Ubiquitin C-terminal hydrolase L1 is required for pancreatic beta cell survival and function in lipotoxic conditions

Diabetic sensory neuropathy and insulin resistance are induced by loss of UCHL1 in Drosophila

Diabetic sensory neuropathy (DSN) is one of the most common complications of type 2 diabetes (T2D), however the molecular mechanistic association between T2D and DSN remains elusive. Here we identify ubiquitin C-terminal hydrolase L1 (UCHL1), a deubiquitinase highly expressed in neurons, as a key molecule underlying T2D and DSN. Genetic ablation of UCHL1 leads to neuronal insulin resistance and T2D-related symptoms in Drosophila. Furthermore, loss of UCHL1 induces DSN-like phenotypes, including numbness to external noxious stimuli and axonal degeneration of sensory neurons in flies' legs. Conversely, UCHL1 overexpression improves DSN-like defects of T2D model flies. UCHL1 governs insulin signaling by deubiquitinating insulin receptor substrate 1 (IRS1) and antagonizes an E3 ligase of IRS1, Cullin 1 (CUL1). Consistent with these results, genetic and pharmacological suppression of CUL1 activity rescues T2D- and DSN-associated phenotypes. Therefore, our findings suggest a complete set of genetic factors explaining T2D and DSN, together with potential remedies for the diseases.

Crucial roles of UCH-L1 on insulin-producing cells and carbohydrate metabolism in Drosophila melanogaster model

Ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) is a highly expressed protein in β cells and has been implicated in β cells' viability and function, however, the role of UCH-L1 in β cells remains unclear. Herein, we examined the functions of UCH-L1 in β cells by utilizing the Drosophila melanogaster model. Our results showed that specific knockdown of dUCH (D.melanogaster homolog of UCH-L1) in Drosophila Insulin-producing cells (D.melanogaster homolog of β cells) induced mitochondria fusion, IPCs death/degeneration, interfered with DILP2 secretion, and triggered the rise of glycogen storage and body weight. Strikingly, the impairment in IPCs cellular activities can be rescued by vitamin C- a strong antioxidant compound, which suggested the relationship between knockdown dUCH and oxidative stress in IPCs; and the potential of this model in screening compounds for β cells function moderation. Since carbohydrate metabolism is an important function of beta cells, we continued to examine the ability to regulate carbohydrate metabolism of knockdown dUCH flies. Our results showed that knockdown dUCH caused the decline of IPCs number under a high-sucrose diet, which finally led to metabolic and physiological disturbances, including total lipid rise, glycogen storage reduction, circulating carbohydrate increase, and weight loss. These symptoms could be early indications of metabolic disorders, particularly β cell dysfunction-related diseases. Taken together, our results indicate that dUCH is essential in the viability and functions of IPCs through the regulation of carbohydrate metabolism in the Drosophila model.

UCHL1 and Proteasome in Blood Serum in Relation to Dietary Habits, Concentration of Selected Antioxidant Minerals and Total Antioxidant Status among Patients with Alzheimer's Disease

Alzheimer's disease (AD) is an incurable neurodegenerative disease. It is the most common form of dementia among the elderly population. So far, no effective methods of its treatment have been found. Research to better understand the mechanism of pathology may provide new methods for early diagnosis. This, in turn, could enable early intervention that could slow or halt disease progression and improve patients' quality of life. Therefore, minimally invasive markers, including serum-based markers, are being sought to improve the diagnosis of AD. One of the important markers may be the concentration of UCHL1 and the proteasome in the blood serum. Their concentration can be affected by many factors, including eating habits. This study was conducted in 110 patients with early or moderate AD, with a mean age of 78.0 ± 8.1 years. The patients were under the care of the Podlasie Center of Psychogeriatrics and the Department of Neurology (Medical University of Białystok, Poland). The control group consisted of 60 healthy volunteers, matched for gender and age. The concentration of UCHL1 and the 20S proteasome subunit were measured by surface plasmon resonance imaging (SPRI). In addition, a nutritional interview was conducted with patients with AD, which assessed the frequency of consumption of 36 groups of products. In the group of patients with AD, compared to the control group, we showed a significantly higher concentration of UCHL1 (56.05 vs. 7.98 ng/mL) and the proteasome (13.02 vs. 5.72 µg/mL). Moreover, we found a low negative correlation between UCHL1 and the proteasome in the control group, and positive in the AD group. The analysis of eating habits showed that the consumption of selected groups of products may affect the concentration of the tested components, and therefore may have a protective effect on AD.

