PDX:PBX complexes are required for normal proliferation of pancreatic cells during development

'Building a perfect body': control of vertebrate organogenesis by PBX-dependent regulatory networks

Pbx genes encode transcription factors that belong to the TALE (three-amino-acid loop extension) superclass of homeodomain proteins. We have witnessed a surge in information about the roles of this gene family as leading actors in the transcriptional control of development. PBX proteins represent a clear example of how transcription factors can regulate developmental processes by combinatorial properties, acting within multimeric complexes to implement activation or repression of transcription depending on their interaction partners. Here, we revisit long-emphasized functions of PBX transcription factors as cofactors for HOX proteins, major architects of the body plan. We further discuss new knowledge on roles of PBX proteins in different developmental contexts as upstream regulators of Hox genes-as factors that interact with non-HOX proteins and can work independently of HOX-as well as potential pioneer factors. Committed to building a perfect body, PBX proteins govern regulatory networks that direct essential morphogenetic processes and organogenesis in vertebrate development. Perturbations of PBX-dependent networks can cause human congenital disease and cancer.

Maternal high-fat diet impairs glucose metabolism, β-cell function and proliferation in the second generation of offspring rats

Background

This study aimed to assess the impact of perinatal high-fat (HF) diet in female Sprague-Dawley rats (F0) on glucose metabolism and islet function in their early life of second-generation of offspring (F2).

Methods

F0 rats were fed with a standard chow (SC) or HF diet for 8 weeks before mating, up to termination of lactation for their first-generation of offspring (F1-SC and F1-HF). F1 females were mated with normal males at the age of week 11, and producing F2 offspring (F2-SC, F2-HF). All the offspring were fed SC diet after weaning for 3 weeks. The glucose level and islet function of F2 offspring were assessed at the age of week 3 and 12.

Results

The F2-HF offspring had a high birth weight and maintained a higher body mass at the age of week 3 and 12, along with an impaired glucose tolerance and lower serum insulin levels compared with the F2-SC. β-cell proliferation was also impaired in the islets of F2-HF rats at the age of week 3 and 12. The pancreatic and duodenal homeobox factor-1 (Pdx1) and Neurogenic differentiation 1 (NeuroD1) expressions were decreased in the islet of F2-HF rats at the age of week 12.

Conclusions

Maternal HF diet during pre-gestation, gestation, and lactation in rats could result in the increased body weight and glucose intolerance in their early life of F2 offspring due to impaired β-cell function and proliferation.

Dissection of transcriptional and cis-regulatory control of differentiation in human pancreatic cancer

The histological grade of carcinomas describes the ability of tumor cells to organize in differentiated epithelial structures and has prognostic and therapeutic impact. Here, we show that differential usage of the genomic repertoire of transcriptional enhancers leads to grade-specific gene expression programs in human pancreatic ductal adenocarcinoma (PDAC). By integrating gene expression profiling, epigenomic footprinting, and loss-of-function experiments in PDAC cell lines of different grade, we identified the repertoires of enhancers specific to high- and low-grade PDACs and the cognate set of transcription factors acting to maintain their activity. Among the candidate regulators of PDAC differentiation, KLF5 was selectively expressed in pre-neoplastic lesions and low-grade primary PDACs and cell lines, where it maintained the acetylation of grade-specific enhancers, the expression of epithelial genes such as keratins and mucins, and the ability to organize glandular epithelia in xenografts. The identification of the transcription factors controlling differentiation in PDACs will help clarify the molecular bases of its heterogeneity and progression.

PDX1 binds and represses hepatic genes to ensure robust pancreatic commitment in differentiating human embryonic stem cells

Inactivation of the Pancreatic and Duodenal Homeobox 1 (PDX1) gene causes pancreatic agenesis, which places PDX1 high atop the regulatory network controlling development of this indispensable organ. However, little is known about the identity of PDX1 transcriptional targets. We simulated pancreatic development by differentiating human embryonic stem cells (hESCs) into early pancreatic progenitors and subjected this cell population to PDX1 chromatin immunoprecipitation sequencing (ChIP-seq). We identified more than 350 genes bound by PDX1, whose expression was upregulated on day 17 of differentiation. This group included known PDX1 targets and many genes not previously linked to pancreatic development. ChIP-seq also revealed PDX1 occupancy at hepatic genes. We hypothesized that simultaneous PDX1-driven activation of pancreatic and repression of hepatic programs underlie early divergence between pancreas and liver. In HepG2 cells and differentiating hESCs, we found that PDX1 binds and suppresses expression of endogenous liver genes. These findings rebrand PDX1 as a context-dependent transcriptional repressor and activator within the same cell type.

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Early pancreatic progenitor (ePP) cells are efficiently derived from hESCs

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High levels of the homeobox transcription factor PDX1 label ePP cells

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PDX1 binds a battery of foregut/midgut and early pancreatic genes in ePP cells

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PDX1 binds and represses hepatic genes

Genome-wide associations between genetic and epigenetic variation influence mRNA expression and insulin secretion in human pancreatic islets

Genetic and epigenetic mechanisms may interact and together affect biological processes and disease development. However, most previous studies have investigated genetic and epigenetic mechanisms independently, and studies examining their interactions throughout the human genome are lacking. To identify genetic loci that interact with the epigenome, we performed the first genome-wide DNA methylation quantitative trait locus (mQTL) analysis in human pancreatic islets. We related 574,553 single nucleotide polymorphisms (SNPs) with genome-wide DNA methylation data of 468,787 CpG sites targeting 99% of RefSeq genes in islets from 89 donors. We identified 67,438 SNP-CpG pairs in cis, corresponding to 36,783 SNPs (6.4% of tested SNPs) and 11,735 CpG sites (2.5% of tested CpGs), and 2,562 significant SNP-CpG pairs in trans, corresponding to 1,465 SNPs (0.3% of tested SNPs) and 383 CpG sites (0.08% of tested CpGs), showing significant associations after correction for multiple testing. These include reported diabetes loci, e.g. ADCY5, KCNJ11, HLA-DQA1, INS, PDX1 and GRB10. CpGs of significant cis-mQTLs were overrepresented in the gene body and outside of CpG islands. Follow-up analyses further identified mQTLs associated with gene expression and insulin secretion in human islets. Causal inference test (CIT) identified SNP-CpG pairs where DNA methylation in human islets is the potential mediator of the genetic association with gene expression or insulin secretion. Functional analyses further demonstrated that identified candidate genes (GPX7, GSTT1 and SNX19) directly affect key biological processes such as proliferation and apoptosis in pancreatic β-cells. Finally, we found direct correlations between DNA methylation of 22,773 (4.9%) CpGs with mRNA expression of 4,876 genes, where 90% of the correlations were negative when CpGs were located in the region surrounding transcription start site. Our study demonstrates for the first time how genome-wide genetic and epigenetic variation interacts to influence gene expression, islet function and potential diabetes risk in humans.

Inter-individual variation in genetics and epigenetics affects biological processes and disease susceptibility. However, most studies have investigated genetic and epigenetic mechanisms independently and to uncover novel mechanisms affecting disease susceptibility there is a highlighted need to study interactions between these factors on a genome-wide scale. To identify novel loci affecting islet function and potentially diabetes, we performed the first genome-wide methylation quantitative trait locus (mQTL) analysis in human pancreatic islets including DNA methylation of 468,787 CpG sites located throughout the genome. Our results showed that DNA methylation of 11,735 CpGs in 4,504 unique genes is regulated by genetic factors located in cis (67,438 SNP-CpG pairs). Furthermore, significant mQTLs cover previously reported diabetes loci including KCNJ11, INS, HLA, PDX1 and GRB10. We also found mQTLs associated with gene expression and insulin secretion in human islets. By performing causality inference tests (CIT), we identified CpGs where DNA methylation potentially mediates the genetic impact on gene expression and insulin secretion. Our functional follow-up experiments further demonstrated that identified mQTLs/genes (GPX7, GSTT1 and SNX19) directly affect pancreatic β-cell function. Together, our study provides a detailed map of genome-wide associations between genetic and epigenetic variation, which affect gene expression and insulin secretion in human pancreatic islets.

