A karyopherin alpha2 nuclear transport pathway is regulated by glucose in hepatic and pancreatic cells

The role of KPNA2 as a monotonically changing differentially expressed gene in the diagnosis, risk stratification, and chemotherapy sensitivity of chronic hepatitis B-liver cirrhosis-hepatocellular carcinoma

PURPOSE

Chronic hepatitis B-liver cirrhosis-hepatocellular carcinoma (CLH), commonly called the "liver cancer trilogy", is a crucial evolutionary phase in the emergence of hepatocellular carcinoma (HCC) in China. Previous studies on early diagnostic biomarkers of HCC were limited to the end-stage of HCC and did not focus on the evolutionary process of CLH.

METHODS

11 monotonically changing differentially expressed genes (MCDEGs) highly correlated with CLH were screened through bioinformatic analysis and KPNA2 was identified for further research. The serum KPNA2 expression in different CLH states was detected by Enzyme linked immunosorbent assay (ELISA). A nomogram model was constructed using univariate and multivariate Cox regression methods.

RESULTS

The single-cell RNA-seq and bulk RNA-seq revealed that KPNA2 related to immune infiltration in HCC and may participate in cell cycle pathways in HCC. The serum KPNA2 expression was monotonically upregulated in CLH and was valuable for diagnosing different CLH states. Besides, chronic hepatitis B(CHB) patients, liver cirrhosis (LC) patients, and HCC patients were classified into subgroups with distinct serum KPNA2 expressions. Accordingly, patients with different serum KPNA2 expressions displayed various clinicopathological features. The AUC value of the nomogram model was 0.959 in predicting the likelihood of developing HCC in CHB patients or LC patients. Finally, we found that KPNA2 expression was negatively correlated with the IC50 of four chemotherapeutic drugs in HCC.

CONCLUSION

KPNA2 was a novel serum biomarker for diagnosing different CLH states, monitoring the dynamic evolution of CLH, and a new therapeutic target for intervening in the progression of CLH.

Novel roles of karyopherin subunit alpha 2 in hepatocellular carcinoma

Hepatocellular carcinoma is the most common type of liver cancer and associated with a high fatality rate. This disease poses a major threat to human health worldwide. A considerable number of genetic and epigenetic factors are involved in the development of hepatocellular carcinoma. However, the molecular mechanism underlying the progression of hepatocellular carcinoma remains unclear. Karyopherin subunit alpha 2 (KPNA2), also termed importin α1, is a member of the nuclear transporter family. In recent years, KPNA2 has been gradually linked to the nuclear transport pathway for a variety of tumor-associated proteins. Furthermore, it promotes tumor development by participating in various pathophysiological processes such as cell proliferation, apoptosis, immune response, and viral infection. In hepatocellular carcinoma, it has been found that KPNA2 expression is significantly higher in liver cancer tissues versus paracancerous tissues. Moreover, it has been identified as a marker of poor prognosis and early recurrence in patients with hepatocellular carcinoma. Nevertheless, the role of KPNA2 in the development of hepatocellular carcinoma remains to be determined. This review summarizes the current knowledge on the pathogenesis and role of KPNA2 in hepatocellular carcinoma, and provides new directions and strategies for the diagnosis, treatment, and prediction of prognosis of this disease.

Kidney Proximal Tubule GLUT2-More than Meets the Eye

Tubulopathy plays a central role in the pathophysiology of diabetic kidney disease (DKD). Under diabetic conditions, the kidney proximal tubule cells (KPTCs) are exposed to an extensive amount of nutrients, most notably glucose; these nutrients deteriorate KPTCs function and promote the development and progression of DKD. Recently, the facilitative glucose transporter 2 (GLUT2) in KPTCs has emerged as a central regulator in the pathogenesis of DKD. This has been demonstrated by identifying its specific role in enhancing glucose reabsorption and glucotoxicity, and by deciphering its effect in regulating the expression of the sodium-glucose transporter 2 (SGLT2) in KPTCs. Moreover, reduction/deletion of KPTC-GLUT2 has been recently found to ameliorate DKD, raising the plausible idea of considering it as a therapeutic target against DKD. However, the underlying molecular mechanisms by which GLUT2 exerts its deleterious effects in KPTCs remain vague. Herein, we review the current findings on the proximal tubule GLUT2 biology and function under physiologic conditions, and its involvement in the pathophysiology of DKD. Furthermore, we shed new light on its cellular regulation during diabetic conditions.

