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Volume: 20 Issue: 6 June 2022

FULL TEXT

ARTICLE
Beta-Cell Golgi Stress Response to Lipotoxicity and Glucolipotoxicity: A Preliminary Study of a Potential Mechanism of Beta-Cell Failure in Posttransplant Diabetes and Intraportal Islet Transplant
Neslihan Başçıl Tütüncü,1 Hasibe Verdi,2 Yaprak Yalçın,2 Pınar Baysan Çebi,2 Sibel Kınık,3 Tanju Tütüncü,4 Fatma Belgin Ataç2

Objectives: Lipotoxicity and glucolipotoxicity are among the most important triggers of beta-cell failure in patients with type 2 and posttransplant diabetes. Because the Golgi apparatus is a vital organelle in secretory cells like beta cells, its behavior under stress conditions determines the cell’s functional capacity.
Materials and Methods: To mimic lipotoxicity and glucolipotoxicity as metabolic stresses for beta-cell failure, rat insulinoma INS-1E cells were treated with palmitic acid, glucose, or both. Cells were cultured in the presence of 5.0, 16.7, or 33 mM glucose with or without 0.5 mM palmitic acid for 8, 16, 24, and 48 hours. Incubation in the presence of any of the 3 concentrations of glucose with 0.5 mM palmitic acid provided glucolipotoxicity. In addition to the endop-lasmic reticulum stress marker (Hspa5), we evaluated changes in Golgi function under experimental metabolic stresses. In doing this, we measured expression levels of the genes coding Golgi structural proteins (Acbd3, Golga2, and Arf1), Golgi glycosylation enzymes sialyltransferaz10 and sialyltransferase 1 (St3gal1), and Golgi stress mediators (Creb3 and Arf4).
Results: Golgi responded to lipotoxicity and glucolipotoxicity by increasing the expression of St3gal1 (P = .05 in both conditions) and Creb3 (P = .022 and P = .01, respectively). The Arf4 gene transcript also increased in glucolipotoxic media (P = .03). Glucotoxicity alone did not induce a change in the transcript levels of Creb3 and Arf4. Lipotoxicity and glucolipotoxicity induced Creb3 and Arf4 expression, which are important Golgi stress response mediators leading to apoptosis.
Conclusions: This preliminary study showed that the Golgi stress response is important in lipotoxic and glucolipotoxic conditions in terms of beta-cell failure. Solving the mystery of intracellular molecular mechanisms leading to beta-cell dysfunction is crucial to understanding the pathophysiology of posttrans-plant diabetes and most probably the failure of intraportal islet transplants in the long term.


Key words : Arf4, Creb3, Metabolic stress, Obesity

Introduction

Obesity, being the most important factor leading to beta-cell failure in type 2 diabetes mellitus, plays a major role in the dramatic increased global prevalence of this disease. Visceral adiposity in obese individuals causes insulin resistance that leads, in time, to inadequate compensated insulin secretion from the beta cells in type 2 diabetes mellitus. In this scenario, lipotoxicity and glucolipotoxicity create severe metabolic burden on beta cells.1-3

Lipotoxicity and glucolipotoxicity are also among the most important triggers of posttransplant diabetes in kidney transplant recipients. Although immunosuppressive medications are the main risk factors for posttransplant diabetes, other risk factors like insulin resistance, obesity, and hypertrigly-ceridemia are also important triggers of beta-cell failure in these patients. Of note, these factors have high prevalence in the general population and are associated with the incidence of type 2 diabetes mellitus. Tacrolimus is shown to be the most diabetogenic immunosuppressant drug, especially in patients with insulin resistance. Tacrolimus has been reported to accelerate beta-cell damage in pathways already altered by insulin resistance. That is, in those who use tacrolimus, beta-cell dysfunction is potentiated in cells under metabolic stress induced by glucolipotoxicity.4-7

