Transcriptional Regulation of Glucose Sensors in Pancreatic β-Cells and Liver: An Update

Pancreatic β-cells and the liver play a key role in glucose homeostasis. After a meal or in a state of hyperglycemia, glucose is transported into the β-cells or hepatocytes where it is metabolized. In the β-cells, glucose is metabolized to increase the ATP:ADP ratio, resulting in the secretion of insulin stored in the vesicle. In the hepatocytes, glucose is metabolized to CO2, fatty acids or stored as glycogen. In these cells, solute carrier family 2 (SLC2A2) and glucokinase play a key role in sensing and uptaking glucose. Dysfunction of these proteins results in the hyperglycemia which is one of the characteristics of type 2 diabetes mellitus (T2DM). Thus, studies on the molecular mechanisms of their transcriptional regulations are important in understanding pathogenesis and combating T2DM. In this paper, we will review a recent update on the progress of gene regulation of glucose sensors in the liver and β-cells.


Introduction
Glucose is one of the most important molecules that acts as a basic fuel for energy source and a substrate for intermediary metabolism as well. Because of its essential role in the metabolism, most cells have evolved to have an apparatus to sense and transport extracellular glucose into the cells. The glucose sensing in mammalian cells is regulated by both direct and indirect pathways. In the postprandial state, temporarily increased glucose has to be disposed of to prevent the cells from gluco-toxicity. After meal, glucose in the blood is absorbed near the portal vein, and metabolized in the liver and pancreas [1].
Blood glucose is transported into the liver and β-cells of pancreas through solute carrier family 2 (SLC2A2, also known as GLUT2) and immediately phosphorylated by glucokinase present in the liver (LGCK) or β-cells (βGCK) which acts as a glucose sensor. Glucose-6-phosphate in the hepatocytes undergoes glycolysis, glycogenesis, pentose phosphate pathway, or hexosamine biosynthetic pathway depending on the metabolic needs. Both SLC2A2 and GCK have high K m values and high capacity and thus are able to sense and transport glucose into hepatocytes or β-cells in proportion to the blood glucose level [2].
In the β-cells, glucose is metabolized and thereby increases intracellular the ATP:ADP ratio which causes suppression of ATP-sensitive K + channel and triggers insulin secretion [3][4][5]. In addition, the gene expression of insulin is stimulated by glucose and is subjected to control at the transcriptional level.
Most of the type 2 diabetes mellitus (T2DM)-associated genes are mainly involved both in β-cell function and peripheral insulin sensitivity. Mutations in the GCK gene are associated with maturity onset diabetes of the young (MODY), a subtype of diabetes characterized by monogenic autosomal dominant transmission, early age of onset (typically less than 25 years of age) and primary defects in β-cell function. MODY are also associated with mutations in the genes encoding transcription factors like, hepatic nuclear factor 4 alpha (HNF4A), HNF1 homeobox A (HNF1A), pancreatic and duodenal homeobox 1 (PDX1), HNF1 homeobox B (HNF1B) and neurogenic differentiation 1 (NEUROD1) [6][7][8]. Furthermore, these transcription factors are known to be involved in the regulation of tissue-specific expression of SLC2A2 and/or GCK genes [9]. Dysfunctional mutation in SLC2A2 gene is also found in one patient with T2DM [10].
The gene expression of SLC2A2 and GCK is affected by metabolic conditions and are also tissue-specific. SLC2A2 is primarily expressed in the liver and β-cells [11,12] and its gene expression is affected by the blood glucose and insulin [13,14]. In diabetic animal models, SLC2A2 mRNA level is increased in the liver [15], whereas it is decreased in β-cells [16,17]. GCK is expressed mainly in the mammalian liver and β-cells, with two alternative promoters that govern tissue-specific expression [18][19][20][21]. The βGCK promoter (upstream promoter) is regulated by glucose, whereas the LGCK promoter (downstream promoter) is regulated by insulin and glucagon [21].
In this review, we will focus on an update on the transcriptional regulation of SLC2A2 and GCK genes in the liver and β-cells. Studying the molecular mechanisms in relation to T2DM will help understand its pathogenesis and find potential drug targets for the development of therapeutic drugs.
HNF1A is an essential transcription factor for the expression of Slc2a2 gene in β-cells. In transgenic mice over-expressing a dominant negative form of HNF1A, the expression of Slc2a2 gene is decreased in the pancreatic islets. In Hnf1a knockout mice, the expression of Slc2a2 gene was decreased in the pancreatic islets, but not affected in the liver [23][24][25].
