Biomedical subjects

Catherine B Chan

Publications and source records attributed to Catherine B Chan.

15 recordsLinked to original sources

PubMed2005

Glucose-regulated glucagon secretion requires insulin receptor expression in pancreatic alpha-cells.

The insulin receptor (IR) and its signaling appear to be essential for insulin secretion from pancreatic beta-cells. However, much less is known about the role of the IR in alpha-cells. To assess the role of the IR in glucagon and insulin secretion, we engineered adeno-viruses for high efficiency small interference RNA (siRNA)-IR expression in isolated mouse pancreatic islets and lentiviruses for siRNA-IR expression in pancreatic alpha- and beta-cell lines (alpha-TC6 and MIN6) with specific, long term stable IR knockdown. Western blot analysis showed that these strategies resulted in 60-80% reduction of IR protein in islets and alpha- and beta-cell lines. Cell growth was reduced by 35-50% in alpha-TC and MIN6 cells stably expressing siRNA-IR, respectively. Importantly, glucagon secretion, in response to glucose (25 to 2.8 mm), was completely abolished in islets expressing siRNA-IR, whereas secretion increased 1.7-fold in islets expressing control siRNA. In contrast, there was no difference in glucose-stimulated insulin secretion when comparing siRNA-IR and siRNA control, with both groups showing a 1.7-fold increase. Islet glucagon and insulin content were also unaffected by IR knockdown. To further explore the role of the IR, siRNA-IR was stably expressed in pancreatic cell lines, which dramatically suppressed glucose-regulated glucagon secretion in alpha-TC6 cells (3.4-fold) but did not affect GSIS in MIN6 cells. Defects in siRNA-IR-expressing alpha-cells were associated with an alteration in the activity of Akt and p70S6K where insulin-induced phosphorylation of protein kinase B/AKt was greatly reduced while p70S6K activation was enhanced, suggesting that the related pathways play important roles in alpha cell function. This study provides direct evidence that appropriate expression of the IR in alpha-cells is required for glucose-dependent glucagon secretion.

Adenoviridae↗

PubMed2005

Use of pedometers to measure physical activity in dogs.

OBJECTIVE: To determine whether pedometers can be used to measure physical activity in dogs. DESIGN: Cross-sectional study. ANIMALS: 26 dogs. PROCEDURE: To determine pedometer accuracy, number of steps recorded with the pedometer as dogs walked, trotted, and ran for a distance of approximately 30 m (100 ft) at each gait was compared with actual number of steps. Dogs and owners then wore pedometers for 7 to 14 days, and dog pedometer output was compared with body condition score, owner-reported activity of the dog, and owner pedometer output. RESULTS: Most owners classified their dogs as active or quite active and indicated that their dogs exercised 3 to 7 days/wk. For all dogs, body condition score was 5, 6, or 7 on a scale from 1 to 9. At a walk, pedometers overestimated actual number of steps by approximately 17% in large and medium dogs and underestimated actual number of steps by approximately 7% in small dogs. No significant differences between pedometer-recorded and actual number of steps were detected when dogs trotted or ran. Number of steps per day for the dogs was significantly correlated with owner-reported activity of the dog (r = 0.305) and number of steps per day for the owners (r = 0.469) and was inversely correlated with body condition score (r = -0.554). CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that pedometers can measure physical activity in dogs with reasonable accuracy. A lower number of steps per day was associated with a higher body condition score, and less active owners generally had less active dogs.

Animals↗

PubMed2005

Transcriptional regulation of lipid metabolism by fatty acids: a key determinant of pancreatic beta-cell function.

BACKGROUND: Optimal pancreatic beta-cell function is essential for the regulation of glucose homeostasis in both humans and animals and its impairment leads to the development of diabetes. Type 2 diabetes is a polygenic disease aggravated by environmental factors such as low physical activity or a hypercaloric high-fat diet. RESULTS: Free fatty acids represent an important factor linking excess fat mass to type 2 diabetes. Several studies have shown that chronically elevated free fatty acids have a negative effect on beta-cell function leading to elevated insulin secretion basally but with an impaired response to glucose. The transcription factors PPARalpha, PPARgamma and SREBP-1c respond to changing fat concentrations in tissues, thereby coordinating the genomic response to altered metabolic conditions to promote either fat storage or catabolism. These transcription factors have been identified in beta-cells and it appears that each may exert influence on beta-cell function in health and disease. CONCLUSION: The role of the PPARs and SREBP-1c as potential mediators of lipotoxicity is an emerging area of interest.

Journal Article↗

PubMed2005

Role of mitochondria in toxic oxidative stress.

