Regulation of the pancreatic duodenal homeobox-1 protein by DNA-dependent protein kinase

Biophysical insights into glucose-dependent transcriptional regulation by PDX1

The pancreatic and duodenal homeobox 1 (PDX1) is a central regulator of glucose-dependent transcription of insulin in pancreatic β cells. PDX1 transcription factor activity is integral to the development and sustained health of the pancreas; accordingly, deciphering the complex network of cellular cues that lead to PDX1 activation or inactivation is an important step toward understanding the etiopathologies of pancreatic diseases and the development of novel therapeutics. Despite nearly 3 decades of research into PDX1 control of Insulin expression, the molecular mechanisms that dictate the function of PDX1 in response to glucose are still elusive. The transcriptional activation functions of PDX1 are regulated, in part, by its two intrinsically disordered regions, which pose a barrier to its structural and biophysical characterization. Indeed, many studies of PDX1 interactions, clinical mutations, and posttranslational modifications lack molecular level detail. Emerging methods for the quantitative study of intrinsically disordered regions and refined models for transactivation now enable us to validate and interrogate the biochemical and biophysical features of PDX1 that dictate its function. The goal of this review is to summarize existing PDX1 studies and, further, to generate a comprehensive resource for future studies of transcriptional control via PDX1.

Cross talk between exosomes and pancreatic β-cells in diabetes

Exosomes are a class of extracellular vesicles with a diameter of 50-100 nm secreted by various cells. They are generated through complex intracellular production mechanisms before being secreted to the extracellular environment. Due to their inclusion of proteins, lipids, and nucleic acids, exosomes play an important role in intercellular communication. Pancreatic β-cells play an irreplaceable role in the body's glucose metabolism. Their dysfunction is one of the causes of diabetes. Exosomes of various cells regulate the function of β-cells by regulating autoimmunity, delivering non-coding RNAs, or directly regulating intracellular signal pathways. This communication between β-cells and other cells plays an important role in the pathogenesis and development of diabetes, and has potential for clinical application. This paper reviews the biological sources and functions of exosomes, as well as intercellular crosstalk between β-cells and other cells that is involved in β-cell failure and regeneration.

Pancreatic β-Cell Senescence: Mechanisms and Association with Diabetes

Senescence occurs as a part of the cellular response to different stressors. With increasing age, continuous exposure to stressors leads to age-induced senescence. Pancreatic β-cell proliferation and glucose homeostasis also decrease with age, which results in a decrease in β cell mass and, eventually, the possible development of diabetes. This process is mediated through impaired cell cycle regulators, along with specific increases in cell cycle inhibitors, telomere shortening, and defective DNA repair mechanisms. Diabetes contributes to β-cell senescence through hyperglycaemia, dyslipidaemia, oxidative stress, and inflammation. β cells isolated from patients with Type 2 diabetes mellitus have been shown to have senescence markers, such as senescence-associated secretory phenotype genes and β-galactosidase. In this paper, the authors discuss the mechanisms of cellular senescence, how senescence is impacted by the diabetic microenvironment, and the possible mechanisms and factors contributing to β-cell senescence.

Secreted PDZD2 exerts an insulinotropic effect on INS-1E cells by a PKA-dependent mechanism

Secreted PDZD2 (sPDZD2) is a signaling molecule generated upon proteolytic processing of the multi-PDZ-containing protein PDZD2. Previous analysis of gene-trap mice deficient in the synthesis of full-length PDZD2, but not the secreted form, revealed a role of PDZD2 in the regulation of glucose-stimulated insulin secretion. Here, using the pancreatic INS-1E β cells as in vitro model, we showed that depletion of PDZD2/sPDZD2 by RNA interference suppressed the expression of β-cell genes Ins1, Glut2 and MafA whereas treatment with recombinant sPDZD2 rescued the suppressive effect. Similar to GLP-1, sPDZD2 stimulated intracellular cAMP levels, activated β-cell gene expression in a PKA-dependent manner and induced the phosphorylation and nuclear localization of PDX1. Depletion of PDX1 inhibited the sPDZD2 insulinotropic effect, which could also be demonstrated in mouse islets. In summary, our findings are consistent with sPDZD2 serving a signaling function in regulating β-cell gene expression.

Phosphorylation of carboxypeptidase B1 protein regulates β-cell proliferation

A reduction in pancreatic islet β-cells leads to the onset of diabetes. Hence, the identification of the mechanisms inducing β-cell proliferation is important for developing a treatment course against the disease. It has been well established that post-translational modifications (PTMs) of proteins affect their functionality. In addition, PTMs have been suggested to play important roles in organ regeneration. Therefore, in this study, we investigated PTMs associated with pancreatic regeneration using two-dimensional electrophoresis. Four carboxypeptidase B1 (CPB1) proteins were identified at different isoelectric points, with the same molecular weight. The motif of CPB1 PTMs was identified by mass spectrophotometry, and the downregulation of CPB1 phosphorylation in pancreatectomy was confirmed. The dephosphorylation of CPB1 induced β-cell proliferation. We thus surmise that the altered PTM of CPB1 is associated with pancreatic regeneration.

The Phosphorylation of PDX-1 by Protein Kinase CK2 Is Crucial for Its Stability

The homeodomain protein PDX-1 is a critical regulator of pancreatic development and insulin production in pancreatic β-cells. We have recently shown that PDX-1 is a substrate of protein kinase CK2; a multifunctional protein kinase which is implicated in the regulation of various cellular aspects, such as differentiation, proliferation, and survival. The CK2 phosphorylation site of PDX-1 is located within the binding region of the E3 ubiquitin ligase adaptor protein PCIF1. To study the interaction between PDX-1 and PCIF1 we used immunofluorescence analysis, co-immunoprecipitation, GST-pull-down studies, and proximity ligation assay (PLA). For the analysis of the stability of PDX-1 we performed a cycloheximide chase. We used PDX-1 in its wild-type form as well as phosphomutants of the CK2 phosphorylation site. In pancreatic β-cells PDX-1 binds to PCIF1. The phosphorylation of PDX-1 by CK2 increases the ratio of PCIF1 bound to PDX-1. The stability of PDX-1 is extended in the absence of CK2 phosphorylation. Our results identified protein kinase CK2 as new important modulator of the stability of PDX-1.

