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

Zonation of Pancreatic Acinar Cells in Diabetic Mice

The islets of Langerhans are dynamic structures that can change in size, number of cells, and molecular function in response to physiological and pathological stress. Molecular cues originating from the surrounding "peri-islet" acinar cells that could facilitate this plasticity have not been explored. Here, we combine single-molecule transcript imaging in the intact pancreas and transcriptomics to identify spatial heterogeneity of acinar cell gene expression. We find that peri-islet acinar cells exhibit a distinct molecular signature in db/db diabetic mice that includes upregulation of trypsin family genes and elevated mTOR activity. This zonated expression program seems to be induced by CCK that is secreted from islet cells. Elevated peri-islet trypsin secretion could facilitate the islet expansion observed in this model via modulation of the islet capsule matrix components. Our study highlights a molecular axis of communication between the pancreatic exocrine and endocrine compartments that may be relevant to islet expansion.

Specific silencing of the REST target genes in insulin-secreting cells uncovers their participation in beta cell survival

The absence of the transcriptional repressor RE-1 Silencing Transcription Factor (REST) in insulin-secreting beta cells is a major cue for the specific expression of a large number of genes. These REST target genes were largely ascribed to a function of neurotransmission in a neuronal context, whereas their role in pancreatic beta cells has been poorly explored. To identify their functional significance, we have generated transgenic mice expressing REST in beta cells (RIP-REST mice), and previously discovered that REST target genes are essential to insulin exocytosis. Herein we characterized a novel line of RIP-REST mice featuring diabetes. In diabetic RIP-REST mice, high levels of REST were associated with postnatal beta cell apoptosis, which resulted in gradual beta cell loss and sustained hyperglycemia in adults. Moreover, adenoviral REST transduction in INS-1E cells led to increased cell death under control conditions, and sensitized cells to death induced by cytokines. Screening for REST target genes identified several anti-apoptotic genes bearing the binding motif RE-1 that were downregulated upon REST expression in INS-1E cells, including Gjd2, Mapk8ip1, Irs2, Ptprn, and Cdk5r2. Decreased levels of Cdk5r2 in beta cells of RIP-REST mice further confirmed that it is controlled by REST, in vivo. Using siRNA-mediated knock-down in INS-1E cells, we showed that Cdk5r2 protects beta cells against cytokines and palmitate-induced apoptosis. Together, these data document that a set of REST target genes, including Cdk5r2, is important for beta cell survival.

The Lou/C rat: a model of spontaneous food restriction associated with improved insulin sensitivity and decreased lipid storage in adipose tissue

The inbred Lou/C rat, originating from the Wistar strain, has been described as a model of resistance to diet-induced obesity, but little is known about its metabolism. Since this knowledge could provide some clues about the etiology of obesity/insulin resistance, this study aimed at characterizing glucose and lipid metabolism in Lou/C vs. Wistar rats. This was achieved by performing glucose and insulin tolerance tests, euglycemic hyperinsulinemic clamps, and characterization of intracellular insulin signaling in skeletal muscle. Substrate-induced insulin secretion was evaluated using perfused pancreas and isolated islets. Finally, body fat composition and the expression of various factors involved in lipid metabolism were determined. Body weight and caloric intake were lower in Lou/C than in Wistar rats, whereas food efficiency was similar. Improved glucose tolerance of Lou/C rats was not related to increased insulin output but was related to improved insulin sensitivity/responsiveness in the liver and in skeletal muscles. In the latter tissue, this was accompanied by improved insulin signaling, as suggested by higher activation of the insulin receptor and of the Akt/protein kinase B pathway. Fat deposition was markedly lower in Lou/C than in Wistar rats, especially in visceral adipose tissue. In the inguinal adipose depot, expression of uncoupling protein-1 was detected in Lou/C but not in Wistar rats, in keeping with a higher expression of peroxisome proliferator-activated receptor-gamma coactivator-1 in these animals. The Lou/C rat is a valuable model of spontaneous food restriction with associated improved insulin sensitivity. Independently from its reduced caloric intake, it also exhibits a preferential channeling of nutrients toward utilization rather than storage.

