Bridge-1, a novel PDZ-domain coactivator of E2A-mediated regulation of insulin gene transcription

A Drosophila model of diabetic neuropathy reveals a role of proteasome activity in the glia

Diabetic peripheral neuropathy (DPN) is the most common chronic, progressive complication of diabetes mellitus. The main symptom is sensory loss; the molecular mechanisms are not fully understood. We found that Drosophila fed a high-sugar diet, which induces diabetes-like phenotypes, exhibit impairment of noxious heat avoidance. The impairment of heat avoidance was associated with shrinkage of the leg neurons expressing the Drosophila transient receptor potential channel Painless. Using a candidate genetic screening approach, we identified proteasome modulator 9 as one of the modulators of impairment of heat avoidance. We further showed that proteasome inhibition in the glia reversed the impairment of noxious heat avoidance, and heat-shock proteins and endolysosomal trafficking in the glia mediated the effect of proteasome inhibition. Our results establish Drosophila as a useful system for exploring molecular mechanisms of diet-induced peripheral neuropathy and propose that the glial proteasome is one of the candidate therapeutic targets for DPN.

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.

PSMD9 ribosomal protein network maintains nucleolar architecture and WT p53 levels

Capitalizing on an unexpected observation that multiple free ribosomal proteins co-purify/pull-down with PSMD9, we report here for the first time that PSMD9 is necessary to maintain the morphology and integrity of the nucleolus. As seen by NPM1 immunofluorescence and electron microscopy, the nucleolar structure is clearly disrupted in PSMD9 null MCF7 breast cancer cells. The resultant stress is pronounced leading to the accumulation of WT p53 and slow growth. A dual insult with Actinomycin D exasperates the nucleolar stress in these cells which fail to recover in stipulated time. This double insult in the WT cells enhances the interaction of PSMD9 with ribosomal subunits. Our data also reveals that in PSMD9 null cells, ribosomal proteins RPS25 and RPL15 fail to localise in the nucleolus. We speculate that the interaction of PSMD9 with multiple free ribosome subunits has at least two important implications: a) PSMD9 plays a role in trafficking of ribosomal proteins into the nucleolus, therefore contributing to the maintenance of structural and morphological organization of the membrane-less nucleolar compartment; b) under conditions that induce nucleolar stress, PSMD9-Ribosomal Protein interaction protects WT MCF7 breast cancer cells from slow growth and eventual death. This possibility renders the domains of PSMD9 to be attractive drug targets in the context of cancer and other multiple ribosome-associated disorders.

LIM-domain transcription complexes interact with ring-finger ubiquitin ligases and thereby impact islet β-cell function

Diabetes is characterized by a loss of β-cell mass, and a greater understanding of the transcriptional mechanisms governing β-cell function is required for future therapies. Previously, we reported that a complex of the Islet-1 (Isl1) transcription factor and the co-regulator single-stranded DNA-binding protein 3 (SSBP3) regulates the genes necessary for β-cell function, but few proteins are known to interact with this complex in β-cells. To identify additional components, here we performed SSBP3 reverse-cross-linked immunoprecipitation (ReCLIP)- and MS-based experiments with mouse β-cell extracts and compared the results with those from our previous Isl1 ReCLIP study. Our analysis identified the E3 ubiquitin ligases ring finger protein 20 (RNF20) and RNF40, factors that in nonpancreatic cells regulate transcription through imparting monoubiquitin marks on histone H2B (H2Bub1), a precursor to histone H3 lysine 4 trimethylation (H3K4me3). We hypothesized that RNF20 and RNF40 regulate similar genes as those regulated by Isl1 and SSBP3 and are important for β-cell function. We observed that Rnf20 and Rnf40 depletion reduces β-cell H2Bub1 marks and uncovered several target genes, including glucose transporter 2 (Glut2), MAF BZIP transcription factor A (MafA), and uncoupling protein 2 (Ucp2). Strikingly, we also observed that Isl1 and SSBP3 depletion reduces H2Bub1 and H3K4me3 marks, suggesting that they have epigenetic roles. We noted that the RNF complex is required for glucose-stimulated insulin secretion and normal mitochondrial reactive oxygen species levels. These findings indicate that RNF20 and RNF40 regulate β-cell gene expression and insulin secretion and establish a link between Isl1 complexes and global cellular epigenetics.

Co-shared genetics and possible risk gene pathway partially explain the comorbidity of schizophrenia, major depressive disorder, type 2 diabetes, and metabolic syndrome

Schizophrenia (SCZ) and major depressive disorder (MDD) in treatment-naive patients are associated with increased risk for type 2 diabetes (T2D) and metabolic syndrome (MetS). SCZ, MDD, T2D, and MetS are often comorbid and their comorbidity increases cardiovascular risk: Some risk genes are likely co-shared by them. For instance, transcription factor 7-like 2 (TCF7L2) and proteasome 26S subunit, non-ATPase 9 (PSMD9) are two genes independently reported as contributing to T2D and SCZ, and PSMD9 to MDD as well. However, there are scarce data on the shared genetic risk among SCZ, MDD, T2D, and/or MetS. Here, we briefly describe T2D, MetS, SCZ, and MDD and their genetic architecture. Next, we report separately about the comorbidity of SCZ and MDD with T2D and MetS, and their respective genetic overlap. We propose a novel hypothesis that genes of the prolactin (PRL)-pathway may be implicated in the comorbidity of these disorders. The inherited predisposition of patients with SCZ and MDD to psychoneuroendocrine dysfunction may confer increased risk of T2D and MetS. We illustrate a strategy to identify risk variants in each disorder and in their comorbid psychoneuroendocrine and mental-metabolic dysfunctions, advocating for studies of genetically homogeneous and phenotype-rich families. The results will guide future studies of the shared predisposition and molecular genetics of new homogeneous endophenotypes of SCZ, MDD, and metabolic impairment.

Novel Nuclear Partnering Role of EPS8 With FOXM1 in Regulating Cell Proliferation

One hallmark of cancer cells is sustaining proliferative signaling that leads to uncontrolled cell proliferation. Both the Forkhead box (FOX) M1 transcription factor and the Epidermal Growth Factor (EGF) receptor Pathway Substrate 8 (EPS8) are known to be activated by mitogenic signaling and their levels upregulated in cancer. Well-known to regulate Rac-mediated actin remodeling at the cell cortex, EPS8 carries a nuclear localization signal but its possible nuclear role remains unclear. Here, we demonstrated interaction of FOXM1 with EPS8 in yeast two-hybrid and immunoprecipitation assays. Immunostaining revealed co-localization of the two proteins during G2/M phase of the cell cycle. EPS8 became nuclear localized when CRM1/Exportin 1-dependent nuclear export was inhibited by Leptomycin B, and a functional nuclear export signal could be identified within EPS8 using EGFP-tagging and site-directed mutagenesis. Downregulation of EPS8 using shRNAs suppressed expression of FOXM1 and the FOXM1-target CCNB1, and slowed down G2/M transition in cervical cancer cells. Chromatin immunoprecipitation analysis indicated recruitment of EPS8 to the CCNB1 and CDC25B promoters. Taken together, our findings support a novel partnering role of EPS8 with FOXM1 in the regulation of cancer cell proliferation and provides interesting insight into future design of therapeutic strategy to inhibit cancer cell proliferation.

