Cyclin D2 protein stability is regulated in pancreatic beta-cells

Nuclear focal adhesion kinase induces APC/C activator protein CDH1-mediated cyclin-dependent kinase 4/6 degradation and inhibits melanoma proliferation

Dysregulation of cyclin-dependent kinases (CDKs) can promote unchecked cell proliferation and cancer progression. Although focal adhesion kinase (FAK) contributes to regulating cell cycle progression, the exact molecular mechanism remains unclear. Here, we found that FAK plays a key role in cell cycle progression potentially through regulation of CDK4/6 protein expression. We show that FAK inhibition increased its nuclear localization and induced G1 arrest in B16F10 melanoma cells. Mechanistically, we demonstrate nuclear FAK associated with CDK4/6 and promoted their ubiquitination and proteasomal degradation through recruitment of CDC homolog 1 (CDH1), an activator and substrate recognition subunit of the anaphase-promoting complex/cyclosome E3 ligase complex. We found the FAK N-terminal FERM domain acts as a scaffold to bring CDK4/6 and CDH1 within close proximity. However, overexpression of nonnuclear-localizing mutant FAK FERM failed to function as a scaffold for CDK4/6 and CDH1. Furthermore, shRNA knockdown of CDH1 increased CDK4/6 protein expression and blocked FAK inhibitor-induced reduction of CDK4/6 in B16F10 cells. In vivo, we show that pharmacological FAK inhibition reduced B16F10 tumor size, correlating with increased FAK nuclear localization and decreased CDK4/6 expression compared with vehicle controls. In patient-matched healthy skin and melanoma biopsies, we found FAK was mostly inactive and nuclear localized in healthy skin, whereas melanoma lesions showed increased active cytoplasmic FAK and elevated CDK4 expression. Taken together, our data demonstrate that FAK inhibition blocks tumor proliferation by inducing G1 arrest, in part through decreased CDK4/6 protein stability by nuclear FAK.

GIGYF1 loss of function is associated with clonal mosaicism and adverse metabolic health

Mosaic loss of chromosome Y (LOY) in leukocytes is the most common form of clonal mosaicism, caused by dysregulation in cell-cycle and DNA damage response pathways. Previous genetic studies have focussed on identifying common variants associated with LOY, which we now extend to rarer, protein-coding variation using exome sequences from 82,277 male UK Biobank participants. We find that loss of function of two genes—CHEK2 and GIGYF1—reach exome-wide significance. Rare alleles in GIGYF1 have not previously been implicated in any complex trait, but here loss-of-function carriers exhibit six-fold higher susceptibility to LOY (OR = 5.99 [3.04–11.81], p = 1.3 × 10⁻¹⁰). These same alleles are also associated with adverse metabolic health, including higher susceptibility to Type 2 Diabetes (OR = 6.10 [3.51–10.61], p = 1.8 × 10⁻¹²), 4 kg higher fat mass (p = 1.3 × 10⁻⁴), 2.32 nmol/L lower serum IGF1 levels (p = 1.5 × 10⁻⁴) and 4.5 kg lower handgrip strength (p = 4.7 × 10⁻⁷) consistent with proposed GIGYF1 enhancement of insulin and IGF-1 receptor signalling. These associations are mirrored by a common variant nearby associated with the expression of GIGYF1. Our observations highlight a potential direct connection between clonal mosaicism and metabolic health.

Mosaic loss of chromosome Y (LOY) is a common form of clonal mosaicism in leukocytes. Here, the authors extend genetic association analyses to rare variation using exome-sequence data from 82,277 males, finding that loss-of-function alleles in GIGYF1 are associated with six-fold higher susceptibility to both LOY and Type 2 Diabetes.

Pancreatic β-Cell Senescence: Mechanisms and Association with Diabetes

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

Genetic predisposition to mosaic Y chromosome loss in blood

Mosaic loss of chromosome Y (LOY) in circulating white blood cells is the most common form of clonal mosaicism1-5, yet our knowledge of the causes and consequences of this is limited. Here, using a computational approach, we estimate that 20% of the male population represented in the UK Biobank study (n = 205,011) has detectable LOY. We identify 156 autosomal genetic determinants of LOY, which we replicate in 757,114 men of European and Japanese ancestry. These loci highlight genes that are involved in cell-cycle regulation and cancer susceptibility, as well as somatic drivers of tumour growth and targets of cancer therapy. We demonstrate that genetic susceptibility to LOY is associated with non-haematological effects on health in both men and women, which supports the hypothesis that clonal haematopoiesis is a biomarker of genomic instability in other tissues. Single-cell RNA sequencing identifies dysregulated expression of autosomal genes in leukocytes with LOY and provides insights into why clonal expansion of these cells may occur. Collectively, these data highlight the value of studying clonal mosaicism to uncover fundamental mechanisms that underlie cancer and other ageing-related diseases.

Tamoxifen suppresses pancreatic β-cell proliferation in mice

Tamoxifen is a mixed agonist/antagonist estrogen analogue that is frequently used to induce conditional gene deletion in mice using Cre-loxP mediated gene recombination. Tamoxifen is routinely employed in extremely high-doses relative to typical human doses to induce efficient gene deletion in mice. Although tamoxifen has been widely assumed to have no influence upon β-cells, the acute developmental and functional consequences of high-dose tamoxifen upon glucose homeostasis and adult β-cells are largely unknown. We tested if tamoxifen influences glucose homeostasis in male mice of various genetic backgrounds. We then carried out detailed histomorphometry studies of mouse pancreata. We also performed gene expression studies with islets of tamoxifen-treated mice and controls. Tamoxifen had modest effects upon glucose homeostasis of mixed genetic background (F1 B6129SF1/J) mice, with fasting hyperglycemia and improved glucose tolerance but without overt effects on fed glucose levels or insulin sensitivity. Tamoxifen inhibited proliferation of β-cells in a dose-dependent manner, with dramatic reductions in β-cell turnover at the highest dose (decreased by 66%). In sharp contrast, tamoxifen did not reduce proliferation of pancreatic acinar cells. β-cell proliferation was unchanged by tamoxifen in 129S2 mice but was reduced in C57Bl6 genetic background mice (decreased by 59%). Gene expression studies revealed suppression of RNA for cyclins D1 and D2 within islets of tamoxifen-treated mice. Tamoxifen has a cytostatic effect on β-cells, independent of changes in glucose homeostasis, in mixed genetic background and also in C57Bl6 mice. Tamoxifen should be used judiciously to inducibly inactivate genes in studies of glucose homeostasis.

