Substrates for cyclic AMP-dependent protein kinase in islets of Langerhans. Studies with forskolin and catalytic subunit

Type II PKAs are anchored to mature insulin secretory granules in INS-1 β-cells and required for cAMP-dependent potentiation of exocytosis

Specificity of the cAMP-dependent protein kinase (PKA) pathway relies on an extremely sophisticated compartmentalization mechanism of the kinase within a given cell, based on high-affinity binding of PKA tetramer pools to different A-Kinase Anchoring Proteins (AKAPs). We and others have previously shown that AKAPs-dependent PKA subcellular targeting is a requisite for optimal cAMP-dependent potentiation of insulin exocytosis. We thus hypothesized that a PKA pool may directly anchor to the secretory compartment to potentiate insulin exocytosis. Here, using immunofluorescence analyses combined to subcellular fractionations and purification of insulin secretory granules (ISGs), we identified discrete subpools of type II PKAs, RIIα and RIIβ PKAs, along with the catalytic subunit, physically associated with ISGs within pancreatic insulin-secreting β-cells. Ultrastructural analysis of native rodent β-cells confirmed in vivo the occurrence of PKA on dense-core ISGs. Isoform-selective disruption of binding of PKAs to AKAPs reinforced the requirement of type II PKA isoforms for cAMP potentiation of insulin exocytosis. This granular localization of PKA was of critical importance since siRNA-mediated depletion of either RIIα or RIIβ PKAs resulted in a significant reduction of cAMP-dependent potentiation of insulin release. The present work provides evidence for a previously unrecognized pool of type II PKAs physically anchored to the β-cell ISGs compartment and supports a non-redundant function for type II PKAs during cAMP potentiation of exocytosis.

Secretagogue-dependent phosphorylation of phogrin, an insulin granule membrane protein tyrosine phosphatase homologue

Phogrin, a 60/64 kDa integral membrane protein localized to dense-core secretory granules of neuroendocrine cells, was found to be reversibly phosphorylated in intact pancreatic beta-cells. Phosphorylation occurred in response to a variety of secretory stimuli, including glucose and depolarizing concentrations of K(+). In MIN6 cells, the glucose dose-response and time course of phogrin phosphorylation paralleled that of insulin secretion. Like secretion, glucose- or K(+)-stimulated phosphorylation required the presence of Ca(2+). The calmodulin antagonist W-7 and the Ca(2+)/calmodulin-dependent kinase II inhibitor KN-93 dose-dependently inhibited both phosphorylation and secretion, while the 'inactive' analogue KN-92 was effective only at significantly higher concentrations. Phosphorylation of phogrin was also stimulated in cells exposed to forskolin, an effect presumably mediated by protein kinase A (cAMP-dependent protein kinase). Under these conditions, phogrin phosphorylation could be dissociated from the secretory response. In MIN6 cells, as in pancreatic islets, cAMP potentiates rather than initiates insulin release. Thus our observations are consistent with a role for phogrin phosphorylation in the signal-transduction pathway at a site proximal to the exocytic event itself, possibly regulating secretory-granule mobilization and recruitment to the exocytic site.

AMP-activated protein kinase is activated by low glucose in cell lines derived from pancreatic beta cells, and may regulate insulin release

The role of the AMP-activated protein kinase (AMPK) cascade in the glucose-sensitive pancreatic beta cell lines HIT-T15 and INS-1 was addressed. In both cell types, removal of glucose leads to a >5-fold activation of AMPK activity. Activation of AMPK was due to phosphorylation, since the effect was reversed by protein phosphatase treatment of the extracts, and was restored by re-addition of MgATP and the purified upstream kinase. When the effects of different concentrations of medium glucose were examined, insulin secretion and AMPK activity were inversely related, and varied over the same concentration range. The activation in response to glucose removal appeared to be due to changes in the concentration of the known regulators of the cascade, i.e. AMP and ATP, since AMPK activation was associated with a large increase in the cellular AMP/ATP ratio, and the two parameters varied over the same range of glucose concentrations. In late-passage HIT-T15 cells that had lost the glucose-dependent insulin secretion response, both AMPK activity and the AMP/ATP ratio also became insensitive to the extracellular glucose concentration. Treatment of INS-1 cells, but not HIT-T15 cells, with AICA riboside (5-aminoimidazole-4-carboxamide riboside) results in accumulation of the ribotide, ZMP (AICA riboside monophosphate), and activation of AMPK. AICA riboside treatment of INS-1 cells, and of isolated rat islets, had both inhibitory and stimulatory effects on insulin secretion. These results show that in beta cell lines the AMP-activated protein kinase, like its yeast homologue the SNF1 complex, can respond to the level of glucose in the medium, and may be involved in regulating insulin release.