MST1: a promising therapeutic target to restore functional beta cell mass in diabetes

The loss of insulin-producing beta cells by apoptosis is a hallmark of all forms of diabetes mellitus. Strategies to prevent beta cell apoptosis and dysfunction are urgently needed to restore the insulin-producing cells and to prevent severe diabetes progression. We recently identified the serine/threonine kinase known as mammalian sterile 20-like kinase 1 (MST1) as a critical regulator of apoptotic beta cell death and dysfunction. MST1 activates several apoptotic signalling pathways, which further stimulate its own cleavage, leading to a vicious cycle of cell death. This led us to hypothesise that MST1 signalling is central to the initiation of beta cell death in diabetes. We found that MST1 is strongly activated in a diabetic beta cell and induces not only its death but also directly impairs insulin secretion through promoting proteasomal degradation of key beta cell transcription factor, pancreatic and duodenal homeobox 1 (PDX1), which is critical for insulin production.Pre-clinical studies in various animal models of diabetes have reported that MST1 deficiency remarkably restores normoglycaemia and beta cell function and prevents the development of diabetes. Importantly, MST1 deficiency can revert fully diabetic beta cells to a non-diabetic state. MST1 may serve as a target for the development of novel therapies for diabetes that trigger the cause of the disease, namely, the destruction of the beta cells. The major current focus of our investigation is to identify and test the efficacy of potent inhibitors of this death signalling pathway to protect beta cells against the effects of autoimmune attack in type 1 diabetes and to preserve beta cell mass and function in type 2 diabetes. This review summarises a presentation given at the 'Can we make a better beta cell?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Heiko Lickert and colleagues, DOI: 10.1007/s00125-016-3949-9 , and by Harry Heimberg and colleagues, DOI: 10.1007/s00125-016-3879-6 ) and a commentary by the Session Chair, Shanta Persaud (DOI: 10.1007/s00125-016-3870-2 ).

Metformin Restrains Pancreatic Duodenal Homeobox-1 (PDX-1) Function by Inhibiting ERK Signaling in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is one of the most potent and perilous diseases known, with a median survival rate of 3-5 months due to the combination of only advanced stage diagnosis and ineffective therapeutic options. Metformin (1,1-Dimethylbiguanide hydrochloride), the leading drug used for type 2 diabetes mellitus, emerges as a potential therapy for PDAC and other human cancers. Metformin exerts its anticancer action via a variety of adenosine monophosphate (AMP)-activated protein kinase (AMPK)- dependent and/or AMPK-independent mechanisms. We present data here showing that metformin downregulated pancreatic transcription factor pancreatic duodenal homeobox-1 (PDX-1), suggesting a potential novel mechanism by which metformin exerts its anticancer action. Metformin inhibited PDX-1 expression at both protein and mRNA levels and PDX-1 transactivity as well in PDAC cells. Extracellular signal-regulated kinase (ERK) was identified as a PDX-1-interacting protein by antibody array screening in GFP-PDX-1 stable HEK293 cells. Co-transfection of ERK1 with PDX-1 resulted in an enhanced PDX-1 expression in HEK293 cells in a dose-dependent manner. Immunoprecipitation/Western blotting analysis confirmed the ERK-PDX-1 interaction in PANC-1 cells stimulated by epidermal growth factor (EGF). EGF induced an enhanced PDX-1 expression in PANC-1 cells and this stimulation was inhibited by MEK inhibitor PD0325901. Metformin inhibited EGF-stimulated PDX-1 expression with an accompanied inhibition of ERK kinase activation in PANC- 1 cells. Taken together, our studies show that PDX-1 is a potential novel target for metformin in PDAC cells and that metformin may exert its anticancer action in PDAC by down-regulating PDX-1 via a mechanism involving inhibition of ERK signaling.

Interferon-γ Decreases Nuclear Localization of Pdx-1 and Triggers β-Cell Dysfunction in Chronic Pancreatitis

Interferon-gamma (IFN-γ) is shown to play a major role in β-cell dysfunction in chronic pancreatitis (CP). However, the underlying mechanisms are to be elucidated. The present study was conducted to determine the role of IFN-γ subverting insulin gene expression in CP. Pancreatic tissues from control (n=15) and CP patients (n=30) were analyzed for nuclear localization of pancreatic and duodenal homeobox transcription factor (Pdx-1) after ascertaining their diabetic status. By immunofluorescence and western blot analysis, the influence of IFN-γ, anti-inflammatory cytokine (interleukin-10), and anti-IFN-γ agent epigallocatechin-3-gallate (EGCG) on nuclear localization of Pdx-1was examined in the islets isolated from resected normal pancreatic tissue. Nuclear localization of Pdx-1 was 20.25±2.19 in the islets of diabetic CP patients and 31.44±2.09 in nondiabetic CP patients as compared with controls (60.45±5.11) and the corresponding distribution of Pdx-1 protein in the nuclear compartment was also decreased. Exposure of normal islets to IFN-γ revealed decreased nuclear localization of Pdx-1. Pretreatment with polyphenolic compound EGCG restored the nuclear localization of Pdx-1. These results suggest that IFN-γ induced β-cell dysfunction is mediated through decreased nuclear localization of Pdx-1.

