Cell-specific expression controlled by the 5'-flanking region of insulin and chymotrypsin genes

From Genotype to Phenotype: How Enhancers Control Gene Expression and Cell Identity in Hematopoiesis

Blood comprises a wide array of specialized cells, all of which share the same genetic information and ultimately derive from the same precursor, the hematopoietic stem cell (HSC). This diversity of phenotypes is underpinned by unique transcriptional programs gradually acquired in the process known as hematopoiesis. Spatiotemporal regulation of gene expression depends on many factors, but critical among them are enhancers-sequences of DNA that bind transcription factors and increase transcription of genes under their control. Thus, hematopoiesis involves the activation of specific enhancer repertoires in HSCs and their progeny, driving the expression of sets of genes that collectively determine morphology and function. Disruption of this tightly regulated process can have catastrophic consequences: in hematopoietic malignancies, dysregulation of transcriptional control by enhancers leads to misexpression of oncogenes that ultimately drive transformation. This review attempts to provide a basic understanding of enhancers and their role in transcriptional regulation, with a focus on normal and malignant hematopoiesis. We present examples of enhancers controlling master regulators of hematopoiesis and discuss the main mechanisms leading to enhancer dysregulation in leukemia and lymphoma.

Proteomic Changes to the Updated Discovery of Engineered Insulin and Its Analogs: Pros and Cons

The destruction of β-cells of the pancreas leads to either insulin shortage or the complete absence of insulin, which in turn causes diabetes Mellitus. For treating diabetes, many trials have been conducted since the 19th century until now. In ancient times, insulin from an animal's extract was taken to treat human beings. However, this resulted in some serious allergic reactions. Therefore, scientists and researchers have tried their best to find alternative ways for managing diabetes with progressive advancements in biotechnology. However, a lot of research trials have been conducted, and they discovered more progressed strategies and approaches to treat type I and II diabetes with satisfaction. Still, investigators are finding more appropriate ways to treat diabetes accurately. They formulated insulin analogs that mimic the naturally produced human insulin through recombinant DNA technology and devised many methods for appropriate delivery of insulin. This review will address the following questions: What is insulin preparation? How were these devised and what are the impacts (both positive and negative) of such insulin analogs against TIDM (type-I diabetes mellitus) and TIIDM (type-II diabetes mellitus)? This review article will also demonstrate approaches for the delivery of insulin analogs into the human body and some future directions for further improvement of insulin treatment.

In vivo evaluation of GG2-GG1/A2 element activity in the insulin promoter region using the CRISPR-Cas9 system

The insulin promoter is regulated by ubiquitous as well as pancreatic β-cell-specific transcription factors. In the insulin promoter, GG2-GG1/A2-C1 (bases - 149 to - 116 in the human insulin promoter) play important roles in regulating β-cell-specific expression of the insulin gene. However, these events were identified through in vitro studies, and we are unaware of comparable in vivo studies. In this study, we evaluated the activity of GG2-GG1/A2 elements in the insulin promoter region in vivo. We generated homozygous mice with mutations in the GG2-GG1/A2 elements in each of the Ins1 and Ins2 promoters by CRISPR-Cas9 technology. The mice with homozygous mutations in the GG2-GG1/A2 elements in both Ins1 and Ins2 were diabetic. These data suggest that the GG2-GG1/A2 element in mice is important for Ins transcription in vivo.

Mutations in the C1 element of the insulin promoter lead to diabetic phenotypes in homozygous mice

Genome editing technologies such as CRISPR-Cas9 are widely used to establish causal associations between mutations and phenotypes. However, CRISPR-Cas9 is rarely used to analyze promoter regions. The insulin promoter region (approximately 1,000 bp) directs β cell-specific expression of insulin, which in vitro studies show is regulated by ubiquitous, as well as pancreatic, β cell-specific transcription factors. However, we are unaware of any confirmatory in vivo studies. Here, we used CRISPR-Cas9 technology to generate mice with mutations in the promoter regions of the insulin I (Ins1) and II (Ins2) genes. We generated 4 homozygous diabetic mice with 2 distinct mutations in the highly conserved C1 elements in each of the Ins1 and Ins2 promoters (3 deletions and 1 replacement in total). Remarkably, all mice with homozygous or heterozygous mutations in other loci were not diabetic. Thus, the C1 element in mice is required for Ins transcription in vivo.

Origins of Brain Insulin and Its Function

The brain or central nervous system (CNS) utilizes a vast amount of energy to sustain its basic functions, and most of the energy in the brain is derived from glucose. Whole-body energy and glucose homeostasis in the periphery of the human body are regulated by insulin, while the brain had been considered as an "insulin-insensitive" organ, because bulk brain glucose uptake is not affected by insulin in either rodents and humans. However, recently it has become clear that the actions of insulin are more widespread in the CNS and are a critical part of normal development, food intake, and energy balance, as well as plasticity throughout adulthood. Moreover, there are substantial evidence demonstrating that brain insulin is derived from pancreas, neurons, and astrocytes. In this chapter, I reviewed recent progress in roles of insulin in the brain, expression of insulin genes, and multiple origins of the brain insulin.

Chromatin remodeling effects on enhancer activity

During organism development, a diversity of cell types emerges with disparate, yet stable profiles of gene expression with distinctive cellular functions. In addition to gene promoters, the genome contains enhancer regulatory sequences, which are implicated in cellular specialization by facilitating cell-type and tissue-specific gene expression. Enhancers are DNA binding elements characterized by highly sophisticated and various mechanisms of action allowing for the specific interaction of general and tissue-specific transcription factors (TFs). However, eukaryotic organisms package their genetic material into chromatin, generating a physical barrier for TFs to interact with their cognate sequences. The ability of TFs to bind DNA regulatory elements is also modulated by changes in the chromatin structure, including histone modifications, histone variants, ATP-dependent chromatin remodeling, and the methylation status of DNA. Furthermore, it has recently been revealed that enhancer sequences are also transcribed into a set of enhancer RNAs with regulatory potential. These interdependent processes act in the context of a complex network of chromatin interactions, which together contributes to a renewed vision of how gene activation is coordinated in a cell-type-dependent manner. In this review, we describe the interplay between genetic and epigenetic aspects associated with enhancers and discuss their possible roles on enhancer function.

