The CDK4-pRB-E2F1 pathway controls insulin secretion

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.

Circulating microRNA (miRNA) Expression Profiling in Plasma of Patients with Gestational Diabetes Mellitus Reveals Upregulation of miRNA miR-330-3p

Gestational diabetes mellitus (GDM) is characterized by insulin resistance accompanied by low/absent beta-cell compensatory adaptation to the increased insulin demand. Although the molecular mechanisms and factors acting on beta-cell compensatory response during pregnancy have been partially elucidated and reported, those inducing an impaired beta-cell compensation and function, thus evolving in GDM, have yet to be fully addressed. MicroRNAs (miRNAs) are a class of small endogenous non-coding RNAs, which negatively modulate gene expression through their sequence-specific binding to 3'UTR of mRNA target. They have been described as potent modulators of cell survival and proliferation and, furthermore, as orchestrating molecules of beta-cell compensatory response and function in diabetes. Moreover, it has been reported that miRNAs can be actively secreted by cells and found in many biological fluids (e.g., serum/plasma), thus representing both optimal candidate disease biomarkers and mediators of tissues crosstalk(s). Here, we analyzed the expression profiles of circulating miRNAs in plasma samples obtained from n = 21 GDM patients and from n = 10 non-diabetic control pregnant women (24-33 weeks of gestation) using TaqMan array microfluidics cards followed by RT-real-time PCR single assay validation. The results highlighted the upregulation of miR-330-3p in plasma of GDM vs non-diabetics. Furthermore, the analysis of miR-330-3p expression levels revealed a bimodally distributed GDM patients group characterized by high or low circulating miR-330 expression and identified as GDM-miR-330^high and GDM-miR-330^low. Interestingly, GDM-miR-330^high subgroup retained lower levels of insulinemia, inversely correlated to miR-330-3p expression levels, and a significant higher rate of primary cesarean sections. Finally, miR-330-3p target genes analysis revealed major modulators of beta-cell proliferation and of insulin secretion, such as the experimentally validated genes E2F1 and CDC42 as well as AGT2R2, a gene involved in the differentiation of mature beta-cells. In conclusion, we demonstrated that plasma miR-330-3p could be of help in identifying GDM patients with potential worse gestational diabetes outcome; in GDM, miR-330-3p may directly be transferred from plasma to beta-cells thus modulating key target genes involved in proliferation, differentiation, and insulin secretion.

Fueling the Cell Division Cycle

Cell division is a complex process with high energy demands. However, how cells regulate the generation of energy required for DNA synthesis and chromosome segregation is not well understood. Recent data suggest that changes in mitochondrial dynamics and metabolic pathways such as oxidative phosphorylation (OXPHOS) and glycolysis crosstalk with, and are tightly regulated by, the cell division machinery. Alterations in energy availability trigger cell-cycle checkpoints, suggesting a bidirectional connection between cell division and general metabolism. Some of these connections are altered in human disease, and their manipulation may help in designing therapeutic strategies for specific diseases including cancer. We review here recent studies describing the control of metabolism by the cell-cycle machinery.

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.

Pancreatic β-Cells From Mice Offset Age-Associated Mitochondrial Deficiency With Reduced KATP Channel Activity

Aging is accompanied by impaired glucose homeostasis and an increased risk of type 2 diabetes, culminating in the failure of insulin secretion from pancreatic β-cells. To investigate the effects of age on β-cell metabolism, we established a novel assay to directly image islet metabolism with NAD(P)H fluorescence lifetime imaging (FLIM). We determined that impaired mitochondrial activity underlies an age-dependent loss of insulin secretion in human islets. NAD(P)H FLIM revealed a comparable decline in mitochondrial function in the pancreatic islets of aged mice (≥24 months), the result of 52% and 57% defects in flux through complex I and II, respectively, of the electron transport chain. However, insulin secretion and glucose tolerance are preserved in aged mouse islets by the heightened metabolic sensitivity of the β-cell triggering pathway, an adaptation clearly encoded in the metabolic and Ca(2+) oscillations that trigger insulin release (Ca(2+) plateau fraction: young 0.211 ± 0.006, aged 0.380 ± 0.007, P < 0.0001). This enhanced sensitivity is driven by a reduction in KATP channel conductance (diazoxide: young 5.1 ± 0.2 nS; aged 3.5 ± 0.5 nS, P < 0.01), resulting in an ∼2.8 mmol/L left shift in the β-cell glucose threshold. The results demonstrate how mice but not humans are able to successfully compensate for age-associated metabolic dysfunction by adjusting β-cell glucose sensitivity and highlight an essential mechanism for ensuring the maintenance of insulin secretion.

Islet biology, the CDKN2A/B locus and type 2 diabetes risk

Type 2 diabetes, fuelled by the obesity epidemic, is an escalating worldwide cause of personal hardship and public cost. Diabetes incidence increases with age, and many studies link the classic senescence and ageing protein p16(INK4A) to diabetes pathophysiology via pancreatic islet biology. Genome-wide association studies (GWASs) have unequivocally linked the CDKN2A/B locus, which encodes p16 inhibitor of cyclin-dependent kinase (p16(INK4A)) and three other gene products, p14 alternate reading frame (p14(ARF)), p15(INK4B) and antisense non-coding RNA in the INK4 locus (ANRIL), with human diabetes risk. However, the mechanism by which the CDKN2A/B locus influences diabetes risk remains uncertain. Here, we weigh the evidence that CDKN2A/B polymorphisms impact metabolic health via islet biology vs effects in other tissues. Structured in a bedside-to-bench-to-bedside approach, we begin with a summary of the evidence that the CDKN2A/B locus impacts diabetes risk and a brief review of the basic biology of CDKN2A/B gene products. The main emphasis of this work is an in-depth look at the nuanced roles that CDKN2A/B gene products and related proteins play in the regulation of beta cell mass, proliferation and insulin secretory function, as well as roles in other metabolic tissues. We finish with a synthesis of basic biology and clinical observations, incorporating human physiology data. We conclude that it is likely that the CDKN2A/B locus influences diabetes risk through both islet and non-islet mechanisms.