SSBP3 Interacts With Islet-1 and Ldb1 to Impact Pancreatic β-Cell Target Genes

Islet-1 (Isl1) is a Lin11, Isl1, Mec3 (LIM)-homeodomain transcription factor important for pancreatic islet cell development, maturation, and function, which largely requires interaction with the LIM domain-binding protein 1 (Ldb1) coregulator. In other tissues, Ldb1 and Isl1 interact with additional factors to mediate target gene transcription, yet few protein partners are known in β-cells. Therefore, we hypothesize that Ldb1 and Isl1 participate in larger regulatory complexes to impact β-cell gene expression. To test this, we used cross-linked immunoprecipitation and mass spectrometry to identify interacting proteins from mouse β-cells. Proteomic datasets revealed numerous interacting candidates, including a member of the single-stranded DNA-binding protein (SSBP) coregulator family, SSBP3. SSBPs potentiate LIM transcription factor complex activity and stability in other tissues. However, nothing was known of SSBP3 interaction, expression, or activity in β-cells. Our analyses confirmed that SSBP3 interacts with Ldb1 and Isl1 in β-cell lines and in mouse and human islets and demonstrated SSBP3 coexpression with Ldb1 and Isl1 pancreas tissue. Furthermore, β-cell line SSBP3 knockdown imparted mRNA deficiencies similar to those observed upon Ldb1 reduction in vitro or in vivo. This appears to be (at least) due to SSBP3 occupancy of known Ldb1-Isl1 target promoters, including MafA and Glp1r. This study collectively demonstrates that SSBP3 is a critical component of Ldb1-Isl1 regulatory complexes, required for expression of critical β-cell target genes.

Polymorphisms of the peroxisome proliferator-activated receptor-γ (rs1801282) and its coactivator-1 (rs8192673) are associated with obesity indexes in subjects with type 2 diabetes mellitus

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The aim of this study was to clarify whether common single nucleotide polymorphisms (SNPs) of the Peroxisome Proliferator-Activated Receptor-γ (PPAR-γ) gene (rs1801282) and the Peroxisome Proliferator-Activated Receptor-γ Coactivator-1 (PGC-1α) gene (rs8192673) are associated with obesity indexes (BMI, waist circumference) in subjects with type 2 diabetes mellitus (T2DM) in Caucasian population. The second aim was to find an association of both polymorphisms with T2DM.

Methods

Two exonic SNPs of both genes rs1801282 of the PPAR-γ gene and rs8192673 of the PGC-1α gene) were genotyped in 881 unrelated Slovene subjects (Caucasians) with T2DM and in 348 subjects without T2DM (control subjects).

Results

Female homozygotes with the CC genotype of the rs8192673 had higher waist circumference in comparison with subjects with other genotypes. Homozygotes (females, males) with wild allele (Pro) of the rs1801282 (Pro12Ala polymorphism) had higher waist circumference in comparison with subjects with other genotypes. In the study, there were no differences in the distributions of the rs8192673 and the rs1801282 genotypes between patients with T2DM and controls. Linear regression analyses for both polymorphisms were performed and demonstrated an independent effect of the rs1801282 of the PPAR-γ on waist circumference in subjects with T2DM, whereas an independent effect on waist circumference was not demonstrated for the rs8192673 of the PGC-1α gene.

Conclusions

In a large sample of the Caucasians the rs8192673 of the PGC-1α gene and the rs1801282 of the PPAR-γ gene were associated with waist circumference in subjects with T2DM.

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.

Regulation of postnatal pancreatic Pdx1 and downstream target genes after gestational exposure to protein restriction in rats

The study carried out in our laboratory demonstrated that protein restriction (low protein, LP) during fetal and neonatal life alters pancreatic development and impairs glucose tolerance later in life. In this study, we examined the role of the transcription factorPdx1, a master regulator of β-cell differentiation and function along with its downstream target genes insulin,Glut2and glucokinase (GK). The role(s) of these genes and protein products on the pancreata of male offspring from mothers exposed to LP diets were assessed during gestation, weaning, and adult life. Pregnant rats were allocated to two dietary treatments: control (C) 20% protein diet or LP, 8% protein diet. At birth, offspring were divided into four groups: C received control diet all life, LP1 received LP diet all life, LP2 changed the LP diet to C at weaning, and LP3 switched to C after being exposed to LP during gestation only. Body weights (bw) were significantly (P<0.001) decreased in all LP groups at birth. At weaning, only the LP3 offspring had their body weight restored to control levels.Pdx1or any of thePdx1-target genes were similar in all diets at day 21. However, at d130Pdx1mRNA expression and protein abundance were significantly decreased (P<0.05) in all LP groups. In addition, insulin mRNA and protein were decreased in LP1 and LP3 groups compared with C,Glut2mRNA and GLUT2 protein levels were decreased in LP3 and GK did not change between groups. Intraperitoneal glucose tolerance test revealed impaired glucose tolerance in LP3 males, concomitant with decreased β-cell mass, islet area, and PDX1 nuclear protein localization. Collectively, this study suggests that restoring proteins in the diet after birth in LP offspring dramatically impairs glucose homeostasis in early adulthood, by alteringPdx1expression and downstream-target genes increasing the risk to develop type 2 diabetes.

Discovery of novel interacting partners of PSMD9, a proteasomal chaperone: Role of an Atypical and versatile PDZ-domain motif interaction and identification of putative functional modules

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The structure and functions of PSMD9, a proteasomal chaperone, are uncharacterized.

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PDZ-like domain of PSMD9 may recognize C-terminal residues in proteins.

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Using conserved C-terminal motifs in human proteome, we identify novel binding partners.

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hnRNPA1, GH, IL6-receptor, S14 and E12 interact with PSMD9 via a specific C-terminal motif.

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We predict and confirm residues in the PDZ domain that are involved in this interaction.

PSMD9 (Proteasome Macropain non-ATPase subunit 9), a proteasomal assembly chaperone, harbors an uncharacterized PDZ-like domain. Here we report the identification of five novel interacting partners of PSMD9 and provide the first glimpse at the structure of the PDZ-domain, including the molecular details of the interaction. We based our strategy on two propositions: (a) proteins with conserved C-termini may share common functions and (b) PDZ domains interact with C-terminal residues of proteins. Screening of C-terminal peptides followed by interactions using full-length recombinant proteins, we discovered hnRNPA1 (an RNA binding protein), S14 (a ribosomal protein), CSH1 (a growth hormone), E12 (a transcription factor) and IL6 receptor as novel PSMD9-interacting partners. Through multiple techniques and structural insights, we clearly demonstrate for the first time that human PDZ domain interacts with the predicted Short Linear Sequence Motif (SLIM) at the C-termini of the client proteins. These interactions are also recapitulated in mammalian cells. Together, these results are suggestive of the role of PSMD9 in transcriptional regulation, mRNA processing and editing, hormone and receptor activity and protein translation. Our proof-of-principle experiments endorse a novel and quick method for the identification of putative interacting partners of similar PDZ-domain proteins from the proteome and for discovering novel functions.

Purification, crystallization and preliminary X-ray data collection of the N-terminal domain of the 26S proteasome regulatory subunit p27 and its complex with the ATPase domain of Rpt5 from Mus musculus

The protein 26S proteasome regulatory subunit p27 is one of the four chaperones that help in the assembly of the 19S regulatory particle (RP) of the 26S proteasome. In the present work, the N-terminus of p27 (residues 1-128) from Mus musculus was cloned, expressed, purified and crystallized alone and in complex with the C-terminal ATPase domain of Rpt5 (residues 173-442). The crystals of p27((1-128)) diffracted to 1.7 Å resolution and belonged to space group P212121, with unit-cell parameters a = 26.79, b = 30.39, c = 145.06 Å. Resolution-dependent Matthews coefficient probability analysis suggested the presence of only one molecule per asymmetric unit, with 40.5% solvent content and a VM value of 2.02 Å(3) Da(-1). The crystal of the p27((1-128))-Rpt5((173-442)) complex diffracted to 4 Å resolution and belonged to space group P222, with unit-cell parameters a = 75.93, b = 76.08, c = 336.85 Å. The presence of four heterodimers in the asymmetric unit with 53.2% solvent content and a VM value of 2.63 Å(3) Da(-1) or five heterodimers in the asymmetric unit with 41.5% solvent content and a VM value of 2.10 Å(3) Da(-1) is assumed.

