The Cdk4-E2f1 pathway regulates early pancreas development by targeting Pdx1+ progenitors and Ngn3+ endocrine precursors

Epigenetic memory as crucial contributing factor in directing the differentiation of human iPSC into pancreatic β-cells in vitro

Impaired insulin secretion contributes to the pathogenesis of type 1 diabetes mellitus through autoimmune destruction of pancreatic β-cells and the pathogenesis of severe forms of type 2 diabetes mellitus through β-cell dedifferentiation and other mechanisms. Replenishment of malfunctioning β-cells via islet transplantation has the potential to induce long-term glycemic control in the body. However, this treatment option cannot widely be implemented in clinical due to healthy islet donor shortage. Emerging β-cell replacement with human-induced pluripotent stem cell (iPSC) provides high remedial therapy hopes. Thus, tremendous progress has been made in developing β-cell differentiation protocols in vitro; however, most of the differentiated iPSC-derived β-cells showed immature phenotypes associated with low efficiency depending on the iPSC lines used, creating a crucial barrier for their clinical implementation. Multiple mechanisms including differences in genetic, cell cycle patterns, and mitochondrial dysfunction underlie the defective differentiation propensity of iPSC into insulin-producing β-cells. Accumulating evidence recently indicated that, following the reprogramming, epigenetic memory inherited from parental cells substantially affects the differentiation capacity of many iPSC lines. Therefore, differences in epigenetic signature are likely to be essential contributing factors influencing the propensity of iPSC differentiation. In this review, we will document the impact of the epigenome on the reprogramming efficacy and differentiation potential of iPSCs and how targeting the epigenetic residual memory could be an additional strategy to improve the differentiation efficiency of existing protocols to generate fully functional hPSC-derived pancreatic β-cells for diabetes therapy and drug screening.

Investigation of the motif activity of transcription regulators in pancreatic β-like cell subpopulations differentiated from human induced pluripotent stem cells

Pancreatic β-cells are composed of different subtypes that play a key role in the control of insulin secretion and thereby control glucose homeostasis. In vitro differentiation of human induced pluripotent stem cells (hiPSCs) into 3D spheroids leads to the generation of β-cell subtypes and thus to the development of islet-like structures. Using this cutting-edge cell model, the aim of the study was to decipher the signaling signature that underlines β-cell subtypes, with a focus on the search for the activity of motifs of important transcription regulators (TRs). The investigation was performed using data from previous single-cell sequencing analysis introduced into the integrated system for motif activity response analysis (ISMARA) of transcription regulators. We extracted the matrix of important TRs activated in the β-cell subpopulation and bi-hormonal-like β-cells. Based on these TRs and their targets, we built specific regulatory networks for main cell subpopulations. Our data confirmed the transcriptomic heterogeneity of the β-cell subtype lineage and suggested a mechanism that could account for the differentiation of β-cell subtypes during pancreas development. We do believe that our findings could be instrumental for understanding the mechanisms that affect the balance of β-cell subtypes, leading to impaired insulin secretion in type 2 diabetes.

3D model of mouse embryonic pancreas and endocrine compartment using stem cell-derived mesoderm and pancreatic progenitors

The developing mouse pancreas is surrounded by mesoderm compartments providing signals that induce pancreas formation. Most pancreatic organoid protocols lack this mesoderm niche and only partially capture the pancreatic cell repertoire. This work aims to generate pancreatic aggregates by differentiating mouse embryonic stem cells (mESCs) into mesoderm progenitors (MPs) and pancreas progenitors (PPs), without using Matrigel. First, mESCs were differentiated into epiblast stem cells (EpiSCs) to enhance the PP differentiation rate. Next, PPs and MPs aggregated together giving rise to various pancreatic cell types, including endocrine, acinar, and ductal cells, and to endothelial cells. Single-cell RNA sequencing analysis revealed a larger endocrine population within the PP + MP aggregates, as compared to PPs alone or PPs in Matrigel aggregates. The PP + MP aggregate gene expression signatures and its endocrine population percentage closely resembled those of the endocrine population found in the mouse embryonic pancreas, which holds promise for studying pancreas development.

E2F transcription factors promote tumorigenicity in pancreatic ductal adenocarcinoma

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers with limited treatment options, illustrating an urgent need to identify new drugable targets in PDACs.

OBJECTIVE

Using the similarities between tumor development and normal embryonic development, which is accompanied by rapid cell expansion, we aimed to identify and characterize embryonic signaling pathways that were reinitiated during tumor formation and expansion.

METHODS AND RESULTS

Here, we report that the transcription factors E2F1 and E2F8 are potential key regulators in PDAC. E2F1 and E2F8 RNA expression is mainly localized in proliferating cells in the developing pancreas and in malignant ductal cells in PDAC. Silencing of E2F1 and E2F8 in PANC-1 pancreatic tumor cells inhibited cell proliferation and impaired cell spreading and migration. Moreover, loss of E2F1 also affected cell viability and apoptosis with E2F expression in PDAC tissues correlating with expression of apoptosis and mitosis pathway genes, suggesting that E2F factors promote cell cycle regulation and tumorigenesis in PDAC cells.

CONCLUSION

Our findings illustrate that E2F1 and E2F8 transcription factors are expressed in pancreatic progenitor and PDAC cells, where they contribute to tumor cell expansion by regulation of cell proliferation, viability, and cell migration making these genes attractive therapeutic targets and potential prognostic markers for pancreatic cancer.