A tumorigenic factor interactome connected through tumor suppressor microRNA-198 in human pancreatic cancer

PURPOSE

The majority of pancreatic cancers overexpress mesothelin (MSLN), which contributes to enhanced proliferation, invasion, and migration. However, the MSLN regulatory network is still unclear. Here, we investigated the regulation of a panel of tumorigenic factors and explored the potential of MSLN-regulated miR-198 treatment in vivo.

EXPERIMENTAL DESIGN

The expression and functional regulation of the tumorigenic factors MSLN, NF-κB, and the homeobox transcription factors (TF) POU2F2 (OCT-2), Pre-B-cell leukemia homeobox factor 1 (PBX-1), valosin-containing protein (VCP), and miR-198 were studied in pancreatic cancer cell lines, patient tumor samples, and xenograft pancreatic cancer mouse models.

RESULTS

We found that miR-198 is downregulated in pancreatic cancer and is involved in an intricate reciprocal regulatory loop with MSLN, which represses miR-198 through NF-κB-mediated OCT-2 induction. Furthermore, miR-198 repression leads to overexpression of PBX-1 and VCP. The dysregulated PBX-1/VCP axis leads to increased tumorigenicity. Reconstitution of miR-198 in pancreatic cancer cells results in reduced tumor growth, metastasis, and increased survival through direct targeting MSLN, PBX-1, and VCP. Most interestingly, reduced levels of miR-198 in human tissue samples are associated with upregulation of these tumorigenic factors (MSLN, OCT-2, PBX-1, VCP) and predict poor survival. Reduced miR-198 expression links this tumor network signature and prognosticates poor patient outcome. High miR-198 disrupts the network and predicts better prognosis and increased survival.

CONCLUSIONS

miR-198 acts as a central tumor suppressor and modulates the molecular makeup of a critical interactome in pancreatic cancer, indicating a potential prognostic marker signature and the therapeutic potential of attacking this tumorigenic network through a central vantage point.

Increased DNA methylation and decreased expression of PDX-1 in pancreatic islets from patients with type 2 diabetes

Mutations in pancreatic duodenal homeobox 1 (PDX-1) can cause a monogenic form of diabetes (maturity onset diabetes of the young 4) in humans, and silencing Pdx-1 in pancreatic β-cells of mice causes diabetes. However, it is not established whether epigenetic alterations of PDX-1 influence type 2 diabetes (T2D) in humans. Here we analyzed mRNA expression and DNA methylation of PDX-1 in human pancreatic islets from 55 nondiabetic donors and nine patients with T2D. We further studied epigenetic regulation of PDX-1 in clonal β-cells. PDX-1 expression was decreased in pancreatic islets from patients with T2D compared with nondiabetic donors (P = 0.0002) and correlated positively with insulin expression (rho = 0.59, P = 0.000001) and glucose-stimulated insulin secretion (rho = 0.41, P = 0.005) in the human islets. Ten CpG sites in the distal PDX-1 promoter and enhancer regions exhibited significantly increased DNA methylation in islets from patients with T2D compared with nondiabetic donors. DNA methylation of PDX-1 correlated negatively with its gene expression in the human islets (rho = -0.64, P = 0.0000029). Moreover, methylation of the human PDX-1 promoter and enhancer regions suppressed reporter gene expression in clonal β-cells (P = 0.04). Our data further indicate that hyperglycemia decreases gene expression and increases DNA methylation of PDX-1 because glycosylated hemoglobin (HbA1c) correlates negatively with mRNA expression (rho = -0.50, P = 0.0004) and positively with DNA methylation (rho = 0.54, P = 0.00024) of PDX-1 in the human islets. Furthermore, while Pdx-1 expression decreased, Pdx-1 methylation and Dnmt1 expression increased in clonal β-cells exposed to high glucose. Overall, epigenetic modifications of PDX-1 may play a role in the development of T2D, given that pancreatic islets from patients with T2D and β-cells exposed to hyperglycemia exhibited increased DNA methylation and decreased expression of PDX-1. The expression levels of PDX-1 were further associated with insulin secretion in the human islets.

Research resource: the pdx1 cistrome of pancreatic islets

The homeodomain transcription factor pancreas duodenal homeobox 1 (Pdx1, also known as insulin promoter factor 1) is a master regulator of pancreas development, as mice or humans lacking Pdx1 function are a pancreatic. Importantly, heterozygous mutations in Pdx1 cause early and late onset forms of diabetes in humans. Despite these central roles in development and adult β-cell function, we have only rudimentary knowledge of the transcriptome targets of Pdx1 that mediate these phenotypes. Therefore, we performed global location analysis of Pdx1 occupancy in pancreatic islets. We used evolutionary conservation of target genes to identify the most relevant Pdx1 targets by performing chromatin immunoprecipitation sequencing on both human and mouse islets. Remarkably, the conserved target set is highly enriched for genes annotated to function in endocrine system and metabolic disorders, various signaling pathways, and cell survival, providing a molecular explanation for many of the phenotypes resulting from Pdx1 deficiency.

Analysis of PBX1 mutations in 192 Chinese women with Müllerian duct abnormalities

We examined the PBX1 gene in 192 Chinese women with Müllerian duct abnormalities and revealed 2 known single nucleotide polymorphisms: c.61 > A in exon 1 and c.998-1330A>G in intron 7. Future studies in large cohorts of different ethnic populations are warranted to establish definite associations between the PBX1 gene and Müllerian duct abnormalities.

Three-amino-acid-loop-extension homeodomain factor Meis3 regulates cell survival via PDK1

Three-amino-acid-loop-extension (TALE) homeodomain proteins including Meis and Pbx families are generally recognized for their roles in growth and differentiation during vertebrate embryogenesis and tumorigenesis. Whereas genetic studies indicate that Pbx1 regulates the development and function of insulin-producing pancreatic β-cells, the role of Meis family members in β-cells is still unknown. Here we show that Meis3 is abundantly expressed in pancreatic islets and β-cells and that it regulates β-cell survival. We further identify the 3-phosphoinositide-dependent protein kinase 1 (PDK1), a well-known kinase involved in the PI3K-Akt signaling pathway, as a direct Meis3 target, which mediates its role in β-cell survival. This regulatory module appears to function broadly as we also identify Meis3 regulation of cell survival and PDK1 expression in ovarian carcinoma cells, suggesting a unique function for Meis3 beyond the traditional roles for TALE homeodomain factors during embryogenesis.

Experimental evidence supporting the lack of primary stem cells in adult pancreatic tissue

PURPOSE

To investigate the origin and localization of pancreatic stem cells in adult pancreatic tissues and to determine the primary mechanism underlying the participation of these cells in repairing pancreatic injuries.

METHODS

Sprague-Dawley rats were divided into experimental and control groups. The experimental group was given intraperitoneal injections of cerulein to induce acute pancreatitis. At 6 h, 1, 2, 3, 5 and 7 days, 5 rats from the experimental group and 2 rats from the control group were sacrificed; all sacrificed animals were intraperitoneally injected with 5-bromo-2'-deoxyuracil nucleotides (BrdU) 6 and 3 h prior to sacrifice. The pathological changes of pancreatic tissue were observed. The stem cell marker nestin and the cell proliferation marker BrdU were detected with immunohistochemistry. Pancreatic duodenal homeobox-1 (PDX-1) was determined by real-time PCR.

RESULTS

(1) The pathological changes of acute pancreatitis can be divided into three phases: the edema and apoptosis phase, the hemorrhagic necrosis phase, and the reconstruction phase. (2) Nestin-positive cells mainly appeared in the interlobular vascular lumen after cerulein injection, and they peaked at day 3 when the positive cells spread all over the pancreatic tissues. (3) BrdU-positive cells began to appear in the area surrounding the interlobular region, and the number of positive cells peaked on day 7. (4) The expression of PDX-1 mRNA initially increased, then decreased and gradually got close to a normal level.

