Glucotoxic conditions induce endoplasmic reticulum stress to cause caspase 3 mediated lamin B degradation in pancreatic β-cells: protection by nifedipine

The Role of Endoplasmic Reticulum Stress in Calcific Aortic Valve Disease

Calcific aortic valve disease (CAVD), which is involved in osteogenic reprogramming of valvular interstitial cells, is the most common form of valve disease. It still lacks effective pharmacologic intervention, as its cellular biological mechanisms remain unclear. Congenital abnormality (bicuspid valve) and older age are considered to be the most powerful risk factors for CAVD. Aortic valve sclerosis (AVS) and calcific aortic stenosis (CAS), 2 subclinical forms of CAVD, represent 2 distinct stages of aortic valve calcification. During the AVS stage, the disease is characterised by endothelial activation/damage, inflammatory response, and lipid infiltration accompanied by microcalcification. The CAS stage is dominated by calcification, resulting in valvular dysfunction and severe obstruction to cardiac outflow, which is life threatening if surgery is not performed in time. Endoplasmic reticulum (ER) stress, a state in which conditions disrupting ER homeostasis cause an accumulation of unfolded and misfolded proteins in the ER lumen, has been shown to promote osteogenic differentiation and aortic valve calcification. Therefore, identifying targets or drugs for suppressing ER stress may be a novel approach for CAVD treatment.

Small Molecule-mediated Insulin Hypersecretion Induces Transient ER Stress Response and Loss of Beta Cell Function

Pancreatic islet beta cells require a fine-tuned endoplasmic reticulum (ER) stress response for normal function; abnormal ER stress contributes to diabetes pathogenesis. Here, we identified a small molecule, SW016789, with time-dependent effects on beta cell ER stress and function. Acute treatment with SW016789 potentiated nutrient-induced calcium influx and insulin secretion, while chronic exposure to SW016789 transiently induced ER stress and shut down secretory function in a reversible manner. Distinct from the effects of thapsigargin, SW016789 did not affect beta cell viability or apoptosis, potentially due to a rapid induction of adaptive genes, weak signaling through the eIF2α kinase PERK, and lack of oxidative stress gene Txnip induction. We determined that SW016789 acted upstream of voltage-dependent calcium channels (VDCCs) and potentiated nutrient- but not KCl-stimulated calcium influx. Measurements of metabolomics, oxygen consumption rate, and G protein-coupled receptor signaling did not explain the potentiating effects of SW016789. In chemical cotreatment experiments, we discovered synergy between SW016789 and activators of protein kinase C and VDCCs, suggesting involvement of these pathways in the mechanism of action. Finally, chronically elevated calcium influx was required for the inhibitory impact of SW016789, as blockade of VDCCs protected human islets and MIN6 beta cells from hypersecretion-induced dysfunction. We conclude that beta cells undergoing this type of pharmacological hypersecretion have the capacity to suppress their function to mitigate ER stress and avoid apoptosis. These results have the potential to uncover beta cell ER stress mitigation factors and add support to beta cell rest strategies to preserve function.

BHPF exposure impairs mouse and human decidualization

Although BHPF has been widely used in plastic manufacturing as a substitute for BPA, current evidence suggests that BHPF also causes harmful effects on reproduction. However, effects of BHPF on mammalian early pregnancy are still poorly defined. This study aimed to explore the effects of BHPF on early pregnancy, especially decidualization and embryonic development in mice and human beings. The results showed that 50 and 100 mg/kg BHPF exposure reduced birth weight, and implantation site weight on the day 8 of pregnancy in mice. Because BHPF inhibits both embryo development and artificial decidualization in mice, suggesting that the detrimental effects of BHPF should be from its effects on embryo development and decidualization. Under in vitro decidualization, 10 μM BHPF inhibits decidualization and leads to disordered expression of Lamin B1 and collagen in mice. In addition, 10 μM BHPF also inhibits decidualization, and causes disordered expression of both collagen III and Lamin B1 under human in vitro decidualization. However, collagen III supplementation can rescue BHPF inhibition on decidualization. Further, our study demonstrates that BHPF impairs human decidualization through the HB-EGF/EGFR/STAT3/Collagen III pathway. Taken together these data suggest that exposure to BHPF impairs mouse and human decidualization during early pregnancy.

