The Cell Cycle Related To B Cell Failure

Published Date: 02 Nov 2017

ABSTRACT

Type 2 diabetes is a progressive metabolic disorder characterized by high blood glucose due to the failure of pancreatic cells to produce sufficient levels of insulin. The pathogenesis of type 2 diabetes is associated with the induction of the unfolded protein response (UPR). While UPR aims to restore tissue homeostasis following stress of the endoplasmic reticulum (ER) prolonged ER stress triggers apoptosis, at least in part through the unfolded protein response (UPR) -activated transcription factor C/EBP homologous protein (CHOP). CHOP is a crucial mediator that links protein misfolding in the ER to oxidative stress and apoptosis in β cells under conditions of increased insulin demand. On the other hand p21 is implicated in the regulation of the UPR by various mechanisms involving inhibition of apoptosis and facilitation of the regeneration capacity of the beta cells. Considerable evidence exists indicating that overexpression of p21 could improve the recovery of β cells from streptozotocin-induced diabetes. In this review we summarized the roles of CHOP and p21 related to ER stress and oxidative stress in the β-cell pathogenesis of type 2 diabetes and suggest directions for future research.

Keywords: ER-stress, UPR, diabetes, apoptosis, CHOP, PERK, p21

Introduction

Type 2 diabetes consists of an array of metabolic dysfunctions that coincide with hyperglycemia and result from the progressive failure of beta cells to secrete insulin at adequate levels [1]. Recent studies suggest that p21 represses β cell – duplication rate and facilitates the recovery of mice from hyperglycemia caused by streptozotocin- induced diabetes [2]. Alternatively chop deletion rescues β cells from ER stress –related apoptosis [3]. A fundamental question regarding the decision for β-cell survival versus apoptosis is which UPR sub-pathways are essential for β-cell function and what key sensors of these responses are protective or disadvantageous to β-cell survival upon short-term or long-term ER stress.

Interplay between Endoplasmic reticulum –homeostasis and protein quality controlThe endoplasmic reticulum (ER) is a highly dynamic organelle that performs folding, modification, and trafficking of secretory and membrane proteins to the Golgi compartment, intracellular calcium homeostasis and lipid biosynthesis. The ER responds to genetic and environmental stimuli and provides a unique folding environment for approximately one third of all proteins. All eukaryotic cells have evolved specific mechanisms to preserve ER functions under conditions of stress [4]. Growing evidence shows that endoplasmic reticulum (ER) stress is an important mechanism linking obesity, insulin resistance and glucose intolerance [5].

The maintenance of ER homeostasis in insulin-secreting beta-cells are extremely important, because when homeostasis is disrupted, misfolded or unfolded proteins may accumulate in the ER, a condition referred to as ER stress [6]. Numerous perturbations in the normal functions of the ER such as hypoxia, alterations in calcium, nutrient availability, mutations in the ER- chaperones, inhibition of protein glycosylation, reduced disulfide bond formations and viral infections, may initiate an evolutionarily conserved cellular response that is designated as the unfolded protein response (UPR), and initially aims to restore homeostasis but can eventually promote cell death if ER dysfunction is acute or prolonged [7].

The UPR is comprised of three distinct biochemical branches linked to three intracellular receptors that upon activation they initiate specific biochemical events: (1) Activated PERK (endoplasmic reticulum kinase) that causes translational attenuation by phosphorylating, eIF2. However, some specific mRNAs, including ATF4, are translated under these conditions. ATF4, induces transcription of genes encoding adaptive functions including the glucose-regulated proteins.[7]; (2) IRE1a (ER-protein kinase), that induces the alternative splicing, and thus activation of the transcription factor XBP1 mRNA and produces XBP1-spliced (XBP1s). The activated transcription factor upregulates many ER chaperones and genes involved in ERAD, various UPR ‘stress genes’ as well as enzymes involved in membrane biogenesis [7];(3) ATF6 (activating transcription factor 6) that translocates to the Golgi, where proteolytic cleavages form a transcriptionally cytosolic fragment activating the induction of the UPR. ATF6 induces genes involved in ER homeostasis and membrane biogenesis [8];

There is an array of four types of specific cellular responses that are induced during the earlier phases of the UPR and aim to overcome stress:

1. Translational attenuation that occurs in order to overcome the load of ER.

2. Induction of UPR-related genes, primarily chaperones such as Bip/GRP78 and GRP74 in order to prevent further accumulation of unfolded/misfolded proteins.