Cooperation between HMGA1, PDX-1, and MafA is Essential for Glucose-Induced Insulin Transcription in Pancreatic Beta Cells

The high-mobility group AT-hook 1 (HMGA1) protein is a nuclear architectural factor that can organize chromatin structures. It regulates gene expression by controlling the formation of stereospecific multiprotein complexes called "enhanceosomes" on the AT-rich regions of target gene promoters. Previously, we reported that defects in HMGA1 caused decreased insulin receptor expression and increased susceptibility to type 2 diabetes mellitus in humans and mice. Interestingly, mice with disrupted HMGA1 gene had significantly smaller islets and decreased insulin content in their pancreata, suggesting that HMGA1 may have a direct role in insulin transcription and secretion. Herein, we investigate the regulatory roles of HMGA1 in insulin transcription. We provide evidence that HMGA1 physically interacts with PDX-1 and MafA, two critical transcription factors for insulin gene expression and beta-cell function, both in vitro and in vivo. We then show that the overexpression of HMGA1 significantly improves the transactivating activity of PDX-1 and MafA on human and mouse insulin promoters, while HMGA1 knockdown considerably decreased this transactivating activity. Lastly, we demonstrate that high glucose stimulus significantly increases the binding of HMGA1 to the insulin (INS) gene promoter, suggesting that HMGA1 may act as a glucose-sensitive element controlling the transcription of the INS gene. Together, our findings provide evidence that HMGA1, by regulating PDX-1- and MafA-induced transactivation of the INS gene promoter, plays a critical role in pancreatic beta-cell function and insulin production.

Transgenic mouse models--a seminal breakthrough in oncogene research

Transgenic mouse models are an integral part of modern cancer research, providing a versatile and powerful means of studying tumor initiation and progression, metastasis, and therapy. The present repertoire of these models is very diverse, with a wide range of strategies used to induce tumorigenesis by expressing dominant-acting oncogenes or disrupting the function of tumor-suppressor genes, often in a highly tissue-specific manner. Much of the current technology used in the creation and characterization of transgenic mouse models of cancer will be discussed in depth elsewhere. However, to gain a complete appreciation and understanding of these complex models, it is important to review the history of the field. Transgenic mouse models of cancer evolved as a new and, compared with the early cell-culture-based techniques, more physiologically relevant approach for studying the properties and transforming capacities of oncogenes. Here, we will describe early transgenic mouse models of cancer based on tissue-specific expression of oncogenes and discuss their impact on the development of this still rapidly growing field.

Tailoring the models of transcription

Molecular biology is a rapidly evolving field that has led to the development of increasingly sophisticated technologies to improve our capacity to study cellular processes in much finer detail. Transcription is the first step in protein expression and the major point of regulation of the components that determine the characteristics, fate and functions of cells. The study of transcriptional regulation has been greatly facilitated by the development of reporter genes and transcription factor expression vectors, which have become versatile tools for manipulating promoters, as well as transcription factors in order to examine their function. The understanding of promoter complexity and transcription factor structure offers an insight into the mechanisms of transcriptional control and their impact on cell behaviour. This review focuses on some of the many applications of molecular cut-and-paste tools for the manipulation of promoters and transcription factors leading to the understanding of crucial aspects of transcriptional regulation.

Clinical utility of insulin and insulin analogs

Diabetes is a pandemic disease characterized by autoimmune, genetic and metabolic abnormalities. While insulin deficiency manifested as hyperglycemia is a common sequel of both Type-1 and Type-2 diabetes (T1DM and T2DM), it does not result from a single genetic defect--rather insulin deficiency results from the functional loss of pancreatic β cells due to multifactorial mechanisms. Since pancreatic β cells of patients with T1DM are destroyed by autoimmune reaction, these patients require daily insulin injections. Insulin resistance followed by β cell dysfunction and β cell loss is the characteristics of T2DM. Therefore, most patients with T2DM will require insulin treatment due to eventual loss of insulin secretion. Despite the evidence of early insulin treatment lowering macrovascular (coronary artery disease, peripheral arterial disease and stroke) and microvascular (diabetic nephropathy, neuropathy and retinopathy) complications of T2DM, controversy exists among physicians on how to initiate and intensify insulin therapy. The slow acting nature of regular human insulin makes its use ineffective in counteracting postprandial hyperglycemia. Instead, recombinant insulin analogs have been generated with a variable degree of specificity and action. Due to the metabolic variability among individuals, optimum blood glucose management is a formidable task to accomplish despite the presence of novel insulin analogs. In this article, we present a recent update on insulin analog structure and function with an overview of the evidence on the various insulin regimens clinically used to treat diabetes.

GPR41 gene expression is mediated by internal ribosome entry site (IRES)-dependent translation of bicistronic mRNA encoding GPR40 and GPR41 proteins

GPR41 is a G protein-coupled receptor activated by short chain fatty acids. The gene encoding GPR41 is located immediately downstream of a related gene encoding GPR40, a receptor for long chain fatty acids. Expression of GPR41 has been reported in a small number of cell types, including gut enteroendocrine cells and sympathetic ganglia, where it may play a role in the maintenance of metabolic homeostasis. We now demonstrate that GPR41, like GPR40, is expressed in pancreatic beta cells. Surprisingly, we found no evidence for transcriptional control elements or transcriptional initiation in the intergenic GPR40-GPR41 region. Rather, using 5'-rapid amplification of cDNA ends analysis, we demonstrated that GPR41 is transcribed from the promoter of the GPR40 gene. We confirmed this finding by generating bicistronic luciferase reporter plasmids, and we were able to map a potential internal ribosome entry site-containing region to a 2474-nucleotide region of the intergenic sequence. Consistent with this, we observed m(7)G cap-independent reporter gene expression upon transfection of RNA containing this region. Thus, GPR41 expression is mediated via an internal ribosome entry site located in the intergenic region of a bicistronic mRNA. This novel sequence organization may be utilized to permit coordinated regulation of the fatty acid receptors GPR40 and GPR41.