CONCLUSION

Primary pancreatic stem cells may not exist in the adult pancreatic tissues. The so-called pancreatic stem cells may actually originate from bone marrow stem cells. When pancreatic tissue is injured, bone marrow stem cells may participate in the repair.

The pancreatic and duodenal homeobox protein PDX-1 regulates the ductal specific keratin 19 through the degradation of MEIS1 and DNA binding

Background

Pancreas organogenesis is the result of well-orchestrated and balanced activities of transcription factors. The homeobox transcription factor PDX-1 plays a crucial role in the development and function of the pancreas, both in the maintenance of progenitor cells and in determination and maintenance of differentiated endocrine cells. However, the activity of homeobox transcription factors requires coordination with co-factors, such as PBX and MEIS proteins. PBX and MEIS proteins belong to the family of three amino acid loop extension (TALE) homeodomain proteins. In a previous study we found that PDX-1 negatively regulates the transcriptional activity of the ductal specific keratin 19 (Krt19). In this study, we investigate the role of different domains of PDX-1 and elucidate the functional interplay of PDX-1 and MEIS1 necessary for Krt19 regulation.

Methodology/Principal Findings

Here, we demonstrate that PDX-1 exerts a dual manner of regulation of Krt19 transcriptional activity. Deletion studies highlight that the NH₂-terminus of PDX-1 is functionally relevant for the down-regulation of Krt19, as it is required for DNA binding of PDX-1 to the Krt19 promoter. Moreover, this effect occurs independently of PBX. Second, we provide insight on how PDX-1 regulates the Hox co-factor MEIS1 post-transcriptionally. We find specific binding of MEIS1 and MEIS2 to the Krt19 promoter using IP-EMSA, and siRNA mediated silencing of Meis1, but not Meis2, reduces transcriptional activation of Krt19 in primary pancreatic ductal cells. Over-expression of PDX-1 leads to a decreased level of MEIS1 protein, and this decrease is prevented by inhibition of the proteasome.

Conclusions/Significance

Taken together, our data provide evidence for a dual mechanism of how PDX-1 negatively regulates Krt19 ductal specific gene expression. These findings imply that transcription factors may efficiently regulate target gene expression through diverse, non-redundant mechanisms.

Pbx acts with Hand2 in early myocardial differentiation

Transcription factors of the basic helix-loop-helix (bHLH) family are critical regulators of muscle cell differentiation. For example, Myod drives skeletal muscle differentiation, and Hand2 potentiates cardiac muscle differentiation. Understanding how these bHLH factors regulate distinct transcriptional targets in a temporally and spatially controlled manner is critical for understanding their activity in cellular differentiation. We previously showed that Pbx homeodomain proteins modulate the activity of Myod to promote the differentiation of fast-twitch skeletal muscle. Here, we test the hypothesis that Pbx proteins are also necessary for cardiac muscle differentiation through interacting with Hand2. We show that Pbx proteins are required for the activation of cardiac muscle differentiation in zebrafish embryos. Loss of Pbx activity leads to delay of myocardial differentiation and subsequent defective cardiac morphogenesis, similar to reduced Hand2 activity. Genetic interaction experiments support the hypothesis that Pbx proteins modulate the activity of Hand2 in myocardial differentiation. Furthermore, we show that Pbx proteins directly bind the promoter of the myocardial differentiation gene myl7 in vitro, supporting a direct role for Pbx proteins in promoting cardiac muscle differentiation. Our findings demonstrate new roles for Pbx proteins in vertebrate cardiac development and also provide new insight into connections between the transcriptional regulation of skeletal and cardiac muscle differentiation programs.

Development of insulin-producing cells from primitive biologic precursors

PURPOSE OF REVIEW

The differentiation of pluripotent and multipotent stem cells into insulin-producing cells has the potential to create a renewable supply of replacement beta cells with tremendous utility in the treatment of diabetes. The purpose of this review is to summarize recent advancements in the field, with emphasis on the limitations of this technology as it relates to the beta cell.

RECENT FINDINGS

Multiple groups have developed successful in-vitro protocols to differentiate human embryonic stem cells and selected tissue specific stem cells into progenitors capable of insulin production and glucose-stimulated insulin secretion. The resulting cells are immature beta cell-like cells that coexpress multiple islet hormones and lack the full complement of genes necessary for normal function. Protocols that include in-vivo maturation in immune-compromised mice produce cells with a more mature phenotype.

SUMMARY

Although tremendous progress has been made in differentiating stem cells into insulin-producing cells, there is still more research needed to produce a fully functional adult beta cell.

The association of PBX1 polymorphisms with overweight/obesity and metabolic alterations in the Korean population

Pre-B-cell leukemia transcription factor 1 (PBX1), which is located on chromosome 1q23, was recently reported to be associated with type 2 diabetes mellitus. We examined whether single nucleotide polymorphisms (SNPs) of the PBX1 gene are associated with overweight/obesity in a Korean population. We genotyped 66 SNPs in the PBX1 gene and investigated their association with clinical phenotypes found in 214 overweight/obese subjects and 160 control subjects using the Affymetrix Targeted Genotyping chip array. Seven SNPs (g.+75186C>T, g.+78350C>A, g.+80646C>T, g.+138004C>T, g.+185219G>A, g.+191272A>C, and g.+265317T>A) were associated with the risk of obesity in three models (codominant, dominant, and recessive) (P=0.007-0.05). Haplotype 1 (CAC) and 3 (TAC) of block 3 and haplotype 2 (GGAAT) of block 10 were also strongly associated with the risk of obesity. In the control group, subjects that had homozygote for the major allele for both g.+185219G>A and g.+191272A>C showed lower high density lipoprotein-cholesterol (HDL-C) level compared to those possessing the minor allele, suggesting that the association between the homozygote for the major allele for both g.+185219G>A and g.+191272A>C and HDL-C is attributable to the increased risk of obesity. This study suggests that the PBX1 gene is a possible risk factor in overweight/obese patients.

Developmental biology of the pancreas: a comprehensive review

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.

Overexpression of the Pdx-1 homeodomain transcription factor impairs glucose metabolism in cultured rat hepatocytes

The Pdx-1 transcription factor plays crucial functions both during pancreas development and in the adult β cells. Previous studies have indicated that ectopic Pdx-1 expression in liver or intestinal primary and immortalized cells is sufficient to promote activation of insulin gene expression. This work is focused on the molecular and physiological consequences of Pdx-1 overexpression in liver cells. We present evidence that Pdx-1 affects the level of expression of one of the four mammalian hexokinase isozymes. These are glucose phosphorylating enzymes involved in essential cellular functions such as glucose sensing, metabolic energy production and apoptosis. Specifically, our data show that over-expression of Pdx-1 in cultured hepatocytes is able to repress the expression of hexokinase 2 (Hxk 2) and the phenomenon is mediated via binding of Pdx-1 to a specific sequence on the Hxk 2 gene promoter. As a consequence, liver cells over-expressing Pdx-1 present interesting alterations concerning glucose metabolism.

Transcription factor PDX-1 in human colorectal adenocarcinoma: A potential tumor marker?

AIM: To examine the expression of pancreatic duodenal homeobox-1 (PDX-1) transcription factor in human colorectal cancer.

METHODS: RT-PCR, Western blotting, and immuno-histochemistry were performed to determine the expression pattern of transcription factor PDX-1 in primary colorectal tumor, hepatic metastasis, and benign colon tissue from a single patient.

RESULTS: The highest PDX-1 transcription levels were detected in the metastasis material. Lower levels of PDX-1 were found to be present in the primary tumor, while normal colon tissue failed to express detectable levels of PDX-1. Western blot data revealed a PDX-1 expression pattern identical to that of mRNA expression. Immunohistochemistry confirmed high metastasis PDX-1 expression, lower levels in the primary tumor, and the presence of only traces of PDX-1 in normal colon tissue.

CONCLUSION: These data argue for further evaluation of PDX-1 as a biomarker for colorectal cancer.