3. Enzymes including protein disulfide isomerase (PDI) and SERCA2 (sarcoplasmic ER Ca+2 –ATPase2) that increase the capacity of endoplasmic reticulum for protein folding. In addition during UPR the transcriptional induction of amino acids occurs, and glutathione biosynthesis that protects against oxidative stress.

4. Stimulation of NFkB activity, that is a transcription factor acting as a mediator of immune and anti-apoptotic response [9].

If these responses are not sufficient for the re-establishment of the cellular homeostasis, ERAD (ER-associated protein degradation) components are induced in order to eliminate the misfolded proteins [9]. Finally, if cellular damage is deemed irreversible and the pro-survival activity of UPR is not sufficient for the retention of cellular homeostasis apoptosis is induced by stimulation of the CCAAT/enhancer-binding homologous protein (CHOP) and activation of the JNK kinase and caspase-12 [9].

Apparently, there is a continuous interplay between survival and death decisions during ER stress and a precise regulation that determines the transition from the prosurvival towards the proapoptotic state of UPR [10, 11]. We have provided evidence that chop regulates the cell cycle regulator p21 during ER stress. Chop suppresses p21and prompt cells into a pro-apoptotic program. Our findings indicate that CHOP relieves the anti-apoptotic activity of p21 during ER stress. Thus, p21 is implicated in the regulation of the UPR by inhibiting induction of apoptosis [12].

ER-stress: proposed mechanism of glucose-induced beta cell dysfunction

Prolonged in vitro exposure of beta cell lines or islets to glucose increases ER stress markers in the majority of studies. This is related to the fact that increased glucose levels over-stimulate insulin production that exhausts the insulin producing and secreting activity of the b cells leading to ER stress. The homeostatic consequences of the resulting UPR becomes apparent by the observation that overproduction of the ER chaperone 78kDa glucose- regulated protein (GRP78) partially prevents glucose-induced beta cell dysfunction in vitro in INS-1 cells (a rat cell line that secretes insulin in response to glucose concentrations in the physiological range) [13].

Evidence for a crosstalk between the generation of ROS and ER stress response

Several experimental results suggest that the production of reactive oxygen species (ROS) results in misfolded protein- mediated cell death. Oxidative protein folding takes place in the .ER oxidoreductases, including protein disulphideisomerase (PDI), PDIR, PDIp, P5 ERp57 and ERp72. In vitro formation, isomerization and reduction of disulphide bonds are catalyzed by PDI. During disulphide bond formation, cysteine residues within the PDI active site accept two electrons from thiol residues in the polypeptide chain substrate. These latter results in the oxidation of the protein, the reduction of the PDI active site and finally in the production of hydrogen peroxide. Thus, reactive oxygen species (ROS) are produced in a cell during synthesis of disulphide bonds in the ER during oxidative protein folding [14]. Furthermore, glutathione is involved in the reduction of mispaired disulfide bonds that may destroy the cellular glutathione pool which is crucial for the neutralization of the reactive oxygen species (ROS) and the blockade of the oxidation of the cytosolic proteins. Because oxidative protein folding occurs in the ER and disturbances in protein folding can have damaging consequences, alterations in redox status or the generation of ROS could directly or indirectly (or both) affect ER homeostasis and protein folding. Consequently, cells with a powerful biosynthetic load, deficient UPR, or defective ER-associated protein degradation (ERAD) are sensitive to oxidative stress [15].

Oxidative stress plays a causal role in glucose-induced beta cell dysfunction both in vivo and in vitro. Analysis of clinical specimens indicates that both, ER stress and oxidative stress are increased in the islets of individuals with type 2 diabetes. Recent studies demonstrate a close interrelationship between oxidative stress and ER-stress in beta cells [15].