Both HNF1A and forkhead box A2 (FOXA2, also known as HNF3B) are responsible for the tissue-specific expression of the human SLC2A2 gene. These factors synergistically increase the promoter activity of human SLC2A2 gene in NIH-3T3 cells. Binding of HNF1A and FOXA2 to +96/+108 and +114/+120 bp region of human SLC2A2 promoter was identified and these binding sites were well conserved in the mouse and rat gene [26]. HNF1A and FOXA2 also upregulate Slc2a2 mRNA in the kidney of diabetic rats [27]. Another HNF1A binding site (+200/+212 bp) was found in the promoter of human SLC2A2 gene. A mutation study revealed that the +200/+212 bp site is more important than the +96/+108 bp one in HNF1A-induced SLC2A2 gene expression. Moreover, E1A binding protein p300 (EP300) potentiates activity of the human SLC2A2 promoter by interacting with the transactivation domain of HNF1A [28].
In transgenic mice or adenoviral transduction of recombinant Foxa2 (AdFoxa2), Slc2a2 mRNA level was decreased in the liver [29, 30], presumably because FOXA2 represses one cut homeobox 1 (Onecut1, also known as Hnf6) gene expression, which is a positive regulator of Slc2a2 gene in the mouse liver [31]. However, mRNA levels of Slc2a2 and Onecut1 were not altered in liver specific Foxa2 knockout mice [32]. In addition, Slc2a2 gene expression was not altered in β-cell specific Foxa2 knockout mice [33]. These reports suggest that transactivating effect of FOXA2 on Slc2a2 promoter may be weak or absent in mouse liver and β-cells. Although PDX1 was shown to bind in vitro to the promoter region of β-cell specific genes, including Slc2a2 (EMSA data), chromatin immunoprecipitation (ChIP) assays indicated that PDX1 did not bind to the promoter of Slc2a2 gene in the β-TC3 cells. These results suggest that selectivity of PDX1 may depend on the cell type specific chromatin structures and/or the presence of interacting proteins [42]. Indeed, PDX1 of which binding to Slc2a2 promoter is reinforced by high-mobility group N 3 (HMGN3), a chromatin binding protein that is highly expressed in β-cells [43].
V-maf musculoaponeurotic fibrosarcoma oncogene homolog B (MAFB) is a critical transcription factor for β-cell differentiation, although its expression was not observed in adult pancreas β-cells [55][56][57]. Slc2a2 and Pdx1 gene expression is reduced in the embryonic pancreas of Mafb knockout mice and MAFB binds to the promoter of Slc2a2 gene in vivo [57]. Because PDX1 is a positive regulator of Slc2a2 gene expression, Slc2a2 gene expression may be regulated by MAFB either directly or indirectly. Furthermore, Slc2a2 expression is decreased in Mafa-deficient mice [58]. However, MAF response element(s) in the Slc2a2 promoter has not been identified.
Glucose increased the Slc2a2 gene expression in the liver and β-cells, both in vivo and in vitro [14,[67][68][69][70]. Recently, we have identified a sterol regulatory element binding transcription factor 1 (SREBF1) response element (SRE) in the promoter of mouse Slc2a2 gene, which is responsive to glucose in primary cultured hepatocytes [69]. Furthermore, glucose-induction of the Slc2a2 gene expression in pancreatic islets was not found in Srebf1 knockout mice [70].
Also, cyclic adenosine monophosphate (cAMP) prevents the glucose-mediated stimulation of Slc2a2 gene expression in hepatocytes. The -312/+49 bp region of the mouse Slc2a2 promoter is responsible for cAMP responsiveness [71]. However, functional cAMP response element(s) within this region has not been identified.
An orphan nuclear receptor NR4A1 (nuclear receptor subfamily 4, group A, member 1) binds to nerve growth factor I-B response element (NBRE) in mouse Slc2a2 promoter (-82/-75 bp) and increases mRNA level in primary cultured hepatocytes and liver. Furthermore, Slc2a2 mRNA induced by NR4A1 is synergistically enhanced by PPARG coactivator 1 alpha (PPARGC1A) [72]. Because NR4A1 expression is highly regulated by the cAMP axis in the liver [72], these results are not consistent with the previous report that cAMP decreases the promoter activity of the Slc2a2 gene [71]. Further studies are needed to elucidate these contradictory results.
Insulin plays a negative role in Slc2a2 gene expression in the liver [14]. Since insulin-FOXO1 pathway is responsible for the negative role in insulin-mediated gene expression [73], it is tempting to speculate that FOXO1 may be a negative regulator of Slc2a2 gene expression. However, there is no evidence that FOXO1 is involved in the gene expression of Slc2a2 in the liver, although its negative role of FOXO1 in the Slc2a2 gene expression was shown in β-cells.
CCAAT/enhancer binding protein (CEBP) is shown to activate rat Slc2a2 promoter in HepG2 cells. The promoter has two CEBP consensus sequences binding CEBPA and CEBPB (Figure 1). These factors synergistically activate the promoter [74].
Kruppel-like factor 7 (KLF7) is known to reduce mRNA level of SLC2A2 in HIT-T15 and HepG2 cells. Because KLF7 is shown to reduce PDX1 gene expression in HIT-T15 cells [75], SLC2A2 gene expression may be regulated by KLF7 either directly or indirectly.
Although tissue-specific transcriptional regulation is not absolutely consistent between human and mouse [76], SLC2A2 gene expression in liver was activated by ONECUT1, FOXA3, PPARG, SREBF1c, NR4A1, CEBPA, CEBPB and KLF7. On the other hand, SLC2A2 gene expression in β-cells was activated by HNF1A, HNF4A, PDX1, HMGN3, MAFA, MAFB, PPARA, PPARG and KLF7 and was suppressed by FOXO1. The gene expression or activity of these regulators in abnormal conditions like high-fat or high-carbohydrate diet and cellular stress may contribute to the etiology of T2DM.