Cadmium inhibits signal transducer and activator of transcription 6 leading to pancreatic β cell apoptosis

The toxic heavy metal cadmium has been proven to cause pancreatic dysfunction and lead to the development of DM. However, the underlying mechanisms have not been completely elucidated. Here, we investigated the effects of cadmium on the pancreatic β cell line MIN6 and explored the underlying mechanisms. The Cell Counting Kit-8 (CCK8) assay and flow cytometry were used to determine cell viability and apoptosis in MIN6 cells. The expression levels of signal transducer and activator of transcription 6 (STAT6) were assessed by western blotting. We further assessed the effects of cadmium on the function of pancreatic β cells under high glucose levels using enzyme-linked immunosorbent assay (ELISA) and western blotting. Insulin secretion and expression were decreased by cadmium in MIN6 cells. In addition, cadmium suppressed cell viability and promoted apoptosis of MIN6 cells, downregulated insulin secretion and genesis of MIN6 cells under high glucose conditions, while inhibiting STAT6. Furthermore, after treatment with IL-4, the activator of STAT6, the MIN6 cell viability suppression and apoptosis promotion effect caused by cadmium were blocked. In conclusion, cadmium inhibits pancreatic β cell MIN6 growth by regulating the activation of STAT6. Our findings reveal a new mechanism of cadmium toxicity in pancreatic β cells.

CARD9 Mediates Pancreatic Islet Beta-Cell Dysfunction Under the Duress of Hyperglycemic Stress

BACKGROUND/AIMS

Published evidence implicates Caspase recruitment domain containing protein 9 (CARD9) in innate immunity. Given its recently suggested roles in obesity and insulin resistance, we investigated its regulatory role(s) in the onset of islet beta cell dysfunction under chronic hyperglycemic (metabolic stress) conditions.

METHODS

Islets from mouse pancreas were isolated by the collagenase digestion method. Expression of CARD9 was suppressed in INS-1 832/13 cells by siRNA transfection using the DharmaFect1 reagent. The degree of activation of Rac1 was assessed by a pull-down assay kit. Interactions between CARD9, RhoGDIβ and Rac1 under metabolic stress conditions were determined by co-immunoprecipitation assay. The degree of phosphorylation of stress kinases was assessed using antibodies directed against phosphorylated forms of the respective kinases.

RESULTS

CARD9 expression is significantly increased following exposure to high glucose, not to mannitol (both at 20 mM; 24 hrs.) in INS-1 832/13 cells. siRNA-mediated knockdown of CARD9 significantly attenuated high glucose-induced activation of Rac1 and phosphorylation of p38MAPK and p65 subunit of NF-κB (RelA), without significantly impacting high glucose-induced effects on JNK1/2 and ERK1/2 activities. CARD9 depletion also suppressed high glucose-induced CHOP expression (a marker for endoplasmic reticulum stress) in these cells. Co-immunoprecipitation studies revealed increased association between CARD9-RhoGDIβ and decreased association between RhoGDIβ-Rac1 in cells cultured under high glucose conditions.

CONCLUSION

Based on these data, we conclude that CARD9 regulates activation of Rac1-p38MAPK-NFκB signaling pathway leading to functional abnormalities in beta cells under metabolic stress conditions.

Nimodipine Exerts Time-Dependent Neuroprotective Effect after Excitotoxical Damage in Organotypic Slice Cultures

During injuries in the central nervous system, intrinsic protective processes become activated. However, cellular reactions, especially those of glia cells, are frequently unsatisfactory, and further exogenous protective mechanisms are necessary. Nimodipine, a lipophilic L-type calcium channel blocking agent is clinically used in the treatment of aneurysmal subarachnoid haemorrhage with neuroprotective effects in different models. Direct effects of nimodipine on neurons amongst others were observed in the hippocampus as well as its influence on both microglia and astrocytes. Earlier studies proposed that nimodipine protective actions occur not only via calcium channel-mediated vasodilatation but also via further time-dependent mechanisms. In this study, the effect of nimodipine application was investigated in different time frames on neuronal damage in excitotoxically lesioned organotypic hippocampal slice cultures. Nimodipine, but not nifedipine if pre-incubated for 4 h or co-applied with NMDA, was protective, indicating time dependency. Since blood vessels play no significant role in our model, intrinsic brain cell-dependent mechanisms seems to strongly be involved. We also examined the effect of nimodipine and nifedipine on microglia survival. Nimodipine seem to be a promising agent to reduce secondary damage and reduce excitotoxic damage.

