Characteristics of a Ca2+-ATPase activity measured in islet homogenates

Proteins altered by elevated levels of palmitate or glucose implicated in impaired glucose-stimulated insulin secretion

Background

Development of type 2 diabetes mellitus (T2DM) is characterized by aberrant insulin secretory patterns, where elevated insulin levels at non-stimulatory basal conditions and reduced hormonal levels at stimulatory conditions are major components. To delineate mechanisms responsible for these alterations we cultured INS-1E cells for 48 hours at 20 mM glucose in absence or presence of 0.5 mM palmitate, when stimulatory secretion of insulin was reduced or basal secretion was elevated, respectively.

Results

After culture, cells were protein profiled by SELDI-TOF-MS and 2D-PAGE. Differentially expressed proteins were discovered and identified by peptide mass fingerprinting. Complimentary protein profiles were obtained by the two approaches with SELDI-TOF-MS being more efficient in separating proteins in the low molecular range and 2D-PAGE in the high molecular range. Identified proteins included alpha glucosidase, calmodulin, gars, glucose-6-phosphate dehydrogenase, heterogenous nuclear ribonucleoprotein A3, lon peptidase, nicotineamide adenine dinucleotide hydrogen (NADH) dehydrogenase, phosphoglycerate kinase, proteasome p45, rab2, pyruvate kinase and t-complex protein. The observed glucose-induced differential protein expression pattern indicates enhanced glucose metabolism, defense against reactive oxygen species, enhanced protein translation, folding and degradation and decreased insulin granular formation and trafficking. Palmitate-induced changes could be related to altered exocytosis.

Conclusion

The identified altered proteins indicate mechanism important for altered β-cell function in T2DM.

Quantitation of plasma membrane calcium pump phosphorylated intermediates by electrophoresis

P-ATPases are characterized by the formation of acid-stable phosphorylated intermediates (EP) during their reaction cycle. We have developed a microscale method to determine EP that involves the phosphorylation of the enzyme using [gamma-(32)P]ATP and precipitation with TCA; separation of the sample by SDS-PAGE, and measurement of the enzyme protein and (32)P-labeled EP by digital analysis of both the stained gel and its autoradiogram, respectively. The principal advantages of this method over typical procedures (filtration and centrifugation) are the low amount of enzyme required and the substantial decrease in the blank values and data scattering produced by unspecific phosphorylation and nonquantitative recovering of the enzyme. Application of this new method to a purified preparation of the plasma membrane calcium ATPase (PMCA) results in overcoming the difficulties of measuring EP at high ATP concentrations. A biphasic behavior of the substrate curve for EP was observed when the study was extended to ATP levels within the physiological range. Since, in principle, the method does not require the use of highly purified preparations, it could be helpful for the study of phosphorylated intermediates especially under conditions in which small amounts of protein are available, e.g., mutated variants of P-ATPases.

An overview of Argentine contributions to diabetes research in the decade of the 1990s

Argentina has a longstanding tradition of diabetes research, beginning with the seminal work of Prof. Bernardo A. Houssay, who was awarded the first Nobel Prize in Medical Sciences for his studies on the relationship between diabetes and pituitary function. Prof. Luis F. Leloir, who was also awarded the Nobel Prize for his work in carbohydrate metabolism, also inspired younger generations of biologists to work in the field of diabetes research. The aim of this paper is to provide a review of the contributions of Argentine researchers during the 1990s. This manuscript includes only reports of Argentine researchers working on diabetes in local laboratories and quoted in Medline. Thus, important contributions not reported in journals included in Medline or produced by Argentine researchers working abroad may have been omitted. The material consists of a brief description of clinical research (epidemiology and costs, metabolic control, associated risk factors, immunological aspects, and other clinical studies) and basic research (animal model with spontaneous diabetes, islet morphology and function in normal and pathological conditions, insulin action, metabolic disorders related to diabetes, and some miscellaneous effects related to drug-induced diabetes). Altogether, a broad idea of the continuous contribution of our national research to the international field of diabetes is provided, as well as a list of Argentine researchers and research centers devoted to the study of diabetes.