There is also a plausible theoretical basis for lipotoxic or glucolipotoxic stress causing destruction of transplanted human islets. In islet transplantation, chronic exposure of normal islets transplanted to the liver exposes them to high portal levels of nutrients and incretins resulting in hypersecretion of undiluted insulin into surrounding hepatocytes. This hyperin-sulinism in the liver elicits a lipogenic response that overloads the hepatocytes with triacylglycerol. Islet cells thus would be chronically exposed to both a uniquely high lipid environment and a high glucose environment. This combination would result in both lipotoxicity and glucolipotoxicity. This metabolic toxic insult to islet grafts in the liver is also implicated in the pathogenesis of long-term beta-cell failure and eventual insulin dependence in islet transplant recipients.8

Endoplasmic reticulum (ER) stress is a well-known and the most studied consequence of lipotoxicity and glucolipotoxicity in the process of beta-cell failure.9,10 Although the ER stress response is well studied in beta-cells, adaptive or maladaptive molecular responses of the Golgi apparatus (GA) during metabolic stress is unknown. The GA is the core of processing and sorting of lipids and proteins en route from the ER to the plasma membrane and other destinations. The capacity of the GA seems to be regulated in accordance with the cellular demands of the ER. It has been known for more than 50 years that the GA is expanded in beta cells in those with diabetes.11,12

In vitro GA stress models have been created by several different GA disruptors, including brefeldin A, monensin, and nigericin. In those in vitro models, TFE3 (basic-helix-loop-helix type transcription factor), a highly phosphorylated cytoplasmic transcription factor, was demonstrated to be one of the regulators of the GA stress response. When dephosphorylated during stress, it translocates to the nucleus and binds to GA stress response elements, resulting in transcriptional activation of Golgi-related genes encoding GA structural proteins, glycosylation enzymes, and vesicular transport component.13

A recently identified pathway of mammalian Golgi stress response is the cAMP-responsive element binding protein 3 (CREB3)-ARF4 pathway. Golgi stress evoked by treatment of cells with brefeldin A causes proteolytic activation of the ER-resident transcription factor CREB3. Consequently, the cytoplasmic domain of CREB3 is released from the ER membrane and transports to the GA, where site 1 and site 2 proteolysis occurs, and then translocates to the nucleus to upregulate the transcription of ARF4, resulting in Golgi-stress induced apoptosis and disruption of the GA.13-19

In conditions of lipotoxicity and glucolipotoxicity, ceramide synthesis increases, and its transport from the ER to GA is overwhelmed. If GA cannot alleviate the toxic accumulation of ceramides, a Golgi stress response is expected to occur. Likewise, increases in insulin demand during glucotoxic states require the GA to adapt itself adequately to the augmented work of insulin folding, sulfation, and transport to the secretory vesicles.8,9 In a study in which an informatics-based approach was used to study the transcriptional signature of GA stress in beta cells, the investigators used existing RNA sequencing and microarray data sets that were generated using human islets from donors with type 1 and type 2 diabetes. The study showed a dysregulation of GA-associated genes, including Atf3, Arf4, Creb3, and Cog6.20

There are limited data on triggers and the molecular signaling pathways of the Golgi stress response in nondiabetic models of stress. Here, we believe that we have provided the first report in the literature to study the response of the beta-cell GA to glucotoxicity, lipotoxicity, and glucolipotoxicity. To test this, we treated INS-1E rat beta cells with palmitic acid, glucose, or both and then investigated the expression of the Golgi stress genes described before in the literature for other Golgi stress models.

Materials and Methods

We conducted this study in the Medical Biology Laboratory of Baskent University Faculty of Medicine (Ankara, Turkey). The study was approved by the Institutional Review Board for Experimental Studies of Baskent University and registered with research number DA16/30.

Materials
All reagents were of analytical grade unless otherwise stated. The tissue culture medium RPMI-1640 (R0883), L-glutamine G6392 (200 mM, 50 mL), sodium pyruvate solution (100 mM, S8636), penicillin/streptomycin (P4333), trypsin EDTA (0.25%, T3924), palmitate or palmic acid (P0500), glucose, bovine serum albumin fraction V (A6003), dimethyl sulfoxide (D9170), fetal calf serum (F7524), HEPES (H3784), phosphate salt buffer (D1408), sodium hydroxide (S2770), diethyl pyrocarbonate (D5758), ethyl alcohol (E7023), boric acid (B6768), EDTA (E5134), glycerol (G5516), orange G (O3756), Tris base (T7527), magnesium chloride (25 mM, M8787), LightCycler TaqMan Master, and LC capillary were purchased from Sigma. 2-Mercaptoethanol (ES-007-E) was purchased from Merck. 3-(4,5-Dimethylthiazol-2-yl)-2,5 diphenyl tetrazolium bromide (MTT), high-fidelity Transcriptor (05081963001) for cDNA synthesis, Taq DNA polymerase (5 U/μL, 11146173001), and dNTP (10 mM × 4, 11581295001) were purchased from Roche. The Qiagen RNeasy mini kit (74104) was used for RNA isolation. Agarose gel electrophoresis agarose (A2114) was purchased from AppliChem.