Karyopherin Alpha 2-Expressing Pancreatic Duct Glands and Intra-Islet Ducts in Aged Diabetic C414A-Mutant-CRY1 Transgenic Mice

Our earlier studies demonstrated that cysteine414- (zinc-binding site of mCRY1-) alanine mutant mCRY1 transgenic mice (Tg mice) exhibit diabetes characterized by the reduction of β-cell proliferation and by β-cell dysfunction, presumably caused by senescence-associated secretory phenotype- (SASP-) like characters of islets. Earlier studies also showed that atypical duct-like structures in the pancreas developed age-dependently in Tg mice. Numerous reports have described that karyopherin alpha 2 (KPNA2) is highly expressed in cancers of different kinds. However, details of the expression of KPNA2 in pancreatic ductal atypia and in normal pancreatic tissues remain unclear. To assess the feature of the expression of KPNA2 in the development of the ductal atypia and islet architectures, we scrutinized the pancreas of Tg mice histopathologically. Results showed that considerable expression of KPNA2 was observed in pancreatic β-cells, suggesting its importance in maintaining the functions of β-cells. In mature stages, the level of KPNA2 expression was lower in islets of Tg mice than in wild-type controls. At 4 weeks, the expression levels of KPNA2 in islets of Tg mice were the same as those in wild-type controls. These results suggest that the reduction of KPNA2 might contribute to β-cell dysfunction in mature Tg mice. Additionally, the formation of mucin-producing intra-islet ducts, islet fibrosis, and massive T cell recruitment to the islet occurred in aged Tg mice. In exocrine areas, primary pancreatic intraepithelial neoplasias (PanINs) with mucinous pancreatic duct glands (PDGs) emerged in aged Tg mice. High expression of KPNA2 was observed in the ductal atypia. By contrast, KPNA2 expression in normal ducts was quite low. Thus, upregulation of KPNA2 seemed to be correlated with progression of the degree of atypia in pancreatic ductal cells. The SASP-like microenvironment inside islets might play stimulatory roles in the formation of ductal metaplasia inside islets and in islet fibrosis in Tg mice.

High Basolateral Glucose Increases Sodium-Glucose Cotransporter 2 and Reduces Sirtuin-1 in Renal Tubules through Glucose Transporter-2 Detection

Under diabetic conditions, sodium-glucose cotransporter 2 (SGLT2) for glucose uptake in proximal tubules (PTs) increases, whereas NAD⁺-dependent protein deacetylase silent mating type information regulation 2 homolog 1 (Sirtuin-1; SIRT1) for PT survival decreases. Therefore, we hypothesized that increased glucose influx by SGLT2 reduces SIRT1 expression. To test this hypothesis, db/db mice with diabetes and high-glucose (HG)-cultured porcine PT LLC-PK1 cells in a two-chamber system were treated with the SGLT2 inhibitor canagliflozin. We also examined SIRT1 and SGLT2 expression in human kidney biopsies. In db/db mice, SGLT2 expression increased with concomitant decreases in SIRT1, but was inhibited by canagliflozin. For determination of the polarity of SGLT2 and SIRT1 expression, LLC-PK1 cells were seeded into Transwell chambers (pore size, 0.4 µm; Becton Dickinson, Oxford, UK). HG medium was added to either or to both of the upper and lower chambers, which corresponded to the apical and basolateral sides of the cells, respectively. In this system, the lower chamber with HG showed increased SGLT2 and decreased SIRT1 expression. Canagliflozin reversed HG-induced SIRT1 downregulation. Gene silencing and inhibitors for glucose transporter 2 (GLUT2) blocked HG-induced SGLT2 expression upregulation. Gene silencing for the hepatic nuclear factor-1α (HNF-1α), whose nuclear translocation was enhanced by HG, blocked HG-induced SGLT2 expression upregulation. Similarly, gene silencing for importin-α1, a chaperone protein bound to GLUT2, blocked HG-induced HNF-1α nuclear translocation and SGLT2 expression upregulation. In human kidney, SIRT1 immunostaining was negatively correlated with SGLT2 immunostaining. Thus, under diabetic conditions, SIRT1 expression in PTs was downregulated by an increase in SGLT2 expression, which was stimulated by basolateral HG through activation of the GLUT2/importin-α1/HNF-1α pathway.

