Basolateral sorting signals differ in their ability to redirect apical proteins to the basolateral cell surface.

Polarized sorting of membrane proteins in epithelial cells is mediated by cytoplasmic basolateral signals or by apical signals in the transmembrane or exoplasmic domains. Basolateral signals were generally found to be dominant over apical determinants. We have generated chimeric proteins with the cytoplasmic domain of either the asialoglycoprotein receptor H1 or the transferrin receptor, two basolateral proteins, fused to the transmembrane and exoplasmic segments of aminopeptidase N, an apical protein, and analyzed them in Madin-Darby canine kidney cells. Whereas both cytoplasmic sequences induced endocytosis of the chimeras, only that of the transferrin receptor mediated basolateral expression in steady state. The H1 fusion protein, although still largely sorted to the basolateral side in biosynthetic surface transport, was subsequently resorted to the apical cell surface. We tested whether the difference in sorting between trimeric wild-type H1 and the dimeric aminopeptidase chimera was caused by the number of sorting signals presented in the oligomers. Consistent with this hypothesis, the H1 signal was fully functional in a tetrameric fusion protein with the transmembrane and exoplasmic domains of influenza neuraminidase. The results suggest that basolateral signals per se need not be dominant over apical determinants for steady-state polarity and emphasize an important contribution of the valence of signals in polarized sorting.

Mitochondrial import of the long and short isoforms of human uncoupling protein 3.

Human uncoupling protein 3 (UCP3) has two RNA transcripts that arise from the differential processing of the same gene product. One encodes the full length protein (UCP3L) while the other encodes a truncated version (UCP3S) lacking the sixth membrane spanning domain. The roles of the two isoforms are not known, but a mutation that decreases the proportion of UCP3L decreases fat oxidation and increases susceptibility to obesity. In the ADP/ATP carrier, a protein closely related to UCP3, the sixth membrane spanning domain is required for insertion into the inner membrane. Therefore, defective membrane insertion of UCP3S may account for the different effects of the two isoforms in vivo. We investigated mitochondrial import of the two UCP3 isoforms. When epitope-tagged versions of UCP3S and UCP3L were expressed in COS7 cells, both were inserted into the mitochondrial inner membrane. Translation in vitro followed by incubation with isolated mitochondria showed that both isoforms were inserted into the inner membrane, however, the insertion of UCP3S was significantly slower.

Repression by Ikaros and Aiolos is mediated through histone deacetylase complexes.

Here we show that the lymphoid lineage-determining factors Ikaros and Aiolos can function as strong transcriptional repressors. This function is mediated through two repression domains and is dependent upon the promoter context and cell type. Repression by Ikaros proteins correlates with hypo-acetylation of core histones at promoter sites and is relieved by histone deacetylase inhibitors. Consistent with these findings, Ikaros and its repression domains can interact in vivo and in vitro with the mSin3 family of co-repressors which bind to histone deacetylases. Based on these and our recent findings of associations between Ikaros and Mi-2-HDAC, we propose that Ikaros family members modulate gene expression during lymphocyte development by recruiting distinct histone deacetylase complexes to specific promoters.

Human deafness dystonia syndrome is a mitochondrial disease.

The human deafness dystonia syndrome results from the mutation of a protein (DDP) of unknown function. We show now that DDP is a mitochondrial protein and similar to five small proteins (Tim8p, Tim9p, Tim10p, Tim12p, and Tim13p) of the yeast mitochondrial intermembrane space. Tim9p, Tim10p, and Tim12p mediate the import of metabolite transporters from the cytoplasm into the mitochondrial inner membrane and interact structurally and functionally with Tim8p and Tim13p. DDP is most similar to Tim8p. Tim8p exists as a soluble 70-kDa complex with Tim13p and Tim9p, and deletion of Tim8p is synthetically lethal with a conditional mutation in Tim10p. The deafness dystonia syndrome thus is a novel type of mitochondrial disease that probably is caused by a defective mitochondrial protein-import system.

Aiolos regulates B cell activation and maturation to effector state.

Aiolos encodes a zinc finger DNA-binding protein that is highly expressed in mature B cells and is homologous to Ikaros. In the periphery of mice homozygous for an Aiolos-null mutation, B cells exhibit an activated cell surface phenotype and undergo augmented antigen receptor (BCR)-mediated in vitro proliferative responses, even at limiting amounts of stimulant. In vivo, T cell-dependent B cell responses, including the formation of germinal centers and elevated serum IgG and IgE, are detected in Aiolos-deficient mice in the absence of immunization. Auto-antibodies and development of B cell lymphomas are frequently seen among aging Aiolos mutants. In sharp contrast to conventional B cells, B cells of the peritoneum, of the marginal zone, and the recirculating bone marrow population are greatly reduced.