Gli-similar proteins: their mechanisms of action, physiological functions, and roles in disease

Gli-similar (Glis) 1-3 proteins constitute a subfamily of Krüppel-like zinc-finger proteins that are closely related to members of the Gli family. Glis proteins have been implicated in several pathologies, including cystic kidney disease, diabetes, hypothyroidism, fibrosis, osteoporosis, psoriasis, and cancer. In humans, a mutation in the Glis2 gene has been linked to the development of nephronophthisis (NPHP), a recessive cystic kidney disease, while mutations in Glis3 lead to an extended multisystem phenotype that includes the development of neonatal diabetes, polycystic kidneys, congenital hypothyroidism, and facial dysmorphism. Glis3 has also been identified as a risk locus for type-1 and type-2 diabetes and additional studies have revealed a role for Glis3 in pancreatic endocrine development, β-cell maintenance, and insulin regulation. Similar to Gli1-3, Glis2 and 3 have been reported to localize to the primary cilium. These studies appear to suggest that Glis proteins are part of a primary cilium-associated signaling pathway(s). It has been hypothesized that Glis proteins are activated through posttranslational modifications and subsequently translocate to the nucleus where they regulate transcription by interacting with Glis-binding sites in the promoter regions of target genes. This chapter summarizes the current state of knowledge regarding mechanisms of action of the Glis family of proteins, their physiological functions, as well as their roles in disease.

Regulation of insulin gene transcription by multiple histone acetyltransferases

Glucose-stimulated insulin gene transcription is mainly regulated by a 340-bp promoter region upstream of the transcription start site by beta-cell-enriched transcription factors Pdx-1, MafA, and NeuroD1. Previous studies have shown that histone H4 hyperacetylation is important for acute up-regulation of insulin gene transcription. Until now, only the histone acetyltransferase (HAT) protein p300 has been shown to be involved in this histone H4 acetylation event. In this report we investigated the role of the additional HAT proteins CREB binding protein (CBP), p300/CBP-associated factor (PCAF), and general control of amino-acid synthesis 5 (GCN5) in regulation of glucose-stimulated insulin gene transcription. Utilizing quantitative chromatin immunoprecipitation analysis, we demonstrate that glucose regulates the binding of p300, CBP, PCAF, and GCN5 to the proximal insulin promoter. siRNA-mediated knockdown of each of these HAT proteins revealed that depletion of p300 and CBP leads to a drastic decrease in histone H4 acetylation at the insulin promoter and in insulin gene expression, whereas knockdown of PCAF and GCN5 leads to a more moderate decrease in histone H4 acetylation and insulin gene expression. These data suggest that high glucose mediates the recruitment of p300, CBP, PCAF, and GCN5 to the insulin promoter and that all four HATs are important for insulin gene expression.

Spontaneous in vivo differentiation of embryonic stem cell-derived pancreatic endoderm-like cells corrects hyperglycemia in diabetic mice

BACKGROUND

Whole pancreas and islet transplantation are currently used for the treatment of type 1 diabetes. However, the major limitations of this potentially curative approach are an inadequate supply of cadaveric pancreata, lifelong immunosuppression, and chronic graft rejection. Therefore, there is an urgent need to develop new sources of insulin-producing cells (IPCs). Here, we investigated whether embryonic stem (ES) cells can be exploited for the derivation of IPCs, and whether their transplantation can correct hyperglycemia in diabetic mice.

METHODS

ES cells engineered to express pancreatic and duodenal homeobox 1 (Pdx1), a critical pancreatic transcription factor, were differentiated into pancreatic endoderm-like cells (PELCs) and evaluated for their potential to correct hyperglycemia after transplantation in diabetic mice.

RESULTS

After systemic injection, PELCs localized to the pancreas, liver, and kidney. They then spontaneously differentiated into IPCs that corrected hyperglycemia in diabetic mice. When transplanted under the kidney capsule, PELC-derived IPCs were equally efficient at correcting hyperglycemia. Real-time noninvasive in vivo bioluminescence imaging (BLI) of rat insulin promoter (RIP)-driven luciferase was used to monitor the fate of the transplanted PELCs. To confirm that the transplanted cells were responsible for the correction of hyperglycemia, kidneys containing the transplanted cells were nephrectomized, causing rapid hyperglycemia. Interestingly, none of the animals transplanted with PELCs developed tumors, a potential consequence of the differentiation and purification procedures.

CONCLUSIONS

Our data suggest that Pdx1-expressing PELCs are capable of spontaneously undergoing differentiation in vivo into IPCs and leading to a sustained correction of hyperglycemia in diabetic mice.

Activin receptor-like kinase and the insulin gene

The biological responses of the transforming growth factor-β (TGF-β) superfamily, which includes Activins and Nodal, are induced by activation of a receptor complex and Smads. A type I receptor, which is a component of the complex, is known as an activin receptor-like kinase (ALK); currently seven ALKs (ALK1-ALK7) have been identified in humans. Activins signaling, which is mediated by ALK4 and 7 together with ActRIIA and IIB, plays a critical role in glucose-stimulated insulin secretion, development/neogenesis, and glucose homeostatic control of pancreatic endocrine cells; the insulin gene is regulated by these signaling pathways via ALK7, which is a receptor for Activins AB and B and Nodal. This review discusses signal transduction of ALKs in pancreatic endocrine cells and the role of ALKs in insulin gene regulation.

Efficient, glucose responsive and islet-specific transgene expression by a modified rat insulin promoter

This study was done to improve efficiency and islet specificity of the rat insulin promoter (RIP). Various RIP lengths were prepared and tested in vitro to drive luciferase reporter gene expression in INS1-cells, alpha-cells, acinar cells, ductal cells and fibroblasts. The CMV promoter was used as a positive control. In addition, the DsRed reporter gene was administered in vivo to rat pancreas by ultrasound-targeted microbubble destruction (UTMD). Confocal microscopy was used to detect the presence and distribution of DsRed within the pancreas after UTMD. A modified RIP3.1 promoter, which includes portions of the insulin gene after its transcription start site is fivefold more active in INS-1 cells than the full-length RIP promoter or the CMV promoter. RIP3.1 is regulated by glucose level and various islet transcription factors in vitro, and exhibits activity in alpha-cells, but not in exocrine cells. In vivo delivery of RIP3.1-DsRed resulted in expression of DsRed protein in beta-cells, and to a lesser extent in alpha-cells under normal glucose conditions. No DsRed signal was present in exocrine pancreas under RIP3.1. A modified RIP, RIP3.1, efficiently and specifically directs gene expression to endocrine pancreas.