Subchronic olanzapine treatment decreases the expression of pancreatic glucose transporter 2 in rat pancreatic β cells

BACKGROUND

Olanzapine is a second generation antipsychotic. A common side effect in humans is weight gain, but the mechanisms are mostly unknown.

AIM

To study the effects of subchronic olanzapine treatment on body weight, fasting plasma glucose (FPG), fasting insulin (FINS), C-peptide, insulin sensitivity index (ISI), and expression of glucose transporter 2 (GLUT2) in rat pancreatic β cells.

MATERIALS AND METHODS

Female Sprague-Dawley rats were randomly divided into two groups: the olanzapine-treated group and the control group (each n = 8). Rats in the olanzapine-treated group intragastrically received olanzapine 5 mg/kg/day for 28 days; the rats in the control group received the same volume of vehicle. FPG and body weight were measured on the 1st, 7th, 14th and 28th day. FINS and C-peptide were measured using immunoradiometric assays at baseline and on the 28th day. GLUT2 mRNA and protein expressions in pancreatic β cells were analyzed by RT-PCR and western blot.

RESULTS

Olanzapine-treated rats had higher body weight (227.4 ± 8.9 vs. 211.0 ± 9.9 g), FPG (5.86 ± 0.42 vs. 4.24 ± 0.29 mmol/L), FINS (17.34 ± 3.64 vs. 10.20 ± 1.50 µIU/mL), and C-peptide (0.154 ± 0.027 vs. 0.096 ± 0.009 ng/mL) than those in controls (all P < 0.05) at the 28th day. Pancreatic β cells of the olanzapine-treated group showed lower ISI (-4.60 ± 0.23 vs. -3.76 ± 0.20) and GLUT2 levels (mRNA: 1.12 ± 0.02 vs. 2.00 ± 0.03; protein: 0.884 ± 0.134 vs. 1.118 ± 0.221) than those in controls (all P < 0.05).

CONCLUSIONS

Subchronic olanzapine treatment inhibited expression of GLUT2 in rat pancreatic β cells. Therefore, it may disturb glucose metabolism via the insulin resistance of β cells, but confirmation in humans is needed.

Mutations in SLC2A2 gene reveal hGLUT2 function in pancreatic β cell development

The structure-function relationships of sugar transporter-receptor hGLUT2 coded by SLC2A2 and their impact on insulin secretion and β cell differentiation were investigated through the detailed characterization of a panel of mutations along the protein. We studied naturally occurring SLC2A2 variants or mutants: two single-nucleotide polymorphisms and four proposed inactivating mutations associated to Fanconi-Bickel syndrome. We also engineered mutations based on sequence alignment and conserved amino acids in selected domains. The single-nucleotide polymorphisms P68L and T110I did not impact on sugar transport as assayed in Xenopus oocytes. All the Fanconi-Bickel syndrome-associated mutations invalidated glucose transport by hGLUT2 either through absence of protein at the plasma membrane (G20D and S242R) or through loss of transport capacity despite membrane targeting (P417L and W444R), pointing out crucial amino acids for hGLUT2 transport function. In contrast, engineered mutants were located at the plasma membrane and able to transport sugar, albeit with modified kinetic parameters. Notably, these mutations resulted in gain of function. G20S and L368P mutations increased insulin secretion in the absence of glucose. In addition, these mutants increased insulin-positive cell differentiation when expressed in cultured rat embryonic pancreas. F295Y mutation induced β cell differentiation even in the absence of glucose, suggesting that mutated GLUT2, as a sugar receptor, triggers a signaling pathway independently of glucose transport and metabolism. Our results describe the first gain of function mutations for hGLUT2, revealing the importance of its receptor versus transporter function in pancreatic β cell development and insulin secretion.