Glucagon induces Ca2+-dependent increase of reduced pyridine nucleotides in mouse pancreatic beta-cells

Glucagon enhances the electrical activity of pancreatic beta-cells. The mechanism of the glucagon-evoked enhancement of electrical activity was investigated in terms of glucose metabolism. ICR mice aged 6-12 weeks were used for experiments. Intracellular Ca2+ increased in parallel with the enhancement of electrical activity. The stimulating effect of glucagon on Ca2+ oscillation was suppressed by calmodulin-antagonists (Chlorpromazine, W-7, and trifluoperazine). To trace the glucagon-evoked change in glucose metabolism, the reduced pyridine nucleotide (NAD(P)H) fluorescence was monitored using the microfluorometry with the excitation of 360 nm and the emission of 465 nm in islet cell clusters mainly consisting of beta-cells. In the presence of 2.5 mM Ca2+ glucagon (8.6 X 10(-8) M) increased the NAD(P)H fluorescence, while in the absence of Ca2+ the hormone had no effect on the fluorescence. Extracellular Ca2+ removal from the glucagon-containing perifusion solution decreased the fluorescence to the level which had been attained before glucagon was added. Chlorpromazine (10 microM) reversed the glucagon-induced increase of NAD(P)H fluorescence as well as removing Ca2+ W-7 (15 microM) and trifluoperazine (30 microM) also suppressed the glucagon-induced increase of NAD(P)H. These results suggest that Ca2+/calmodulin system is involved in the acceleration of glycogenolysis by glucagon in beta-cells. On the basis of these observations, the mechanism of glucagon-induced enhancement of electrical activity and the relative ineffectiveness of glucagon at low glucose concentrations were discussed.

Identification of functional ionotropic glutamate receptor proteins in pancreatic beta-cells and in islets of Langerhans

The presence of ionotropic glutamate receptor proteins in islets of Langerhans and pancreatic beta-cell lines (MIN6, HIT T15, RINm5F) was investigated. For this purpose immunoblot analysis of beta-cell membranes was performed with subunit-specific antibodies. We identified NMDAR1 subunits of the NMDA and KA-2 subunits of the kainate receptors, but did not detect GluR1 subunits of the AMPA receptor. The receptor subunits present were shown to be glycosylated. beta-cell membranes contained specific binding sites for glutamate receptor ligands, and NMDA increased insulin secretion. These results demonstrate that ionotropic glutamate receptor proteins, similar to those in the central nervous system, are expressed in rat pancreatic beta-cells.

Stimulus-secretion coupling in pancreatic beta cells

Insulin secretion is triggered by a rise in the intracellular Ca2+ concentration that results from the activation of voltage-gated Ca2+ channels in the beta-cell plasma membrane. Multiple types of beta-cell Ca2+ channel have been identified in both electrophysiological and molecular biological studies, but it appears that the L-type Ca2+ channel plays a dominant role in regulating Ca2+ influx. Activity of this channel is potentiated by protein kinases A and C and is inhibited by GTP-binding proteins, which may mediate the effects of potentiators and inhibitors of insulin secretion on Ca2+ influx, respectively. The mechanisms by which elevation of intracellular Ca2+ leads to the release of insulin granules is not fully understood but appears to involve activation of Ca2+/calmodulin-dependent protein kinase. Phosphorylation by either protein kinase A or C, probably at different substrates, potentiates insulin secretion by acting at some late stage in the secretory process. There is also evidence that small GTP-binding proteins are involved in regulating exocytosis in beta cells. The identification and characterisation of the proteins involved in exocytosis in beta cells and clarification of the mechanism(s) of action of Ca2+ is clearly an important goal for the future.