β-Cell-Specific E2f1 Deficiency Impairs Glucose Homeostasis, β-Cell Identity, and Insulin Secretion

The loss of pancreatic β-cell identity has emerged as an important feature of type 2 diabetes development, but the molecular mechanisms are still elusive. Here, we explore the cell-autonomous role of the cell-cycle regulator and transcription factor E2F1 in the maintenance of β-cell identity, insulin secretion, and glucose homeostasis. We show that the β-cell-specific loss of E2f1 function in mice triggers glucose intolerance associated with defective insulin secretion, altered endocrine cell mass, downregulation of many β-cell genes, and concomitant increase of non-β-cell markers. Mechanistically, epigenomic profiling of the promoters of these non-β-cell upregulated genes identified an enrichment of bivalent H3K4me3/H3K27me3 or H3K27me3 marks. Conversely, promoters of downregulated genes were enriched in active chromatin H3K4me3 and H3K27ac histone marks. We find that specific E2f1 transcriptional, cistromic, and epigenomic signatures are associated with these β-cell dysfunctions, with E2F1 directly regulating several β-cell genes at the chromatin level. Finally, the pharmacological inhibition of E2F transcriptional activity in human islets also impairs insulin secretion and the expression of β-cell identity genes. Our data suggest that E2F1 is critical for maintaining β-cell identity and function through sustained control of β-cell and non-β-cell transcriptional programs.

ARTICLE HIGHLIGHTS

β-Cell-specific E2f1 deficiency in mice impairs glucose tolerance. Loss of E2f1 function alters the ratio of α- to β-cells but does not trigger β-cell conversion into α-cells. Pharmacological inhibition of E2F activity inhibits glucose-stimulated insulin secretion and alters β- and α-cell gene expression in human islets. E2F1 maintains β-cell function and identity through control of transcriptomic and epigenetic programs.

CDK4-E2F3 signals enhance oxidative skeletal muscle fiber numbers and function to affect myogenesis and metabolism

Understanding how skeletal muscle fiber proportions are regulated is vital to understanding muscle function. Oxidative and glycolytic skeletal muscle fibers differ in their contractile ability, mitochondrial activity, and metabolic properties. Fiber-type proportions vary in normal physiology and disease states, although the underlying mechanisms are unclear. In human skeletal muscle, we observed that markers of oxidative fibers and mitochondria correlated positively with expression levels of PPARGC1A and CDK4 and negatively with expression levels of CDKN2A, a locus significantly associated with type 2 diabetes. Mice expressing a constitutively active Cdk4 that cannot bind its inhibitor p16INK4a, a product of the CDKN2A locus, were protected from obesity and diabetes. Their muscles exhibited increased oxidative fibers, improved mitochondrial properties, and enhanced glucose uptake. In contrast, loss of Cdk4 or skeletal muscle-specific deletion of Cdk4's target, E2F3, depleted oxidative myofibers, deteriorated mitochondrial function, and reduced exercise capacity, while increasing diabetes susceptibility. E2F3 activated the mitochondrial sensor PPARGC1A in a Cdk4-dependent manner. CDK4, E2F3, and PPARGC1A levels correlated positively with exercise and fitness and negatively with adiposity, insulin resistance, and lipid accumulation in human and rodent muscle. All together, these findings provide mechanistic insight into regulation of skeletal muscle fiber-specification that is of relevance to metabolic and muscular diseases.

TRPM7 kinase is required for insulin production and compensatory islet responses during obesity

Most overweight individuals do not develop diabetes due to compensatory islet responses to restore glucose homeostasis. Therefore, regulatory pathways that promote β cell compensation are potential targets for treatment of diabetes. The transient receptor potential cation channel subfamily M member 7 protein (TRPM7), harboring a cation channel and a serine/threonine kinase, has been implicated in controlling cell growth and proliferation. Here, we report that selective deletion of Trpm7 in β cells disrupted insulin secretion and led to progressive glucose intolerance. We indicate that the diminished insulinotropic response in β cell-specific Trpm7-knockout mice was caused by decreased insulin production because of impaired enzymatic activity of this protein. Accordingly, high-fat-fed mice with a genetic loss of TRPM7 kinase activity displayed a marked glucose intolerance accompanied by hyperglycemia. These detrimental glucoregulatory effects were engendered by reduced compensatory β cell responses because of mitigated protein kinase B (AKT)/ERK signaling. Collectively, our data identify TRPM7 kinase as a potentially novel regulator of insulin synthesis, β cell dynamics, and glucose homeostasis under obesogenic diet.

The transcription factor E2F1 controls the GLP-1 receptor pathway in pancreatic β cells

The glucagon-like peptide 1 (Glp-1) has emerged as a hormone with broad pharmacological potential in type 2 diabetes (T2D) treatment, notably by improving β cell functions. The cell-cycle regulator and transcription factor E2f1 is involved in glucose homeostasis by modulating β cell mass and function. Here, we report that β cell-specific genetic ablation of E2f1 (E2f1β-/-) impairs glucose homeostasis associated with decreased expression of the Glp-1 receptor (Glp1r) in E2f1β-/- pancreatic islets. Pharmacological inhibition of E2F1 transcriptional activity in nondiabetic human islets decreases GLP1R levels and blunts the incretin effect of GLP1R agonist exendin-4 (ex-4) on insulin secretion. Overexpressing E2f1 in pancreatic β cells increases Glp1r expression associated with enhanced insulin secretion mediated by ex-4. Interestingly, ex-4 induces retinoblastoma protein (pRb) phosphorylation and E2f1 transcriptional activity. Our findings reveal critical roles for E2f1 in β cell function and suggest molecular crosstalk between the E2F1/pRb and GLP1R signaling pathways.