SGLT2 inhibitors therapy protects glucotoxicity-induced β-cell failure in a mouse model of human KATP-induced diabetes through mitigation of oxidative and ER stress

Progressive loss of pancreatic β-cell functional mass and anti-diabetic drug responsivity are classic findings in diabetes, frequently attributed to compensatory insulin hypersecretion and β-cell exhaustion. However, loss of β-cell mass and identity still occurs in mouse models of human KATP-gain-of-function induced Neonatal Diabetes Mellitus (NDM), in the absence of insulin secretion. Here we studied the temporal progression and mechanisms underlying glucotoxicity-induced loss of functional β-cell mass in NDM mice, and the effects of sodium-glucose transporter 2 inhibitors (SGLT2i) therapy. Upon tamoxifen induction of transgene expression, NDM mice rapidly developed severe diabetes followed by an unexpected loss of insulin content, decreased proinsulin processing and increased proinsulin at 2-weeks of diabetes. These early events were accompanied by a marked increase in β-cell oxidative and ER stress, without changes in islet cell identity. Strikingly, treatment with the SGLT2 inhibitor dapagliflozin restored insulin content, decreased proinsulin:insulin ratio and reduced oxidative and ER stress. However, despite reduction of blood glucose, dapagliflozin therapy was ineffective in restoring β-cell function in NDM mice when it was initiated at >40 days of diabetes, when loss of β-cell mass and identity had already occurred. Our data from mouse models demonstrate that: i) hyperglycemia per se, and not insulin hypersecretion, drives β-cell failure in diabetes, ii) recovery of β-cell function by SGLT2 inhibitors is potentially through reduction of oxidative and ER stress, iii) SGLT2 inhibitors revert/prevent β-cell failure when used in early stages of diabetes, but not when loss of β-cell mass/identity already occurred, iv) common execution pathways may underlie loss and recovery of β-cell function in different forms of diabetes. These results may have important clinical implications for optimal therapeutic interventions in individuals with diabetes, particularly for those with long-standing diabetes.

Chemical Chaperone PBA Attenuates ER Stress and Upregulates SOCS3 Expression as a Regulator of Leptin Signaling

Endoplasmic reticulum (ER) is very sensitive to the nutritional and energy states of the cells. Disruption of ER homeostasis leads to the accumulation of unfolded/misfolded proteins in the ER lumen, which is defined as ER stress. ER stress triggers the unfolded protein response (UPR). It is suggested that chronic ER stress is associated with obesity and leptin resistance. We investigated the role of ER stress and the effect of the ER stress inhibitor phenylbutyric acid (PBA) of ER stress, in obesity, as well as their impact on leptin signaling. This study involved twenty-four lean and twenty-four leptin-deficient (ob/ob) mice divided into PBA- and vehicle-treated groups. Pancreatic islets were isolated, incubated with leptin for 48 h, and assayed for the expression of CHOP and XBP1s (UPR signaling indicators) and SOCS3 (regulator of leptin signaling) by RT-qPCR. The expression levels of XBP1s and CHOP were markedly increased in the ob/ob controls compared to other groups with and without leptin treatment. No significant differences in the XBP1s and CHOP expression levels were found between the PBA-treated ob/ob and lean mice. SOCS3 expression was significantly upregulated in the PBA-treated ob/ob mice compared to the ob/ob controls after leptin treatment; but no significant difference in the SOCS3 expression was found between the PBA-treated ob/ob and lean mice with and without leptin treatment. Our findings suggested that ER stress plays an important role in the pathology of obesity, while PBA reduces ER stress and may potentially ameliorate leptin signaling.

Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

Shikonin induces tumor apoptosis in glioma cells via endoplasmic reticulum stress, and Bax/Bak mediated mitochondrial outer membrane permeability

ETHNOPHARMACOLOGICAL RELEVANCE

Shikonin, one of the main active ingredients of Chinese herbal medicine Lithospermum erythrorhizon, has been widely used to treat various disease including virus infection and inflammation in clinical. Its anti-tumor activity has been recorded in "Chinese herbal medicine". Recently, some studies about its anti-glioma effects have been reported. However, little is known about the molecular pharmacological activity of Shikonin in glioma.