Transcriptome landmarks of the functional maturity of rat beta-cells, from lactation to adulthood

Research on the postnatal development of pancreatic beta-cells has become an important subject in recent years. Understanding the mechanisms that govern beta-cell postnatal maturation could bring new opportunities to therapeutic approaches for diabetes. The weaning period consists of a critical postnatal window for structural and physiologic maturation of rat beta-cells. To investigate transcriptome changes involved in the maturation of beta-cells neighboring this period, we performed microarray analysis in fluorescence-activated cell-sorted (FACS) beta-cell-enriched populations. Our results showed a variety of gene sets including those involved in the integration of metabolism, modulation of electrical activity, and regulation of the cell cycle that play important roles in the maturation process. These observations were validated using reverse hemolytic plaque assay, electrophysiological recordings, and flow cytometry analysis. Moreover, we suggest some unexplored pathways such as sphingolipid metabolism, insulin-vesicle trafficking, regulation of transcription/transduction by miRNA-30, trafficking proteins, and cell cycle proteins that could play important roles in the process mentioned above for further investigation.

Two-way communication between the metabolic and cell cycle machineries: the molecular basis

The relationship between cellular metabolism and the cell cycle machinery is by no means unidirectional. The ability of a cell to enter the cell cycle critically depends on the availability of metabolites. Conversely, the cell cycle machinery commits to regulating metabolic networks in order to support cell survival and proliferation. In this review, we will give an account of how the cell cycle machinery and metabolism are interconnected. Acquiring information on how communication takes place among metabolic signaling networks and the cell cycle controllers is crucial to increase our understanding of the deregulation thereof in disease, including cancer.

Non-canonical functions of cell cycle cyclins and cyclin-dependent kinases

The roles of cyclins and their catalytic partners, the cyclin-dependent kinases (CDKs), as core components of the machinery that drives cell cycle progression are well established. Increasing evidence indicates that mammalian cyclins and CDKs also carry out important functions in other cellular processes, such as transcription, DNA damage repair, control of cell death, differentiation, the immune response and metabolism. Some of these non-canonical functions are performed by cyclins or CDKs, independently of their respective cell cycle partners, suggesting that there was a substantial divergence in the functions of these proteins during evolution.

Regulation of cell polarity determinants by the Retinoblastoma tumor suppressor protein

In addition to their canonical roles in the cell cycle, RB family proteins regulate numerous developmental pathways, although the mechanisms remain obscure. We found that Drosophila Rbf1 associates with genes encoding components of the highly conserved apical-basal and planar cell polarity pathways, suggesting a possible regulatory role. Here, we show that depletion of Rbf1 in Drosophila tissues is indeed associated with polarity defects in the wing and eye. Key polarity genes aPKC, par6, vang, pk, and fmi are upregulated, and an aPKC mutation suppresses the Rbf1-induced phenotypes. RB control of cell polarity may be an evolutionarily conserved function, with important implications in cancer metastasis.

p16(Ink4a)-induced senescence of pancreatic beta cells enhances insulin secretion

Cellular senescence is thought to contribute to age-associated deterioration of tissue physiology. The senescence effector p16(Ink4a) is expressed in pancreatic beta cells during aging and limits their proliferative potential; however, its effects on beta cell function are poorly characterized. We found that beta cell-specific activation of p16(Ink4a) in transgenic mice enhances glucose-stimulated insulin secretion (GSIS). In mice with diabetes, this leads to improved glucose homeostasis, providing an unexpected functional benefit. Expression of p16(Ink4a) in beta cells induces hallmarks of senescence--including cell enlargement, and greater glucose uptake and mitochondrial activity--which promote increased insulin secretion. GSIS increases during the normal aging of mice and is driven by elevated p16(Ink4a) activity. We found that islets from human adults contain p16(Ink4a)-expressing senescent beta cells and that senescence induced by p16(Ink4a) in a human beta cell line increases insulin secretion in a manner dependent, in part, on the activity of the mechanistic target of rapamycin (mTOR) and the peroxisome proliferator-activated receptor (PPAR)-γ proteins. Our findings reveal a novel role for p16(Ink4a) and cellular senescence in promoting insulin secretion by beta cells and in regulating normal functional tissue maturation with age.

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.