Cell culture conditions
Rat insulinoma INS-1 cells were kindly provided by Prof. Dr. Pierre Maehler (Geneva University, Geneva, Switzerland). Cells were grown in RPMI-1640 medium buffered with 10 mM HEPES (pH 7.3) containing 10% (vol/vol) fetal calf serum, 2 mM L-glutamine, 1 mM sodium pyruvate, 50 μM 2-mercaptoethanol, and 100 U/mL penicillin/streptomycin at 37 °C in an atmosphere of 5% CO2 and 95% humidified air. The maintenance culture was passaged once a week by gentle trypsinization, and cells were seeded at a density of 4 × 104 cells/cm2 in 25-cm2 Falcon bottles (Orange Scientific, 4420100) with an 8-mL complete medium.

Cells were then cultured in the presence of 5.0, 16.7, or 33 mM glucose with or without 0.5 mM palmitic acid for 8, 16, 24, and 48 hours. Incubation in the presence of any of the 3 concentrations of glucose with 0.5 mM palmitic acid mimicked glucolipotoxicity conditions. Thus, we had 3 groups of cells: cells treated with increasing levels of glucose were the glucotoxicity group, cells treated with high glucose and palmitate together were the glucolipotoxicity group, and cells treated with palmitate only were the lipotoxicity group. In the 3 groups of cells, we examined both the cellular viability and the gene expression levels of GA stress proteins. Four different incubation durations (8, 16, 24, and 48 hours) were designed to mimic chronicity of the specific metabolic stresses. Each individual experiment was repeated 4 times.

Palmitic acid and glucose preparation
Saturated palmitic acid was prepared with bovine serum albumin using a modification of the method described previously.21 A stock of 1 M glucose solution was obtained by mixing commercially purchased glucose solution with the medium. From here, 33 mM glucose solution was prepared to be used in the study.

3-(4,5-Dimethylthiazol-2-yl)-2,5 diphenyl tetrazolium bromide assay for cell viability analysis
Cell viability was determined by the MTT assay, which is based on the conversion of MTT to dark blue formazan crystals by mitochondrial dehydrogenase enzyme. We added MTT and phosphate-buffered saline (pH 7.4) into each well, with wells allowed to incubate at 37 °C for 4 hours. After the solution was discarded, we added dimethyl sulfoxide. We measured optical densities at 490-nm spectral wavelength using a microplate spectrop-hotometer (micro-enzyme-linked immunosorbent assay reader; EPOCH gen 5 2.0). These results (presented as mean ± SD of 3 independent experiments) are shown in Table 1.

3-(4,5-Dimethylthiazol-2-yl)-2,5 diphenyl tetrazo­lium bromide is a stable tetrazolium salt that can be converted to soluble formazan in the presence of NAD(P)H produced during glycolysis in a metabolically active cell. The percentage of viable cells in a medium was determined by measuring the absorbance of formazan by optic density.