Characterisation of endoplasmic reticulum and plasma membrane Ca(2+)-ATPases in pancreatic beta-cells and in islets of Langerhans

We have investigated the plasma membrane (PMCA) and endoplasmic reticulum (SERCA) Ca(2+)-ATPases involved in active transport of Ca2+ in pancreatic beta-cell lines (MIN6, HIT T15, RINm5F) and in islets of Langerhans. Under selective membrane phosphorylation conditions (at low ATP concentration, in the presence of Ca2+ and La3+ and in the absence of Mg2+ at 4 degrees C) the only labelled proteins are the phosphoenzyme intermediates of the Ca(2+)-ATPases. Under these conditions, beta-cell membranes incorporated 32P from [gamma-32P]ATP into two proteins with molecular mass on acidic SDS-polyacrylamide gels of around 115 and 150 kDa. The 150 kDa band was identified as PMCA (i) by reaction with a monoclonal anti-human erythrocyte plasma membrane Ca(2+)-ATPase antibody; (ii) by its typical tryptic cleavage pattern which generated an 80 kDa band; (iii) by lack of inhibition of its autophosphorylation by SERCA-specific inhibitors. The 115 kDa band was identified as SERCA (i) by reaction with a polyclonal anti-rat fast skeletal muscle Ca(2+)-ATPase antibody; (ii) by the concentration-dependent inhibition of its autophosphorylation by thapsigargin and 2,5-di(t-butyl)-1,4-benzohydroquinone (tBHQ), which are specific inhibitors of SERCA. The 115 kDa band was further characterised as the SERCA-2b isoform by reaction with a polyclonal rabbit antibody against the 12 C-terminal amino acids of SERCA-2b.

Effect of pH upon Ca(2+)-ATPase activity of rat pancreatic islets: its possible contribution to the inhibitory effect of different insulin secretagogues

This work was undertaken in an attempt to elucidate the possible mechanism by which insulin secretagogues produce a fast and transient drop in the Ca(2+)-ATPase activity of the pancreatic islet membrane. For this purpose, the enzyme activity was measured in either homogenates or partially purified membranes of islets previously incubated under different experimental conditions. Ca(2+)-ATPase activity measured in homogenates of islets preincubated with 8 mM glucose decreased significantly compared to control islets incubated with 2.8 mM glucose. The inhibition was also observed when the enzyme activity was measured in homogenates of islets preincubated with 2.8 mM glucose plus 20 mM propionic acid as well as with glucose 2.8 mM in a buffer equilibrated with a gas mixture of O2 and either 12% or 30% CO2. Ca(2+)-ATPase activity decreased significantly in partially purified islet membranes preincubated for 3 min with glucose (2 and 8 mM), 15 mM KCl and 2 mM tolbutamide. These substances did not affect the Ca(2+)-ATPase activity when added directly to the enzyme assay medium. The enzyme activity also decreased when measured in membranes preincubated at pH 6.5. The addition of 1 mM ATP to the preincubation medium protected the Ca(2+)-ATPase activity from the inhibition induced by glucose, KCl and tolbutamide as well as from the one produced by acidic pH in the medium. On account of these results, we suggest that insulin secretagogues, as well as either acidification of B-cell cytosol or islet membrane incubation medium, produce changes at the islet membrane level which promote a decrease in the Ca(2+)-ATPase activity. A shift of the E1-E2 equilibrium of the phosphoenzyme towards E1 may account for such decreased activity. Changes in Ca(2+)-ATPase activity could either favour the decrease or the increase in the cytosolic concentration of Ca2+ in B-cells. Therefore, negative and positive modulation of its activity might allow Ca(2+)-ATPase to play a role in the switch-on and -off mechanism for intracellular Ca2+ signal regulation of B-cell secretion of insulin.