CDK4 is an essential insulin effector in adipocytes

Insulin resistance is a fundamental pathogenic factor that characterizes various metabolic disorders, including obesity and type 2 diabetes. Adipose tissue contributes to the development of obesity-related insulin resistance through increased release of fatty acids, altered adipokine secretion, and/or macrophage infiltration and cytokine release. Here, we aimed to analyze the participation of the cyclin-dependent kinase 4 (CDK4) in adipose tissue biology. We determined that white adipose tissue (WAT) from CDK4-deficient mice exhibits impaired lipogenesis and increased lipolysis. Conversely, lipolysis was decreased and lipogenesis was increased in mice expressing a mutant hyperactive form of CDK4 (CDK4(R24C)). A global kinome analysis of CDK4-deficient mice following insulin stimulation revealed that insulin signaling is impaired in these animals. We determined that insulin activates the CCND3-CDK4 complex, which in turn phosphorylates insulin receptor substrate 2 (IRS2) at serine 388, thereby creating a positive feedback loop that maintains adipocyte insulin signaling. Furthermore, we found that CCND3 expression and IRS2 serine 388 phosphorylation are increased in human obese subjects. Together, our results demonstrate that CDK4 is a major regulator of insulin signaling in WAT.

E2F1 mediates sustained lipogenesis and contributes to hepatic steatosis

E2F transcription factors are known regulators of the cell cycle, proliferation, apoptosis, and differentiation. Here, we reveal that E2F1 plays an essential role in liver physiopathology through the regulation of glycolysis and lipogenesis. We demonstrate that E2F1 deficiency leads to a decrease in glycolysis and de novo synthesis of fatty acids in hepatocytes. We further demonstrate that E2F1 directly binds to the promoters of key lipogenic genes, including Fasn, but does not bind directly to genes encoding glycolysis pathway components, suggesting an indirect effect. In murine models, E2F1 expression and activity increased in response to feeding and upon insulin stimulation through canonical activation of the CDK4/pRB pathway. Moreover, E2F1 expression was increased in liver biopsies from obese, glucose-intolerant humans compared with biopsies from lean subjects. Finally, E2f1 deletion completely abrogated hepatic steatosis in different murine models of nonalcoholic fatty liver disease (NAFLD). In conclusion, our data demonstrate that E2F1 regulates lipid synthesis and glycolysis and thus contributes to the development of liver pathology.

Functional genomics of the CDKN2A/B locus in cardiovascular and metabolic disease: what have we learned from GWASs?

Genome-wide association studies (GWASs) provide an unprecedented opportunity to examine, on a large scale, the association of common genetic variants with complex diseases like type 2 diabetes (T2D) and cardiovascular disease (CVD), thus allowing the identification of new potential disease loci. Using this approach, numerous studies have associated SNPs on chromosome 9p21.3 situated near the cyclin-dependent kinase inhibitor 2A/B (CDKN2A/B) locus with the risk for coronary artery disease (CAD) and T2D. However, identifying the function of the nearby gene products (CDKN2A/B and ANRIL) in the pathophysiology of these conditions requires functional genomic studies. We review the current knowledge, from studies using human and mouse models, describing the function of CDKN2A/B gene products, which may mechanistically link the 9p21.3 risk locus with CVD and diabetes.

Diabetes mellitus--advances and challenges in human β-cell proliferation

The treatment of diabetes mellitus represents one of the greatest medical challenges of our era. Diabetes results from a deficiency or functional impairment of insulin-producing β cells, alone or in combination with insulin resistance. It logically follows that the replacement or regeneration of β cells should reverse the progression of diabetes and, indeed, this seems to be the case in humans and rodents. This concept has prompted attempts in many laboratories to create new human β cells using stem-cell strategies to transdifferentiate or reprogramme non-β cells into β cells or to discover small molecules or other compounds that can induce proliferation of human β cells. This latter approach has shown promise, but has also proven particularly challenging to implement. In this Review, we discuss the physiology of normal human β-cell replication, the molecular mechanisms that regulate the cell cycle in human β cells, the upstream intracellular signalling pathways that connect them to cell surface receptors on β cells, the epigenetic mechanisms that control human β-cell proliferation and unbiased approaches for discovering novel molecules that can drive human β-cell proliferation. Finally, we discuss the potential and challenges of implementing strategies that replace or regenerate β cells.

Emerging links between E2F control and mitochondrial function

The family of E2F transcription factors is the key downstream target of the retinoblastoma tumor suppressor protein (pRB), which is frequently inactivated in human cancer. E2F is best known for its role in cell-cycle regulation and triggering apoptosis. However, E2F binds to thousands of genes and, thus, could directly influence a number of biologic processes. Given the plethora of potential E2F targets, the major challenge in the field is to identify specific processes in which E2F plays a functional role and the contexts in which a particular subset of E2F targets dictates a biologic outcome. Recent studies implicated E2F in regulation of expression of mitochondria-associated genes. The loss of such regulation results in severe mitochondrial defects. The consequences become evident during irradiation-induced apoptosis, where E2F-deficient cells are insensitive to cell death despite induction of canonical apoptotic genes. Thus, this novel function of E2F may have a major impact on cell viability, and it is independent of induction of apoptotic genes. Here, we discuss the implications of these findings in cancer biology.