The functional role of the novel biomarker karyopherin α 2 (KPNA2) in cancer

In recent years, Karyopherin α 2 (KPNA2) has emerged as a potential biomarker in multiple cancer forms. The aberrant high levels observed in cancer tissue have been associated with adverse patient characteristics, prompting the idea that KPNA2 plays a role in carcinogenesis. This notion is supported by studies in cancer cells, where KPNA2 deregulation has been demonstrated to affect malignant transformation. By virtue of its role in nucleocytoplasmic transport, KPNA2 is implicated in the translocation of several cancer-associated proteins. We provide an overview of the clinical studies that have established the biomarker potential of KPNA2 and describe its functional role with an emphasis on established associations with cancer.

Nerveless and gutsy: intestinal nutrient sensing from invertebrates to humans

The increasingly recognized role of gastrointestinal signals in the regulation of food intake, insulin production and peripheral nutrient storage has prompted a surge of interest in studying how the gastrointestinal tract senses and responds to nutritional information. Identification of metabolically important intestinal nutrient sensors could provide potential new drug targets for the treatment of diabetes, obesity and gastrointestinal disorders. From a more fundamental perspective, the study of intestinal chemosensation is revealing novel, non-neuronal modes of communication involving differentiated epithelial cells. It is also identifying signalling mechanisms downstream of not only canonical receptors but also nutrient transporters, thereby supporting a chemosensory role for “transceptors” in the intestine. This review describes known and proposed mechanisms of intestinal carbohydrate, protein and lipid sensing, best characterized in mammalian systems. It also highlights the potential of invertebrate model systems such as C. elegans and Drosophila melanogaster by summarizing known examples of molecular evolutionary conservation. Recently developed genetic tools in Drosophila, an emerging model system for the study of physiology and metabolism, allow the temporal, spatial and high-throughput manipulation of putative intestinal sensors. Hence, fruit flies may prove particularly suited to the study of the link between intestinal nutrient sensing and metabolic homeostasis.

Detection of extracellular glucose by GLUT2 contributes to hypothalamic control of food intake

The sugar transporter GLUT2, present in several tissues of the gut-brain axis, has been reported to be involved in the control of food intake. GLUT2 is a sugar transporter sustaining energy production in the cell, but it can also function as a receptor for extracellular glucose. A glucose-signaling pathway is indeed triggered, independently of glucose metabolism, through its large cytoplasmic loop domain. However, the contribution of the receptor function over the transporter function of GLUT2 in the control of food intake remains to be determined. Thus, we generated transgenic mice that express a GLUT2-loop domain, blocking the detection of glucose but leaving GLUT2-dependent glucose transport unaffected. Inhibiting GLUT2-mediated glucose detection augmented daily food intake by a mechanism that increased the meal size but not the number of meals. Peripheral hormones (ghrelin, insulin, leptin) were unaffected, leading to a focus on central aspects of feeding behavior. We found defects in c-Fos activation by glucose in the arcuate nucleus and changes in the amounts of TRH and orexin neuropeptide mRNA, which are relevant to poorly controlled meal size. Our data provide evidence that glucose detection by GLUT2 contributes to the control of food intake by the hypothalamus. The sugar transporter receptor, i.e., "transceptor" GLUT2, may constitute a drug target to treat eating disorders and associated metabolic diseases, particularly by modulating its receptor function without affecting vital sugar provision by its transporter function.

