Foxa3 (HNF-3gamma) binds to and activates the rat proglucagon gene promoter but is not essential for proglucagon gene expression

The role of FOXA family transcription factors in glucolipid metabolism and NAFLD

Abnormal glucose metabolism and lipid metabolism are common pathological processes in many metabolic diseases, such as nonalcoholic fatty liver disease (NAFLD). Many studies have shown that the forkhead box (FOX) protein subfamily FOXA has a role in regulating glucolipid metabolism and is closely related to hepatic steatosis and NAFLD. FOXA exhibits a wide range of functions ranging from the initiation steps of metabolism such as the development of the corresponding metabolic organs and the differentiation of cells, to multiple pathways of glucolipid metabolism, to end-of-life problems of metabolism such as age-related obesity. The purpose of this article is to review and discuss the currently known targets and signal transduction pathways of FOXA in glucolipid metabolism. To provide more experimental evidence and basis for further research and clinical application of FOXA in the regulation of glucolipid metabolism and the prevention and treatment of NAFLD.

Stage-specific transcriptomic changes in pancreatic α-cells after massive β-cell loss

Background

Loss of pancreatic insulin-secreting β-cells due to metabolic or autoimmune damage leads to the development of diabetes. The discovery that α-cells can be efficiently reprogrammed into insulin-secreting cells in mice and humans has opened promising avenues for innovative diabetes therapies. β-cell loss triggers spontaneous reprogramming of only 1–2% of α-cells, limiting the extent of regeneration. Most α-cells are refractory to conversion and their global transcriptomic response to severe β-cell loss as well as the mechanisms opposing their reprogramming into insulin producers are largely unknown. Here, we performed RNA-seq on FAC-sorted α-cells to characterize their global transcriptional responses at different time points after massive β-cell ablation.

Results

Our results show that α-cells undergo stage-specific transcriptional changes 5- and 15-days post-diphtheria toxin (DT)-mediated β-cell ablation. At 5 days, α-cells transiently upregulate various genes associated with interferon signaling and proliferation, including Interferon Induced Protein with Tetratricopeptide Repeats 3 (Ifit3). Subsequently, at 15 days post β-cell ablation, α-cells undergo a transient downregulation of genes from several pathways including Insulin receptor, mTOR and MET signaling.

Conclusions

The results presented here pinpoint novel markers discriminating α-cells at different stages after acute β-cell loss, and highlight additional signaling pathways that are modulated in α-cells in this context.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07812-x.

Glucagon: the renewal of an old hormone in the pathophysiology of diabetes

Type 2 diabetes (T2D) is one of the most common diseases, affecting 5-10% of the population in most countries; the progression of its prevalence has been constant over the past 50 years in all countries worldwide, creating a major public health problem in terms of disease management and financial burden. Although the pathophysiology of T2D has been attributed for decades to insulin resistance and decreased insulin secretion, particularly in response to glucose, the contributing role of glucagon in hyperglycemia has been highlighted since the early 1970s by demonstrating its glycogenolytic, gluconeogenic and ketogenic properties. More recently, the importance of glucagon in diabetes has been highlighted in a model of streptozotocin-induced diabetic mice becoming euglycemic in the absence of glucagon receptors and without insulin treatment. Understanding the dysregulation of α-cells in diabetes will be critical to better define the pathophysiology of diabetes and develop new antidiabetic treatment.

Expression of Id1 in adult, regenerating and developing pancreas

Several key transcription factors are necessary for alpha cell development in the pancreas. In this study, we describe the expression of Inhibition of DNA-binding protein 1 (Id1) in the developing as well as the normal adult pancreas. We found co-expression of Id1 with bone morphogenetic protein (BMP) receptor in alpha cells. Inhibition of BMP4 signaling with a specific neutralizing antibody slightly decreases the proportion of glucagon cells in the adult pancreas but had a significant effect in a model of pancreas regeneration. In late embryonic pancreas, Id1 co-localized with GATA4, a transcription factor known for its critical function in glucagon cell development. However, in early postnatal period, the expression of Id1 and GATA4 diverged with Id1 identified in glucagon cells and GATA4 restricted to the acinar pancreas.