CDKN2A-Mutated Pancreatic Ductal Organoids from Induced Pluripotent Stem Cells to Model a Cancer Predisposition Syndrome

Patient-derived induced pluripotent stem cells (iPSCs) provide a unique platform to study hereditary disorders and predisposition syndromes by resembling germline mutations of affected individuals and by their potential to differentiate into nearly every cell type of the human body. We employed plucked human hair from two siblings with a family history of cancer carrying a pathogenic CDKN2A variant, P16-p.G101W/P14-p.R115L, to generate patient-specific iPSCs in a cancer-prone ancestry for downstream analytics. The differentiation capacity to pancreatic progenitors and to pancreatic duct-like organoids (PDLOs) according to a recently developed protocol remained unaffected. Upon inducible expression of KRAS^G12Dusing a piggyBac transposon system in CDKN2A-mutated PDLOs, we revealed structural and molecular changes in vitro, including disturbed polarity and epithelial-to-mesenchymal (EMT) transition. CDKN2A-mutated KRAS^G12DPDLO xenotransplants formed either a high-grade precancer lesion or a partially dedifferentiated PDAC-like tumor. Intriguingly, P14/P53/P21 and P16/RB cell-cycle checkpoint controls have been only partly overcome in these grafts, thereby still restricting the tumorous growth. Hereby, we provide a model for hereditary human pancreatic cancer that enables dissection of tumor initiation and early development starting from patient-specific CDKN2A-mutated pluripotent stem cells.

Cinnamaldehyde Improves Metabolic Functions in Streptozotocin-Induced Diabetic Mice by Regulating Gut Microbiota

Purpose

The aim of the present study was to examine the protective effects of cinnamaldehyde (CA) on type 1 diabetes mellitus (T1DM) and explore the underlying molecular mechanisms by using multiple omics technology.

Methods

T1DM was induced by streptozotocin in the mice. Immunostaining was performed to evaluate glycogen synthesis in the liver and morphological changes in the heart. Gut microbiota was analyzed using 16S rRNA gene amplification sequencing. The serum metabolomics were determined by liquid chromatography-mass spectrometry. The relevant gene expression levels were determined by quantitative real-time PCR.

Results

CA treatment significantly improved the glucose metabolism and insulin sensitivity in T1DM mice. CA increased glycogen synthesis in the liver and protected myocardial injury in T1DM mice. CA affected the gut microbiota particularly by increasing the relative abundance of Lactobacillus johnsonii and decreasing the relative abundance of Lactobacillus murinus in T1DM mice. The glucose level was positively correlated with 88 functional pathways of gut microbiota and negatively correlated with 2 functional pathways of gut microbiota. Insulin resistance was positively correlated with 11 functional pathways. The analysis of serum metabolomics showed that CA treatment significantly increased the levels of taurochenodeoxycholic acid, tauroursodeoxycholic acid, tauro-α-muricholic acid and tauro-β-muricholic acid, taurodeoxycholic acid, taurocholic acid and taurohyodeoxycholic acid in T1DM mice. Taurohyodeoxycholic acid level was highly correlated with the blood glucose levels. Furthermore, the abundance of Faecalibacterium prausnitzii was positively correlated with AKT2, insulin like growth factor 1 receptor, E2F1 and insulin receptor substrate 1 mRNA expression levels, while taurohyodeoxycholic acid level was negatively correlated with IRS1 mRNA expression level.

Conclusion

Our results indicated that CA may interfere with gut microbiota to affect host metabolomics, especially the bile acids, so as to directly or indirectly modulate the expression levels of glucose metabolism-related genes, thus subsequently reducing the blood glucose level in the T1DM mice.

TGF-β Signaling in Pancreatic Islet β Cell Development and Function

Pancreatic islet beta cells (β-cells) synthesize and secrete insulin in response to rising glucose levels and thus are a prime target in both major forms of diabetes. Type 1 diabetes ensues due to autoimmune destruction of β-cells. On the other hand, the prevailing insulin resistance and hyperglycemia in type 2 diabetes (T2D) elicits a compensatory response from β-cells that involves increases in β-cell mass and function. However, the sustained metabolic stress results in β-cell failure, characterized by severe β-cell dysfunction and loss of β-cell mass. Dynamic changes to β-cell mass also occur during pancreatic development that involves extensive growth and morphogenesis. These orchestrated events are triggered by multiple signaling pathways, including those representing the transforming growth factor β (TGF-β) superfamily. TGF-β pathway ligands play important roles during endocrine pancreas development, β-cell proliferation, differentiation, and apoptosis. Furthermore, new findings are suggestive of TGF-β's role in regulation of adult β-cell mass and function. Collectively, these findings support the therapeutic utility of targeting TGF-β in diabetes. Summarizing the role of the various TGF-β pathway ligands in β-cell development, growth and function in normal physiology, and during diabetes pathogenesis is the topic of this mini-review.

Oncogenic miR-20b-5p contributes to malignant behaviors of breast cancer stem cells by bidirectionally regulating CCND1 and E2F1

BACKGROUND

Breast cancer is the leading cause of cancer mortality in women worldwide. Therefore, it is of great significance to identify the biological mechanism of tumorigenesis and explore the development of breast cancer to achieve a better prognosis for individuals suffering from breast cancer. MicroRNAs (miRNAs) have become a hot topic in cancer research, but the underlying mechanism of its involvement in cancer remains unclear.