Gangliosides in vivo reduce diabetes incidence in non-obese diabetic mice.

Non-obese diabetic mice were treated daily with a mixture of gangliosides from day 30 until day 250 of life or until the mice became diabetic. Ganglioside treatment reduced diabetes incidence from 80-90% to 47% and from 20-30% to zero in female and male mice respectively. Gangliosides did not affect the frequency of perivasculitis. It is concluded that gangliosides can reduce diabetes incidence in non-obese diabetic mice.

Effect of helper and/or cytotoxic T-lymphocyte depletion on low-dose streptozocin-induced diabetes in C57BL/6J mice.

The low-dose streptozocin (STZ) model of diabetes has been reported to involve direct STZ beta-cytotoxicity and/or immunologically mediated beta-cell destruction. Because the T-lymphocyte dependency of such a model is controversial, we further assessed the role of T-lymphocytes by determining the occurrence and magnitude of hyperglycemia as well as the pancreatic insulin contents in both STZ-injected nude C57BL/6J male mice and STZ-injected euthymic C57BL/6J male mice selectively depleted in helper and/or cytotoxic T-lymphocytes with monoclonal antibodies (MoAbs). The effectiveness of MoAb treatment was assessed in lymph node cells by flow-microfluorometry analysis and in spleen cells by concanavalin A stimulation, allospecific cytotoxic T-lymphocyte activity, and T-lymphocyte lymphokine production. Sixteen days after the first STZ injection, hyperglycemia (plasma glucose greater than 200 mg/dl) occurred in significantly fewer helper T-lymphocyte-depleted mice (P less than .005) or helper and cytotoxic T-lymphocyte-depleted mice (P less than .001) than in non-MoAb-treated mice. However, a progressive increase in the number of mice with hyperglycemia ensued in all MoAb-treated groups, and 2 mo after STZ was administered, the prevalence of hyperglycemia, mean plasma glucose levels, and pancreatic insulin contents did not differ significantly from the values obtained in the non-MoAb-treated animals. Similarly, STZ-injected C57BL/6J male nude mice developed hyperglycemia that was associated with a marked decrease in pancreatic insulin contents on a time course comparable with that of STZ-injected euthymic C57BL/6J male mice depleted in helper or in helper and cytotoxic T-lymphocytes by MoAbs.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation by insulin of glucokinase gene transcription in liver of diabetic rats.

The purpose of this work was to investigate the molecular mechanism responsible for the induction of hepatic glucokinase in diabetic rats acutely treated with insulin. Experimental diabetes was provoked by injection of streptozotocin 8-10 days before the experiments. Regular insulin was given by three intraperitoneal injections at 8-h intervals, and the time course of glucokinase induction was followed over a time period of 24 h. The amount of glucokinase in liver was estimated by Western blotting of total cytosol protein with affinity-purified antibodies, as well as by conventional enzyme activity assay. Both measurements showed that glucokinase was reduced by more than 90% in the livers of diabetic rats as compared to normal controls. Following insulin administration, the amount (and activity) of glucokinase increased in a time-dependent fashion, after an initial lag of 4 h, to reach 65% of the nondiabetic control level 24 h after the initial dose of insulin. Northern blot analysis with a cloned cDNA probe was used to quantitate glucokinase mRNA. In contrast with the slow onset of enzyme accumulation, the amount of glucokinase mRNA was shown to be increased dramatically as early as 1 h after insulin administration. The abundance of specific mRNA increased until 8 h after the initial dose of insulin. Subsequently, the level of the mRNA decayed rapidly so that little message was left after 16 h and virtually none after 24 h. Run-on transcription experiments with isolated nuclei showed that the rate of transcription of the glucokinase gene was increased about 20-fold within 45 min of insulin administration and returned to the prestimulation level after 8 h. From these data, it was concluded that the induction of glucokinase resulted primarily from a burst in the transcriptional activity of the gene, leading to a short-term accumulation of glucokinase mRNA. The more sustained elevation of the enzyme level can be accounted for by the long half-life of the enzyme (greater than 30 h). The virtually immediate activation of glucokinase gene transcription suggests a direct effect of insulin on the liver cell.

Differences in pancreatic enzyme release from ventral and dorsal areas of the rat pancreas.