Dynamic recruitment of functionally distinct Swi/Snf chromatin remodeling complexes modulates Pdx1 activity in islet β cells

Pdx1 is a transcription factor of fundamental importance to pancreas formation and adult islet β cell function. However, little is known about the positive- and negative-acting coregulators recruited to mediate transcriptional control. Here, we isolated numerous Pdx1-interacting factors possessing a wide range of cellular functions linked with this protein, including, but not limited to, coregulators associated with transcriptional activation and repression, DNA damage response, and DNA replication. Because chromatin remodeling activities are essential to developmental lineage decisions and adult cell function, our analysis focused on investigating the influence of the Swi/Snf chromatin remodeler on Pdx1 action. The two mutually exclusive and indispensable Swi/Snf core ATPase subunits, Brg1 and Brm, distinctly affected target gene expression in β cells. Furthermore, physiological and pathophysiological conditions dynamically regulated Pdx1 binding to these Swi/Snf complexes in vivo. We discuss how context-dependent recruitment of coregulatory complexes by Pdx1 could impact pancreas cell development and adult islet β cell activity.

Down-regulation of pancreatic and duodenal homeobox-1 by somatostatin receptor subtype 5: a novel mechanism for inhibition of cellular proliferation and insulin secretion by somatostatin

Somatostatin (SST) is a regulatory peptide and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells. SST’s actions are mediated by a family of seven transmembrane domain G protein-coupled receptors that comprise five distinct subtypes (SSTR1-5). SSTR5 is one of the major SSTRs in the islets of Langerhans. Homeodomain-containing transcription factor pancreatic and duodenal homeobox-1 (PDX-1) is essential for pancreatic development, β cell differentiation, maintenance of normal β cell functions in adults and tumorigenesis. Recent studies show that SSTR5 acts as a negative regulator for PDX-1 expression and that SSTR5 mediates somatostatin’s inhibitory effect on cell proliferation and insulin expression/excretion through down-regulating PDX-1 expression. SSTR5 exerts its inhibitory effect on PDX-1 expression at both the transcriptional level by down-regulating PDX-1 mRNA and the post-translational level by enhancing PDX-1 ubiquitination. Identification of PDX-1 as a transcriptional target for SSTR5 may help in guiding the choice of therapeutic cancer treatments.

MST1 is a key regulator of beta cell apoptosis and dysfunction in diabetes

Apoptotic cell death is a hallmark of the loss of insulin producing beta-cells in all forms of diabetes mellitus. Current treatment fails to halt the decline in functional beta-cell mass. Strategies to prevent beta-cell apoptosis and dysfunction are urgently needed. Here, we identified Mammalian Sterile 20-like kinase 1 (MST1) as a critical regulator of apoptotic beta-cell death and function. MST1 was strongly activated in beta-cells under diabetogenic conditions and correlated with beta-cell apoptosis. MST1 specifically induced the mitochondrial-dependent pathway of apoptosis in beta-cells through up-regulation of the BH3-only protein Bim. MST1 directly phosphorylated PDX1 at Thr11, resulting in its ubiquitination, degradation and impaired insulin secretion. Mst1 deficiency completely restored normoglycemia, beta-cell function and survival in vitro and in vivo. We show MST1 as novel pro-apoptotic kinase and key mediator of apoptotic signaling and beta-cell dysfunction, which may serve as target for the development of novel therapies for diabetes.

Pancreatic and duodenal homeobox-1 nuclear localization is regulated by glucose in dispersed rat islets but not in insulin-secreting cell lines

The transcription factor Pancreatic and Duodenal Homeobox-1 (PDX-1) plays a major role in the development and function of pancreatic β-cells and its mutation results in diabetes. In adult β-cells, glucose stimulates transcription of the insulin gene in part by regulating PDX-1 expression, stability and activity. Glucose is also thought to modulate PDX-1 nuclear translocation but in vitro studies examining nucleo-cytoplasmic shuttling of endogenous or ectopically expressed PDX-1 in insulin-secreting cell lines have led to conflicting results. Here we show that endogenous PDX-1 undergoes translocation from the cytoplasm to the nucleus in response to glucose in dispersed rat islets but not in insulin-secreting MIN6, HIT-T15, or INS832/13 cells. Interestingly, however, we found that a PDX-1-GFP fusion protein can shuttle from the cytoplasm to the nucleus in response to glucose stimulation in HIT-T15 cells. Our results suggest that the regulation of endogenous PDX-1 sub-cellular localization by glucose is observed in primary islets and that care should be taken when interpreting data from insulin-secreting cell lines.

Increased expression of phosphorylated NBS1, a key molecule of the DNA damage response machinery, is an adverse prognostic factor in patients with de novo myelodysplastic syndromes

The expression of activated forms of key proteins of the DNA damage response machinery (pNBS1, pATM and γH2AX) was assessed by means of immunohistochemistry in bone marrow biopsies of 74 patients with de novo myelodysplastic syndromes (MDS) and compared with 15 cases of de novo acute myeloid leukemia (AML) and 20 with reactive bone marrow histology. Expression levels were significantly increased in both MDS and AML, compared to controls, being higher in high-risk than in low-risk MDS. Increased pNBS1 and γH2AX expression possessed a significant negative prognostic impact for overall survival in MDS patients, whereas pNBS1 was an independent marker of poor prognosis.