METHODS

The miRNA profile between breast cancer stem cells (BCSCs, CD44⁺CD24^-/low) and control MCF-7 breast cancer cells was obtained in a previous study. Based on biological analysis, miR-20b-5p was hypothesized to be a key factor due to the malignant behavior of BCSCs. Then, agomir-20b-5p and antagomir-20b-5p were transfected into MCF-7 and T47D breast cancer cells to detect cell migration, wound healing and proliferation, and lentivirus vectors silencing or overexpressing miR-20b-5p were transfected into T47D-CSCs to detect proliferation and apoptosis. The effect of miR-20b-5p on xenograft growth was investigated in vivo by transfection of a lentivirus-overexpression vector into T47D cells. The target genes were predicted by the online programs picTar, miRanda and TargetScan and verified by dual luciferase assay, and changes in protein expression were detected by western blot.

RESULTS

MiR-20b-5p had the highest degree in both the miRNA-gene network and miRNA-GO network to regulate BCSCs. Overexpression of miR-20b-5p significantly promoted the migration and wound healing ability of MCF-7 cells and T47D cells compared with the control (P < 0.05). In addition, miR-20b-5p facilitated the proliferation of MCF-7 cells and T47D-CSCs (P < 0.05) and inhibited the apoptosis of T47D-CSCs (P < 0.05). Moreover, miR-20b-5p promoted xenograft growth compared with the control group (P < 0.05). Accordingly, potential targets of both CCND1 and E2F1 were predicted by bioinformatics analysis. MiR-20b-5p directly targeted both CCND1 and E2F1 in a dual luciferase assay, while antagomir-20b-5p downregulated the protein levels of CCND1 and E2F1.

CONCLUSIONS

Oncogenic miR-20b-5p was confirmed to promote the malignant behaviors of breast cancer cells and BCSCs. The underlying mechanism lies in that miR-20b-5p overall enhanced both CCND1 and E2F1 targets via bidirectional regulation probably involving direct downregulation and indirect upregulation.

E2F1 Mediates the Retinoic Acid-Induced Transcription of Tshz1 during Neuronal Differentiation in a Cell Division-Dependent Manner

The involvement of cell division in cellular differentiation has long been accepted. Cell division may be required not only for the expansion of a differentiated cell population but also for the execution of differentiation processes. Nonetheless, knowledge regarding how specific differentiation processes are controlled in a cell division-dependent manner is far from complete. Here, we determined the involvement of cell division in neuronal differentiation. We initially confirmed that cell division is an essential event for the neuronal differentiation of P19 embryonic carcinoma cells. We investigated the induction mechanisms of Tshz1, whose expression is induced by retinoic acid (RA) in a cell division-dependent manner. Promoter analysis of Tshz1 revealed a specific region required for RA-dependent transcription. A series of experiments was used to identify E2F1 as the induction factor for the RA-dependent transcription of Tshz1 We propose that E2F1 mediates neuronal differentiation in a cell division-dependent manner.

Phosphorylation of NEUROG3 Links Endocrine Differentiation to the Cell Cycle in Pancreatic Progenitors

During pancreatic development, proliferating pancreatic progenitors activate the proendocrine transcription factor neurogenin 3 (NEUROG3), exit the cell cycle, and differentiate into islet cells. The mechanisms that direct robust NEUROG3 expression within a subset of progenitor cells control the size of the endocrine population. Here we demonstrate that NEUROG3 is phosphorylated within the nucleus on serine 183, which catalyzes its hyperphosphorylation and proteosomal degradation. During progression through the progenitor cell cycle, NEUROG3 phosphorylation is driven by the actions of cyclin-dependent kinases 2 and 4/6 at G₁/S cell-cycle checkpoint. Using models of mouse and human pancreas development, we show that lengthening of the G₁ phase of the pancreatic progenitor cell cycle is essential for proper induction of NEUROG3 and initiation of endocrine cell differentiation. In sum, these studies demonstrate that progenitor cell-cycle G₁ lengthening, through its actions on stabilization of NEUROG3, is an essential variable in normal endocrine cell genesis.

Association of polymorphic markers of genes FTO, KCNJ11, CDKAL1, SLC30A8, and CDKN2B with type 2 diabetes mellitus in the Russian population

Background

The association of type 2 diabetes mellitus (T2DM) with the KCNJ11, CDKAL1, SLC30A8, CDKN2B, and FTO genes in the Russian population has not been well studied. In this study, we analysed the population frequencies of polymorphic markers of these genes.

Methods

The study included 862 patients with T2DM and 443 control subjects of Russian origin. All subjects were genotyped for 10 single nucleotide polymorphisms (SNPs) of the genes using real-time PCR (TaqMan assays). HOMA-IR and HOMA-β were used to measure insulin resistance and β-cell secretory function, respectively.

Results

The analysis of the frequency distribution of polymorphic markers for genes KCNJ11, CDKAL1, SLC30A8 and CDKN2B showed statistically significant associations with T2DM in the Russian population. The association between the FTO gene and T2DM was not statistically significant. The polymorphic markers rs5219 of the KCNJ11 gene, rs13266634 of the SLC30A8 gene, rs10811661 of the CDKN2B gene and rs9465871, rs7756992 and rs10946398 of the CDKAL1 gene showed a significant association with impaired glucose metabolism or impaired β-cell function.