RNA isolation, reverse transcription, and real-time polymerase chain reaction
We plated and allowed the INS-1E cells to grow on glass coverslips. At the end of the incubation with different treatments, total RNA was isolated from INS-1E cells with the QIAGEN-RNeasy mini kit and treated with RNase-free DNase I. We performed qualitative control of the isolated RNAs by agarose gel electrophoresis and by EPOCH spectrophotometry (Gen5 program); this program was also used to quantify the isolated RNAs in each microliter. One microgram of RNA was reverse transcribed using a Transcriptor high-fidelity DNA synthesis kit from Roche according to the manufacturer’s instructions. Rat β-actin primers were used as housekeeping genes to control availability of the cDNA. After this stage, gene expression levels for Hspa5 (heat shock protein family A member 5, previous name Grp78); GA genes encoding structural proteins Acbd3, Golga2, and Arf1; 2 GA glycosylation enzymes (ST3 beta-galactoside alpha-2,3-sialyltransferase1 [sialyltransferase 4A] gene St3gal1 and ST3 beta-galactoside alpha-2,3-sialyltransferase 6 [sialyltransferase10] gene St3gal6); and the GA stress mediators Arf1, Creb3, and Arf4 were analyzed by a real-time polymerase chain reaction detection system.

Expression levels of the GA structural protein genes, the GA glycosylation enzymes, and Arf1, Creb3 and Arf4 were measured in all experimental conditions. Relative gene expression level was determined using the 2-ΔΔCt method.

Statistical analyses
Descriptive statistical values for the study groups are expressed as mean ± SD for continuous variables and as proportions for ordinal variables.

To compare the treatment groups with the untreated control group (basal level group) with regard to gene expression levels, we used t tests. For statistical analysis of MTT results, we used the Kruskal-Wallis test among groups and the post hoc Mann-Whitney U test when appropriate, as variances of study groups were not homogenous. To compare gene expression levels among the dependent groups, we used the Friedman test and the Wilcoxon signed rank test as post hoc test.

Statistical significance was considered when 2-sided P values were ≤.05. Data were analyzed using the SPSS software version 18.0.

Results

Effect of palmitate and glucose on INS-1E rat beta-cell viability
In INS-1E cells that received lipotoxic medium with 0.5 mM palmitate, a toxic effect was shown, with significantly reduced cell viability of about 60% to 70%, starting at 8 hours of incubation to 48 hours of incubation (P = .001) (Table 1).

In INS-1E cells treated with 5, 16.7, or 33 mM glucose (glucotoxic media), after about 8 hours of incubation, there was approximately a 60% to 65% loss of viable cells (P = .004) (Table 1). However, unlike cells treated with 0.5 mM palmitate, prolongation of treatment duration with all 3 concentrations of glucose revealed some recovery, especially at hour 16 and hour 24 of treatment, with MTT test demonstrating a lower percentage of loss of viable cells (about 40%-50%) at these hours. This finding was in agreement with previously published reports.20-22

Coadministration of 0.5 mM palmitate with any of the concentrations of glucose (glucolipotoxicity) resulted in a loss of INS-1E beta-cell viability by 60% to 80% starting at 8 hours of incubation to 24 hours (P < .001). Interestingly, at hour 48 of glucoli-potoxicity with 33 mM glucose and 0.5 mM palmitate, there was recovery of viable cells (Table 1).

Effect of palmitate and glucose on INS-1E rat beta-cell Golgi apparatus stress markers
Hspa5 gene expression as a marker of ER stress response in glucotoxic, lipotoxic, and glucolipo-toxic media In glucotoxic medium, Hspa5 gene transcript levels did not increase from basal levels with any of the glucose concentrations. A small but statistically insignificant increase of about 1.6 times was detected at 48 hours of incubation with 16.7 mM glucose (Table 2).

In lipotoxic media, Hspa5 transcript levels displayed an early significant increase of about 2.5 to 3.5 times versus basal levels (P = .006), which remained increased until 48 hours of treatment in lipotoxic media (Table 3).

With regard to glucolipotoxicity experiments, Hspa5 transcript levels increased about 2 to 2.5 times in cells treated with 5 and 16.7 mM glucose plus palmitate starting from hour 8 (P = .02 and P = .008, respectively) to the hour 48 of incubation. However, in cells treated with 33 mM glucose, the transcript levels did not increase until hour 24 (P > 0.05). Although this result was not significant, prolonged glucolipotoxicity at hour 48 caused a 2-time increased Hspa5 transcript level (Table 4). This finding can be interpreted in accordance with the MTT assay results, in which prolonged glucolipotoxicity with very high glucose levels can induce ER stress response in the surviving viable cells after longer incubation.