Islet amyloid toxicity: From genesis to counteracting mechanisms

Islet amyloid polypeptide (IAPP or amylin) is a hormone co-secreted with insulin by pancreatic β-cells and is the major component of islet amyloid. Islet amyloid is found in the pancreas of patients with type 2 diabetes (T2D) and may be involved in β-cell dysfunction and death, observed in this disease. Thus, investigating the aspects related to amyloid formation is relevant to the development of strategies towards β-cell protection. In this sense, IAPP misprocessing, IAPP overproduction, and disturbances in intra- and extracellular environments seem to be decisive for IAPP to form islet amyloid. Islet amyloid toxicity in β-cells may be triggered in intra- and/or extracellular sites by membrane damage, endoplasmic reticulum stress, autophagy disruption, mitochondrial dysfunction, inflammation, and apoptosis. Importantly, different approaches have been suggested to prevent islet amyloid cytotoxicity, from inhibition of IAPP aggregation to attenuation of cell death mechanisms. Such approaches have improved β-cell function and prevented the development of hyperglycemia in animals. Therefore, counteracting islet amyloid may be a promising therapy for T2D treatment.

Ovarian Transcriptomic Analysis Reveals Differential Expression Genes Associated with Cell Death Process after Selection for Ovulation Rate in Rabbits

Simple Summary

Transcriptomic analysis showed nineteen potential biomarkers in ovarian tissue from females belonged to a rabbit line selected for ovulation rate for 10 generations and the control line. These females differed not only in ovulation rate but also in prenatal survival since similar litter size were observed.

Abstract

Litter size is an essential trait in rabbit meat production but with low heritability. A selection experiment for ovulation rate has been performed for 10 generations to improve litter size in rabbits. The selected line increased two ova more than the control line but nevertheless a negative correlation was observed with prenatal survival. A transcriptomic study was performed, using microarrays, in ovarian tissue from females belonging to the selected line and the control line. Our results showed 1357 differential expressed genes and nineteen potential biomarkers associated with prenatal mortality, which could explain differences between litter size in rabbits. Cell death was the most relevant process.

Ubiquitin carboxyl-terminal hydrolase L1 promotes hypoxia-inducible factor 1-dependent tumor cell malignancy in spheroid models

Hypoxia-inducible factor 1 (HIF-1) is a critical heterodimeric transcription factor for tumor malignancy. Recently, ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) has been reported to function as a deubiquitinating enzyme for the stabilization of its α subunit (HIF-1α). In the present study, we showed that UCHL1 inhibition can be an effective therapeutic strategy against HIF-1-dependent tumor malignancy. In 2D monolayer culture, a UCHL1 inhibitor suppressed HIF activity and decreased the transcription of HIF downstream genes by inhibiting the UCHL1-mediated accumulation of HIF-1α. Phenotypically, UCHL1 inhibition remarkably blocked cell migration. In 3D spheroid culture models, ectopic expression of UCHL1 significantly upregulated malignancy-related factors such as solidity, volume, as well as viable cell number in an HIF-1α-dependent manner. Conversely, inhibition of the UCHL1-HIF-1 pathway downregulated these malignancy-related factors and also abolished UCHL1-mediated cell proliferation and invasiveness. Finally, inhibition of UCHL1 promoted HIF-1α degradation and lowered the expression of HIF-1 target genes in the 3D model, as also observed in 2D monolayer culture. Our research indicates that the UCHL1-HIF-1 pathway plays a crucial role in tumor malignancy, making it a promising therapeutic target for cancer chemotherapy.

