) and have a very significant enzymatic activity. The features of the enzyme (Km and Vmax values, sensitivity to acidic pH, partial latency, and double immunoreactive band) are similar to those of the hepatic form. On the other hand, hardly detectable levels of glucose-6-phosphatase activity and protein were present in the parent glucose insensitive RINm5F cell line. The mRNA of the glucose-6-phosphate transporter was also more abundant in the INS-1 cells. The results support the view that the glucose-6-phosphatase system of the beta-cell is associated with the regulation of insulin secretion.

Beta-glucuronidase latency in isolated murine hepatocytes

The physiological function of microsomal beta-glucuronidase is unclear. Substrates may be either glucuronides produced in the lumen of endoplasmic reticulum (ER) or those taken up by hepatocytes. In the latter case, efficient inward transport of glucuronides at the plasma membrane and the ER membrane would be required. Therefore, the potential role of beta-glucuronidase in ER was investigated. Isolated mouse hepatocytes and mouse and rat liver microsomal vesicles were used in the experiments. Selective permeabilization of the plasma membrane of isolated hepatocytes with saponin or digitonin resulted in an almost 4-fold elevation in the rate of beta-nitrophenol glucuronide hydrolysis, while the permeabilization of plasma membrane plus ER membrane by Triton X-100 caused a further 2-fold elevation. In microsomal vesicles, the p-nitrophenol glucuronide or phenolphthalein glucuronide beta-glucuronidase activity showed about 50% latency as revealed by alamethicin or Triton X-100 treatment. A light-scattering study indicated that the microsomes are relatively impermeable to both glucuronides and to glucuronate. On the basis of our results, the role of liver microsomal beta-glucuronidase in the deconjugation of glucuronides taken up by the liver seems unlikely. Hydrolysis of the glucuronides produced in the ER lumen may play a role in substrate supply for ascorbate synthesis or in "proofreading" of glucuronidation.

A convenient diagnostic function test of peripheral blood neutrophils in glycogen storage disease type Ib

Neutrophils from patients suffering from glycogen storage disease type Ib (GSD-Ib) show several defects. one of which is a decreased rate of glucose utilization. In this study, we established experimental conditions to show the stimulation of the neutrophil respiratory burst by extracellular glucose. With phorbol-myristate-acetate as stimulus of the burst, the activity of the NADPH oxidase in GSD-Ib neutrophils hardly increased on addition of glucose. In control and GSD-type Ia neutrophils, a clear increase was observed. The lack of response to extracellular glucose in GSD-Ib neutrophils is correlated with the inability to raise intracellular glucose-6-P levels on glucose addition, thereby limiting the activity of the generation of NADPH in the hexose-monophosphate shunt. Our study shows the usefulness of this test for the diagnosis of neutrophil function abnormality in GSD-Ib patients.

Glycolytic metabolites and intracellular signaling in the pancreatic beta cell

In the pathways modulating the secretion of insulin and other physiologically important molecules, the critical role played by calcium in the moment-to-moment regulation of secretory processes may be modulated by additional factors, and these factors may include the glycolytic metabolites. We studied these early glucose breakdown products for effects on calcium release and inositol 1,4, 5-trisphosphate (IP3) binding to the IP3 receptor in a pancreatic beta cell preparation. The physiological significance of the response was also examined in terms of the insulinotropic effects of these metabolites. In studies of calcium release from the pancreatic beta cell, the metabolite 2,3-bisphosphoglycerate (DPG) exerted a statistically significant stimulatory effect on calcium release. A lesser but nonetheless significant effect also occurred in the presence of 3-phosphoglycerate and glucose-6-phosphate. The DPG-induced effect was concentration dependent. It is likely that the effects of DPG and other glycolytic metabolites on pancreatic beta cell signaling are physiologically significant inasmuch as we were also able to demonstrate that DPG and other glycolytic metabolites promoted the release of insulin from the pancreatic beta cell.