Expression levels of Golgi apparatus structural genes in glucotoxic, lipotoxic, and glucolipotoxic media
Expression levels of the GA structural genes Acbd3 (Gcp60), Golga2 (Gm130), and Arf1 did not significantly change in all palmitate and glucose treatment groups compared with basal levels. Although no significant change was detected compared with the basal conditions without metabolic intervention, there was a significant increase in Arf1 after hour 16 of glucotoxicity with 16.7 mM glucose compared with that shown at hour 8 (P = .04). This result may have been because of the loss/degradation of Arf1 transcript during the early treatment hours and then a small increase in transcript levels in the subsequent hours of glucotoxic stimulation. The Arf1 transcript level in the glucolipotoxic media with 33 mM glucose and 0.5 mM palmitate, on the other hand, was significantly decreased during treatment times after hour 8 (P = .044). This suppression of Arf1 transcription may have been because of loss/degradation of the preformed transcripts in cells with prolonged glucolipotoxicity (Table 4).

Expression levels of Golgi apparatus glycosylation enzymes in glucotoxic, lipotoxic, and glucolipotoxic media
Among the GA glycosylation enzymes investigated in our study, sialyltransferaz10 (St3gal6) expression levels did not significantly increase from basal levels for all treatment groups (Tables 2-6). Transcript levels decreased significantly during metabolic interventions with palmitate (P = .029; Table 3), with 5 mM and 16.7 mM glucose plus palmitate (P = .039), and with 33 mM glucose plus palmitate (P = .019) (Table 5, Table 6, and Table 4). We interpreted the decreased St3gal6 transcript levels during lipotoxicity and glucolipotoxicity similar to that shown with Arf1.

With regard to the glycosylation enzyme sialyltransferase (St3gal1), we observed a statistically significant increase (up to about 3 times; Table 3) versus basal level during the first 16 hours in cells treated with palmitate (P = .05). We observed a significant variation in St3gal1 transcript levels during the lipotoxic treatment periods (P = .007, Table 3). The St3gal1 gene expression level increased in all groups treated with palmitate plus glucose, with the most prominent expression increase at hour 16 (Tables 4-6). St3gal1 transcript levels also significantly varied during all glucolipotoxic treatments, demonstrating a progressive increase in the first 16 hours and then decreasing to hour 48. That is, changes in St3gal1 transcript levels were significant with glucolipotoxic treatment, demonstrating a progressive increase in the first 16 hours and then decreasing to hour 48 (Tables 4-6). Levels were significant only when palmitate was combined with 33 mM glucose (P = .05), but not with the other glucose levels, probably because of high expression level variance.

Although not significant, the St3gal1 transcript level also increased during treatment with the low concentration of glucose (5 mM), which increased up to 1.5 times in the first 16 hours (Table 2). Treatment with higher glucose concentrations revealed a more delayed increase at 48 hours of treatment(Table 2).

Thus, GA adapts and responds, to some extent, to lipotoxicity and glucolipotoxicity by increasing the expression of one of its structural genes coding sialyltransferase 1.

Expression levels of Golgi apparatus stress response mediators in glucotoxic, lipotoxic, and glucolipotoxic media
We next aimed to investigate the expression levels of genes that have been previously described as Golgi stress response mediators,10-12 that is, Creb3 and Arf4.

Arf4 transcript levels showed a nonsignificant increase of about 1.5 times from basal levels in the lipotoxic media during the first 24 hours. There was a striking increase in the Arf4 gene expression level in cells treated with 0.5 mM palmitate and 33 mM glucose compared with the basal level (Figure 1, Table 4). Although this increase from the basal level was shown for all treatment durations with 0.5 mM palmitate and 33 mM glucose, Arf4 levels with high glucose treatment was statistical significant, especially with 8 and 24 hours of treatment (P = .05 and P = .03, respectively). Arf4 transcript levels significantly changed (P = .038) during the treatment periods only with 5 mM glucose and 0.5 mM palmitate, with an early increase in the first 24 hours and a decline at 48 hours (Table 5). In all other intervention groups, there was no significant variation among the treatment durations.