Inhibition of UCHL1 by LDN-57444 attenuates Ang II-Induced atrial fibrillation in mice

Atrial fibrillation (AF) is the most common human arrhythmia in clinical practice and may be promoted by atrial inflammation and fibrosis. Ubiquitination is an important posttranslational modification process that is reversed by deubiquitinating enzymes (DUBs). DUBs play critical roles in modulating the degradation, activity, trafficking, and recycling of substrates. However, less research has focused on the role of DUBs in AF. Here, we investigated the effect of ubiquitin C-terminal hydrolase 1 (UCHL1), an important DUB, on the development of AF induced by angiotensin II (Ang II). Male wild-type mice were treated with the UCHL1 inhibitor LDN57444 (LDN) at a dose of 40 μg/kg and infused with Ang II (2000 ng/kg/min) for 3 weeks. Our results showed that Ang II-infused wild-type (WT) mice had higher systolic blood pressure and an increased incidence and duration of AF. Conversely, this effect was attenuated in LDN-treated mice. Moreover, the administration of LDN significantly reduced Ang II-induced left atrial dilation, fibrosis, inflammatory cell infiltration, and reactive oxygen species (ROS) production. Mechanistically, LDN treatment inhibited the activation of multiple signaling pathways (the AKT, ERK1/2, HIF-1α, and TGF-β/smad2/3 pathways) and the expression of CX43 protein in atrial tissues compared with that in vehicle-treated control mice. Overall, our study identified UCHL1 as a novel regulator that contributes to Ang II-induced AF and suggests that the administration of LDN may represent a potential therapeutic approach for treating hypertensive AF.

Downregulated UCHL1 Accelerates Gentamicin-Induced Auditory Cell Death via Autophagy

The clinical use of aminoglycoside antibiotics is partly limited by their ototoxicity. The pathogenesis of aminoglycoside-induced ototoxicity still remains unknown. Here, RNA-sequencing was conducted to identify differentially expressed genes in rat cochlear organotypic cultures treated with gentamicin (GM), and 232 and 43 genes were commonly up- and downregulated, respectively, at day 1 and 2 after exposure. Ubiquitin carboxyl-terminal hydrolase isozyme L1 (Uchl1) was one of the downregulated genes whose expression was prominent in spiral ganglion cells (SGCs), lateral walls, as well as efferent nerve terminal and nerve fibers. We further investigated if a deficit of Uchl1 in organotypic cochlea and the House Ear Institute-Organ of Corti 1 (HEI-OC1) cells accelerates ototoxicity. We found that a deficit in Uchl1 accelerated GM-induced ototoxicity by showing a decreased number of SGCs and nerve fibers in organotypic cochlear cultures and HEI-OC1 cells. Furthermore, Uchl1-depleted HEI-OC1 cells revealed an increased number of autophagosomes accompanied by decreased lysosomal fusion. These data indicate that the downregulation of Uchl1 following GM treatment is deleterious to auditory cell survival, which results from the impaired autophagic flux. Our results provide evidence that UCHL1-dependent autophagic flux may have a potential as an otoprotective target for the treatment of GM-induced auditory cell death.

Diabetes relief in mice by glucose-sensing insulin-secreting human α-cells

Cell identity switches, where terminally-differentiated cells convert into different cell-types when stressed, represent a widespread regenerative strategy in animals, yet they are poorly documented in mammals. In mice, some glucagon-producing pancreatic α-cells and somatostatin-producing δ-cells become insulin expressers upon ablation of insulin-secreting β-cells, promoting diabetes recovery. Whether human islets also display this plasticity, especially in diabetic conditions, remains unknown. Here we show that islet non-β-cells, namely α-cells and PPY-producing γ–cells, obtained from deceased non-diabetic or diabetic human donors, can be lineage-traced and reprogrammed by the transcription factors Pdx1 and MafA to produce and secrete insulin in response to glucose. When transplanted into diabetic mice, converted human α-cells reverse diabetes and remain producing insulin even after 6 months. Surprisingly, insulin-producing α-cells maintain α-cell markers, as seen by deep transcriptomic and proteomic characterization. These observations provide conceptual evidence and a molecular framework for a mechanistic understanding of in situ cell plasticity as a treatment for diabetes and other degenerative diseases.