Prep1 deficiency induces protection from diabetes and increased insulin sensitivity through a p160-mediated mechanism

We have examined glucose homeostasis in mice hypomorphic for the homeotic transcription factor gene Prep1. Prep1-hypomorphic (Prep1(i/i)) mice exhibit an absolute reduction in circulating insulin levels but normal glucose tolerance. In addition, these mice exhibit protection from streptozotocin-induced diabetes and enhanced insulin sensitivity with improved glucose uptake and insulin-dependent glucose disposal by skeletal muscle. This muscle phenotype does not depend on reduced expression of the known Prep1 transcription partner, Pbx1. Instead, in Prep1(i/i) muscle, we find normal Pbx1 but reduced levels of the recently identified novel Prep1 interactor p160. Consistent with this reduction, we find a muscle-selective increase in mRNA and protein levels of PGC-1alpha, accompanied by enhanced expression of the GLUT4 transporter, responsible for insulin-stimulated glucose uptake in muscle. Indeed, using L6 skeletal muscle cells, we induced the opposite effects by overexpressing Prep1 or p160, but not Pbx1. In vivo skeletal muscle delivery of p160 cDNA in Prep1(i/i) mice also reverses the molecular phenotype. Finally, we show that Prep1 controls the stability of the p160 protein. We conclude that Prep1 controls insulin sensitivity through the p160-GLUT4 pathway.

Expression of zebrafish pax6b in pancreas is regulated by two enhancers containing highly conserved cis-elements bound by PDX1, PBX and PREP factors

Background

PAX6 is a transcription factor playing a crucial role in the development of the eye and in the differentiation of the pancreatic endocrine cells as well as of enteroendocrine cells. Studies on the mouse Pax6 gene have shown that sequences upstream from the P0 promoter are required for expression in the lens and the pancreas; but there remain discrepancies regarding the precise location of the pancreatic regulatory elements.

Results

Due to genome duplication in the evolution of ray-finned fishes, zebrafish has two pax6 genes, pax6a and pax6b. While both zebrafish pax6 genes are expressed in the developing eye and nervous system, only pax6b is expressed in the endocrine cells of the pancreas. To investigate the cause of this differential expression, we used a combination of in silico, in vivo and in vitro approaches. We show that the pax6b P0 promoter targets expression to endocrine pancreatic cells and also to enteroendocrine cells, retinal neurons and the telencephalon of transgenic zebrafish. Deletion analyses indicate that strong pancreatic expression of the pax6b gene relies on the combined action of two conserved regulatory enhancers, called regions A and C. By means of gel shift assays, we detected binding of the homeoproteins PDX1, PBX and PREP to several cis-elements of these regions. In constrast, regions A and C of the zebrafish pax6a gene are not active in the pancreas, this difference being attributable to sequence divergences within two cis-elements binding the pancreatic homeoprotein PDX1.

Conclusion

Our data indicate a conserved role of enhancers A and C in the pancreatic expression of pax6b and emphasize the importance of the homeoproteins PBX and PREP cooperating with PDX1, in activating pax6b expression in endocrine pancreatic cells. This study also provides a striking example of how adaptative evolution of gene regulatory sequences upon gene duplication progressively leads to subfunctionalization of the paralogous gene pair.

Evaluating the association of common PBX1 variants with type 2 diabetes

BACKGROUND

PBX1 is a biological candidate gene for type 2 diabetes at the 1q21-q24 susceptibility locus. The aim of this study was to evaluate the association of common PBX1 variants with type 2 diabetes in French Caucasian subjects.

METHODS

Employing a case-control design, we genotyped 39 SNPs spanning the PBX1 locus in 3,093 subjects to test for association with type 2 diabetes.

RESULTS

Several PBX1 SNPs, including the G21S coding SNP rs2275558, were nominally associated with type 2 diabetes but the strongest result was obtained with the intron 2 SNP rs2792248 (P = 0.004, OR 1.20 [95% CI 1.06-1.37]). The SNPSpD multiple testing correction method gave a significance threshold of P = 0.002 for the 39 SNPs genotyped, indicating that the rs2792248 association did not survive multiple testing adjustment. SNP rs2792248 did not show evidence of association with the French 1q linkage signal (P = 0.31; weighted NPL score 2.16). None of the PBX1 SNPs nominally associated with type 2 diabetes were associated with a range of quantitative metabolic traits in the normoglycemic control subjects

CONCLUSION

The available data does not support a major influence of common PBX1 variants on type 2 diabetes susceptibility or quantitative metabolic traits. In order to make progress in identifying the elusive susceptibility variants in the 1q region it will be necessary to carry out further large association studies, meta-analyses of existing data from individual studies, and deep resequencing of the 1q region.

Hypoxia induces upregulation of the deoxyribonuclease I gene in the human pancreatic cancer cell line QGP-1

We have previously demonstrated that ischemia caused by acute myocardial infarction induces an abrupt increase of serum deoxyribonuclease I (DNase I) activity. In this study, we examined whether hypoxia can affect the levels of DNase I activity and/or its transcripts in vitro. We first exposed the human pancreatic cancer cell line QGP-1, which is the first documented DNase-I-producing cell line, to hypoxia (2% O2), and found that this induced a significant increase in both the activity and transcripts of DNase I. This response was mediated by increased transcription only from exon 1a of the two alternative transcription-initiating exons utilized simultaneously in the human DNase I gene (DNASE1); exposure of QGP-1 cells to hypoxia for 24 h resulted in a 15-fold increase of DNASE1 transcripts starting from exon 1a compared with the expression level under normoxic conditions. Promoter, electrophoretic mobility shift, and chromatin immunoprecipitation assays with QGP-1 cells exposed to hypoxia or normoxia showed that the region just upstream from exon 1a was involved in this response in a hypoxia-induced factor-1-independent, but at least in a Sp1 transcription factor-dependent manner possibly through enhanced binding of Sp1 protein to the promoter. These results indicate that DNASE1 expression is upregulated by hypoxia in the cells.

c-Kit in early onset of diabetes: a morphological and functional analysis of pancreatic beta-cells in c-KitW-v mutant mice

c-Kit tyrosine receptor kinase, a well-established stem cell marker, is expressed in a variety of tissues including the pancreas. The involvement of c-Kit in fetal rat and human endocrine pancreatic development, survival, and function has been well characterized but primarily using in vitro experimental approaches. Therefore, the aim of the current study was to examine whether deficiency of a functional c-Kit receptor would have physiological and functional implications in vivo. We characterized the c-Kit mutant mouse, c-Kit(W-v/+), to evaluate the in vivo role of c-Kit in beta-cell growth and function. Here we report that male c-Kit(W-v/+) mice, at 8 wk of age, showed high fasting blood glucose levels and impaired glucose tolerance, which was associated with low levels of insulin secretion after glucose stimulation in vivo and in isolated islets. Morphometric analysis revealed that beta-cell mass was significantly reduced (50%) in male c-Kit(W-v/+) mice when compared with controls (c-Kit(+/+)) (P < 0.05). In parallel, a reduction in pancreatic duodenal homeobox-1 and insulin gene expression in whole pancreas as well as isolated islets of c-Kit(W-v/+) male mice was noted along with a decrease in pancreatic insulin content. Furthermore, the reduction in beta-cell mass in male c-Kit(W-v/+) mice was associated with a decrease in beta-cell proliferation. Interestingly, these changes were not observed in female c-Kit(W-v/+) mice until 40 wk of age. Our results clearly demonstrate that the c-Kit receptor is involved in the regulation of glucose metabolism, likely through an important role in beta-cell development and function.

Pbx homeodomain proteins pattern both the zebrafish retina and tectum

BACKGROUND

Pbx genes encode TALE class homeodomain transcription factors that pattern the developing neural tube, pancreas, and blood. Within the hindbrain, Pbx cooperates with Hox proteins to regulate rhombomere segment identity. Pbx cooperates with Eng to regulate midbrain-hindbrain boundary maintenance, and with MyoD to control fast muscle cell differentiation. Although previous results have demonstrated that Pbx is required for proper eye size, functions in regulating retinal cell identity and patterning have not yet been examined.

