Pancreatic islet development

Yield, cell composition, and function of islets isolated from different ages of neonatal pigs

The yield, cell composition, and function of islets isolated from various ages of neonatal pigs were characterized using in vitro and in vivo experimental models. Islets from 7- and 10-day-old pigs showed significantly better function both in vitro and in vivo compared to islets from 3- and 5-day-old pigs however, the islet yield from 10-day-old pigs were significantly less than those obtained from the other pigs. Since islets from 3-day-old pigs were used in our previous studies and islets from 7-day-old pigs reversed diabetes more efficiently than islets from other groups, we further evaluated the function of these islets post-transplantation. B6 rag-/- mouse recipients of various numbers of islets from 7-day-old pigs achieved normoglycemia faster and showed significantly improved response to glucose challenge compared to the recipients of the same numbers of islets from 3-day-old pigs. These results are in line with the findings that islets from 7-day-old pigs showed reduced voltage-dependent K⁺ (Kv) channel activity and their ability to recover from post-hypoxia/reoxygenation stress. Despite more resident immune cells and immunogenic characteristics detected in islets from 7-day-old pigs compared to islets from 3-day-old pigs, the combination of anti-LFA-1 and anti-CD154 monoclonal antibodies are equally effective at preventing the rejection of islets from both age groups of pigs. Collectively, these results suggest that islets from various ages of neonatal pigs vary in yield, cellular composition, and function. Such parameters may be considered when defining the optimal pancreas donor for islet xenotransplantation studies.

Insulin-producing cells derived from human embryonic stem cells: comparison of definitive endoderm- and nestin-positive progenitor-based differentiation strategies

Human embryonic stem cells (hESCs) are pluripotent and capable of undergoing multilineage differentiation into highly specialized cells including pancreatic islet cells. Thus, they represent a novel alternative source for targeted therapies and regenerative medicine for diabetes. Significant progress has been made in differentiating hESCs toward pancreatic lineages. One approach is based on the similarities of pancreatic β cell and neuroepithelial development. Nestin-positive cells are selected as pancreatic β cell precursors and further differentiated to secrete insulin. The other approach is based on our knowledge of developmental biology in which the differentiation protocol sequentially reproduces the individual steps that are known in normal β cell ontogenesis during fetal pancreatic development. In the present study, the hESC cell line PKU1.1 was induced to differentiate into insulin-producing cells (IPCs) using both protocols. The differentiation process was dynamically investigated and the similarities and differences between both strategies were explored. Our results show that IPCs can be successfully induced with both differentiation strategies. The resulting IPCs from both protocols shared many similar features with pancreatic islet cells, but not mature, functional β cells. However, these differently-derived IPC cell types displayed specific morphologies and different expression levels of pancreatic islet development-related markers. These data not only broaden our outlook on hESC differentiation into IPCs, but also extend the full potential of these processes for regenerative medicine in diabetes.

Long-range enhancers are required to maintain expression of the autoantigen islet-specific glucose-6-phosphatase catalytic subunit-related protein in adult mouse islets in vivo

OBJECTIVE

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is selectively expressed in islet beta-cells and is a major autoantigen in both mouse and human type 1 diabetes. This study describes the use of a combination of transgenic and transfection approaches to characterize the gene regions that confer the islet-specific expression of IGRP.

RESEARCH DESIGN AND METHODS

Transgenic mice were generated containing the IGRP promoter sequence from -306, -911, or -3911 to +3 ligated to a LacZ reporter gene. Transgene expression was monitored by 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside staining of pancreatic tissue.

RESULTS

In all the transgenic mice, robust LacZ expression was detected in newborn mouse islets, but expression became mosaic as animals aged, suggesting that additional elements are required for the maintenance of IGRP gene expression. VISTA analyses identified two conserved regions in the distal IGRP promoter and one in the third intron. Transfection experiments demonstrated that all three regions confer enhanced luciferase reporter gene expression in beta TC-3 cells when ligated to a minimal IGRP promoter. A transgene containing all three conserved regions was generated by using a bacterial recombination strategy to insert a LacZ cassette into exon 5 of the IGRP gene. Transgenic mice containing a 15-kbp fragment of the IGRP gene were then generated. This transgene conferred LacZ expression in newborn mouse islets; however, expression was still suppressed as animals aged.