A new conditional mouse mutant reveals specific expression and functions of connexin36 in neurons and pancreatic beta-cells

Connexin36 (Cx36) is the main connexin isoform expressed in neurons of the central nervous system (CNS) and in pancreatic beta-cells, i.e. two types of excitable cells that share - in spite of their different origins - a number of common features. Previous studies on Cx36 deficient mice have documented that loss of Cx36 resulted in phenotypic abnormalities in both the CNS and the pancreas which, however, could not be attributed to specific cell types due to the general deletion nature of the animal model used. Attempts to address this limitation using cell type specific deletions generated by the Cre/loxP strategy have so far been complicated by the lack of Cx36 expression from the floxed allele. We have now generated a conditional Cx36 deficient mouse mutant in which the coding region of Cx36 is flanked by loxP sites, followed by a cyan fluorescent protein (CFP) reporter gene. Here we show that Cx36 was still expressed from the floxed allele in neurons and pancreatic beta-cells. In these cells, a 30-60% decrease of this protein, relative to the expression level of the wildtype allele, did not significantly perturb cell coupling. The deletion of Cx36 by ubiquitously and cell type specifically expressed Cre recombinases revealed that CFP functions as a reliable reporter for Cx36 expression in brain neurons and to some extent in retina neurons, but not in pancreas. Loss of Cx36 by Cre-mediated recombination was documented at transcript and protein levels. Cell type specific deletion of Cx36 in the endocrine pancreas revealed major alterations in the basal as well as the glucose-induced insulin secretion, hence specifically attributing to pancreatic Cx36 an important regulatory role in the control of beta-cell function. Cell type specific deletion of Cx36 in the CNS by suitable Cre recombinases should also help to elucidate the functional role of Cx36 in different neuronal subtypes.

Reg genes are CCK2 receptor targets in ElasCCK2 mice pancreas

We previously demonstrated that expression of the gastrin receptor, CCK2R, in pancreatic acini of transgenic ElasCCK2 mice induced alteration of acinar morphology and differentiation, increased sensitivity to a carcinogen and development of preneoplastic lesions and tumours. Reg proteins are suggested to be involved in pancreatic cancer and in regeneration of endocrine pancreas. Reg I gene is a known target of gastrin. We examined whether an expression of CCK2R in the pancreatic acini of ElasCCK2 mice is linked to induction of Reg proteins expression. We analyzed Reg expression by Western-blot and immunohistochemistry in pancreas from ElasCCK2 and control mice. Islet neogenesis, glucose homeostasis, insulin secretion and content were also evaluated. Reg I is exclusively produced in acini in ElasCCK2 and control mice. In tumoral pancreas, Reg I and Reg III proteins are expressed in duct-like cells in preneoplastic lesions or in the periphery of tumours and in adjacent acini. The expression of Reg III proteins is increased in ElasCCK2 pancreas before the development of preneoplastic lesions in a subpopulation of islet cells and in small islet-like cell clusters dispersed within the acinar tissue. Several criteria of an enhanced neogenesis are fulfilled in ElasCCK2 pancreas. Moreover, ElasCCK2 mice have an improved response to glucose load, an increased insulin secretion and a doubling of insulin content compared to control mice. We show that Reg proteins are targets of CCK2R activation and are induced during early steps of carcinogenesis in ElasCCK2 mice pancreas. Alterations of exocrine tissue homeostasis in ElasCCK2 pancreas concomitantly activate regenerative responses of the endocrine pancreas possibly linked to paracrine actions of Reg III proteins.