Improved metabolic regulation is associated with retinoblastoma protein gene haploinsufficiency in mice

Retinoblastoma protein (pRb) is involved in the control of energy metabolism, and its inactivation protects mice against high-fat diet-induced diabesity. Here, we tested the hypothesis that partial deficiency in the Rb gene could confer metabolic advantages in front of acute challenges to metabolism and as mice age on a regular diet. Rb haploinsufficient (Rb(+/-)) mice and wild-type (WT) littermates were studied from weaning and characterized at 1.5-2.5 mo of age (young adults) and 6-7.5 mo of age (mature adults). Whereas no differences in body weight or composition were observed at young age, mature adult Rb(+/-) mice were leaner than WT littermates, displaying 36% reduced body fat content. At both ages studied, Rb(+/-) mice displayed improved blood lipids, enhanced sensitivity to the blood glucose-lowering effect of insulin and to the anorectic effect of leptin, and a reduced respiratory exchange ratio, indicative of an increased use of fatty acids as a fuel. Insulin sensitivity and oral fat tolerance were better maintained with age in the Rb(+/-) than the WT mice. Mature adult Rb(+/-) mice displayed gene expression changes consistent with increased fatty acid oxidation in white adipose tissue and skeletal muscle and paramount signs of browning in the inguinal white adipose tissue. In conclusion, Rb haploinsufficiency provides metabolic advantages in front of acute metabolic stressors and ameliorates body fat gain and metabolic impairments that normally accompany transition from young to mature adult age.

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.

Cdkn2a/p16Ink4a regulates fasting-induced hepatic gluconeogenesis through the PKA-CREB-PGC1α pathway

Type 2 diabetes (T2D) is hallmarked by insulin resistance, impaired insulin secretion, and increased hepatic glucose production. The worldwide increasing prevalence of T2D calls for efforts to understand its pathogenesis in order to improve disease prevention and management. Recent genome-wide association studies have revealed strong associations between the CDKN2A/B locus and T2D risk. The CDKN2A/B locus contains genes encoding cell cycle inhibitors, including p16(Ink4a), which have not yet been implicated in the control of hepatic glucose homeostasis. Here, we show that p16(Ink4a) deficiency enhances fasting-induced hepatic glucose production in vivo by increasing the expression of key gluconeogenic genes. p16(Ink4a) downregulation leads to an activation of PKA-CREB-PGC1α signaling through increased phosphorylation of PKA regulatory subunits. Taken together, these results provide evidence that p16(Ink4a) controls fasting glucose homeostasis and could as such be involved in T2D development.

Targeting the Warburg effect with a novel glucose transporter inhibitor to overcome gemcitabine resistance in pancreatic cancer cells

Gemcitabine resistance remains a significant clinical challenge. Here, we used a novel glucose transporter (Glut) inhibitor, CG-5, as a proof-of-concept compound to investigate the therapeutic utility of targeting the Warburg effect to overcome gemcitabine resistance in pancreatic cancer. The effects of gemcitabine and/or CG-5 on viability, survival, glucose uptake and DNA damage were evaluated in gemcitabine-sensitive and gemcitabine-resistant pancreatic cancer cell lines. Mechanistic studies were conducted to determine the molecular basis of gemcitabine resistance and the mechanism of CG-5-induced sensitization to gemcitabine. The effects of CG-5 on gemcitabine sensitivity were investigated in a xenograft tumor model of gemcitabine-resistant pancreatic cancer. In contrast to gemcitabine-sensitive pancreatic cancer cells, the resistant Panc-1 and Panc-1(GemR) cells responded to gemcitabine by increasing the expression of ribonucleotide reductase M2 catalytic subunit (RRM2) through E2F1-mediated transcriptional activation. Acting as a pan-Glut inhibitor, CG-5 abrogated this gemcitabine-induced upregulation of RRM2 through decreased E2F1 expression, thereby enhancing gemcitabine-induced DNA damage and inhibition of cell survival. This CG-5-induced inhibition of E2F1 expression was mediated by the induction of a previously unreported E2F1-targeted microRNA, miR-520f. The addition of oral CG-5 to gemcitabine therapy caused greater suppression of Panc-1(GemR) xenograft tumor growth in vivo than either drug alone. Glut inhibition may be an effective strategy to enhance gemcitabine activity for the treatment of pancreatic cancer.

Metabolic adaptation to cancer growth: from the cell to the organism

Tumour cells proliferate much faster than normal cells; nearly all anticancer treatments are toxic to both cell types, limiting their efficacy. The altered metabolism resulting from cellular transformation and cancer progression supports cellular proliferation and survival, but leaves cancer cells dependent on a continuous supply of energy and nutrients. Hence, many metabolic enzymes have become targets for new cancer therapies. In addition to its well-described roles in cell-cycle progression and cancer, the cyclin/CDK-pRB-E2F1 pathway contributes to lipid synthesis, glucose production, insulin secretion, and glycolytic metabolism, with strong effects on overall metabolism. Notably, these cell-cycle regulators trigger the adaptive "metabolic switch" that underlies proliferation.

Role of Ink4a/Arf locus in beta cell mass expansion under physiological and pathological conditions

The ARF/INK4A (Cdkn2a) locus includes the linked tumour suppressor genes p16INK4a and p14ARF (p19ARF in mice) that trigger the antiproliferative activities of both RB and p53. With beta cell self-replication being the primary source for new beta cell generation in adult animals, the network by which beta cell replication could be increased to enhance beta cell mass and function is one of the approaches in diabetes research. In this review, we show a general view of the regulation points at transcriptional and posttranslational levels of Cdkn2a locus. We describe the molecular pathways and functions of Cdkn2a in beta cell cycle regulation. Given that aging reveals increased p16Ink4a levels in the pancreas that inhibit the proliferation of beta cells and decrease their ability to respond to injury, we show the state of the art about the role of this locus in beta cell senescence and diabetes development. Additionally, we focus on two approaches in beta cell regeneration strategies that rely on Cdkn2a locus negative regulation: long noncoding RNAs and betatrophin.