PHA-4/FoxA cooperates with TAM-1/TRIM to regulate cell fate restriction in the C. elegans foregut

A key question in development is how pluripotent progenitors are progressively restricted to acquire specific cell fates. Here we investigate how embryonic blastomeres in C. elegans develop into foregut (pharynx) cells in response to the selector gene PHA-4/FoxA. When pha-4 is removed from pharyngeal precursors, they exhibit two alternative responses. Before late-gastrulation (8E stage), these cells lose their pharyngeal identity and acquire an alternative fate such as ectoderm (Specification stage). After the Specification stage, mutant cells develop into aberrant pharyngeal cells (Morphogenesis/Differentiation stage). Two lines of evidence suggest that the Specification stage depends on transcriptional repression of ectodermal genes by pha-4. First, pha-4 exhibits strong synthetic phenotypes with the B class synMuv gene tam-1 (Tandam Array expression Modifier 1) and with a mediator of transcriptional repression, the NuRD complex (NUcleosome Remodeling and histone Deacetylase). Second, pha-4 associates with the promoter of the ectodermal regulator lin-26 and is required to repress lin-26 expression. We propose that restriction of early blastomeres to the pharyngeal fate depends on both repression of ectodermal genes and activation of pharyngeal genes by PHA-4.

FoxO1 is required for the regulation of preproglucagon gene expression by insulin in pancreatic alphaTC1-9 cells

Forkhead/winged helix box gene, group O-1 (FoxO1) is a member of a family of nuclear transcription factors regulated by insulin-dependent phosphorylation and implicated in the development of the endocrine pancreas. We show here firstly that FoxO1 protein is expressed in both primary mouse islet alpha and beta cells. Examined in clonal alphaTC1-9 cells, insulin caused endogenous FoxO1 to translocate from the nucleus to the cytoplasm. Demonstrating the importance of nuclear exclusion of FoxO1 for the inhibition of preproglucagon gene expression, FoxO1 silencing by RNA interference reduced preproglucagon mRNA levels by >40% in the absence of insulin and abolished the decrease in mRNA levels elicited by the hormone. Electrophoretic mobility shift assay and chromatin immunoprecipitation revealed direct binding of FoxO1 to a forkhead consensus binding site, termed GL3, in the preproglucagon gene promoter region, localized -1798 bp upstream of the transcriptional start site. Deletion or mutation of this site diminished FoxO1 binding and eliminated transcriptional regulation by glucose or insulin. FoxO1 silencing also abolished the acute regulation by insulin, but not glucose, of glucagon secretion, demonstrating the importance of FoxO1 expression in maintaining the alpha-cell phenotype.

In control of biology: of mice, men and Foxes

Forkhead proteins comprise a highly conserved family of transcription factors, named after the original forkhead gene in Drosophila. To date, over 100 forkhead genes have been identified in a large variety of species, all sharing the evolutionary conserved 'forkhead' DNA-binding domain, and the cloning and characterization of forkhead genes have continued in recent years. Forkhead transcription factors regulate the expression of countless genes downstream of important signalling pathways in most, if not all, tissues and cell types. Recent work has provided novel insights into the mechanisms that contribute to their functional diversity, including functional protein domains and interactions of forkheads with other transcription factors. Studies using loss- and gain-of-function models have elucidated the role of forkhead factors in developmental biology and cellular functions such as metabolism, cell division and cell survival. The importance of forkhead transcription factors is underlined by the developmental defects observed in mutant model organisms, and multiple human disorders and cancers which can be attributed to mutations within members of the forkhead gene family. This review provides a comprehensive overview of current knowledge on forkhead transcription factors, from structural organization and regulatory mechanisms to cellular and developmental functions in mice and humans. Finally, we will discuss how novel insights gained from involvement of 'Foxes' in the mechanisms underlying human pathology may create new opportunities for treatment strategies.