Pancreatic beta cells: Relationship between survival signaling and apoptosis signaling

Many genetic and environmental variations trigger apoptotic pathways that eventually eliminate damaged cells. The decision for a protective or destructive stress response to cells depends on the nature and extends of the stress and the cell type [16]. The implications of cellular stress responses are numerous and will be discussed in this review with focus on diabetes.

Beta cells (β-cells) are a type of cells in the pancreas that are localized in the islets of Langerhans. They constitute 65-80% of the cells in the islets [17]. The other type of endocrine cells in the pancreas is the alpha cells (15-20%) which produce glycagon and elevates the glucose levels in the blood [18]. Pancreatic beta cells, the only source of insulin in the body, are either lost or become dysfunctional during the progression of diabetes that leads to high glucose levels in the blood. Targeting, and restoring the function of beta cells constitutes a major therapeutic strategy for the treatment of diabetes [17]. Beta cell mass is dynamically regulated and is maintained through a delicate balance of regeneration and apoptosis [19].

A marked increase in beta cell number occurs under conditions that include obesity and insulin resistance. The increase in beta cell mass can take place either through an increase in the cell number by neogenesis and proliferation (hyperplasia), or through an increase in the cell volume (hypertrophy). Pancreatic b-cell mass is regulated by interference with at least four independent mechanisms that contribute to the emergence and function of b cells that are responsible to cope with conditions related to the increased demand for insulin production: (i) by induction of β cell replication, (ii) by increase in β-cell size, (iii) by de novo production of β-cell (neogenesis) (iv) by inhibition of β-cell apoptosis [20].

ER stress is implicated in β cell dysfunction.

Hereditary syndromes such as the Wolfram syndrome illustrate the intrinsic link between ER stress and diabetes. Wolfram syndrome is a neurodegenerative disorder that is characterized by diabetes mellitus caused by nonautoimmune loss of beta cells, deafness and optic atrophy [21]. Wolfram syndrome is activated by mutations in the Wfs1 gene [22, 23]. Despite that WFS1 is not a direct marker of the UPR, analysis of Wfs1-deficient mice demonstrates that WFS1 function is linked with the preservation of ER homeostasis. WFS1 is induced by XBP1 through an unfolded protein stress response element-like motif in SH-SY5Y cells. In mutant mice the pancreatic β cells are subjected to ER-stress as shown by the phosphorylation of eIF2 and the production of the spliced form of XBP1. Mutations in Wfs1 repress intracellular calcium signaling, beyond glucose stimulation associated with insufficient insulin secretion and activate UPR-regulated genes, while ultimately inhibits cell cycle and stimulates apoptosis [21-23]. WFS1 alters the maturation of plasma membrane proteins or stability of ER membrane proteins. The finding that WFS1 and Na(+)/K(+)ATPase β1 proteins interact physically is necessary for the trafficking ofthe subunit to the cell surface. Decreased levels of this ATPase subunit were traced in the fractions of plasma membrane of Wfs1 mutant cells and of Wfs1 knockdown cells [24]. Accordingly loss of function in the WFS1 induces ER stress-cell failure associated with diabetes. Thus, WFS1 suggests roles in membrane trafficking, protein processing and/or regulation of ER calcium homeostasis [24]. This discovery evaluated that WFS1 has a protective role against ER stress and reveals in the negative regulation of pancreatic beta cell death in diabetes

CDKs, regulators of the cell cycle related to β-cell failure

Cell-cycle progression is orchestrated by a series of events, cyclins and their partners, the cyclin-dependent kinases (CDK). The activation of the kinase activity of the CDKs is responsible for the sequential progression towards the phases of the cell cycle. p21 is a member of the CIP/Kip family of cyclin-dependent kinase inhibitors (CKIs), which also includes p27 and p57 [25]. These three CKIs contain a conserved region at the N terminal transcriptional activation domain (NH2 terminus) that is essential for the inhibition of cyclin/Cdk complexes, whereas the COOH terminal regions are variable in length and function [26]. CKIs bind and inhibit a wide range of cyclin/Cdk complexes, especially those containing Cdk2 [27]. p21 expression up-regulates tumor-associated genes and proteins implicated in age-related diseases [28] and stimulates viral promoters, including those of HIV and CMV [29,30]. The expression of this p21 is tightly mediated by the tumor suppressor protein p53, through which this protein mediates the p53-dependent cell cycle arrest at the G1 phase of the cell cycle, in response to a variety of stress-inducing conditions. In parallel with its cytostatic function as an inhibitor of cell cycle progression p21 also operates as an inhibitor of apoptosis exhibiting a pro-survival activity as it protects cells from p53- dependent and p53-independent apoptosis [31]. This dual activity of p21 to regulate both cell cycle and susceptibility to apoptosis appears to play a regulatory role in the transition of UPR from its prosurvival towards its proapoptotic mode of action [12].