Beta Cell Glucokinase (βGCK)
GCK plays a critical role in maintaining the postprandial glucose level near 5 mM, which is achieved by glucose stimulated insulin secretion (GSIS) from β-cells and glucose metabolism in the liver [77]. βGCK is a primary determinant of blood glucose level because it senses glucose for GSIS.
Upregulation of βGck gene expression by glucose is mediated by insulin. In this mechanism, insulin receptor B type, PI3K class 1a and p70 s6 kinase pathway are known to be involved in glucose-regulated βGck transcription [78]. Furthermore, βGck gene expression is increased when MIN6 cells were cultured at 30 mM glucose [79]. However, βGck mRNA level is not changed in rats which are subjected to fasting/refeeding although LGck gene expression is significantly increased [80]. Indeed, the 4 kb promoter reporter construct of the βGck gene was not activated either by glucose (30 mM) or insulin (20 nM) [81]. These studies indicate that the role of glucose or insulin in the activation of βGCK may occur by stabilization rather than upregulation of βGck gene expression in insulinoma cells [21].
As shown in Figure 2, the gene expression of βGCK is regulated by various transcription factors. In its 5'-flanking region, there are three upstream promoter elements (UPEs) and two Pal motifs. Particulary, UPE-3 and Pal motifs are well conserved in the rat, mouse and human genes [19, 82,83]. The Pal motifs consist of inverted repeats separated by 1 bp (TGGTCACCA). The promoter activity of βGck gene was decreased by the introduction of mutation in the Pal motifs. These Pal motifs are pivotal determinants for the neural/neuroendocrine cell-specific expression of the Gck promoter [82,83].
UPEs have AT-rich sequence, which is known to be responsible for PDX1 activation. PDX1 is a master regulator for maintaining function and differentiation of β-cell. In the presence of glucose, PDX1 is phosphorylated and translocated into nucleus [84][85][86]. Expression of PDX1 increased the reporter activity of βGck promoter in CHO cells. PDX1 binding site is conserved at UPE3 region in human βGCK promoter and may play an essential role for β-cell function [87]. However, β-cell specific disruption of Pdx1 did not affect the expression of βGck although Slc2a2 expression was down-regulated [39]. The transcriptional role of PDX1 on the βGCK gene is not fully understood.
In addition, insulin-like growth factor 1 (IGF1) is known to induce βGck gene expression by phosphorylating FOXO1. FOXO1 response element (FRE) is located at -550/-543 bp of rat βGck promoter and FOXO1 binding to FRE is decreased by IGF1 in vitro [90].
Although PDX1, NEUROD1 and NK2 homeobox 2 (NKX2-2) bind to the region of -285/-5 bp in the mouse βGck promoter, their respective response elements in the βGck promoter have not been characterized [91]. Consistent with a role of NKX2-2 on the βGck promoter, Nkx2-2 knockout mice revealed a reduction in the βGck mRNA level. In addition, NKX2-2 appeared to play an important role for the differentiation of β-cells [92].