Notch1 activation up-regulates pancreatic and duodenal homeobox-1

Transcription factor pancreatic and duodenal homeobox-1 (PDX-1) plays an essential role in pancreatic development, β-cell differentiation, maintenance of normal β-cell function and tumorigenesis. PDX-1 expression is tightly controlled through a variety of mechanisms under different cellular contexts. We report here that overexpression of Notch1 intracellular domain (NICD), an activated form of Notch1, enhanced PDX-1 expression in both PDX-1 stable HEK293 cells and mouse insulinoma β-TC-6 cells, while NICD shRNA inhibited the enhancing effect. NICD-enhanced PDX-1 expression was accompanied by increased insulin expression/secretion and cell proliferation in β-TC-6 cells, which was reversed by NICD shRNA. Cre activation-induced specific expression of NICD in islet β cells of transgenic β^NICD+/+ mice induced increased expression of PDX-1, insulin and proliferating cell nuclear antigen (PCNA) and decreased expression of p27 with accompanied fasting hyperinsulinemia and hypoglycemia and altered responses to intraperitoneal glucose tolerance test. Systemically delivered NICD shRNA suppressed islet expression of PDX-1 and reversed the hypoglycemia and hyperinsulinemia. Moreover, expression levels of NICD were correlated with those of PDX-1 in human pancreatic neuroendocrine tumor. Thus, Notch1 acts as a positive regulator for PDX-1 expression, cooperates with PDX-1 in the development of insulin overexpression and islet cell neoplasia and represents a potential therapeutic target for islet neoplasia.

Per-Arnt-Sim kinase regulates pancreatic duodenal homeobox-1 protein stability via phosphorylation of glycogen synthase kinase 3β in pancreatic β-cells

In pancreatic β-cells, glucose induces the binding of the transcription factor pancreatic duodenal homeobox-1 (PDX-1) to the insulin gene promoter to activate insulin gene transcription. At low glucose levels, glycogen synthase kinase 3β (GSK3β) is known to phosphorylate PDX-1 on C-terminal serine residues, which triggers PDX-1 proteasomal degradation. We previously showed that the serine/threonine Per-Arnt-Sim domain-containing kinase (PASK) regulates insulin gene transcription via PDX-1. However, the mechanisms underlying this regulation are unknown. In this study, we aimed to identify the role of PASK in the regulation of PDX-1 phosphorylation, protein expression, and stability in insulin-secreting cells and isolated rodent islets of Langerhans. We observed that glucose induces a decrease in overall PDX-1 serine phosphorylation and that overexpression of WT PASK mimics this effect. In vitro, PASK directly phosphorylates GSK3β on its inactivating phosphorylation site Ser(9). Overexpression of a kinase-dead (KD), dominant negative version of PASK blocks glucose-induced Ser(9) phosphorylation of GSK3β. Accordingly, GSK3β Ser(9) phosphorylation is reduced in islets from pask-null mice. Overexpression of WT PASK or KD GSK3β protects PDX-1 from degradation and results in increased PDX-1 protein abundance. Conversely, overexpression of KD PASK blocks glucose-induction of PDX-1 protein. We conclude that PASK phosphorylates and inactivates GSK3β, thereby preventing PDX-1 serine phosphorylation and alleviating GSK3β-mediated PDX-1 protein degradation in pancreatic β-cells.

Ku70 functions in addition to nonhomologous end joining in pancreatic β-cells: a connection to β-catenin regulation

The genesis of β-cells predominantly occurs through self-replication; therefore, understanding the regulation of cell proliferation is essential. We previously showed that the lack of nonhomologous end joining (NHEJ) DNA repair factor ligase IV leads to an accumulation of DNA damage that permanently halts β-cell proliferation and dramatically decreases insulin production, causing overt diabetes in a hypomorphic p53(R172P) background. In the present study, to further delineate the function of NHEJ, we analyzed mice deficient for another key NHEJ factor, Ku70, to discover the effect of cellular responses to DNA damage in pancreatic β-cells on cellular proliferation and glucose homeostasis. Analysis of Ku70(-/-) pancreatic β-cells revealed an accumulation of DNA damage and activation of p53-dependent cellular senescence similar to the results found in our earlier ligase IV deficiency study. To our surprise, Ku70(-/-) mice had significantly increased β-cell proliferation and islet expansion, heightened insulin levels, and decreased glycemia. This augmented β-cell proliferation was accompanied by an increased β-catenin level, which we propose to be responsible for this phenotype. This study highlights Ku70 as an important player not only in maintaining genomic stability through NHEJ-dependent functions, but also in regulating pancreatic β-cell proliferation, a novel NHEJ-independent function.

Identification of DNA-dependent protein kinase catalytic subunit as a novel interaction partner of lymphocyte enhancer factor 1

Lymphocyte enhancer factor 1 (LEF1), a member of the LEF/T-cell-specific factor (TCF) family of the high mobility group domain transcription factors, acts downstream in canonical Wnt signaling. Aberrant transactivation of LEF1 contributes to the tumorigenesis of colonic neoplasms, sebaceous skin tumors, and lymphoblastic leukemia. LEF1-associated proteins are crucial for regulating its transcriptional activity. In this study, glutathione-S-transferase pull-down assay and mass spectrometry enabled identification of the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) as a novel interaction partner for LEF1. The interaction between LEF1 and DNA-PKcs was confirmed using in vivo co-immunoprecipitation. Furthermore, double immunofluorescence observations showed that LEF1 and DNA-PKcs colocalized in the nuclei of colon adenocarcinoma cell lines. Identification of the interaction between LEF1 and DNA-PKcs may provide clues for a novel therapy for cancer treatment as well as for understanding LEF1-mediated transcriptional regulation.