Hyperfiltration in ubiquitin C-terminal hydrolase L1-deleted mice

Neuronal ubiquitin C-terminal hydrolase L1 (UCHL1) is a deubiquitinating enzyme that maintains intracellular ubiquitin pools and promotes axonal transport. Uchl1 deletion in mice leads to progressive axonal degeneration, affecting the dorsal root ganglion that harbors axons emanating to the kidney. Innervation is a crucial regulator of renal hemodynamics, though the contribution of neuronal UCHL1 to this is unclear. Immunofluorescence revealed significant neuronal UCHL1 expression in mouse kidney, including periglomerular axons. Glomerular filtration rate trended higher in 6-week-old Uchl1-/- mice, and by 12 weeks of age, these displayed significant glomerular hyperfiltration, coincident with the onset of neurodegeneration. Angiotensin converting enzyme inhibition had no effect on glomerular filtration rate of Uchl1-/- mice indicating that the renin–angiotensin system does not contribute to the observed hyperfiltration. DCE-MRI revealed increased cortical renal blood flow in Uchl1-/- mice, suggesting that hyperfiltration results from afferent arteriole dilation. Nonetheless, hyperglycemia, cyclooxygenase-2, and nitric oxide synthases were ruled out as sources of hyperfiltration in Uchl1-/- mice as glomerular filtration rate remained unchanged following insulin treatment, and cyclooxygenase-2 and nitric oxide synthase inhibition. Finally, renal nerve dysfunction in Uchl1-/- mice is suggested given increased renal nerve arborization, decreased urinary norepinephrine, and impaired vascular reactivity. Uchl1-deleted mice demonstrate glomerular hyperfiltration associated with renal neuronal dysfunction, suggesting that neuronal UCHL1 plays a crucial role in regulating renal hemodynamics.

Ubiquitin C-terminal hydrolase isozyme L1 is associated with shelterin complex at interstitial telomeric sites

BACKGROUND

Ubiquitin C-terminal hydrolase isozyme L1 (UCHL1) is primarily expressed in neuronal cells and neuroendocrine cells and has been associated with various diseases, including many cancers. It is a multifunctional protein involved in deubiquitination, ubiquitination and ubiquitin homeostasis, but its specific roles are disputed and still generally undetermined.

RESULTS

Herein, we demonstrate that UCHL1 is associated with genomic DNA in certain prostate cancer cell lines, including DU 145 cells derived from a brain metastatic site, and in HEK293T embryonic kidney cells with a neuronal lineage. Chromatin immunoprecipitation and sequencing revealed that UCHL1 localizes to TTAGGG repeats at telomeres and interstitial telomeric sequences, as do TRF1 and TRF2, components of the shelterin complex. A weak or transient interaction between UCHL1 and the shelterin complex was confirmed by immunoprecipitation and proximity ligation assays. UCHL1 and RAP1, also known as TERF2IP and a component of the shelterin complex, were bound to the nuclear scaffold.

CONCLUSIONS

We demonstrated a novel feature of UCHL1 in binding telomeres and interstitial telomeric sites.