Modulatory mechanism of ACTH on insulin secretion: effect on cytosolic Ca2+, membrane potential and Ca(2+-ATPase activity

The aim of this work was to get some insight into the mechanism by which ACTH produces its enhancing effect on glucose-induced insulin secretion. For this purpose we have determined: a) the release of insulin by isolated rat islets incubated with 3.3 or 16.6 mM glucose with or without the addition of 500 pg/ml ACTH, together with the changes induced by ACTH on b) cytosolic [Ca2+] of isolated B cells, c) islet plasma membrane Ca(2+)-ATPase activity and d) changes in membrane potential of single mouse islets. ACTH significantly enhanced the release of insulin elicited by either 3.3 or 16.6 mM glucose. This hormone concentration also induced a significant increase in the cytosolic [Ca2+] in isolated B cells. ACTH did not produce B cell membrane depolarization. Conversely, ACTH produced a significant decrease in islet plasma membrane Ca(2+)-ATPase activity. These results suggest that ACTH in concentrations similar to those attained by the endogenous peptide at the islet interstitium exerts its positive modulation on glucose-induced secretion of insulin, at least partly through its increasing effect on cytosolic [Ca2+] of B cells. The latter might be the consequence of the decreasing effect of ACTH on Ca(2+)-ATPase activity rather than to stimulation of voltage-dependent Ca(2+)-channels.

Presynaptic ecto- and postsynaptic endo-calcium-adenosine-triphosphatases in synaptosomes: doubts about biochemical interpretation of localization

Ecto- and endo-Ca-adenosine-triphosphatase (ATPase) activity was identified as electron-dense lead or cerium phosphate precipitate in the rat cortical synaptosomes by transmission electron microscopy and enzyme histochemistry. The formation of the deposit was dependent on the presence of ATP (the substrate), Ca (activator) and levamisole, quercetin or ouabain (inhibitors of different phosphatases and ATPases). Reaction products were found at the external surface of the presynaptic membrane, both surfaces of the postsynaptic membrane, in the synaptic cleft and in the free mitochondrial membranes. In the presence of ATP and the three inhibitors together, the quantity of the precipitate decreased markedly, but we still found some deposit on the external surface of the presynaptic membrane (this activity is probably due to the so-called ecto-ATPase) and on the internal surface of the postsynaptic one (endo-ATPase). The distinction between ecto- and endo-ATPases in biochemical fractions solely upon biochemical differential measurements must be interpreted with caution.

Rapidly induced modulation of beta-N-acetylglucosaminidase activity in pancreatic islets

The aim of this work was to determine the possible rapid modulatory effect of glucose on the activity of pancreatic islet lysosomal enzymes. For this purpose, beta-N-acetylglucosaminidase and beta-galactosidase activities were measured in homogenates of isolated rat islets after a 5, 15, 30 or 60-min exposure to either 3.3 or 16.6 mM glucose. The enzyme activities were determined spectrofluorometrically by means of their respective 4-methylumbelliferyl derivatives as substrates. beta-N-acetylglucosaminidase activity measured in freshly isolated non-incubated islets was 5.482 +/- 0.281 mumol/mg protein/h at 37 degrees C. In islets incubated with 3.3 mM glucose, this activity dropped significantly after 5 min and remained almost constant until the end of the incubation period. In islets incubated with 16.6 mM glucose, beta-N-acetylglucosaminidase activity also decreased significantly at 5 min, and attained its lowest value after 15 min of incubation. After this interval, the activity began to recover and thereafter gained a value close to that measured in non-incubated islets by 60 minutes' time. Despite this ultimate recovery, the enzyme activities measured were significantly lower than those found in islets incubated with 3.3 mM glucose. beta-galactose activity in freshly isolated non-incubated islets was 0.515 +/- 0.094 mumol/mg protein/h at 37 degrees C. This value remained almost unchanged throughout the incubation period in the presence of either 3.3 or 16.6 mM glucose. These results show that beta-N-acetylglucosaminidase activity, a lysosomal hydrolase of pancreatic rat islets,--and only this enzyme--is modulated by glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