Negative regulation of pancreatic and duodenal homeobox-1 by somatostatin receptor subtype 5

Somatostatin receptor subtype 5 (SSTR5) mediates the inhibitory effect of somatostatin and its analogs on insulin expression/secretion and islet cell proliferation. We provide biochemical and genetic evidence that SSTR5 exerted its physiological actions via down-regulating pancreatic and duodenal homeobox-1 (PDX-1), a β-cell-specific homeodomain-containing transcription factor. Cotransfection of SSTR5 with PDX-1 resulted in dose-dependent inhibition of PDX-1 expression in human embryonic kidney 293 cells. SSTR5 agonist RPL-1980 inhibited PDX-1 expression and abolished glucagon-like peptide 1-stimulated PDX-1 expression in mouse insulinoma β-TC-6 cells. SSTR5 knockdown by short hairpin RNA led to increased PDX-1 expression that was accompanied by enhanced insulin secretion stimulated by high glucose in β-TC6 cells and alternated expressions of cell cycle proteins that favor cell proliferation in mouse insulinoma MIN6 cells. Quantitative RT-PCR analysis showed that cotransfected SSTR5 inhibited PDX-1 mRNA expression, whereas knockdown of SSTR5 increased PDX-1 mRNA expression. In addition, we found that cotransfected wild-type SSTR5 increased PDX-1 ubiquitination in human embryonic kidney 293 cells, whereas SSTR5 P335L, a hypofunctional single nucleotide polymorphism of SSTR5, inhibited PDX-1 ubiquitination. SSTR5 knockout resulted in increased expression of PDX-1, insulin, and proliferating cell nuclear antigen in the islets of sstr(-/-) mice. Immunohistochemistry analysis showed that SSTR5 P335L was associated with elevated expression of PDX-1 in human pancreatic neuroendocrine tumor. Taken together, our studies demonstrated that SSTR5 is a negative regulator for PDX-1 expression and that SSTR5 may mediate the inhibitory effects of somatostatin and its analogs on insulin expression/secretion and cell proliferation via down-regulating PDX-1 at both transcriptional and posttranslational levels.

Pdx1 is post-translationally modified in vivo and serine 61 is the principal site of phosphorylation

Maintaining sufficient levels of Pdx1 activity is a prerequisite for proper regulation of blood glucose homeostasis and beta cell function. Mice that are haploinsufficient for Pdx1 display impaired glucose tolerance and lack the ability to increase beta cell mass in response to decreased insulin signaling. Several studies have shown that post-translational modifications are regulating Pdx1 activity through intracellular localization and binding to co-factors. Understanding the signaling cues converging on Pdx1 and modulating its activity is therefore an attractive approach in diabetes treatment. We employed a novel technique called Nanofluidic Proteomic Immunoassay to characterize the post-translational profile of Pdx1. Following isoelectric focusing in nano-capillaries, this technology relies on a pan specific antibody for detection and it therefore allows the relative abundance of differently charged protein species to be examined simultaneously. In all eukaryotic cells tested we find that the Pdx1 protein separates into four distinct peaks whereas Pdx1 protein from bacteria only produces one peak. Of the four peaks in eukaryotic cells we correlate one of them to a phosphorylation Using alanine scanning and mass spectrometry we map this phosphorylation to serine 61 in both Min6 cells and in exogenous Pdx1 over-expressed in HEK293 cells. A single phosphorylation is also present in cultured islets but it remains unaffected by changes in glucose levels. It is present during embryogenesis but is not required for pancreas development.

Pcif1 modulates Pdx1 protein stability and pancreatic β cell function and survival in mice

The homeodomain transcription factor pancreatic duodenal homeobox 1 (Pdx1) is a major mediator of insulin transcription and a key regulator of the β cell phenotype. Heterozygous mutations in PDX1 are associated with the development of diabetes in humans. Understanding how Pdx1 expression levels are controlled is therefore of intense interest in the study and treatment of diabetes. Pdx1 C terminus-interacting factor-1 (Pcif1, also known as SPOP) is a nuclear protein that inhibits Pdx1 transactivation. Here, we show that Pcif1 targets Pdx1 for ubiquitination and proteasomal degradation. Silencing of Pcif1 increased Pdx1 protein levels in cultured mouse β cells, and Pcif1 heterozygosity normalized Pdx1 protein levels in Pdx1(+/-) mouse islets, thereby increasing expression of key Pdx1 transcriptional targets. Remarkably, Pcif1 heterozygosity improved glucose homeostasis and β cell function and normalized β cell mass in Pdx1(+/-) mice by modulating β cell survival. These findings indicate that in adult mouse β cells, Pcif1 limits Pdx1 protein accumulation and thus the expression of insulin and other gene targets important in the maintenance of β cell mass and function. They also provide evidence that targeting the turnover of a pancreatic transcription factor in vivo can improve glucose homeostasis.

Hydrogen peroxide stimulates nuclear import of the POU homeodomain protein Oct-1 and its repressive effect on the expression of Cdx-2

Background

The ubiquitously expressed POU homeodomain protein Oct-1 serves as a sensor for stress induced by irradiation. We found recently that in pancreatic and intestinal endocrine cells, Oct-1 also functions as a sensor for cyclic AMP (cAMP). The caudal homeobox gene Cdx-2 is a transactivator of proglucagon (gcg) and pro-insulin genes. Oct-1 binds to Cdx-2 promoter and represses its expression. cAMP elevation leads to increased nuclear exclusion of Oct-1, associated with reduced recruitment of nuclear co-repressors to the Cdx-2 promoter and increased Cdx-2 expression.

Results

We show in this study that inducing oxidative stress by hydrogen peroxide (H₂O₂) increased nuclear Oct-1 content in both pancreatic α and β cell lines, as well as in a battery of other cells. This increase was then attributed to accelerated nuclear import of Oct-1, assessed by Fluorescence Recovery After Photobleaching (FRAP) using green fluorescence protein (EGFP) tagged Oct-1 molecule. H₂O₂ treatment was then shown to stimulate the activities of DNA-dependent protein kinase (DNA-PK) and c-jun N-terminal kinase (JNK). Finally, increased Oct-1 nuclear content upon H₂O₂ treatment in a pancreatic α cell line was associated with reduced Cdx-2 and gcg mRNA expression.