The methodology for studying coordinated calcium concentration changes in a pancreatic beta cell line

Intracellular calcium concentration was imaged in beta Tc cells with the aid of Fluo-3 indicator and the Meridian ACAS 570 interactive laser cytometry. This cell line does not respond by an elevation in [Ca2+]in to increase in extracellular [glucose], but does respond to 10 microM forskolin. It was found that forskolin increases the mean [Ca2+]in and produces calcium spikes. Time series analysis was performed on individual pixels. Autocorrelation revealed that forskolin induces oscillation in [Ca2+]in. Cross-correlation analysis showed that all the intracellular pixels along the line scan are highly correlated, indicating that the increase in [Ca2+]in encompasses the entire cell.

Characterization of Ca2+/calmodulin-dependent protein kinase in rat pancreatic islets

We have attempted to identify islet Ca2+/calmodulin-dependent protein kinase (CaM kinase) by comparing its activity with purified brain CaM kinase II. Islet CaM kinase, in the presence of calmodulin and Ca2+, phosphorylated major endogenous substrates of 102, 57 and 53 kDa and also exogenous glycogen synthase; brain CaM kinase II phosphorylated glycogen synthase and peptides of 57 and 53 kDa. Alloxan (1 mM) inhibited the phosphorylation of glycogen synthase and the 102, 57 and 53 kDa islet peptides by islet CaM kinase; the phosphorylation of glycogen synthase and the 57 and 53 kDa substrates by brain CaM kinase II was also inhibited by alloxan. The Ca2+ and calmodulin-dependencies of phosphorylation of the endogenous islet substrates differed. In the presence of 400 nM calmodulin, half-maximal phosphorylation was attained at Ca2+ concentrations of 80 +/- 9, 401 +/- 61 and 459 +/- 59 nM for the 102, 57 and 53 kDa substrates respectively. In the presence of 10 microM Ca2+, half-maximal phosphorylation was attained at calmodulin concentrations of 9 +/- 2, 38 +/- 2.5 and 37 +/- 2 nM for the 102, 57 and 53 kDa substrates respectively. Differential centrifugation located the 102 kDa substrate in the post-100,000 g supernatant and the 57 and 53 kDa substrates in the particulate fraction. These data suggest that islet CaM kinase is similar to, if not identical with, brain CaM kinase II, but that phosphorylation of the endogenous 102 kDa substrate occurs by a distinct kinase which shows different sensitivities to Ca2+ and calmodulin. This kinase probably corresponds to CaM kinase III and the 102 kDa peptide to elongation factor 2 (EF-2), since the 102 kDa peptide was shown to undergo ADP-ribosylation in the presence of diphtheria toxin and NAD+.

The role of cytosolic free Ca2+ and protein kinase C in acetylcholine-induced insulin release in the clonal beta-cell line, HIT-T15

We examined the contribution of signal-transduction pathways to acetylcholine-induced insulin release in the clonal beta-cell line HIT-T15. To assess the importance of changes in cytosolic free Ca2+ [( Ca2+]i), we studied time courses of the effects of glucose and acetylcholine on [Ca2+]i and insulin release in quin 2-loaded HIT cells. Incubation in the presence of glucose (2 mM) resulted in a sustained increase in [Ca2+]i in HIT cells from 98 +/- 7 nM to 195 +/- 12 nM measured after 9 min, whereas subsequent addition of acetylcholine (50 microM) produced a transient increase in [Ca2+]i which reached a peak after 30 s (at 274 +/- 10 nM), returning to pre-stimulus levels after 3 min. In contrast, incubation of HIT cells with acetylcholine in the presence of glucose produced a sustained increase in insulin release over and above that stimulated by glucose alone; after 10 min acetylcholine had potentiated glucose-stimulated insulin release by an additional increment of 135%. The transient increase in [Ca2+]i induced by acetylcholine was dose-dependent, and was prevented by omission of glucose or extracellular Ca2+ from the incubation medium. It was also inhibited by inclusion of 50 microM-verapamil in the incubation medium (by 87 +/- 3%) or by decreasing the Na+ concentration in the medium (by 73 +/- 6%). To evaluate the role of the protein kinase C pathway, we have pretreated HIT cells with the phorbol ester 12-O-tetradecanoylphorbol acetate (TPA), to deplete the protein kinase C activity, and have compared their secretory activity with that of control cells. Protein kinase C activity was decreased by 73% in HIT cells cultured in the presence of 200 nM-TPA for 22-24 h. TPA pre-treatment also significantly decreased the insulin content of HIT cells, but had no effect on cell number or the increases in [Ca2+]i induced by glucose or acetylcholine. TPA-pre-treated cells responded comparatively less well to secretagogues than did control cells: glucose-stimulated insulin release was decreased by 40%, whereas potentiation by TPA was significantly decreased by 50% in comparison with control cells (P less than 0.05, n = 24). Acetylcholine (50 microM) potentiated glucose-stimulated insulin release by 61% in control cells. This effect was abolished in HIT cells pre-treated with TPA, whereas these cells still retained their normal secretory response to stimulation by forskolin. These data suggest that an early increase in [Ca2+]i may be important for the initial increase in insulin release induced by acetylcholine in HIT cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterisation of the cellular substrates for transglutaminase in normal liver and hepatocellular carcinoma