Liver Glucokinase (LGCK)
The gene expression of LGck is decreased in streptozotocin-induced diabetic rats and restored by insulin administration [93]. In addition, LGck gene expression is increased by insulin and decreased by the glucagons-cAMP system in primary cultured hepatocytes [94]. Insulin-induction of LGck gene expression is shown to be blocked by LY294002 or wortmannin, a PI3K inhibitor [95]. Furthermore, LGck gene expression is inhibited by a dominant negative form of insulin receptor substrate 1 (IRS1) [96]. These studies support that insulin is a principal regulator of LGck gene expression.
SREBF1c, one of the master regulators of lipogenesis, is dramatically induced by insulin [97,98].

Administration of recombinant adenovirus of Srebf1c to streptozotocin-induced diabetic mice restored
LGck and lipogenic enzymes normalizing blood glucose level despite that insulin is absent [99]. In addition, adenoviral expression of dominant negative form of SREBF1c in primary cultured hepatocytes decreased insulin-induction of LGck gene, suggesting a direct participation of SREBF1c in the LGck gene expression [100]. Furthermore, direct binding site of SREBF1c on rat LGck promoter is identified [101]. However, LGck gene expression is still increased by refeeding the Srebf1c knockout mice [102]. Moreover, LGck gene expression is not changed even though Srebf1c was knockdown by siRNA although fatty acid synthase mRNA level is decreased. These studies suggest that SREBF1c is not likely to be a mediator of LGck gene expression [103,104]. Further studies are needed to answer these contradictory results.
HNF4A is known to be an important transcription factor for glucose and lipid homeostasis [35,105]. HNF4A increases the LGCK gene expression and its binding site HRE (HNF response element) is identified in human [106] and rat [107]. During the fasting period, LGCK transcription by HNF4A is repressed by FOXO1 which acts as a corepressor, whereas the suppression is restored by feeding where FOXO1 is phosphorylated and extruded to cytosol by insulin [106].
Hypoxia inducible factor 1 alpha subunit (HIF1A) also affects promoter activity of rat LGck gene. HIF1A binding site is localized at -87/-80 bp region of the promoter. Both insulin [108] and hypoxia [21, 109,110] upregulate LGck gene expression by increasing HIF1A level and its DNA-binding activity. Transactivation by HIF1A was also enhanced by co-expression of HNF4A and EP300. Moreover, HIF1A interacts with HNF4A and each of these factors also interacts with EP300. It was suggested that synergy and cooperative interactions between HIF1A, HNF4A and EP300 might be necessary for insulin-stimulated LGck expression [109].
Signal transducer and activator of transcription 5B (STAT5B) is known to be regulated by insulin [111,112]. STAT5B phosphorylated by insulin is translocated to the nucleus and increases its binding to the cis-acting elements, thereby increasing the transcription of target genes. Activated STAT5B by insulin increased LGCK gene expression [113,114]. In humans, the binding site of STAT5B is characterized at -1368/-1360 bp region, but the mouse STAT5 response element (STAT5RE) was unknown. The binding affinity of STAT5B to the human LGCK promoter is also increased by insulin and the activation by insulin occurs in a janus kinase (JAK)-independent manner. Thus, it is suggested that STAT5B plays an important role in the insulin-mediated upregulation of LGck gene [113]. However, insulin is known to increase the transcription of LGck in primary cultured hepatocytes although the tyrosine phosphorylation of STAT5 was not detectable [114]. Abbreviations: STAT5B, signal transducer and activator of transcription 5B; HNF4A, hepatic nuclear factor 4 alpha; FOXO1, forkhead box O1; ONECUT1, one cut homeobox 1 (also known as HNF6); SREBF1c, sterol regulatory element binding transcription factor 1c; LXRA, liver X receptor alpha (also known as NR1H3); RXRA, retinoid X receptor alpha; NR0B2, nuclear receptor subfamily 0, group B, member 2 (also known as SHP); EP300, E1A binding protein p300; PPARG, peroxisome proliferator-activated receptor gamma; USF2, upstream stimulatory factor 2; HIF1A, hypoxia induced factor 1 alpha subunit; STAT5RE, STAT5B response element; HRE, HNF response element; FRE, FOXO1 response element; SRE, SREBF response element; PPRE, PPAR response element; HIFRE, HIF1A response element; LXRE, LXR response element.
During the fasting period, sirtuin 1 (SIRT1), an NAD + -dependent deacetylase, decreases the rat LGck gene expression by deacetylating FOXO1, which results in an increase in binding of FOXO1 to FRE (-537/-529 bp) of its promoter. They also observed that resveratrol enhances interaction of between FOXO1 and HNF4A, causing a decrease in the binding affinity of HNF4A to the HRE [115].
Upstream stimulating factor 2 (USF2) is responsible for the regulation of LGck gene expression by binding to the P2 element (-89/-81 bp), and thus, the transcription factor may be in part responsible for the glucose homeostasis [116].
TZDs are anti-diabetic drugs improving glucose utilization and insulin sensitivity. Troglitazone is a synthetic ligand of PPARG and is shown to upregulate LGck gene expression. The PPRE is located at the -116/-104 bp of LGck gene promoter [117].
Recently, liver X receptor alpha (LXRA, also known as NR1H3) was shown to upregulate LGck gene expression by binding to the LXR response element (LXRE) (-52/-39 bp) in its promoter. In addition, LXRA increases LGck gene expression by inducing SREBF1c and increasing transcriptional activity of PPARG. Furthermore, NR0B2 induced by LXRA plays a role in the fine-tuning the LGck gene expression [118]. Because the binding site of LXRA and HNF4A seems to be overlapped [107], detailed studies are needed to elaborate their precise roles of these factors with regard to specific metabolic conditions.
In mice, ONECUT1 binding site in the LGck gene promoter is localized at -7613/-7622 bp and -877/-868 bp, suggesting a possible link between Onecut1 deficiency and development of T2DM. This study could explain why T2DM occurs in Onecut1 knockout mice [119,120].