CONCLUSIONS

The data suggest that long-range enhancers 5' or 3' of the IGRP gene are required for the maintenance of IGRP gene expression in adult mice.

Autonomic involvement in the permanent metabolic programming of hyperinsulinemia in the high-carbohydrate rat model

Exposure to a high-carbohydrate (HC) milk formula during the suckling period results in permanent metabolic programming of hyperinsulinemia in HC rats. Previous studies have shown that hyperinsulinemia in HC rats involves a programmed hyperresponsiveness to glucose. However, the immediate onset and persistence of enhanced insulin secretion throughout life suggests a role for numerous factors that control insulin secretion. Present in vivo and in vitro studies have shown a role for altered autonomic activity, including increased parasympathetic and decreased sympathetic activities, in the maintenance of hyperinsulinemia in 100-day-old HC rats. HC rats were shown to be more sensitive to cholinergic-induced potentiation of glucose-stimulated insulin secretion (GSIS) in response to acetylcholine and showed increased sensitivity to blockade of cholinergic-induced insulin secretion by the muscarinic-type 3 receptor-specific antagonist 4-diphenylacetoxy-N-methylpiperidine. In addition, HC rats were less sensitive to adrenergic-induced inhibition of insulin secretion by oxymetazoline, whereas treatment with yohimbine resulted in increased GSIS. Furthermore, HC rats showed greater reductions in plasma insulin levels after vagotomy, as well as an attenuation of yohimbine-induced potentiation of GSIS, suggesting that yohimbine-mediated changes are mediated by parasympathetic activity. Changes in autonomic regulation of GSIS are supported by increased mRNA levels of the parasympathetic signaling molecules muscarinic-type 3 receptor, phospholipase Cbeta1, and protein kinase C-alpha and decreased levels of alpha(2a)-adrenergic receptors in islets from adult HC rats. In conclusion, metabolic programming of hyperinsulinemia throughout adulthood of HC rats involves changes in autonomic activity in response to the HC dietary intervention in the suckling period.

Role of the autonomic nervous system in the development of hyperinsulinemia by high-carbohydrate formula feeding to neonatal rats

An early dietary intervention in the form of a high-carbohydrate (HC) milk formula in neonatal rat pups results in immediate onset of hyperinsulinemia. While increased insulin secretion in HC rats has been shown to be related to hypersensitivity to glucose, the immediate onset of hyperinsulinemia and its persistence throughout the suckling period suggest involvement of multiple systems that enhance insulin secretion in response to increased demand. Evidence presented here in 12-day-old HC rats indicates that altered activity of the autonomic nervous system contributes to enhanced insulin secretory responses to glucose stimulation through increased parasympathetic and decreased sympathetic signaling. Both in vivo and in vitro studies have shown that HC rats secrete significantly higher levels of insulin in response to glucose in the presence of acetylcholine, a cholinergic agonist, while sensitivity to inhibition of insulin secretion by oxymetazoline, an alpha(2a)-adrenergic receptor (alpha(2a)AR) agonist, was reduced. In addition, HC rats showed increased sensitivity to blockade of cholinergic-induced insulin secretion by the muscarinic type 3 receptor (M3R) antagonist 4-diphenylacetoxy-N-methylpiperidine methobromide, as well as increased potentiation of glucose-stimulated insulin secretion by treatment with yohimbine. Increases in islets levels of M3R, phospholipase C-beta1, and protein kinase Calpha mRNAs, as well as decreased alpha(2a)AR mRNA, in 12-day-old HC rats provide a mechanistic connection to the changes in insulin secretion seen in HC rats. In conclusion, altered autonomic regulation of insulin secretion, due to the HC nutritional intervention, contributes to the development of hyperinsulinemia in 12-day-old HC rats.