Activation of NLRP3 Inflammasome by Advanced Glycation End Products Promotes Pancreatic Islet Damage

Accumulation of advanced glycation end products (AGEs) contributes to ageing and age-related diseases, especially type 2 diabetes. The NLRP3 inflammasome, as a vital component of the innate immune system, is implicated in the pathogenesis of type 2 diabetes. However, the role of the NLRP3 inflammasome in AGE-induced pancreatic islet damage remains largely unclear. Results showed that administration of AGEs (120 mg/kg for 6 weeks) in C57BL/6J mice induced an abnormal response to glucose (as measured by glucose tolerance and insulin release), pancreatic β-cell ultrastructural lesion, and cell death. These effects were associated with an excessive superoxide anion level, significant increased protein expression levels for NADPH oxidase 2 (NOX2), thioredoxin-interacting protein (TXNIP), NLRP3, and cleaved IL-1β, enhanced caspase-1 activity, and a significant increase in the levels of TXNIP–NLRP3 protein interaction. Ablation of the NLRP3 inflammasome or treatment with antioxidant N-acetyl-cysteine (NAC) clearly ameliorated these effects. In conclusion, our results reveal a possible mechanism for AGE-induced pancreatic islet damage upon NLRP3 inflammasome activation.

Reduced Insulin Production Relieves Endoplasmic Reticulum Stress and Induces β Cell Proliferation

Pancreatic β cells are mostly post-mitotic, but it is unclear what locks them in this state. Perturbations including uncontrolled hyperglycemia can drive β cells into more pliable states with reduced cellular insulin levels, increased β cell proliferation, and hormone mis-expression, but it is unknown whether reduced insulin production itself plays a role. Here, we define the effects of ∼50% reduced insulin production in Ins1(-/-):Ins2(f/f):Pdx1Cre(ERT):mTmG mice prior to robust hyperglycemia. Transcriptome, proteome, and network analysis revealed alleviation of chronic endoplasmic reticulum (ER) stress, indicated by reduced Ddit3, Trib3, and Atf4 expression; reduced Xbp1 splicing; and reduced phospho-eIF2α. This state was associated with hyper-phosphorylation of Akt, which is negatively regulated by Trib3, and with cyclinD1 upregulation. Remarkably, β cell proliferation was increased 2-fold after reduced insulin production independently of hyperglycemia. Eventually, recombined cells mis-expressed glucagon in the hyperglycemic state. We conclude that the normally high rate of insulin production suppresses β cell proliferation in a cell-autonomous manner.

Type 2 diabetes as a protein misfolding disease

Type 2 diabetes (T2D) is a highly prevalent and chronic metabolic disorder. Recent evidence suggests that formation of toxic aggregates of the islet amyloid polypeptide (IAPP) might contribute to β-cell dysfunction and disease. However, the mechanism of protein aggregation and associated toxicity remains unclear. Misfolding, aggregation, and accumulation of diverse proteins in various organs is the hallmark of the group of protein misfolding disorders (PMDs), including highly prevalent illnesses affecting the central nervous system (CNS) such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this review we discuss the current understanding of the mechanisms implicated in the formation of protein aggregates in the endocrine pancreas and associated toxicity in the light of the long-standing knowledge from neurodegenerative disorders associated with protein misfolding.

Proteomic analysis of fetal programming-related obesity markers

The objectives of this study were to analyze fetal programming in rat brain using proteomic analysis and to identify fetal programming-related obesity markers. Sprague-Dawley rats were divided into four feeding groups: (i) the Ad Libitum (AdLib)/AdLib group was given a normal diet during pregnancy and the lactation period; (ii) the AdLib/maternal food restriction group (FR) was subjected to 50% FR during the lactation period; (iii) the FR/AdLib group was subjected to 50% FR during pregnancy; and (iv) the FR/FR group was subjected to 50% FR during pregnancy and the lactation period. Offspring from each group were sacrificed at 3 weeks of age and whole brains were dissected. To obtain a maximum number of protein markers related to obesity, 2DE and Pathway Studio bioinformatics analysis were performed. The identities of the markers among the selected and candidate proteins were confirmed by Western blotting and immunohistochemistry. Proteomic and bioinformatics analyses revealed that expression of ubiquitin carboxy-terminal hydrolase L1 (UCHL1) and Secernin 1 (SCRN1) were significantly different in the FR/AdLib group compared with the AdLib/AdLib group for both male and female offspring. These findings suggest that UCHL1 and SCRN1 may be used as fetal programming-related obesity markers.