Formation of the digestive system in zebrafish: III. Intestinal epithelium morphogenesis

Recent analysis of a novel strain of transgenic zebrafish (gutGFP) has provided a detailed description of the early morphological events that occur during the development of the liver and pancreas. In this paper, we aim to complement these studies by providing an analysis of the morphological events that shape the zebrafish intestinal epithelium. One of our goals is to provide a framework for the future characterization of zebrafish mutant phenotypes in which intestinal epithelial morphogenesis has been disrupted. Our analysis encompasses the period between 26 and 126 h post-fertilization (hpf) and follows the growth, lumen formation and differentiation of a continuous layer of endoderm into a functional intestinal epithelium with three morphologically distinct segments: the intestinal bulb, mid-intestine and posterior intestine. Between 26 hpf and 76 hpf, the entire intestinal endoderm is a highly proliferative organ. To make a lumen, the zebrafish endoderm cells undergo apical membrane biogenesis, adopt a bilayer configuration and form small cavities that coalesce without cell death. Thereafter, the endoderm cells polarize and differentiate into distinct cell lineages. Enteroendocrine cells are distinguished first at 52 hpf in the caudal region of the intestine in a new stable transgenic line, Tg[nkx2.2a:mEGFP]. The differentiation of mucin-containing goblet cells is first evident at 100 hpf and is tightly restricted to a middle segment of the intestine, designated the mid-intestine, that is also demarcated by the presence of enterocytes with large supranuclear vacuoles. Meanwhile, striking expansion of the lumen in the rostral intestine forms the intestinal bulb. Here the epithelium elaborates folds and proliferating cells become progressively restricted to a basal compartment analogous to the crypts of Lieberkühn in mammals. At 126 hpf, the posterior intestine remains an unfolded monolayer of simple columnar epithelium.

Characterization of a novel Foxa (hepatocyte nuclear factor-3) site in the glucagon promoter that is conserved between rodents and humans

The pancreatic islet hormone glucagon stimulates hepatic glucose production and thus maintains blood glucose levels in the fasting state. Transcription factors of the Foxa [Fox (forkhead box) subclass A; also known as HNF-3 (hepatocyte nuclear factor-3)] family are required for cell-specific activation of the glucagon gene in pancreatic islet alpha-cells. However, their action on the glucagon gene is poorly understood. In the present study, comparative sequence analysis and molecular characterization using protein-DNA binding and transient transfection assays revealed that the well-characterized Foxa-binding site in the G2 enhancer element of the rat glucagon gene is not conserved in humans and that the human G2 sequence lacks basal enhancer activity. A novel Foxa site was identified that is conserved in rats, mice and humans. It mediates activation of the glucagon gene by Foxa proteins and confers cell-specific promoter activity in glucagon-producing pancreatic islet alpha-cell lines. In contrast with previously identified Foxa-binding sites in the glucagon promoter, which bind nuclear Foxa2, the novel Foxa site was found to bind preferentially Foxa1 in nuclear extracts of a glucagon-producing pancreatic islet alpha-cell line, offering a mechanism that explains the decrease in glucagon gene expression in Foxa1-deficient mice. This site is located just upstream of the TATA box (between -30 and -50), suggesting a role for Foxa proteins in addition to direct transcriptional activation, such as a role in opening the chromatin at the start site of transcription of the glucagon gene.

Side Population cells isolated from different tissues share transcriptome signatures and express tissue-specific markers

Side Population (SP) cells, isolated from murine adult bone marrow (BM) based on the exclusion of the DNA dye Hoechst 33342, exhibit potent hematopoietic stem cell (HSC) activity when compared to Main Population (MP) cells. Furthermore, SP cells derived from murine skeletal muscle exhibit both hematopoietic and myogenic potential in vivo. The multipotential capacity of SP cells isolated from variable tissues is supported by an increasing number of studies. To investigate whether the SP phenotype is associated with a unique transcriptional profile, we characterized gene expression of SP cells isolated from two biologically distinct tissues, bone marrow and muscle. Comparison of SP cells with differentiated MP cells within a tissue revealed that SP cells are in an active transcriptional and translational status and underexpress genes reflecting tissue-specific functions. Direct comparison of gene expression of SP cells isolated from different tissues identified genes common to SP cells as well as genes specific to SP cells within a particular tissue and further define a muscle and bone marrow environment. This study reports gene expression of muscle SP cells, common features and differences between SP cells isolated from muscle and bone marrow, and further identifies common signaling pathways that might regulate SP cell functions.

