P21cip-overexpression in the mouse beta cells leads to the improved recovery from streptozotocin-induced diabetes

Overexpression of UBE2E2 in Mouse Pancreatic β-Cells Leads to Glucose Intolerance via Reduction of β-Cell Mass

Genome-wide association studies have identified several gene polymorphisms, including UBE2E2, associated with type 2 diabetes. Although UBE2E2 is one of the ubiquitin-conjugating enzymes involved in the process of ubiquitin modifications, the pathophysiological roles of UBE2E2 in metabolic dysfunction are not yet understood. Here, we showed upregulated UBE2E2 expression in the islets of a mouse model of diet-induced obesity. The diabetes risk allele of UBE2E2 (rs13094957) in noncoding regions was associated with upregulation of UBE2E2 mRNA in the human pancreas. Although glucose-stimulated insulin secretion was intact in the isolated islets, pancreatic β-cell-specific UBE2E2-transgenic (TG) mice exhibited reduced insulin secretion and decreased β-cell mass. In TG mice, suppressed proliferation of β-cells before the weaning period and while receiving a high-fat diet was accompanied by elevated gene expression levels of p21, resulting in decreased postnatal β-cell mass expansion and compensatory β-cell hyperplasia, respectively. In TG islets, proteomic analysis identified enhanced formation of various types of polyubiquitin chains, accompanied by increased expression of Nedd4 E3 ubiquitin protein ligase. Ubiquitination assays showed that UBE2E2 mediated the elongation of ubiquitin chains by Nedd4. The data suggest that UBE2E2-mediated ubiquitin modifications in β-cells play an important role in regulating glucose homeostasis and β-cell mass.

Stress-induced β cell early senescence confers protection against type 1 diabetes

During the progression of type 1 diabetes (T1D), β cells are exposed to significant stress and, therefore, require adaptive responses to survive. The adaptive mechanisms that can preserve β cell function and survival in the face of autoimmunity remain unclear. Here, we show that the deletion of the unfolded protein response (UPR) genes Atf6α or Ire1α in β cells of non-obese diabetic (NOD) mice prior to insulitis generates a p21-driven early senescence phenotype and alters the β cell secretome that significantly enhances the leukemia inhibitory factor-mediated recruitment of M2 macrophages to islets. Consequently, M2 macrophages promote anti-inflammatory responses and immune surveillance that cause the resolution of islet inflammation, the removal of terminally senesced β cells, the reduction of β cell apoptosis, and protection against T1D. We further demonstrate that the p21-mediated early senescence signature is conserved in the residual β cells of T1D patients. Our findings reveal a previously unrecognized link between β cell UPR and senescence that, if leveraged, may represent a novel preventive strategy for T1D.

Decreased proliferation of aged rat beta cells corresponds with enhanced expression of the cell cycle inhibitor p27KIP1

BACKGROUND

Over 400 million people are diabetic. Type 1 and type 2 diabetes are characterized by decreased functional β-cell mass and, consequently, decreased glucose-stimulated insulin secretion. A potential intervention is transplantation of β-cell containing islets from cadaveric donors. A major impediment to greater application of this treatment is the scarcity of transplant-ready β-cells. Therefore, inducing β-cell proliferation ex vivo could be used to expand functional β-cell mass prior to transplantation. Various molecular pathways are sufficient to induce proliferation of young β-cells; however, aged β-cells are refractory to these proliferative signals. Given that the majority of cadaveric donors fit an aged demographic, defining the mechanisms that impede aged β-cell proliferation is imperative.

RESULTS

We demonstrate that aged rat (5-month-old) β-cells are refractory to mitogenic stimuli that otherwise induce young rat (5-week-old) β-cell proliferation. We hypothesized that this change in proliferative capacity could be due to differences in cyclin-dependent kinase inhibitor expression. We measured levels of p16^INK4a , p15^INK4b , p18^INK4c , p19^INK4d , p21^CIP1 , p27^KIP1 and p57^KIP2 by immunofluorescence analysis. Our data demonstrates an age-dependent increase of p27^KIP1 in rat β-cells by immunofluorescence and was validated by increased p27^KIP1 protein levels by western blot analysis. Interestingly, HDAC1, which modulates the p27^KIP1 promoter acetylation state, is downregulated in aged rat islets. These data demonstrate increased p27^KIP1 protein levels at 5 months of age, which may be due to decreased HDAC1 mediated repression of p27^KIP1 expression.