An overview of type 2 diabetes and importance of vitamin D3-vitamin D receptor interaction in pancreatic β-cells

One significant health issue that plagues contemporary society is that of Type 2 diabetes (T2D). This disease is characterised by higher-than-average blood glucose levels as a result of a combination of insulin resistance and insufficient insulin secretions from the β-cells of pancreatic islets of Langerhans. Previous developmental research into the pancreas has identified how early precursor genes of pancreatic β-cells, such as Cpal, Ngn3, NeuroD, Ptf1a, and cMyc, play an essential role in the differentiation of these cells. Furthermore, β-cell molecular characterization has also revealed the specific role of β-cell-markers, such as Glut2, MafA, Ins1, Ins2, and Pdx1 in insulin expression. The expression of these genes appears to be suppressed in the T2D β-cells, along with the reappearance of the early endocrine marker genes. Glucose transporters transport glucose into β-cells, thereby controlling insulin release during hyperglycaemia. This stimulates glycolysis through rises in intracellular calcium (a process enhanced by vitamin D) (Norman et al., 1980), activating 2 of 4 proteinases. The rise in calcium activates half of pancreatic β-cell proinsulinases, thus releasing free insulin from granules. The synthesis of ATP from glucose by glycolysis, Krebs cycle and oxidative phosphorylation plays a role in insulin release. Some studies have found that the β-cells contain high levels of the vitamin D receptor; however, the role that this plays in maintaining the maturity of the β-cells remains unknown. Further research is required to develop a more in-depth understanding of the role VDR plays in β-cell function and the processes by which the beta cell function is preserved.

Cyclin D2 is sufficient to drive β cell self-renewal and regeneration

Diabetes results from an inadequate mass of functional β cells, due to either β cell loss caused by autoimmune destruction (type I diabetes) or β cell failure in response to insulin resistance (type II diabetes). Elucidating the mechanisms that regulate β cell mass may be key to developing new techniques that foster β cell regeneration as a cellular therapy to treat diabetes. While previous studies concluded that cyclin D2 is required for postnatal β cell self-renewal in mice, it is not clear if cyclin D2 is sufficient to drive β cell self-renewal. Using transgenic mice that overexpress cyclin D2 specifically in β cells, we show that cyclin D2 overexpression increases β cell self-renewal post-weaning and results in increased β cell mass. β cells that overexpress cyclin D2 are responsive to glucose stimulation, suggesting they are functionally mature. β cells that overexpress cyclin D2 demonstrate an enhanced regenerative capacity after injury induced by streptozotocin toxicity. To understand if cyclin D2 overexpression is sufficient to drive β cell self-renewal, we generated a novel mouse model where cyclin D2 is only expressed in β cells of cyclin D2^−/− mice. Transgenic overexpression of cyclin D2 in cyclin D2^−/− β cells was sufficient to restore β cell mass, maintain normoglycaemia, and improve regenerative capacity when compared with cyclin D2^−/− littermates. Taken together, our results indicate that cyclin D2 is sufficient to regulate β cell self-renewal and that manipulation of its expression could be used to enhance β cell regeneration.

Effects of metformin on compensatory pancreatic β-cell hyperplasia in mice fed a high-fat diet

Metformin has been widely used for the treatment of type 2 diabetes. However, the effect of metformin on pancreatic β-cells remains controversial. In this study, we investigated the impacts of treatment with metformin on pancreatic β-cells in a mouse model fed a high-fat diet (HFD), which triggers adaptive β-cell replication. An 8-wk treatment with metformin improved insulin resistance and suppressed the compensatory β-cell hyperplasia induced by HFD-feeding. In contrast, the increment in β-cell mass arising from 60 wk of HFD feeding was similar in mice treated with and those treated without metformin. Interestingly, metformin suppressed β-cell proliferation induced by 1 wk of HFD feeding without any changes in insulin resistance. Metformin directly suppressed glucose-induced β-cell proliferation in islets and INS-1 cells in accordance with a reduction in mammalian target of rapamycin phosphorylation. Taken together, metformin suppressed HFD-induced β-cell proliferation independent of the improvement of insulin resistance, partly via direct actions.

β Cells Persist in T1D Pancreata Without Evidence of Ongoing β-Cell Turnover or Neogenesis

CONTEXT

The cellular basis of persistent β-cell function in type 1 diabetes (T1D) remains enigmatic. No extensive quantitative β-cell studies of T1D pancreata have been performed to test for ongoing β-cell regeneration or neogenesis.

OBJECTIVE

We sought to determine the mechanism of β-cell persistence in T1D pancreata.

DESIGN

We studied T1D (n = 47) and nondiabetic control (n = 59) pancreata over a wide range of ages from the Juvenile Diabetes Research Foundation Network of Pancreatic Organ Donors with Diabetes via high-throughput microscopy.

INTERVENTION AND MAIN OUTCOME MEASURES

We quantified β-cell mass, β-cell turnover [via Ki-67 and terminal deoxynucleotide transferase-mediated dUTP nick end labeling (TUNEL)], islet ductal association, and insulin/glucagon coexpression in T1D and control pancreata.

RESULTS

Residual insulin-producing β cells were detected in some (but not all) T1D cases of varying disease duration. Several T1D pancreata had substantial numbers of β cells. Although β-cell proliferation was prominent early in life, it dramatically declined after infancy in both nondiabetic controls and T1D individuals. However, β-cell proliferation was equivalent in control and T1D pancreata. β-cell death (assessed by TUNEL) was extremely rare in control and T1D pancreata. Thus, β-cell turnover was not increased in T1D. Furthermore, we found no evidence of small islet/ductal neogenesis or α-cell to β-cell transdifferentiation in T1D pancreata, regardless of disease duration.

CONCLUSION

Longstanding β-cell function in patients with T1D appears to be largely a result of β cells that persist, without any evidence of attempted β-cell regeneration, small islet/ductal neogenesis, or transdifferentiation from other islet endocrine cell types.

GLP-1 signalling compensates for impaired insulin signalling in regulating beta cell proliferation in βIRKO mice

AIMS/HYPOTHESIS

We aimed to investigate potential interactions between insulin and glucagon-like peptide (GLP)-1 signalling pathways in the regulation of beta cell-cycle dynamics in vivo, in the context of the therapeutic potential of GLP-1 to modulate impaired beta cell function.