The erythrocyte calcium pump is inhibited by non-enzymic glycation: studies in situ and with the purified enzyme

In a previous paper we demonstrated that incubation of either intact erythrocytes or erythrocytes membranes with glucose decreases the activity of the membrane Ca(2+)-ATPase [González Flecha, Bermúdez, Cédola, Gagliardino and Rossi (1990) Diabetes 39, 707-711]. The aim of the present work was to obtain information about the mechanism of this inhibition. For this purpose, experiments were carried out with purified Ca(2+)-ATPase, inside-out vesicles and membranes from human erythrocytes. Incubation of the purified Ca(2+)-ATPase with glucose led to a decay in the enzyme activity of up to 50% of the control activity under the conditions used. The decrease in ATPase activity was concomitant with labelling by [6-3H]glucose of the purified Ca2+ pump; the kinetic properties of both processes were almost identical, suggesting that inhibition is a consequence of the incorporation of glucose into the Ca(2+)-ATPase molecule. In inside-out vesicles, glucose also promoted inhibition of Ca(2+)-ATPase activity as well as of active Ca2+ transport. Arabinose, xylose, mannose, ribose, fructose and glucose 6-phosphate (but not mannitol) were also able to inactive the ATPase. The activation energy for both the decrease in ATPase activity by glucose and the labelling of the pump with [6-3H]glucose was about 65 kJ/mol. Furthermore, inorganic phosphate enhanced the inactivation of the Ca(2+)-ATPase by glucose. This evidence strongly suggests that inhibition is a non-enzymically catalysed process. Inactivation of the Ca(2+)-ATPase by glucose was enhanced by reductive alkylation with sodium borohydride. Aminoguanidine, an inhibitor of the formation of the advanced end products of glycosylation, did not prevent the deleterious effect of glucose on the enzyme activity. Therefore it is concluded that inactivation of the Ca2+ pump is a consequence of the glycation of this protein.

Differential reactivity of lysine residues of the red blood cell Ca2+ pump involved in the E1-E2 conformational equilibrium

1. Modification of Lys residues of the Ca(2+)-ATPase from human red blood cells with methyl acetimidate (MA) inhibited up to 70% of the Ca(2+)-ATPase activity. Furthermore, calmodulin-activated p-nitrophenyl phosphatase activity was fully inhibited at non-limiting concentrations of MA. 2. Treatment with MA inhibited phosphorylation of the Ca(2+)-ATPase. 3. When the enzyme was treated with 7.2 mM-MA in the presence of 100 microM-Ca2+, Ca(2+)-ATPase activity was decreased by 33%, whereas when the membranes were treated with MA in the presence of 50 microM-VO4(3-), this activity was decreased by only 8%. 4. When membranes were either proteolysed or preincubated with 1 mM-Ca2+, MA quickly inactivated the Ca(2+)-ATPase (k = 1.2 min-1). On the other hand, inactivation of membranes preincubated in the absence of Ca2+ and Mg2+ was slow (k = 0.08 min-1). 5. When the activity was measured in the absence of calmodulin, MA decreased to the same extent the values of KCa (the apparent dissociation constant for Ca2+) and Vmax, but in the presence of calmodulin the treatment decreased Vmax. only. 6. The results are consistent with the idea that MA reacts readily with the Ca(2+)-ATPase when the enzyme is in an E1 conformation, but not an E2 conformation, and that, reciprocally, treatment of the enzyme with MA shifts the enzyme to E1. 7. Provided that Ca2+ is present, ATP, with low apparent affinity (K0.5 = 195 microM), protected against inactivation by MA. However, MA treatment did not change the Km values of either the high-affinity or the low-affinity site for ATP, suggesting that protection results from a shift to a conformation in which the Lys residues are inaccessible to MA.