SIGNIFICANCE

As the majority of transplant-ready β-cells come from aged donors, it is imperative that we understand why aged β-cells are refractory to mitogenic stimuli. Our findings demonstrate that increased p27^KIP1 expression occurs early in β-cell aging, which corresponds with impaired β-cell proliferation. Furthermore, the correlation between HDAC1 and p27 levels suggests that pathways that activate HDAC1 in aged β-cells could be leveraged to decrease p27^KIP1 levels and enhance β-cell proliferation.

Harnessing the Endogenous Plasticity of Pancreatic Islets: A Feasible Regenerative Medicine Therapy for Diabetes?

Diabetes is a chronic metabolic disease caused by an absolute or relative deficiency in functional pancreatic β-cells that leads to defective control of blood glucose. Current treatments for diabetes, despite their great beneficial effects on clinical symptoms, are not curative treatments, leading to a chronic dependence on insulin throughout life that does not prevent the secondary complications associated with diabetes. The overwhelming increase in DM incidence has led to a search for novel antidiabetic therapies aiming at the regeneration of the lost functional β-cells to allow the re-establishment of the endogenous glucose homeostasis. Here we review several aspects that must be considered for the development of novel and successful regenerative therapies for diabetes: first, the need to maintain the heterogeneity of islet β-cells with several subpopulations of β-cells characterized by different transcriptomic profiles correlating with differences in functionality and in resistance/behavior under stress conditions; second, the existence of an intrinsic islet plasticity that allows stimulus-mediated transcriptome alterations that trigger the transdifferentiation of islet non-β-cells into β-cells; and finally, the possibility of using agents that promote a fully functional/mature β-cell phenotype to reduce and reverse the process of dedifferentiation of β-cells during diabetes.

Increased double strand breaks in diabetic β-cells with a p21 response that limits apoptosis

DNA damage and DNA damage response (DDR) pathways in β-cells have received little attention especially in the context of type-2 diabetes. We postulate that p21 plays a key role in DDR by preventing apoptosis, associated through its overexpression triggered by DNA stand breaks (DSBs). Our results show that β-cells from chronic diabetic mice had a greater extent of DSBs as compared to their non-diabetic counterparts. Comet assays and nuclear presence of γH2AX and 53bp1 revealed increased DNA DSBs in 16 weeks old (wo) db/db β-cells as compared to age matched non-diabetic β-cells. Our study of gene expression changes in MIN6 cell line with doxorubicin (Dox) induced DNA damage, showed that the DDR was similar to primary β-cells from diabetic mice. There was significant overexpression of DDR genes, gadd45a and p21 after a 24-hr treatment. Western blot analysis revealed increased cleaved caspase3 over time, suggesting higher frequency of apoptosis due to Dox-induced DNA strand breaks. Inhibition of p21 by pharmacological inhibitor UC2288 under DNA damage conditions (both in Dox-induced MIN6 cells and older db/db islets) significantly increased the incidence of β-cell apoptosis. Our studies confirmed that while DNA damage, specifically DSBs, induced p21 overexpression in β-cells and triggered the p53/p21 cellular response, p21 inhibition exacerbated the frequency of apoptosis.

Molecular Modelling of Islet β-Cell Adaptation to Inflammation in Pregnancy and Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM), a metabolic disease that develops with the increase in insulin resistance during late pregnancy, is currently one of the most common complications affecting pregnancy. The polygenic nature of GDM, together with the interplay between different genetic variants with nutritional and environmental factors has hindered the full understanding of the etiology of this disease. However, an important genetic overlap has been found with type 2 diabetes mellitus (T2DM) and, as in the case of T2DM, most of the identified loci are associated with β-cell function. Early detection of GDM and adequate interventions to control the maternal glycemia are necessary to avoid the adverse outcomes for both the mother and the offspring. The in utero exposure to the diabetic milieu predispose these children for future diseases, among them T2DM, originating a vicious circle implicated in the increased prevalence of both GDM and T2DM. The involvement of inflammatory processes in the development of GDM highlights the importance of pancreatic β-cell factors able to favor the adaptation processes required during gestation, concomitantly with the protection of the islets from an inflammatory milieu. In this regard, two members of the Pax family of transcription factors, PAX4 and PAX8, together with the chromatin remodeler factor HMG20A, have gained great relevance due to their involvement in β-cell mass adaptation together with their anti-inflammatory properties. Mutations in these factors have been associated with GDM, highlighting these as novel candidates for genetic screening analysis in the identification of women at risk of developing GDM.