Implication of glutamate in the kinetics of insulin secretion in rat and mouse perfused pancreas

It is intriguing that the kinetics of glucose-stimulated insulin secretion from the in situ perfused pancreas differ between the rat and the mouse. Here we confirm that insulin release in the rat is clearly biphasic, whereas in the mouse glucose essentially elicits a transient monophasic insulin release. Glucose-derived glutamate has been suggested to participate in the full development of the secretory response. The present report shows that the expression of glutamate dehydrogenase is lower in mouse than in rat or human islets, paralleling the insulin secretion profile. Addition of glutamic acid dimethyl ester mainly enhances insulin release at an intermediate glucose concentration in the rat pancreas. In the mouse preparation, glutamic acid dimethyl ester induces a sustained secretory response, both at 7.0 and 16.7 mmol/l glucose. These results are compatible with a role for glucose-derived glutamate principally in the sustained phase of nutrient-stimulated insulin secretion.

Junctional communication of pancreatic beta cells contributes to the control of insulin secretion and glucose tolerance

Proper insulin secretion requires the coordinated functioning of the numerous beta cells that form pancreatic islets. This coordination depends on a network of communication mechanisms whereby beta cells interact with extracellular signals and adjacent cells via connexin channels. To assess whether connexin-dependent communication plays a role in vivo, we have developed transgenic mice in which connexin 32 (Cx32), one of the vertebrate connexins found in the pancreas, is expressed in beta cells. We show that the altered beta-cell coupling that results from this expression causes reduced insulin secretion in response to physiologically relevant concentrations of glucose and abnormal tolerance to the sugar. These alterations were observed in spite of normal numbers of islets, increased insulin content, and preserved secretory response to glucose by individual beta cells. Moreover, glucose-stimulated islets showed improved electrical synchronization of these cells and increased cytosolic levels of Ca(2+). The results show that connexins contribute to the control of beta cells in vivo and that their excess is detrimental for insulin secretion.

In vitro influences between pancreatic adenocarcinoma cells and pancreatic islets

BACKGROUND

Interactions have been found between exocrine pancreatic adenocarcinoma and islets of Langerhans. Growth of pancreatic adenocarcinoma cells can be regulated by islet hormones such as insulin and somatostatin. Conversely, dysfunction of endocrine pancreas frequently accompanies the exocrine malignancy. The mechanisms underlying these interactions have not been defined.

MATERIALS AND METHODS

Human pancreatic adenocarcinoma cells (HPAF cells) were cocultured with isolated rat pancreatic islets in two-compartment wells. HPAF cells and islets cultured in separate wells served as controls. In separate experiments, HPAF cells were incubated with two concentrations of exogenous insulin, including one reflecting the levels of insulin secretion seen in the coculture experiments.

RESULTS

Proliferation of HPAF cells was increased by about 50% following a 2- or 5-day incubation with pancreatic islets (P < 0.05). Coculture of HPAF cells and pancreatic islets was associated with a greater reduction in glucose concentrations (P < 0. 01) and an increase in lactate accumulation (P < 0.05) in the culture media. Insulin concentrations in the media were significantly decreased during the first 2-3 days of the coculture incubation (P < 0.05). In contrast, insulin secretion from control islets was not significantly decreased until the fifth day of the experiment. The growth of HPAF cells was stimulated by both concentrations of exogenous insulin (P < 0.05). The insulin-stimulated HPAF cells also showed an enhanced glucose consumption and lactate production (P < 0.05).

CONCLUSIONS

Pancreatic islets regulate both growth and glucose metabolism of adjacent exocrine cancer cells. beta-cell-derived insulin may be one of the factors inducing these effects. Insulin release from islet beta-cells is compromised in the presence of exocrine cancer cells.