Pdx-1 modulates histone H4 acetylation and insulin gene expression in terminally differentiated alpha-TC-1 cells

OBJECTIVES

Islet-associated transcription factors play a critical role in regulating pancreatic endocrine cell differentiation and islet hormone gene expression. Both alpha- and beta-cells differentiate from a common endocrine precursor cell. Therefore, it is important to reveal the differential gene expression profiles between alpha- and beta-cells that can direct their terminal cell fates. alpha-TC-1 and beta-TC-1 are 2 terminally differentiated islet tumor cell lines derived from islets transformed by promoter-specific driven SV40 T antigen overexpression. In this study, we demonstrated that Pdx-1 is a potent transcriptional regulator of endogenous insulin gene expression in alpha-TC-1 cells.

METHODS

Reverse transcriptase-polymerase chain reaction and chromatin immunoprecipitation assays were used to analyze gene expression patterns and chromatin modifications in the insulin promoter.

RESULTS

Differential transcription factor expression profiles of alpha-TC-1 and beta-TC-1 cells revealed that INSM-1 and Pdx-1 transcription factors were expressed exclusively in beta-TC-1 cells. Overexpression of Ad-Pdx-1 in alpha-TC-1 cells induced insulin gene expression. Chromatin immunoprecipitation assays in Ad-Pdx-1 alpha-TC-1 cells demonstrated Pdx-1 occupancy and the hyperacetylation of histone H4 in the insulin promoter region.

CONCLUSIONS

Collectively, these experiments revealed that Pdx-1 is a potent transcriptional regulator that is capable of modulating histone H4 acetylation and activates the insulin gene in a terminally differentiated glucagonoma cell line.

Intake of trans fatty acid–rich hydrogenated fat during pregnancy and lactation inhibits the hypophagic effect of central insulin in the adult offspring

OBJECTIVE

Using rats we examined whether maternal intake of hydrogenated fat rich in trans fatty acids affects brain fatty acid profile, hypothalamic content of insulin receptor and insulin receptor substrate-1 proteins, and the hypophagic effect of centrally administered insulin in 3-mo-old male progeny.

METHODS

Throughout pregnancy and lactation, Wistar rats ate isocaloric/normolipidic diets with soybean oil (control) or soybean oil-derived hydrogenated fat (trans diet) as a fat source. Upon weaning, the trans offspring continued on the trans diet (trans group) or were switched to a control diet (trans-control group).

RESULTS

Compared with control rats, trans rats had lower brain levels of eicosapentaenoic acid. Compared with trans rats, trans-control rats had increased levels of total polyunsaturated fatty acids and arachidonic acid and decreased levels of trans fatty acids, saturated fatty acids, and monounsaturated fatty acids. Insulin receptor and insulin receptor substrate-1 levels were significantly lower (44% and 38%, respectively) in trans than in control rats. In trans-control rats, insulin receptor was 26% lower (P < 0.05), whereas insulin receptor substrate-1 was 50% lower, than in control rats. Insulin decreased 24-h feeding in control (27%) and trans (38%) rats but failed to do so in trans-control rats. The latter group had increased serum glucose levels.

CONCLUSIONS

The data suggest that the early (intrauterine/perinatal) exposure to hydrogenated fat rich in trans fatty acids programmed the hypothalamic feeding control mechanisms. As young adults, only trans-control animals showed loss of insulin-induced hypophagia, indicating that the mismatch between early and later nutritional environments was relevant. However, the trans group also showed signs of altered appetite signaling mechanisms, suggesting that the early adaptations may have deleterious consequences later in life.