Regulation of intracellular calcium is closely linked to glucose metabolism in J774 macrophages

The effects of 2-deoxy-D-glucose (2dGlc) and glucose deprivation were investigated in the J774 murine macrophage-like cell line. 2dGlc addition or glucose deprivation for 4 min led to an inhibition in the transient increase in cytoplasmic free Ca2+ ([Ca2+]i) that otherwise occurs in response to three different agonists: IgG, ATP and platelet activating factor. This inhibition was preceded by a partial release of Ca2+ from intracellular, thapsigargin-sensitive stores. In contrast, the transition from 5 to 30 mM glucose caused a decrease in [Ca2+]i and a corresponding increase in thapsigargin-sensitive sequestered Ca2+. The effects of an alternate glycolytic inhibitor, NaF, and a mitochondrial inhibitor, rotenone, were also tested. These inhibitors caused neither a release of Ca2+ from intracellular stores nor an inhibition in any of the agonist responses. The capacitative influx of extracellular Ca2+ following depletion of intracellular stores was also found to be selectively inhibited by the prior addition of 2dGlc or with glucose deprivation. In addition, when an elevated plateau of [Ca2+]i was established by the irreversible depletion of intracellular Ca2+ stores, the addition of 2dGlc caused a decrease in the on-going capacitative entry of Ca2+.

The inositol 1,4,5-trisphosphate receptor is localized on specialized sub-regions of the endoplasmic reticulum in rat liver

Inositol 1,4,5-trisphosphate (InsP3) is involved in the mobilization of Ca2+ from intracellular non-mitochondrial stores. In rat liver, it has been shown that the InsP3-binding site co-purifies with the plasma membrane. This suggests that in the liver the InsP3 receptor (InsP3R) associates with plasma membrane. We studied the subcellular distribution of the liver InsP3R by measuring the maximal binding capacity of [3H]InsP3 and using antibodies against the 14 C-terminal residues of the type 1 InsP3R. The antibodies recognized a large amount of an InsP3R protein of 260 kDa in a membrane fraction which is also enriched with [3H]InsP3-binding sites and with markers of the basal, the lateral and the bile-canalicular membrane and the plasma-membrane Ca2+ pump (PMCA). The fractions enriched in markers of the endoplasmic reticulum (ER) and the Ca2+ pump of the ER (SERCA2b) contained low levels of InsP3 receptors. The immunofluorescent labelling of cultured hepatocytes with anti-InsP3R antibodies indicated that the receptor is concentrated in the perinuclear area and in some regions near the plasma membrane. The fraction enriched with InsP3R is also contaminated with markers of the ER and with SERCA2b. It was exposed to alkaline medium (pH 10.5) to extract endogenous actin and membrane-associated proteins before being subfractionated by Percoll-gradient centrifugation. The alkaline treatment allowed partial separation of the markers of the ER from the markers of the plasma membrane. The InsP3R was recovered in the heavy subfraction, which was also enriched with markers for the ER and with the SERCA2b and contained low levels of markers of the plasma membrane. These data indicate that the InsP3R is neither localized on the plasma membrane itself nor homogeneously distributed on the ER membrane. This supports the view that part of the receptor is localized on a specialized sub-region of the ER which interacts with the plasma membrane.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

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Permeability of rat liver microsomal membrane to glucose 6-phosphate