Islet α-, β-, and δ-cell development is controlled by the Ldb1 coregulator, acting primarily with the islet-1 transcription factor

Ldb1 and Ldb2 are coregulators that mediate Lin11-Isl1-Mec3 (LIM)-homeodomain (HD) and LIM-only transcription factor-driven gene regulation. Although both Ldb1 and Ldb2 mRNA were produced in the developing and adult pancreas, immunohistochemical analysis illustrated a broad Ldb1 protein expression pattern during early pancreatogenesis, which subsequently became enriched in islet and ductal cells perinatally. The islet-enriched pattern of Ldb1 was similar to pan-endocrine cell-expressed Islet-1 (Isl1), which was demonstrated in this study to be the primary LIM-HD transcription factor in developing and adult islet cells. Endocrine cell-specific removal of Ldb1 during mouse development resulted in a severe reduction of hormone⁺ cell numbers (i.e., α, β, and δ) and overt postnatal hyperglycemia, reminiscent of the phenotype described for the Isl1 conditional mutant. In contrast, neither endocrine cell development nor function was affected in the pancreas of Ldb2(-/-) mice. Gene expression and chromatin immunoprecipitation (ChIP) analyses demonstrated that many important Isl1-activated genes were coregulated by Ldb1, including MafA, Arx, insulin, and Glp1r. However, some genes (i.e., Hb9 and Glut2) only appeared to be impacted by Ldb1 during development. These findings establish Ldb1 as a critical transcriptional coregulator during islet α-, β-, and δ-cell development through Isl1-dependent and potentially Isl1-independent control.

Overweight condition and waist circumference and a candidate gene within the 12q24 locus

Aims

Obesity and obesity-associated phenotypes are linked to the chromosome12q24 locus, the non-insulin-dependent-diabetes 2 (NIDDM2) locus. The gene of proteasome modulator 9 (PSMD9) lies in the NIDDM2 region and is linked to type 2 diabetes (T2D), microvascular and macrovascular complications of T2D.

We aimed at studying whether the PSMD9 T2D risk single nucleotide polymorphisms (SNPs) IVS3+nt460, IVS3+nt437, and 197G are linked to obesity, overweight status and waist circumference in Italian T2D families.

Methods and results

We screened 200 Italians T2D siblings/families for PSMD9 variants. Using Merlin software, we performed non-parametric linkage analysis to test for linkage with obesity and overweight condition and variance component analysis to test for linkage with waist circumference in our T2D siblings/families dataset.

Our study shows that the PSMD9 SNPs IVS3+nt460, IVS3+nt437, and 197G are in linkage with overweight condition and waist circumference in Italians. The statistical power tests performed via simulations on real data confirm that the results are not due to random chance.

Conclusions

In summary, the linkage strategy using a homogeneous family/subject dataset can identify a gene contributing to a complex trait. PMSD9 may be at least one of the genes responsible for the linkage to obesity and obesity-associated phenotypes at the locus 12q24 in other populations.

Proteasome modulator 9 is linked to microvascular pathology of T2D

The locus 12q24 is linked to type 2 diabetes (T2D) and to changes in retinal vascular caliber in Caucasians. Proteasome Modulator 9 gene (PSMD9) lies in the 12q24 locus and is implicated in diabetes onset and in degradation of intracellular proteins in antigenic peptides in the immune response to antigen presentation by MHC class I cells. Within PSMD9, we reported a linkage to T2D and to MODY3 in Italian families. We recently demonstrated a linkage of the PSMD9 T2D risk SNPs with T2D-nephropathy, T2D-neuropathy, retinopathy, hypercholesterolemia, and macrovascular pathology. We aimed at studying the presence of the linkage signal of the PSMD9 T2D risk SNPs IVS3 + nt460, IVS3 + nt437, E197G to microvascular pathology associated to T2D in Italian siblings/families. We screened 200 T2D siblings/families for the PSMD9 above-mentioned variants and performed a parametric and non-parametric linkage study by Merlin software. Our results show significant LOD score in linkage with microvascular pathology for the PSMD9 SNPs studied using the non-parametric and parametric linkage analysis. The strongest signal is present under the recessive model. Our statistical power relies on the presence of T2D affected siblings, which represent an ideal dataset to identify linkage with a recessive disease model. Our simulation analysis confirms that the results are not due to random chance. In summary, the PSMD9 IVS3 + nt460, IVS3 + nt437, E197G SNPs are linked via the recessive model to microvascular pathology of T2D in Italians. A possible role of PSMD9 in microvascular pathology may be related to a causative pathogenetic role in inflammation as part of an autoimmune process.

Proteasome modulator 9 and carpal tunnel syndrome

Proteasome modulator 9 (PSMD9) is linked to type 2 diabetes (T2D). The author studied whether PSMD9 IVS3 + nt460 A > G, IVS3 + nt437 T > C and E197 are linked to carpal tunnel syndrome in T2D Italian families. Non-parametric linkage, linkage disequilibrium-based and independent SNPs-based linkage analyses were performed. The PSMD9 SNPs show linkage to carpal tunnel syndrome.

PSMD9 is linked to type 2 diabetes neuropathy

OBJECTIVE

Diabetic neuropathy, a long-term complication of type 2 diabetes (T2D), has a genetic inheritance component to its manifestation. We aimed at identifying whether the proteasome modulator 9 (PSMD9) gene responsible for linkage with T2D and maturity-onset diabetes of the young 3 in Italian families may play a role in the inheritance of diabetic neuropathy in T2D as well.

METHODS

We characterized the Italian families with T2D for presence and/or absence of diabetic neuropathy. The diabetic neuropathy is reported as unknown in all cases in which either the diagnosis was unclear or the data were not available for the subject studied. Thus, in our 200 Italian families, the data presence was less than 100%. We tested in the 200 Italians families for the presence of the linkage of the PSMD9 single nucleotide polymorphisms (SNPs) with diabetic neuropathy. The nonparametric as well as the parametric linkage analysis was performed by using the Merlin software; the LOD score and correspondent P value were calculated. For the significance and/or trends toward significance linkage score, 1000 replicates were performed to determine the empirical P value.

RESULTS

We report the linkage study of the PSMD9 SNPs [intervening sequence IVS3+nt460A/G, IVS3+nt437C/T and E197G] in Italian families with T2D neuropathy.

CONCLUSION

In summary, the PSMD9 gene SNPs studied contribute to the genetic risk of diabetic neuropathy in our Italian data set, showing statistical significance for an additive model-based inheritance.

Proteasome modulator 9 SNPs are linked to hypertension in type 2 diabetes families

Background

Chromosome 12q24 was recently associated with hypertension. Proteasome Modulator 9 (PSMD9) lies in the 12q24 locus and is in linkage with MODY3, type 2 diabetes (T2D), microvascular and macrovascular pathology, carpal tunnel syndrome, and hypercholesterolemia in Italian families.

Aims

Our goal was to determine whether PSMD9 is linked to elevated blood pressure/hypertension in T2D families.

Methods

We characterized the Italian T2D families' members for presence and/or absence of elevated blood pressure (≥ 130/80) and/or hypertension. The phenotypes were described as unknown in all cases in which the diagnosis was either unclear or the data were not available for the subject studied. We tested in the 200 Italians families for the presence of the linkage of the PSMD9 T2D risk single nucleotide polymorphisms (SNPs) IVS3+nt460 A > G, IVS3+nt437 T > C and E197G A > G with elevated blood pressure/hypertension. The non-parametric linkage analysis was performed for this qualitative phenotype by using the Merlin software; the Lod score and correspondent P-value were calculated. Parametric linkage analysis was also performed. For the significant linkage score, 1000 replicates were run to calculate the empirical P-value.