AIM

This study aimed to systematically uncover and validate the pharmacological mechanism of Shikonin against glioma.

MATERIAL AND METHODS

Network pharmacology approach, survival analysis, and Pearson co-expression analysis were performed to uncover and test the pharmacological mechanisms of Shikonin in glioma. Apoptosis assay, Caspase-3 activity assay and immunoblot analysis were practiced to validate the mechanisms.

RESULTS

Network pharmacology results suggested, anti-glioma effect of Shikonin by interfering endoplasmic reticulum (ER) stress-mediated tumor apoptosis targeting Caspase-3, and Bax/Bak-induced mitochondrial outer membrane permeabilization (MOMP) triggering cancer cell apoptosis. Survival analysis suggested the association of CASP3 with glioma (P < 0.05). Pearson correlation analysis indicated possible interaction of CASP3 with PERK through positive feedback regulation. Shikonin or in combination with 14G2a induced cell apoptosis in oligodendroglioma Hs683 cells in a dose-dependent manner with at a maximum apoptosis rate of 33%-37.5%, and 73%-77% respectively. Immunoblot analysis showed that Shikonin increased Caspase-3 activity to about 4.29 times, and increased 9 times when it combined with 14G2a. Shikonin increased also the expression levels of the proteins PERK and CHOP by about 4.4 and 5.6 folds, respectively, when it combined with 14G2a.

CONCLUSIONS

This study highlights the pharmacological mechanisms of Shikonin in the induction of tumor apoptosis in glioma cells.

Tectorigenin enhances PDX1 expression and protects pancreatic β-cells by activating ERK and reducing ER stress

Pancreas/duodenum homeobox protein 1 (PDX1) is an important transcription factor that regulates islet β-cell proliferation, differentiation, and function. Reduced expression of PDX1 is thought to contribute to β-cell loss and dysfunction in diabetes. Thus, promoting PDX1 expression can be an effective strategy to preserve β-cell mass and function. Previously, we established a PDX1 promoter-dependent luciferase system to screen agents that can promote PDX1 expression. Natural compound tectorigenin (TG) was identified as a promising candidate that could enhance the activity of the promoter for the PDX1 gene. In this study, we first demonstrated that TG could promote the expression of PDX1 in β-cells via activating extracellular signal-related kinase (ERK), as indicated by increased phosphorylation of ERK; this effect was observed under either normal or glucotoxic/lipotoxic conditions. We then found that TG could suppress induced apoptosis and improved the viability of β-cells under glucotoxicity and lipotoxicity by activation of ERK and reduction of reactive oxygen species and endoplasmic reticulum (ER) stress. These effects held true in vivo as well: prophylactic or therapeutic use of TG could obviously inhibit ER stress and decrease islet β-cell apoptosis in the pancreas of mice given a high-fat/high-sucrose diet (HFHSD), thus dramatically maintaining or restoring β-cell mass and islet size, respectively. Accordingly, both prophylactic and therapeutic use of TG improved HFHSD-impaired glucose metabolism in mice, as evidenced by ameliorating hyperglycemia and glucose intolerance. Taken together, TG, as an agent promoting PDX1 expression exhibits strong protective effects on islet β-cells both in vitro and in vivo.

Nifedipine Upregulates ATF6-α, Caspases -12, -3, and -7 Implicating Lipotoxicity-Associated Renal ER Stress

Nifedipine (NF) is reported to have many beneficial effects in antihypertensive therapy. Recently, we found that NF induced lipid accumulation in renal tubular cells. Palmitic acid-induced renal lipotoxicity was found to be partially mediated by endoplasmic reticular (ER) stress, while it can also be elicited by NF in kidney cells; we examined the induction of suspected pathways in both in vitro and in vivo models. NRK52E cells cultured in high-glucose medium were treated with NF (30 µM) for 24–48 h. ER stress-induced lipotoxicity was explored by staining with thioflavin T and Nile red, transmission electron microscopy, terminal uridine nick-end labeling, and Western blotting. ER stress was also investigated in rats with induced chronic kidney disease (CKD) fed NF for four weeks. NF induced the production of unfolded protein aggregates, resulting in ER stress, as evidenced by the upregulation of glucose-regulated protein, 78 kDa (GRP78), activating transcription factor 6α (ATF6α), C/EBP-homologous protein (CHOP), and caspases-12, -3, and -7. In vitro early apoptosis was more predominant than late apoptosis. Most importantly, ATF6α was confirmed to play a unique role in NF-induced ER stress in both models. CKD patients with hypertension should not undergo NF therapy. In cases where it is required, alleviation of ER stress should be considered to avoid further damaging the kidneys.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