The transglutaminase-mediated incorporation of [14C]methylamine into tissue slices obtained from normal rat liver and diethylnitrosamine-induced hepatocellular carcinomas was used as a means of characterising the endogenous substrates of the transglutaminase enzymes present in these tissues. The amount of radiolabel incorporated was found to be similar in both tissues with the major radiolabelled protein identified as a high molecular weight polymer unable to traverse a 3.0% (w/v) acrylamide gel and with a molecular weight of at least 5 x 10(6) Da. Measurement of the crosslink, epsilon-(gamma-glutamyl)lysine, in the hepatocellular carcinoma and in normal liver indicated a 3-fold reduction in the levels found in tumour tissue when compared to normal liver. In contrast, the levels of covalently bound polyamines present in the hepatocellular carcinoma were found to be comparable or greater than those found in normal liver. Considering that there is a selective reduction (approx. 5-fold) in the activity of the cytosolic transglutaminase present in hepatocellular carcinomas with no change in the activity of the particulate enzyme (Hand et al. (1988) Biochim. Biophys. Acta 970, 137-145) these results suggests that the two enzymes may be differentially activated and that they may act on different substrates within the cell.

Effect of secretagogues on cytosolic free Ca2+ and insulin release at different extracellular Ca2+ concentrations in the hamster clonal beta-cell line HIT-T15

We have examined the relationship between extracellular Ca2+, cytosolic free Ca2+ and insulin release in the clonal beta-cell line HIT-T15. Glucose-stimulated insulin release was dependent on the extracellular Ca2+ concentration in a dose-related manner; the threshold medium Ca2+ concentration for glucose-stimulated insulin release was 0.5 mM. Both forskolin and 12-O-tetradecanoylphorbol 13-acetate (TPA) increased insulin release in the presence of glucose at all extracellular Ca2+ concentration tested (0.1-2.5 mM) but not in the absence of Ca2+. Thus, the threshold medium Ca2+ concentration for glucose-stimulated insulin release was reduced to 0.1 mM by forskolin or TPA. Step-wise increases in the medium Ca2+ concentration in the presence of an initiator of insulin release resulted in a dose-related increase in cytosolic free Ca2+. In the presence of 10 mM glucose, cytosolic free Ca2+ in HIT cells was increased from 60 +/- 5 nM in Ca2+-free medium to 290 +/- 46 nM in medium containing 2.5 mM Ca2+. The effects of increasing extracellular Ca2+ in the presence of 40 mM K+ were similar but considerably more pronounced. Inclusion of either TPA or forskolin in the incubation medium had no significant effect on the steady-state cytosolic free Ca2+ levels in the absence of glucose but in the presence of 10 mM glucose forskolin caused modest (11-18%) increases in steady-state cytosolic free Ca2+ levels at extracellular Ca2+ concentrations of 0.25 mM or above. In contrast, in the presence of glucose TPA significantly reduced the steady-state levels of cytosolic free Ca2+ by 17-21% at extracellular Ca2+ concentrations of 0.25 mM or above. These data provide further evidence that insulin release mediated by activation of beta-cell protein kinases involves primarily an increase in sensitivity of the secretory system to intracellular Ca2+.