Creb3 transcript levels significantly changed compared with basal level with lipotoxic and glucolipotoxic media (Figure 2, Tables 3-6). An increase of more than 2 times was shown in the metabolic stress condition induced by palmitate during the first 24 hours (P = .022). The Creb3 expression level in groups treated with 5 mM glucose plus palmitate increased compared with basal level, with 2.6-fold increase at 48 hours of incubation (P = .01, Figure 2). Variations in Creb3 transcript levels over the treatment durations were also significant (P = .07), with a progressive increase after hour 8 and reaching a maximum at hour 48 (Table 5).

With any glucose concentration, we observed no significant change in either Arf4 and Creb3 transcript levels (Figures 1 and 2 and Table 2).

Discussion

Lipotoxicity and glucolipotoxicity are important triggers of beta-cell failure in those with type 2 diabetes and in those with posttransplant diabetes and may also be important in the loss of the functional capacity in the long term of transplanted beta cells. In this study, we investigated the beta-cell GA stress response under lipotoxic and glucolipotoxic conditions, which mimics the metabolic environment of insulin resistance in people with obesity and those who have high nutrient overload by portal system.

To our knowledge, this is the first report of elevated expression levels of the beta-cell GA stress mediators Creb3 and Arf4 during prolonged glucolipotoxicity and elevated level of Creb3 under lipotoxic stimuli. Both Creb3 and Arf4 are shown to be important mediators in the GA stress response in other cell types resulting in cellular apoptosis. At present, there is only 1 recent informatics-based study, which was from Bone and associates, which demonstrated dysregulation of GA-associated genes (including Atf3, Arf4, Creb3, and Cog6) in human islets from donors with type 1 and type 2 diabetes.20

Similar to previous data, we also demonstrated that palmitic acid is more toxic to INS-1E rat pancreatic beta cells than glucose.22-25 Palmitic acid induces an ER stress response. In the present study, we also showed that palmitic acid induces GA stress.

In addition to the defined GA stress proteins, we also found that levels of the beta-cell GA enzyme st3gal1 increased early after treatment in lipotoxic and glucolipotoxic environments, as well as showed an ER stress response. Further research on the role of this enzyme during metabolic stress conditions is needed.

Mature pancreatic beta cells are specialized cells that are functionally programmed to secrete insulin in response to changes in plasma glucose concentration. Glucose is the most important stimulus for insulin synthesis and secretion, as well as beta-cell hyperplasia.26-28 In accordance with this biological identity, high glucose concentrations did result in changes of either ER or GA stress protein levels in our experimental models.

We could not demonstrate an adaptive increase in the transcript levels of the other GA structural genes (Acbd3, Golga2, and Arf1) at any dose or duration of metabolic stress tested.

Both CREB3 and ARF4 are important mediators of cell death in previous GA stress models in the literature.13-19 ARFs are evolutionary conserved and ubiquitously expressed guanosine triphosphatases. They fulfill critical roles in the secretory pathway for the antegrade and retrograde transport of cargo between ER and GA and in the endocytic system.29 Transcript levels of Arf1 and Arf5 were demonstrated to be upregulated under GA stress to oppose the fragmentation of GA and apoptosis, whereas Arf4 expression was shown to be associated with cell death.16-19

In our study with INS-1E rat beta cells, we demonstrated a significant increase in Creb3 transcript levels in cells after prolonged lipotoxicity and glucolipotoxicity durations, during which a dramatic loss of cellular viability was shown. We also found that glucolipotoxicity was an important stimulus for changed transcript levels of Arf4 in beta cells. Transcript levels tended to increase in the lipotoxic media. Our findings regarding the increased Creb3 and Arf4 transcript levels in glucolipotoxicity and lipotoxicity may indicate a GA stress response that induces apoptosis in beta cells under metabolic stress with excess lipid. However, glucotoxicity of any degree did not trigger an increase in either Creb3 or Arf4 transcription in the cell models.