The association of calcium channel blockers with β-cell function in type 2 diabetic patients: A cross-sectional study

Type 2 diabetes mellitus (T2DM) patients are often accompanied with hypertension. However, the association of antihypertensive drugs with β-cell function has not been well studied. To investigate this question, the authors performed a cross-sectional study involving 882 hypertensive T2DM patients. To assess β-cell function, patients were given 75g glucose orally and C-peptide levels before and 1, 2, and 3 hours after glucose intake were measured. Homa-β was computed by Homeostasis Model Assessment model to evaluate β-cell function using fasting C-peptide and glucose levels in the plasma. Multivariable-adjusted analysis was performed to evaluate the association of antihypertensive drugs with C-peptide levels, HbA1c, and Homa-β. Among 882 hypertensive patients, 547 (62.0%) received antihypertensive treatment. Multivariate-adjusted analysis demonstrated that use of calcium channel blockers (CCBs) was negatively associated with HbA1c levels (CCBs: 0.95 [95% CI: 0.92-0.98], P = 0.002). Our data further illustrated that the C-peptide levels before and 1, 2, and 3 hours of OGTT were 1.10-, 1.18-, 1.19-, and 1.15-fold increase in T2DM patients taking CCBs (P = 0.084 for fasting C-peptide levels; P ≤ 0.024 for C-peptide levels at 1, 2, and 3 hours after OGTT) in comparison with non-CCB users. Nevertheless, usage of any other antihypertensive drugs did neither associated with HbA1c nor associated with C-peptide levels (P ≥ 0.11). In conclusion, CCB treatment was negatively associated with HbA1c levels but positively associated with β-cell function in hypertensive T2DM patients, implying that CCBs could be considered to treat hypertensive T2DM patients with reduced β-cell function.

Glucotoxicity promotes aberrant activation and mislocalization of Ras-related C3 botulinum toxin substrate 1 [Rac1] and metabolic dysfunction in pancreatic islet β-cells: reversal of such metabolic defects by metformin

Emerging evidence suggests that long-term exposure of insulin-secreting pancreatic β-cells to hyperglycemic (HG; glucotoxic) conditions promotes oxidative stress, which, in turn, leads to stress kinase activation, mitochondrial dysfunction, loss of nuclear structure and integrity and cell apoptosis. Original observations from our laboratory have proposed that Rac1 plays a key regulatory role in the generation of oxidative stress and downstream signaling events culminating in the onset of dysfunction of pancreatic β-cells under the duress of metabolic stress. However, precise molecular and cellular mechanisms underlying the metabolic roles of hyperactive Rac1 remain less understood. Using pharmacological and molecular biological approaches, we now report mistargetting of biologically-active Rac1 [GTP-bound conformation] to the nuclear compartment in clonal INS-1 cells, normal rat islets and human islets under HG conditions. Our findings also suggest that such a signaling step is independent of post-translational prenylation of Rac1. Evidence is also presented to highlight novel roles for sustained activation of Rac1 in HG-induced expression of Cluster of Differentiation 36 [CD36], a fatty acid transporter protein, which is implicated in cell apoptosis. Finally, our findings suggest that metformin, a biguanide anti-diabetic drug, at a clinically relevant concentration, prevents β-cell defects [Rac1 activation, nuclear association, CD36 expression, stress kinase and caspase-3 activation, and loss in metabolic viability] under the duress of glucotoxicity. Potential implications of these findings in the context of novel and direct regulation of islet β-cell function by metformin are discussed.