Control of metabolism by p53 - Cancer and beyond

p53 is an important tumour suppressor gene, with loss of p53 contributing to the development of most human cancers. However, the activation of p53 in response to stress signals underpins a role for p53 in diverse aspects of health and disease. Activities of p53 that regulate metabolism can play a role in maintaining homeostasis and protecting cells from damage - so preventing disease development. By contrast, either loss or over-activation of p53 can contribute to numerous metabolic pathologies, including aging, obesity and diabetes.

mTORC2 Signaling: A Path for Pancreatic β Cell's Growth and Function

The mechanistic target of rapamycin (mTOR) signaling pathway is an evolutionary conserved pathway that senses signals from nutrients and growth factors to regulate cell growth, metabolism and survival. mTOR acts in two biochemically and functionally distinct complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2), which differ in terms of regulatory mechanisms, substrate specificity and functional outputs. While mTORC1 signaling has been extensively studied in islet/β-cell biology, recent findings demonstrate a distinct role for mTORC2 in the regulation of pancreatic β-cell function and mass. mTORC2, a key component of the growth factor receptor signaling, is declined in β cells under diabetogenic conditions and in pancreatic islets from patients with type 2 diabetes. β cell-selective mTORC2 inactivation leads to glucose intolerance and acceleration of diabetes as a result of reduced β-cell mass, proliferation and impaired glucose-stimulated insulin secretion. Thereby, many mTORC2 targets, such as AKT, PKC, FOXO1, MST1 and cell cycle regulators, play an important role in β-cell survival and function. This indicates mTORC2 as important pathway for the maintenance of β-cell homeostasis, particularly to sustain proper β-cell compensatory response in the presence of nutrient overload and metabolic demand. This review summarizes recent emerging advances on the contribution of mTORC2 and its associated signaling on the regulation of glucose metabolism and functional β-cell mass under physiological and pathophysiological conditions in type 2 diabetes.

The Diabetes-Linked Transcription Factor PAX4: From Gene to Functional Consequences

Paired box 4 (PAX4) is a key factor in the generation of insulin producing β-cells during embryonic development. In adult islets, PAX4 expression is sequestered to a subset of β-cells that are prone to proliferation and more resistant to stress-induced apoptosis. The importance of this transcription factor for adequate pancreatic islets functionality has been manifested by the association of mutations in PAX4 with the development of diabetes, independently of its etiology. Overexpression of this factor in adult islets stimulates β-cell proliferation and increases their resistance to apoptosis. Additionally, in an experimental model of autoimmune diabetes, a novel immunomodulatory function for this factor has been suggested. Altogether these data pinpoint at PAX4 as an important target for novel regenerative therapies for diabetes treatment, aiming at the preservation of the remaining β-cells in parallel to the stimulation of their proliferation to replenish the β-cell mass lost during the progression of the disease. However, the adequate development of such therapies requires the knowledge of the molecular mechanisms controlling the expression of PAX4 as well as the downstream effectors that could account for PAX4 action.

Is p53 Involved in Tissue-Specific Insulin Resistance Formation?

p53 constitutes an extremely versatile molecule, primarily involved in sensing the variety of cellular stresses. Functional p53 utilizes a plethora of mechanisms to protect cell from deleterious repercussions of genotoxic insults, where senescence deserves special attention. While the impressive amount of p53 roles has been perceived solely by the prism of antioncogenic effect, its presence seems to be vastly connected with metabolic abnormalities underlain by cellular aging, obesity, and inflammation. p53 has been found to regulate multiple biochemical processes such as glycolysis, oxidative phosphorylation, lipolysis, lipogenesis, β-oxidation, gluconeogenesis, and glycogen synthesis. Notably, p53-mediated metabolic effects are totally up to results of insulin action. Accumulating amount of data identifies p53 to be a factor activated upon hyperglycemia or excessive calorie intake, thus contributing to low-grade chronic inflammation and systemic insulin resistance. Prominent signs of its actions have been observed in muscles, liver, pancreas, and adipose tissue being associated with attenuation of insulin signalling. p53 is of crucial importance for the regulation of white and brown adipogenesis simultaneously being a repressor for preadipocyte differentiation. This review provides a profound insight into p53-dependent metabolic actions directed towards promotion of insulin resistance as well as presenting experimental data regarding obesity-induced p53-mediated metabolic abnormalities.