The transcriptional co-regulator HCF-1 is required for INS-1 β-cell glucose-stimulated insulin secretion

The transcriptional co-regulator host cell factor-1 (HCF-1) plays critical roles in promoting cell cycle progression in diverse cell types, and in maintaining self-renewal of embryonic stem cells, but its role in pancreatic β-cell function has not been investigated. Immunhistochemistry of mouse pancreas revealed nuclear expression of HCF-1 in pancreatic islets. Reducing HCF-1 expression in the INS-1 pancreatic β-cell line resulted in reduced cell proliferation, reduced glucose-stimulated insulin secretion, and reduced expression of the critical β-cell transcription factor Pdx1. HCF-1 is a known co-activator of the E2F1 transcription factor, and loss of E2F1 results in pancreatic β-cell dysfunction and reduced expression of Pdx1. Therefore we wondered whether HCF-1 might be required for E2F1 regulation of Pdx1. Chromatin immunoprecipitation experiments revealed that HCF-1 and E2F1 co-localize to the Pdx1 promoter. These results indicate that HCF-1 represents a novel transcriptional regulator required for maintaining pancreatic β-cell function.

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.

The multiple connections between pRB and cell metabolism

The pRB tumor suppressor is traditionally seen as an important regulator of the cell cycle. pRB represses the transcriptional activation of a diverse set of genes by the E2F transcription factors and prevents inappropriate S-phase entry. Advances in our understanding of pRB have documented roles that extend beyond the cell cycle and this review summarizes recent studies that link pRB to the control of cell metabolism. pRB has been shown to regulate glucose tolerance, mitogenesis, glutathione synthesis, and the expression of genes involved in central carbon metabolism. Several studies have demonstrated that pRB directly targets a set of genes that are crucial for nucleotide metabolism, and this seems likely to represent one of the ways by which pRB influences the G1/S-phase transition and S-phase progression.

PPARβ Regulates Liver Regeneration by Modulating Akt and E2f Signaling

The current study tests the hypothesis that peroxisome proliferator-activated receptor β (PPARβ) has a role in liver regeneration due to its effect in regulating energy homeostasis and cell proliferation. The role of PPARβ in liver regeneration was studied using two-third partial hepatectomy (PH) in Wild-type (WT) and PPARβ-null (KO) mice. In KO mice, liver regeneration was delayed and the number of Ki-67 positive cells reached the peak at 60 hr rather than at 36-48 hr after PH shown in WT mice. RNA-sequencing uncovered 1344 transcriptomes that were differentially expressed in regenerating WT and KO livers. About 70% of those differentially expressed genes involved in glycolysis and fatty acid synthesis pathways failed to induce during liver regeneration due to PPARβ deficiency. The delayed liver regeneration in KO mice was accompanied by lack of activation of phosphoinositide-dependent kinase 1 (PDK1)/Akt. In addition, cell proliferation-associated increase of genes encoding E2f transcription factor (E2f) 1-2 and E2f7-8 as well as their downstream target genes were not noted in KO livers 36-48 hr after PH. E2fs have dual roles in regulating metabolism and proliferation. Moreover, transient steatosis was only found in WT, but not in KO mice 36 hr after PH. These data suggested that PPARβ-regulated PDK1/Akt and E2f signaling that controls metabolism and proliferation is involved in the normal progression of liver regeneration.

Metabolic control in cancer cells

An important hallmark of cancer cells is a profound change in metabolism. Indeed, most tumor cells are characterized by higher rates of glycolysis, lactate production, and biosynthesis of lipids and other macromolecules. Our group, among others, has previously demonstrated a close relationship between metabolic responses and proliferative stimuli, showing that cell cycle regulators have a major role in the control of metabolism. Changes in this coordinated response might lead to abnormal metabolic changes during tumor development and cancer progression. In this paper we review the dual role of cell cycle regulators in the control of both proliferation and metabolism in normal and in cancer cells. We show participation of the E2F1-CDK4 axis in the modulation of oxidative metabolism, in the positive regulation of lipid synthesis, and the regulation glycolysis. These three metabolic pathways are, interestingly fundamental in providing synthetic processes, energy production and cell signaling events, which are crucial factors for cancer cell survival.

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.

Increased dosage of Ink4/Arf protects against glucose intolerance and insulin resistance associated with aging

Recent genome-wide association studies have linked type-2 diabetes mellitus to a genomic region in chromosome 9p21 near the Ink4/Arf locus, which encodes tumor suppressors that are up-regulated in a variety of mammalian organs during aging. However, it is unclear whether the susceptibility to type-2 diabetes is associated with altered expression of the Ink4/Arf locus. In the present study, we investigated the role of Ink4/Arf in age-dependent alterations of insulin and glucose homeostasis using Super-Ink4/Arf mice which bear an extra copy of the entire Ink4/Arf locus. We find that, in contrast to age-matched wild-type controls, Super-Ink4/Arf mice do not develop glucose intolerance with aging. Insulin tolerance tests demonstrated increased insulin sensitivity in Super-Ink4/Arf compared with wild-type mice, which was accompanied by higher activation of the insulin receptor substrate (IRS)-PI3K-AKT pathway in liver, skeletal muscle and heart. Glucose uptake studies in Super-Ink4/Arf mice showed a tendency toward increased (18)F-fluorodeoxyglucose uptake in skeletal muscle compared with wild-type mice (P = 0.079). Furthermore, a positive correlation between glucose uptake and baseline glucose levels was observed in Super-Ink4/Arf mice (P < 0.008) but not in wild-type mice. Our studies reveal a protective role of the Ink4/Arf locus against the development of age-dependent insulin resistance and glucose intolerance.

