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Open Access Publications from the University of California

Scholarly Works (18 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2015)

The circadian system plays an essential role in regulation of glucose homeostasis, and the disruption of this system leads to deleterious health consequences, such as Type 2 Diabetes Mellitus (T2DM). Disruption of circadian organization in humans is strongly associated with development of metabolic dysfunction and increases the propensity for development of T2DM. Importantly, the correlation between circadian disruption and T2DM is partly attributed to loss of beta-cell function and mass, called beta-cell failure. To address this association, we first hypothesized that exposure to environmental conditions associated with disruption of circadian rhythms and susceptibility to T2DM in humans disrupts islet clock and beta-cell function. We reported that prolonged exposure to LL disrupts islet circadian clock function through impairment in the amplitude, phase, and interislet synchrony of clock transcriptional oscillations. We also reported that exposure to LL leads to diminished glucose-stimulated insulin secretion due to a decrease in insulin secretory pulse mass. Second, obesity-mediated insulin resistance is an important contributory factor to induction and progression of T2DM. We, therefore, hypothesized that disruption of circadian rhythms compromises pancreatic beta cell functional and morphological adaption to diet-induced obesity leading to development of T2DM. We found that concomitant exposure to LL and HFD resulted in development of hyperglycemia characterized by loss of circadian rhythms in insulin secretion, compromised beta cell function, and induction of beta cell apoptosis, suggesting that circadian disruption and diet-induced obesity synergize to promote development of beta cell failure, likely mediated as a consequence of impaired beta cell clock function. Third, as accumulating evidence pointed out that metabolic signals can feed back into the circadian system, we tested whether the impaired glucose homeostasis disturbs the circadian system in return. We examined the expression of circadian rhythms in the human islet amyloid polypeptide transgenic (HIP) rats: a validated non-obese, beta-cell dysfuntion model of T2DM. Diurnal and circadian rhythms of locomotor activity, circadian entrainment, and clock gene oscillation in SCN, pancreatic islets and aorta all remains intact in the HIP rats. In summary, these studies examined mechanisms by which circadian disruption predisposes to beta-cell failure in the development of T2DM. We also showed that glucose intolerance caused by beta-cell dysfunction might not be sufficient to cause circadian deficits, which emphasized the causal role of circadian disruption in the development of T2DM.

    Cover page: Disruption of circadian rhythm impairs pancreatic islet function and increases susceptibility to beta-cell failure, while pathological complications of Type 2 Diabetes Mellitus have scant effects on the circadian system
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2009)

    Objective

    Obesity is a known risk factor for type 2 diabetes. However, most obese individuals do not develop diabetes because they adapt to insulin resistance by increasing beta-cell mass and insulin secretion. Islet pathology in type 2 diabetes is characterized by beta-cell loss, islet amyloid derived from islet amyloid polypeptide (IAPP), and increased beta-cell apoptosis characterized by endoplasmic reticulum (ER) stress. We hypothesized that IAPP-induced ER stress distinguishes successful versus unsuccessful islet adaptation to insulin resistance.

    Research design and methods

    To address this, we fed wild-type (WT) and human IAPP transgenic (HIP) rats either 10 weeks of regular chow or a high-fat diet and prospectively examined the relations among beta-cell mass and turnover, beta-cell ER stress, insulin secretion, and insulin sensitivity.

    Results

    A high-fat diet led to comparable insulin resistance in WT and HIP rats. WT rats compensated with increased insulin secretion and beta-cell mass. In HIP rats, in contrast, neither beta-cell function nor mass compensated for the increased insulin demand, leading to diabetes. The failure to increase beta-cell mass in HIP rats was the result of ER stress-induced beta-cell apoptosis that increased in proportion to diet-induced insulin resistance.