The role of p21 in the regulation of other cellular beyond the cell cycle has recently attracted increasing attention. Recent findings indicate that p21 is essential for damage- induced paracrine antiapoptotic activity because of the ability of p21 to imitate the transcriptional damage response without DNA damage. p21 interacts with CDK8 that is a CDK family member regulating several transcriptional programs involved in carcinogenesis but has no involved in cell cycle progression. Recent evidence showed that pharmacological inhibition of CDK8 blocked different chemotherapy-induced tumor-promoting paracrine activities of normal and tumor cells, in vitro and in vivo [32].

Maintenance of glucose homeostasis in the p21-null mouse

The ability of p21 to regulate cell cycle and sensitivity against apoptosis-inducing signals [33] prompted several investigators to explore a potential association between p21 and diabetes. Under diabetic conditions oxidative stress is provoked whereas, the implication of p21 in the glucose-related toxicity was examined. Isolated rat pancreatic islet cells were treated with H2O2 to provoke oxidative stress. Also in Zucker diabetic fatty rats p21 was overexpressed in islet cells using adenovirus. Analysis of gene expression in both experiments suggested that p21 mRNA expression was induced whereas insulin mRNA was decreased. The sum of these findings support that the expression of cyclin-dependent kinase inhibitor p21, which can be induced by oxidative stress, increases in pancreatic islet cells upon development of diabetes. By suppressing cell proliferation and insulin biosynthesis, the p21 induction is likely to be implicated in the beta-cell glucose toxicity. [34]

In vitro experiments showed that p21-deficient mouse islets treated with beta cell mitogens exhibited higher rate of DNA synthesis as compared to p21-expressing islets that were treated with the same mitogens which is consistent with the role of p21 in inhibiting cell cycle progression [35]. However, in vivo studies conducted by the same group in p21-deficient mice demonstrated that beta cell function, replication rates and glucose homeostasis are not altered as compared to the controls. This result suggests that either p21 does not act alone to arrest the beta cell cycle or that its function is complemented by another protein that can substitute for the loss of p21 in b cells in vivo [35]. Interestingly though, in this study, p21 deficient mice displayed lower insulin levels than controls implying some dysfunction of the b cells which however did not become clinically relevant, at least under the conditions used by the investigators in this study. It is conceivable though that p21 –deficiency might have sensitized mice to the harmful consequences of ER stress. This is in line with recent findings showing that p53-deficient cells and animals, that among others have reduced p21 levels, are more sensitive to chronic and acute ER stress [36]

Overexpression of p21 in β-Cells Inhibits β-Cell Proliferation

Beta cell proliferation is a very important contributor to the dynamic nature of adult beta cell mass. p21 is an inhibitor of the cell cycle progression, however, p21 ablation in islets has no effects on islet mass and glucose metabolism [35]. To determine whether p21 contributes to the β-cell failure associated with type 2 diabetes, the effects of p21 in the pancreatic β-cell regeneration were examined by an alternative approach. Specifically the consequence of p21 overexpression in β cell proliferation was investigated by using a tetracycline-based inducible system (Insulin-rtTA/TET-p21); In this system p21 is controlled by the RIPII-rtTA promoter and thus, it was targeted in the pancreatic b-cells. Therefore, following doxycycline (Dox) administration the specific overexpression of p21 in islet β cells is induced, which can inhibit the proliferation of β cells. Chronic administration of doxycycline in these mice slowly elevated serum glucose levels and eventually caused diabetes that apparently was associated with the reduction in b-cell content and insulin production [2].