The proximal islet-specific glucose-6-phosphatase catalytic subunit-related protein autoantigen promoter is sufficient to initiate but not maintain transgene expression in mouse islets in vivo

We have previously reported the discovery of an islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) that is predominantly expressed in islet beta-cells. IGRP has recently been identified as a major autoantigen in a mouse model of type 1 diabetes. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion gene expression in transiently transfected islet-derived hamster insulinoma tumor and betaTC-3 cells revealed that the promoter region located between -306 and +3 confers high-level reporter gene expression. To determine whether this same promoter region is sufficient to confer islet beta-cell-specific gene expression in vivo, it was ligated to a beta-galactosidase reporter gene, and transgenic mice expressing the resulting fusion gene were generated. In two independent founder lines, this -306 to +3 promoter region was sufficient to drive beta-galactosidase expression in newborn mouse islets, predominantly in beta-cells, which was initiated during the expected time in development, around embryonic day 12.5. However, unlike the endogenous IGRP gene, beta-galactosidase expression was also detected in the cerebellum. Moreover, beta-galactosidase expression was almost completely absent in adult mouse islets, suggesting that cis-acting elements elsewhere in the IGRP gene are required for determining appropriate IGRP tissue-specific expression and for the maintenance of IGRP gene expression in adult mice.

Differential regulation of islet-specific glucose-6-phosphatase catalytic subunit-related protein gene transcription by Pax-6 and Pdx-1

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is selectively expressed in islet beta cells and is a major autoantigen in a mouse model of type I diabetes. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion gene expression through transient transfection of islet-derived betaTC-3 cells revealed that a promoter region, located between -273 and -254, is essential for high IGRP-CAT fusion gene expression. The sequence of this promoter region does not match that for any known islet-enriched transcription factor. However, data derived from gel retardation assays, a modified ligation-mediated polymerase chain reaction in situ footprinting technique and a SDS-polyacrylamide separation/renaturation procedure led to the hypothesis that this protein might be Pax-6, a conclusion that was confirmed by gel supershift assays. Additional experiments revealed a second non-consensus Pax-6 binding site in the -306/-274 IGRP promoter region. Pax-6 binding to these elements is unusual in that it appears to require both its homeo and paired domains. Interestingly, loss of Pax-6 binding to the -273/ -246 element is compensated by Pax-6 binding to the -306/-274 element and vice versa. Gel retardation assays revealed that another islet-enriched transcription factor, namely Pdx-1, binds four non-consensus elements in the IGRP promoter. However, mutation of these elements has little effect on IGRP fusion gene expression. Although chromatin immunoprecipitation assays show that both Pax-6 and Pdx-1 bind to the IGRP promoter within intact cells, in contrast to the critical role of these factors in beta cell-specific insulin gene expression, IGRP gene transcription appears to require Pax-6 but not Pdx-1.

The concerted activities of Pax4 and Nkx2.2 are essential to initiate pancreatic beta-cell differentiation

Pancreatic beta cells play a central role in maintaining glucose homeostasis because they secrete insulin in response to increased level of blood glucose; failure of this capacity constitutes a major component of the pathogenesis of diabetes. The identification of key regulators of pancreatic beta-cell differentiation is relevant for the overall understanding of this process and for future experiments aimed at regenerating insulin-producing beta cells from pancreatic or embryonic stem cells. Several studies using transgenic or knockout mice have established that the development and function of pancreatic beta cells are controlled by several genes encoding specific transcription factors. By inactivating the homeobox gene Pax4, we previously demonstrated that its function is required for the formation of mature insulin-producing cells. Here, we show that during pancreas ontogeny, Pax4 is expressed in differentiating endocrine cells, including beta cells. Pax4 activity appears essential for appropriate initiation of beta-cell differentiation because loss of Pax4 prevents the expression of Pdx1, HB9 and insulin in beta-cell precursors. This role of Pax4 appears to be accomplished via its genetic interaction with another homeobox gene, Nkx2.2.

Glutamate signaling in peripheral tissues

The hypothesis that l-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis - as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.