Ciclopirox enhances pancreatic islet health by modulating the unfolded protein response in diabetes

Pancreatic dysfunction during diabetes is linked to the induction of endoplasmic reticulum (ER) stress on pancreatic beta (β) cells. Our laboratory recently discovered that p21 protects from diabetes by modifying the outcome of ER stress response. In the present study, we explored the antidiabetic activity of ciclopirox (CPX), an iron chelator and recently described activator of p21 expression. The effects of CPX in beta cell survival and function were assessed in cultured islets in vitro as well as in diabetic mice in vivo. The consequences of CPX in high glucose-induced insulin release and reactive oxygen species (ROS) production were also evaluated. Islet survival assays confirmed the significance of p21 in the regulation of glucotoxicity and suggested that CPX counteracts glucotoxicity in a manner that depends on p21. In vivo, administration of CPX in wild-type (WT) diabetic mice restored glucose homeostasis. In WT-cultured islets, CPX suppressed the expression of ER stress markers BiP, GRP94, and CHOP and reduced the levels of ROS during culture at high glucose. This reduction of ER stress may be associated with the ability of CPX to inhibit insulin release. Iron citrate stimulated insulin release, which was inhibited by CPX that functions as an iron chelator. It is conceivable that inhibition of insulin production constrains ER stress in islets promoting their survival and thus protecting from diabetes in vivo. This unfolded protein response (UPR)-antagonizing activity of CPX suggests application for the management not only of diabetes but also of other conditions related to ER stress.

Synergy Between Gαz Deficiency and GLP-1 Analog Treatment in Preserving Functional β-Cell Mass in Experimental Diabetes

A defining characteristic of type 1 diabetes mellitus (T1DM) pathophysiology is pancreatic β-cell death and dysfunction, resulting in insufficient insulin secretion to properly control blood glucose levels. Treatments that promote β-cell replication and survival, thus reversing the loss of β-cell mass, while also preserving β-cell function, could lead to a real cure for T1DM. The α-subunit of the heterotrimeric Gz protein, Gαz, is a tonic negative regulator of adenylate cyclase and downstream cAMP production. cAMP is one of a few identified signaling molecules that can simultaneously have a positive impact on pancreatic islet β-cell proliferation, survival, and function. The purpose of our study was to determine whether mice lacking Gαz might be protected, at least partially, from β-cell loss and dysfunction after streptozotocin treatment. We also aimed to determine whether Gαz might act in concert with an activator of the cAMP-stimulatory glucagon-like peptide 1 receptor, exendin-4 (Ex4). Without Ex4 treatment, Gαz-null mice still developed hyperglycemia, albeit delayed. The same finding held true for wild-type mice treated with Ex4. With Ex4 treatment, Gαz-null mice were protected from developing severe hyperglycemia. Immunohistological studies performed on pancreas sections and in vitro apoptosis, cytotoxicity, and survival assays demonstrated a clear effect of Gαz signaling on pancreatic β-cell replication and death; β-cell function was also improved in Gαz-null islets. These data support our hypothesis that a combination of therapies targeting both stimulatory and inhibitory pathways will be more effective than either alone at protecting, preserving, and possibly regenerating β-cell mass and function in T1DM.

PAX4 Defines an Expandable β-Cell Subpopulation in the Adult Pancreatic Islet

PAX4 is a key regulator of pancreatic islet development whilst in adult acute overexpression protects β-cells against stress-induced apoptosis and stimulates proliferation. Nonetheless, sustained PAX4 expression promotes β-cell dedifferentiation and hyperglycemia, mimicking β-cell failure in diabetic patients. Herein, we study mechanisms that allow stringent PAX4 regulation endowing favorable β-cell adaptation in response to changing environment without loss of identity. To this end, PAX4 expression was monitored using a mouse bearing the enhanced green fluorescent protein (GFP) and cre recombinase construct under the control of the islet specific pax4 promoter. GFP was detected in 30% of islet cells predominantly composed of PAX4-enriched β-cells that responded to glucose-induced insulin secretion. Lineage tracing demonstrated that all islet cells were derived from PAX4⁺ progenitor cells but that GFP expression was confined to a subpopulation at birth which declined with age correlating with reduced replication. However, this GFP⁺ subpopulation expanded during pregnancy, a state of active β-cell replication. Accordingly, enhanced proliferation was exclusively detected in GFP⁺ cells consistent with cell cycle genes being stimulated in PAX4-overexpressing islets. Under stress conditions, GFP⁺ cells were more resistant to apoptosis than their GFP^- counterparts. Our data suggest PAX4 defines an expandable β-cell sub population within adult islets.