Overexpression of p21 in mouse islets improves the recovery from streptozotocin-induced diabetes.

In the same study the recovery of the b-cells from sreptozotocin (STZ)-induced toxicity was also evaluated. STZhas preferential toxicity toward pancreatic β cells that is known to induce insulin-dependent diabetes, possibly via DNA damage in experimental animals. Diabetes induction was accomplished by using a single high STZ dose, to achieve optimal toxicity to pancreatic β cells.. A single injection of a high dose of STZ (200 mg/kg) was administered intraperitonealy in mice on the eighth day of dox treatment and blood glucose levels were measured. Surprisingly, the p21- overexpressing transgenic mice apparently promoted the recovery from STZ injury. Also p21-overexpressing mice were far more capable to increase weight and survive, and recover glucose homeostasis in the blood than control littermates. This demonstrates that when b-cell self-duplication is repressed, such as when p21 is overexpressed, islet damage from STZ treatment may re-activate alternative mechanisms to induce efficiently b-cell regeneration for diabetic recovery [2].

Structural changes in islets in p21-Overexpressing Transgenic Mice In order to determine which cells in the islets are responsible for β cell regeneration morphometric analysis identified a network of transcription factors, Ngn3 (Pax4, Arx, Nkx2.2, NeuroD1 and Pax6) and putative progenitor markers CD133 and c-Met, to be expressed in the islets of p21 overexpressing transgenic mice after STZ induced islet injury.. However, the expression of these genes was not detected in the islets of control mice following STZ injury. These results indicated that the progenitor cell activation could be induced under conditions where b-cell self-duplication is inhibited [2].

p21-Overexpressing Transgenic Mice co-expressing endocrine hormonesAfter STZ treatment, the levels of endocrine markers such as Glycagon, Insulin and Somatostatin were increased. [2] It has been verified that newly differentiated endocrine cells express Insulin, Glucagon, and Somatostatin spontaneously during early pancreas development [38,39]. It is familiar that pancreatic beta cells are the sole source of the insulin hormone Insulin. Insylin and glycogen activate beta cells whereas somatostatin inhibits beta cells [40]. The progenitor islet cells of STZ-double transgenic mice co- express insulin, glucagon and somatostatin in much higher levels than the untreated double transgenic mice. Therefore, recent data suggest that b- cell regeneration in the adult islets might be the cause of the STZ- induced diabetic recovery in p21 transgenic mice, which might induce the activation of early pancreatic developmental pathways [2].

Biological significance of p21 associated with diabetic nephropathy

Renal mesangial cell hypertrophy is a characteristic of diabetic nephropathy as well as a response to renal stress or injury and is characterized by increased protein synthesis. Considering that p21 controls cell cycle progression and affects DNA replication while allowing protein synthesis to continue, the implication of p21 in hypertrophy disease was considered. P21 inhibition by specific phosphorothioate antisense oligodeoxynucleotide (ODN) decreases p21 protein levels in human mesangial cell (MC) P21 inhibition by specific phosphorothioate antisense oligodeoxynucleotide (ODN) decreases p21 protein levels in human mesangial cell (MC).It is known that IGF-1, plays important roles in cellular functions including cell proliferation and hypertrophy, Also Antisense p21 ODN caused attenuation of IGF-1–induced p21 levels of MC cekks result in attenuating the hypertrophic effect. These data demonstrate the use of antisense p21 in renal disease and suggest that attenuation of p21 may ultimately prove useful in the therapy of glomerular hypertrophic diseases [41].

ER stress-associated apoptosis factor CHOP associated withType 2 diabetes

CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP) -10/growth arrest and DNA damage 153 is a dominant-negative member of the C/EBP transcription family. Chop is a leucine zipper transcription factor that is present, mostly in the cytosol, at low levels under normal conditions. Induction of CHOP after stress conditions accumulates in the nucleus [42]. CHOP was identified as an ER stress–induced transcription factor that induces apoptosis in response to ER stress. CHOP expression is strongly stimulated through IRE1- and PERK-mediated signaling [12, 45].The proapoptotic effect of CHOP that is transduced by Bim [44] emerges when ER stress cannot be subdued by the efforts of the prosurvival module of the response system, and the levels of misfolded proteins remain high. In this case, CHOP stimulates a transcription program that facilitates a pro-apoptotic program. This includes expression of proapoptotic Bim and repression of antiapoptotic Bcl-2 which represents a mechanism that is aligned with similar pro-apoptotic efforts of JNK. CHOP also induces death receptor 5 (DR5), which further sensitizes cells to apoptosis stimulation by a variety of conditions that cause ER stress [44, 45].