    Conclusions

    IAPP-induced ER stress distinguishes the successful versus unsuccessful islet adaptation to a high-fat diet in rats. These studies are consistent with the hypothesis that IAPP oligomers contribute to increased beta-cell apoptosis and beta-cell failure in humans with type 2 diabetes.
      Cover page: Successful Versus Failed Adaptation to High-Fat Diet–Induced Insulin Resistance The Role of IAPP-Induced β-Cell Endoplasmic Reticulum Stress
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2018)
      Development of cell replacement therapies in diabetes requires understanding of the molecular underpinnings of β-cell maturation. The circadian clock regulates diverse cellular functions important for regulation of β-cell function and turnover. However, postnatal ontogenesis of the islet circadian clock and its potential role in β-cell maturation remain unknown. To address this, we studied wild-type Sprague-Dawley as well as Period1 luciferase transgenic (Per1:LUC) rats to determine circadian clock function, clock protein expression, and diurnal insulin secretion during islet development and maturation process. We additionally studied β-cell-specific Bmal1-deficient mice to elucidate a potential role of this key circadian transcription factor in β-cell functional and transcriptional maturation. We report that emergence of the islet circadian clock 1) occurs during the early postnatal period, 2) depends on the establishment of global behavioral circadian rhythms, and 3) leads to the induction of diurnal insulin secretion and gene expression. Islet cell maturation was also characterized by induction in the expression of circadian transcription factor BMAL1, deletion of which altered postnatal development of glucose-stimulated insulin secretion and the associated transcriptional network. Postnatal development of the islet circadian clock contributes to early-life β-cell maturation and should be considered for optimal design of future β-cell replacement strategies in diabetes.
        Cover page: Postnatal Ontogenesis of the Islet Circadian Clock Plays a Contributory Role in β-Cell Maturation Process
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2013)
        The islet in type 2 diabetes (T2DM) is characterized by a deficit in β-cells, increased β-cell apoptosis, and extracellular amyloid deposits derived from islet amyloid polypeptide (IAPP). In the absence of longitudinal studies, it is unknown if the low β-cell mass in T2DM precedes diabetes onset (is a risk factor for diabetes) or develops as a consequence of the disease process. Although insulin resistance is a risk factor for T2DM, most individuals who are insulin resistant do not develop diabetes. By inference, an increased β-cell workload results in T2DM in some but not all individuals. We propose that the extent of the β-cell mass that develops during childhood may underlie subsequent successful or failed adaptation to insulin resistance in later life. We propose that a low innate β-cell mass in the face of subsequent insulin resistance may expose β-cells to a burden of insulin and IAPP biosynthetic demand that exceeds the cellular capacity for protein folding and trafficking. If this threshold is crossed, intracellular toxic IAPP membrane permeant oligomers (cylindrins) may form, compromising β-cell function and inducing β-cell apoptosis.
          Cover page: β-Cell Failure in Type 2 Diabetes: A Case of Asking Too Much of Too Few?
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2013)
          There is a correlation between circadian disruption, type 2 diabetes mellitus (T2DM), and islet failure. However, the mechanisms underlying this association are largely unknown. Pancreatic islets express self-sustained circadian clocks essential for proper β-cell function and survival. We hypothesized that exposure to environmental conditions associated with disruption of circadian rhythms and susceptibility to T2DM in humans disrupts islet clock and β-cell function. To address this hypothesis, we validated the use of Per-1:LUC transgenic rats for continuous longitudinal assessment of islet circadian clock function ex vivo. Using this methodology, we subsequently examined effects of the continuous exposure to light at night (LL) on islet circadian clock and insulin secretion in vitro in rat islets. Our data show that changes in the light-dark cycle in vivo entrain the phase of islet clock transcriptional oscillations, whereas prolonged exposure (10 weeks) to LL disrupts islet circadian clock function through impairment in the amplitude, phase, and interislet synchrony of clock transcriptional oscillations. We also report that exposure to LL leads to diminished glucose-stimulated insulin secretion due to a decrease in insulin secretory pulse mass. Our studies identify potential mechanisms by which disturbances in circadian rhythms common to modern life can predispose to islet failure in T2DM.
            Cover page: Consequences of Exposure to Light at Night on the Pancreatic Islet Circadian Clock and Function in Rats
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2016)
            Laser capture microdissection (LCM) is a powerful method to isolate specific populations of cells for subsequent analysis such as gene expression profiling, for example, microarrays or ribonucleic (RNA)-Seq. This technique has been applied to frozen as well as formalin-fixed, paraffin-embedded (FFPE) specimens with variable outcomes regarding quality and quantity of extracted RNA. The goal of the study was to develop the methods to isolate high-quality RNA from islets of Langerhans and pancreatic duct glands (PDG) isolated by LCM. We report an optimized protocol for frozen sections to minimize RNA degradation and maximize recovery of expected transcripts from the samples using quantitative real-time polymerase chain reaction (RT-PCR) by adding RNase inhibitors at multiple steps during the experiment. This technique reproducibly delivered intact RNA (RIN values 6-7). Using quantitative RT-PCR, the expected profiles of insulin, glucagon, mucin6 (Muc6), and cytokeratin-19 (CK-19) mRNA in PDGs and pancreatic islets were detected. The described experimental protocol for frozen pancreas tissue might also be useful for other tissues with moderate to high levels of intrinsic ribonuclease (RNase) activity.
              Cover page: Recovery of high-quality RNA from laser capture microdissected human and rodent pancreas
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2016)
                Cover page: Recovery of high-quality RNA from laser capture microdissected human and rodent pancreas
                • Article
                • Peer Reviewed
                UC San Francisco Previously Published Works (2015)
                Pancreatic β cells secrete insulin in response to postprandial increases in glucose levels to prevent hyperglycemia and inhibit insulin secretion under fasting conditions to protect against hypoglycemia. β cells lack this functional capability at birth and acquire glucose-stimulated insulin secretion (GSIS) during neonatal life. Here, we have shown that during postnatal life, the de novo DNA methyltransferase DNMT3A initiates a metabolic program by repressing key genes, thereby enabling the coupling of insulin secretion to glucose levels. In a murine model, β cell-specific deletion of Dnmt3a prevented the metabolic switch, resulting in loss of GSIS. DNMT3A bound to the promoters of the genes encoding hexokinase 1 (HK1) and lactate dehydrogenase A (LDHA) - both of which regulate the metabolic switch - and knockdown of these two key DNMT3A targets restored the GSIS response in islets from animals with β cell-specific Dnmt3a deletion. Furthermore, DNA methylation-mediated repression of glucose-secretion decoupling genes to modulate GSIS was conserved in human β cells. Together, our results reveal a role for DNA methylation to direct the acquisition of pancreatic β cell function.
                  Cover page: DNA methylation directs functional maturation of pancreatic β cells
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2012)
                  Pancreatic duct glands (PDGs) have been hypothesized to give rise to pancreatic intraepithelial neoplasia (PanIN). Treatment with the glucagon-like peptide (GLP)-1 analog, exendin-4, for 12 weeks induced the expansion of PDGs with mucinous metaplasia and columnar cell atypia resembling low-grade PanIN in rats. In the pancreata of Pdx1-Cre; LSL-Kras(G12D) mice, exendin-4 led to acceleration of the disruption of exocrine architecture and chronic pancreatitis with mucinous metaplasia and increased formation of murine PanIN lesions. PDGs and PanIN lesions in rodent and human pancreata express the GLP-1 receptor. Exendin-4 induced proproliferative signaling pathways in human pancreatic duct cells, cAMP-protein kinase A and mitogen-activated protein kinase phosphorylation of cAMP-responsive element-binding protein, and increased cyclin D1 expression. These GLP-1 effects were more pronounced in the presence of an activating mutation of Kras and were inhibited by metformin. These data reveal that GLP-1 mimetic therapy may induce focal proliferation in the exocrine pancreas and, in the context of exocrine dysplasia, may accelerate formation of neoplastic PanIN lesions and exacerbate chronic pancreatitis.
                    Cover page: Chronic GLP-1 Receptor Activation by Exendin-4 Induces Expansion of Pancreatic Duct Glands in Rats and Accelerates Formation of Dysplastic Lesions and Chronic Pancreatitis in the KrasG12D Mouse Model
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2009)