Conclusion

In the Russian population, genes, which affect insulin synthesis and secretion in the β-cells of the pancreas, play a central role in the development of T2DM.

Loss of Cyclin-dependent Kinase 2 in the Pancreas Links Primary β-Cell Dysfunction to Progressive Depletion of β-Cell Mass and Diabetes

The failure of pancreatic islet β-cells is a major contributor to the etiology of type 2 diabetes. β-Cell dysfunction and declining β-cell mass are two mechanisms that contribute to this failure, although it is unclear whether they are molecularly linked. Here, we show that the cell cycle regulator, cyclin-dependent kinase 2 (CDK2), couples primary β-cell dysfunction to the progressive deterioration of β-cell mass in diabetes. Mice with pancreas-specific deletion of Cdk2 are glucose-intolerant, primarily due to defects in glucose-stimulated insulin secretion. Accompanying this loss of secretion are defects in β-cell metabolism and perturbed mitochondrial structure. Persistent insulin secretion defects culminate in progressive deficits in β-cell proliferation, reduced β-cell mass, and diabetes. These outcomes may be mediated directly by the loss of CDK2, which binds to and phosphorylates the transcription factor FOXO1 in a glucose-dependent manner. Further, we identified a requirement for CDK2 in the compensatory increases in β-cell mass that occur in response to age- and diet-induced stress. Thus, CDK2 serves as an important nexus linking primary β-cell dysfunction to progressive β-cell mass deterioration in diabetes.

E2F1, a Novel Regulator of Metabolism

In the past years, several lines of evidence have shown that cell cycle regulatory proteins also can modulate metabolic processes. The transcription factor E2F1 is a central player involved in cell cycle progression, DNA-damage response, and apoptosis. Its crucial role in the control of cell fate has been extensively studied and reviewed before; however, here, we focus on the participation of E2F1 in the regulation of metabolism. We summarize recent findings about the cell cycle-independent roles of E2F1 in various tissues that contribute to global metabolic homeostasis and highlight that E2F1 activity is increased during obesity. Finally, coming back to the pivotal role of E2F1 in cancer development, we discuss how E2F1 links cell cycle progression with different metabolic adaptations required for cell growth and survival.

PDGF Facilitates Direct Lineage Reprogramming of Hepatocytes to Functional β-Like Cells Induced by Pdx1 and Ngn3

Islet transplantation has been proven to be an effective treatment for patients with type 1 diabetes, but a lack of islet donors limits the use of transplantation therapies. It has been previously demonstrated that hepatocytes can be converted into insulin-producing β-like cells by introducing pancreatic transcription factors, indicating that direct hepatocyte reprogramming holds potential as a treatment for diabetes. However, the efficiency at which functional β-cells can be derived from hepatocyte reprogramming remains low. Here we demonstrated that the combination of Pdx1 and Ngn3 can trigger reprogramming of mouse and human liver cells to insulin-producing cells that exhibit the characteristics of pancreatic β-cells. Treatment with PDGF-AA was found to facilitate Pdx1 and Ngn3-induced reprogramming of hepatocytes to β-like cells with the ability to secrete insulin in response to glucose stimulus. Importantly, this reprogramming strategy could be applied to adult mouse primary hepatocytes, and the transplantation of β-like cells derived from primary hepatocyte reprogramming could ameliorate hyperglycemia in diabetic mice. These findings support the possibility of developing transplantation therapies for type 1 diabetes through the use of β-like cells derived from autologous hepatocyte reprogramming.

Initiation of stem cell differentiation involves cell cycle-dependent regulation of developmental genes by Cyclin D

Coordination of differentiation and cell cycle progression represents an essential process for embryonic development and adult tissue homeostasis. These mechanisms ultimately determine the quantities of specific cell types that are generated. Despite their importance, the precise molecular interplays between cell cycle machinery and master regulators of cell fate choice remain to be fully uncovered. Here, we demonstrate that cell cycle regulators Cyclin D1-3 control cell fate decisions in human pluripotent stem cells by recruiting transcriptional corepressors and coactivator complexes onto neuroectoderm, mesoderm, and endoderm genes. This activity results in blocking the core transcriptional network necessary for endoderm specification while promoting neuroectoderm factors. The genomic location of Cyclin Ds is determined by their interactions with the transcription factors SP1 and E2Fs, which result in the assembly of cell cycle-controlled transcriptional complexes. These results reveal how the cell cycle orchestrates transcriptional networks and epigenetic modifiers to instruct cell fate decisions.

The cell cycle as a brake for β-cell regeneration from embryonic stem cells

The generation of insulin-producing β cells from stem cells in vitro provides a promising source of cells for cell transplantation therapy in diabetes. However, insulin-producing cells generated from human stem cells show deficiency in many functional characteristics compared with pancreatic β cells. Recent reports have shown molecular ties between the cell cycle and the differentiation mechanism of embryonic stem (ES) cells, assuming that cell fate decisions are controlled by the cell cycle machinery. Both β cells and ES cells possess unique cell cycle machinery yet with significant contrasts. In this review, we compare the cell cycle control mechanisms in both ES cells and β cells, and highlight the fundamental differences between pluripotent cells of embryonic origin and differentiated β cells. Through critical analysis of the differences of the cell cycle between these two cell types, we propose that the cell cycle of ES cells may act as a brake for β-cell regeneration. Based on these differences, we discuss the potential of modulating the cell cycle of ES cells for the large-scale generation of functionally mature β cells in vitro. Further understanding of the factors that modulate the ES cell cycle will lead to new approaches to enhance the production of functional mature insulin-producing cells, and yield a reliable system to generate bona fide β cells in vitro.