METHODS

Beta cell-specific insulin receptor knockout (βIRKO) mice, which exhibit beta cell dysfunction and an age-dependent decrease in beta cell mass, were treated with the dipeptidyl peptidase-4 inhibitor vildagliptin. Following this, glucose homeostasis and beta cell proliferation were evaluated and underlying molecular mechanisms were investigated.

RESULTS

The sustained elevation in circulating GLP-1 levels, caused by treatment of the knockout mice with vildagliptin for 6 weeks, significantly improved glucose tolerance secondary to enhanced insulin secretion and proliferation of beta cells. Treating βIRKO beta cell lines with the GLP-1 analogue, exendin-4, promoted Akt phosphorylation and protein expression of cyclins A, D1 and E two- to threefold, in addition to cyclin D2. Pancreases from the vildagliptin-treated βIRKO mice exhibited increased cyclin D1 expression, while cyclin D2 expression was impaired.

CONCLUSIONS/INTERPRETATION

Activation of GLP-1 signalling compensates for impaired growth factor (insulin) signalling and enhances expression of cyclins to promote beta cell proliferation. Together, these data indicate the potential of GLP-1-related therapies to enhance beta cell proliferation and promote beneficial outcomes in models with dysfunctional beta cells.

Incretin Therapies Do Not Expand β-Cell Mass or Alter Pancreatic Histology in Young Male Mice

The impact of incretins upon pancreatic β-cell expansion remains extremely controversial. Multiple studies indicate that incretin-based therapies can increase β-cell proliferation, and incretins have been hypothesized to expand β-cell mass. However, disagreement exists on whether incretins increase β-cell mass. Moreover, some reports indicate that incretins may cause metaplastic changes in pancreatic histology. To resolve these questions, we treated a large cohort of mice with incretin-based therapies and carried out a rigorous analysis of β-cell turnover and pancreatic histology using high-throughput imaging. Young mice received exenatide via osmotic pump, des-fluoro-sitagliptin, or glipizide compounded in diet for 2 weeks (short-term) on a low-fat diet (LFD) or 4.5 months (long-term) on a LFD or high-fat diet (HFD). Pancreata were quantified for β-cell turnover and mass. Slides were examined for gross anatomical and microscopic changes to exocrine pancreas. Short-term des-fluoro-sitagliptin increased serum insulin and induced modest β-cell proliferation but no change in β-cell mass. Long-term incretin therapy in HFD-fed mice resulted in reduced weight gain, improved glucose homeostasis, and abrogated β-cell mass expansion. No evidence for rapidly dividing progenitor cells was found in islets or pancreatic parenchyma, indicating that incretins do not induce islet neogenesis or pancreatic metaplasia. Contrasting prior reports, we found no evidence of β-cell mass expansion after acute or chronic incretin therapy. Chronic incretin administration was not associated with histological abnormalities in pancreatic parenchyma; mice did not develop tumors, pancreatitis, or ductal hyperplasia. We conclude that incretin therapies do not generate β-cells or alter pancreatic histology in young mice.

Epidermal Growth Factor Receptor Signaling Regulates β Cell Proliferation in Adult Mice

A thorough understanding of the signaling pathways involved in the regulation of β cell proliferation is an important initial step in restoring β cell mass in the diabetic patient. Here, we show that epidermal growth factor receptor 1 (EGFR) was significantly up-regulated in the islets of C57BL/6 mice after 50% partial pancreatectomy (PPx), a model for workload-induced β cell proliferation. Specific deletion of EGFR in the β cells of adult mice impaired β cell proliferation at baseline and after 50% PPx, suggesting that the EGFR signaling pathway plays an essential role in adult β cell proliferation. Further analyses showed that β cell-specific depletion of EGFR resulted in impaired expression of cyclin D1 and impaired suppression of p27 after PPx, both of which enhance β cell proliferation. These data highlight the importance of EGFR signaling and its downstream signaling cascade in postnatal β cell growth.

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.

Extreme obesity induces massive beta cell expansion in mice through self-renewal and does not alter the beta cell lineage

AIMS/HYPOTHESIS

Understanding the developmental biology of beta cell regeneration is critical for developing new diabetes therapies. Obesity is a potent but poorly understood stimulus for beta cell expansion. Current models of obesity are complicated by developmental compensation or concurrent diabetes, limiting their usefulness for identifying the lineage mechanism(s) of beta cell expansion. We aimed to determine whether acute inducible obesity stimulates beta cell expansion and to determine the lineage mechanism of beta cell growth in obesity.

METHODS

We created whole-body tamoxifen-inducible leptin receptor (LepR)-deficient mice (Ubc-Cre (ERT2) LepR (loxP/loxP) ) as a novel model of acute obesity. Beta cell mass and proliferation were quantified after short-term LepR deletion. Clonal analysis of beta cell expansion using the Brainbow2.1 reporter was performed 6 months post tamoxifen initiation.

RESULTS

LepR deficiency induced a doubling of body mass within 3 weeks, with moderate glucose intolerance (unlike typical LepR mutant mice [db/db], which have frank diabetes). Beta cell mass expanded threefold through increased beta cell proliferation, without evidence for contribution from specialised progenitors or stem cells (via sequential thymidine labelling and Brainbow2.1 reporter). Thus, self-renewal is the primary lineage mechanism in obesity-induced beta cell expansion. However, even the rapid beta cell proliferation could not exceed the restrictions of the replication refractory period.

CONCLUSIONS/INTERPRETATION

In summary, we created a novel model of inducible obesity demonstrating that even extreme metabolic demand does not alter beta cell lineage.