CHOP is a fundamental factor that triggers apoptosis. Recent findings showed that high levels of p21 induce cell cycle arrest and inhibit apoptosis that facilitates the pro-survival role of the UPR during the initial stages of ER stress. CHOP- dependent inhibition of p21 expression levels is consistent with the pro-apoptotic effects of ER stress [12]. Extension of these findings to diabetes- related ER stress remains to be seen.

Importance of chronic ER stress – mediated β cell apoptosis in AKITA mouse.

The Akita mouse harbors a mutation in the insulin 2 gene (Ins2) (Cys96Tyr) which results in disrupting a disulfide bond between A and B chains and causes a drastic conformation change of this molecule. Therefore, mutant insulin is not secreted and is degraded by the ERAD pathway that is associated with the induction of ER-stress, β cell dysfunction and finally β cell apoptosis. [46].

Role of chop deletion to the homozygous Ser51Ala mutant eIF2a

Accumulation of unfolded proteins within the endoplasmic reticulum (ER) attenuates mRNA translation through PERK-mediated phosphorylation of eukaryotic initiation factor 2 on the Ser51 residue of the a subunit (eIF2a). Diabetes associated with the loss of PERK activity is speculated to be due to a postnatal loss of β cells through apoptotic cell death arising from a failure to properly regulate the endoplasmic reticulum unfolded protein response (UPR) [47]. PERK deficiency may have a significant impact on physiological states associated with ER stress [48].

To elucidate the role of eIF2a phosphorylation, homozygous Ser51Ala mutant eIF2a mice were generated. Mice with a homozygous mutation at the eIF2α phosphorylation site (Ser51Ala) exhibited a neonatal lethal phenotype associated with accumulation of misfolded proteins and defective transport, oxidative stress related to mitochondrial damage, decreased expression of UPR, anti-oxidant and beta cell-associated genes, and finally with defective gluconeogenesis related to severe β-cell deficiency. These results indicate that the UPR has a broader role than maintaining functional ER protein and is essential for in vivo glucose homeostasis by various mechanisms [49].

Role of chop deletion to the heterozygous Ser51Ala mutant eIF2a

While heterozygous eIF2αs/A animals did not spontaneously manifest β-cell failure, upon feeding them with a 45% high-fat diet, these mice developed diabetes characterized by increased fasting blood glucose, glucose intolerance and a β-cell secretion defect. It was shown that this insulin secretion defect was due to an elevated rate of glucose-stimulated translation that compromised the activity of the unfolded protein folding response and led to the distention of the ER compartment, the prolonged association of misfolded proinsulin with the ER chaperone BiP and finally with the reduced secretory granule content [3].

Increasing evidence indicates that ER stress is associated with β-cell dysfunction failure of the β cells under conditions of increased insulin demand due to high-fat diet and insulin resistance.These findings demonstrated that regulation of mRNA translation via phosphorylation of eIF2α is crucial for the maintenance of cellular functional integrity of endoplasmic reticulum[3].While eIF2 phosphorylation inhibits protein translation, it is required for transcriptional induction of the majority of the ER stress-inducible genes associated with the UPR, including those encoding for the glucose-regulated proteins, such as GRP78/BiP, ERp72, GRP94, and CHOP. CHOP expression is increased in β cells from diabetic mice and humans. As ER distention and β-cell death in homozygous Ser51Ala eIF2α islets were apparent, β-cell failure, it is evident that there are enviromentical stimuli that induce eIF2α phosphorylation early in development that is fundamental for β-cell survival [50] The beta cell requirement for eIF2α phosphorylation is not mediated through Atf4 mRNA because mice lacking ATF4 have no reported defects in glycemic control [51]. Β cells from mice with homozygous Ser51Ala eIF2α underwent apoptosis in part signalled through CHOP. However Β cells from mice with heterozygous Ser51Ala eIF2α/chop-/- β cell mass was increased, β cell function was improved, and glucose intolerance was prevented compared with islets from Ser51Ala eIF2α/chop+/+ mice [3]. These supports that CHOP is involved in apoptosis of β cells However, as CHOP is not the only death signal promoted by ER stress, in the absence of chop other apoptotic pathways are evoked. CHOP deletion prevents β cell apoptosis and improves β cell function by preventing oxidative damage and preserve homeostasis in ER [3].