GLUT2 mutations, translocation, and receptor function in diet sugar managing

Cloned 20 years ago, GLUT2 is a facilitative glucose transporter in the liver, pancreas, intestine, kidney, and brain. It ensures large bidirectional fluxes of glucose in and out the cell due to its low affinity and high capacity. It also transports other dietary sugars, such as fructose and galactose, within the range of physiological concentrations. Sugars and hormones regulate its gene expression. The contribution of GLUT2 to human metabolic diseases previously appeared modest. However, in the past decade, three major features of the GLUT2 protein have been revealed. First, GLUT2 mutations cause the severe but rare Fanconi-Bickel syndrome, mainly characterized by glycogenosis. Recently, a GLUT2 polymorphism has been associated with preferences for sugary food. Second, the GLUT2 location at the cell surface is regulated; this governs cellular activities dependent on glucose in the intestine and possibly those in the liver and pancreas. For instance, GLUT2 translocation from an intracellular pool to the apical membrane after a sugar meal transiently increases sugar uptake by enterocytes (reviewed in 32). Third, GLUT2 functions as a membrane receptor of sugar. Independently of glucose metabolism, GLUT2 detects the presence of extracellular sugar and transduces a signal to modulate cell functions, including beta-cell insulin secretion, renal reabsorption, and intestinal absorption according to the sugar environment. These recent developments are examined here in heath and metabolic disease, highlighting various unanswered questions.

The N-terminal nuclear localization sequences of liver X receptors alpha and beta bind to importin alpha and are essential for both nuclear import and transactivating functions

Liver X receptors (LXRs) alpha and beta are nuclear receptors, which form obligate heterodimers with the retinoid X receptor (RXR). The LXRs regulate both redundantly and non-redundantly the transcription of genes controlling cholesterol metabolism and transport as well as lipogenesis. Previously, we showed that mutations in putative N-terminal nuclear localization sequences (NLSs) within both LXRs inhibit nuclear import. Through in vitro studies, we show here that these NLSs bind importin alpha and are both necessary and sufficient for the nuclear import of LXRs. Imaging, transactivation, and electro-mobility shift experiments show that RXR rescues the nuclear import of the LXRalpha NLS mutant yet does not restore its transcriptional activity despite intact DNA binding. In contrast, RXR partially rescues the import of the LXRbeta NLS mutant, but has no effect on its transcriptional activity due to the loss of DNA binding. Experiments with NLS mutant RXR confirmed that RXR may dominate the nuclear import of the RXR/LXRalpha heterodimer, whereas LXRbeta dominates the nuclear import of the RXR/LXRbeta heterodimer. Intriguingly, our data indicate differences between LXRalpha and LXRbeta in their interaction with RXR and in the role their NLSs play in transactivating functions independent of nuclear import.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Regulation of the fructose transporter GLUT5 in health and disease

Fructose is now such an important component of human diets that increasing attention is being focused on the fructose transporter GLUT5. In this review, we describe the regulation of GLUT5 not only in the intestine and testis, where it was first discovered, but also in the kidney, skeletal muscle, fat tissue, and brain where increasing numbers of cell types have been found to have GLUT5. GLUT5 expression levels and fructose uptake rates are also significantly affected by diabetes, hypertension, obesity, and inflammation and seem to be induced during carcinogenesis, particularly in the mammary glands. We end by highlighting research areas that should yield information needed to better understand the role of GLUT5 during normal development, metabolic disturbances, and cancer.