UCHL1 deficiency exacerbates human islet amyloid polypeptide toxicity in β-cells: evidence of interplay between the ubiquitin/proteasome system and autophagy

The islet in type 2 diabetes mellitus (T2DM) is characterized by a deficit in β-cells and increased β-cell apoptosis attributable at least in part to intracellular toxic oligomers of IAPP (islet amyloid polypeptide). β-cells of individuals with T2DM are also characterized by accumulation of polyubiquitinated proteins and deficiency in the deubiquitinating enzyme UCHL1 (ubiquitin carboxyl-terminal esterase L1 [ubiquitin thiolesterase]), accounting for a dysfunctional ubiquitin/proteasome system. In the present study, we used mouse genetics to elucidate in vivo whether a partial deficit in UCHL1 enhances the vulnerability of β-cells to human-IAPP (hIAPP) toxicity, and thus accelerates diabetes onset. We further investigated whether a genetically induced deficit in UCHL1 function in β-cells exacerbates hIAPP-induced alteration of the autophagy pathway in vivo. We report that a deficit in UCHL1 accelerated the onset of diabetes in hIAPP transgenic mice, due to a decrease in β-cell mass caused by increased β-cell apoptosis. We report that UCHL1 dysfunction aggravated the hIAPP-induced defect in the autophagy/lysosomal pathway, illustrated by the marked accumulation of autophagosomes and cytoplasmic inclusions positive for SQSTM1/p62 and polyubiquitinated proteins with lysine 63-specific ubiquitin chains. Collectively, this study shows that defective UCHL1 function may be an early contributor to vulnerability of pancreatic β-cells for protein misfolding and proteotoxicity, hallmark defects in islets of T2DM. Also, given that deficiency in UCHL1 exacerbated the defective autophagy/lysosomal degradation characteristic of hIAPP proteotoxicity, we demonstrate a previously unrecognized role of UCHL1 in the function of the autophagy/lysosomal pathway in β-cells.

Human IAPP amyloidogenic properties and pancreatic β-cell death

A hallmark of type 2 diabetes mellitus (T2DM) is the presence of extracellular amyloid deposits in the islets of Langerhans. These deposits are formed by the human islet amyloid polypeptide, hIAPP (or amylin), which is a hormone costored and cosecreted with insulin. Under normal conditions, the hormone remains in solution but, in the pancreas of T2DM individuals, it undergoes misfolding giving rise to oligomers and cross-β amyloid fibrils. Accumulating evidence suggests that the amyloid deposits that accompany type 2 diabetes mellitus are not just a trivial epiphenomenon derived from the disease progression. Rather, hIAPP aggregation induces processes that impair the functionality and viability of β-cells and may lead to apoptosis. The present review article aims to summarize a few aspects of the current knowledge of this amyloidogenic polypeptide. In the first place, the physicochemical properties which condition its propensity to misfold and form aggregates. Secondly, how these properties confer hIAPP the capacity to interfere with some signaling of the pancreatic β-cell, interact with membranes, form channels or affect natural ion channels, including calcium channels. Finally, how misfolded hIAPP cytotoxicity results in apoptosis. A number of pathophysiological changes of the T2DM islet can be related to the amyloidogenic properties of hIAPP. However, in a certain way, the in vivo aggregation of the polypeptide also reflects a failure of chaperones and, in general, of cellular proteostasis, supporting the view that T2DM may also be considered as a conformational disorder.