RESULTS

Analysis of retinal ganglion cell axon pathfinding and outgrowth in pbx2/4 null embryos demonstrated a key role for pbx genes in regulating neural cell behavior. To identify Pbx-dependent genes involved in regulating retino-tectal pathfinding, we conducted a microarray screen for Pbx-dependent transcripts in zebrafish, and detected genes that are specifically expressed in the eye and tectum. A subset of Pbx-dependent retinal transcripts delineate specific domains in the dorso-temporal lobe of the developing retina. Furthermore, we determined that some Pbx-dependent transcripts also require Meis1 and Gdf6a function. Since gdf6a expression is also dependent on Pbx, we propose a model in which Pbx proteins regulate expression of the growth factor gdf6a, which in turn regulates patterning of the dorso-temporal lobe of the retina. This, in concert with aberrant tectal patterning in pbx2/4 null embryos, may lead to the observed defects in RGC outgrowth.

CONCLUSION

These data define a novel role for Pbx in patterning the vertebrate retina and tectum in a manner required for proper retinal ganglion cell axon outgrowth.

Pbx proteins cooperate with Engrailed to pattern the midbrain-hindbrain and diencephalic-mesencephalic boundaries

Pbx proteins are a family of TALE-class transcription factors that are well characterized as Hox co-factors acting to impart segmental identity to the hindbrain rhombomeres. However, no role for Pbx in establishing more anterior neural compartments has been demonstrated. Studies done in Drosophila show that Engrailed requires Exd (Pbx orthologue) for its biological activity. Here, we present evidence that zebrafish Pbx proteins cooperate with Engrailed to compartmentalize the midbrain by regulating the maintenance of the midbrain-hindbrain boundary (MHB) and the diencephalic-mesencephalic boundary (DMB). Embryos lacking Pbx function correctly initiate midbrain patterning, but fail to maintain eng2a, pax2a, fgf8, gbx2, and wnt1 expression at the MHB. Formation of the DMB is also defective as shown by a caudal expansion of diencephalic epha4a and pax6a expression into midbrain territory. These phenotypes are similar to the phenotype of an Engrailed loss-of-function embryo, supporting the hypothesis that Pbx and Engrailed act together on a common genetic pathway. Consistent with this model, we demonstrate that zebrafish Engrailed and Pbx interact in vitro and that this interaction is required for both the eng2a overexpression phenotype and Engrailed's role in patterning the MHB. Our data support a novel model of midbrain development in which Pbx and Engrailed proteins cooperatively pattern the mesencephalic region of the neural tube.

TALE-family homeodomain proteins regulate endodermal sonic hedgehog expression and pattern the anterior endoderm

sonic hedgehog (shh) is expressed in anterior endoderm, where it is required to repress pancreas gene expression and to pattern the endoderm, but the pathway controlling endodermal shh expression is unclear. We find that expression of meis3, a TALE class homeodomain gene, coincides with shh expression in the endoderm of zebrafish embryos. Using a dominant negative construct or anti-sense morpholino oligos (MOs) to disrupt meis3 function, we observe ectopic insulin expression in anterior endoderm. This phenotype is also observed when meis3 MOs are targeted to the endoderm, suggesting that meis3 acts within the endoderm to restrict insulin expression. We also find that meis3 is required for endodermal shh expression, indicating that meis3 acts upstream of shh to restrict insulin expression. Loss of pbx4, a TALE gene encoding a Meis cofactor, produces the same phenotype as loss of meis3, consistent with Meis3 acting in a complex with Pbx4 as reported in other systems. Lastly, we observe a progressive anterior displacement of endoderm-derived organs upon disruption of meis3 or pbx4, apparently as a result of underdevelopment of the pharyngeal region. Our data indicate that meis3 and pbx4 regulate shh expression in anterior endoderm, thereby influencing patterning and growth of the foregut.

Specific targeting of pancreatic islet cells in vivo by insulin-promoter-driven adenoviral conjugated reporter genes

BACKGROUND

The objective of this study was to determine whether rat insulin promoter (RIP) could, in a mouse model, direct expression of an adenovirus-mediated reporter gene specifically into pancreatic islets via systemic delivery.

METHODS

Five hundred and eight base pairs of the RIP DNA sequence were constructed into an adenoviral vector containing a lacZ reporter gene (Adeno-RIP-lacZ). The cytomegalovirus (CMV) promoter was constructed to drive lacZ reporter-gene expression (Adeno-CMV-lacZ) and used as controls. In vitro transient transfection assays were performed to determine levels of reporter-gene expression and compared with that of liposome-mediated plasmid transfection. SCID mice were bred and housed in the barrier BL-4 animal facility. At 2 months of age, the human pancreatic cancer cell PANC-1 was intraperitoneally injected into male mice. Two months after the tumor cell inoculation, mice were injected with 10(7) adenoviral particles via tail veins. After gene delivery, mice were sacrificed at different time points to determine transgene expression levels. Complete necropsies were performed. Morphological alterations were determined using hematoxylin and eosin (H&E) staining, and distribution of the reporter lacZ gene was determined by immunohistochemistry analyses.

RESULTS

Adenoviral-driven reporter-gene expression resulted in more than 5 times higher transgene expression compared with conventional plasmid transfections. In Adeno-RIP-lacZ-injected mice, lacZ expression was specifically detected in pancreatic islets. By contrast, in Adeno-CMV-lacZ-injected mice, lacZ gene expression was observed in multiple organs and tissues. Mononuclear cell infiltration and liver cell inflammation were found in Adeno-CMV-lacZ-treated mice. Similar phenomena were observed in islet cells of Adeno-RIP-lacZ-treated mice. A significantly higher level of reporter-gene expression was also found at the edge of in-vivo-inoculated human pancreatic tumors.

CONCLUSION

These results demonstrate that RIP-directed reporter-gene expression was found specifically in mouse pancreatic islets and implanted human pancreatic cancer cells. These data thus demonstrate that the combination of an adenoviral vector and a tissue-specific promoter could lead to an enhanced and more specific transgene expression in vivo.

Pax6 is regulated by Meis and Pbx homeoproteins during pancreatic development

Pancreatic development depends on the transcription factor Pax6, which controls islet cell differentiation and hormone production. To understand the regulation of Pax6 pancreatic expression, we have identified a minimal Pax6 pancreatic enhancer and show that it contains a composite binding site for Meis and Pbx homeoproteins. We further show that Meis proteins are expressed during pancreatic development, and together with Pbx, are able to form a synergistic binding complex on the Pax6 pancreatic enhancer. When tested in transgenic mice, both the Meis and Pbx sites are essential for Pax6 pancreatic enhancer activity, and the composite site can be functionally replaced by a consensus Meis-Pbx sequence. In addition, analysis of Pbx1 and Pbx2 knockout mice demonstrates that, during pancreatic islet formation, Pax6 expression becomes dependent upon Pbx1 and Pbx2 function. As Meis homeoproteins have been previously demonstrated to regulate Pax6 expression during lens development, these results suggest a conserved mechanism of Pax6 regulation by Meis homeoproteins in two different organs.

Refined localization of the FAT1 quantitative trait locus on pig chromosome 4 by marker-assisted backcrossing

BACKGROUND

A major QTL for fatness and growth, denoted FAT1, has previously been detected on pig chromosome 4q (SSC4q) using a Large White - wild boar intercross. Progeny that carried the wild boar allele at this locus had higher fat deposition, shorter length of carcass, and reduced growth. The position and the estimated effects of the FAT1 QTL for growth and fatness have been confirmed in a previous study. In order to narrow down the QTL interval we have traced the inheritance of the wild boar allele associated with high fat deposition through six additional backcross generations.

RESULTS

Progeny-testing was used to determine the QTL genotype for 10 backcross sires being heterozygous for different parts of the broad FAT1 region. The statistical analysis revealed that five of the sires were segregating at the QTL, two were negative while the data for three sires were inconclusive. We could confirm the QTL effects on fatness/meat content traits but not for the growth traits implying that growth and fatness are controlled by distinct QTLs on chromosome 4. Two of the segregating sires showed highly significant QTL effects that were as large as previously observed in the F2 generation. The estimates for the remaining three sires, which were all heterozygous for smaller fragments of the actual region, were markedly smaller. With the sample sizes used in the present study we cannot with great confidence determine whether these smaller effects in some sires are due to chance deviations, epistatic interactions or whether FAT1 is composed of two or more QTLs, each one with a smaller phenotypic effect. Under the assumption of a single locus, the critical region for FAT1 has been reduced to a 3.3 cM interval between the RXRG and SDHC loci.