Dexamethasone sensitizes the neonatal intestine to fructose induction of intestinal fructose transporter (Slc2A5) function

The recent dramatic increase in fructose consumption is tightly correlated with an equally dramatic surge in the incidence of type 2 diabetes and obesity in children, but little is known about dietary fructose metabolism and absorption in neonates. The expression of the rat intestinal fructose transporter GLUT5 [Slc2A5, a member of the glucose transporter family (GLUT)] can be specifically induced by its substrate fructose, but only after weaning begins at 14 d of age. In suckling rats younger than 14 d old, dietary fructose cannot enhance GLUT5 expression. The aim of this study was to identify the mechanisms allowing fructose to stimulate GLUT5 during weaning. After intestines were perfused with fructose or glucose (control), using microarray hybridization we showed that of 5K genes analyzed in 10-d-old pups, only 13 were fructose responsive. Previous work found approximately 50 fructose-responsive genes in 20-d-old pups. To identify fructose-responsive genes whose expression also changed with age, intestines of 10- and 20-d-old littermate pups perfused with fructose were compared by microarray. Intestines of 10- and 20-d-old pups perfused with glucose were used to segregate age- but not fructose-responsive genes. About 28 genes were up- and 22 down-regulated in 20- relative to 10-d-old pups, under conditions of fructose perfusion, and many were found, by cluster analysis, to be regulated by corticosterone. When dexamethasone was injected into suckling pups before fructose perfusion, the expression of GLUT5 but not that of the sodium glucose cotransporter (SGLT) 1 and of GLUT2, as well as the uptake of fructose but not of glucose increased dramatically. Thus, dexamethasone, which allows dietary fructose to precociously stimulate intestinal fructose absorption, can mimic the effect of age and modify developmental timing mechanisms regulating GLUT5.

Sugar sensing by enterocytes combines polarity, membrane bound detectors and sugar metabolism

Sugar consumption and subsequent sugar metabolism are known to regulate the expression of genes involved in intestinal sugar absorption and delivery. Here we investigate the hypothesis that sugar-sensing detectors in membranes facing the intestinal lumen or the bloodstream can also modulate intestinal sugar absorption. We used wild-type and GLUT2-null mice, to show that dietary sugars stimulate the expression of sucrase-isomaltase (SI) and L-pyruvate kinase (L-PK) by GLUT2-dependent mechanisms, whereas the expression of GLUT5 and SGLT1, did not rely on the presence of GLUT2. By providing sugar metabolites, sugar transporters, including GLUT2, fuelled a sensing pathway. In Caco2/TC7 enterocytes, we could disconnect the sensing triggered by detector from that produced by metabolism, and found that GLUT2 generated a metabolism-independent pathway to stimulate the expression of SI and L-PK. In cultured enterocytes, both apical and basolateral fructose could increase the expression of GLUT5, conversely, basolateral sugar administration could stimulate the expression of GLUT2. Finally, we located the sweet-taste receptors T1R3 and T1R2 in plasma membranes, and we measured their cognate G alpha Gustducin mRNA levels. Furthermore, we showed that a T1R3 inhibitor altered the fructose-induced expression of SGLT1, GLUT5, and L-PK. Intestinal gene expression is thus controlled by a combination of at least three sugar-signaling pathways triggered by sugar metabolites and membrane sugar receptors that, according to membrane location, determine sugar-sensing polarity. This provides a rationale for how intestine adapts sugar delivery to blood and dietary sugar provision.

Expression pattern and action analysis of genes associated with the responses to chemical stimuli during rat liver regeneration

AIM: To study the genes associated with the responses to chemokines, nutrients, inorganic substances, organic substances and xenobiotics after rat partial hepatectomy (PH) at transcriptional level.

METHODS: The associated genes involved in the five kinds of responses were obtained from database and literature, and the gene expression changes during liver regeneration in rats were checked by the Rat Genome 230 2.0 array.

RESULTS: It was found that 60, 10, 9, 6, 26 genes respectively participating in the above five kinds of responses were associated with liver regeneration. The numbers of initially and totally expressed genes occurring in the initial phase of liver regeneration (0.5-4 h after PH), G₀/G₁ transition (4-6 h after PH), cell proliferation (6-66 h after PH), cell differentiation and structure-functional reconstruction (66-168 h after PH) were 51, 19, 52, 6 and 51, 43, 98, 68 respectively, illustrating that the associated genes were mainly triggered in the initiation and transition stages, and functioned at different phases. According to their expression similarity, these genes were classified into 5 groups: only up-regulated (47), predominantly up-regulated (18), only down-regulated (24), predominantly down-regulated (10), and up- and down-regulated (8). The total times of their up-regulated and down-regulated expression were 441 and 221, demonstrating that the number of up-regulated genes is more than that of the down-regulated genes. Their time relevance and gene expression patterns were classified into 14 and 26 groups, showing that the cell physiological and biochemical activities were staggered, diversified and complicated during liver regeneration in rats.