Lamin-like Proteins Negatively Regulate Plant Immunity through NAC WITH TRANSMEMBRANE MOTIF1-LIKE9 and NONEXPRESSOR OF PR GENES1 in Arabidopsis thaliana

Nuclear lamins are involved in multiple biological processes in metazoan cells. The proteins of the CROWDED NUCLEI (CRWN) family are considered lamin-like candidates in Arabidopsis, although the functions of these proteins are largely unknown. In this article we show that crwn1 crwn2 double mutant displays an enhanced resistance against virulent bacterial pathogens, and both virulent bacteria and salicylic acid (SA) induce transcription of CRWN1 gene as well as proteasome-mediated degradation of CRWN1 protein. We also show that CRWN1 interacts with NAC WITH TRANSMEMBRANE MOTIF1-LIKE9 (NTL9), a NAC transcription factor involved in plant immunity. The interaction between CRWN1 and NTL9 enhances the binding of NTL9 to the promoter of the PATHOGENESIS-RELATED1 (PR1) gene, and inhibits PR1 expression. Further genetic experiments indicated that the defense-related phenotypes of crwn1 crwn2 double mutant are dependent on NONEXPRESSOR OF PR GENES1 (NPR1), a transcriptional cofactor of PR1. These findings revealed a regulatory network composed of lamin-like protein CRWN1, NTL9, and NPR1 for the regulation of PR1 expression.

Tiam1/Vav2-Rac1 axis: A tug-of-war between islet function and dysfunction

Glucose-stimulated insulin secretion [GSIS] from the islet β-cell involves a well-orchestrated interplay between metabolic and cationic events. It is well established that intracellular generation of adenine and guanine nucleotide triphosphates [e.g., ATP and GTP] represents one of the requisite signaling steps in GSIS. The small molecular mass GTP-binding proteins [G-proteins; e.g., Rac1 and Cdc42] have been shown to regulate islet β-cell function including actin cytoskeletal remodeling and fusion of insulin granules with the plasma membrane for GSIS to occur. In this context, several regulatory factors for these G-proteins have been identified in the pancreatic β-cell; these include guanine nucleotide exchange factors [GEFs] and guanine nucleotide dissociation inhibitors [GDI]. Recent pharmacological and molecular biological evidence identified Tiam1 and Vav2 as GEFs for Rac1 in promoting physiological insulin secretion. Paradoxically, emerging evidence in multiple cell types, including the islet β-cell, suggests key roles for Rac1 in the onset of cellular dysfunction under conditions of metabolic stress and diabetes. Furthermore, functional inactivation of either Tiam1 or Vav2 appears to attenuate sustained activation of Rac1 and its downstream signaling events [activation of stress kinases] under conditions of metabolic stress. Together, these findings suggest both "friendly" and "non-friendly" roles for Tiam1/Vav2-Rac1 signaling axis in islet β-cell in health and diabetes. Our current understanding of the field and the knowledge gaps that exist in this area of islet biology are heighted herein. Furthermore, potential caveats in the specificity and selectivity of pharmacological inhibitors that are available currently are discussed in this Commentary.

Quantitative proteomics reveals novel protein interaction partners of PP2A catalytic subunit in pancreatic β-cells

Protein phosphatase 2A (PP2A) is one of the major serine/threonine phosphatases. We hypothesize that PP2A regulates signaling cascades in pancreatic β-cells in the context of glucose-stimulated insulin secretion (GSIS). Using co-immunoprecipitation (co-IP) and tandem mass spectrometry, we globally identified the protein interaction partners of the PP2A catalytic subunit (PP2Ac) in insulin-secreting pancreatic β-cells. Among the 514 identified PP2Ac interaction partners, 476 were novel. This represents the first global view of PP2Ac protein-protein interactions caused by hyperglycemic conditions. Additionally, numerous PP2Ac partners were found involved in a variety of signaling pathways in the β-cell function, such as insulin secretion. Our data suggest that PP2A interacts with various signaling proteins necessary for physiological insulin secretion as well as signaling proteins known to regulate cell dysfunction and apoptosis in the pancreatic β-cells.