Protein phosphorylation in electrically permeabilized islets of Langerhans. Effects of Ca2+, cyclic AMP, a phorbol ester and noradrenaline

The incorporation of 32P from [gamma-32P]ATP into intracellular proteins was studied in electrically permeabilized rat islets of Langerhans. Ca2+ (10 microM), cyclic AMP (100 microM) and a protein kinase C-activating phorbol ester, phorbol 13-myristate 12-acetate (PMA; 100 nM) produced marked changes in the phosphorylation state of a number of proteins in permeabilized islets after incubation for 1 min at 37 degrees C. Ca2+ modified the effects of cyclic AMP and PMA on protein phosphorylation. Noradrenaline (10 microM) had no detectable effects on Ca2+-dependent protein phosphorylation, but significantly inhibited Ca2+-induced insulin secretion from electrically permeabilized islets. These results suggest that electrically permeabilized islets offer a useful model in which to study rapid events in protein phosphorylation as a mechanism of stimulus-secretion coupling. If the rapid Ca2+-induced effects on protein phosphorylation are involved in the control of insulin secretion, the results of this study also imply that part of the catecholamine inhibition of insulin secretion occurs at a stage in the secretory pathway beyond the activation of the regulated protein kinases.

Expression of the cytosolic and particulate forms of transglutaminase during chemically induced rat liver carcinogenesis

Transglutaminase (EC 2.3.2.13) activity in chemically induced rat hepatocellular carcinomas was reduced by some 65% when compared to normal rat livers. The majority of the remaining activity (approx. 85%) was found in the particulate fraction. The use of non-ionic detergent to extract the transglutaminase activity present in both normal and tumour tissue followed by its separation on a Mono-Q column revealed two distinct peaks of activity. These peaks of activity were equivalent to those previously identified as a membrane-bound transglutaminase and the more characteristic cytosolic or tissue transglutaminase. The ratio of the activity of the cytosolic enzyme to that of the membrane-bound enzyme in normal liver was calculated as 5:1. In hepatocellular carcinomas, this ratio was reduced to 0.4:1. No significant change in the activity of the membrane-bound enzyme was detectable in tumour tissue. Comparison of the cytosolic enzyme found in hepatocellular carcinomas with that found in normal liver indicated no change in its molecular weight, Km,app for putrescine incorporation into N,N'-dimethylcasein and sensitivity to activation by Ca2+. These observations suggest that the reduction in transglutaminase activity observed in the hepatocellular carcinoma is due to a selective reduction in the expression of the cytosolic transglutaminase.

Transglutaminase-catalysed cross-linking of proteins phosphorylated in the intact glucose-stimulated pancreatic beta-cell

Incubation of intact islets in the presence of [32P]Pi and stimulatory levels of glucose followed by separation of phosphorylated islet proteins by SDS-polyacrylamide gel electrophoresis revealed the presence of a high molecular weight phosphopolymer which did not transverse a 3% (w/v) acrylamide gel. The majority of this phosphopolymer (approx. 70%) was present in the 600 x g sedimented fraction of islet homogenates. Islet homogenates obtained from intact islets previously incubated with [32P]Pi and stimulatory levels of glucose when incubated under conditions that activated the islet transglutaminase resulted in an increase in the amount of phosphopolymer present in the 600 x g sedimented fraction. Inhibitors of transglutaminase activity which are known to inhibit glucose-stimulated insulin release led to a significant reduction in the fraction of phosphopolymer present in the glucose-stimulated intact islet. These findings suggest that protein cross-linking and phosphorylation reactions may be closely linked in the pancreatic beta-cell.