CONCLUSION: The chemotaxis was enhanced mainly in the forepart and metaphase of LR. The response of regenerating liver to nutrients and chemical substances was increased, whereas that to xenobiotics was not strong. One hundred and seven genes associated with LR play important roles in the responses to chemical substances.

The role of GLUT2 in dietary sugar handling

GLUT2 is a facilitative glucose transporter located in the plasma membrane of the liver, pancreatic, intestinal, kidney cells as well as in the portal and the hypothalamus areas. Due to its low affinity and high capacity, GLUT2 transports dietary sugars, glucose, fructose and galactose in a large range of physiological concentrations, displaying large bidirectional fluxes in and out the cells. This review focuses on the roles of GLUT2. The first identified function of GLUT2 is its capacity to fuel metabolism and to provide metabolites stimulating the transcription of glucose sensitive genes. Recently, two other functions of GLUT2 are uncovered. First, the insertion of GLUT2 into the apical membrane of enterocytes induces the acute regulation of intestinal sugar absorption after a meal. Second, the GLUT2 protein itself initiates a protein signalling pathway triggering a glucose signal from the plasma membrane to the transcription machinery.

Analysis of nuclear transport signals in the human apurinic/apyrimidinic endonuclease (APE1/Ref1)

The mammalian abasic-endonuclease1/redox-factor1 (APE1/Ref1) is an essential protein whose subcellular distribution depends on the cellular physiological status. However, its nuclear localization signals have not been studied in detail. We examined nuclear translocation of APE1, by monitoring enhanced green fluorescent protein (EGFP) fused to APE1. APE1's nuclear localization was significantly decreased by deleting 20 amino acid residues from its N-terminus. Fusion of APE1's N-terminal 20 residues directed nuclear localization of EGFP. An APE1 mutant lacking the seven N-terminal residues (ND7 APE1) showed nearly normal nuclear localization, which was drastically reduced when the deletion was combined with the E12A/D13A double mutation. On the other hand, nearly normal nuclear localization of the full-length E12A/D13A mutant suggests that the first 7 residues and residues 8–13 can independently promote nuclear import. Both far-western analyses and immuno-pull-down assays indicate interaction of APE1 with karyopherin alpha 1 and 2, which requires the 20 N-terminal residues and implicates nuclear importins in APE1's nuclear translocation. Nuclear accumulation of the ND7 APE1(E12A/D13A) mutant after treatment with the nuclear export inhibitor leptomycin B suggests the presence of a previously unidentified nuclear export signal, and the subcellular distribution of APE1 may be regulated by both nuclear import and export.

Synapse to nucleus signaling during long-term synaptic plasticity; a role for the classical active nuclear import pathway

The requirement for transcription during long-lasting plasticity indicates that signals generated at the synapse must be transported to the nucleus. We have investigated whether the classical active nuclear import pathway mediates intracellular retrograde signal transport in Aplysia sensory neurons and rodent hippocampal neurons. We found that importins localize to distal neuronal processes, including synaptic compartments, where they are well positioned to mediate synapse to nucleus signaling. In Aplysia, stimuli known to produce long-lasting but not short-lasting facilitation triggered importin nuclear translocation. In hippocampal neurons, NMDA receptor activation but not depolarization induced importin nuclear translocation. We further showed that LTP-inducing stimuli recruited active nuclear import in hippocampal slices. Together with our finding that long-term facilitation of Aplysia sensory-motor synapses required active nuclear import, our results indicate that regulation of the active nuclear import pathway plays a critical role in transporting synaptically generated signals into the nucleus during learning-related forms of plasticity.