The role of Brn4/Pou3f4 and Pax6 in forming the pancreatic glucagon cell identity

Brain 4 (Brn4/Pou3f4) and Pax6 are POU-homeodomain and paired-homeodomain transcription factors, respectively, that are expressed in the brain and the glucagon-expressing cells in the pancreas. Brn4 expression begins at embryonic day 10 in the pancreas, just before pax6 and both appear in the glucagon immunoreactive cells. At a later time point, E19, no Brn4 co-localization is observed with insulin or somatostatin but a rare pancreatic polypeptide (PP)-producing cell can be found, while Pax6 is found in all endocrine cells. These data suggest that brn4 is the only alpha-cell specific transcription factor yet identified; therefore, we sought to analyze alpha-cell development and function in mice with a targeted disruption of the brn4 gene. In homozygous brn4(-/-) mice, pancreatic bud formation, glucagon cell numbers and physiological measurements all appear normal. Examination of other transcription factors found in the glucagon cells showed normal Pax6 and Nkx2.2 immunoreactivity, suggesting that Brn4 does not regulate these transcription factors. Pax6 mutant mice (pax6(Sey/Sey)), with a natural inactivating mutation in pax6, have few endocrine cells but normal numbers of Brn4 and Nkx2.2 cells. The pancreatic phenotype of the pax6 mutants can be rescued with a YAC clone containing the human Pax6 gene.

Cell and molecular biology of the small intestine: new insights into differentiation, growth and repair

PURPOSE OF REVIEW

This paper will discuss recent research that has provided new insights into the molecular and cell biology of the small intestine.

RECENT FINDINGS

Differentiation of the epithelial cell lineages, including the enterocytes, enteroendocrine, Goblet and Paneth cells, from the stem cells is better understood. Important interactions have been demonstrated between these cells, luminal bacteria, and underlying mesenchymal tissue. Intestine-specific gene expression is regulated by transcription factors that are becoming well characterized, including CDX1, CDX2 and HNF1. The actions of growth factors such as GLP-2 and EGF are now known to be complex, demonstrating multiple effects in this tissue at a number of levels.

SUMMARY

Progress in the cellular and molecular biology of the small intestine is producing many intriguing new findings.

Contrasting patterns of expression of transcription factors in pancreatic alpha and beta cells

Pancreatic alpha and beta cells are derived from the same progenitors but play opposing roles in the control of glucose homeostasis. Disturbances in their function are associated with diabetes mellitus. To identify many of the proteins that define their unique pathways of differentiation and functional features, we have analyzed patterns of gene expression in alphaTC1.6 vs. MIN6 cell lines by using oligonucleotide microarrays. Approximately 9-10% of >11,000 transcripts examined showed significant differences between the two cell types. Of >700 known transcripts enriched in either cell type, transcription factors and their regulators (TFR) was one of the most significantly different categories. Ninety-six members of the basic zipper, basic helix-loop-helix, homeodomain, zinc finger, high mobility group, and other transcription factor families were enriched in alpha cells; in contrast, homeodomain proteins accounted for 51% of a total of 45 TFRs enriched in beta cells. Our analysis thus highlights fundamental differences in expression of TFR subtypes within these functionally distinct islet cell types. Interestingly, the alpha cells appear to express a large proportion of factors associated with progenitor or stem-type cells, perhaps reflecting their earlier appearance during pancreatic development. The implications of these findings for a better understanding of alpha and beta cell dysfunction in diabetes mellitus are also considered.

Aberrant regulation of human intestinal proglucagon gene expression in the NCI-H716 cell line

Despite interest in understanding glucagon-like peptide-1 (GLP-1) production, the factors important for GLP-1 biosynthesis remain poorly understood. We examined control of human proglucagon gene expression in NCI-H716 cells, a cell line that secretes GLP-1 in a regulated manner. Insulin, phorbol myristate acetate, or forskolin, known regulators of rodent proglucagon gene expression, had no effect, whereas sodium butyrate decreased levels of NCI-H716 proglucagon mRNA transcripts. The inhibitory effect of sodium butyrate was mimicked by trichostatin A but was not detected with sodium acetate or isobutyrate. The actions of butyrate were not diminished by the ERK1/2 inhibitor PD98059, p38 inhibitor SB203580, or soluble guanylate cyclase inhibitor LY83583 or following treatment of cells with KT5823, a selective inhibitor of cGMP-dependent protein kinase. NCI-H716 cells expressed multiple proglucagon gene transcription factors including isl-1, pax-6, pax-2, cdx-2/3, pax-4, hepatocyte nuclear factor (HNF)-3 alpha, HNF-3beta, HNF-3 gamma, and Nkx2.2. Nevertheless, the butyrate-dependent inhibition of proglucagon gene expression was not associated with coordinate changes in transcription factor expression and both the human and rat transfected proglucagon promoters were transcriptionally inactive in NCI-H716 cells. Hence, NCI-H716 cells may not be a physiologically optimal model for studies of human enteroendocrine proglucagon gene transcription.