Glucose Induces Mouse β-Cell Proliferation via IRS2, MTOR, and Cyclin D2 but Not the Insulin Receptor

An important goal in diabetes research is to understand the processes that trigger endogenous β-cell proliferation. Hyperglycemia induces β-cell replication, but the mechanism remains debated. A prime candidate is insulin, which acts locally through the insulin receptor. Having previously developed an in vivo mouse hyperglycemia model, we tested whether glucose induces β-cell proliferation through insulin signaling. By using mice lacking insulin signaling intermediate insulin receptor substrate 2 (IRS2), we confirmed that hyperglycemia-induced β-cell proliferation requires IRS2 both in vivo and ex vivo. Of note, insulin receptor activation was not required for glucose-induced proliferation, and insulin itself was not sufficient to drive replication. Glucose and insulin caused similar acute signaling in mouse islets, but chronic signaling differed markedly, with mammalian target of rapamycin (MTOR) and extracellular signal-related kinase (ERK) activation by glucose and AKT activation by insulin. MTOR but not ERK activation was required for glucose-induced proliferation. Cyclin D2 was necessary for glucose-induced β-cell proliferation. Cyclin D2 expression was reduced when either IRS2 or MTOR signaling was lost, and restoring cyclin D2 expression rescued the proliferation defect. Human islets shared many of these regulatory pathways. Taken together, these results support a model in which IRS2, MTOR, and cyclin D2, but not the insulin receptor, mediate glucose-induced proliferation.

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.

Bisphenol-A treatment during pregnancy in mice: a new window of susceptibility for the development of diabetes in mothers later in life

Evidence now exists supporting the hypothesis that endocrine-disrupting chemicals (EDCs) can harmfully impact glucose metabolism. Thus, EDCs are beginning to be considered important contributors to the increased incidence of diabetes, obesity, or both. The possible effect of exposure to EDCs during pregnancy on glucose homeostasis in mothers later in life is presently unknown. Here we show that several months after delivery, mothers treated with the widespread EDC bisphenol-A (BPA) during gestation, at environmentally relevant doses, exhibit profound glucose intolerance and altered insulin sensitivity as well as increased body weight. These mice presented a decreased insulin secretion both in vivo and in vitro together with reduced pancreatic β-cell mass. The proliferation capacity was decreased in association with a diminished expression of the cell cycle activators: cyclin D2 and cyclin-dependent kinase-4. In addition, the rate of β-cells apoptosis was increased as well as the expression of the cell cycle inhibitors p16 and p53. Conversely, no effects on glucose metabolism or insulin sensitivity were observed when female nonpregnant mice were treated with BPA at the same doses. Taken together, these findings reveal that BPA exposure during gestation has harmful long-term implications in glucose metabolism for the mother. This finding highlights a new window of susceptibility for EDC exposure that may be important for the development of type 2 diabetes.

M2 macrophages promote beta-cell proliferation by up-regulation of SMAD7

Determination of signaling pathways that regulate beta-cell replication is critical for beta-cell therapy. Here, we show that blocking pancreatic macrophage infiltration after pancreatic duct ligation (PDL) completely inhibits beta-cell proliferation. The TGFβ superfamily signaling inhibitor SMAD7 was significantly up-regulated in beta cells after PDL. Beta cells failed to proliferate in response to PDL in beta-cell-specific SMAD7 mutant mice. Forced expression of SMAD7 in beta cells by itself was sufficient to promote beta-cell proliferation in vivo. M2, rather than M1 macrophages, seem to be the inducers of SMAD7-mediated beta-cell proliferation. M2 macrophages not only release TGFβ1 to directly induce up-regulation of SMAD7 in beta cells but also release EGF to activate EGF receptor signaling that inhibits TGFβ1-activated SMAD2 nuclear translocation, resulting in TGFβ signaling inhibition. SMAD7 promotes beta-cell proliferation by increasing CyclinD1 and CyclinD2, and by inducing nuclear exclusion of p27. Our study thus reveals a molecular pathway to potentially increase beta-cell mass through enhanced SMAD7 activity induced by extracellular stimuli.

Successful β cells islet regeneration in streptozotocin-induced diabetic baboons using ultrasound-targeted microbubble gene therapy with cyclinD2/CDK4/GLP1

Both major forms of diabetes mellitus (DM) involve β-cell destruction and dysfunction. New treatment strategies have focused on replenishing the deficiency of β-cell mass common to both major forms of diabetes by islet transplantation or β-cell regeneration. The pancreas, not the liver, is the ideal organ for islet regeneration, because it is the natural milieu for islets. Since islet mass is known to increase during obesity and pregnancy, the concept of stimulating pancreatic islet regeneration in vivo is both rational and physiologic. This paper proposes a novel approach in which non-viral gene therapy is targeted to pancreatic islets using ultrasound targeted microbubble destruction (UTMD) in a non-human primate model (NHP), the baboon. Treated baboons received a gene cocktail comprised of cyclinD2, CDK, and GLP1, which in rats results in robust and durable islet regeneration with normalization of blood glucose, insulin, and C-peptide levels. We were able to generate important preliminary data indicating that gene therapy by UTMD can achieve in vivo normalization of the intravenous (IV) glucose tolerance test (IVGTT) curves in STZ hyperglycemic-induced conscious tethered baboons. Immunohistochemistry clearly demonstrated evidence of islet regeneration and restoration of β-cell mass.

D-type Cyclins are important downstream effectors of cytokine signaling that regulate the proliferation of normal and neoplastic mammary epithelial cells

In response to the ligand-mediated activation of cytokine receptors, cells decide whether to proliferate or to undergo differentiation. D-type Cyclins (Cyclin D1, D2, or D3) and their associated Cyclin-dependent kinases (CDK4, CDK6) connect signals from cytokines to the cell cycle machinery, and they propel cells through the G1 restriction point and into the S phase, after which growth factor stimulation is no longer essential to complete cell division. D-type Cyclins are upregulated in many human malignancies including breast cancer to promote an uncontrolled proliferation of cancer cells. After summarizing important aspects of the cytokine-mediated transcriptional regulation and the posttranslational modification of D-type Cyclins, this review will highlight the physiological significance of these cell cycle regulators during normal mammary gland development as well as the initiation and promotion of breast cancer. Although the vast majority of published reports focus almost exclusively on the role of Cyclin D1 in breast cancer, we summarize here previous and recent findings that demonstrate an important contribution of the remaining two members of this Cyclin family, in particular Cyclin D3, for the growth of ErbB2-associated breast cancer cells in humans and in mouse models. New data from genetically engineered models as well as the pharmacological inhibition of CDK4/6 suggest that targeting the combined functions of D-type Cyclins could be a suitable strategy for the treatment of ErbB2-positive and potentially other types of breast cancer.