Distribution and ultrastructure of the autonomic nerves in the mouse pancreas

Peripheral innervation of the mouse pancreas was studied by scanning and transmission electron microscopy, as well as by light microscopy (cholinesterase technique). Major nerve bundles usually ran with arteries in the connective tissue septa. They gave off delicate branches that formed plexuses around arteries and arterioles. When reaching the capillaries, nerve fibers left the arterioles and formed very loose networks in the interacinar spaces. The nerves accompanying the arteries also sent off branches toward the islets of Langerhans and formed a dense plexus around the islets. A few delicate nerve fibers were also present around the pancreatic ducts. Thus, the intrapancreatic nerves formed four plexuses: perivascular, periductal, periacinar and peri-insular. The plexuses were networks of unmyelinated nerve fibers consisting of axons with varicosities and Schwann cells. Intrapancreatic ganglia were found in the interlobular connective tissue; ganglia were often closely associated to islets of Langerhans. Our findings indicate that the "interstitial cells" described by light microscopists correspond to Schwann cells. Axons in the nerve plexuses contain transmitter vesicles and therefore represent an autonomic terminal apparatus. The rich innervation of arterioles and islets suggests that neural regulation of secretory function is mediated by control of pancreatic blood flow.

Suppressive role of the islet-acinar axis in the perfused rat pancreas

BACKGROUND

The stimulating effects of insulin on the exocrine pancreas are well known. The effects of other islet hormones, however, are controversial. The aim of the present study was to determine whether the islet-acinar axis, as a whole, is stimulatory or inhibitory. Because we have shown that retrograde perfusion reverses the islet-acinar directed microcirculation, retrograde perfusion was expected to remove the overall effects of islet hormones from the acinar tissue.

METHODS

Rat pancreata were perfused (7 mmol/L glucose plus 3 mmol/L mixed amino acids) either anterogradely or retrogradely. Pancreatic juice flow, protein output, and amylase output were measured.

RESULTS

When perfusion was switched from anterograde to retrograde, juice flow increased threefold without changes in protein and amylase output. When cerulein (10(-10) mol/L) was infused, retrograde protein and amylase responses were larger than anterograde responses (each, n = 7; 2.71 +/- 0.23 vs. 1.71 +/- 0.11 mg/40 minutes; 173 +/- 17 vs. 98 +/- 8 U/40 minutes; mean +/- SE; both, P < 0.01). Somatostatin-14 and rat pancreatic polypeptide (each, 10(-9) mol/L) reduced the retrograde protein and amylase responses, but not juice flow, to the anterograde response levels. Conversely, these peptides did not affect exocrine function during anterograde perfusion.

CONCLUSIONS

A suppressive role of the islet-acinar axis via endogenous somatostatin and/or pancreatic polypeptide is suggested.

Effect of alloxan diabetes on exocrine pancreatic secretion in the anesthetized rabbit

The influence of the endocrine pancreas on the exocrine pancreatic secretion of both electrolytes and enzymes was studied in rabbits made diabetic by alloxan administration. No alterations were observed in the flow of pancreatic juice. Bicarbonate concentrations were considerably increased, whereas chloride concentrations were clearly reduced in the alloxan-diabetic rabbits compared with the control animals. Insulin treatment restored anion levels to normal. Similar, although less pronounced changes were seen in the output values of bicarbonate and chloride. There were no significant differences between the control and alloxan-treated animals in the output of sodium and potassium; however, potassium concentrations exhibited a significant rise both in untreated and insulin-treated diabetic rabbits compared with the controls. Total protein and amylase secretion decreased markedly in the diabetic animals. The secretion of amylase was not brought back to control values by additional administration of insulin. These results suggest that the endocrine pancreas plays an extremely important role in exocrine pancreatic function.