Glucotoxic and diabetic conditions induce caspase 6-mediated degradation of nuclear lamin A in human islets, rodent islets and INS-1 832/13 cells

Nuclear lamins form the lamina on the interior surface of the nuclear envelope, and regulate nuclear metabolic events, including DNA replication and organization of chromatin. The current study is aimed at understanding the role of executioner caspase 6 on lamin A integrity in islet β-cells under duress of glucotoxic (20 mM glucose; 24 h) and diabetic conditions. Under glucotoxic conditions, glucose-stimulated insulin secretion and metabolic cell viability were significantly attenuated in INS-1 832/13 cells. Further, exposure of normal human islets, rat islets and INS-1 832/13 cells to glucotoxic conditions leads to caspase 6 activation and lamin A degradation, which is also observed in islets from the Zucker diabetic fatty rat, a model for type 2 diabetes (T2D), and in islets from a human donor with T2D. Z-Val-Glu-Ile-Asp-fluoromethylketone, a specific inhibitor of caspase 6, markedly attenuated high glucose-induced caspase 6 activation and lamin A degradation, confirming that caspase 6 mediates lamin A degradation under high glucose exposure conditions. Moreover, Z-Asp-Glu-Val-Asp-fluoromethylketone, a known caspase 3 inhibitor, significantly inhibited high glucose-induced caspase 6 activation and lamin A degradation, suggesting that activation of caspase 3 might be upstream to caspase 6 activation in the islet β-cell under glucotoxic conditions. Lastly, we report expression of ZMPSTE24, a zinc metallopeptidase involved in the processing of prelamin A to mature lamin A, in INS-1 832/13 cells and human islets; was unaffected by high glucose. We conclude that caspases 3 and 6 could contribute to alterations in the integrity of nuclear lamins leading to metabolic dysregulation and failure of the islet β-cell.

Protein prenylation in islet β-cell function in health and diabetes: Putting the pieces of the puzzle together

Post-translational prenylation involves incorporation of 15-(farnesyl) or 20-(geranylgeranyl) carbon derivatives of mevalonic acid into highly conserved C-terminal cysteines of proteins. The farnesyl transferase (FTase) and the geranylgeranyl transferase (GGTase) mediate incorporation of farnesyl and geranylgeranyl groups, respectively. At least 300 proteins are prenylated in the human genome; the majority of these are implicated in cellular processes including growth, differentiation, cytoskeletal function and vesicle trafficking. From a functional standpoint, isoprenylation is requisite for targeting of modified proteins to relevant cellular compartments for regulation of effector proteins. Pharmacological and molecular biological studies have provided compelling evidence for key roles of this signaling pathway in physiological insulin secretion in normal rodent and human islets. Recent evidence indicates that inhibition of prenylation results in mislocalization of unprenylated proteins, and surprisingly, they remain in active (GTP-bound) conformation. Sustained activation of G proteins has been reported in mice lacking GGTase, suggesting alternate mechanisms for the activation of non-prenylated G proteins. These findings further raise an interesting question if mislocalized, non-prenylated and functionally active G proteins cause cellular pathology since aberrant protein prenylation has been implicated in the onset of cardiovascular disease and diabetes. Herein, we overview the existing evidence to implicate prenylation in islet function and potential defects in this signaling pathways in the diabetic β-cell. We will also identify critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signaling steps in β cells in models of impaired insulin secretion, metabolic stress and diabetes.

Phagocyte-like NADPH oxidase (Nox2) promotes activation of p38MAPK in pancreatic β-cells under glucotoxic conditions: Evidence for a requisite role of Ras-related C3 botulinum toxin substrate 1 (Rac1)

It is well established that glucotoxicity (caused by high glucose concentrations; HG) underlies pathogenesis of islet dysfunction in diabetes. We have recently demonstrated that Nox2 plays a requisite role in the generation of reactive oxygen species (ROS) under HG conditions, resulting in mitochondrial dysregulation and loss of islet β-cell function. Herein, we investigated roles of Nox2 in the regulation of downstream stress kinase (p38MAPK) activation under HG conditions (20mM; 24h) in normal rodent islets and INS-1 832/13 cells. We observed that gp91-ds-tat, a specific inhibitor of Nox2, but not its inactive analog, significantly attenuated HG-induced Nox2 activation, ROS generation and p38MAPK activation, thus suggesting that Nox2 activation couples with p38MAPK activation. Since Rac1, is an integral member of the Nox2 holoenzyme, we also assessed the effects of Rac1 inhibitors (EHT 1864, NSC23766 and Ehop-016) on HG-induced p38MAPK activation in isolated β-cells. We report a significant inhibition of p38MAPK phosphorylation by Rac1 inhibitors, implying a regulatory role for Rac1 in promoting the Nox2-p38MAPK signaling axis in the β-cell under the duress of HG. 2-Bromopalmitate, a known inhibitor of protein (Rac1) palmitoylation, significantly reduced HG-induced p38MAPK phosphorylation. However, GGTI-2147, a specific inhibitor of geranylgeranylation of Rac1, failed to exert any significant effects on HG-induced p38MAPK activation. In conclusion, we present the first evidence that the Rac1-Nox2 signaling module plays novel regulatory roles in HG-induced p38MAPK activation and loss in glucose-stimulated insulin secretion (GSIS) culminating in metabolic dysfunction and the onset of diabetes.