                    Objective

                    We sought to establish the extent and mechanisms by which sitagliptin and metformin singly and in combination modify islet disease progression in human islet amyloid polypeptide transgenic (HIP) rats, a model for type 2 diabetes.

                    Research design and methods

                    HIP rats were treated with sitagliptin, metformin, sitagliptin plus metformin, or no drug as controls for 12 weeks. Fasting blood glucose, insulin sensitivity, and beta-cell mass, function, and turnover were measured in each group.

                    Results

                    Sitagliptin plus metformin had synergistic effects to preserve beta-cell mass in HIP rats. Metformin more than sitagliptin inhibited beta-cell apoptosis. Metformin enhanced hepatic insulin sensitivity; sitagliptin enhanced extrahepatic insulin sensitivity with a synergistic effect in combination. beta-Cell function was partially preserved by sitagliptin plus metformin. However, sitagliptin treatment was associated with increased pancreatic ductal turnover, ductal metaplasia, and, in one rat, pancreatitis.

                    Conclusions

                    The combination of metformin and sitagliptin had synergistic actions to preserve beta-cell mass and function and enhance insulin sensitivity in the HIP rat model of type 2 diabetes. However, adverse actions of sitagliptin treatment on exocrine pancreas raise concerns that require further evaluation.
                      Cover page: Beneficial Endocrine but Adverse Exocrine Effects of Sitagliptin in the Human Islet Amyloid Polypeptide Transgenic Rat Model of Type 2 Diabetes Interactions With Metformin
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