Re-thinking cell cycle regulators: the cross-talk with metabolism

Analysis of genetically engineered mice deficient in cell cycle regulators, including E2F1, cdk4, and pRB, showed that the major phenotypes are metabolic perturbations. These key cell cycle regulators contribute to lipid synthesis, glucose production, insulin secretion, and glycolytic metabolism. It has been shown that deregulation of these pathways can lead to metabolic perturbations and related metabolic diseases, such as obesity and type II diabetes. The cyclin–cdk–Rb–E2F1 pathway regulates adipogenesis in addition to its well-described roles in cell cycle regulation and cancer. It was also shown that E2F1 directly participates in the regulation of pancreatic growth and function. Similarly, cyclin D3, cdk4, and cdk9 are also adipogenic factors with strong effects on whole organism metabolism. These examples support the emerging notion that cell cycle regulatory proteins also modulate metabolic processes. These cell cycle regulators are activated by insulin and glucose, even in non-proliferating cells. Most importantly, these cell cycle regulators trigger the adaptive metabolic switch that normal and cancer cells require in order to proliferate. These changes include increased lipid synthesis, decreased oxidative metabolism, and increased glycolytic metabolism. In summary, these factors are essential regulators of anabolic biosynthetic processes, blocking at the same time oxidative and catabolic pathways, which is reminiscent of cancer cell metabolism.

E2f1-deficient NOD/SCID mice have dry mouth due to a change of acinar/duct structure and the down-regulation of AQP5 in the salivary gland

Non-obese diabetic (NOD) mice have been used as a model for dry mouth. NOD mice lacking the gene encoding E2f1, a transcription factor, develop hyposalivation more rapidly progressively than control NOD mice. However, the model mice are associated with an underlying disease such as diabetes. We have now established E2f1-deficient NOD/severe combined immunodeficiency disease (NOD/SCID.E2f1(-/-)) mice to avoid the development of diabetes (Matsui-Inohara et al., Exp Biol Med (Maywood) 234(12):1525-1536, 2009). In this study, we investigated the pathophysiological features of dry mouth using NOD/SCID.E2f1(-/-) mice. In NOD/SCID.E2f1(-/-) mice, the volume of secreted saliva stimulated with pilocarpine is about one third that of control NOD/SCID mice. In behavioral analysis, NOD/SCID.E2f1(-/-) mice drank plenty of water when they ate dry food, and the frequency and time of water intake were almost double compared with control NOD/SCID mice. Histological analysis of submandibular glands with hematoxylin-eosin stain revealed that NOD/SCID.E2f1(-/-) mice have more ducts than NOD/SCID mice. In western blot analysis, the expression of aquaporin 5 (AQP5), a marker of acinar cells, in parotid and in submandibular glands of NOD/SCID.E2f1(-/-) mice was lower than in NOD/SCID mice. Immunohistochemical analysis of parotid and submandibular acini revealed that the localization of AQP5 in NOD/SCID.E2f1(-/-) mice differs from that in NOD/SCID mice; AQP5 was leaky and diffusively localized from the apical membrane to the cytosol in NOD/SCID.E2f1(-/-) mice. The ubiquitination of AQP5 was detected in submandibular glands of NOD/SCID.E2f1(-/-) mice. These findings suggest that the change of acinar/duct structure and the down-regulation of AQP5 in the salivary gland cause the pathogenesis of hyposalivation in NOD/SCID.E2f1(-/-) mice.

Evidence for autoregulation and cell signaling pathway regulation from genome-wide binding of the Drosophila retinoblastoma protein

The retinoblastoma (RB) tumor suppressor protein is a transcriptional cofactor with essential roles in cell cycle and development. Physical and functional targets of RB and its paralogs p107/p130 have been studied largely in cultured cells, but the full biological context of this family of proteins' activities will likely be revealed only in whole organismal studies. To identify direct targets of the major Drosophila RB counterpart in a developmental context, we carried out ChIP-Seq analysis of Rbf1 in the embryo. The association of the protein with promoters is developmentally controlled; early promoter access is globally inhibited, whereas later in development Rbf1 is found to associate with promoter-proximal regions of approximately 2000 genes. In addition to conserved cell-cycle-related genes, a wholly unexpected finding was that Rbf1 targets many components of the insulin, Hippo, JAK/STAT, Notch, and other conserved signaling pathways. Rbf1 may thus directly affect output of these essential growth-control and differentiation pathways by regulation of expression of receptors, kinases and downstream effectors. Rbf1 was also found to target multiple levels of its own regulatory hierarchy. Bioinformatic analysis indicates that different classes of genes exhibit distinct constellations of motifs associated with the Rbf1-bound regions, suggesting that the context of Rbf1 recruitment may vary within the Rbf1 regulon. Many of these targeted genes are bound by Rbf1 homologs in human cells, indicating that a conserved role of RB proteins may be to adjust the set point of interlinked signaling networks essential for growth and development.