The promoter of the pri-miR-375 gene directs expression selectively to the endocrine pancreas

microRNAs (miRNAs) are known to play an essential role in controlling a broad range of biological processes including animal development. Accordingly, many miRNAs are expressed preferentially in one or a small number of cell types. Yet the mechanisms responsible for this selectivity are not well understood. The aim of this study was to elucidate the molecular basis of cell-specific expression of the pri-miR-375 gene, which is selectively expressed in pancreatic islets, and has been implicated both in the development of islets, and the function of mature pancreatic beta cells. An evolutionarily conserved 768 bp region of DNA upstream of the pri-miR-375 gene was linked to GFP and luciferase reporter genes, and expression monitored in transgenic mice and transfected cultured cells. Deletion and targeted mutagenesis analysis was used to evaluate the functional significance of sequence blocks within the upstream fragment. 5′-RACE analysis was used for mapping the pri-miR-375 gene transcription start site. The conserved 768 bp region was able to direct preferential expression of a GFP reporter gene to pancreatic islets in transgenic mice. Deletion analysis using a luciferase reporter gene in transfected cultured cell lines confirmed the cell specificity of the putative promoter region, and identified several key cis-elements essential for optimal activity, including E-boxes and a TATA sequence. Consistent with this, 5′-RACE analysis identified a transcription start site within this DNA region, 24 bp downstream of the TATA sequence. These studies define the promoter of the pri-miR-375 gene, and show that islet-specific expression of the pri-miR-375 gene is controlled at the transcriptional level. Detailed analysis of the transcriptional mechanisms controlling expression of miRNA genes will be essential to permit a comprehensive understanding of the complex role of miRNAs such as miR-375 in developmental processes.

Transcriptional regulation of the salivary histatin gene: finding of a strong positive regulatory element and its binding protein

Histatins are salivary proteins found and expressed in human salivary glands. They play a role in the non-immune system of antimicrobial defense, for instance, against Candida albicans. The transcriptional regulatory sequences of the histatin gene, HIS1, have remained obscure for a long time. Here, we cloned the putative promoter from human genomic DNA and tested it in a luciferase reporter system. This promoter is much more active in salivary gland cells than in other cell types. Analysis of deletion mutants revealed that the region encompassing -2254 to -1748 is a strong positive transcriptional element, and its functional core sequence (termed HTN27 box) works in correct and reverse orientations in synergy with downstream sequences, the region spanning -680 to +28 and a proximal promoter. The plus single-stranded HTN27 box is specifically bound by a 100 kDa protein that is present in HSG cells, but not in HeLa cells. These findings indicate that the regulation of the histatin gene expression may be intricate, and it seems to have a cell-type preference in the salivary gland cells.

Regulation of insulin gene expression and insulin production in Nile tilapia (Oreochromis niloticus)

Compared to mammals, little is known about insulin gene expression in fish. Using transient transfection experiments and mammalian insulinoma cell lines we demonstrate that transcription of the Nile tilapia (Oreochromis niloticus) insulin gene is (a) regulated in a beta-cell-specific manner; and (b) not sensitive to the glucose stimulations. Deletion analysis of the 1575 bp 5' insulin gene flanking sequence revealed that cooperative interactions between regulatory elements within the proximal (-1 to -396 bp) and the distal (-396 bp to -1575 bp) promoter regions were necessary for induction of the beta-cell-specific transcription. Effects of glucose and arginine on endogenous insulin secretion, translation, and transcription in isolated tilapia Brockmann bodies were determined using Northern hybridization, Western analysis, and quantitative RT-PCR. Similar to the regulation of mammalian insulin, we found that increases of glucose (1-70 mM) and arginine (0.4-25 mM) induced insulin secretion. However, transcription of the insulin gene was activated only by extremely high concentrations of glucose and arginine added simultaneously. When stimulated for 24 h with low concentrations of both inducers or with either of them added separately, tilapia beta-cells were able to replenish secreted insulin and to maintain insulin stores at a constant level without elevations of the insulin mRNA levels. Since the basal level of insulin mRNA was approximately 3.7-fold higher in tilapia beta-cells than it is in mammalian beta-cells, insulin production in tilapia cells probably relies on an enlarged intracellular insulin mRNA pool and does not require the transcriptional activation of the insulin gene.

Co-expressing Pdx1 and Ngn3 induces few beta-like cells in the liver of mice

We have assessed whether expressing pancreatic transcription factors Pdx1 and Ngn3 in the liver could induce beta-like cells for curing type I diabetes. When co-expressed in the liver of insulin reporter mice using adenovirus, few cells in the liver were turned into beta-like cells which were marked by lacZ expression by X-gal staining. Insulin promoter assay showed that Pdx1 highly activates insulin promoter when combined with Ngn3. In ex vivo experiments to determine permissiveness of stem/progenitor and fully differentiated cells by ectopic transcription factors, it was found that more islet genes were induced in ES or bone marrow SP cells than fully differentiated NIH3T3 or MEF cells by Pdx1 or Ngn3. Our results suggest that synergistic action of Pdx1 and Ngn3 on inducing various islet genes as well as cell intrinsic factors played a crucial role in directing few liver cells into beta-like cells in vivo.

Mechanisms of action of glucagon-like peptide 1 in the pancreas

Glucagon-like peptide 1 (GLP-1) is a hormone that is encoded in the proglucagon gene. It is mainly produced in enteroendocrine L cells of the gut and is secreted into the blood stream when food containing fat, protein hydrolysate, and/or glucose enters the duodenum. Its particular effects on insulin and glucagon secretion have generated a flurry of research activity over the past 20 years culminating in a naturally occurring GLP-1 receptor (GLP-1R) agonist, exendin 4 (Ex-4), now being used to treat type 2 diabetes mellitus (T2DM). GLP-1 engages a specific guanine nucleotide-binding protein (G-protein) coupled receptor (GPCR) that is present in tissues other than the pancreas (brain, kidney, lung, heart, and major blood vessels). The most widely studied cell activated by GLP-1 is the insulin-secreting beta cell where its defining action is augmentation of glucose-induced insulin secretion. Upon GLP-1R activation, adenylyl cyclase (AC) is activated and cAMP is generated, leading, in turn, to cAMP-dependent activation of second messenger pathways, such as the protein kinase A (PKA) and Epac pathways. As well as short-term effects of enhancing glucose-induced insulin secretion, continuous GLP-1R activation also increases insulin synthesis, beta cell proliferation, and neogenesis. Although these latter effects cannot be currently monitored in humans, there are substantial improvements in glucose tolerance and increases in both first phase and plateau phase insulin secretory responses in T2DM patients treated with Ex-4. This review will focus on the effects resulting from GLP-1R activation in the pancreas.