Hepatocyte growth factor/c-Met signaling is required for β-cell regeneration

Hepatocyte growth factor (HGF) is a mitogen required for β-cell replication during pregnancy. To determine whether HGF/c-Met signaling is required for β-cell regeneration, we characterized mice with pancreatic deletion of the HGF receptor, c-Met (PancMet KO mice), in two models of reduced β-cell mass and regeneration: multiple low-dose streptozotocin (MLDS) and partial pancreatectomy (Ppx). We also analyzed whether HGF administration could accelerate β-cell regeneration in wild-type (WT) mice after Ppx. Mouse islets obtained 7 days post-Ppx displayed significantly increased c-Met, suggesting a potential role for HGF/c-Met in β-cell proliferation in situations of reduced β-cell mass. Indeed, adult PancMet KO mice displayed markedly reduced β-cell replication compared with WT mice 7 days post-Ppx. Similarly, β-cell proliferation was decreased in PancMet KO mice in the MLDS mouse model. The decrease in β-cell proliferation post-Ppx correlated with a striking decrease in D-cyclin levels. Importantly, PancMet KO mice showed significantly diminished β-cell mass, decreased glucose tolerance, and impaired insulin secretion compared with WT mice 28 days post-Ppx. Conversely, HGF administration in WT Ppx mice further accelerated β-cell regeneration. These results indicate that HGF/c-Met signaling is critical for β-cell proliferation in situations of diminished β-cell mass and suggest that activation of this pathway can enhance β-cell regeneration.

Expression profiling of cell cycle genes in human pancreatic islets with and without type 2 diabetes

Microarray gene expression data were used to analyze the expression pattern of cyclin, cyclin-dependent kinase (CDKs) and cyclin-dependent kinase inhibitor (CDKIs) genes from human pancreatic islets with and without type 2 diabetes (T2D). Of the cyclin genes, CCNI was the most expressed. Data obtained from microarray and qRT-PCR showed higher expression of CCND1 in diabetic islets. Among the CDKs, CDK4, CDK8 and CDK9 were highly expressed, while CDK1 was expressed at low level. High expression of CDK18 was observed in diabetic islets. Of the CDKIs, CDKN1A expression was higher in diabetic islets in both microarray and qRT-PCR. Expression of CDKN1A, CDKN2A, CCNI2, CDK3 and CDK16 was correlated with age. Finally, eight SNPs in these genes were associated with T2D in the DIAGRAM database. Our data provide a comprehensive expression pattern of cell cycle genes in human islets. More human studies are required to confirm and reproduce animal studies.

Cytoplasmic-nuclear trafficking of G1/S cell cycle molecules and adult human β-cell replication: a revised model of human β-cell G1/S control

Harnessing control of human β-cell proliferation has proven frustratingly difficult. Most G1/S control molecules, generally presumed to be nuclear proteins in the human β-cell, are in fact constrained to the cytoplasm. Here, we asked whether G1/S molecules might traffic into and out of the cytoplasmic compartment in association with activation of cell cycle progression. Cdk6 and cyclin D3 were used to drive human β-cell proliferation and promptly translocated into the nucleus in association with proliferation. In contrast, the cell cycle inhibitors p15, p18, and p19 did not alter their location, remaining cytoplasmic. Conversely, p16, p21, and p27 increased their nuclear frequency. In contrast once again, p57 decreased its nuclear frequency. Whereas proliferating β-cells contained nuclear cyclin D3 and cdk6, proliferation generally did not occur in β-cells that contained nuclear cell cycle inhibitors, except p21. Dynamic cytoplasmic-nuclear trafficking of cdk6 was confirmed using green fluorescent protein-tagged cdk6 and live cell imaging. Thus, we provide novel working models describing the control of cell cycle progression in the human β-cell. In addition to known obstacles to β-cell proliferation, cytoplasmic-to-nuclear trafficking of G1/S molecules may represent an obstacle as well as a therapeutic opportunity for human β-cell expansion.

Adaptive β-cell proliferation increases early in high-fat feeding in mice, concurrent with metabolic changes, with induction of islet cyclin D2 expression

Type 2 diabetes (T2D) is caused by relative insulin deficiency, due in part to reduced β-cell mass (11, 62). Therapies aimed at expanding β-cell mass may be useful to treat T2D (14). Although feeding rodents a high-fat diet (HFD) for an extended period (3-6 mo) increases β-cell mass by inducing β-cell proliferation (16, 20, 53, 54), evidence suggests that adult human β-cells may not meaningfully proliferate in response to obesity. The timing and identity of the earliest initiators of the rodent compensatory growth response, possible therapeutic targets to drive proliferation in refractory human β-cells, are not known. To develop a model to identify early drivers of β-cell proliferation, we studied mice during the first week of HFD exposure, determining the onset of proliferation in the context of diet-related physiological changes. Within the first week of HFD, mice consumed more kilocalories, gained weight and fat mass, and developed hyperglycemia, hyperinsulinemia, and glucose intolerance due to impaired insulin secretion. The β-cell proliferative response also began within the first week of HFD feeding. Intriguingly, β-cell proliferation increased before insulin resistance was detected. Cyclin D2 protein expression was increased in islets by day 7, suggesting it may be an early effector driving compensatory β-cell proliferation in mice. This study defines the time frame and physiology to identify novel upstream regulatory signals driving mouse β-cell mass expansion, in order to explore their efficacy, or reasons for inefficacy, in initiating human β-cell proliferation.

Human pancreatic β-cell G1/S molecule cell cycle atlas

Expansion of pancreatic β-cells is a key goal of diabetes research, yet induction of adult human β-cell replication has proven frustratingly difficult. In part, this reflects a lack of understanding of cell cycle control in the human β-cell. Here, we provide a comprehensive immunocytochemical "atlas" of G1/S control molecules in the human β-cell. This atlas reveals that the majority of these molecules, previously known to be present in islets, are actually present in the β-cell. More importantly, and in contrast to anticipated results, the human β-cell G1/S atlas reveals that almost all of the critical G1/S cell cycle control molecules are located in the cytoplasm of the quiescent human β-cell. Indeed, the only nuclear G1/S molecules are the cell cycle inhibitors, pRb, p57, and variably, p21: none of the cyclins or cdks necessary to drive human β-cell proliferation are present in the nuclear compartment. This observation may provide an explanation for the refractoriness of human β-cells to proliferation. Thus, in addition to known obstacles to human β-cell proliferation, restriction of G1/S molecules to the cytoplasm of the human β-cell represents an unanticipated obstacle to therapeutic human β-cell expansion.