Effects of a phorbol ester and clomiphene on protein phosphorylation and insulin secretion in rat pancreatic islets

The potentiation of glucose-stimulated insulin release induced by 100 nM-12-O-tetradecanoylphorbol 13-acetate (TPA) was inhibited by clomiphene, an inhibitor of protein kinase C (PK C), in a dose-dependent manner. Clomiphene at concentrations up to 50 microM had a modest inhibitory action (27%) on insulin release stimulated by 10 mM-glucose alone, but had no effect on the potentiation of insulin release induced by forskolin. Islet PK C activity, associated with a particulate fraction, was stimulated maximally by 100 nM-TPA. This stimulation was blocked by clomiphene in a dose-dependent manner, with 50% inhibition at 30 microM. Incubation of intact islets with TPA after preincubation with [32P]Pi and 10 mM-glucose to label intracellular ATP resulted primarily in enhanced phosphorylation of a 37 kDa protein (mean value, +/- S.E.M., 36,700 +/- 600 Da; n = 7). This increased phosphorylation was blocked by the simultaneous inclusion of clomiphene. Subcellular fractionation revealed the presence of the 37 kDa phosphoprotein in a 24,000 g particulate fraction of islet homogenates. Neither clomiphene nor TPA affected the rate of glucose oxidation by islets. These results show that the phosphorylation state of a 37 kDa membrane protein parallels the modulation of insulin release induced by TPA and clomiphene and support a role for PK C in the insulin-secretory mechanism.

Regulation and specificity of glucose-stimulated insulin gene expression in human islets of Langerhans

The insulin response of cultured human islets of Langerhans was measured at both mRNA and polypeptide levels in response to natural and pharmacological stimuli. We report a dosage dependent stimulation of both mRNA levels and insulin secretion by extracellular glucose, and present evidence that islet responsiveness can be divided into two temporal phases: an early response, apparently under post-transcriptional control, and a late phase in which insulin messenger accumulates. Although glucose effects in man are similar to rodents, there are important differences, especially with respect to modulation of glucose stimulation by activators of beta-cell protein kinases.

Distinct mechanisms for two amplification systems of insulin release

The mechanisms whereby activation of the cyclic AMP-dependent protein kinase A or the Ca2+-phospholipid-dependent protein kinase C amplifies insulin release were studied with mouse islets. Forskolin and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) were used to stimulate adenylate cyclase and protein kinase C respectively. The sulphonylurea tolbutamide was used to initiate insulin release in the presence of 3 mM-glucose. Tolbutamide alone inhibited 86Rb+ efflux, depolarized beta-cell membrane, triggered electrical activity, accelerated 45Ca2+ influx and efflux and stimulated insulin release. Forskolin alone only slightly inhibited 86Rb+ efflux, but markedly increased the effects of tolbutamide on electrical activity, 45Ca2+ influx and efflux, and insulin release. In the absence of Ca2+, only the inhibition of 86Rb+ efflux persisted. TPA (100 nM) alone slightly accelerated 45Ca2+ efflux and insulin release without affecting 45Ca2+ influx or beta-cell membrane potential. It increased the effects of tolbutamide on 45Ca2+ efflux and insulin release without changing 86Rb+ efflux, 45Ca2+ influx or electrical activity. Omission of extracellular Ca2+ suppressed all effects due to the combination of TPA and tolbutamide, but not those of TPA alone. Though ineffective alone, 10 nM-TPA amplified the releasing action of tolbutamide without affecting its ionic and electrical effects. In conclusion, the two amplification systems of insulin release involve at least partially distinct mechanisms. The cyclic AMP but not the protein kinase C system initiating signal (Ca2+ influx) triggered by the primary secretagogue.

Mechanisms of leucine- and theophylline-stimulated insulin biosynthesis in isolated rat pancreatic islets

To extend previous observations on the mechanisms of translational regulation of insulin biosynthesis [Welsh, Scherberg, Gilmore & Steiner (1986) Biochem. J. 235, 459-467], we have now compared the intracellular distributions of insulin mRNA after stimulation of insulin biosynthesis by glucose, leucine or theophylline. In comparison with low glucose (3.3 mM) only, the presence of 10 mM-leucine + 3.3 mM-glucose resulted in the transfer of insulin mRNA from the pool of the uninitiated mRNA to the free polysome/monosome fraction and an increase in the amount of insulin mRNA associated with the microsomal fraction. Islets exposed to 5 mM-theophylline + 3.3 mM-glucose also showed a decreased content of uninitiated insulin mRNA in the cytosol, but these islets showed no increase in insulin mRNA in the microsomal fraction. These results suggest that leucine, a nutrient stimulant of insulin biosynthesis, acts essentially by the same mechanisms as those of glucose, whereas theophylline acts only to stimulate initiation rates.

