Browsing by Author "Sohn, Paul"

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    Cigarette smoke exposure impairs β-cell function through activation of oxidative stress and ceramide accumulation
    (Elsevier, 2020-07) Tong, Xin; Chaudhry, Zunaira; Lee, Chih-Chun; Bone, Robert N.; Kanojia, Sukrati; Maddatu, Judith; Sohn, Paul; Weaver, Staci A.; Robertson, Morgan A.; Petrache, Irina; Evans-Molina, Carmella; Kono, Tatsuyoshi; Medicine, School of Medicine
    Objectives Epidemiological studies indicate that first- and second-hand cigarette smoke (CS) exposure are important risk factors for the development of type 2 diabetes (T2D). Additionally, elevated diabetes risk has been reported to occur within a short period of time after smoking cessation, and health risks associated with smoking are increased when combined with obesity. At present, the mechanisms underlying these associations remain incompletely understood. The objective of this study was to test the impact of CS exposure on pancreatic β-cell function using rodent and in vitro models. Methods Beginning at 8 weeks of age, C57BL/6 J mice were concurrently fed a high-fat diet (HFD) and exposed to CS for 11 weeks, followed by an additional 11 weeks of smoking cessation with continued HFD. Glucose tolerance testing was performed during CS exposure and during the cessation period. Cultured INS-1 β-cells and primary islets were exposed ex vivo to CS extract (CSE), and β-cell function and viability were tested. Since CS increases ceramide accumulation in the lung and these bioactive sphingolipids have been implicated in pancreatic β-cell dysfunction in diabetes, islet and β-cell sphingolipid levels were measured in islets from CS-exposed mice and in CSE-treated islets and INS-1 cells using liquid chromatography-tandem mass spectrometry. Results Compared to HFD-fed, ambient air-exposed mice, HFD-fed and CS-exposed mice had reduced weight gain and better glucose tolerance during the active smoking period. Following smoking cessation, CS-mice exhibited rapid weight gain and had accelerated worsening of their glucose tolerance. CS-exposed mice had higher serum proinsulin/insulin ratios, indicative of β-cell dysfunction, significantly lower β-cell mass (p = 0.017), reduced β-cell proliferation (p = 0.006), and increased islet ceramide content compared to non-smoking control mice. Ex vivo exposure of isolated islets to CSE was sufficient to increase islet ceramide levels, which was correlated with reduced insulin gene expression and glucose-stimulated insulin secretion, and increased β-cell oxidative and endoplasmic reticulum (ER) stress. Treatment with the antioxidant N-acetylcysteine markedly attenuated the effects of CSE on ceramide levels, restored β-cell function and survival, and increased cyclin D2 expression, while also reducing activation of β-cell ER and oxidative stress. Conclusions Our results indicate that CS exposure leads to impaired insulin production, processing, secretion and reduced β-cell viability and proliferation. These effects were linked to increased β-cell oxidative and ER stress and ceramide accumulation. Mice fed HFD continued to experience detrimental effects of CS exposure even during smoking cessation. Elucidation of the mechanisms by which CS exposure impairs β-cell function in synergy with obesity will help design therapeutic and preventive interventions for both active and former smokers.