Nifedipine and nimodipine protect dopaminergic substantia nigra neurons against axotomy-induced cell death in rat vibrosections via modulating inflammatory responses

Neurodegeneration of cholinergic and dopaminergic neurons is a major hallmark in Alzheimer's or Parkinson's disease, respectively. A dysregulation in calcium homeostasis may be part of this process and counteracting calcium influx may have neuroprotective properties in both diseases. Therefore, we investigated the putative neuroprotective or neurotoxic activity of L-type calcium channel (LTCC) inhibitors on cholinergic and dopaminergic neurons in a rat organotypic vibrosection model. Sagittal or coronal vibrosections (200 μm thick) of postnatal day 10 rats were cultured on 0.4 μm semipermeable membranes for 2 weeks with 10 ng/ml nerve growth factor (NGF) and/or glial-cell line derived neurotrophic factor (GDNF) to maintain survival of cholinergic or dopaminergic neurons, respectively. Thereafter, sections were incubated with 0.1, 1 or 10 μM isradipine, nicardipine or verapamil for 2 weeks to explore cytotoxicity. Alternatively, in order to explore neuroprotective activity, vibrosections were incubated without growth factors but with isradipine or verapamil or with nicardipine, nimodipine or nifedipine from the beginning for 4 weeks. Our data show that all LTCC inhibitors exhibited no neurotoxic effect on cholinergic and dopaminergic neurons. Further, LTCC inhibitors did not have any neuroprotective activity on cholinergic neurons. However, nimodipine and nifedipine significantly enhanced the survival of dopaminergic substantia nigra (SN) but not ventral tegmental area (VTA) neurons, while nicardipine, isradipine and verapamil had no effect. Nifedipine (and more potently GDNF) reduced inflammatory cytokines (macrophage inflammatory protein-2, tumor necrosis factor-α), but did not influence oxidative stress or caspase-3 activity and did not interfere with iron-mediated overload. Our data show that nifedipine and nimodipine are very potent to enhance the survival of axotomized SN neurons, possibly influencing inflammatory processes.

High glucose exposure promotes activation of protein phosphatase 2A in rodent islets and INS-1 832/13 β-cells by increasing the posttranslational carboxylmethylation of its catalytic subunit

Existing evidence implicates regulatory roles for protein phosphatase 2A (PP2A) in a variety of cellular functions, including cytoskeletal remodeling, hormone secretion, and apoptosis. We report here activation of PP2A in normal rat islets and insulin-secreting INS-1 832/13 cells under the duress of hyperglycemic (HG) conditions. Small interfering RNA-mediated knockdown of the catalytic subunit of PP2A (PP2Ac) markedly attenuated glucose-induced activation of PP2A. HG, but not nonmetabolizable 3-O-methyl glucose or mannitol (osmotic control), significantly stimulated the methylation of PP2Ac at its C-terminal Leu-309, suggesting a novel role for this posttranslational modification in glucose-induced activation of PP2A. Moreover, knockdown of the cytosolic leucine carboxymethyl transferase 1 (LCMT1), which carboxymethylates PP2Ac, significantly attenuated PP2A activation under HG conditions. In addition, HG conditions, but not 3-O-methyl glucose or mannitol, markedly increased the expression of LCMT1. Furthermore, HG conditions significantly increased the expression of B55α, a regulatory subunit of PP2A, which has been implicated in islet dysfunction under conditions of oxidative stress and diabetes. Thapsigargin, a known inducer of endoplasmic reticulum stress, failed to exert any discernible effects on the carboxymethylation of PP2Ac, expression of LCMT1 and B55α, or PP2A activity, suggesting no clear role for endoplasmic reticulum stress in HG-induced activation of PP2A. Based on these findings, we conclude that exposure of the islet β-cell to HG leads to accelerated PP2A signaling pathway, leading to loss in glucose-induced insulin secretion.