Paradoxical effects of endogenous and exogenous insulin on amino acid transport activity in the isolated rat pancreas: somatostatin-14 inhibits insulin action

Regulatory effects of insulin, somatostatin and cholecystokinin on amino acid transport in the isolated perfused rat pancreas have been studied using a rapid dual isotope dilution technique. Uni-directional L-serine transport (15 s) was quantified relative to an extracellular tracer D-mannitol over a wide range of substrate concentrations. In pancreata perfused with 2.5 mmol/l D-glucose, a weighted nonlinear regression analysis of overall transport indicated an apparent Km = 14.4 +/- 1.6 mmol/l and Vmax = 25.9 +/- 1.4 mumol.min-1.g-1 (n = 6). Although L-serine transport was stimulated during perfusion with 100 microU/ml bovine insulin, endogenous insulin (7-25 ng.min-1.g-1) released during continuous perfusion with either 8.8 mmol/l or 16.8 mmol/l D-glucose had no such effect. Exogenous somatostatin-14 (250 pg/ml) or cholecystokinin octapeptide (CCK-8, 3 x 10(-11) mol/l) appeared to increase only the Km for transport. Only CCK-8 evoked a notable protein output (2.9 +/- 0.3 mg.30 min-1.g-1) and juice flow (68 +/- 10 microliters.30 min-1.g-1, n = 3) from the exocrine pancreas. When pancreata were perfused with bovine insulin (100 microU/ml) and somatostatin-14 (250 pg/ml), the stimulatory action of exogenous insulin on L-serine transport was abolished. If endogenous insulin and somatostatin, released concurrently in response to 16.8 mmol/l D-glucose, were conveyed to the exocrine epithelium via an islet-acinar portal axis, it is conceivable that somatostatin modulates the stimulatory action of insulin on basolateral amino acid transport in the exocrine pancreas.

Ionic dependence of amino-acid transport in the exocrine pancreatic epithelium: calcium dependence of insulin action

Rapid unidirectional transport (15 sec) of L-serine and 2-methylaminoisobutyric acid (MeAIB) was studied in the isolated perfused rat pancreas using a dual-tracer dilution technique. Time-course experiments in the presence of normal cation gradients revealed a time-dependent transstimulation of L-serine influx and transinhibition of MeAIB influx. Transport of the model nonmetabolized System A analog MeAIB was Na+ dependent and significantly inhibited during perfusion with 1 mM ouabain. Although transport of L-serine was largely Na+ independent, ouabain caused a time-dependent inhibition of transport. Influx of both amino acids appeared to be inhibited by the ionophore monensin but unaffected by a lowered extracellular potassium concentration. Removal of extracellular calcium had no effect on influx of the natural substrate L-serine, whereas stimulation of transport by exogenous insulin (100 microU/ml) was entirely dependent upon extracellular calcium and unaffected by ouabain. Paradoxically, exogenous insulin had no effect on the time-course of MeAIB influx. The characteristics of L-serine influx described in earlier studies together with our present findings suggest that insulin may modulate the activity of System asc in the exocrine pancreatic epithelium by a calcium-dependent mechanism.

Insulin resistance is accompanied by impairment of amylase-gene expression in the exocrine pancreas of the obese Zucker rat

Insulin plays a major role in the control of pancreatic amylase biosynthesis. In this study we determined glucose metabolism by pancreatic acini as well as the pancreatic content of both amylase protein and amylase mRNA during development of insulin resistance in the obese Zucker rat. At age 4 weeks there were no abnormalities detected in the above parameters, although the obese animals were already hyperinsulinaemic. At 6 weeks glucose metabolism was decreased by 50% in acini from obese rats, whereas pancreatic amylase-gene expression was only slightly impaired. At 22 weeks glucose metabolism was decreased by 50%, amylase content by 55% and amylase mRNA by 60% in acinar tissue of obese rats. As expected, hyperinsulinaemia increased markedly with age. Thus development of severe insulin resistance was associated with impairment of amylase-gene expression. To decrease insulin resistance, one group of adult obese rats was treated with Ciglitazone for 4 weeks. A lowered plasma insulin concentration without alteration of food intake was taken as evidence of decreased insulin resistance. This was associated with normalization of glucose metabolism and a marked increase of both amylase content of pancreatic tissue and amylase mRNA. In conclusion, both the increase of insulin resistance with age and its partial reversal by Ciglitazone treatment appear to modulate pancreatic amylase-gene expression in the obese Zucker rat.