Notch Signaling in Pancreatic Development

The Notch signaling pathway plays a significant role in embryonic cell fate determination and adult tissue homeostasis. Various studies have demonstrated the deep involvement of Notch signaling in the development of the pancreas and the lateral inhibition of Notch signaling in pancreatic progenitor differentiation and maintenance. The targeted inactivation of the Notch pathway components promotes premature differentiation of the endocrine pancreas. However, there is still the contrary opinion that Notch signaling specifies the endocrine lineage. Here, we review the current knowledge of the Notch signaling pathway in pancreatic development and its crosstalk with the Wingless and INT-1 (Wnt) and fibroblast growth factor (FGF) pathways.

Transcriptional control of stem cell fate by E2Fs and pocket proteins

E2F transcription factors and their regulatory partners, the pocket proteins (PPs), have emerged as essential regulators of stem cell fate control in a number of lineages. In mammals, this role extends from both pluripotent stem cells to those encompassing all embryonic germ layers, as well as extra-embryonic lineages. E2F/PP-mediated regulation of stem cell decisions is highly evolutionarily conserved, and is likely a pivotal biological mechanism underlying stem cell homeostasis. This has immense implications for organismal development, tissue maintenance, and regeneration. In this article, we discuss the roles of E2F factors and PPs in stem cell populations, focusing on mammalian systems. We discuss emerging findings that position the E2F and PP families as widespread and dynamic epigenetic regulators of cell fate decisions. Additionally, we focus on the ever expanding landscape of E2F/PP target genes, and explore the possibility that E2Fs are not simply regulators of general ‘multi-purpose’ cell fate genes but can execute tissue- and cell type-specific gene regulatory programs.

Rb and p107 are required for alpha cell survival, beta cell cycle control and glucagon-like peptide-1 action

AIMS/HYPOTHESIS

Diabetes mellitus is characterised by beta cell loss and alpha cell expansion. Analogues of glucagon-like peptide-1 (GLP-1) are used therapeutically to antagonise these processes; thus, we hypothesised that the related cell cycle regulators retinoblastoma protein (Rb) and p107 were involved in GLP-1 action.

METHODS

We used small interfering RNA and adenoviruses to manipulate Rb and p107 expression in insulinoma and alpha-TC cell lines. In vivo we examined pancreas-specific Rb knockout, whole-body p107 knockout and Rb/p107 double-knockout mice.

RESULTS

Rb, but not p107, was downregulated in response to the GLP-1 analogue, exendin-4, in both alpha and beta cells. Intriguingly, this resulted in opposite outcomes of cell cycle arrest in alpha cells but proliferation in beta cells. Overexpression of Rb in alpha and beta cells abolished or attenuated the effects of exendin-4 supporting the important role of Rb in GLP-1 modulation of cell cycling. Similarly, in vivo, Rb, but not p107, deficiency was required for the beta cell proliferative response to exendin-4. Consistent with this finding, Rb, but not p107, was suppressed in islets from humans with diabetes, suggesting the importance of Rb regulation for the compensatory proliferation that occurs under insulin resistant conditions. Finally, while p107 alone did not have an essential role in islet homeostasis, when combined with Rb deletion, its absence potentiated apoptosis of both alpha and beta cells resulting in glucose intolerance and diminished islet mass with ageing.

CONCLUSIONS/INTERPRETATION

We found a central role of Rb in the dual effects of GLP-1 in alpha and beta cells. Our findings highlight unique contributions of individual Rb family members to islet cell proliferation and survival.

Telomere dysfunction suppresses multiple endocrine neoplasia in mice

Multiple endocrine neoplasia (MEN) syndrome is typified by the occurrence of tumors in two or more hormonal tissues. Whereas the genetics of MEN syndrome is relatively well understood, the tumorigenic mechanisms for these cancers remain relatively obscure. The Cdk4 (R24C) mouse model develops highly penetrant pituitary tumors and endocrine pancreas adenomas, and, as such, this model is appropriate to gain insight into mechanisms underlying MEN. Using this model, here we provide evidence supporting an important role for telomerase in the pathogenesis of MEN. We observed increased aneuploidy in Cdk4 (R/R) fibroblasts along with significantly elevated telomerase activity and telomere length in Cdk4 (R/R) islets and embryonic fibroblasts. To better understand the role of telomerase, we generated Cdk4 (R24C) mice with inactivation of the mTERC locus, which codes for the essential RNA component of the enzyme telomerase (mTERC (-/-) Cdk4 (R/R) mice). Embryonic fibroblasts and islets derived from mTERC (-/-) Cdk4 (R/R) mice exhibit reduced telomere length and proliferative capacity. Further, mTERC (-/-) Cdk4 (R/R) fibroblasts display reduced transformation potential. Importantly, mTERC (-/-) Cdk4 (R/R) mice display significantly reduced spontaneous tumorigenesis. Strikingly, we observed dramatic suppression of pituitary tumors and endocrine pancreas adenomas in mTERC (-/-) Cdk4 (R/R) mice. Telomere dysfunction suppressed tumor initiation and increased latency of tumor development while not affecting the progression of established tumors. In summary, these results are suggestive of an important role for telomerase in tumor development in the Cdk4 (R24C) mouse model, specifically in the genesis of tumors in the pituitary and the endocrine pancreas.