Comparative analysis of insulin gene promoters: implications for diabetes research

DNA sequences that regulate expression of the insulin gene are located within a region spanning approximately 400 bp that flank the transcription start site. This region, the insulin promoter, contains a number of cis-acting elements that bind transcription factors, some of which are expressed only in the beta-cell and a few other endocrine or neural cell types, while others have a widespread tissue distribution. The sequencing of the genome of a number of species has allowed us to examine the manner in which the insulin promoter has evolved over a 450 million-year period. The major findings are that the A-box sites that bind PDX-1 are among the most highly conserved regulatory sequences, and that the conservation of the C1, E1, and CRE sequences emphasize the importance of MafA, E47/beta2, and cAMP-associated regulation. The review also reveals that of all the insulin gene promoters studied, the rodent insulin promoters are considerably dissimilar to the human, leading to the conclusion that extreme care should be taken when extrapolating rodent-based data on the insulin gene to humans.

Development of a novel beta-cell specific promoter system for the identification of insulin-producing cells in in vitro cell cultures

Recently, it has been reported that islet transplantation into patients with Type 1 diabetes may achieve insulin independence for a year or longer [Shapiro et al., Islet transplantation in seven patients with type 1 diabetes mellitus using a glucocorticoid-free immunosuppressive regimen, N Engl J Med. 343 (2000) 230-238]. However, the amount of donor islet tissue is limited, therefore, multiple approaches are being explored to generate insulin-producing cells in vitro. Some promising results have been obtained using mouse and human stem cells and progenitor cells [Soria et al., From stem cells to beta cells: new strategies in cell therapy of diabetes mellitus, Diabetologia. 4 (2001) 407-415; Lechner et al., Stem/progenitor cells derived from adult tissues: potential for the treatment of diabetes mellitus, Am J Physiol Endocrinol Metab. 284 (2003) 259-266; Bonner-Weir et al., In vitro cultivation of human islets from expanded ductal tissue, Proc Natl Acad Sci U S A, 97 (2000) 7999-8004; Assady et al., Insulin production by human embryonic stem cells, 50 (2001) Diabetes 1691-1697]. However, the efficiency of obtaining populations with high numbers of differentiated cells has been poor. In order to improve the efficiency of producing and selecting insulin-producing cells from undifferentiated cells, we have designed a novel beta-cell specific and glucose responsive promoter system designated pGL3.hINS-363 3x. This artificial promoter system exhibits significant luciferase activity not only in insulin-producing MIN6 m9 cells but also in isolated human islets. The pGL3.hINS-363 3x construct shows no activity in non-insulin-producing cells in low glucose conditions (2 mM glucose) but demonstrates significant activity and beta-cell specificity in high glucose conditions (16 mM glucose). Furthermore, pGL3.hINS-363 3x shows significant promoter activity in differentiated AR42J cells that can produce insulin after activin A and betacellulin treatment. Here, we describe a novel beta-cell specific and glucose responsive artificial promoter system designed for analyzing and sorting beta-like insulin-producing cells that have differentiated from stem cells or other progenitor cells.

Biochemistry and clinical role of trypsinogens and pancreatic secretory trypsin inhibitor

Trypsinogens and PSTI/TATI/SPINK1 are expressed, usually together, at high levels by the pancreas but also by many other normal and malignant tissues. The present review describes studies on the expression and putative functions of trypsinogens and PSTI/TATI/SPINK1 in the human body. The clinical aspects are discussed, including the correlations between expression of trypsinogens and PSTI/TATI/SPINK1 in tissues, serum, and urine of patients with pancreatitis or cancer and clinicopathological characteristics, i.e., the roles of trypsinogens and PSTI/TATI/SPINK1 in spontaneous and hereditary pancreatitis, tumor progression, and prognosis.

The effect of overexpression of Pdx1 and Foxa2 on the differentiation of human embryonic stem cells into pancreatic cells

Human embryonic stem cells (HESCs) are pluripotent cells that may serve as a source of cells for transplantation medicine and as a tool to study human embryogenesis. Using genetic manipulation methodologies, we have investigated the potential of HESCs to differentiate into the various pancreatic cell types. We initially created various HESCs carrying the enhanced green fluorescent protein (eGFP) reporter gene under the control of either the insulin promoter or the pancreatic and duodenal homeobox factor-1 (Pdx1) promoter. Our analysis revealed that during the differentiation of HESCs into embryoid bodies (EBs), we could detect green fluorescent cells when eGFP is regulated by Pdx1 promoter but not by insulin promoter. To examine whether we can induce differentiation into pancreatic cells, we have established human embryonic stem cell lines that constitutively express either Pdx1 or the endodermal transcription factor Foxa2. Following differentiation into EBs, the constitutive expression of Pdx1 enhanced the differentiation of HESCs toward pancreatic endocrine and exocrine cell types. Thus, we have demonstrated expression of several transcription factors that are downstream of Pdx1 and various molecular markers for the different pancreatic cell types. However, the expression of the insulin gene could be demonstrated only when the cells differentiated in vivo into teratomas. We conclude that although overexpression of Pdx1 enhanced expression of pancreatic enriched genes, induction of insulin expression may require additional signals that are only present in vivo.

Tissue-specific and glucose-responsive expression of the pancreatic derived factor (PANDER) promoter

Pancreatic derived factor (PANDER) is a recently identified cytokine-like protein that is dominantly expressed in the islets of Langerhans of the pancreas. To investigate the mechanism of tissue-specific regulation of PANDER, we identified and characterized the promoter region. The transcriptional start site was identified 520 bp upstream of the translational start codon by 5'-RLM-RACE. Computer algorithms identified several islet-associated and glucose-responsive binding motifs that included A and E boxes, hepatocyte nuclear factors 1 and 4, Oct-1, and signal transducer and activator of transcription 3, and 5. Reporter gene analysis revealed cell type-specific PANDER promoter expression in islet and liver-derived cell lines. Levels of PANDER mRNA were directly concordant to the observed cell type-specific PANDER promoter gene expression. The minimal element was mapped to the 5'-UTR and located between +200 and +491 relative to the transcriptional start site and imparted maximal gene expression. In addition, several putative glucose-responsive binding sites were further functionally characterized to reveal critical regulatory elements of PANDER. The PANDER promoter was demonstrated to be glucose-responsive in a dose-dependent manner in murine insulinoma beta-TC3 cells and primary murine islets, but unresponsive in glucagon-secreting alpha-TC3 cells. Our findings revealed that the 5'-UTR of PANDER contains the minimal element for gene expression and imparts both tissue-specificity and glucose-responsiveness. The regulation of PANDER gene expression mimics that of insulin and suggests a potential biological function of PANDER involved in metabolic homeostasis.