Transfection of a eukaryotic vector expressing a mutant CDK4 up-regulates POLD1 expression in SMMC-7702 cells

AIM: To construct a eukaryotic expression vector encoding a mutant CDK4 protein and to investigate the effect of transfection of this vector on POLD1 expression in SMMC-7702 cells.

METHODS: The mutant CDK4 gene was amplified by RT-PCR from total RNA isolated from the human hepatocarcinoma cell line SMMC-7721, digested, and inserted into the eukaryotic expression vector pEGFP-C1. The resultant recombinant plasmid was confirmed by sequencing. After the recombinant plasmid was transfected into SMMC-7702 cells using Lipofectamine 2000, the expression of fusion protein was observed by fluorescence microscopy, and expression of CDK4 and POLD1 mRNAs was detected by real-time PCR.

RESULTS: The eukaryotic expression plasmid GFP-CDK4 was successfully constructed. The mutant CDK4 gene contained 5 base mutation sites, 4 base insertions and 2 deletions, which caused 7 amino acids to change. Compared to non-tranfected cells or cells transfected with the pEGFP-C1 vector, cell proliferation was significantly higher in cells transfected with the recombinant vector (0.826 ± 0.08 vs 0.596 ± 0.06, 0.609 ± 0.10, F = 7.033, P < 0.05). The expression levels of CDK4 and POLD1 genes in cells transfected with the recombinant vector was significantly higher than those in the two control groups (1.94 ± 0 .11 vs 1.01 ± 0.00, 1.05 ± 0.12, F = 54.046, P < 0.01; 0.54 ± 0.04 vs 0.30 ± 0.07, 0.25 ± 0.06, F = 11.788, P < 0.05). Similar results were also obtained for the protein expression levels of CDK4 (0.65 ± 0.03 vs 0.41 ± 0.03, 0.39 ± 0.05, F = 14.665, P < 0.05) and P125 (0.54 ± 0.04 vs 0.30 ± 0.07, 0.25 ± 0.06, F = 11.788, P < 0.05).

CONCLUSION: Tranfection of the eukaryotic expression plasmid GFP-CDK4 significantly increases the proliferation and invasion of SMMC-7702 cells possibly by up-regulating POLD1 expression.

Bone marrow p16INK4a-deficiency does not modulate obesity, glucose homeostasis or atherosclerosis development

Objective

A genomic region near the CDKN2A locus, encoding p16^INK4a, has been associated to type 2 diabetes and atherosclerotic vascular disease, conditions in which inflammation plays an important role. Recently, we found that deficiency of p16^INK4a results in decreased inflammatory signaling in murine macrophages and that p16^INK4a influences the phenotype of human adipose tissue macrophages. Therefore, we investigated the influence of immune cell p16^INK4a on glucose tolerance and atherosclerosis in mice.

Methods and Results

Bone marrow p16^INK4a-deficiency in C57Bl6 mice did not influence high fat diet-induced obesity nor plasma glucose and lipid levels. Glucose tolerance tests showed no alterations in high fat diet-induced glucose intolerance. While bone marrow p16^INK4a-deficiency did not affect the gene expression profile of adipose tissue, hepatic expression of the alternative markers Chi3l3, Mgl2 and IL10 was increased and the induction of pro-inflammatory Nos2 was restrained on the high fat diet. Bone marrow p16^INK4a-deficiency in low density lipoprotein receptor-deficient mice did not affect western diet-induced atherosclerotic plaque size or morphology. In line, plasma lipid levels remained unaffected and p16^INK4a-deficient macrophages displayed equal cholesterol uptake and efflux compared to wild type macrophages.

Conclusion

Bone marrow p16^INK4a-deficiency does not affect plasma lipids, obesity, glucose tolerance or atherosclerosis in mice.

PPARγ contributes to PKM2 and HK2 expression in fatty liver

Rapidly proliferating cells promote glycolysis in aerobic conditions, to increase growth rate. Expression of specific glycolytic enzymes, namely pyruvate kinase M2 and hexokinase 2, concurs to this metabolic adaptation, as their kinetics and intracellular localization favour biosynthetic processes required for cell proliferation. Intracellular factors regulating their selective expression remain largely unknown. Here we show that the peroxisome proliferator-activated receptor gamma transcription factor and nuclear hormone receptor contributes to selective pyruvate kinase M2 and hexokinase 2 gene expression in PTEN-null fatty liver. Peroxisome proliferator-activated receptor gamma expression, liver steatosis, shift to aerobic glycolysis and tumorigenesis are under the control of the Akt2 kinase in PTEN-null mouse livers. Peroxisome proliferator-activated receptor gamma binds to hexokinase 2 and pyruvate kinase M promoters to activate transcription. In vivo rescue of peroxisome proliferator-activated receptor gamma activity causes liver steatosis, hypertrophy and hyperplasia. Our data suggest that therapies with the insulin-sensitizing agents and peroxisome proliferator-activated receptor gamma agonists, thiazolidinediones, may have opposite outcomes depending on the nutritional or genetic origins of liver steatosis.