CONCLUSION

We have further characterized the FAT1 QTL on pig chromosome 4 and refined its map position considerably, from a QTL interval of 70 cM to a maximum region of 20 cM and a probable region as small as 3.3 cM. The flanking markers for the small region are RXRG and SDHC and the orthologous region of FAT1 in the human genome is located on HSA1q23.3 and harbors approximately 20 genes. Our strategy to further refine the map position of this major QTL will be i) to type new markers in our pigs that are recombinant in the QTL interval and ii) to perform Identity-By-Descent (IBD) mapping across breeds that have been strongly selected for lean growth.

Reprogramming liver-stem WB cells into functional insulin-producing cells by persistent expression of Pdx1- and Pdx1-VP16 mediated by lentiviral vectors

Adenovirus-mediated transient expression of the pancreatic duodenal homeobox transcription factor Pdx1 in mouse liver activates pancreatic endocrine and exocrine genes, the latter reportedly resulting in severe hepatitis. Expression of a super-active form of Pdx1 or Pdx1-VP16 selectively transdifferentiates hepatic WB cells into functional pancreatic beta-like insulin-producing cells, without evidence of exocrine differentiation. No study has systematically compared the transdifferentiation efficiency of Pdx1 and Pdx1-VP16 at the cellular and molecular level. Comparisons can be ambiguous when vectors harboring a transcription factor cDNA have differing extents and duration of gene expression. In view of the remarkable capacity of lentiviral vector (LV) for delivering and integrating transgene into both dividing and nondividing cells, we transduced rat hepatic stem cell-like WB cells with LV-Pdx1 or LV-Pdx1-VP16, and then used the limiting-dilution technique to clone single-cell-derived cell lines that stably express either Pdx1 or Pdx1-VP16. With these cell lines, we studied: (a) the expression of Pdx1 or Pdx1-VP16 protein by Western blotting and immunocytochemistry; (b) the repertoire of long-term expression of Pdx1- or Pdx1-VP16-induced pancreatic gene expression using RT-PCR methods; and (c) their capacity to serve as beta-cell surrogates in restoring euglycemia in streptozotocin-treated diabetic mice. We found that cell lines expressing either Pdx1 or Pdx1-VP16 long-term exhibited similar profiles for expression of genes related to pancreatic development and beta-cell function, and reversed hyperglycemia in diabetic mice. We also examined short-term expression of Pdx1 or Pdx1-VP16, and the results demonstrated that expression of Pdx1-VP16 is more efficient in initiating liver-to-endocrine pancreas transdifferentiation. Our findings demonstrate: (a) that the LV system is highly effective in producing persistent expression of Pdx1 or Pdx1-VP16 in WB hepatic cells; and (b) long-term, persistent expression of either Pdx1 or Pdx1-VP16 is similarly effective in converting hepatic stem cells into pancreatic endocrine precursor cells that, upon transplantation into diabetic mice, become functional insulin-producing cells and restore euglycemia.

SOX6 attenuates glucose-stimulated insulin secretion by repressing PDX1 transcriptional activity and is down-regulated in hyperinsulinemic obese mice

In obesity-related insulin resistance, pancreatic islets compensate for insulin resistance by increasing secretory capacity. Here, we report the identification of sex-determining region Y-box 6 (SOX6), a member of the high mobility group box superfamily of transcription factors, as a co-repressor for pancreatic-duodenal homeobox factor-1 (PDX1). SOX6 mRNA levels were profoundly reduced by both a long term high fat feeding protocol in normal mice and in genetically obese ob/ob mice on a normal chow diet. Interestingly, we show that SOX6 is expressed in adult pancreatic insulin-producing beta-cells and that overexpression of SOX6 decreased glucose-stimulated insulin secretion, which was accompanied by decreased ATP/ADP ratio, Ca(2+) mobilization, proinsulin content, and insulin gene expression. In a complementary fashion, depletion of SOX6 by small interfering RNAs augmented glucose-stimulated insulin secretion in insulinoma mouse MIN6 and rat INS-1E cells. These effects can be explained by our mechanistic studies that show SOX6 acts to suppress PDX1 stimulation of the insulin II promoter through a direct protein/protein interaction. Furthermore, SOX6 retroviral expression decreased acetylation of histones H3 and H4 in chromatin from the promoter for the insulin II gene, suggesting that SOX6 may decrease PDX1 stimulation through changes in chromatin structure at specific promoters. These results suggest that perturbations in transcriptional regulation that are coordinated through SOX6 and PDX1 in beta-cells may contribute to the beta-cell adaptation in obesity-related insulin resistance.

Endocrine pancreatic tissue plasticity in obese humans is associated with cytoplasmic expression of PBX-1 in pancreatic ductal cells

In vivo lineage tracing experiments in mice have recently cast doubt on the potential islet neogenesis from ductal precursors in adult mammals. We examined, in human obesity, a model for pancreatic endocrine tissue plasticity, the gene and protein expression of PBX-1-a transcription factor expressed in regenerating rat ductules and potentially implicated in the pancreatic development, alone or in association with PDX-1. When comparing gene expression, by quantitative real-time RT-PCR, in pancreatic exocrine tissue from obese non-diabetic subjects with increased islet mass, we found that Pbx-1 and Pdx-1 were up-regulated (5.9+/-1.2 and 2.4+/-0.6 versus non-obese). Immunohistochemistry confirmed PBX-1 over-expression and its cytoplasmic sequestration in ductal cells of obese subjects, associated with pronounced islet neogenesis (cytokeratin 19/chromogranin A double labeling). cDNA microarray analysis also showed up-regulation of other genes implicated in islet regeneration, including betacellulin, laminin, TGFa, NeuroD1, Pax6, substantiating the role of the islet neogenesis pathway in human obesity.

Variable Meis-dependence among paralog group-1 Hox proteins

Optimal function of Hox transcription factors may require Meis and Pbx cofactors. Here we test the in vivo Meis-dependence of two zebrafish paralog group-1 (PG1) Hox proteins. Misexpression of Hoxb1a induces ectopic gene expression throughout the anterior nervous system, while Hoxb1b induces ectopic expression primarily in hindbrain rhombomere 2. These activities are drastically reduced when endogenous Meis function is disrupted, demonstrating that both proteins are Meis-dependent. Upon addition of Meis3, Hoxb1b mimics the more severe Hoxb1a phenotype, indicating that Hoxb1b requires higher Meis levels than Hoxb1a. Using chimeric proteins we map this difference to the N-terminus, which contains the transcription activation domain. Lastly, we demonstrate strong genetic interactions between meis and PG1 hox genes, as well as between meis and pbx genes, in vivo. Our results are consistent with PG1 hox genes requiring pbx and meis function in vivo and reveal that different Hox proteins have distinct Meis requirements.

Reduced PDX-1 expression impairs islet response to insulin resistance and worsens glucose homeostasis

In type 2 diabetes mellitus, insulin resistance and an inadequate pancreatic beta-cell response to the demands of insulin resistance lead to impaired insulin secretion and hyperglycemia. Pancreatic duodenal homeodomain-1 (PDX-1), a transcription factor required for normal pancreatic development, also plays a key role in normal insulin secretion by islets. To investigate the role of PDX-1 in islet compensation for insulin resistance, we examined glucose disposal, insulin secretion, and islet cell mass in mice of four different genotypes: wild-type mice, mice with one PDX-1 allele inactivated (PDX-1+/-, resulting in impaired insulin secretion), mice with one GLUT4 allele inactivated (GLUT4+/-, resulting in insulin resistance), and mice heterozygous for both PDX-1 and GLUT4 (GLUT4+/-;PDX-1+/-). The combination of PDX-1 and GLUT4 heterozygosity markedly prolonged glucose clearance. GLUT4+/-;PDX-1+/- mice developed beta-cell hyperplasia but failed to increase their beta-cell insulin content. These results indicate that PDX-1 heterozygosity (approximately 60% of normal protein levels) abrogates the beta-cell's compensatory response to insulin resistance, impairs glucose homeostasis, and may contribute to the pathogenesis of type 2 diabetes.