Overexpression of inducible cyclic AMP early repressor inhibits transactivation of genes and cell proliferation in pancreatic beta cells

Transcriptional control mediated by the cyclic AMP-responsive element (CRE) represents an important mechanism of gene regulation. To test our hypothesis that increased inducible cyclic AMP early repressor (ICER) Igamma inhibits function of CRE-binding proteins and thus disrupts CRE-mediated transcription in pancreatic beta cells, we generated transgenic mice with beta-cell-directed expression of ICER Igamma, a powerful repressor that is greatly increased in diabetes. Three transgenic lines clearly show that increased ICER Igamma expression in beta cells results in early severe diabetes. From birth islets were severely disorganized with a significantly increased proportion of alpha cells throughout the islet. Diabetes results from the combined effects of impaired insulin expression and a decreased number of beta cells. The decrease in beta cells appears to result from impaired proliferation rather than from increased apoptosis after birth. Cyclin A gene expression is impaired by the strong inhibition of ICER; the suppression of cyclin A results in a substantially decreased proliferation of beta cells in the postnatal period. These results suggest that CRE and CRE-binding factors have an important role in pancreatic beta-cell physiology not only directly by regulation of gene trans-activation but also indirectly by regulation of beta-cell mass.

Role of intrinsic DNA binding specificity in defining target genes of the mammalian transcription factor PDX1

PDX1 is a homeodomain transcription factor essential for pancreatic development and mature beta cell function. Homeodomain proteins typically recognize short TAAT DNA motifs in vitro: this binding displays paradoxically low specificity and affinity, given the extremely high specificity of action of these proteins in vivo. To better understand how PDX1 selects target genes in vivo, we have examined the interaction of PDX1 with natural and artificial binding sites. Comparison of PDX1 binding sites in several target promoters revealed an evolutionarily conserved pattern of nucleotides flanking the TAAT core. Using competitive in vitro DNA binding assays, we defined three groups of binding sites displaying high, intermediate and low affinity. Transfection experiments revealed a striking correlation between the ability of each sequence to activate transcription in cultured beta cells, and its ability to bind PDX1 in vitro. Site selection from a pool of oligonucleotides (sequence NNNTAATNNN) revealed a non-random preference for particular nucleotides at the flanking locations, resembling natural PDX1 binding sites. Taken together, the data indicate that the intrinsic DNA binding specificity of PDX1, in particular the bases adjacent to TAAT, plays an important role in determining the spectrum of target genes.

Transcriptional profiles of latent human immunodeficiency virus in infected individuals: effects of Tat on the host and reservoir

The persistence of human immunodeficiency virus (HIV) in optimally treated infected individuals poses a major therapeutic problem. In latently infected cells, one of the observed phenotypes is absent elongation of viral transcription. Thus, the positive elongation factor b (P-TEFb), which is usually recruited by NF-kappaB or Tat, is not present on the HIV long terminal repeat (LTR). Although most attempts to activate these proviruses centered on NF-kappaB, we investigated effects of Tat. To this end, we generated transgenic mice, which secreted a chimera between Tat and the green fluorescent protein from beta cells of the pancreas. This extracellular Tat distributed widely, entered nuclei of resting cells, and specifically transactivated the HIV LTR. No deleterious side effects of Tat were found. Next, we determined that Tat can activate latent proviruses in optimally treated infected individuals. In their cells, T-cell activation or exogenous Tat could induce viral replication equivalently. Thus, P-TEFb could activate the majority of the latent HIV, in this case by Tat.

The transcription factor PDX-1 is post-translationally modified by O-linked N-acetylglucosamine and this modification is correlated with its DNA binding activity and insulin secretion in min6 beta-cells

The pancreatic/duodenal homeobox-1 protein (PDX-1, also called STF-1, IPF-1) is a transcription factor that plays an important role in pancreatic function and development. Here, we have overexpressed and purified PDX-1 from baculovirus/sf-9 cells, transiently transfected Cos-7 cells and native Min6 cells and demonstrated that the protein is posttranslationally modified by O-linked N-acetylglucosamine (O-GlcNAc). The approaches we used include binding of the protein to the lectin WGA, labeling with galactosyltransferase and UDP-[(3)H]gal and probing with the O-GlcNAc-specific antibody, RL-2. PNGase F treatment and structural analysis indicate that the carbohydrate is beta-linked O-GlcNAc. Mapping of [(3)H]gal-labeled tryptic peptides indicates that PDX-1 has two major sites for O-GlcNAcylation. In Min6 cells, elevated glucose concentration leads to an increase in protein O-GlcNAcylation and this hyperglycosylation correlates with an increase in DNA binding activity of PDX-1 and insulin secretion. On the other hand, the GFAT inhibitor azaserine reduces intracellular O-GlcNAc levels and profoundly attenuates glucose-stimulated insulin secretion. These data suggest that O-GlcNAcylation may be involved in the regulation of PDX-1 DNA binding activity and in glucose-stimulated insulin secretion in beta-cells.

Regulation of human insulin gene transcription by the immunosuppressive drugs cyclosporin A and tacrolimus at concentrations that inhibit calcineurin activity and involving the transcription factor CREB

Cyclosporin A and tacrolimus are important immunosuppressive drugs. They share a diabetogenic action as one of their most serious adverse effects. In a single study, tacrolimus (100 nM) inhibited human insulin gene transcription in the beta-cell line HIT. Using transfections of a human insulin-reporter gene into HIT cells, the present study shows that this inhibition is seen only at high concentrations of tacrolimus and is not caused by cyclosporin A. However, after stimulation by the major second messengers in the regulation of the insulin gene, cAMP and depolarization-induced calcium influx, both tacrolimus and cyclosporin A inhibited human insulin gene transcription in a concentration-dependent manner with IC(50) values of 1 nM and 30 nM, respectively. A further analysis offers a mechanism for this effect by revealing that the activation by cAMP and calcium of human insulin gene transcription is mediated by the transcription factor cAMP-responsive element binding protein (CREB) whose activity is inhibited by the immunosuppressants. These data demonstrate for the first time that cAMP- and calcium-induced activity of the human insulin gene is mediated by CREB and blocked by both tacrolimus and cyclosporin A at concentrations that inhibit calcineurin phosphatase activity. Since also the immunosuppressive effects of cyclosporin A and tacrolimus are thought to be secondary to inhibition of calcineurin, the present study suggests that inhibition of human insulin gene transcription by the immunosuppressants is clinically important and may contribute to their diabetogenic effect.