The exocrine secretory function of the ventral and dorsal areas of the rat pancreas was investigated separately. In the isolated perfused rat pancreas, caerulein (10(-12), 10(-11), and 10(-10) M) and carbachol (10(-7) and 10(-6) M) caused a greaterfold increase in pancreatic secretion from the dorsal than the ventral area. Caerulein (10(-9) M) and carbachol (10(-5) M) were supramaximal concentrations with respect to secretion from the dorsal area, but this was not the case in the ventral area. Similar findings were also observed when ventral and dorsal secretions were studied in vivo in anesthetized animals. By contrast, in dispersed acini the sensitivity and responsiveness of tissue from the dorsal and ventral areas were similar to both secretagogues. The possible influence of the endocrine pancreas via the insuloacinar portal system was studied in the perfused pancreas of diabetic rats treated with insulin in vivo. Carbachol (10(-6) M)-stimulated secretion from the ventral area was similar (except for amylase) to that of controls, while a significantly reduced (P less than 0.001) output was measured from the dorsal pancreas. These results demonstrate that in the intact pancreas differences of exocrine secretion exist between the ventral and dorsal areas and suggest that insulin may contribute, at least in part, to the expression of such differences.

Tris(hydroxymethyl)aminomethane inhibits the synthesis and processing of proinsulin in isolated rat pancreatic islets without affecting release of insulin stores.

Isolated rat islets of Langerhans were pulse-labeled (5 min, [3H]leucine) and then exposed to 10 or 50 mM tris(hydroxymethyl)aminomethane (Tris) at pH 7.4 during an 85-min chase period. There was a dose-related inhibition of the conversion of labeled proinsulin to insulin by Tris. At 50 mM, Tris also inhibited the release of newly synthesized (labeled) proinsulin and insulin. These inhibitory effects of Tris were almost absent if the islets were exposed to 50 mM Tris during only the last 60 min of the 85-min chase period. Both proinsulin and total islet protein synthesis (as indexed by incorporation of [3H]leucine) were inhibited acutely by 50 mM Tris (5-min exposure); after 85 min of exposure to 50 mM Tris, the inhibition of proinsulin biosynthesis was more marked than that of total islet protein. In contrast to its effects on newly synthesized products, 50 mM Tris failed to inhibit the release of immunoreactive insulin during an 85-min incubation. However, when islets were exposed to 50 mM Tris for a longer period, a partial inhibition of immunoreactive insulin release was observed as from 120 min. Insulin released from islets consists of a mixture of older stored material and of newly synthesized products, the latter being released preferentially. These results are consistent with a selective effect of 50 mM Tris on the production of newly synthesized insulin. During the first 120 min of exposure to Tris, islet reserves of newly synthesized products will be depleted thereby leading to a new, reduced, rate of release of immunoreactive material consisting only of older insulin stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-specific expression of glucokinase: identification of the gene product in liver and pancreatic islets.

The tissue distribution of glucokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) was examined by protein blotting analysis. Antibodies raised against rat liver glucokinase recognized a single protein subunit with an apparent Mr of 56,500 on nitrocellulose blots of cytosol protein from liver, separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A protein of identical electrophoretic mobility was detected by immunoblotting of cytosol protein from pancreatic islets. Hepatic glucokinase and the immunoreactive islet product bound to and were eluted from DEAE-cellulose at the same ionic strength. Glucokinase was displayed as a set of two spots with apparent pI values of 5.54 and 5.64 by immunoblotting after two-dimensional gel electrophoresis. The two isoforms appeared equally abundant in liver extract, whereas the component with a pI of 5.64 was predominant in islets. By quantitative immunoblotting, glucokinase was estimated to represent 0.1% of total cytosol protein in liver and 1/20th as much in islets. The glucokinase activity of both liver and islet cytosols was suppressed by the antibodies to hepatic glucokinase. Immunoblotting of cytosol protein from intestinal mucosa, exocrine pancreas, epididymal adipose tissue, kidney, brain, and spleen failed to reveal the glucokinase protein. Thus, significant expression of the glucokinase gene appears restricted to the liver and pancreatic islets.

Calmodulin in various insulin secreting cell types.

The involvement of the Ca2+ binding protein, calmodulin, in the regulation of insulin release was studied. Calmodulin was measured in isolated rat islets, rat insulinoma cells, the insulin secreting cell line (RINm5F) and in islets isolated from normal and diabetic Chinese hamsters. Total content of calmodulin was determined by a radioimmunoassay using a rabbit anti-calmodulin serum and was found to lie in the range of 4 to 7 micrograms/ml protein. When rat islets were maintained in tissue culture for 6 days at 2.8 or 8.3 mM glucose, the content of calmodulin of the two groups was similar. Likewise there was no difference in calmodulin content between islets from normal and diabetic hamsters. This study suggests that a variation of the total cellular calmodulin does not play a role in the process of insulin secretion.