Results

The PSMD9 gene SNPs studied are in linkage with elevated blood pressure/hypertension in our Italian families.

Conclusions

We conclude that the PSMD9 gene and/or any variant in linkage disequilibrium with the SNPs studied contribute to the linkage to hypertension within our family dataset. This is the first report of PSMD9 linkage to hypertension within the 12q24 locus.

Proteasome modulator 9 gene is linked to diabetic and non-diabetic retinopathy in T2D

BACKGROUND

Diabetic retinopathy is a long-term complication of type 2 diabetes (T2D). Non-diabetic retinopathy may be present in T2D patients as well and non-T2D patients with hypertension and/or atherosclerosis. The aim of this study was to identify linkage of the proteasome modulator 9 (PSMD9) T2D risk variants IVS3+nt460, IVS3+nt437, E197G to diabetic retinopathy and retinopathy including also atherosclerotic or hypertensive retinopathy in Italian T2D families.

MATERIALS AND METHODS

A total of 126 siblings of our 200 T2D siblings/families were characterized for diabetic retinopathy or retinopathy. The clinical characterization is based on a fundus oculi exam and on fluorangiography of the participating subjects. Diabetic retinopathy includes both pre-proliferative and proliferative retinopathy. The common gene variants were directly amplified by PCR and by fluorescent-based automation. A parametric and non-parametric linkage study of the gene variants with diabetic retinopathy and retinopathy was then performed using Merlin software. Finally, 1000 simulation analyses were performed to test for the statistical power of the significant results (P-value ≤ 0.05).

RESULTS

This study shows a linkage of the PSMD9 IVS3+nt460 (rs74421874), IVS3+nt437 (rs3825172) and E197G (rs14259) single nucleotide polymorphisms (SNPs) to diabetic and non-diabetic retinopathy by using the non-parametric as well as the parametric linkage analysis. The strongest signal is present for the PSMD9 variants with the diabetic retinopathy, in particular under the additive model. The 1,000 simulations performed for each significant test confirmed that the results are not due to random chance.

CONCLUSIONS

In summary, the PSMD9 IVS3+nt460, IVS3+nt437, E197G SNPs are linked to diabetic retinopathy and non-diabetic retinopathy in Italians.

T2D-nephropathy linkage within 12q24 locus

The 12q24 locus is linked to microcirculation pathology and type 2 diabetes (T2D) with nephropathy and entails the PSMD9 gene linked to T2D. We tested the PSMD9 T2D risk SNPs IVS3+nt460A>G, IVS3+nt437T>C and E197G for linkage to T2D-diabetic nephropathy in 200 Italians T2D families and show linkage to diabetic nephropathy.

Hypercholesterolemia and a candidate gene within the 12q24 locus

Background

The 12q24 locus entails at least one gene responsible for hypercholesterolemia. Within the 12q24 locus lies the gene of proteasome modulator 9 (PSMD9). PSMD9 is in linkage with type 2 diabetes (T2D), T2D-nephropathy and macrovascular pathology in Italian families and PSMD9 rare mutations contribute to T2D.

Aims

In the present study, we aimed at determining whether the PSMD9 T2D risk single nucleotide polymorphisms (SNPs) IVS3 + nt460 A > G, IVS3 + nt437 T > C and E197G A > G are linked to hypercholesterolemia in 200 T2D Italian families.

Methods

We characterized 200 Italian families for presence and/or absence of hypercholesterolemia characterized by LDL levels ≥ 100 mg/dl in drug-naïve patients and/or presence of a diagnosis of hypercholesterolemia in a patient treated with statin medication. The phenotypes were described as unknown in all cases in which the diagnosis was either unclear or the data were missing. We tested in the 200 Italians families for evidence of linkage of the PSMD9 SNPs with hypercholesterolemia. The non-parametric linkage analysis was performed for the qualitative phenotype by using the Merlin software; the Lod score and correspondent P-value were calculated. For the significant linkage score, 1000 replicates were performed to calculate the empirical P-value.

Results

The PSMD9 gene SNPs studied show linkage to hypercholesterolemia. The results are not due to random chance.

Conclusions

PSMD9 should be tested in all populations reporting linkage to hypercholesterolemia within the chromosome 12q24 locus. The impact of this gene on hypercholesterolemia and contribution to cardio- and cerebrovascular events may be high.

Proteasome modulator 9 and macrovascular pathology of T2D

Aims

Coronary artery disease (CAD) and stroke share a major linkage at the chromosome 12q24 locus. The same chromosome region entails at least a major risk gene for type 2 diabetes (T2D) within NIDDM2, the non-insulin-dependent-diabetes 2 locus. The gene of Proteasome Modulator 9 (PSMD9) lies in the NIDDM2 region and is implicated in diabetes in mice. PSMD9 mutations rarely cause T2D and common variants are linked to both late-onset T2D and maturity-onset-diabetes of the young (MODY3). In this study, we aimed at determining whether PSMD9 is linked to macrovascular pathology of T2D.

Methods and Results

In our 200 T2D families from Italy, we characterized the clinical phenotype of macrovascular pathology by defining the subjects for presence or absence of CAD, stroke and/or transitory ischemic attacks (TIA), plaques of the large arterial vessels (macro-vasculopathy) and arterial angioplasty performance. We then screened 200 T2D siblings/families for PSMD9 +nt460A/G, +nt437C/T and exon E197G A/G single nucleotide polymorphisms (SNPs) and performed a non-parametric linkage study to test for linkage for coronary artery disease, stroke/TIA, macro-vasculopathy and macrovascular pathology of T2D. We performed 1,000 replicates to test the power of our significant results. Our results show a consistent significant LOD score in linkage with all the above-mentioned phenotypes. Our 1000 simulation analyses, performed for each single test, confirm that the results are not due to random chance.

Conclusions

In summary, the PSMD9 IVS3+nt460A/G, +nt437C/T and exon E197G A/G SNPs are linked to CAD, stroke/TIA and macrovascular pathology of T2D in Italians.

Bridge-1 is expressed in human breast carcinomas: silencing of Bridge-1 decreases Smad2, Smad3 and Smad4 expression in MCF-7 cells, a human breast cancer cell line

PURPOSE

The aim of the study was to investigate the expression of Bridge-1 in human breast carcinomas, and to determine the in vitro regulation of Bridge-1 by activin A and the influence of Bridge-1 on activin A signaling in the human breast cancer cell line MCF-7.

METHOD

Bridge-1 expression in human breast carcinomas was shown after staining paraffin slides with a specific antibody against Bridge-1. To gain insight into Bridge-1 function, immortalized, human breast cancer cells (MCF-7 cell line) were stimulated with activin A and the expression of Bridge-1 was analyzed by real-time PCR and Western blot. Next, Bridge-1 was downregulated via siRNA treatment in MCF-7 cells and the expression of Bridge-1, Smad2, 3 and 4 was investigated by real-time PCR and Western blot.

RESULTS

Human breast carcinoma cells showed nuclear and cytoplasmic localization of Bridge-1. Activin A stimulation of the immortalized human breast adenocarcinoma cell line MCF-7 showed an increase in Bridge-1 expression by real-time PCR and Western blot. Downregulation of Bridge-1 by Bridge-1-siRNA resulted in a decreased expression of Smad2, 3 and 4 of up to 50% compared to the treatment with non-targeting siRNA.