Transgenic studies on the regulation of the anterior pituitary gland function by the hypothalamus

The anterior pituitary gland is composed of five different cell types secreting hormones whose functions include the regulation of post-natal growth (growth hormone, GH), lactation (prolactin, PRL), reproduction (luteinising hormone, LH, and follicle stimulating hormone, FSH), metabolism (thyroid stimulating hormone, TSH), and stress (adrenocorticotrophic hormone, ACTH). The synthesis and secretion of the anterior pituitary hormones is under the control of neuropeptides released from the hypothalamus into a capillary portal plexus which flows through the external zone of the median eminence to the anterior lobe. This review describes the ways that gene transfer technologies have been applied to whole animals in order to study the regulation of anterior pituitary function by the hypothalamus. The extensive studies on these neuronal systems, within the context of the physiological integrity of the intact organism, not only exemplify the successful application of transgenic technologies to neuroendocrine systems, but also illustrate the problems that have been encountered, and the challenges that lie ahead.

Activation of the insulin gene promoter through a direct effect of hepatocyte nuclear factor 4 alpha

Maturity onset diabetes of the young, subtype 1 (MODY1), is associated with defective glucose-dependent insulin secretion from pancreatic beta cells. MODY1 is caused by mutation in the transcription factor hepatocyte nuclear factor 4 alpha (HNF4 alpha). To understand better the MODY1 phenotype, we tested whether HNF4 alpha was able to modulate directly the insulin gene promoter. Transfection of cultured 293T cells with an HNF4 alpha expression vector led to 10-fold activation of a cotransfected reporter plasmid containing the rat insulin I gene promoter. Computer analysis revealed a potential HNF4 alpha-binding site between nucleotides -57 and -69 of the promoter; mutation of this sequence led to reduced ability of HNF4 alpha to activate the promoter. The ability of HNF4 alpha to bind this sequence was confirmed using gel shift analysis. In transfected INS-1 beta cells, mutation of either the HNF1 alpha site or the HNF4 alpha site in the insulin gene promoter led to 50-75% reduction in reporter gene activity; expression of dominant negative HNF4 alpha led to significant reduction in the activity of wild type and both mutated promoters. Thus, in addition to the previously described indirect action of HNF4 alpha on insulin gene expression mediated through elevated HNF1 alpha levels, HNF4 alpha also activates the insulin gene directly, through a previously unrecognized cis element.

Insulin-enhanced liposome-mediated gene transfer into a gastric carcinoma cell line

1. Liposome-mediated transfection is useful due to no DNA constraints, lower immunogenicity and easy preparation. However, it has the disadvantage of low transfection efficiency. We aimed to test whether lipofection efficiency could be enhanced in gastrointestinal cell lines by the growth-promoting effect of insulin. 2. To assess the effect of insulin on lipofection efficiency and the cell cycle, expression of green fluorescent protein (GFP) and DNA distribution in gastric (MKN1), colonic (HT29) and pancreatic (BxPC3) carcinoma cell lines was analysed using flow cytometry. 3. The percentage of positive cells with GFP was significantly higher in MKN1 cells in culture medium with 5 mg/mL insulin than without insulin, whereas the percentage was the same in HT29 and BxPC3 cells with insulin as without insulin. The percentage of S phase fraction MKN1 cells with insulin was greater than without insulin, whereas the percentage of S phase fractions of HT29 and BxPC3 cells was the same with or without insulin. Lipofection efficiency correlated with the percentage of S phase fraction. 4. Insulin has the potential to enhance efficiency of lipofection into a sensitive cell line by increasing cellular proliferation.

Insulators prevent transcriptional interference between two promoters in a double gene construct for transgenesis

In transgenesis, the expression of two transgenes is often subject to mutual interference by each of the two expression cassettes when they are driven by different transcriptional regulatory elements in a single construct. To study this problem, we constructed vectors consisting of two expression units, one contains a strong ubiquitous promoter and the other contains a tissue-specific transcriptional element. The expression pattern of each transgene was examined in transfected cell lines and also in transgenic mice. In both cases, two expression units in a single construct were expressed in an independent manner and were controlled by their respective regulatory element only if we placed insulators at both ends of one expression unit. These results indicate that usage of insulators is a valuable tool for transfection of double gene constructs in transgenesis.

The human prepro-orexin gene regulatory region that activates gene expression in the lateral region and represses it in the medial regions of the hypothalamus

Prepro-orexin is a precursor of the neuropeptides orexin-A and -B, which are localized in the neuronal population of the lateral hypothalamic area (LHA). We wished to elucidate the mechanisms by which the prepro-orexin gene is specifically activated in orexin neurons in the LHA. The 3.2-kb 5'-flanking region of the human prepro-orexin gene is sufficient for the specific expression of an Escherichia coli lacZ reporter gene in orexin neurons. Therefore, we examined a series of reporter constructs harboring this 3.2-kb regulatory region or its deletion in a reporter transgenic mouse assay. There are two phylogenetically conserved regions located 287 bp (orexin regulatory element (OE) 1) and 2.5 kb (OE2) upstream of the transcription initiation site of the human prepro-orexin gene. In transgenic mice, both OE1 and OE2 are necessary for expressing the human prepro-orexin gene in the LHA and for repressing its expression in the medial regions of the hypothalamus. Through serial deletion analysis of OE1, we found that the 57-bp core region of OE1 is critical for its spatial gene regulatory function in vivo. Mutation analysis further demonstrated that without contribution from the OE1 core region, the lacZ reporter is expressed ectopically in the medial regions of the hypothalamus. Thus, OE1 contains crucial cis-acting elements regulating prepro-orexin gene expression specifically in the LHA.