Regulation of P21 during diabetes-associated stress of the endoplasmic reticulum

Endoplasmic reticulum (ER) stress plays a major role in the pathogenesis of diabetes by inducing β-cell apoptosis in the islets of Langerhans. In this study, we show that the transcription factor CHOP, which is instrumental for the induction of ER-stress-associated apoptosis and the pancreatic dysfunction in diabetes, regulates the expression of P21 (WAF1), a cell cycle regulator with anti-apoptotic activity that promotes cell survival. Deficiency of P21 sensitizes pancreatic β-cells to glucotoxicity, while in mice genetic ablation of P21 accelerates experimental diet-induced diabetes, results indicative of a protective role for P21 in the development of the disease. Conversely, pharmacological stimulation of P21 expression by nutlin-3a, an inhibitor of P53-MDM2 interaction, restores pancreatic function and facilitates glucose homeostasis. These findings indicate that P21 acts as an inhibitor of ER-stress-associated tissue damage and that stimulation of P21 activity can be beneficial for the management of diabetes and probably of other conditions in which ER-stress-associated death is undesirable.

Activated FoxM1 attenuates streptozotocin-mediated β-cell death

The forkhead box transcription factor FoxM1, a positive regulator of the cell cycle, is required for β-cell mass expansion postnatally, during pregnancy, and after partial pancreatectomy. Up-regulation of full-length FoxM1, however, is unable to stimulate increases in β-cell mass in unstressed mice or after partial pancreatectomy, probably due to the lack of posttranslational activation. We hypothesized that expression of an activated form of FoxM1 could aid in recovery after β-cell injury. We therefore derived transgenic mice that inducibly express an activated version of FoxM1 in β-cells (RIP-rtTA;TetO-hemagglutinin (HA)-Foxm1(Δ)(NRD) mice). This N-terminally truncated form of FoxM1 bypasses 2 posttranslational controls: exposure of the forkhead DNA binding domain and targeted proteasomal degradation. Transgenic mice were subjected to streptozotocin (STZ)-induced β-cell ablation to test whether activated FoxM1 can promote β-cell regeneration. Mice expressing HA-FoxM1(ΔNRD) displayed decreased ad libitum-fed blood glucose and increased β-cell mass. β-Cell proliferation was actually decreased in RIP-rtTA:TetO-HA-Foxm1(NRD) mice compared with that in RIP-rtTA mice 7 days after STZ treatment. Unexpectedly, β-cell death was decreased 2 days after STZ treatment. RNA sequencing analysis indicated that activated FoxM1 alters the expression of extracellular matrix and immune cell gene profiles, which may protect against STZ-mediated death. These studies highlight a previously underappreciated role for FoxM1 in promoting β-cell survival.

β-Cell-Specific Glucocorticoid Reactivation Attenuates Inflammatory β-Cell Destruction

Progression and severity of type 1 diabetes is dependent upon inflammatory induction of nitric oxide production and consequent pancreatic β-cell damage. Glucocorticoids (GCs) are highly effective anti-inflammatory agents but have been precluded in type 1 diabetes and in islet transplantation protocols because they exacerbated insulin resistance and suppressed β-cell insulin secretion at the high-doses employed clinically. In contrast, physiological-range elevation of GC action within β-cells ameliorated lipotoxic β-cell failure in transgenic mice overexpressing the intracellular enzyme 11β-hydroxysteroid dehydrogenase type 1 (MIP-HSD1(tg/+) mice). Here, we tested the hypothesis that elevated β-cell 11beta-HSD1 protects against the β-cell destruction elicited by streptozotocin (STZ), a toxin that dose-dependently mimics aspects of inflammatory and autoimmune β-cell destruction. MIP-HSD1(tg/+) mice exhibited an episodic protection from the severe hyperglycemia caused by a single high dose of STZ associated with higher and sustained β-cell survival, maintained β-cell replicative potential, higher plasma and islet insulin levels, reduced inflammatory macrophage infiltration and increased anti-inflammatory T regulatory cell content. MIP-HSD1(tg/+) mice also completely resisted mild hyperglycemia and insulitis induced by multiple low-dose STZ administration. In vitro, MIP-HSD1(tg/+) islets exhibited attenuated STZ-induced nitric oxide production, an effect reversed with a specific 11beta-HSD1 inhibitor. GC regeneration selectively within β-cells protects against inflammatory β-cell destruction, suggesting therapeutic targeting of 11beta-HSD1 may ameliorate processes that exacerbate type 1 diabetes and that hinder islet transplantation.