Effect of Promoter Polymorphisms on SLC2A2 and GCK Transcription
Single nucleotide polymorphisms (SNPs) in the promoter regions can affect the binding of transcription factors regulating the transcription of genes. Some significant promoter SNPs of SLC2A2 and GCK genes were reported in T2DM patients.

Promoter Polymorphisms in SLC2A2 Gene
Three common SNPs, -149C > A (rs5393), -122T > C (rs5394) and -44G > A (rs5396), in SLC2A2 promoter were identified in a Danish population. These SNPs were not significantly different in the genotype frequency between T2DM patients and control subjects. In addition, clinical characteristics of T2DM were not significantly associated [121]. However, SNPs rs5393 and rs5394 of SLC2A2 could be high risk genotypes to predict the conversion of T2DM in an obese Finnish subject who had impaired glucose tolerance [122]. This discrepancy is not explained at present, but it may be due to differences in the ethnic group or study population. Detailed functional studies of the effects of promoter SNPs on SLC2A2 transcription are needed.

Promoter Polymorphisms in GCK Gene
Recently, a functional βGCK promoter mutation (-71G > C) was identified in Slovakian and British patients with GCK-MODY phenotype who have no abnormality of the GCK coding seqeuence. The mutation was associated with increased fasting plasma glucose (FPG) levels. In addition, the βGCK promoter of the -71C allele showed remarkable reductions of promoter reporter activity in INS-1 cells due to the decreased SP1 binding to -82/-67 bp region ( Figure 2). These data suggested that the mutation was cosegregated with fasting hyperglycemia due to loss of SP1 binding [123].
Two SNPs, -6612G > A (rs4607517) [124,125] and -30G > A (rs1799884) [126][127][128][129], in the βGCK promoter were significantly associated with fasting plasma glucose level in several populations. Moreover, the -30G > A SNP has been associated with reduced β-cell function [130], impaired glucose tolerance [131] and T2DM [126]. These data suggested that rare alleles of these two SNPs may also inhibit the βGCK transcription. However, functional studies of these two SNPs are needed to clarify cause-effect in terms of βGCK transcription.

Conclusions
Although muscle or adipose tissue have been considered as the principal organs of glucose disposal, immediate handling of hyperglycemia in β-cells or liver is also important because high levels of glucose are toxic to various tissues. Thus, studies on the regulation of glucose sensors in the liver and