Mangiferin facilitates islet regeneration and β-cell proliferation through upregulation of cell cycle and β-cell regeneration regulators

Mangiferin, a xanthonoid found in plants including mangoes and iris unguicularis, was suggested in previous studies to have anti-hyperglycemic function, though the underlying mechanisms are largely unknown. This study was designed to determine the therapeutic effect of mangiferin by the regeneration of β-cells in mice following 70% partial pancreatectomy (PPx), and to explore the mechanisms of mangiferin-induced β-cell proliferation. For this purpose, adult C57BL/6J mice after 7-14 days post-PPx, or a sham operation were subjected to mangiferin (30 and 90 mg/kg body weight) or control solvent injection. Mangiferin-treated mice exhibited an improved glycemia and glucose tolerance, increased serum insulin levels, enhanced β-cell hyperplasia, elevated β-cell proliferation and reduced β-cell apoptosis. Further dissection at the molecular level showed several key regulators of cell cycle, such as cyclin D1, D2 and cyclin-dependent kinase 4 (Cdk4) were significantly up-regulated in mangiferin-treated mice. In addition, critical genes related to β-cell regeneration, such as pancreatic and duodenal homeobox 1 (PDX-1), neurogenin 3 (Ngn3), glucose transporter 2 (GLUT-2), Forkhead box protein O1 (Foxo-1), and glucokinase (GCK), were found to be promoted by mangiferin at both the mRNA and protein expression level. Thus, mangiferin administration markedly facilitates β-cell proliferation and islet regeneration, likely by regulating essential genes in the cell cycle and the process of islet regeneration. These effects therefore suggest that mangiferin bears a therapeutic potential in preventing and/or treating the diabetes.

Grg3/TLE3 and Grg1/TLE1 induce monohormonal pancreatic β-cells while repressing α-cell functions

In the pancreas, α- and β-cells possess a degree of plasticity. In vitro differentiation of pluripotent cells yields mostly α- and polyhormonal β-like cells, indicating a gap in understanding of how functional monohormonal β-cells are formed and of the endogenous repressive mechanisms used to maintain β-cell identity. We show that the corepressor Grg3 is expressed in almost all β-cells throughout embryogenesis to adulthood. However, Grg3 is expressed in fewer nascent α-cells and is progressively lost from α-cells as endocrine cells mature into adulthood. We show that mouse Grg3(+/-) β-cells have increased α-specific gene expression, and Grg3(+/-) pancreata have more α-cells and more polyhormonal cells, indicating that Grg3 is required for the physiologic maintenance of monohormonal β-cell identity. Ectopic expression of Grg3 in α-cells represses glucagon and Arx, and the addition of Pdx1 induces Glut2 expression and glucose-responsive insulin secretion. Furthermore, we found that Grg1 is the predominant Groucho expressed in human β-cells but acts functionally similarly to Grg3. Overall, we find that Grg3 and Grg1 establish a monohormonal β-cell identity, and Groucho family members may be useful tools or markers for making functional β-cells.

Osteocalcin promotes β-cell proliferation during development and adulthood through Gprc6a

Expanding β-cell mass through β-cell proliferation is considered a potential therapeutic approach to treat β-cell failure in diabetic patients. A necessary step toward achieving this goal is to identify signaling pathways that regulate β-cell proliferation in vivo. Here we show that osteocalcin, a bone-derived hormone, regulates β-cell replication in a cyclin D1-dependent manner by signaling through the Gprc6a receptor expressed in these cells. Accordingly, mice lacking Gprc6a in the β-cell lineage only are glucose intolerant due to an impaired ability to produce insulin. Remarkably, this regulation occurs during both the perinatal peak of β-cell proliferation and in adulthood. Hence, the loss of osteocalcin/Gprc6a signaling has a profound effect on β-cell mass accrual during late pancreas morphogenesis. This study extends the endocrine role of osteocalcin to the developmental period and establishes osteocalcin/Gprc6a signaling as a major regulator of β-cell endowment that can become a potential target for β-cell proliferative therapies.

Retinoblastoma tumor suppressor protein in pancreatic progenitors controls α- and β-cell fate

Pancreatic endocrine cells expand rapidly during embryogenesis by neogenesis and proliferation, but during adulthood, islet cells have a very slow turnover. Disruption of murine retinoblastoma tumor suppressor protein (Rb) in mature pancreatic β-cells has a limited effect on cell proliferation. Here we show that deletion of Rb during embryogenesis in islet progenitors leads to an increase in the neurogenin 3-expressing precursor cell population, which persists in the postnatal period and is associated with increased β-cell mass in adults. In contrast, Rb-deficient islet precursors, through repression of the cell fate factor aristaless related homeobox, result in decreased α-cell mass. The opposing effect on survival of Rb-deficient α- and β-cells was a result of opposing effects on p53 in these cell types. As a consequence, loss of Rb in islet precursors led to a reduced α- to β-cell ratio, leading to improved glucose homeostasis and protection against diabetes.