The glucose sensor in HIT cells is the glucose transporter

The nature of the rate-limiting step for glucose utilization by the clonal insulin-producing cell line HIT-T15 has been investigated. In contrast to the situation in islets of Langerhans, we find that the HIT cell glucose metabolism is limited by the rate of entry of glucose into the cell. This is evidenced by the low rate of sugar transport and by the marked reduction in the rate of glucose utilization elicited by inhibitors of the glucose transporter. As judged by competition with glucose, the HIT cell glucose transporter also transports mannose, 2-deoxyglucose and 3-O-methylglucose but not L-glucose or N-acetylglucosamine. The Km for glucose of the glucose transporter, measured as the concentration of glucose required for a half-maximal rate of glucose utilization, is 4.3 mM, similar to the concentration reported to give half-maximal insulin release. Glucose-stimulated insulin release from HIT cells is inhibited by phloretin or cytochalasin B but not by mannoheptulose. We conclude that the secretory responses of HIT cells are consistent with the substrate-site hypothesis, but that, in contrast to normal B-cells, the glucose sensor which confers concentration-dependence and specificity to sugar-stimulated insulin release, is the glucose transporter.

Identification of nuclear substrates of the cyclic AMP-dependent protein kinase in Dictyostelium discoideum

A search for nuclear substrates of the cAMP-dependent protein kinase (cAMP-d PK) of Dictyostelium discoideum led to the identification of several such proteins. Identification was based initially on increased phosphorylation of the proteins in nuclear extracts catalyzed by added cAMP-d PK. One protein of Mr 38,000 was phosphorylated also in intact nuclei and in vivo; the amount of phosphoprotein or the level of phosphorylation increased during development. Both the Mr 38,000 protein and another substrate of Mr 195,000 were found in the nuclei of prespore and prestalk cells. Phosphorylation of other potential substrates of the cAMP-d PK was either prespore or prestalk cell-specific.

Glucose regulates preproinsulin messenger RNA levels in a clonal cell line of simian virus 40-transformed B cells

In HIT-T15 cells grown in the absence of glucose, Northern blot analysis of total RNA revealed a major 0.5 kb preproinsulin (ppI) mRNA transcript which co-migrated with the mature transcript from a human insulinoma. In 4 h tissue cultures, glucose (2-20 mM) stimulated HIT cell ppI mRNA levels in a markedly dose-dependent manner. Glucose-stimulated ppI mRNA was (i) inhibited by actinomycin D, suggesting that regulation may be in part transcriptional, and (ii) potentiated by agents known to activate B cell protein kinases. HIT cells represent a unique model for investigating long term regulation of insulin gene expression and biosynthesis.

Restoration of the A cell response to glucose in isolated rat islets of Langerhans

This study investigated the modulatory effects of forskolin, phorbol 12-myristate 13-acetate (PMA) and arginine on pancreatic glucagon secretion in response to changes in glucose concentrations. Glucose, on its own (0, 5, 9 and 18 mM), did not modify glucagon secretion from A cell-rich isolated rat islets of Langerhans. In the presence of 20 microM forskolin, glucagon release was stimulated dose-dependently on lowering the external glucose concentration to 0 mM. Sensitivity to glucose was achieved in the presence of either PMA or arginine; both agents also significantly enhanced glucagon release at all glucose concentrations tested. The response of the B cells in these experiments were as expected from the available literature. These results indicate that the endogenous rate of glucagon secretion in the isolated islet preparation was minimal and was insensitive to glucose, sensitivity of the A cells to glucose could be restored by either arginine or agents which alter the concentration or activity of proposed cellular second messengers.