Light-scattering measurements of osmotically induced changes in the size of rat liver microsomal vesicles pre-equilibrated in a low-osmolality buffer revealed the following. (1) The increase in extravesicular osmolality by addition of glucose 6-phosphate or mannose 6-phosphate (25 mM each) caused a rapid shrinking of microsomal vesicles. After shrinkage, a rapid swelling phase (t1/2 approx. 22 s) was present with glucose 6-phosphate but absent with mannose 6-phosphate, indicating that the former had entered microsomal vesicles, but the latter had not. (2) Almost identical results were obtained in the absence of any glucose 6-phosphate hydrolysis, i.e. with microsomes pre-treated with 100 microM-vanadate. (3) The anion-channel blocker 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid (DIDS) suppressed the glucose 6-phosphate-induced swelling phase. (4) The swelling phase was more prolonged as the glucose 6-phosphate concentration increased (t1/2 = 16 +/- 3, 22 +/- 3 and 35 +/- 4 s with 25 mM, 37.5 mM- and 50 mM-glucose 6-phosphate respectively). The behaviour of glucose-6-phosphatase activity of intact and disrupted microsomes measured in the presence of high concentrations (less than 30 mM) of substrate also indicated the saturation of the glucose 6-phosphate permeation system by extravesicular concentrations of glucose 6-phosphate higher than 20-30 mM. Additional experiments showed that vanadate-treated microsomes pre-equilibrated with 0.1 mM- and 1.0 mM-glucose 6-phosphate (and [1-14C]glucose 6-phosphate as a tracer) rapidly (t1/2 less than 20 s) released [1-14C]glucose 6-phosphate when diluted in a glucose 6-phosphate-free medium. The efflux of [1-14C]glucose 6-phosphate was largely prevented by DIDS, allowing an evaluation of the intravesicular space of glucose 6-phosphate of approx. 1.0 microliter/mg of microsomal protein.

Role of a nonmitochondrial Ca2+ pool in the synergistic stimulation by cyclic AMP and vasopressin of Ca2+ uptake in isolated rat hepatocytes

The subcellular distribution of 45Ca2+ accumulated by isolated rat hepatocytes exposed to dibutyryl cyclic AMP (dbcAMP) followed by vasopressin (Vp) was studied by means of a nondisruptive technique. When treated with dbcAMP followed by vasopressin, hepatocytes obtained from fed rats accumulated an amount of Ca2+ approximately fivefold higher than that attained under control conditions. Ca2+ released from the mitochondrial compartment by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) accounted for only a minor portion of the accumulated Ca2+. The largest portion was released by the Ca2+ ionophore A23187 and was attributable to a nonmitochondrial compartment. DbcAMP + Vp-treatment also caused a maximal stimulation of glucose production and a twofold increase in cellular glucose 6-phosphate levels. In hepatocytes obtained from fasted rats, dbcAMP + Vp-stimulated Ca2+ accumulation was lower, although with the same subcellular distribution, and was associated with a minimal glucose production. In the presence of gluconeogenetic substrates (lactate plus pyruvate) hepatocytes from fasted rats were comparable to cells isolated from fed animals. However, Ca2+ accumulation and glucose 6-phosphate production could be dissociated in the absence of dbcAMP, in the presence of lactate/pyruvate alone. Under this condition in fact Vp induced only a minimal accumulation of Ca2+ in hepatocytes isolated from fasted rats, although glucose production was markedly increased. Moreover, treatment of fed rat hepatocytes with 1 mM ATP caused a maximal activation of glycogenolysis, but only a moderate stimulation of cellular Ca2+ accumulation. In this case, sequestration of Ca2+ occurred mainly in the mitochondrial compartment. By contrast, the addition of ATP to dbcAMP-pretreated hepatocytes induced a large accumulation of Ca2+ in a nonmitochondrial pool. Additional experiments using the fluorescent Ca2+ indicator Fura-2 showed that dbcAMP pretreatment can enlarge and prolong the elevation of cytosolic free Ca2+ caused by Vp. A nonmitochondrial Ca2+ pool thus appears mainly responsible for the Ca2+ accumulation stimulated by dbcAMP and Vp in isolated hepatocytes, and cyclic AMP seems able to activate Ca2+ uptake in such a nonmitochondrial pool.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Glucose 6-phosphate stimulation of MgATP-dependent Ca2+ uptake by rat kidney microsomes