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    Endoplasmic reticulum stress alters ryanodine receptor function in the murine pancreatic β cell
    (American Society for Biochemistry and Molecular Biology, 2018-11-12) Yamamoto, Wataru R.; Bone, Robert N.; Sohn, Paul; Syed, Farooq; Reissaus, Christopher A.; Mosley, Amber L.; Wijeratne, Aruna B.; True, Jason D.; Tong, Xin; Kono, Kono; Evans-Molina, Carmella; Biochemistry and Molecular Biology, School of Medicine
    Alterations in endoplasmic reticulum (ER) calcium (Ca2+) levels diminish insulin secretion and reduce β-cell survival in both major forms of diabetes. The mechanisms responsible for ER Ca2+ loss in β cells remain incompletely understood. Moreover, a specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP3R) dysfunction in the pathophysiology of diabetes remains largely untested. To this end, here we applied intracellular and ER Ca2+ imaging techniques in INS-1 β cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP3R function. Our results revealed that the RyR is sensitive mainly to ER stress–induced dysfunction, whereas cytokine stress specifically alters IP3R activity. Consistent with this observation, pharmacological inhibition of the RyR with ryanodine and inhibition of the IP3R with xestospongin C prevented ER Ca2+ loss under ER and cytokine stress conditions, respectively. However, RyR blockade distinctly prevented β-cell death, propagation of the unfolded protein response (UPR), and dysfunctional glucose-induced Ca2+ oscillations in tunicamycin-treated INS-1 β cells and mouse islets and Akita islets. Monitoring at the single-cell level revealed that ER stress acutely increases the frequency of intracellular Ca2+ transients that depend on both ER Ca2+ leakage from the RyR and plasma membrane depolarization. Collectively, these findings indicate that RyR dysfunction shapes ER Ca2+ dynamics in β cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca2+ may be an early pathogenic event in diabetes.
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    Histone Deacetylase Inhibitors Prevent Cytokine-Induced β Cell Dysfunction Through Restoration of Stromal Interaction Molecule 1 Expression and Activation of Store-Operated Calcium Entry
    (bioRxiv, 2023-12-08) Lee, Chih-Chun; Kono, Tatsuyoshi; Syed, Farooq; Weaver, Staci A.; Sohn, Paul; Wu, Wenting; Chang, Garrick; Liu, Jing; Slak Rupnik, Marjan; Evans-Molina, Carmella; Pediatrics, School of Medicine
    Histone deacetylase inhibitors (HDIs) modulate β cell function in preclinical models of diabetes; however, the mechanisms underlying these beneficial effects have not been determined. In this study, we investigated the impact of the HDI sodium butyrate (NaB) on β cell function and calcium (Ca2+) signaling using ex vivo and in vitro models of diabetes. Our results show that NaB significantly improved glucose-stimulated insulin secretion in islets from human organ donors with type 2 diabetes and in cytokine-treated INS-1 β cells. Consistently, NaB partially rescued glucose-stimulated Ca2+ oscillations in mouse islets treated with proinflammatory cytokines. Because the oscillatory phenotype of Ca2+ in the β cell is governed by changes in endoplasmic reticulum (ER) Ca2+ levels, next we explored the relationship between NaB and store-operated calcium entry (SOCE), a rescue mechanism that acts to refill ER Ca2+ levels through STIM1-mediated gating of plasmalemmal Orai channels. We found that NaB treatment preserved basal ER Ca2+ levels and restored SOCE in IL-1β-treated INS-1 cells. Furthermore, we linked these changes with the restoration of STIM1 levels in cytokine-treated INS-1 cells and mouse islets, and we found that NaB treatment was sufficient to prevent β cell death in response to IL-1β treatment. Mechanistically, NaB counteracted cytokine-mediated reductions in phosphorylation levels of key signaling molecules, including AKT, ERK1/2, glycogen synthase kinase-3α (GSK-3α), and GSK-3β. Taken together, these data support a model whereby HDI treatment promotes β cell function and Ca2+ homeostasis under proinflammatory conditions through STIM1-mediated control of SOCE and AKT-mediated inhibition of GSK-3.