PDX-1 and the pancreas

Many transcription factors are critical for ensuring proper embryonic development of the endocrine pancreas and normal islet function. The transcription factor pancreatic duodenal homeobox 1 (PDX-1) is uniformly expressed in early pancreatic buds of embryos as well as the beta and delta cells of the islets of Langerhans. PDX-1 has also been found in dispersed endocrine cells of the duodenum in adults and plays a key role in pancreas formation. It has been reported that null mutation of PDX-1 in mice results in a failure of the pancreatic bud to expand; thus, the mice die 2-3 days after birth from hyperglycemia and dehydration. Heterozygous PDX-1 mice developed a pancreas but were diabetic. It has been shown that PDX-1 is required for maintaining the pancreatic islet functions by activating gene transcriptions including insulin, somatostatin (SST), islet amyloid polypeptide, glucose transporter type 2, and glucokinase. PDX-1 serves a dual role in pancreatic development. It initially contributes to pancreatic formation during embryogenesis and subsequently regulates the pancreatic islet cell physiology in mature islet cells. Understanding the underlying molecular mechanisms of pancreas formation, especially the function of PDX-1, may contribute to the enhanced treatment and prevention of debilitating diseases such as diabetes, insulinomas, and pancreatic carcinomas.

Upstream stimulatory factor (USF) and neurogenic differentiation/beta-cell E box transactivator 2 (NeuroD/BETA2) contribute to islet-specific glucose-6-phosphatase catalytic-subunit-related protein (IGRP) gene expression

Islet-specific glucose-6-phosphatase (G6Pase) catalytic-subunit-related protein (IGRP) is a homologue of the catalytic subunit of G6Pase, the enzyme that catalyses the final step of the gluconeogenic pathway. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion-gene expression through transient transfection of islet-derived beta TC-3 cells revealed that multiple promoter regions, located between -306 and -97, are required for maximal IGRP-CAT fusion-gene expression. These regions correlated with trans -acting factor-binding sites in the IGRP promoter that were identified in beta TC-3 cells in situ using the ligation-mediated PCR (LMPCR) footprinting technique. However, the LMPCR data also revealed additional trans -acting factor-binding sites located between -97 and +1 that overlap two E-box motifs, even though this region by itself conferred minimal fusion-gene expression. The data presented here show that these E-box motifs are important for IGRP promoter activity, but that their action is only manifest in the presence of distal promoter elements. Thus mutation of either E-box motif in the context of the -306 to +3 IGRP promoter region reduces fusion-gene expression. These two E-box motifs have distinct sequences and preferentially bind NeuroD/BETA2 (neurogenic differentiation/beta-cell E box transactivator 2) and upstream stimulatory factor (USF) in vitro, consistent with the binding of both factors to the IGRP promoter in situ, as determined using the chromatin-immunoprecipitation (ChIP) assay. Based on experiments using mutated IGRP promoter constructs, we propose a model to explain how the ubiquitously expressed USF could contribute to islet-specific IGRP gene expression.

Programming of islet functions in the progeny of hyperinsulinemic/obese rats

Neonatal female rat pups that were raised artificially on a high-carbohydrate (HC) milk formula during their suckling period developed hyperinsulinemia immediately, maintained chronic hyperinsulinemia in the postweaning period on laboratory diet, and developed obesity in adulthood. Pups (second-generation HC [2-HC]) born to such female rats (first-generation HC [1-HC]) spontaneously developed chronic hyperinsulinemia and adult-onset obesity (HC phenotype) without the requirement for any dietary intervention in their suckling period. Leftward shift in the insulin secretory response to a glucose stimulus, increase in hexokinase activity, and increased preproinsulin gene transcription were observed in islets from 28-day-old 2-HC rats, and these adaptations are similar to those reported for islets from 12-day-old and 100-day-old 1-HC rats. Unlike 1-HC islets, the ability to secrete moderate amounts of insulin in the absence of glucose and calcium and the incretin input for augmentation of insulin secretion were not observed in 2-HC islets. These results show that a dietary modification in the early postnatal life of the 1-HC female rat sets up a vicious cycle of spontaneous transfer of the HC phenotype to its progeny, implicating a new component to the growing list of factors that contribute to the fetal origins of adult-onset diseases.