(1) The features of MgATP-dependent Ca2+ accumulation under stimulation with glucose 6-phosphate were studied in rat kidney microsomes. (2) Ca2+ accumulated in the presence of MgATP alone does not exceed approx. 2 nmol/mg protein. (3) Glucose 6-phosphate markedly stimulates Ca2+ accumulation, up to steady-state levels approx. 15-fold higher than in its absence. (4) The hydrolysis of glucose 6-phosphate by glucose-6-phosphatase is essential for the stimulation, as shown by inhibiting the glucose 6-phosphate hydrolysis with adequate concentrations of vanadate. Inorganic phosphate is accumulated in microsomal vesicles during glucose 6-phosphate-stimulated Ca2+ uptake in equimolar amounts with respects to Ca2+. (5) Increasing concentrations of glucose 6-phosphate result in increasing stimulations of Ca2+ uptake, until a maximal Ca2(+)-loading capacity of approx. 27 nmol/mg microsomal protein is reached. It is suggested that the enlargement of the kidney microsomal Ca2+ pool induced by glucose 6-phosphate (an important metabolite in kidney) might play a role in the regulation of Ca2+ homeostasis in kidney tubular cells.

Ca2+ mobilization by vasopressin and glucagon in perfused livers. Effect of prior intoxication with bromotrichloromethane

Perfused livers isolated from rats treated with BrCCl3 for up to 15 min were used as an experimental tool to investigate the role of the hepatic endoplasmic reticulum in Ca2+ mobilization elicited by vasopressin and glucagon. BrCCl3-treatment caused extensive impairment (37 to 92%) of Ca2+ pumps of isolated liver microsomes, while Ca2+ pumps of mitochondria and plasma membrane vesicles remained undamaged. In perfused livers of BrCCl3-treated rats, the efflux of Ca2+ and the concomitant stimulation of O2 consumption and glucose release induced by vasopressin were decreased. The extent of the decrease paralleled the duration of BrCCl3-treatment. The decrease of Ca2+ efflux following vasopressin addition was closely correlated with the decrease of active Ca2+ accumulation by isolated microsomes (r = 0.99, P less than 0.001). The Ca2+ efflux elicited by glucagon was also decreased after BrCCl3-treatment, whereas stimulation of O2 consumption and glucose release were retained. The possibility that BrCCl3-treatment might impair the production of the intracellular Ca2+-mobilizing messenger IP3 is unlikely, since vasopressin still induced the formation of inositol phosphates, including IP3, in isolated hepatocytes obtained from BrCCl3-treated rats. Thus, this work supports the hypothesis that the Ca2+ stored in the liver ER is the major pool of intracellular Ca2+ available for mobilization by vasopressin, glucagon and other effectors.

MgATP-dependent, glucose 6-phosphate-stimulated liver microsomal Ca2+ accumulation: difference between rough and smooth microsomes

Some features of the MgATP-dependent Ca2+-accumulating capacity of rough as compared to smooth liver microsomal fraction were studied. Smooth microsomes accumulate somewhat higher amounts of Ca2+ than rough ones in the presence of MgATP. In the presence of glucose 6-phosphate, which markedly stimulates MgATP-dependent Ca2+ accumulation in both fractions, smooth microsomes exhibit a much higher Ca2+-accumulating capacity than rough ones. Possible reasons of the differences observed between the two fractions were investigated. Smooth microsomes exhibit a higher Ca2+-dependent ATPase activity, suggesting a higher Ca2+ inward transport into smooth vesicles. Also, following the inhibition of active Ca2+ transport by means of vanadate, smooth microsomes appear to release the Ca2+ previously accumulated--both in the absence (i.e., with MgATP only) and in the presence of glucose 6-phosphate--at a lower rate than rough ones. This indicates a lower passive backflux of Ca2+ accumulated in smooth vesicles. On the basis of these data, differences can be envisaged with respect to cellular Ca2+ handling by different domains of endoplasmic reticulum in the liver.