Conclusion

These observations suggest that Oct-1 functions as a sensor for both metabolic and stress/survival signaling pathways via altering its nuclear-cytoplasmic shuttling.

CK2 phosphorylation of Pdx-1 regulates its transcription factor activity

The duodenal homeobox-1 protein Pdx-1 is one of the regulators for the transcription of the insulin gene. Pdx-1 is a phosphoprotein, and there is increasing evidence for the regulation of some of its functions by phosphorylation. Here, we asked whether protein kinase CK2 might phosphorylate Pdx-1 and how this phosphorylation could be implicated in the functional regulation of Pdx-1. We used fragments of Pdx-1 as well as phosphorylation mutants for experiments with protein kinase CK2. Transactivation was measured by reporter assays using the insulin promoter. Our data showed that Pdx-1 is phosphorylated by protein kinase CK2 at amino acids thr(231) and ser(232), and this phosphorylation was implicated in the regulation of the transcription factor activity of Pdx-1. Furthermore, inhibition of protein kinase CK2 by specific inhibitors led to an elevated release of insulin from pancreatic beta-cells. Thus, these findings identify CK2 as a novel mediator of the insulin metabolism.

Adaptation and failure of pancreatic beta cells in murine models with different degrees of metabolic syndrome

The events that contribute to the expansion of beta-cell mass and enhanced beta-cell function in insulin-resistant states have not been elucidated fully. Recently, we showed that beta-cell adaptation failed dramatically in adult, insulin-resistant POKO mice, which contrasts with the appropriate expansion of beta cells in their ob/ob littermates. Thus, we hypothesised that characterisation of the islets in these mouse models at an early age should provide a unique opportunity to: (1) identify mechanisms involved in sensing insulin resistance at the level of the beta cells, (2) identify molecular effectors that contribute to increasing beta-cell mass and function, and (3) distinguish primary events from secondary events that are more likely to be present at more advanced stages of diabetes. Our results define the POKO mouse as a model of early lipotoxicity. At 4 weeks of age, it manifests with inappropriate beta-cell function and defects in proliferation markers. Other well-recognised pathogenic effectors that were observed previously in 16-week-old mice, such as increased reactive oxygen species (ROS), macrophage infiltration and endoplasmic reticulum (ER) stress, are also present in both young POKO and young ob/ob mice, indicating the lack of predictive power with regards to the severity of beta-cell failure. Of interest, the relatively preserved lipidomic profile in islets from young POKO mice contrasted with the large changes in lipid composition and the differences in the chain length of triacylglycerols in the serum, liver, muscle and adipose tissue in adult POKO mice. Later lipotoxic insults in adult beta cells contribute to the failure of the POKO beta cell. Our results indicate that the rapid development of insulin resistance and beta-cell failure in POKO mice makes this model a useful tool to study early molecular events leading to insulin resistance and beta-cell failure. Furthermore, comparisons with ob/ob mice might reveal important adaptive mechanisms in beta cells with either therapeutic or diagnostic potential.

Inhibition of forkhead box O1 protects pancreatic beta-cells against dexamethasone-induced dysfunction

Forkhead Box O1 (FoxO1) is a key transcription regulator of insulin/IGF-I signaling pathway, and its activity can be increased by dexamethasone (DEX) in several cell types. However, the role of FoxO1 in DEX-induced pancreatic beta-cell dysfunction has not been fully understood. Therefore, in this study, we investigated whether FoxO1 could mediate DEX-induced beta-cell dysfunction and the possible underlying mechanisms in pancreatic beta-cell line RINm5F cells and primary rat islet. We found that DEX markedly increased FoxO1 mRNA and protein expression and decreased FoxO1 phosphorylation through the Akt pathway, which resulted in an increase in active FoxO1 in RINm5F cells and isolated rat islets. Activated FoxO1 subsequently inhibited pancreatic duodenal homeobox-1 expression and induced nuclear exclusion of pancreatic duodenal homeobox-1. Knockdown of FoxO1 by RNA interference restored the expression of pancreatic duodenal homeobox-1 and prevented DEX-induced dysfunction of glucose-stimulated insulin secretion in rat islets. Together, the results of present study demonstrate that FoxO1 is integrally involved in DEX-induced inhibition of pancreatic duodenal homeobox-1 and glucose-stimulated insulin secretion dysfunction in pancreatic islet beta-cells. Inhibition of FoxO1 can effectively protect beta-cells against DEX-induced dysfunction.

Glucose-responsive gene expression system for gene therapy

Regulation of gene expression by glucose is an important mechanism for mammals in adapting to their nutritional environment. Glucose, the primary fuel for most cells, modulates gene expression that is crucial in the cellular adaptation to glycemic variation. Transcription of the genes for insulin and glycolytic and lipogenic enzymes is stimulated by glucose in pancreatic beta-cells and liver. Recent findings further support the key role of the carbohydrate-responsive element binding protein in the regulation of glycolytic and lipogenic genes by glucose and dietary carbohydrates. Herein, we review the transcriptional regulation of glucose-responsive genes, and recent advances in the gene therapy using glucose-responsive gene expression for diabetes.