Upregulation of p21 activates the intrinsic apoptotic pathway in β-cells

Diabetes manifests from a loss in functional β-cell mass, which is regulated by a dynamic balance of various cellular processes, including β-cell growth, proliferation, and death as well as secretory function. The cell cycle machinery comprised of cyclins, kinases, and inhibitors regulates proliferation. However, their involvement during β-cell stress during the development of diabetes is not well understood. Interestingly, in a screen of multiple cell cycle inhibitors, p21 was dramatically upregulated in INS-1-derived 832/13 cells and rodent islets by two pharmacological inducers of β-cell stress, dexamethasone and thapsigargin. We hypothesized that β-cell stress upregulates p21 to activate the apoptotic pathway and suppress cell survival signaling. To this end, p21 was adenovirally overexpressed in pancreatic rat islets and 832/13 cells. As expected, p21 overexpression resulted in decreased [(3)H]thymidine incorporation. Flow cytometry analysis in p21-transduced 832/13 cells verified lower replication, as indicated by a decreased cell population in the S phase and a block in G2/M transition. The sub-G0 cell population was higher with p21 overexpression and was attributable to apoptosis, as demonstrated by increased annexin-positive stained cells and cleaved caspase-3 protein. p21-mediated caspase-3 cleavage was inhibited by either overexpression of the antiapoptotic mitochondrial protein Bcl-2 or siRNA-mediated suppression of the proapoptotic proteins Bax and Bak. Therefore, an intact intrinsic apoptotic pathway is central for p21-mediated cell death. In summary, our findings indicate that β-cell apoptosis can be triggered by p21 during stress and is thus a potential target to inhibit for protection of functional β-cell mass.

Enhanced beta cell proliferation in mice overexpressing a constitutively active form of Akt and one allele of p21Cip

AIMS/HYPOTHESIS

The ability of pancreatic beta cells to proliferate is critical both for normal tissue maintenance and in conditions where there is an increased demand for insulin. Protein kinase B(Akt) plays a major role in promoting proliferation in many cell types, including the insulin-producing beta cells. We have previously reported that mice overexpressing a constitutively active form of Akt(caAkt (Tg)) show enhanced beta cell proliferation that is associated with increased protein levels of cyclin D1, cyclin D2 and cyclin-dependent kinase inhibitor 1A (p21(Cip)). In the present study, we sought to assess the mechanisms responsible for augmented p21(Cip) levels in caAkt(Tg) mice and test the role of p21(Cip) in the proliferative responses induced by activation of Akt signalling.

METHODS

To gain a greater understanding of the relationship between Akt and p21(Cip), we evaluated the mechanisms involved in the modulation of p2(Cip) by Akt and the in vivo role of reduced p21(Cip) in proliferative responses induced by Akt.

RESULTS

Our experiments showed that Akt signalling regulates p21(Cip) transcription and protein stability. caAkt(Tg) /p21(Cip+/-) mice exhibited fasting and fed hypoglycaemia as well as hyperinsulinaemia when compared with caAkt(Tg) mice. Glucose tolerance tests revealed improved glucose tolerance in caAkt(Tg)/p21(Cip+/-) mice compared with caAkt (Tg). These changes resulted from increased proliferation, survival and beta cell mass in caAkt(Tg)/p21(Cip+/-) compared with caAkt(Tg) mice.

CONCLUSIONS/INTERPRETATION

Our data indicate that increased p21(Cip) levels in caAkt(Tg) mice act as a compensatory brake, protecting beta cells from unrestrained proliferation. These studies imply that p21(Cip) could play important roles in the adaptive responses of beta cells to proliferate in conditions such as in insulin resistance.