CONCLUSIONS

This study is the first to demonstrate the expression of Bridge-1 in human breast carcinomas. Bridge-1 expression is increased by activin A stimulation and itself seems to influence activin A signaling by affecting the expression of Smad2, 3 and 4.

The activation of the rat insulin gene II by BETA2 and PDX-1 in rat insulinoma cells is repressed by Pax6

The transcriptional transactivator Pax6 binds the pancreatic islet cell-specific enhancer sequence (PISCES) of the rat insulin I gene. However the human, mouse, and rat insulin gene II promoters do not contain a PISCES element. To analyze the role of Pax6 in those PISCES-less promoters, we investigated its influence on rat insulin gene II expression and included in our studies the main activators: pancreatic and duodenal homeobox protein-1 (PDX-1) and BETA2/E47. Luciferase assays, Northern blots, and RIA were used to study effects of Pax6 overexpression, gel shift and chromatin precipitation assays to study its binding to the DNA, and yeast two-hybrid assays and glutathione S transferase capture assays to investigate its interactions with PDX-1 and BETA2. Finally, glucose-dependent intracellular transport of Pax6 was demonstrated by fluorescence microscopy. Overexpression of Pax6 prevents activation of the rat insulin II gene by BETA2 and PDX-1 and hence suppresses insulin synthesis and secretion. In vitro, Pax6 binds to the A-boxes, thereby blocking binding of PDX-1, and at the same time, its paired domain interacts with BETA2. Fluorescence microscopy demonstrated that the nuclear-cytoplasmic localization of Pax6 and PDX-1 are oppositely regulated by glucose. From the results, it is suggested that at low concentrations of glucose, Pax6 is localized in the nucleus and prevents the activation of the insulin gene by occupying the PDX-1 binding site and by interacting with BETA2.

PSMD9 is linked to MODY3

Type 2 diabetes has a replicated linkage on chromosome12q24.2 (NIDDM2 locus/non-insulin-dependent diabetes mellitus 2 locus), near the HNF-1alpha/MODY3 gene. The MODY3 gene is not responsible for this linkage. PSMD9--contributing to T2D in Italians by rare unique mutations and by the common haplotype A/T/G--lies in the NIDDM2 region. By genotyping the two markers D12S1721/D12S2073 nearby the MODY3 gene in our unrelated T2D cases, we previously excluded that the PSMD9 SNPs are in linkage disequilibrium (LD) with the MODY3 gene. In the present study, we aimed at identifying whether the PSMD9 A/T/G haplotype is present in the Italy-1 and Italy-3 MODY3 families and whether it cosegregates with diabetes/MODY3. We raised the question whether there is a digenic additive model within the MODY3 families to which the PSMD9 A/T/G haplotype contributes. We demonstrated that the PSMD9 A/T/G haplotype is linked to the MODY3 established mutations in the Italy-1 and Italy-3 families. By non-parametric and parametric linkage analyses, and LD modeling, in the Italy-1 and Italy-3 families we hereby show that the MODY3 mutation and the PSMD9 IVS3 + nt460A/IVS3 + nt437T/G197 SNPs act in an additional model to cause diabetes. Since in the two MODY3 Italian families the PSMD9 A/T/G haplotype is linked to MODY3, it contributes to MODY3/diabetes via an additional model. All MODY3 families should be tested for the PSMD9 A/T/G haplotype. The potential clinical impact of our study is of relevance.

Increased basal insulin secretion in Pdzd2-deficient mice

Expression of the multi-PDZ protein Pdzd2 (PDZ domain-containing protein 2) is enriched in pancreatic islet beta cells, but not in exocrine or alpha cells, suggesting a role for Pdzd2 in the regulation of pancreatic beta-cell function. To explore the in vivo function of Pdzd2, Pdzd2-deficient mice were generated. Homozygous Pdzd2 mutant mice were viable and their gross morphology appeared normal. Interestingly, Pdzd2-deficient mice showed enhanced glucose tolerance in intraperitoneal glucose tolerance tests and their plasma insulin levels indicated increased basal insulin secretion after fasting. Moreover, insulin release from mutant pancreatic islets was found to be twofold higher than from normal islets. To verify the functional defect in vitro, Pdzd2 was depleted in INS-1E cells using two siRNA duplexes. Pdzd2-depleted INS-1E cells also displayed increased insulin secretion at low concentrations of glucose. Our results provide the first evidence that Pdzd2 is required for normal regulation of basal insulin secretion.

PSMD9 gene in the NIDDM2 locus is linked to type 2 diabetes in Italians

Type 2 diabetes (T2D) has a replicated linkage on chromosome12q24.2, the non-insulin-dependent-diabetes 2 (NIDDM2) locus. PSMD9 (which rarely causes T2D in Italians) lies in the NIDDM2 region and is implicated in beta cell insulin transcription and diabetes onset in mice. Thus, PSMD9 is a candidate T2D gene for the NIDDM2 locus. We aimed at identifying any linkage of the PSMD9 A/T/G haplotype, or of any of its single variants, to Italian T2D siblings/families. We screened 201 T2D siblings/families for PSMD9 variants and performed a parametric and non-parametric linkage study, including linkage disequilibrium (LD) modeling and simulation analyses. Our results show a consistent significant LOD score in linkage with T2D for each single PSMD9 SNP variant (IVS3 + nt460A, P = 3.546E - 007, IVS3 + nt437T, P = 7.723E - 008, and 197G, P = 4.921E - 007) and for the haplotype (A/T/G for the above-cited variants, P = 3.078E - 015) using the non-parametric analysis, as well as the LD modeling test (P = 4.178E - 009) and the parametric linkage analysis. The strongest signal is present under the recessive model (P = 4.905E - 011). Our statistical power in the present study relies on the presence of T2D affected siblings, which represent an ideal dataset to identify linkage with a recessive disease model. Our 1,000 simulation analyses, performed for each single test, confirm that the results are not due to random chance. In summary, the A/T/G haplotype in PSMD9 is linked via the recessive allelic model to T2D in Italians. By our observation and testing, the linkage strategy can identify a gene contributing to T2D in a homogeneous subject dataset.

PDZ-domain containing-2 (PDZD2) drives the maturity of human fetal pancreatic progenitor-derived islet-like cell clusters with functional responsiveness against membrane depolarization

We recently reported the isolation and characterization of a population of pancreatic progenitor cells (PPCs) from early trimester human fetal pancreata. The PPCs, being the forerunners of adult pancreatic cell lineages, were amenable to growth and differentiation into insulin-secreting islet-like cell clusters (ICCs) upon stimulation by adequate morphogens. Of note, a novel morphogenic factor, PDZ-domain containing-2 (PDZD2) and its secreted form (sPDZD2) were ubiquitously expressed in the PPCs. Our goals for this study were to evaluate the potential role of sPDZD2 in stimulating PPC differentiation and to establish the optimal concentration for such stimulation. We found that 10(-9)M sPDZD2 promoted PPC differentiation, as evidenced by the upregulation of the pancreatic endocrine markers (PDX-1, NGN3, NEURO-D, ISL-1, NKX 2.2, NKX 6.1) and INSULIN mRNA. Inhibited endogenous production of sPDZD2 suppressed expression of these factors. Secreted PDZD2 treatment significantly elevated the C-peptide content of the ICCs and increased the basal rate of insulin secretion. However, they remained unresponsive to glucose stimulation, reflected by a minimal increase in GLUT-2 and GLUCOKINASE mRNA expression. Interestingly, sPDZD2 treatment induced increased expression of the L-type voltage-gated calcium channel (Ca(v)1.2) in the ICCs, triggering calcium ion influx under KCl stimulation and conferring an ability to secrete insulin in response to KCl. Pancreatic progenitor cells from 10- and 13-week fetal pancreata showed peak expression of endogenous sPDZD2, implying that sPDZD2 has a specific role in islet development during the first trimester. In conclusion, our data suggest that sPDZD2 promotes functional maturation of human fetal PPC-derived ICCs, thus enhancing its transplanting potentials.