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    Impaired Store-Operated Calcium Entry and STIM1 Loss Lead to Reduced Insulin Secretion and Increased Endoplasmic Reticulum Stress in the Diabetic β-Cell
    (American Diabetes Association, 2018-11) Kono, Tatsuyoshi; Tong, Xin; Taleb, Solaema; Bone, Robert N.; Iida, Hitoshi; Lee, Chih-Chun; Sohn, Paul; Gilon, Patrick; Roe, Michael W.; Evans-Molina, Carmella; Medicine, School of Medicine
    Store-operated Ca2+ entry (SOCE) is a dynamic process that leads to refilling of endoplasmic reticulum (ER) Ca2+ stores through reversible gating of plasma membrane Ca2+ channels by the ER Ca2+ sensor Stromal Interaction Molecule 1 (STIM1). Pathogenic reductions in β-cell ER Ca2+ have been observed in diabetes. However, a role for impaired SOCE in this phenotype has not been tested. We measured the expression of SOCE molecular components in human and rodent models of diabetes and found a specific reduction in STIM1 mRNA and protein levels in human islets from donors with type 2 diabetes (T2D), islets from hyperglycemic streptozotocin-treated mice, and INS-1 cells (rat insulinoma cells) treated with proinflammatory cytokines and palmitate. Pharmacologic SOCE inhibitors led to impaired islet Ca2+ oscillations and insulin secretion, and these effects were phenocopied by β-cell STIM1 deletion. STIM1 deletion also led to reduced ER Ca2+ storage and increased ER stress, whereas STIM1 gain of function rescued β-cell survival under proinflammatory conditions and improved insulin secretion in human islets from donors with T2D. Taken together, these data suggest that the loss of STIM1 and impaired SOCE contribute to ER Ca2+ dyshomeostasis under diabetic conditions, whereas efforts to restore SOCE-mediated Ca2+ transients may have the potential to improve β-cell health and function.
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    Pancreatic Beta Cell Identity Regulated by the Endoplasmic Reticulum Calcium Sensor Stromal Interaction Molecule 1
    (2021-12) Sohn, Paul; Evans-Molina, Carmella; Elmendorf, Jeffrey; Linnemann, Amelia; Sankar, Uma
    Type 2 diabetes mellitus is a chronic disorder characterized by hyperglycemia, insulin resistance, and insufficient insulin secretion from the pancreatic β cells. To maintain adequate levels of insulin secretion, β cells rely on highly coordinated control of luminal ER Ca2+. Stromal Interaction Molecule 1 (STIM1) is an ER Ca2+ sensor that serves to replenish ER Ca2+ stores in response to depletion by gating plasmalemmal Orai1 channels in a process known as store-operated calcium entry (SOCE). We developed a method for the direct measurement of SOCE in pancreatic β cells and found that deletion of STIM1 in INS-1 cells (STIM1KO) is sufficient to block Ca2+ influx in response to store-depletion. To determine the physiological importance of β cell STIM1, we created mice with pancreatic β cell specific deletion of STIM1 (STIM1Δβ) and placed them on a high fat diet (HFD) with 60% of kilocalories derived from fat. After 8 weeks of HFD, female, but not male, STIM1Δβ mice exhibited increased body weight and fat mass as well as significant glucose intolerance and impaired insulin secretion without observable differences in insulin tolerance. Immunohistochemical analysis revealed a reduction of β cell mass and an increase of α cell mass; ELISA of islet lysates revealed a similar significant reduction in insulin content and increased glucagon content. RNA-sequencing performed on STIM1Δβ islets revealed differentially expressed genes for functions related to apoptosis, lipid metabolism, and epithelial cell differentiation, as well as loss of β cell identity. Proteomics analysis of STIM1KO cells phenocopied the metabolic findings, revealing significantly increased glucagon expression. Analysis of islet RNA-sequencing results showed modulation of pathways related to 17-β estradiol (E2) signaling, with notable downregulation of G-protein coupled estrogen receptor 1 (GPER1) expression. Consistently, treatment of female wild-type islets with pharmacological SOCE inhibitors led to reduced expression GPER1, while STIM1KO cells showed lower mobilization of intracellular cAMP levels in response to GPER agonist treatment. Taken together, these findings identify a novel interaction between SOCE and E2 signaling in the female islet and suggest that loss of STIM1 and impairments in SOCE may contribute to diabetes pathophysiology through loss of β cell identity.