Oxidative stress and mitochondrial oxidative damage have been implicated in the etiology of numerous common diseases. The critical mitochondrial events responsible for oxidative stress-mediated cell death (toxic oxidative stress), however, have yet to be defined. Several oxidative events implicated in toxic oxidative stress include alterations in mitochondrial lipids (e.g., cardiolipin), mitochondrial DNA, and mitochondrial proteins (eg. aconitase and uncoupling protein 2). Furthermore, recent findings indicate the enrichment of mitochondrial membranes with vitamin E protects cells against the toxic effects of oxidative stress. This review briefly summarizes the role of these mitochondrial events in toxic oxidative stress, including: 1) the protective role of mitochondrial vitamin E in toxic oxidative stress, 2) the role of mitochondrial DNA in toxic oxidative stress, 3) the interaction between cardiolipin and cytochrome c in mitochondrial regulation of apoptosis, 4) the role of mitochondrial aconitase in oxidative neurodegeneration, and 5) the role of mitochondrial uncoupling protein 2 in the pathogenesis of type 2 diabetes.

Aconitate Hydratase↗

PubMed2004

Free fatty acid-induced beta-cell defects are dependent on uncoupling protein 2 expression.

Chronic exposure to elevated free fatty acids (lipotoxicity) induces uncoupling protein (UCP2) in the pancreatic beta-cell, and therefore a causal link between UCP2 and beta-cell defects associated with obesity may exist. Recently, we showed that lipid treatment in vivo and in vitro in UCP2(-/-) mice/islets does not result in any loss in beta-cell glucose sensitivity. We have now assessed the mechanism of maintained beta-cell function in UCP2(-/-) mice by exposing islets to 0.4 mM palmitate for 48 h. Palmitate treatment increased triglyceride concentrations in wild type (WT) but not UCP2(-/-) islets because of higher palmitate oxidation rates in the UCP2(-/-) islets. Dispersed beta-cells from the palmitate-exposed WT islets had reduced glucose-stimulated hyperpolarization of the mitochondrial membrane potential compared with both control WT and palmitate-exposed UCP2(-/-) beta-cells. The glucose-stimulated increases in the ATP/ADP ratio and cytosolic Ca2+ are attenuated in palmitate-treated WT but not UCP2(-/-) beta-cells. Exposure to palmitate reduced glucose-stimulated insulin secretion (GSIS) in WT islets, whereas UCP2(-/-) islets had enhanced GSIS. Overexpression of recombinant UCP2 but not enhanced green fluorescent protein in beta-cells resulted in a loss of glucose-stimulated hyperpolarization of the mitochondrial membrane potential and GSIS similar to that seen in WT islets exposed to palmitate. Reactive oxygen species (ROS) are known to increase the activity of UCP2. We showed that ROS levels were elevated in control UCP2(-/-) islets as compared with WT and UCP2(-/-) islets overexpressing UCP2 and that palmitate increased ROS in WT and UCP2(-/-) islets overexpressing UCP2 but not in UCP2(-/-) islets. Thus, UCP2(-/-) islets resisted the toxic effects of palmitate by maintaining glucose-dependent metabolism-secretion coupling. We propose that higher free fatty acid oxidation rates prevent accumulation of triglyceride in UCP2(-/-) islets, such accumulation being a phenomenon associated with lipotoxicity.

Adenosine Diphosphate↗

PubMed2004

The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation.

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.

Animals↗

PubMed2004

Health benefits of a pedometer-based physical activity intervention in sedentary workers.

BACKGROUND: Inactivity is a leading contributor to chronic health problems. Here, we examined the effects of a pedometer-based physical activity intervention (Prince Edward Island-First Step Program, PEI-FSP) on activity and specific health indices in 106 sedentary workers. METHODS: Participants were recruited from five workplaces where most jobs were moderately-highly sedentary. Using subjects as their own control, physical activity (pedometer-determined steps per day) was compared before and after a 12-week intervention. Changes in body mass index (BMI), waist girth, resting heart rate, and blood pressure were evaluated. RESULTS: The PEI-FSP was completed by 59% of participants. Steps per day increased from 7,029 +/- 3,100 (SD) at baseline to a plateau of 10,480 +/- 3,224 steps/day by 3.96 +/- 3.28 weeks of the intervention. The amount that participants were able to increase their steps per day was not related to their baseline BMI. On average, participants experienced significant decreases in BMI, waist girth, and resting heart rate. Reductions in waist girth and heart rate were significantly related to the increase in steps per day. In contrast, reductions in BMI were predicted by the initial steps per day. CONCLUSIONS: The PEI-FSP increased physical activity in a sedentary population. Importantly, those with a higher BMI at baseline achieved relatively similar increases in their physical activity as participants with a lower BMI.