PDX-1/VP16 fusion protein, together with NeuroD or Ngn3, markedly induces insulin gene transcription and ameliorates glucose tolerance

Diabetes is the most prevalent and serious metabolic disease, and the number of diabetic patients worldwide is increasing. The reduction of insulin biosynthesis in pancreatic beta-cells is closely associated with the onset and progression of diabetes, and thus it is important to search for ways to induce insulin-producing cells in non-beta-cells. In this study, we showed that a modified form of the pancreatic and duodenal homeobox factor 1 (PDX-1) carrying the VP16 transcriptional activation domain (PDX-1/VP16) markedly increases insulin biosynthesis and induces various pancreas-related factors in the liver, especially in the presence of NeuroD or neurogenin 3 (Ngn3). Furthermore, in streptozotocin-induced diabetic mice, PDX-1/VP16 overexpression, together with NeuroD or Ngn3, drastically ameliorated glucose tolerance. Thus PDX-1/VP16 expression, together with NeuroD or Ngn3, markedly induces insulin gene transcription and ameliorates glucose tolerance. This approach warrants further investigation and may have utility in the treatment of diabetes.

beta-Cell Pdx1 expression is essential for the glucoregulatory, proliferative, and cytoprotective actions of glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) regulates energy intake, gastrointestinal motility, and nutrient disposal. The relative importance of the islet beta-cell for GLP-1 actions remains unclear. We determined the role of the islet beta-cell and the pancreatic duodenal homeobox-1 (Pdx1) transcription factor for GLP-1 receptor (GLP-1R)-dependent actions through analysis of mice with beta-cell-specific inactivation of the Pdx1 gene (beta-cell(Pdx1-/-) mice). The GLP-1R agonist exendin-4 (Ex-4) reduced glycemic excursion following intraperitoneal (i.p.) glucose challenge in control littermates (beta-cell(Pdx1+/+) mice) but not in beta-cell(Pdx1-/-) mice. Similarly, Ex-4 failed to increase levels of plasma insulin, pancreatic insulin content, and pancreatic insulin mRNA transcripts in beta-cell(Pdx1-/-) mice. Furthermore, Ex-4 significantly increased beta-cell proliferation and reduced beta-cell apoptosis in beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) mice. Moreover, Ex-4 increased the levels of insulin and amylin mRNA transcripts and augmented glucose-stimulated insulin secretion in islets from beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) islets. Surprisingly, Ex-4 failed to reduce levels of plasma glucagon in beta-cell(Pdx1-/-) mice. These findings demonstrate that Pdx1 expression is essential for integrating GLP-1R-dependent signals regulating alpha-cell glucagon secretion and for the growth, differentiated function, and survival of islet beta-cells.

Constitutively active PDX1 induced efficient insulin production in adult murine liver

To generate insulin-producing cells in the liver, recombinant adenovirus containing a constitutively active mutant of PDX1 (PDX1-VP16), designed to activate target genes without the need for protein partners, was prepared and administered intravenously to streptozotocin (STZ)-treated diabetic mice. The effects were compared with those of administering wild-type PDX1 (wt-PDX1) adenovirus. Administration of these adenoviruses at 2x10(8)pfu induced similar levels of PDX1 protein expression in the liver. While wt-PDX1 expression exerted small effects on blood glucose levels, treatment with PDX1-VP16 adenovirus efficiently induced insulin production in hepatocytes, resulting in reversal of STZ-induced hyperglycemia. The effects were sustained through day 40 when exogenous PDX1-VP16 protein expression was undetectable in the liver. Endogenous PDX1 protein came to be expressed in the liver, which is likely to be the mechanism underlying the sustained effects. On the other hand, albumin and transferrin expressions were observed in insulin-producing cells in the liver, suggesting preservation of hepatocytic functions. Thus, transient expression of an active mutant of PDX1 in the liver induced sustained PDX1 and insulin expressions without loss of hepatocytic function.

High glucose is necessary for complete maturation of Pdx1-VP16-expressing hepatic cells into functional insulin-producing cells

Pdx1 has been shown to convert hepatocytes into both exocrine and endocrine pancreatic cells in mice, but it fails to selectively convert hepatocytes into pure insulin-producing cells (IPCs). The molecular mechanisms underlying the transdifferentiation remain unclear. In this study, we generated a stably transfected rat hepatic cell line named WB-1 that expresses an active form of Pdx1 along with a reporter gene, RIP-eGFP. Our results demonstrate that Pdx1 induces the expression of multiple genes related to endocrine pancreas development and islet function in these liver cells. We do not however find any expression of the late-stage genes (Pax4, Pax6, Isl-1, and MafA) related to beta-cell development, and the cells do not secrete insulin upon the glucose challenge. Yet when WB-1 cells are transplanted into diabetic NOD-scid mice, these genes become activated and hyperglycemia is completely reversed. Detailed comparison of gene expression profiles between pre- and posttransplanted WB-1 cells demonstrates that the WB-1 cells have similar properties as that seen in pancreatic beta-cells. In addition, in vitro culture in high-glucose medium is sufficient to induce complete maturation of WB-1 cells into functional IPCs. In summary, we find that Pdx1-VP16 is able to selectively convert hepatic cells into pancreatic endocrine precursor cells. However, complete transdifferentiation into functional IPCs requires additional external factors, including high glucose or hyperglycemia. Thus, transdifferentiation of hepatocytes into functional IPCs may serve as a viable therapeutic option for patients with type 1 diabetes.

Expression of stem cell markers and transcription factors during the remodeling of the rat pancreas after duct ligation

Ligation of the pancreatic duct has been shown to induce islet cell neogenesis from duct cells in the adult rat pancreas. The transcription factors that regulate islet cell neogenesis and the phenotype of putative precursor cells involved in neogenesis are unknown. We, therefore, studied the expression of the transcription factors Pdx1, Pbx1, Meis2, Nkx2.2 and the putative stem cell markers c-Kit and nestin in rat pancreata 3, 5 and 7 days after duct ligation. Immunocytochemical staining revealed a subpopulation of cells in the ligated portion of the pancreas that was positive for the putative stem cell markers c-Kit and nestin. The c-Kit immunoreactivity was upregulated, reaching a peak at day 3, while nestin expression peaked at day 7. The c-Kit-positive cells were located among the duct and islet cells, while nestin-expressing cells were found scattered in the duct epithelium at day 3 and around the ducts at day 7. Both c-Kit- and nestin-positive cells showed high proliferative activity, as determined by BrdU labeling. Pdx1 and Nkx2.2 were found predominantly in the duct cells of the ligated pancreas. There were significant changes in the expression patterns of Pbx1 and Meis2 in the ductular complexes. The findings indicate that the stem cell markers c-Kit and nestin as well as the transcription factors Pdx1 and Nkx2.2 are upregulated in compartments of the pancreas that are involved in islet cell neogenesis after duct ligation.