Activity of the insulo-acinar axis in the isolated perfused rat pancreas.

The object of the present investigation was to determine whether insulin secreted by the endocrine pancreas and carried in the insulo-acinar portal system has a direct effect on pancreatic enzyme secretion. For this purpose, the isolated rat pancreas was perfused in a nonrecirculating system. The perfusate contained 3 mM glucose, and either caerulein or vaso-active intestinal polypeptide was used to stimulate exocrine secretion. The amount of insulin reaching the exocrine pancreas was reduced by two different experimental procedures. In the first, use was made of streptozotocin-diabetic rats treated with insulin in vivo. Treatment was such that the contents of amylase and lipase, vastly altered in the untreated diabetic state, were normalized before the perfusion studies. In the second procedure, insulin reaching the exocrine pancreas was reduced by antiinsulin serum in the perfusate. In these procedures, the reduced insulin bioavailability was associated with a reduction in caerulein- and vasoactive intestinal polypeptide-stimulated enzyme release, which was shown as a reduction of maximum responsiveness to caerulein without alteration of sensitivity. By contrast, in dispersed pancreatic acini where the insulo-acinar axis was completely disrupted, amylase secretion from diabetic and nondiabetic tissue was identical over a wide range of caerulein concentrations, showing that the secretory defect seen in the perfusion studies was not inherent to the exocrine tissue. The results show that basal insulin secretion has a direct effect on pancreatic enzyme output and that the insulo-acinar axis may play an important role in the regulation of acinar cell function.

Sulfonylureas in insulin-dependent (type I) diabetes: evidence for an extrapancreatic effect in vivo.

The effect of glibenclamide treatment on insulin-mediated glucose disposal was studied in eight C-peptide-negative type I diabetic patients. The patients were studied twice by the euglycemic insulin clamp technique. One of the two experiments was preceded by glibenclamide treatment at the dose of 5 mg, three times daily for 15 days; half of the patients had the first test before and the second test after sulfonylurea treatment, and vice versa. Insulin was infused for four periods of 2 h each sequentially at 0.5, 1.0, 2.0, and 5.0 mU kg-1 min-1; for each insulin infusion period, the steady state plasma free insulin levels were comparable with or without glibenclamide. The mean +/- SEM plasma glucose concentration was 88 +/- 2 mg/dl in both experiments. The insulin-mediated glucose disposal rate was greater with glibenclamide during the first insulin infusion period (which generated plasma free insulin levels within the physiological range) 2.68 +/- 0.32 mg kg-1 min-1 with glibenclamide vs. 1.97 +/- 0.20 mg kg-1 min-1 without glibenclamide (P less than 0.005). However, glucose disposal rates did not differ in the diabetic patients with or without glibenclamide treatment during the second, third, and fourth insulin infusion periods, which generated plasma free insulin levels in the supraphysiological range. These results provide evidence for an extrapancreatic effect of glibenclamide at low insulin concentrations during euglycemic clamping in patients with insulin-dependent diabetes mellitus. However, this effect was not reflected clinically in either an increased rate of hypoglycemic reactions or decreased insulin needs during the short term period of treatment.

Glucose-insulin interactions on exocrine secretion from the perfused rat pancreas.

The effects of glucose and insulin on pancreatic enzyme release have been investigated using the isolated perfused rat pancreas. Basal and caerulein-stimulated secretion was significantly less in the presence of 15 mM glucose than with 5 mM glucose, except at a supramaximal concentration of caerulein (10(-9) M) where secretion was similar in both groups. Addition of exogenous insulin also caused a reduction in enzyme secretion, but the time of onset of the inhibitory action was delayed compared to that observed with glucose. Furthermore, it was found that the effects of 15 mM glucose and exogenous insulin were not additive at the concentration used in these experiments, and that the inhibitory action of insulin was glucose-dependent. Such glucose-insulin interactions must play an important role in the modulation of pancreatic enzyme secretion.

Increased insulin responsiveness in vivo and in vitro consequent to induced hyperinsulinemia in the rat.