Mitochondrial T3 receptor p43 regulates insulin secretion and glucose homeostasis

Thyroid hormone is a major determinant of energy expenditure and a key regulator of mitochondrial activity. We have previously identified a mitochondrial triiodothyronine receptor (p43) that acts as a mitochondrial transcription factor of the organelle genome, which leads, in vitro and in vivo, to a stimulation of mitochondrial biogenesis. Here we generated mice specifically lacking p43 to address its physiological influence. We found that p43 is required for normal glucose homeostasis. The p43(-/-) mice had a major defect in insulin secretion both in vivo and in isolated pancreatic islets and a loss of glucose-stimulated insulin secretion. Moreover, a high-fat/high-sucrose diet elicited more severe glucose intolerance than that recorded in normal animals. In addition, we observed in p43(-/-) mice both a decrease in pancreatic islet density and in the activity of complexes of the respiratory chain in isolated pancreatic islets. These dysfunctions were associated with a down-regulation of the expression of the glucose transporter Glut2 and of Kir6.2, a key component of the K(ATP) channel. Our findings establish that p43 is an important regulator of glucose homeostasis and pancreatic β-cell function and provide evidence for the first time of a physiological role for a mitochondrial endocrine receptor.

RETINOBLASTOMA-RELATED PROTEIN controls the transition to autotrophic plant development

Seedling establishment is a crucial phase during plant development when the germinating heterotrophic embryo switches to autotrophic growth and development. Positive regulators of embryonic development need to be turned off, while the cell cycle machinery is activated to allow cell cycle entry and organ primordia initiation. However, it is not yet understood how the molecular mechanisms responsible for the onset of cell division, metabolism changes and cell differentiation are coordinated during this transition. Here, we demonstrate that the Arabidopsis thaliana RETINOBLASTOMA-RELATED protein (RBR) ortholog of the animal tumor suppressor retinoblastoma (pRB) not only controls the expression of cell cycle-related genes, but is also required for persistent shut-down of late embryonic genes by increasing their histone H3K27 trimethylation. Seedlings with reduced RBR function arrest development after germination, and stimulation with low amounts of sucrose induces transcription of late embryonic genes and causes ectopic cell division. Our results suggest a model in which RBR acts antagonistically to sucrose by negatively regulating the cell cycle and repressing embryonic genes. Thus, RBR is a positive regulator of the developmental switch from embryonic heterotrophic growth to autotrophic growth. This establishes RBR as a new integrator of metabolic and developmental decisions.

Physiological relevance of cell cycle kinases

The basic biology of the cell division cycle and its control by protein kinases was originally studied through genetic and biochemical studies in yeast and other model organisms. The major regulatory mechanisms identified in this pioneer work are conserved in mammals. However, recent studies in different cell types or genetic models are now providing a new perspective on the function of these major cell cycle regulators in different tissues. Here, we review the physiological relevance of mammalian cell cycle kinases such as cyclin-dependent kinases (Cdks), Aurora and Polo-like kinases, and mitotic checkpoint regulators (Bub1, BubR1, and Mps1) as well as other less-studied enzymes such as Cdc7, Nek proteins, or Mastl and their implications in development, tissue homeostasis, and human disease. Among these functions, the control of self-renewal or asymmetric cell division in stem/progenitor cells and the ability to regenerate injured tissues is a central issue in current research. In addition, many of these proteins play previously unexpected roles in metabolism, cardiovascular function, or neuron biology. The modulation of their enzymatic activity may therefore have multiple therapeutic benefits in human disease.

E2F transcription factor-1 regulates oxidative metabolism

Cells respond to stress by coordinating proliferative and metabolic pathways. Starvation restricts cell proliferative (glycolytic) and activates energy productive (oxidative) pathways. Conversely, cell growth and proliferation require increased glycolytic and decreased oxidative metabolism levels. E2F transcription factors regulate both proliferative and metabolic genes. E2Fs have been implicated in the G1/S cell-cycle transition, DNA repair, apoptosis, development and differentiation. In pancreatic β-cells, E2F1 gene regulation facilitated glucose-stimulated insulin secretion. Moreover, mice lacking E2F1 (E2f1(-/-)) were resistant to diet-induced obesity. Here, we show that E2F1 coordinates cellular responses by acting as a regulatory switch between cell proliferation and metabolism. In basal conditions, E2F1 repressed key genes that regulate energy homeostasis and mitochondrial functions in muscle and brown adipose tissue. Consequently, E2f1(-/-) mice had a marked oxidative phenotype. An association between E2F1 and pRB was required for repression of genes implicated in oxidative metabolism. This repression was alleviated in a constitutively active CDK4 (CDK4(R24C)) mouse model or when adaptation to energy demand was required. Thus, E2F1 represents a metabolic switch from oxidative to glycolytic metabolism that responds to stressful conditions.

RB1, development, and cancer

The RB1 gene is the first tumor suppressor gene identified whose mutational inactivation is the cause of a human cancer, the pediatric cancer retinoblastoma. The 25 years of research since its discovery has not only illuminated a general role for RB1 in human cancer, but also its critical importance in normal development. Understanding the molecular function of the RB1 encoded protein, pRb, is a long-standing goal that promises to inform our understanding of cancer, its relationship to normal development, and possible therapeutic strategies to combat this disease. Achieving this goal has been difficult, complicated by the complexity of pRb and related proteins. The goal of this review is to explore the hypothesis that, at its core, the molecular function of pRb is to dynamically regulate the location-specific assembly or disassembly of protein complexes on the DNA in response to the output of various signaling pathways. These protein complexes participate in a variety of molecular processes relevant to DNA including gene transcription, DNA replication, DNA repair, and mitosis. Through regulation of these processes, RB1 plays a uniquely prominent role in normal development and cancer.