Protein kinase C delta (PKCδ) affects proliferation of insulin-secreting cells by promoting nuclear extrusion of the cell cycle inhibitor p21Cip1/WAF1

BACKGROUND

High fat diet-induced hyperglycemia and palmitate-stimulated apoptosis was prevented by specific inhibition of protein kinase C delta (PKCδ) in β-cells. To understand the role of PKCδ in more detail the impact of changes in PKCδ activity on proliferation and survival of insulin-secreting cells was analyzed under stress-free conditions.

METHODOLOGY AND PRINCIPAL FINDINGS

Using genetic and pharmacological approaches, the effect of reduced and increased PKCδ activity on proliferation, apoptosis and cell cycle regulation of insulin secreting cells was examined. Proteins were analyzed by Western blotting and by confocal laser scanning microscopy. Increased expression of wild type PKCδ (PKCδWT) significantly stimulated proliferation of INS-1E cells with concomitant reduced expression and cytosolic retraction of the cell cycle inhibitor p21(Cip1/WAF1). This nuclear extrusion was mediated by PKCδ-dependent phosphorylation of p21(Cip1/WAF1) at Ser146. In kinase dead PKCδ (PKCδKN) overexpressing cells and after inhibition of endogenous PKCδ activity by rottlerin or RNA interference phosphorylation of p21(Cip1/WAF1) was reduced, which favored its nuclear accumulation and apoptotic cell death of INS-1E cells. Human and mouse islet cells express p21(Cip1/WAF1) with strong nuclear accumulation, while in islet cells of PKCδWT transgenic mice the inhibitor resides cytosolic.

CONCLUSIONS AND SIGNIFICANCE

These observations disclose PKCδ as negative regulator of p21(Cip1/WAF1), which facilitates proliferation of insulin secreting cells under stress-free conditions and suggest that additional stress-induced changes push PKCδ into its known pro-apoptotic role.

Participation of Akt, menin, and p21 in pregnancy-induced beta-cell proliferation

β-Cell mass increases during pregnancy to accommodate for insulin resistance. This increase is mainly due to β-cell proliferation, a process that requires intact prolactin receptor (Prlr) signaling. Signaling molecules that are known to regulate β-cell proliferation include Jak2, Akt, the tumor suppressor menin, and cell cycle proteins. Whether these pathways are involved in prolactin-mediated β-cell proliferation is unknown. Using the heterozygous prolactin receptor-null (Prlr(+/-)) mice, we isolated pancreatic islets from both Prlr(+/+) and Prlr(+/-) mice on d 0 and 15 of pregnancy and examined the expression levels of these signaling molecules. In the wild-type mice (Prlr(+/+)), both phospho-Jak2 and phospho-Akt expression in pancreatic islets increased during pregnancy, which were attenuated in the pregnant Prlr(+/-) mice. During pregnancy, menin expression was reduced by 50 and 20% in the Prlr(+/+) and the Prlr(+/-) mice, respectively, and the pregnant Prlr(+/-) mice had higher islet p18 levels than the Prlr(+/+) mice. Interestingly, between d 0 and 15 of pregnancy, expression of cyclin inhibitory protein p21(cip) was increased in the Prlr(+/+) mice, but this increase was blunted in the Prlr(+/-) mice. Lastly, we did not find any difference in the expression levels of cyclins D1, D2, and inhibitory kinases between the pregnant Prlr(+/+) and Prlr(+/-) mice. Therefore, we conclude that during pregnancy, placental hormones act through the prolactin receptor to increase β-cell mass by up regulating β-cell proliferation by engaging Jak2, Akt, menin/p18, and p21. Future studies will determine the relative contribution of these molecules in maintaining normal glucose homeostasis during pregnancy.

β-cell regeneration: the pancreatic intrinsic faculty

Type I diabetes (T1D) patients rely on cumbersome chronic injections of insulin, making the development of alternate durable treatments a priority. The ability of the pancreas to generate new β-cells has been described in experimental diabetes models and, importantly, in infants with T1D. Here we discuss recent advances in identifying the origin of new β-cells after pancreatic injury, with and without inflammation, revealing a surprising degree of cell plasticity in the mature pancreas. In particular, the inducible selective near-total destruction of β-cells in healthy adult mice uncovers the intrinsic capacity of differentiated pancreatic cells to spontaneously reprogram to produce insulin. This opens new therapeutic possibilities because it implies that β-cells can differentiate endogenously, in depleted adults, from heterologous origins.