Cytochemistry of Ca(++)-dependent adenosine triphosphatase (Ca-ATPase) in rat anterior pituitary cells

In the present study, we demonstrate the localization of Ca(++)-ATPase in the anterior pituitary of the male rat. Ca(++)-ATPase was mainly distributed on the membrane system of the granular cells, which included the plasma membrane, the outer mitochondrial membrane, the enveloping membrane of secretory granules, the smooth endoplasmic reticulum and some components of the Golgi complex. No reaction product was detected on the membrane of the rough endoplasmic reticulum or that surrounding the lysosomes. A positive reaction was clearly observed on the membranes surrounding 'large' secretory granules, while that present on the membranes of the 'small' granules was comparatively weak. The cells which contained the 'large' granules were interpreted as growth hormone-secreting cells and those in which the 'small' granules were located as gonadotrophs. There were either no reaction or one that was barely detectable on the plasma membrane of the folliculo-stellate cells. These data along with our previous findings (Soji, 1982, 1984) suggest that the membranous enzymes are not uniformly distributed over all pituitary cells but rather are specific for a given cell population(s).

Characteristics of a presynaptic plasma membrane Ca2(+)-ATPase activity from electric organ

Ca2(+)-ATPase activity was measured in electric organ synaptosomal homogenates and their derived presynaptic plasma membranes using a low ionic strength medium, low in Ca2+ and Mg2+, and devoid of K+. The enzyme activity showed a high apparent affinity for Ca2+ (KCa:0.5 microM) and was: (1) 5-fold stimulated by 120 nM calmodulin, (2) highly sensitive to LaCl3 inhibition, and (3) not affected by 20 mM NaN3 or 0.1 mM ouabain. The addition of Mg2+ promoted the disappearance of Ca2(+)-ATPase activity. Incubation of synaptosomal homogenates in the above-mentioned assay medium with [gamma -32P]ATP resulted in the appearance of a 140 kDa band as revealed by SDS-gel electrophoresis. Labeling of this band with 32P was inhibited by 1 mM EGTA or 10 mM NH2OH, indicating that the isotope incorporation required the presence of Ca2+ and the formation of an acyl-phosphate derivative. The results indicate that the Ca2(+)-ATPase activity from synaptosomal homogenates had characteristics corresponding to those of the enzyme that catalyzes an outward transport of Ca2+ in nerve terminals. Preincubation of synaptosomes in Ca2+ plus K+, a depolarizing procedure, induced a large and rapid decrease in the Ca2(+)-ATPase activity, possibly mediated via Ca2+ entry through voltage-gated Ca2+ channels. Furthermore, the muscarinic cholinergic agonist oxotremorine (at 15 microM concentration) did not significantly affect either the enzyme activity or the intensity of the Ca2(+)-dependent 32P incorporation into the 140 kDa band, suggesting that the enzyme is not coupled to muscarinic binding sites.

Effect of different insulin secretagogues and blocking agents on islet cell Ca2+-ATPase activity

Plasma membrane Ca2+-ATPase activity was measured in rat islet homogenates. The enzyme was inhibited, in a dose-dependent manner, when the islets were preincubated for 5 min with different concentrations of glucose (2 to 16 mM). This inhibition disappeared almost entirely after 15 min incubation, regardless of the glucose concentration in the medium. Simultaneous measurement of insulin in the medium revealed a stimulatory effect of glucose upon insulin secretion. The Ca2+-ATPase activity was also inhibited when the islets were preincubated for 3 min with other stimulators of insulin secretion such as gliclazide (76 microM), tolbutamide (1.5 mM), glucagon (1.4 microM) + theophylline (10 mM) and ketoisocaproic acid (15 mM). Conversely, the activity of the enzyme was significantly enhanced when the islets were preincubated briefly with the insulin secretion blocker, somatostatin (1.4 microM). Neither glucose nor any of the other substances tested when added directly to the enzyme assay medium modified significantly the Ca2+-ATPase activity measured in the islet homogenates. These results would suggest that the activity of the islet plasma membrane is modulated by one or more of the intracellular metabolites produced when the islets are challenged by the insulin stimulator or blocking agents.