GATA factors promote ER integrity and β-cell survival and contribute to type 1 diabetes risk

Pancreatic β-cell survival remains poorly understood despite decades of research. GATA transcription factors broadly regulate embryogenesis and influence survival of several cell types, but their role in adult β-cells remains undefined. To investigate the role of GATA factors in adult β-cells, we derived β-cell-inducible Gata4- and Gata6-knockout mice, along with whole-body inducible Gata4 knockouts. β-Cell Gata4 deletion modestly increased the proportion of dying β-cells in situ with ultrastructural abnormalities suggesting endoplasmic reticulum (ER) stress. Notably, glucose homeostasis was not grossly altered in Gata4- and Gata6-knockout mice, suggesting that GATA factors do not have essential roles in β-cells. Several ER stress signals were up-regulated in Gata4 and Gata6 knockouts, most notably CHOP, a known regulator of ER stress-induced apoptosis. However, ER stress signals were not elevated to levels observed after acute thapsigargin administration, suggesting that GATA deficiency only caused mild ER stress. Simultaneous deletion of Gata4 and CHOP partially restored β-cell survival. In contrast, whole-body inducible Gata4 knockouts displayed no evidence of ER stress in other GATA4-enriched tissues, such as heart. Indeed, distinct GATA transcriptional targets were differentially expressed in islets compared with heart. Such β-cell-specific findings prompted study of a large meta-analysis dataset to investigate single nucleotide polymorphisms harbored within the human GATA4 locus, revealing several variants significantly associated with type 1 diabetes mellitus. We conclude that GATA factors have important but nonessential roles to promote ER integrity and β-cell survival in a tissue-specific manner and that GATA factors likely contribute to type 1 diabetes mellitus pathogenesis.

Effect of aging on islet beta-cell function and its mechanisms in Wistar rats

Type 2 diabetes mellitus is characterized by islet β-cell dysfunction and its incidence increases with age. However, the mechanisms underlying the effect of aging on islet β-cell function are not fully understood. We characterized β-cell function in 4-month-old (young), 14-month-old (adult), and 24-month-old (old) male Wistar rats, and found that islet β-cell function decreased gradually with age. Old rats displayed oral glucose intolerance and exhibited a decrease in glucose-stimulated insulin release (GSIR) and palmitic acid-stimulated insulin release (PSIR). Furthermore, total superoxide dismutase (T-SOD), CuZn superoxide dismutase (CuZn-SOD), and glutathione peroxidase (GSH-Px) activity decreased, whereas serum malondialdehyde (MDA) levels increased in the older rats. Moreover, we detected a significant reduction in β-cell proliferation and an increase in the frequency of apoptotic β-cells in the islets of rats in the old group. Finally, Anxa1 expression in the islets of old rats was significantly upregulated. These data provide new insights into the development of age-related β-cell dysfunction in rats.

PERK is required in the adult pancreas and is essential for maintenance of glucose homeostasis

Germ line PERK mutations are associated with diabetes mellitus and growth retardation in both rodents and humans. In contrast, late embryonic excision of PERK permits islet development and was found to prevent onset of diabetes, suggesting that PERK may be dispensable in the adult pancreas. To definitively establish the functional role of PERK in adult pancreata, we generated mice harboring a conditional PERK allele in which excision is regulated by tamoxifen administration. Deletion of PERK in either young adult or mature adult mice resulted in hyperglycemia associated with loss of islet and β cell architecture. PERK excision triggered intracellular accumulation of proinsulin and Glut2, massive endoplasmic reticulum (ER) expansion, and compensatory activation of the remaining unfolded-protein response (UPR) signaling pathways specifically in pancreatic tissue. Although PERK excision increased β cell death, this was not a result of decreased proliferation as previously reported. In contrast, a significant and specific increase in β cell proliferation was observed, a result reflecting increased cyclin D1 accumulation. This work demonstrates that contrary to expectations, PERK is required for secretory homeostasis and β cell survival in adult mice.

Transient overexpression of cyclin D2/CDK4/GLP1 genes induces proliferation and differentiation of adult pancreatic progenitors and mediates islet regeneration

The molecular mechanism of β-cell regeneration remains poorly understood. Cyclin D2/CDK4 expresses in normal β cells and maintains adult β-cell growth. We hypothesized that gene therapy with cyclin D2/CDK4/GLP-1 plasmids targeted to the pancreas of STZ-treated rats by ultrasound-targeted microbubble destruction (UTMD) would force cell cycle re-entry of residual G(0)-phase islet cells into G(1)/S phase to regenerate β cells. A single UTMD treatment induced β-cell regeneration with reversal of diabetes for 6 mo without evidence of toxicity. We observed that this β-cell regeneration was not mediated by self-replication of pre-existing β cells. Instead, cyclin D2/CDK4/GLP-1 initiated robust proliferation of adult pancreatic progenitor cells that exist within islets and terminally differentiate to mature islets with β cells and α cells.

Viral cyclins mediate separate phases of infection by integrating functions of distinct mammalian cyclins

Gammaherpesvirus cyclins have expanded biochemical features relative to mammalian cyclins, and promote infection and pathogenesis including acute lung infection, viral persistence, and reactivation from latency. To define the essential features of the viral cyclin, we generated a panel of knock-in viruses expressing various viral or mammalian cyclins from the murine gammaherpesvirus 68 cyclin locus. Viral cyclins of both gammaherpesvirus 68 and Kaposi's sarcoma-associated herpesvirus supported all cyclin-dependent stages of infection, indicating functional conservation. Although mammalian cyclins could not restore lung replication, they did promote viral persistence and reactivation. Strikingly, distinct and non-overlapping mammalian cyclins complemented persistence (cyclin A, E) or reactivation from latency (cyclin D3). Based on these data, unique biochemical features of viral cyclins (e.g. enhanced kinase activation) are not essential to mediate specific processes during infection. What is essential for, and unique to, the viral cyclins is the integration of the activities of several different mammalian cyclins, which allows viral cyclins to mediate multiple, discrete stages of infection. These studies also demonstrated that closely related stages of infection, that are cyclin-dependent, are in fact genetically distinct, and thus predict that cyclin requirements may be used to tailor potential therapies for virus-associated diseases.