Glucagon-like peptide 2 function in domestic animals

Glucagon-like peptide 2 (GLP-2) is a member of family of peptides derived from the proglucagon gene expressed in the intestines, pancreas and brain. Tissue-specific posttranslational processing of proglucagon leads to GLP-2 and GLP-1 secretion from the intestine and glucagon secretion from the pancreas. GLP-2 and GLP-1 are co-secreted from the enteroendocrine L-cells located in distal intestine in response to enteral nutrient ingestion, especially carbohydrate and fat. GLP-2 secretion is mediated by direct nutrient stimulation of the L-cells and indirect action from enteroendocrine and neural inputs, including GIP, gastrin-releasing peptide (GRP) and the vagus nerve. GLP-2 is secreted as a 33-amino acid peptide and is rapidly cleaved by dipeptidylpeptidase IV (DPP-IV) to a truncated peptide which acts as a weak agonist with competitive antagonistic properties. GLP-2 acts to enhance nutrient absorption by inhibiting gastric motility and secretion and stimulating nutrient transport. GLP-2 also suppresses food intake when infused centrally. The trophic actions of GLP-2 are specific for the intestine and occur via stimulation of crypt cell proliferation and suppression of apoptosis in mucosal epithelial cells. GLP-2 reduces gut permeability, bacterial translocation and proinflammatory cytokine expression under conditions of intestinal inflammation and injury. The effects of GLP-2 are mediated by a G-protein-linked receptor that is localized to the intestinal mucosa and hypothalamus. The intestinal localization of the GLP-2R to neural and endocrine cells, but not enterocytes, suggests that its actions are mediated indirectly via a secondary signaling mechanism. The implications of GLP-2 in domestic animal production are largely unexplored. However, GLP-2 may have therapeutic application in treatment of gastrointestinal injury and diarrheal diseases that occur in developing neonatal and weanling animals.

Transcriptional activation of the proglucagon gene by lithium and beta-catenin in intestinal endocrine L cells

The proglucagon gene encodes several peptide hormones that regulate blood glucose homeostasis, growth of the small intestine, and satiety. Among them, glucagon-like peptide 1 (GLP-1) lowers blood glucose levels in patients with diabetes and inhibits eating and drinking in fasted rats. Although proglucagon transcription and GLP-1 synthesis were shown to be activated by forskolin and other protein kinase A (PKA) activators, deleting or mutating the cAMP-response element (CRE) only moderately attenuates the proglucagon gene promoter in response to PKA activation. Therefore, PKA may activate proglucagon transcription via a mechanism independent of the CRE motif. Recently, PKA was shown to phosphorylate and inactivate GSK-3beta, a key mediator in the Wnt signaling pathway. We show here that lithium, an inhibitor of GSK-3beta, activates proglucagon gene transcription and stimulates GLP-1 synthesis in an intestinal endocrine L cell line, GLUTag. The activation was also observed in primary fetal rat intestinal cell (FRIC) cultures, but not in a pancreatic A cell line. Co-transfection of beta-catenin, a downstream effector of GSK-3beta activities, activated the proglucagon gene promoter without a CRE. Furthermore, forskolin and 8-Br-cAMP phosphorylated GSK-3beta at serine 9 in intestinal proglucagon-producing cells, and both lithium and forskolin induced the accumulation of free beta-catenin in these cell lines. These observations indicate that the proglucagon gene is among the targets of the Wnt signaling pathway.

Gene expression cascades in pancreatic development

The specialized endocrine and exocrine cells of the pancreas originally derive from a pool of apparently identical cells in the early gut endoderm. Serial changes in their gene expression program, controlled by a hierarchy of pancreatic transcription factors, direct this progression from multipotent progenitor cell to mature pancreatic cell. When the cells differentiate, this hierarchy of factors coalesces into a network of factors that maintain the differentiated phenotype of the cells. As we develop an understanding of the pancreatic transcription factors, we are also acquiring the tools with which we can ultimately control pancreatic cell differentiation.