Adult↗

PubMed2004

Gene and protein kinase expression profiling of reactive oxygen species-associated lipotoxicity in the pancreatic beta-cell line MIN6.

Oligonucleotide microarrays were used to define oleic acid (OA)-regulated gene expression and proteomic technology to screen protein kinases in MIN6 insulinoma cells. The effects of oxidative stress caused by OA and potential protective effects of N-acetyl-L-cysteine (NAC), a scavenger of reactive oxygen species (ROS), on global gene expression and beta-cell function were investigated. Long-term exposure of MIN6 cells to OA led to a threefold increase in basal insulin secretion, a 50% decrease in insulin content, an inhibition of glucose-stimulated insulin secretion (GSIS), and a twofold increase in the level of ROS. The addition of NAC normalized both the OA-induced insulin content and ROS elevation, but it failed to restore GSIS. Microarray studies and subsequent quantitative PCR analysis showed that OA consistently regulated the expression of 45 genes involved in metabolism, cell growth, signal transduction, transcription, and protein processing. The addition of NAC largely normalized the expression of the OA-regulated genes involved in cell growth and differentiation but not other functions. A protein kinase screen showed that OA regulated the expression and/or phosphorylation levels of kinases involved in stress-response mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and cell cycle control pathways. Importantly, these findings indicate that chronic OA exposure can impair beta-cell function through ROS-dependent and -independent mechanisms.

Acetylcysteine↗

PubMed2004

Uncoupling protein 2 and islet function.

Stressors such as chronic hyperglycemia or hyperlipidemia may lead to insufficient insulin secretion in susceptible individuals, contributing to type 2 diabetes. The molecules mediating this effect are just beginning to be identified. Uncoupling protein (UCP)-2 may be one such negative modulator of insulin secretion. Accumulating evidence shows that beta-cell UCP2 expression is upregulated by glucolipotoxic conditions and that increased activity of UCP2 decreases insulin secretion. Mitochondrial superoxide has been identified as a posttranslational regulator of UCP2 activity in islets; thus, UCP2 may provide protection to beta-cells at one level while simultaneously having detrimental effects on insulin secretion. Interestingly, the latter appears to be the dominant outcome, because UCP2 knockout mice display an increased beta-cell mass and retained insulin secretion capacity in the face of glucolipotoxicity.

Animals↗

PubMed2003

Mitochondrial functional state in clonal pancreatic beta-cells exposed to free fatty acids.

Excessive free fatty acid (FFA) exposure represents a potentially important diabetogenic condition that can impair insulin secretion from pancreatic beta-cells. Because mitochondrial oxidative phosphorylation is a main link between glucose metabolism and insulin secretion, in the present work we investigated the effects of the FFA oleate (OE) on mitochondrial function in the clonal pancreatic beta-cell line, MIN6. Both the long term (72 h) and short term (immediately after application) impact of OE exposure on beta-cells was investigated. After 72 h of exposure to OE (0.4 mm, 0.5% bovine serum albumin) cells were washed and permeabilized, and mitochondrial function (respiration, phosphorylation, membrane potential formation, production of reactive oxygen species) was measured in the absence or presence of OE. MIN6 cells exposed to OE for 72 h showed impaired glucose-stimulated insulin secretion and decreased cellular ATP. Mitochondria in OE-exposed cells retained normal functional characteristics in FFA-free medium; however, they were significantly more sensitive to the acute uncoupling effect of OE treatment. The mitochondria of OE-exposed cells displayed increased depolarization caused by acute OE treatment, which is attributable to the elevation in the FFA-transporting function of uncoupling protein 2 and the dicarboxylate carrier. These cells also had an increased production of reactive oxygen species in complex I of the mitochondrial respiratory chain that could be activated by FFA. A high level of reduction of respiratory complex I augmented acute FFA-induced uncoupling in a way compatible with activation of mitochondrial uncoupling protein by intramitochondrial superoxide. A stronger augmentation was observed in OE-exposed cells. Together, these events may underlie FFA-induced depression of the ATP/ADP ratio in beta-cells, which accounts for the defective glucose-stimulated insulin secretion associated with lipotoxicity.

Animals↗

PubMed2003

Cross-sectional relationship of pedometer-determined ambulatory activity to indicators of health.