Dynamic modularity in protein interaction networks predicts breast cancer outcome

Changes in the biochemical wiring of oncogenic cells drives phenotypic transformations that directly affect disease outcome. Here we examine the dynamic structure of the human protein interaction network (interactome) to determine whether changes in the organization of the interactome can be used to predict patient outcome. An analysis of hub proteins identified intermodular hub proteins that are co-expressed with their interacting partners in a tissue-restricted manner and intramodular hub proteins that are co-expressed with their interacting partners in all or most tissues. Substantial differences in biochemical structure were observed between the two types of hubs. Signaling domains were found more often in intermodular hub proteins, which were also more frequently associated with oncogenesis. Analysis of two breast cancer patient cohorts revealed that altered modularity of the human interactome may be useful as an indicator of breast cancer prognosis.

The nuclear receptor coactivator AIB3 is a modulator of HOMA beta-cell function in nondiabetic children

OBJECTIVE

The amplified in breast cancer-3 protein (AIB3) is a nuclear coactivator involved in proliferation, apoptosis and development. AIB3 loss of function causes deficient insulin secretion in mice, indicating that AIB3 participates in beta-cell regulation. Our objective was to evaluate genetic variants located on AIB3 associated with beta-cell function in children and to analyse the effect of AIB3 overexpression on gene expression in insulin 1 (INS-1) beta-pancreatic cells.

DESIGN

Polymorphisms from AIB3 were genotyped in 148 children with normal or low birthweights for gestational age. The effect of AIB3 overexpression on gene expression was analysed by real-time polymerase chain reaction (PCR) in INS-1 cells.

RESULTS

AIB3 variants were associated with homeostasis model assessment of beta-cell function (HOMA-beta-cell) in children with normal or low birthweights for gestational age, but not with HOMA of insulin resistance (HOMA-IR), or with birthweight. AIB3 overexpression increased the expression of genes involved in signalling, such as IRS-1, IRS-2, IGF-II receptor or Foxo1, or of genes that control insulin secretion, such as Cplx2, Glut2 or Kv3.1 in INS-1 cells.

CONCLUSIONS

Our results suggest that AIB3 contributes to the maintenance of beta-cell function in nondiabetic children and regulates gene expression in INS-1 cells.

The autocrine human secreted PDZ domain-containing protein 2 (sPDZD2) induces senescence or quiescence of prostate, breast and liver cancer cells via transcriptional activation of p53

Tumor suppressive actions of the autocrine human secreted PDZ domain-containing protein 2 (sPDZD2) have been reported, but the mechanisms remain enigmatic. Here, we showed that sPDZD2 induced senescence of prostate cancer DU145 cells, quiescence of breast cancer MCF-7 and liver cancer Hep-G2 cells, via transcriptional activation of mutant or wild-type p53. Furthermore, sPDZD2 sensitized mutant p53-positive DU145 cells and wild-type p53-positive MCF-7 cells to apoptosis induction through genotoxic stress imposed by sub-lethal concentration of hydrogen peroxide. Together, our findings suggest a potential autocrine pathway of p53 activation by transcriptional regulation, and a new approach to reactivate p53 for cancer therapy.

PSMD9 gene variants within NIDDM2 may rarely contribute to type 2 diabetes

Multiple genome-wide scans in different populations have linked the chromosome 12q24 region, known as NIDDM2 (non-insulin-dependent-diabetes, locus 2), to type 2 diabetes. Within NIDDM2 we examined the PSMD9 (proteasome modulator 9/Bridge-1) gene that encodes a PDZ-domain transcriptional coactivator of insulin production. Our goal was to identify a potential contribution of the PSMD9 gene to type 2 diabetes in Italians. We directly sequenced the entire gene PSMD9 in Italian type 2 diabetes patients (n = 237) and controls subjects (n = 215) and performed an association study with the identified gene variants. We found five single nucleotide polymorphisms (SNPs), A17V, IVS1+nt29, IVS3+nt460, IVS3+nt437, and E197G, which are not associated with disease in our case-control study. Furthermore, we identified two PSMD9 gene variants in type 2 diabetes patients, which produced nonconservative amino acid substitutions S143G and N166S within the PDZ domain and two other gene variants. Three out of four of these variants are absent from the control subjects screened. We propose that the three PSMD9 gene variants (S143G, N166S and G > A at IVS3+nt102), absent in control subjects, contribute rarely to late-onset type 2 diabetes in Italians. In fact, the frequency rate of such variants in unrelated cases equals 0.016. We may not exclude that PSMD9 gene variants may contribute, either commonly or rarely, to an increased risk of type 2 diabetes in other populations.

The rice bHLH protein OsIRO2 is an essential regulator of the genes involved in Fe uptake under Fe-deficient conditions

Iron (Fe) deficiency is a major abiotic stress in crop production. Although responses to Fe deficiency in graminaceous plants, such as increased production and secretion of mugineic acid family phytosiderophores (MAs), have been described, the gene regulation mechanisms related to these responses are largely unknown. To elucidate the regulation mechanisms of the genes related to Fe acquisition in graminaceous plants, we characterized the Fe-deficiency-inducible basic helix-loop-helix transcription factor OsIRO2 in rice. In yeast cells, OsIRO2 functioned as a transcriptional activator. In rice, overexpression of OsIRO2 resulted in increased MAs secretion, whereas repression of OsIRO2 resulted in lower MAs secretion and hypersensitivity to Fe deficiency. Northern blots revealed that the expression of the genes involved in the Fe(III)-MAs transport system was dependent on OsIRO2. The expression of the genes for nicotianamine synthase, a key enzyme in MAs synthesis, was notably affected by the level of OsIRO2 expression. Microarray analysis demonstrated that OsIRO2 regulates 59 Fe-deficiency-induced genes in roots. Some of these genes, including two transcription factors upregulated by Fe deficiency, possessed the OsIRO2 binding sequence in their upstream regions. OsIRO2 possesses a homologous sequence of the Fe-deficiency-responsive cis-acting elements (IDEs) in its upstream region. We propose a novel gene regulation network for Fe-deficiency responses, including OsIRO2, IDEs and the two transcription factors.

Insulin protects islets from apoptosis via Pdx1 and specific changes in the human islet proteome

Insulin is both a hormone regulating energy metabolism and a growth factor. We and others have shown that physiological doses of insulin initiate complex signals in primary human and mouse beta-cells, but the functional significance of insulin's effects on this cell type remains unclear. In the present study, the role of insulin in beta-cell apoptosis was examined. Exogenous insulin completely prevented apoptosis induced by serum withdrawal when given at picomolar or low nanomolar concentrations but not at higher concentrations, indicating that physiological concentrations of insulin are antiapoptotic and that insulin signaling is self-limiting in islets. Insulin treatment was associated with the nuclear localization of Pdx1 and the prosurvival effects of insulin were largely absent in islets 50% deficient in Pdx1, providing direct evidence that Pdx1 is a signaling target of insulin. Physiological levels of insulin did not increase Akt phosphorylation, and the protective effects of insulin were only partially altered in islets lacking 80% of normal Akt activity, suggesting the presence of additional insulin-regulated antiapoptotic pathways. Proteomic analysis of insulin-treated human islets revealed significant changes in multiple proteins. Bridge-1, a Pdx1-binding partner and regulator of beta-cell survival, was increased significantly at low insulin doses. Together, these data suggest that insulin can act as a master regulator of islet survival by regulating Pdx1.