Insulin gene transcription is mediated by interactions between the p300 coactivator and PDX-1, BETA2, and E47

Pancreatic beta-cell-type-specific expression of the insulin gene requires both ubiquitous and cell-enriched activators, which are organized within the enhancer region into a network of protein-protein and protein-DNA interactions to promote transcriptional synergy. Protein-protein-mediated communication between DNA-bound activators and the RNA polymerase II transcriptional machinery is inhibited by the adenovirus E1A protein as a result of E1A's binding to the p300 coactivator. E1A disrupts signaling between the non-DNA-binding p300 protein and the basic helix-loop-helix DNA-binding factors of insulin's E-element activator (i.e., the islet-enriched BETA2 and generally distributed E47 proteins), as well as a distinct but unidentified enhancer factor. In the present report, we show that E1A binding to p300 prevents activation by insulin's beta-cell-enriched PDX-1 activator. p300 interacts directly with the N-terminal region of the PDX-1 homeodomain protein, which contains conserved amino acid sequences essential for activation. The unique combination of PDX-1, BETA2, E47, and p300 was shown to promote synergistic activation from a transfected insulin enhancer-driven reporter construct in non-beta cells, a process inhibited by E1A. In addition, E1A inhibited the level of PDX-1 and BETA2 complex formation in beta cells. These results indicate that E1A inhibits insulin gene transcription by preventing communication between the p300 coactivator and key DNA-bound activators, like PDX-1 and BETA2:E47.

Molecular adaptations in islets from neonatal rats reared artificially on a high carbohydrate milk formula

Four day-old rat pups artificially raised on a high carbohydrate (HC) milk formula during their suckling period immediately develop hyperinsulinemia which persists into adulthood despite weaning onto lab chow on day 24. The present study investigates the molecular adaptations in islets isolated from neonatal rats in response to this dietary treatment during their suckling period. There is a significant increase in the level of preproinsulin mRNA and insulin biosynthesis in 12 day-old HC islets compared to islets from age-matched mother-fed (MF) control rats. Pancreatic duodenal homeobox factor-1 (PDX-1) modulates pancreatic ontogeny as well as preproinsulin gene expression in islets from neonatal rats. The mRNA level, DNA binding activity and protein content of PDX-1 are significantly increased in HC islets. The stress-activated protein kinase-2 and phosphatidylinositol 3-kinase have been reported to modulate PDX-1 activity in islets. The mRNA levels of these kinases are increased in HC islets. The mRNA level of upstream stimulatory factor (a modulator of PDX-1 gene expression) is also significantly increased in HC islets. These results indicate that the upregulation of several molecular events, including increases in the gene expression of preproinsulin, transcription factors and kinases may contribute to the chronic hyperinsulinemic state in the HC rats.

Cloning and characterization of the human and rat islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) genes

Islet-specific glucose-6-phosphatase (G6Pase) catalytic subunit-related protein (IGRP) is a homolog of the catalytic subunit of G6Pase, the enzyme that catalyzes the terminal step of the gluconeogenic pathway. Its catalytic activity, however, has not been defined. Since IGRP gene expression is restricted to islets, this suggests a possible role in the regulation of islet metabolism and, hence, insulin secretion induced by metabolites. We report here a comparative analysis of the human, mouse, and rat IGRP genes. These studies aimed to identify conserved sequences that may be critical for IGRP function and that specify its restricted tissue distribution. The single copy human IGRP gene has five exons of similar length and coding sequence to the mouse IGRP gene and is located on human chromosome 2q28-32 adjacent to the myosin heavy chain 1B gene. In contrast, the rat IGRP gene does not appear to encode a protein as a result of a series of deletions and insertions in the coding sequence. Moreover, rat IGRP mRNA, unlike mouse and human IGRP mRNA, is not expressed in islets or islet-derived cell lines, an observation that was traced by fusion gene analysis to a mutation of the TATA box motif in the mouse/human IGRP promoters to TGTA in the rat sequence. The results provide a framework for the further analysis of the molecular basis for the tissue-restricted expression of the IGRP gene and the identification of key amino acid sequences that determine its biological activity.