Effect of chop-null mutation in a HF diet/ model of T2DMice eIF2αs/s and eIF2αs/A mice were fed with 45% HF diet for 35–41 weeks. Body mass, glucose tolerance and levels of serum insulin were examined. In these models chop deletion increased obesity, and the number of functional β cells as a consequence of prevented glucose intolerance [3]

Effect of chop-null mutation in a HF diet/streptozotocin model of T2DWild-type and Chop-null mice were treated with streptozotocin (STZ), an agent with the ability to destroy pancreatic β-cell , were fed with a high cholesterol 60% HF diet for 5–6 weeks. Results indicated that in wild type mice β cell mass was reduced and developed hyperglycemia,. However in Chop-null mice improvement of β cell function was observed and glucose homeostasis was maintained. The findings indicated that Chop-null mutation in a HF diet–fed, STZ-treated nongenetic model of T2D has an impact to β cell function and recovery [3].

Effect of chop-null mutation in Leprdb/db mice.

db/db mouse are due to a mutation of a point mutation in the gene for the leptin receptor. It is characterized by insulin resistant, hypertriglyceridemic, and it has impaired glucose tolerance. Accumulated levels of UPR markes were detected in the islets of db/db mice [52]. Furthermore to investigate whether chop null mutation is involved in the maintenance of functional β cells, heterozygous Leprdb/+, and homozygous Leprdb/db mice were studied. Chop deletion in Leprdb/dbmice upregulates expression of UPR and antioxidative response markers and decreases expression of proapoptotic genes. The consequences in Leprdb/dbmice are the delayed glucose tolerance, induce β cell proliferation and prevention of β cell apoptosis [3].

Chop-null mutation protects β cells from ER- stress and oxidative stress

Chop+/+ islets and chop-/- islets, were treated with tunicamycin to inhibit N-linked glycosylation of proteins and promote ER-stress.These showed that there was an important increase in the oxidative degradation of proteins and lipids in wild-type islets, instead in Chop−/− [3]. Alternative treatment Chop+/+ islets and chop-/- islets with the oxidant H2O2, comparable amounts of oxidative damage was observed in islets from β-cells from wild type and chop null mice [3]. Consequently, Chop deletion decreases the oxidative damage that results from unfolded protein response in the ER, but not from general oxidative stress. The sum of these findings demonstrated that chop deletion improves β cell function, prevents apoptosis by improving the capacity of ER to produce folded proteins [3].

CONCLUSIONS

The cellular and molecular mechanisms governing ER-stress is fundamental for understanding the consequences caused by type 2 diabetes. Growing evidence indicates that hyper-activation of the UPR indispensable for ER homeostasis has a role in β cell failure and dysfunction during T2D and genetic types of diabetes considering the unfolded protein response as a mediator in the development of its complications. Paradoxically there are interrelationships between type 2 diabetes and balance between survival pathway and apoptosis pathway related to ER- stress. It is currently believed that overexpression of p21 is responsible for the recovering of STZ- induced diabetes so its implicates to the survival pathway. Alternatively Chop deletion determines the transcriptional profile of the cell to preserve the functional capacity of the ER and reduce accumulation of ROS, so it implicates to the apoptotic pathway. Our findings should encourage the search for p21 and chop sensors of ER stress signaling that have the potential to recover the functional capacity of the ER for the treatment of numerous diseases associated with unfolded protein in the endoplasmic reticulum. Complete understanding of these balanced mechanisms will lead to novel therapeutic modalities for diabetes.