Inhibition of prostate cancer cell growth by human secreted PDZ domain-containing protein 2, a potential autocrine prostate tumor suppressor

A possible role of the PDZ domain-containing protein 2 (PDZD2) in prostate tumorigenesis has been suggested. Besides, PDZD2 is posttranslationally cleaved by a caspase-dependent mechanism to form a secreted PDZ domain-containing protein 2 (sPDZD2) with unknown functions in humans. In this study, we demonstrate the endogenous expression of PDZD2 and secretion of sPDZD2 in cancerous DU145, PC-3, 22Rv1, LNCaP, and immortalized RWPE-1 prostate epithelial cells. Inhibition of endogenous sPDZD2 production and secretion by DU145, PC-3, 22Rv1, and RWPE-1 cells via the caspase-3 inhibitor Z-DEVD-FMK resulted in increased cell proliferation, which was abrogated by treatment with exogenous recombinant sPDZD2. Whereas sPDZD2-induced antiproliferation in DU145, PC-3, and 22Rv1 cells, it induced apoptosis in LNCaP cells. The data suggest that endogenous sPDZD2, produced by caspase-3-mediated cleavage from PDZD2, may function as a novel autocrine growth suppressor for human prostate cancer cells. The antiproliferative effect of sPDZD2 was apparently mediated through slowing the entry of DU145, PC-3, and 22Rv1 cells into the S phase of the cell cycle. In DU145 cells, this can be attributed to stimulated p53 and p21(CIP1/WAF1) expression by sPDZD2. On the other hand, the apoptotic effect of sPDZD2 on LNCaP cells was apparently mediated via p53-independent Bad stimulation. Together our results indicate the presence of p53-dependent and p53-independent PDZD2/sPDZD2 autocrine growth suppressive signaling pathways in human prostate cancer cells and suggest a novel therapeutic approach of harnessing the latent tumor-suppressive potential of an endogenous autocrine signaling protein like sPDZD2 to inhibit prostate cancer growth.

Regulation of insulin gene transcription by the immediate-early growth response gene Egr-1

Changes in extracellular glucose levels regulate the expression of the immediate-early response gene and zinc finger transcription factor early growth response-1 (Egr-1) in insulin-producing pancreatic beta-cells, but key target genes of Egr-1 in the endocrine pancreas have not been identified. We found that overexpression of Egr-1 in clonal (INS-1) beta-cells increased transcriptional activation of the rat insulin I promoter. In contrast, reductions in Egr-1 expression levels or function with the introduction of either small interfering RNA targeted to Egr-1 (siEgr-1) or a dominant-negative form of Egr-1 decreased insulin promoter activation, and siEgr-1 suppressed insulin gene expression. Egr-1 did not directly interact with insulin promoter sequences, and mutagenesis of a potential G box recognition sequence for Egr-1 did not impair the Egr-1 responsiveness of the insulin promoter, suggesting that regulation of insulin gene expression by Egr-1 is probably mediated through additional transcription factors. Overexpression of Egr-1 increased, and reduction of Egr-1 expression decreased, transcriptional activation of the glucose-responsive FarFlat minienhancer within the rat insulin I promoter despite the absence of demonstrable Egr-1-binding activity to FarFlat sequences. Notably, augmenting Egr-1 expression levels in insulin-producing cells increased the mRNA and protein expression levels of pancreas duodenum homeobox-1 (PDX-1), a major transcriptional regulator of glucose-responsive activation of the insulin gene. Increasing Egr-1 expression levels enhanced PDX-1 binding to insulin promoter sequences, whereas mutagenesis of PDX-1-binding sites reduced the capacity of Egr-1 to activate the insulin promoter. We propose that changes in Egr-1 expression levels in response to extracellular signals, including glucose, can regulate PDX-1 expression and insulin production in pancreatic beta-cells.

Secreted PDZD2 exerts concentration-dependent effects on the proliferation of INS-1E cells

PDZD2 (PDZ domain containing 2) is a multi-PDZ protein expressed in pancreas and many other tissues. PDZD2 shows extensive homology to pro-interleukin-16 (pro-IL-16) and is localized mainly to the endoplasmic reticulum. We have recently demonstrated that PDZD2, like pro-IL-16, is proteolytically cleaved at its C-terminus to generate a secreted protein, sPDZD2 (for secreted PDZD2). To understand the possible functional role of PDZD2 in pancreas, we investigated the cellular distribution of PDZD2 in adult pancreas using an antiserum that recognizes both the full-length and secreted forms of PDZD2. Immunohistochemical analysis revealed a strong expression of PDZD2 in pancreatic islet beta cells but not alpha cells. Consistent with the beta-cell-enriched expression of PDZD2, immunoblot analysis indicated expression of both full-length PDZD2 and sPDZD2 in the insulinoma cell line INS-1E. A recombinant sPDZD2 protein was synthesized for study of its functional effect on INS-1E cells. In culture media with limiting serum, co-incubation with sPDZD2 stimulated the proliferation of INS-1E cells. The mitogenic effect of sPDZD2 was concentration-dependent, and was associated with a slight inhibition of the insulin promoter activity at high sPDZD2 concentrations. As a potential mitogen of beta-like cells, sPDZD2 may be useful for the optimization of beta-cell growth and differentiation in vitro.

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.

Overexpression of the Coactivator Bridge-1 Results in Insulin Deficiency and Diabetes

Multiple forms of heritable diabetes are associated with mutations in transcription factors that regulate insulin gene transcription and the development and maintenance of pancreatic β-cell mass. The coactivator Bridge-1 (PSMD9) regulates the transcriptional activation of glucose-responsive enhancers in the insulin gene in a dose-dependent manner via PDZ domain-mediated interactions with E2A transcription factors. Here we report that the pancreatic overexpression of Bridge-1 in transgenic mice reduces insulin gene expression and results in insulin deficiency and severe diabetes. Dysregulation of Bridge-1 signaling increases pancreatic apoptosis with a reduction in the number of insulin-expressing pancreatic β-cells and an expansion of the complement of glucagon-expressing pancreatic α-cells in pancreatic islets. Increased expression of Bridge-1 alters pancreatic islet, acinar, and ductal architecture and disrupts the boundaries between endocrine and exocrine cellular compartments in young adult but not neonatal mice, suggesting that signals transduced through this coactivator may influence postnatal pancreatic islet morphogenesis. Signals mediated through the coactivator Bridge-1 may regulate both glucose homeostasis and pancreatic β-cell survival. We propose that coactivator dysfunction in pancreatic β-cells can limit insulin production and contribute to the pathogenesis of diabetes.

The coactivator Bridge-1 increases transcriptional activation by pancreas duodenum homeobox-1 (PDX-1)

Well-orchestrated transcriptional regulation of pancreatic beta cells is essential for insulin production and glucose homeostasis. Pancreas duodenum homeobox-1 (PDX-1) is a key regulator of glucose-dependent insulin production and glucose metabolism. We find that PDX-1 interacts with the PDZ-domain coactivator Bridge-1 in yeast interaction trap assays. Rat Bridge-1 and PDX-1 interact directly in GST pull-down assays via Bridge-1 interactions with the amino-terminal transactivation domain of PDX-1. Bridge-1 also interacts with wild-type and mutant human PDX-1 (IPF-1) proteins and strongly interacts with the amino-terminal PDX-1 P63fsdelC (MODY4) mutant protein. Transcriptional activation by PDX-1 is increased by addition of Bridge-1 in multiple contexts, including synergistic activation of a Gal4 reporter by Gal4-Bridge-1 and Gal4-PDX-1 fusion proteins, activation of the somatostatin promoter TAAT1 enhancer, and synergistic activation of the rat insulin I promoter FarFlat enhancer by PDX-1, E12, and E47. We propose that the coactivator Bridge-1 modulates PDX-1 functions in the regulation of its target genes.