Identification of PCIF1, a POZ domain protein that inhibits PDX-1 (MODY4) transcriptional activity

Hox factors are evolutionarily conserved homeodomain-containing transcription factors that activate and repress gene expression in a precise temporally and spatially regulated manner during development and differentiation. Pancreatic-duodenal homeobox 1 (PDX-1) is a Hox-type protein that is a critical requirement for normal pancreas development and for proper differentiation of the endocrine pancreas. In humans, PDX-1 gene mutation causes pancreatic agenesis and early- and late-onset type 2 diabetes. PDX-1 consists of an N-terminal transactivation domain, a homeodomain responsible for DNA binding and nuclear localization, and a conserved C terminus that is mutated in human diabetes but whose function is poorly understood. We have identified a novel POZ domain protein, PDX-1 C terminus-interacting factor 1 (PCIF1)/SPOP, that interacts with PDX-1 both in vitro and in vivo. PCIF1 is localized to the nucleus in a speckled pattern, and coexpression of PDX-1 alters the subnuclear distribution of PCIF1. Functionally, PCIF1 inhibits PDX-1 transactivation of established target gene promoters in a specific and dose-dependent manner that requires critical amino acids in the PDX-1 C terminus. PCIF1 is expressed in adult pancreatic insulin-producing beta cells, and overexpression of PCIF1 inhibits the rat insulin 1 and rat insulin 2 promoters in the MIN6 insulinoma beta cell line. The coexpression of PCIF1 with PDX-1 in beta cells and the ability of PCIF1 to repress PDX-1 transactivation suggest that modulation of PDX-1 function by PCIF1 may regulate normal beta cell differentiation.

Transcription Factors in Islet Development and Physiology: Role of PDX-1 in Beta-Cell Function

Differentiation of early foregut endoderm into pancreatic endocrine and exocrine cells depends on a cascade of gene activation events controlled by various transcription factors. The first molecular marker identified that specifies the early pancreatic epithelium is the homeodomain-containing transcription factor PDX-1. Its absence in mice and humans during development leads to agenesis of the pancreas. Later, it becomes restricted primarily to beta cells where it regulates the expression of beta cell-specific genes, and, most importantly, mediates the glucose effect on insulin gene transcription. Although exposure of beta cells to high glucose concentrations for relatively short periods stimulates insulin gene expression, chronic exposure has adverse effects on many beta-cell functions, including insulin gene transcription. These events appear to correlate with pdx-1 gene expression and its ability to bind the insulin gene. We consider that loss of PDX-1 function or altered pdx-1 gene expression due to mutations or functional impairment of transcription factors controlling its expression can lead to diabetes.

Gene regulatory factors in pancreatic development

The intensity of research on pancreatic development has increased markedly in the past 5 years, primarily for two reasons: we now know that the insulin-producing beta-cells normally arise from an endodermally derived, pancreas-specified precursor cell, and successful transplants of islet cells have been performed, relieving patients with type I diabetes of symptoms for extended periods after transplantation. Combining in vitro beta-cell formation from a pancreatic biopsy of a diabetic patient or from other stem-cell sources followed by endocrine cell transplantation may be the most beneficial route for a future diabetes therapy. However, to achieve this, a thorough understanding of the genetic components regulating the development of beta-cells is required. The following review discusses our current understanding of the transcription factor networks necessary for pancreatic development and how several genetic interactions coming into play at the earliest stages of endodermal development gradually help to build the pancreatic organ. Developmental Dynamics 229:176-200, 2004.

PDX-1 expression in pancreatic ductal cells after partial pancreatectomy in adult rats

To investigate the protein and mRNA expression of pancreas/duodenal homeobox-1 (PDX-1), a transcription factor as a marker for pancreatic stem cells, in pancreatic ductal cells of rats after partial (90%) pancreatectomy and evaluated the significance of the PDX-1 expression. Western blot and Reverse transcriptase-polymerase chain reaction (RT-PCR) were used to detect the expression of PDX-1 protein and mRNA respectively. PDX-1 protein was only faintly detected in pancreatic ductal cells on the day 1 after partial pancreatectomy. On the day 2 and 3 after operation in operation group, a 2-3 fold increased PDX-1 protein was observed, corresponding to the characteristic 42-kD protein in Western blot. There was significant difference between operation group and sham-operation group (P<0.05). PDX-1 protein expression on the day 5 and 7 after operation had already been no difference from control group (P>0.05). RT-PCR revealed the PDX-1 mRNA expression showed no significant difference between operation group at various time points and sham-operation group (P> 0.05). These results indicate that there was overexpression of PDX-1 in the cells of pancreatic epithelium during the regeneration of remnant pancreas after partial pancreatectomy in adult rats, suggesting the pancreatic stem cells in pancreatic ductal epithelial cells are involved in the regeneration of remnant pancreas and the expression of PDX-1 in ductal cells was regulated posttranscription.

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

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

Glucose regulates insulin gene transcription by hyperacetylation of histone h4

Induction of insulin gene expression in response to high blood glucose levels is essential for maintaining glucose homeostasis. Although several transcription factors including Beta-2, Ribe3b1, and Pdx-1 have been shown to play a role in glucose stimulation of insulin gene expression, the exact molecular mechanism(s) by which this regulation occurs is unknown. Previous data demonstrate that the transcription factors Beta-2/NeuroD1 and Pdx-1, which are involved in glucose-stimulated insulin gene expression, interact with the histone acetylase p300, suggesting a role for histone acetylation in glucose regulation of the insulin gene expression. We report that exposure of mouse insulinoma 6 cells to high concentrations of glucose results in hyperacetylation of histone H4 at the insulin gene promoter, which correlates with the increased level of insulin gene transcription. In addition, we demonstrate that hyperacetylation of histone H4 in response to high concentrations of glucose also occurs at the glucose transporter-2 gene promoter. Using histone deacetylase inhibitors, we show that increases in histone H4 acetylation cause stimulation of insulin gene transcription even in the absence of high concentrations of glucose. Furthermore, we show that fibroblasts, which lack insulin gene expression, also lack histone acetylation at the insulin gene promoter. In summary, our data support the idea that high concentrations of glucose stimulate insulin gene expression by causing hyperacetylation of histone H4 at the insulin gene promoter.

Experimental conversion of liver to pancreas

BACKGROUND

The liver and the pancreas arise from adjacent regions of endoderm in embryonic development. Pdx1 is a key transcription factor that is essential for the development of the pancreas and is not expressed in the liver. The aim of this study was to determine whether a gene overexpression protocol based on Pdx1 would be able to cause conversion of liver to pancreas.

RESULTS

We show that a modified form of Pdx1, carrying the VP16 transcriptional activation domain, can cause conversion of liver to pancreas, both in vivo and in vitro. Transgenic Xenopus tadpoles carrying the construct TTR-Xlhbox8-VP16:Elas-GFP were prepared. Xlhbox8 is the Xenopus homolog of Pdx1, the TTR (transthyretin) promoter directs expression to the liver, and the GFP is under the control of an elastase promoter and provides a real-time visible marker of pancreatic differentiation. In the transgenic tadpoles, part or all of the liver is converted to pancreas, containing both exocrine and endocrine cells, while liver differentiation products are lost from the regions converted to pancreas. The timing of events is such that the liver is differentiating by the time Xlhbox8-VP16 is expressed, so we consider this a transdifferentiation event rather than a reprogramming of embryonic development. Furthermore, this same construct will bring about transdifferentiation of human hepatocytes in culture, with formation of both exocrine and endocrine cells.

CONCLUSIONS

We consider that the conversion of liver to pancreas could be the basis of a new type of therapy for insulin-dependent diabetes. Although expression of the transgene is transient, once the ectopic pancreas is established, it persists thereafter.

beta-cell neogenesis induced by adenovirus-mediated gene delivery of transcription factor pdx-1 into mouse pancreas

beta-cell neogenesis is expected to provide a new therapy for diabetes. Numerous studies have demonstrated that transcriptional regulation involving pdx-1 is essential for endocrine neogenesis in vivo and in vitro. Therefore, it is possible that ectopic expression of pdx-1 in the pancreas could induce endocrine neogenesis. To test this possibility, we performed safe and efficient gene delivery of the pdx-1 gene into the mouse pancreas through the common bile duct using adenoviral vectors, and examined the effects of the ectopic expression of pdx-1. Here we show that adenovirus-mediated expression of pdx-1 can activate the endogenous pdx-1 gene, leading to beta-cell neogenesis and ductal proliferation. This technique is similar to the endoscopic retrograde cholangiopancreatography, which has been already established as a safe procedure for humans. Thus, beta-cell neogenesis induced by adenovirus-mediated expression of pdx-1 provides a novel strategy for gene therapy for a cure for diabetes mellitus.

Gene expression cascades in pancreatic development

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