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    Sodium butyrate prevents cytokine‐induced β‐cell dysfunction through restoration of stromal interaction molecule 1 expression and activation of store‐operated calcium entry
    (Wiley, 2024) Lee, Chih-Chun; Kono, Tatsuyoshi; Syed, Farooq; Weaver, Staci A.; Sohn, Paul; Wu, Wenting; Chang, Garrick; Liu, Jing; Slak Rupnik, Marjan; Evans-Molina, Carmella; Pediatrics, School of Medicine
    Sodium butyrate (NaB) improves β-cell function in preclinical models of diabetes; however, the mechanisms underlying these beneficial effects have not been fully elucidated. In this study, we investigated the impact of NaB on β-cell function and calcium (Ca2+) signaling using ex vivo and in vitro models of diabetes. Our results show that NaB significantly improved glucose-stimulated insulin secretion in islets from human organ donors with type 2 diabetes and in cytokine-treated INS-1 β cells. Consistently, NaB improved glucose-stimulated Ca2+ oscillations in mouse islets treated with proinflammatory cytokines. Because the oscillatory phenotype of Ca2+ in the β cell is governed by changes in endoplasmic reticulum (ER) Ca2+ levels, we explored the relationship between NaB and store-operated calcium entry (SOCE), a rescue mechanism that acts to refill ER Ca2+ levels through STIM1-mediated gating of plasmalemmal Orai channels. We found that NaB treatment preserved basal ER Ca2+ levels and restored SOCE in IL-1β-treated INS-1 cells. Furthermore, we linked these changes with the restoration of STIM1 levels in cytokine-treated INS-1 cells and mouse islets, and we found that NaB treatment was sufficient to prevent β-cell death in response to IL-1β treatment. Mechanistic experiments revealed that NaB mediated these beneficial effects in the β-cell through histone deacetylase (HDAC) inhibition, iNOS suppression, and modulation of AKT-GSK-3 signaling. Taken together, these data support a model whereby NaB treatment promotes β-cell function and Ca2+ homeostasis under proinflammatory conditions through pleiotropic effects that are linked with maintenance of SOCE. These results also suggest a relationship between β-cell SOCE and gut microbiome-derived butyrate that may be relevant in the treatment and prevention of diabetes.
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    Stromal Interaction Molecule 1 Maintains β-Cell Identity and Function in Female Mice Through Preservation of G-Protein–Coupled Estrogen Receptor 1 Signaling
    (American Diabetes Association, 2023) Sohn, Paul; McLaughlin, Madeline R.; Krishnan, Preethi; Wu, Wenting; Slak Rupnik, Marjan; Takasu, Akira; Senda, Toshiya; Lee, Chih-Chun; Kono, Tatsuyoshi; Evans-Molina, Carmella; Anatomy, Cell Biology and Physiology, School of Medicine
    Altered endoplasmic reticulum (ER) Ca2+ signaling has been linked with β-cell dysfunction and diabetes development. Store-operated Ca2+ entry replenishes ER Ca2+ through reversible gating of plasma membrane Ca2+ channels by the ER Ca2+ sensor, stromal interaction molecule 1 (STIM1). For characterization of the in vivo impact of STIM1 loss, mice with β-cell-specific STIM1 deletion (STIM1Δβ mice) were generated and challenged with high-fat diet. Interestingly, β-cell dysfunction was observed in female, but not male, mice. Female STIM1Δβ mice displayed reductions in β-cell mass, a concomitant increase in α-cell mass, and reduced expression of markers of β-cell maturity, including MafA and UCN3. Consistent with these findings, STIM1 expression was inversely correlated with HbA1c levels in islets from female, but not male, human organ donors. Mechanistic assays demonstrated that the sexually dimorphic phenotype observed in STIM1Δβ mice was due, in part, to loss of signaling through the noncanonical 17-β estradiol receptor (GPER1), as GPER1 knockdown and inhibition led to a similar loss of expression of β-cell maturity genes in INS-1 cells. Together, these data suggest that STIM1 orchestrates pancreatic β-cell function and identity through GPER1-mediated estradiol signaling.