Production of transgenic rodents by the microinjection of cloned DNA into fertilized one-cell eggs

Transgenic technologies that enable rapid movement between genotype and phenotype through specific loss-of-function, overexpression, or misexpression phenotypes will be crucial in the elucidation of gene sequences emerging from genome projects. This article describes detailed procedures for the generation of transgenic mice and rats by the injection of cloned DNA into the pronuclei of fertilized one-cell eggs.

Regulation of insulin gene transcription

Recent studies of the insulin gene promoter and the transcription factors that regulate it have expanded our understanding of both how the production of insulin is restricted to the pancreatic beta -cell, and how that production is regulated by physiologic signals such as glucose. A picture is emerging in which an elaborate set of transcription factors binds to specific sequences along the promoter and recruits additional transcriptional co-activators to build a functional transcriptional activation complex that is unique to beta -cells. Surprisingly, however, genetic experiments in mice have demonstrated an unexpected degree of redundancy in the factors that control insulin gene expression, and have revealed the presence of a network of transcription factors that coordinate the expression of factors forming the insulin gene activation complex.

The genomic structure and promoter analysis of the human ABF-1 gene

The human ABF-1 gene is expressed in activated B-cells and Epstein-Barr virus-immortalized lymphoblastoid cell lines. ABF-1 represents the only member belonging to the basic helix-loop-helix (bHLH) family of transcription factors whose expression pattern is restricted to B-cells. ABF-1 forms heterodimeric complexes with E2A to modulate gene transcription. We report the cloning and characterization of the human ABF-1 gene and the promoter region. The gene spans more than 3 kb and contains two exons. Exon 1 contains 274 bp of a 5'-untranslated sequence (UTR) while exon 2 contains 1097 bp of 3'-UTR. Promoter analysis of the 5'-flanking region revealed no apparent B-cell-restricted control elements within approximately 700 bp, but clearly demonstrated the presence of a functional minimal promoter residing immediately upstream of the transcription start site. Analysis of the region containing the minimal promoter activity identified no CCAAT or TATA sequence. Lastly, we have assigned the ABF-1 gene to human chromosome 8q21.1 using fluorescence in situ hybridization (FISH). The cloning of the human ABF-1 gene will facilitate further biochemical and genetic studies of its function in the regulation of B-cell differentiation.

The homeodomain of PDX-1 mediates multiple protein-protein interactions in the formation of a transcriptional activation complex on the insulin promoter

Activation of insulin gene transcription specifically in the pancreatic beta cells depends on multiple nuclear proteins that interact with each other and with sequences on the insulin gene promoter to build a transcriptional activation complex. The homeodomain protein PDX-1 exemplifies such interactions by binding to the A3/4 region of the rat insulin I promoter and activating insulin gene transcription by cooperating with the basic-helix-loop-helix (bHLH) protein E47/Pan1, which binds to the adjacent E2 site. The present study provides evidence that the homeodomain of PDX-1 acts as a protein-protein interaction domain to recruit multiple proteins, including E47/Pan1, BETA2/NeuroD1, and high-mobility group protein I(Y), to an activation complex on the E2A3/4 minienhancer. The transcriptional activity of this complex results from the clustering of multiple activation domains capable of interacting with coactivators and the basal transcriptional machinery. These interactions are not common to all homeodomain proteins: the LIM homeodomain protein Lmx1.1 can also activate the E2A3/4 minienhancer in cooperation with E47/Pan1 but does so through different interactions. Cooperation between Lmx1.1 and E47/Pan1 results not only in the aggregation of multiple activation domains but also in the unmasking of a potent activation domain on E47/Pan1 that is normally silent in non-beta cells. While more than one activation complex may be capable of activating insulin gene transcription through the E2A3/4 minienhancer, each is dependent on multiple specific interactions among a unique set of nuclear proteins.

Transcription factor BETA2 acts cooperatively with E2A and PDX1 to activate the insulin gene promoter

The insulin gene is efficiently expressed only in pancreatic beta cells. Using reverse transcriptase-polymerase chain reaction analysis, we show that insulin mRNA levels are at least 10(5)-fold higher in beta cells than non-beta cells. To examine the underlying mechanisms, we expressed beta cell transcription factors by transfection of non-beta cells. Separate expression of BETA2, E2A, or PDX1 led to modest (<10-fold) activation of the insulin promoter, whereas co-expression of the three proteins produced synergistic, high level activation (160-fold). This level of activity is approximately 25% that observed in transfected beta cell lines. Of the three factors studied, BETA2 appears to play a dominant role. Efficient transcription required a C-terminal activation domain of BETA2 and an N-terminal region, which does not function as an independent activation domain. The myogenic basic helix-loop-helix (bHLH) protein MyoD was unable to bind and activate the promoter, even when its DNA binding region was replaced with that of BETA2. Our results demonstrate the central importance of BETA2 in insulin gene transcription and the importance of sequences outside the canonical DNA binding domain in permitting efficient DNA binding and cell-specific activity of the insulin gene promoter.

Human insulin gene is a target gene of hepatocyte nuclear factor-1alpha (HNF-1alpha) and HNF-1beta

Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes characterized by autosomal dominant inheritance, early-onset, and impaired insulin secretion. The type 3 and type 5 forms of MODY result from mutations in the genes encoding the transcription factor, hepatocyte nuclear factor (HNF)-1alpha and HNF-1beta, respectively. The mechanism by which mutations in one allele of the HNF-1 gene impair pancreatic beta cell function is unclear. We studied the effects of wild-type and four mutant (L12H, R263C, P379fsdelCT, and L584S585fsinsTC) HNF-1alpha, which were identified in Japanese subjects with MODY3 on human insulin gene transcription. Both wild-type (WT) HNF-1alpha and HNF-1beta bound to the oligonucleotide containing the A3 element sequence in the human insulin promoter and transactivated the insulin-luciferase reporter gene by 30- and 31-fold, respectively. In contrast, binding of L12H, R263C and L584S585fsinsTC-HNF-1alpha to the probe was impaired. Transactivation activity by the four mutant HNF-1alpha was reduced (4.3 to 43.3% of WT). These data suggest that the insulin gene is a candidate target gene of HNF-1alpha/HNF-1beta and the impairment of insulin gene transcription by mutations in the HNF-1 gene might be involved in the pathogenesis of MODY.