Human papillomavirus type 18 E6 protein binds the cellular PDZ protein TIP-2/GIPC, which is involved in transforming growth factor beta signaling and triggers its degradation by the proteasome

Several viral proteins expressed by DNA or RNA transforming viruses have the particular property of binding via their C-terminal end to various cellular proteins with PDZ domains. This study is focused on the PDZ protein TIP-2/GIPC, which was originally identified in two-hybrid screens performed with two different baits: the human T-cell leukemia virus type 1 Tax oncoprotein and the regulator of G signaling RGS-GAIP. Further studies have shown that TIP-2/GIPC is also able to associate with the cytoplasmic domains of various transmembrane proteins. In this report we show that TIP-2/GIPC interacts with the E6 protein of human papillomavirus type 18 (HPV-18). This event triggers polyubiquitination and proteasome-mediated degradation of the cellular protein. In agreement with this observation, silencing of E6 by RNA interference in HeLa cells causes an increase in the intracellular TIP-2/GIPC level. This PDZ protein has been previously found to be involved in transforming growth factor beta (TGF-beta) signaling by favoring expression of the TGF-beta type III receptor at the cell membrane. In line with this activity of TIP-2/GIPC, we observed that depletion of this protein in HeLa cells hampers induction of the Id3 gene by TGF-beta treatment and also diminishes the antiproliferative effect of this cytokine. Conversely, silencing of E6 increases the expression of Id3 and blocks proliferation of HeLa cells. These results support the notion that HPV-18 E6 renders cells less sensitive to the cytostatic effect of TGF-beta by lowering the intracellular amount of TIP-2/GIPC.

Pancreas duodenum homeobox-1 transcriptional activation requires interactions with p300

The homeodomain transcription factor, pancreas duodenum homeobox (PDX)-1, is essential for pancreas development, insulin production, and glucose homeostasis. Mutations in pdx-1(ipf-1) are associated both with maturity-onset diabetes of the young and type 2 diabetes. PDX-1 interacts with multiple transcription factors and coregulators, including the coactivator p300, to activate the transcription of the insulin gene and other target genes within pancreatic beta-cells. In characterizing the protein-protein interactions of PDX-1 and p300, we identified mutations in PDX-1 that disrupt its function and are associated with increased or decreased interactions with p300. Several mutant PDX-1 proteins that are associated with heritable forms of diabetes in humans, in particular the mutant P63fsdelC, exhibited increased binding to a carboxy-terminal segment of p300 in the setting of decreased DNA-binding activities, suggesting that sequestration of p300 by mutant PDX-1 proteins may be an additional mechanism by which insulin gene expression is reduced in heterozygous carriers of pdx-1(ipf-1) mutations. The introduction of the point mutations S66A/Y68A in the highly conserved amino-terminal PDX-1 transactivation domain reduced the ability of PDX-1 to interact with p300, substantially diminished the transcriptional activation of PDX-1, and reduced the synergistic activation of glucose-responsive insulin promoter enhancer sequences by PDX-1, E12, and E47. We propose that interactions of PDX-1 with p300 are required for the transcriptional activation of PDX-1 target genes. Impairment of interactions between PDX-1 and p300 in pancreatic beta-cells may limit insulin production and lead to the development of diabetes.

Proteolytic cleavage of PDZD2 generates a secreted peptide containing two PDZ domains

PDZD2 (PDZ-domain-containing 2; also known as PAPIN, AIPC and PIN1) is a ubiquitously expressed multi-PDZ-domain protein. We have shown that PDZD2, which shows extensive homology to pro-interleukin-16 (pro-IL-16), is localized mainly to the endoplasmic reticulum (ER). Pro-IL-16 is cleaved in a caspase-3-dependent mechanism to generate the secreted cytokine IL-16. The abundant expression of PDZD2 in the ER, and its sequence similarity to pro-IL-16, suggests that similar post-translational processing of PDZD2 may occur. Indeed, western blotting and mass spectrometry analysis of conditioned medium from cells transfected with epitope-tagged PDZD2 show that there is secretion of a PDZD2 peptide of approximately 37 kDa (sPDZD2, for secreted PDZD2) that contains two PDZ domains. Expression of PDZD2 was detected in several tissues. Furthermore, sPDZD2 secretion is suppressed by the mutation of a sequence that shows similarity to caspase recognition motifs or by treatment with a caspase inhibitor. In summary, PDZD2 is the first reported multi-PDZ protein that is processed by proteolytic cleavage to generate a secreted peptide containing two PDZ domains.

Transcriptional coactivation of bone-specific transcription factor Cbfa1 by TAZ

Core-binding factor 1 (Cbfa1; also called Runx2) is a transcription factor belonging to the Runt family of transcription factors that binds to an osteoblast-specific cis-acting element (OSE2) activating the expression of osteocalcin, an osteoblast-specific gene. Using the yeast two-hybrid system, we identified a transcriptional coactivator, TAZ (transcriptional coactivator with PDZ-binding motif), that binds to Cbfa1. A functional relationship between Cbfa1 and TAZ is demonstrated by the coimmunoprecipitation of TAZ by Cbfa1 and by the fact that TAZ induces a dose-dependent increase in the activity of osteocalcin promoter-luciferase constructs by Cbfa1. A dominant-negative construct of TAZ in which the coactivation domains have been deleted reduces osteocalcin gene expression down to basal levels. NIH 3T3, MC 3T3, and ROS 17/2.8 cells showed the expected nuclear localization of Cbfa1, whereas TAZ was distributed throughout the cytoplasm with some nuclear localization when transfected with either Cbfa1 or TAZ. Upon cotransfection by both Cbfa1 and TAZ, the transfected TAZ shows predominant nuclear localization. The dominant-negative construct of TAZ shows minimal nuclear localization upon cotransfection with Cbfa1. These data indicate that TAZ is a transcription coactivator for Cbfa1 and may be involved in the regulation of osteoblast differentiation.

Identification of functional PDZ domain binding sites in several human proteins

TIP-15 was previously identified as a cellular protein that can bind to the C-terminal end of the HTLV-1 Tax protein via its two PDZ domains. The sequence of the N-terminal part of TIP-15 is identical to that of the synaptic protein PSD-95. Both proteins are likely to be produced from the same gene by alternative splicing. Whereas expression of the PSD-95 mRNA was detected only with brain RNAs, that of TIP-15 was detected with RNAs from thymus, brain, skeletal muscle and Jurkat cells. The TIP-15 protein exhibits an apparent molecular weight of 40 kD and is weakly expressed in T cell lines. A two-hybrid screen performed with TIP-15 as bait revealed the presence of a PDZ binding site (PDZ-BS) in the following proteins: Lysyl tRNA synthetase, 6-phosphogluconolactonase (6-GPL), Stress-activated protein kinase 3 (SAPK3), NET-1, Diacylglycerol kinase zeta, MTMR1, MCM7, and hSec8. The sequence at the C-terminal ends of these proteins matches the X-S/T-X-V-COOH consensus previously defined for PDZ-BSs, with the exception of 6-GPL and SAPK3 which include a leucine as the C-terminal residue. For Lysyl tRNA synthetase, NET1, MTMR1 and hSec8, binding to TIP-15 was confirmed by co-immunoprecipitation experiments performed with the extracts of transfected COS7 cells. These results show the existence of functional PDZ-BSs in these proteins, but future studies will be necessary to establish whether or not TIP-15 represents a physiological partner. The significance of the presence of a PDZ-BS in these various proteins is discussed with respect to their function.