Synergistic Effect of Hyperglycemia and p27(kip1) Suppression on Adult Mouse Islet Beta Cell Replication

The complementary role of hyperglycemia and p27(kip1) suppression on islet beta cell regeneration was investigated in a syngeneic mouse model. p27(kip1) gene silencing was performed by infecting islets of C57BL/6 with shRNA lentiviral particles. At 54 hours after viral infection, p27(kip1) protein content in cultured targeting islets was 22% of that in freshly isolated islets. Six days after transplantation to diabetic mice, targeting islet graft had considerably more cells with Ki67-staining nuclei than nontargeting islets. The mice in the targeting-islet group had a significantly shorter duration of temporary hyperglycaemia than mice in the non-targeting-islet group. The long-term ex vivo beneficial effect of p27(kip1) silencing on graft function was also indicated by the significantly higher cumulative cure rate for diabetes in mice receiving 200 targeting islets than that in mice receiving 200 non-targeting islets. Our data suggest that hyperglycemia and persistent p27(kip1) suppression have a synergistic effect on islet beta cell replication in adult mice.

Type 2 diabetes and the aging pancreatic beta cell

The incidence of and susceptibility to Type 2 diabetes increases with age, but the underlying mechanism(s) within beta cells that contribute to this increased susceptibility have not been fully elucidated. Here we review how aging affects the proliferative and regenerative capacity of beta cells and how this impacts beta cell mass. In addition we review changes that occur in beta cell function with age. Although we focus on the different rodent models that have provided insight into the characteristics of the aging beta cell, the limited knowledge from non-rodent models is also reviewed. Further studies are needed in order to identify potential beta cell targets for preventing or slowing the progression of diabetes that occurs with age.

Activation of protein kinase C-ζ in pancreatic β-cells in vivo improves glucose tolerance and induces β-cell expansion via mTOR activation

OBJECTIVE

PKC-ζ activation is a key signaling event for growth factor-induced β-cell replication in vitro. However, the effect of direct PKC-ζ activation in the β-cell in vivo is unknown. In this study, we examined the effects of PKC-ζ activation in β-cell expansion and function in vivo in mice and the mechanisms associated with these effects.

RESEARCH DESIGN AND METHODS

We characterized glucose homeostasis and β-cell phenotype of transgenic (TG) mice with constitutive activation of PKC-ζ in the β-cell. We also analyzed the expression and regulation of signaling pathways, G1/S cell cycle molecules, and β-cell functional markers in TG and wild-type mouse islets.

RESULTS

TG mice displayed increased plasma insulin, improved glucose tolerance, and enhanced insulin secretion with concomitant upregulation of islet insulin and glucokinase expression. In addition, TG mice displayed increased β-cell proliferation, size, and mass compared with wild-type littermates. The increase in β-cell proliferation was associated with upregulation of cyclins D1, D2, D3, and A and downregulation of p21. Phosphorylation of D-cyclins, known to initiate their rapid degradation, was reduced in TG mouse islets. Phosphorylation/inactivation of GSK-3β and phosphorylation/activation of mTOR, critical regulators of D-cyclin expression and β-cell proliferation, were enhanced in TG mouse islets, without changes in Akt phosphorylation status. Rapamycin treatment in vivo eliminated the increases in β-cell proliferation, size, and mass; the upregulation of cyclins Ds and A in TG mice; and the improvement in glucose tolerance-identifying mTOR as a novel downstream mediator of PKC-ζ-induced β-cell replication and expansion in vivo. CONCLUSIONS PKC:-ζ, through mTOR activation, modifies the expression pattern of β-cell cycle molecules leading to increased β-cell replication and mass with a concomitant enhancement in β-cell function. Approaches to enhance PKC-ζ activity may be of value as a therapeutic strategy for the treatment of diabetes.

Glucose regulates cyclin D2 expression in quiescent and replicating pancreatic β-cells through glycolysis and calcium channels

Understanding the molecular triggers of pancreatic β-cell proliferation may facilitate the development of regenerative therapies for diabetes. Genetic studies have demonstrated an important role for cyclin D2 in β-cell proliferation and mass homeostasis, but its specific function in β-cell division and mechanism of regulation remain unclear. Here, we report that cyclin D2 is present at high levels in the nucleus of quiescent β-cells in vivo. The major regulator of cyclin D2 expression is glucose, acting via glycolysis and calcium channels in the β-cell to control cyclin D2 mRNA levels. Furthermore, cyclin D2 mRNA is down-regulated during S-G(2)-M phases of each β-cell division, via a mechanism that is also affected by glucose metabolism. Thus, glucose metabolism maintains high levels of nuclear cyclin D2 in quiescent β-cells and modulates the down-regulation of cyclin D2 in replicating β-cells. These data challenge the standard model for regulation of cyclin D2 during the cell division cycle and suggest cyclin D2 as a molecular link between glucose levels and β-cell replication.

Immunofluorescent detection of two thymidine analogues (CldU and IdU) in primary tissue

Accurate measurement of cell division is a fundamental challenge in experimental biology that becomes increasingly complex when slowly dividing cells are analyzed. Established methods to detect cell division include direct visualization by continuous microscopy in cell culture, dilution of vital dyes such as carboxyfluorescein di-aetate succinimidyl ester (CFSE), immuno-detection of mitogenic antigens such as ki67 or PCNA, and thymidine analogues. Thymidine analogues can be detected by a variety of methods including radio-detection for tritiated thymidine, immuno-detection for bromo-deoxyuridine (BrdU), chloro-deoxyuridine (CldU) and iodo-deoxyuridine (IdU), and chemical detection for ethinyl-deoxyuridine (EdU). We have derived a strategy to detect sequential incorporation of different thymidine analogues (CldU and IdU) into tissues of adult mice. Our method allows investigators to accurately quantify two successive rounds of cell division. By optimizing immunostaining protocols our approach can detect very low dose thymidine analogues administered via the drinking water, safe to administer to mice for prolonged periods of time. Consequently, our technique can be used to detect cell turnover in very long-lived tissues. Optimal immunofluoresent staining results can be achieved in multiple tissue types, including pancreas, skin, gut, liver, adrenal, testis, ovary, thyroid, lymph node, and brain. We have also applied this technique to identify oncogenic transformation within tissues. We have further applied this technique to determine if transit-amplifying cells contribute to growth or renewal of tissues. In this sense, sequential administration of thymidine analogues represents a novel approach for studying the origins and survival of cells involved in tissue homeostasis.