Lipotoxic endoplasmic reticulum stress, β cell failure, and type 2 diabetes mellitus

Failure of the unfolded protein response (UPR) to maintain optimal folding of pro-insulin in the endoplasmic reticulum (ER) leads to unresolved ER stress and β cell death. This contributes not only to some rare forms of diabetes, but also to type 2 diabetes mellitus (T2DM). Many key findings, elaborated over the past decade, are based on the lipotoxicity model, entailing chronic exposure of β cells to elevated levels of fatty acids (FAs). Here, we update recent progress on how FAs initiate ER stress, particularly via disruption of protein trafficking, and how this leads to apoptosis. We also highlight differences in how β cells are impacted by the classic UPR, versus the more selective UPR that arises as part of a broader response to lipotoxicity.

Lipotoxicity in the pancreatic beta cell: not just survival and function, but proliferation as well?

Free fatty acids (FFAs) exert both positive and negative effects on beta cell survival and insulin secretory function, depending on concentration, duration, and glucose abundance. Lipid signals are mediated not only through metabolic pathways, but also through cell surface and nuclear receptors. Toxicity is modulated by positive signals arising from circulating factors such as hormones, growth factors and incretins, as well as negative signals such as inflammatory mediators and cytokines. Intracellular mechanisms of lipotoxicity include metabolic interference and cellular stress responses such as oxidative stress, endoplasmic reticulum (ER) stress, and possibly autophagy. New findings strengthen an old hypothesis that lipids may also impair compensatory beta cell proliferation. Clinical observations continue to support a role for lipid biology in the risk and progression of both type 1 (T1D) and type 2 diabetes (T2D). This review summarizes recent work in this important, rapidly evolving field.

Multi-parameter single-cell kinetic analysis reveals multiple modes of cell death in primary pancreatic β-cells

Programmed β-cell death plays an important role in both type 1 and type 2 diabetes. Most of what is known about the mechanisms of β-cell death comes from single time-point, single parameter measurements of bulk populations of mixed cells. Such approaches are inadequate for determining the true extent of the heterogeneity in death mechanisms. Here, we characterized the timing and order of molecular events associated with cell death in single β-cells under multiple diabetic stress conditions, including hyperglycemia, cytokine exposure, nutrient deprivation and endoplasmic reticulum (ER) stress. We simultaneously measured the kinetics of six distinct cell death mechanisms by using a caspase-3 sensor and three vital dyes, together with brightfield imaging. We identified several cell death modes where the order of events that usually define apoptosis were not observed. This we termed 'partial apoptosis'. Remarkably, complete classical apoptosis, defined as cells with plasma membrane blebbing, caspase-3 activity, nuclear condensation and membrane annexin V labeling prior to loss of plasma membrane integrity, was found in only half of the cytokine-treated primary β-cells and never in cells stressed by serum removal. By contrast, in the MIN6 cell line, death occurred almost exclusively through complete classical apoptosis. Ambient glucose modulated the cell death mode and kinetics in primary β-cells. Taken together, our data define the kinetic progression of β-cell death mechanisms under different conditions and illustrate the heterogeneity and plasticity of cell death modes in β-cells. We conclude that apoptosis is not the primary mode of adult primary β-cell death.

Signaling molecules involved in lipid-induced pancreatic beta-cell dysfunction

The increasing incidence of type 2 diabetes mellitus is partially due to the rising obesity rates and the elevated levels of free fatty acids (FFAs). It is known that FFAs are putative mediators of beta-cell dysfunction, which is characterized with impaired glucose-stimulated insulin secretion and increased apoptosis, being defined as lipotoxicity. To date, many factors and their related signal pathways have been reported to be involved in FFA-induced beta-cell dysfunction. However, the entire blueprint is still not obtained. Some essential and newfound effectors, including the sterol regulatory element-binding protein (SREBP)-1c, farnesoid X receptor (FXR), forkhead box-containing protein O (FoxO) 1, ubiquitin C-terminal hydrolase L (UCHL) 1, N-myc downstream-regulated gene (NDRG) 2, perilipin family proteins, silent information regulator 2 protein 1 (Sirt1), pituitary adenylate cyclase-activating polypeptide (PACAP), and ghrelin are described in this review, which may help to further understand the molecular network for lipotoxicity.