Polyamine-enhanced casein kinase II in mouse pancreatic islets

The occurrence of polyamine-stimulated protein kinase (casein kinase II) in cytosol of mouse pancreatic islets was investigated. Islet protein phosphorylation was enhanced by spermidine, spermine, lysine-rich histone and polylysine; the major endogenous substrates in the cytosol were three proteins of Mr 50,000, 55,000 and 100,000. Cadaverine and putrescine were without effects. A Mr 100,000 protein is a major substrate for Ca2+-calmodulin-dependent protein kinase, and Mr 50,000 and 55,000 proteins are substrates for cyclic adenosine 3',5'-cyclic monophosphate (AMP) dependent protein kinase in mouse islets. However, neither cyclic-AMP-dependent protein kinase inhibitor nor trifluoperazine inhibited polyamine-enhanced protein phosphorylation. Both basal and polyamine-enhanced protein phosphorylation patterns were identical when either [gamma-32P] adenosine 5'-triphosphate (ATP) or [gamma-32P] guanosine 5'-triphosphate (GTP) was used as phosphate donors, indicative of the presence of a polyamine-stimulated casein kinase II in pancreatic islets. It is suggested that polyamines and polyamine-enhanced casein kinase II activity may have an important role in regulation of protein phosphorylation in pancreatic islets.

Insulin secretory responses of a clonal cell line of simian virus 40-transformed B cells

We have evaluated the potential of the clonal insulin-secretory cell line HIT-T15 as a model system for investigating stimulus-secretion coupling in pancreatic B cells. In contrast to other cell lines, HIT cell insulin secretion was consistently stimulated 2- to 3-fold by D-glucose. The maximally effective concentration of glucose was 10 mmol/l; between 2 and 10 mmol/l glucose the increase in insulin release was paralleled by an increased rate of glucose oxidation. The main characteristics of glucose-stimulated insulin release by HIT cells were essentially similar to those of normal islets. Thus, the response was specific for metabolizable sugars (D-mannose and D-glyceraldehyde stimulated insulin release but L-glucose and D-galactose were ineffective); markedly dependent on extracellular Ca2+ concentration; potentiated by forskolin, glucagon, acetylcholine and 12-O-tetradecanoyl phorbol 13-acetate; inhibited by adrenaline or somatostatin; showed a biphasic pattern of release in perifusion experiments, with both phases being potentiated by forskolin. The secretory response of the HIT cells to amino acids was also similar to that of normal islets. Thus, L-leucine and its deamination product 2-ketoisocaproate were effective stimuli, whereas L-isoleucine and L-glutamine were ineffective. Insulin release from HIT cells could also be evoked by the sulphonylureas glibenclamide and tolbutamide and by an increase in concentration of extracellular K+ to 40 mmol/l. The content of cyclic AMP in HIT cells was increased modestly by glucose but not by an increase in extracellular K+. Forskolin elicited a 4-fold increase in cyclic AMP content. We conclude that HIT cells retain the essential features of the insulin secretory response of normal B cells and represent an important tool for further biochemical characterization of the secretory system.

Effects of dehydrouramil on protein phosphorylation and insulin secretion in rat islets of Langerhans

Dehydrouramil hydrate hydrochloride (DHU), a stable analogue of alloxan, inhibited the phosphorylation of an endogenous protein of Mr 53,000 catalysed by a Ca2+-calmodulin-dependent protein kinase in extracts of islets of Langerhans. The concentration of DHU required for 50% inhibition was 0.09 mM. DHU did not inhibit islet cyclic AMP-dependent protein kinase and caused only slight inhibition of Ca2+-phospholipid-dependent protein kinase. Inhibition of Ca2+-calmodulin-dependent protein kinase was neither prevented nor reversed by dithiothreitol. DHU did not affect the ability of calmodulin to activate cyclic AMP phosphodiesterase. In intact islets, pre-exposure to DHU impaired the insulin-secretory response to glucose and blocked the potentiatory effect on insulin secretion of forskolin, an activator of adenylate cyclase, and of tetradecanoylphorbol acetate (TPA), an activator of Ca2+-phospholipid-dependent protein kinase. The increase in islet cyclic AMP elicited by forskolin was not affected by DHU. The data are consistent with the hypothesis that protein phosphorylation catalysed by a Ca2+-calmodulin-dependent protein kinase may play a central role in the regulation of insulin secretion.