Palmitic acid with or without glucose seemed to trigger both ER and GA stress responses. Although the molecular mechanisms of how toxic accumulation of lipids affects beta-cell survival are not well known, both the excess intracellular free fatty acids and ceramide may affect membrane lipid composition and various intracellular signaling pathways and unopposed ceramide toxicity may trigger an intrinsic apoptotic process. There are various studies that have already investigated the molecular mechanisms of ceramide toxicity leading to ER stress.30-35

The GA is among the most important organelles in biochemical processing of ceramide. The GA synthesizes sphingolipids from ceramide in trans-cisternae. Through this method, it both detoxifies ceramide and produces sphingomyelin and other glycosphingolipids, which are important lipid components of the cell membrane. Thus, ceramide is converted to nontoxic sphingolipids in the GA.36-38 Under lipotoxicity and/or glucolipotoxicity conditions, increased ceramide input on the GA is expected to trigger adaptive mechanisms in the GA to increase GA structural proteins, which are necessary enzymatic machinery to convert ceramide to sphingolipids and proteins of the vesicular transport system.

Conclusions

In rat beta cells that were subjected to metabolic stress of lipotoxicity and glucolipotoxicity, but not glucotoxicity alone, we found that changes in Creb3 and Arf4 gene transcription indicated them to be important mediators of the GA stress response. This metabolic stress may also be detrimental in the beta cells of renal transplant recipients, especially in those who are given tacrolimus, as well as in the liver in islet transplant recipients in the long term. In this respect, future studies should test the effects of glucolipotoxicity and lipotoxicity on beta cells in terms of organelle stress, which is frequently observed in transplant patients who use tacrolimus. Future studies should also test beta-cell viability of islet grafts in the liver and investigate the possible detrimental effects of postprandial lipemia of portal system on beta cells or the presence of hepatic steatosis on nearby beta cells.

A limitation was that we did not compare gene expression levels versus corresponding protein levels. However, with no similar study so far performed as far as we know, our study provides a preliminary investigation of the role of Golgi under metabolic stress, specifically, lipotoxicity, glucotoxicity, and glucolipotoxicity, similar to environments encountered in diabetes. Future studies should investigate the intracellular variations of key proteins during metabolic stress of beta cells in diabetes to clarify the role of the GA in this complex disease. Solving the mystery of intracellular molecular mechanisms leading to beta-cell dysfunction is crucial to understanding the pathophysiology posttransplant diabetes and most probably the failure of intraportal islet transplants in the long term.


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Volume : 20
Issue : 6
Pages : 585 - 594
DOI : 10.6002/ect.2022.0027

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From the 1Department of Endocrinology and Metabolism, the 2Department of Medical Biology, and the 3Department of Medical Biology and Department of Pediatric Endocrinology and Metabolism, Baskent University Faculty of Medicine; and the 4Department of Medical Biology, Ankara City Hospital, and the Department of General Surgery, Baskent University Faculty of Medicine, Ankara, Turkey
Acknowledgements: This study was supported by a grant from the Baskent University Research Fund. The authors have no declarations of potential conflicts of interest.
Corresponding author: Neslihan Başçıl Tütüncü, Baskent University Faculty of Medicine, Department of Endocrinology and Metabolism. Bahcelievler 5, Sokak, No 48, Ankara, Turkey 06550
Phone: +90 5327979955
E-mail: neslibascil@yahoo.com

Table 1. Cell Viability Assay During Treatment With Glucose and Palmitate

Table 2. Effect of Glucose Alone on INS-1E Rat Beta-Cell Golgi Apparatus Stress Markers

Table 3. Effect of Palmitate on INS-1E Rat Beta-Cell Golgi Apparatus Stress Markers

Table 4. Effect of Glucolipotoxicity With 33 mM Glucose and 0.5 mM Palmitate on INS-1E rat Beta-Cell Golgi Apparatus Stress Markers

Table 5. Effect of Glucolipotoxicity With 5 mM Glucose and 0.5 mM Palmitate on INS-1E rat Beta-Cell Golgi Apparatus Stress Markers

Table 6. Effect of Glucolipotoxicity With 16.7 mM Glucose and 0.5 mM Palmitate on INS-1E rat Beta-Cell Golgi Apparatus Stress Markers

Figure 1. Levels of Expression of Golgi Apparatus Stress Response Mediator Arf4 in Glucotoxic, Lipotoxic, and Glucolipotoxic Media

Figure 2. Levels of Expression of Golgi Apparatus Stress Response Mediator Creb3 in Glucotoxic, Lipotoxic, and Glucolipotoxic Media