The homeodomain-interacting protein kinase 2 regulates insulin promoter factor-1/pancreatic duodenal homeobox-1 transcriptional activity

The homeodomain transcription factor insulin promoter factor (IPF)-1/pancreatic duodenal homeobox (PDX)-1 plays a crucial role in both pancreas development and maintenance of beta-cell function. Targeted disruption of the Ipf1/Pdx1 gene in beta-cells of mice leads to overt diabetes and reduced Ipf1/Pdx1 gene expression results in decreased insulin expression and secretion. In humans, mutations in the IPF1 gene have been linked to diabetes. Hence, the identification of molecular mechanisms regulating the transcriptional activity of this key transcription factor is of great interest. Herein we analyzed homeodomain-interacting protein kinase (Hipk) 2 expression in the embryonic and adult pancreas by in situ hybridization and RT-PCR. Moreover, we functionally characterized the role of HIPK2 in regulating IPF1/PDX1 transcriptional activity by performing transient transfection experiments and RNA interference. We show that Hipk2 is expressed in the developing pancreatic epithelium from embryonic d 12-15 but that the expression becomes preferentially confined to pancreatic endocrine cells at later developmental stages. Moreover, we show that HIPK2 positively influences IPF1/PDX1 transcriptional activity and that the kinase activity of HIPK2 is required for this effect. We also demonstrate that HIPK2 directly phosphorylates the C-terminal portion of IPF1/PDX1. Taken together, our data provide evidence for a new mechanism by which IPF1/PDX1 transcriptional activity, and thus possibly pancreas development and/or beta-cell function, is regulated.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

The 45-kDa form of Pdx-1 does not result from post-translational modifications

Pdx-1 is a key regulator of glucose-stimulated insulin gene transcription in beta-cells. The regulation of Pdx-1 in response to glucose has previously been associated with a remarkable shift in electrophoretic mobility on SDS-PAGE from 31 to 45kDa. This has been attributed to different post-translational modifications including phosphorylation, sumoylation or glycosylation. However, and in contrast with previous studies, we describe in this paper that Pdx-1 produced in Escherichia coli, by in vitro transcription/translation or exogenously expressed in eukaryotic cells, migrates with an apparent molecular mass of 45kDa despite a calculated mass of 31kDa. Moreover, we show that the migration of endogenous Pdx-1 obtained from a mouse beta-cell line as well as from human primary islets is not dependent on glucose concentration. Taken together, these data, validated by mass spectrometry techniques, establish that anomalous migration of Pdx-1 on SDS-PAGE does not result from post-translational modifications.

DNA-PK contributes to the phosphorylation of AIRE: importance in transcriptional activity

The autoimmune regulator (AIRE) protein is a key mediator of the central tolerance for tissue specific antigens and is involved in transcriptional control of many antigens in thymic medullary epithelial cells (mTEC). Mutations in the AIRE gene cause a rare disease named autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). Here we report using GST pull-down assay, mass-spectrometry and co-immunoprecipitation that a heterotrimeric complex of DNA-Dependent Protein Kinase (DNA-PK), consisting of Ku70, Ku80 and DNA-PK catalytic subunit (DNA-PKcs), is a novel interaction partner for AIRE. In vitro phosphorylation assays show that the residues Thr68 and Ser156 are DNA-PK phosphorylation sites in AIRE. In addition, we demonstrate that DNA-PKcs is expressed in AIRE positive mTEC cell population and that introduction of mutations into the AIRE phosphorylation sites decrease the capacity of AIRE to activate transcription from reporter promoters. In conclusion, our results suggest that phosphorylation of the AIRE protein at Thr68 and Ser156 by DNA-PK influences AIRE transactivation ability and might have impact on other aspects of the functional regulation of the AIRE protein.

DNA damage and cellular differentiation: more questions than responses

Studies on DNA damage responses in proliferating cells have revealed the relationship between sensing and repair of the DNA lesions and the regulation of the cell cycle, leading to the discovery and molecular characterization of the DNA damage-activated cell cycle checkpoints. Much less is known about the DNA damage response in progenitors of differentiated cells, in which cell cycle arrest is a critical signal to trigger the differentiation program, and in terminally differentiated cells, which are typically post-mitotic. How DNA lesions are detected, processed and repaired in these cells, the functional impact of DNA damage on transcription of differentiation-specific genes, how these events are coordinated at the molecular level, the consequence of defective DNA damage response on tissue-specific functions and its potential relationship with age-related diseases are currently open questions. In particular the biological complexity inherent to the global genome reprogramming of tissue progenitors, such as embryonic or adult stem cells, suggests the importance of an accurate DNA damage response at the transcription level in these cells to ensure the genomic integrity of regenerating tissues.

Functional role of Zic2 phosphorylation in transcriptional regulation

Zic2 is a transcriptional activator that plays a crucial role in mammalian forebrain development. It activates the transcription of target genes by DNA binding and recruitment of RNA helicase A (RHA). We recently reported that the Zic2-RHA interaction is decreased by phosphatase treatment in vitro. We have now identified the phosphorylation site (serine 200) in mouse Zic2. Zic2S200A was defective in RHA-binding, and its transcriptional activation ability was diminished. These data indicate that Zic2S200 is a target for phosphorylation by DNA-dependent protein kinase, regulating Zic2-mediated transcriptional activation.

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

Emerging evidence over the past decade indicates a central role for transcription factors in the embryonic development of pancreatic islets and the consequent maintenance of normal glucose homeostasis. Pancreatic and duodenal homeobox 1 (Pdx1) is the best studied and perhaps most important of these factors. Whereas deletion or inactivating mutations of the Pdx1 gene causes whole pancreas agenesis in both mice and humans, even haploinsufficiency of the gene or alterations in its expression in mature islet cells causes substantial impairments in glucose tolerance and the development of a late-onset form of diabetes known as maturity onset diabetes of the young. The study of Pdx1 has revealed crucial phenotypic interrelationships of the varied cell types within the pancreas, particularly as these impinge upon cellular differentiation in the embryo and neogenesis and regeneration in the adult. In this review, we describe the actions of Pdx1 in the developing and mature pancreas and attempt to unify these actions with its known roles in modulating transcriptional complex formation and chromatin structure at the molecular genetic level.