The effect of constant, controlled hyperinsulinemia on in vivo and in vitro insulin responsiveness has been investigated in rats that have received insulin infusions through chronically implanted jugular vein catheters. Constant rates of insulin infusion during days 1-4 resulted in stable plasma insulin concentrations. The plasma glucose initially fell from 122 +/- 3 to 53 +/- 4 mg/dl. While the infusion rate was maintained constant, the plasma glucose continued to fall over the subsequent days so that on day 4 the plasma glucose, 40 +/- 2 mg/dl, was significantly lower than that in the same animals on day 1 (P less than 0.02). Subsequently, the rate of insulin infusion was decreased to maintain the plasma glucose level in the 35-40 mg/dl range. Plasma catecholamine levels were high in insulin-infused rats. On the eighth day an in vivo insulin tolerance test (0.5 U/kg) was performed. Insulin-infused rats responded with a hypoglycemia that was both more pronounced and longer sustained than in saline-infused controls. Insulin responsiveness in vitro has been measured in isolated adipocytes. Adipocytes from epididymal fat pads were of similar size in the two groups of animals. Glucose uptake by adipocytes from insulin-infused rats was similar to that of controls under basal (zero insulin) conditions, but showed an increase in the maximum response to insulin. Glucose incorporation into total lipid and fatty acid was greater in adipocytes from insulin-infused rats than in controls under both basal (zero insulin) and insulin-stimulated conditions. Activities of the lipogenic enzymes acetyl CoA carboxylase and fatty acid synthetase were markedly increased in epididymal fat pads of insulin-infused rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Dispersed adult rat pancreatic islet cells in culture: A, B, and D cell function.

The availability of suitably characterized dispersed islet cell preparations may assist in studies of islet function. Since freshly dispersed adult rat islet cells failed to respond appropriately to secretagogues (no alteration in insulin, glucagon, or somatostatin release after glucose change; modest response to IBMX), these cells were established in primary monolayer culture. We then tested the hypothesis that islet function is at least partially determined by islet structure. B cells which had attached to Petri dishes during a culture period of four days were well preserved at the ultrastructural level, with mitochondria clustered at the cell face attached to the Petri dish and secretory granules concentrated towards the portion of the cell facing the medium. Since it was not possible to estimate cellular hormone content or hormone release as a function of the number of specific types of cells, fractional rates of release and hormone content ratios were compared with those for intact islets maintained in culture in parallel. Whereas the ratio of somatostatin:insulin content was similar for islets and cells (approximately 0.7:100), the dispersed cell population appeared depleted in glucagon (glucagon:insulin ratios being 17:100 for islets and 4:100 for cells) reflecting either degranulation or relative loss of A cells. In contrast to the lack of responsiveness seen with freshly dispersed islet cells, the cultured cells released insulin in response to glucose and glucose plus IBMX in a fashion comparable to that seen with cultured islets. Proinsulin biosynthesis (incorporation of [3H] leucine) was higher in cultured cells than islets. Somatostatin release was lower from dispersed cells than from islets while the opposite was true for glucagon.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose-induced somatostatin secretion by the rat pancreas is not potentiated by glucose.

The effect of previous exposure to glucose on subsequent glucose-stimulated insulin and somatostatin secretion has been investigated using the isolated perfused rat pancreas. As expected, when the pancreases of non-diabetic rats were exposed to 16.7 mM glucose on two occasions, 20 min apart, insulin secretion during the second period of exposure to high glucose was greater than that during the first period. By contrast, there was no potentiation of somatostatin secretion during the second glucose stimulation with respect to that of the first. Indeed, when the basal glucose concentration was low (1.4 or 2.8 mM) somatostatin secretion during the second glucose stimulation was lower than that during the first. Since exogenous insulin is known to inhibit glucose-induced somatostatin secretion, it seemed possible that lack of visible potentiation of glucose-induced somatostatin secretion by glucose could have been due to partial D cell inhibition by simultaneously augmented insulin secretion during the second glucose stimulation. In an attempt to exclude such an interaction between B and D cells, somatostatin secretion was also studied in the pancreases of spontaneously diabetic, Wistar (BB) rats (these animals are insulin deficient and are maintained by daily injections of insulin). However, even though insulin secretion was not detectable from these pancreases, glucose potentiation of glucose-induced somatostatin secretion did not occur. Although the pancreatic B and D cells are known to respond in a similar manner to many secretagogues the present results show that glucose potentiation of glucose-stimulated somatostatin secretion is not found under circumstances where potentiation of insulin secretion does occur. In addition, the absence of potentiated somatostatin secretion could not be attributed to partial inhibition of the D cell by insulin.