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

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

Targeted overexpression of CKI-insensitive cyclin-dependent kinase 4 increases functional β-cell number through enhanced self-replication in zebrafish

β-Cells of the islet of Langerhans produce insulin to maintain glucose homeostasis. Self-replication of β-cells is the predominant mode of postnatal β-cell production in mammals, with about 20% of rodent β cells dividing in a 24-hour period. However, replicating β-cells are rare in adults. Induction of self-replication of existing β-cells is a potential treatment for diabetes. In zebrafish larvae, β-cells rarely self-replicate, even under conditions that favor β-cell genesis such overnutrition and β-cell ablation. It is not clear why larval β-cells are refractory to replication. In this study, we tested the hypothesis that insufficient activity of cyclin-dependent kinase 4 may be responsible for the low replication rate by ectopically expressing in β-cells a mutant CDK4 (CDK4(R24C)) that is insensitive to inhibition by cyclin-dependent kinase inhibitors. Our data show that expression of CDK4(R24C) in β-cells enhanced β-cell replication. CDK4(R24C) also dampened compensatory β-cell neogenesis in larvae and improved glucose tolerance in adult zebrafish. Our data indicate that CDK4 inhibition contributes to the limited β-cell replication in larval zebrafish. To our knowledge, this is the first example of genetically induced β-cell replication in zebrafish.

Novel pancreatic endocrine maturation pathways identified by genomic profiling and causal reasoning

We have used a previously unavailable model of pancreatic development, derived in vitro from human embryonic stem cells, to capture a time-course of gene, miRNA and histone modification levels in pancreatic endocrine cells. We investigated whether it is possible to better understand, and hence control, the biological pathways leading to pancreatic endocrine formation by analysing this information and combining it with the available scientific literature to generate models using a casual reasoning approach. We show that the embryonic stem cell differentiation protocol is highly reproducible in producing endocrine precursor cells and generates cells that recapitulate many aspects of human embryonic pancreas development, including maturation into functional endocrine cells when transplanted into recipient animals. The availability of whole genome gene and miRNA expression data from the early stages of human pancreatic development will be of great benefit to those in the fields of developmental biology and diabetes research. Our causal reasoning algorithm suggested the involvement of novel gene networks, such as NEUROG3/E2F1/KDM5B and SOCS3/STAT3/IL-6, in endocrine cell development We experimentally investigated the role of the top-ranked prediction by showing that addition of exogenous IL-6 could affect the expression of the endocrine progenitor genes NEUROG3 and NKX2.2.

Attenuation of the retinoblastoma pathway in pancreatic neuroendocrine tumors due to increased cdk4/cdk6

PURPOSE

In mice, genetic changes that inactivate the retinoblastoma tumor suppressor pathway often result in pancreatic neuroendocrine tumors (Pan-NETs). Conversely, in humans with this disease, mutations in genes of the retinoblastoma pathway have rarely been detected, even in genome-wide sequencing studies. In this study, we took a closer look at the role of the retinoblastoma pathway in human Pan-NETs.

EXPERIMENTAL DESIGN

Pan-NET tumors from 92 patients were subjected to immunohistochemical staining for markers of the retinoblastoma pathway. To search for amplifications of retinoblastoma pathway genes, genomic DNAs from 26 tumors were subjected to copy number analysis. Finally, a small-molecule activator of the retinoblastoma pathway was tested for effects on the growth of two Pan-NET cell lines.

RESULTS

A majority of tumors expressed high amounts of Cdk4 or its partner protein cyclin D1. High amounts of phosphorylated Rb1 were present in tumors that expressed high levels of Cdk4 or cyclin D1. The copy numbers of Cdk4 or the analogous kinase gene Cdk6 were increased in 19% of the tumors. Growth of the human Pan-NET cell line QGP1 was inhibited in a xenograft mouse model by the Cdk4/6 inhibitor, PD 0332991, which reactivates the retinoblastoma pathway.

CONCLUSIONS

Inactivation of the retinoblastoma pathway was indicated for most Pan-NETs. Gene amplification and overexpression of Cdk4 and Cdk6 suggests that patients with Pan-NETs may respond strongly to Cdk4/6 inhibitors that are entering clinical trials.

Agaricus bisporus lectins mediates islet β-cell proliferation through regulation of cell cycle proteins

This study was designed to determine the therapeutic effect of Agaricus bisporus lectins (ABL) by the regeneration of β-cells in mice following 70% partial pancreatectomy (PPx), and to explore the mechanisms of ABL-induced β-cell proliferation. Adult C57BL/6J mice were subjected to a 70% PPx operation or a sham operation, and mice received 10 mg/kg body weight of ABL or saline immediately after surgery. Blood glucose concentrations and insulin secretion levels were measured. To determine the growth rates of β-cells and duct cells, immunohistological analysis of pancreatic tissues was performed. Key cell cycle proteins and β-cell specific genes were measured by realtime polymerase chain reaction, Western blotting and immunohistological staining. In this study, a significant decrease in blood glucose concentrations, increase in glucose tolerance and expanded β-cell mass were observed in the ABL-treated mice. At the same time, after ABL treatment, increased β-cell proliferation rates were observed. Further studies on the expression of cyclin D1, cyclin D2 and Cdk4 demonstrated that these genes were significantly up-regulated in the ABL-treated mice. Meanwhile, Cdk4 activity was also enhanced. Moreover, the expression of PDX-1 (pancreatic and duodenal homeobox 1), Ngn3 (neurogenin 3), insulin, GLUT-1 (glucose transporter 1) and glucokinase was also increased in the ABL-treated mice. These findings demonstrate that ABL administration could partially reverse the impaired β-cell growth potential by regulating cell cycle proteins. Induction of islet β-cell proliferation by ABL suggests the therapeutic potential in preventing and/or treating diabetes.