Aging induces a distinct gene expression program in mouse islets

The role of aging in the pathogenesis of type 2 diabetes remains poorly understood. In the past adult β-cells were assumed to undergo frequent turnover. However, we find that β-cell turnover declines to very low levels in middle-aged mice. We therefore hypothesized that aged islets could exhibit a distinct gene expression program. We compared gene expression in islets from young mice to islets from aged mice under basal conditions. Aging was associated with differential expression of many genes in islets, including mRNAs encoding for chromatin remodeling components, RNA binding proteins, and pancreatic endocrine transcription factors. We previously observed that cell cycle entry of β-cells is severely restricted by middle age, with minimal of β-cell proliferation in response to regenerative stimuli such as 50% partial pancreatectomy. To characterize the effect of age in adaptive β-cell proliferation, we measured gene expression in islets from young mice after pancreatectomy. As expected, partial pancreatectomy induced differential expression of many genes, including those encoding Reg (regenerating) proteins. Surprisingly, partial pancreatectomy also induced expression of Reg genes in islets from aged mice, which have greatly reduced capacity for adaptive β-cell proliferation. However, there was little overlap (besides the Reg genes) in between the partial pancreatectomy induced islet genes in young mice versus old mice. Thus, partial pancreatectomy does not induce the same gene expression program in young mice vs old mice. Taken together, our results reveal that aged islets exhibit a unique gene expression signature that could contribute to the limited regenerative capacity of mature β-cells.

Gastrointestinal hormones and the regulation of β-cell mass

Type 2 diabetes occurs due to a relative deficit in β-cell mass or function. Glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), and gastrin are gastrointestinal hormones that are secreted in response to nutrient intake, regulating digestion, insulin secretion, satiety, and β-cell mass. In this review, we focus upon β-cell mass regulation. β-cell mass expands through β-cell proliferation and islet neogenesis; β-cell mass is lost via apoptosis. GLP-1 and GIP are well-studied gastrointestinal hormones and influence β-cell proliferation, apoptosis, and islet neogenesis. CCK regulates β-cell apoptosis and mitogenesis, and gastrin stimulates islet neogenesis. GLP-1 and GIP bind to G protein-coupled receptors and regulate β-cell mass via multiple signaling pathways. The protein kinase A pathway is central to this process because it directly regulates proliferative and anti-apoptotic genes and transactivates several signaling cascades, including Akt and mitogen-activated protein kinases. However, the signaling pathways downstream of G protein-coupled CCK receptors that influence β-cell mass remain unidentified. Gastrointestinal hormones integrate nutrient signals from the gut to the β-cell, regulating insulin secretion and β-cell mass adaptation.

Induction of human beta-cell proliferation and engraftment using a single G1/S regulatory molecule, cdk6

OBJECTIVE

Most knowledge on human beta-cell cycle control derives from immunoblots of whole human islets, mixtures of beta-cells and non-beta-cells. We explored the presence, subcellular localization, and function of five early G1/S phase molecules-cyclins D1-3 and cdk 4 and 6-in the adult human beta-cell.

RESEARCH DESIGN AND METHODS

Immunocytochemistry for the five molecules and their relative abilities to drive human beta-cell replication were examined. Human beta-cell replication, cell death, and islet function in vivo were studied in the diabetic NOD-SCID mouse.

RESULTS

Human beta-cells contain easily detectable cdks 4 and 6 and cyclin D3 but variable cyclin D1. Cyclin D2 was only marginally detectable. All five were principally cytoplasmic, not nuclear. Overexpression of the five, alone or in combination, led to variable increases in human beta-cell replication, with the cdk6/cyclin D3 combination being the most robust (15% versus 0.3% in control beta-cells). A single molecule, cdk6, proved to be capable of driving human beta-cell replication in vitro and enhancing human islet engraftment/proliferation in vivo, superior to normal islets and as effectively as the combination of cdk6 plus a D-cyclin.

CONCLUSIONS

Human beta-cells contain abundant cdk4, cdk6, and cyclin D3, but variable amounts of cyclin D1. In contrast to rodent beta-cells, they contain little or no detectable cyclin D2. They are primarily cytoplasmic and likely ineffective in basal beta-cell replication. Unexpectedly, cyclin D3 and cdk6 overexpression drives human beta-cell replication most effectively. Most importantly, a single molecule, cdk6, supports robust human beta-cell proliferation and function in vivo.

Cyclin D2 is essential for the compensatory beta-cell hyperplastic response to insulin resistance in rodents

OBJECTIVE

A major determinant of the progression from insulin resistance to the development of overt type 2 diabetes is a failure to mount an appropriate compensatory beta-cell hyperplastic response to maintain normoglycemia. We undertook the present study to directly explore the significance of the cell cycle protein cyclin D2 in the expansion of beta-cell mass in two different models of insulin resistance.

RESEARCH DESIGN AND METHODS

We created compound knockouts by crossing mice deficient in cyclin D2 (D2KO) with either the insulin receptor substrate 1 knockout (IRS1KO) mice or the insulin receptor liver-specific knockout mice (LIRKO), neither of which develops overt diabetes on its own because of robust compensatory beta-cell hyperplasia. We phenotyped the double knockouts and used RT-qPCR and immunohistochemistry to examine beta-cell mass.

RESULTS

Both compound knockouts, D2KO/LIRKO and D2KO/IRS1KO, exhibited insulin resistance and hyperinsulinemia and an absence of compensatory beta-cell hyperplasia. However, the diabetic D2KO/LIRKO group rapidly succumbed early compared with a relatively normal lifespan in the glucose-intolerant D2KO/IRS1KO mice.

CONCLUSIONS

This study provides direct genetic evidence that cyclin D2 is essential for the expansion of beta-cell mass in response to a spectrum of insulin resistance and points to the cell-cycle protein as a potential therapeutic target that can be harnessed for preventing and curing type 2 diabetes.

Target gene identification via nuclear receptor binding site prediction

In spite of numerous advances in recent years, the complete list of direct target genes for nuclear receptors remains elusive. The integrated application of new computational and experimental methods reviewed in this chapter provides insight into the complex network of regulatory pathways mediated by nuclear receptors which is expected to improve the understanding of the physiology and the pathology of metabolism, development, homeostasis, and other fundamental processes.