Dynamic binding of Ku80, Ku70 and NF90 to the IL-2 promoter in vivo in activated T-cells

IL-2 gene expression in activated T-cells is initiated by chromatin remodeling at the IL-2 proximal promoter and conversion of a transcriptional repressor into a potent transcriptional activator. A purine-box regulator complex was purified from activated Jurkat T-cell nuclei based on sequence-specific DNA binding to the antigen receptor response element (ARRE)/nuclear factor of activated T-cells (NF-AT) target DNA sequence in the proximal IL-2 promoter. ARRE DNA-binding subunits were identified as NF90, NF45 and systemic lupus erythematosis autoantigens, Ku80 and Ku70. Monoclonal antibodies to Ku80, Ku70 and NF90 specifically inhibit constitutive and inducible ARRE DNA-binding activity in Jurkat T-cells. Ku80, Ku70 and NF90 bind specifically to the IL-2 gene promoter in vivo, as demonstrated by chromatin immunoprecipitation. Activation of Jurkat T-cells and mouse primary spleen cells induces binding of Ku80 and NF90 to the IL-2 promoter in vivo, and decreases binding of Ku70 to the IL-2 promoter in vivo, and these dynamic changes are inhibited by immunosuppressants cyclosporin A and triptolide. Dynamic changes in binding of Ku80, Ku70 and NF90 to the IL-2 proximal promoter in vivo correlate with chromatin remodeling and transcriptional initiation in activated T-cells.

ZIC2-dependent Transcriptional Regulation Is Mediated by DNA-dependent Protein Kinase, Poly(ADP-ribose) Polymerase, and RNA Helicase A

The Zic family of zinc finger proteins is essential for animal development, as demonstrated by the holoprosencephaly caused by mammalian Zic2 mutation. To determine the molecular mechanism of Zic-mediated developmental control, we characterized two types of high molecular weight complexes, including Zic2. Complex I was composed of DNA-dependent protein kinase catalytic subunit (DNA-PKcs), Ku70/80, and poly(ADP-ribose) polymerase; complex II contained Ku70/80 and RNA helicase A; all the components interacted directly with Zic2 protein. Immunoprecipitation, subnuclear localization, and in vitro phosphorylation analyses revealed that the DNA-PKcs in complex I played an essential role in the assembly of complex II. Stepwise exchange from complex I to complex II depended on phosphorylation of Zic2 by DNA-PK and poly-(ADP-ribose) polymerase. Phosphorylated Zic2 protein made a stable complex with RNA helicase A, and complex II could interact with RNA polymerase II. Phosphorylation-dependent transformation of Zic2-containing molecular complexes may occur in transcriptional regulation.

Islet beta cell failure in type 2 diabetes

The major focus of this Review is on the mechanisms of islet beta cell failure in the pathogenesis of obesity-associated type 2 diabetes (T2D). As this demise occurs within the context of beta cell compensation for insulin resistance, consideration is also given to the mechanisms involved in the compensation process, including mechanisms for expansion of beta cell mass and for enhanced beta cell performance. The importance of genetic, intrauterine, and environmental factors in the determination of "susceptible" islets and overall risk for T2D is reviewed. The likely mechanisms of beta cell failure are discussed within the two broad categories: those with initiation and those with progression roles.

PARP-2 Interacts with TTF-1 and Regulates Expression of Surfactant Protein-B

Thyroid transcription factor 1 (TTF-1/Nkx-2.1) plays a critical role in lung morphogenesis and regulates the expression of lung-specific genes, including the surfactant proteins required for pulmonary function after birth. The activity of TTF-1 is influenced by its interactions with other transcription factors and coactivators, including CBP/p300 and SRC-1. In this study, we have identified poly(ADP-ribose) polymerases (PARP-2 and PARP-1) as TTF-1 interacting proteins that influence its transcriptional activity. Endogenous PARP-2 was coimmunoprecipitated from transformed mouse lung epithelial cell (MLE15) extracts with TTF-1 and was identified by mass spectrometry. PARP-1 and Ku70/Ku80 were also coimmunoprecipitated from the cell extracts with TTF-1. The E domain of PARP-2 interacted via the C-terminal domain of TTF-1. Both PARP-1 and PARP-2 enhanced the activity of the promoter of surfactant protein-B (Sftpb gene) but not other surfactant proteins in vitro. PARP-2 was selectively expressed in epithelial cells of the conducting and peripheral lung tubules of the fetal mouse lung from embryonic day 12.5 and was detected in bronchial epithelial cells in the adult lung at cellular sites consistent with that of surfactant protein B. PARP-2 and PARP-1 interact with TTF-1 and regulate the expression of surfactant protein B, a protein required for lung function.

Phosphorylation marks IPF1/PDX1 protein for degradation by glycogen synthase kinase 3-dependent mechanisms

The transcription factor IPF1/PDX1 plays a crucial role in both pancreas development and maintenance of beta-cell function. Targeted disruption of this transcription factor in beta-cells leads to diabetes, whereas reduced expression levels affect insulin expression and secretion. Therefore, it is essential to determine molecular mechanisms underlying the regulation of this key transcription factor on mRNA levels and, most importantly, on protein levels. Here we show that a minor portion of IPF1/PDX1 is phosphorylated on serine 61 and/or serine 66 in pancreatic beta-cells. This phosphorylated form of IPF1/PDX1 preferentially accumulates following proteasome inhibition, an effect that is prevented by inhibition of glycogen synthase kinase 3 (GSK3) activity. Oxidative stress, which is associated with the diabetic state, (i) increases IPF1/PDX1 Ser61 and/or Ser66 phosphorylation and (ii) increases the degradation rate and decreases the half-life of IPF-1/PDX-1 protein. In addition, we provide evidence that GSK3 activity participates in oxidative stress-induced effects on beta-cells. Thus, this current study uncovers a new mechanism that might contribute to diminished levels of IPF1/PDX1 protein and beta-cell dysfunction during the progression of diabetes.