The DNA binding activity of the RIPE3b1 transcription factor of insulin appears to be influenced by tyrosine phosphorylation

The RIPE3b1 DNA binding factor plays a critical role in pancreatic islet beta cell-specific and glucose-regulated transcription of the insulin gene. Recently it was shown that RIPE3b1 binding activity in beta cell nuclear extracts is reduced by treatment with either calf intestinal alkaline phosphatase (CIAP) or a brain-enriched phosphatase preparation (BPP) (Zhao, L., Cissell, M. A., Henderson, E., Colbran, R., and Stein, R. (2000) J. Biol. Chem. 275, 10532-10537). Evidence is presented here suggesting that a tyrosine phosphatase(s) influences the ability of RIPE3b1 to bind to the insulin C1 element in beta cells. We found that RIPE3b1 binding was inhibited upon incubating beta cell nuclear extracts at 30 degrees C. In contrast, PDX-1 and MLTF-1 transcription factor binding activity was unaffected under these conditions. The loss in RIPE3b1 binding activity was prevented by inhibitors of tyrosine phosphatases (sodium orthovanadate and sodium molybdate) but not by inhibitors of serine/threonine phosphatases (sodium fluoride, okadaic acid, and microcystin LR). CIAP- and BPP-catalyzed inhibition of RIPE3b1 binding was also blocked by these tyrosine phosphatase inhibitors. Collectively, the data suggested that removal of a tyrosine(s) within RIPE3b1 prevented activator binding to insulin C1 control element sequences. The presence of a key phosphorylated tyrosine(s) within this transcription factor was further supported by the ability of the 4G10 anti-phosphotyrosine monoclonal antibody to immunoprecipitate RIPE3b1 DNA binding activity. We discuss how tyrosine phosphorylation, a very rare and highly significant regulatory modification, may control RIPE3b1 activator function.

Adaptive changes in insulin secretion by islets from neonatal rats raised on a high-carbohydrate formula

Artificial rearing of neonatal rats on a high-carbohydrate (HC) milk formula resulted in the immediate onset of hyperinsulinemia. This study examines, in islets of 12-day-old HC rats, adaptive changes that support the hyperinsulinemic state. Increases in plasma glucagon-like peptide-1 (GLP-1) levels and islet GLP-1 receptor mRNA supported increased insulin secretion by HC islets. Isolated HC islets, but not mother-fed (MF) islets, secreted moderate amounts of insulin in a glucose- and Ca(2+)-independent manner. Under stringent Ca(2+)-free conditions and in the presence of glucose, GLP-1 plus acetylcholine augmented insulin release to a larger extent in HC islets. Levels of adenylyl cyclase type VI mRNA and activities of protein kinase A, protein kinase C, and calcium calmodulin kinase II were increased in HC islets. A tenfold increase in norepinephrine concentration was required to inhibit insulin secretion in HC islets compared with MF islets, indicating reduced sensitivity to adrenergic signals. This study shows that significant alterations at proximal and distal sites of the insulin secretory pathway in HC islets may support the hyperinsulinemic state of these rats.

Activin A increases Pax4 gene expression in pancreatic beta cell lines

Activin A, a member of the TGFbeta superfamily, has many physiological and developmental functions. In pancreatic beta cell cultures, activin promotes cell differentiation and insulin production. The author has found activin increases gene expression of the PAX4, one of the major transcription factors determining pancreatic beta cell differentiation. This effect was mediated, at least in part, by the type IB activin receptor (ALK4). Moreover, the activity of human insulin promoter-reporter system was controlled by PAX4 and its isoform PAX4 delta(G239-P251) in a unique fashion; positively by low concentrations, and negatively by high concentrations of these proteins. And the repression activities were different between these proteins. These findings confirm the importance of activin signal transduction in pancreatic beta cell development and function.