OBJECTIVE: To describe the cross-sectional relationship between an objective measure of walking (pedometer-determined steps/day) and general indicators of health, a prior diagnosis of one or more components of the metabolic syndrome, and self-reported occupational activity in a generally sedentary working population. RESEARCH METHODS AND PROCEDURES: Steps/day were compared with previous diagnosis of one or more components of the metabolic syndrome (by self-administered questionnaire) and with general health indicators including BMI, waist circumference, resting heart rate, and blood pressure in 182 subjects in Prince Edward Island, Canada. Study participants were volunteer employees recruited from five workplaces where, in general, the job types were moderately or highly sedentary. RESULTS: Steps/day were 7230 +/- SD 3447 for women (n = 153) and 8265 +/- 2849 (n = 21) for men. Pedometer-determined steps/day were associated inversely with BMI (r = -0.4005, p < 0.0001) in all participants and waist circumference in females only (r = -0.4303, p < 0.0001). There was a low correlation between steps/day and diastolic blood pressure in the whole sample (r = -0.2140, p = 0.0383). Participants who reported a prior diagnosis of one or more components of the metabolic syndrome (hypertension, hypercholesterolemia, heart disease, or type 2 diabetes) took fewer steps/day than healthy participants (p = 0.0254). Pedometer-determined steps/day were positively associated with self-reported occupational activity (p = 0.0002). DISCUSSION: Fewer steps/day are associated with increased BMI, waist circumference, diastolic blood pressure, and components of the metabolic syndrome. Low occupational activity is a contributing factor to low total ambulatory activity.

Adult↗

PubMed2002

Inhibition of Kv2.1 voltage-dependent K+ channels in pancreatic beta-cells enhances glucose-dependent insulin secretion.

Voltage-dependent (Kv) outward K(+) currents repolarize beta-cell action potentials during a glucose stimulus to limit Ca(2+) entry and insulin secretion. Dominant-negative "knockout" of Kv2 family channels enhances glucose-stimulated insulin secretion. Here we show that a putative Kv2.1 antagonist (C-1) stimulates insulin secretion from MIN6 insulinoma cells in a glucose- and dose-dependent manner while blocking voltage-dependent outward K(+) currents. C-1-blocked recombinant Kv2.1-mediated currents more specifically than currents mediated by Kv1, -3, and -4 family channels (Kv1.4, 3.1, 4.2). Additionally, C-1 had little effect on currents recorded from MIN6 cells expressing a dominant-negative Kv2.1 alpha-subunit. The insulinotropic effect of acute Kv2.1 inhibition resulted from enhanced membrane depolarization and augmented intracellular Ca(2+) responses to glucose. Immunohistochemical staining of mouse pancreas sections showed that expression of Kv2.1 correlated highly with insulin-containing beta-cells, consistent with the ability of C-1 to block voltage-dependent outward K(+) currents in isolated mouse beta-cells. Antagonism of Kv2.1 in an ex vivo perfused mouse pancreas model enhanced first- and second-phase insulin secretion, whereas glucagon secretion was unaffected. The present study demonstrates that Kv2.1 is an important component of beta-cell stimulus-secretion coupling, and a compound that enhances, but does not initiate, beta-cell electrical activity by acting on Kv2.1 would be a useful antidiabetic agent.

Animals↗

PubMed2002

The effects of high-fat diet on exercise-induced changes in metabolic parameters in Zucker fa/fa rats.

The objectives of this study were to document the effects of moderate aerobic exercise on insulin secretion and other metabolic indices in fa/fa rats and to determine if a high-fat (HF) diet altered these effects. Six-week-old fa/fa and lean Zucker rats were either sedentary or exercised by daily swimming for 4 weeks. Half of the exercised and sedentary rats were fed a diet with 16% fat and 44% carbohydrate, while the control groups were fed a diet with 4.5% fat and 49% carbohydrate. At the end of 4 weeks, caloric intake, weight gain, plasma hormone and nutrient levels, and oral glucose tolerance were measured. The pancreatic islet beta-cell function was assessed by measuring glucose-stimulated insulin secretion, glucose phosphorylating activity, and free fatty acid (FFA) oxidation in cultured islets. In fa/fa rats fed the control diet, exercise reduced weight gain, caloric intake, and fasting plasma triglyceride (TG) concentrations without affecting fasting glucose and insulin concentrations. HF diet blocked the effects of exercise on weight gain and food intake and worsened insulin resistance of fa/fa rats. In vitro, neither exercise nor HF diet alone affected islet beta-cell function. However, in combination, exercise and high dietary fat reduced glucokinase sensitivity to glucose and increased islet cell response to mannoheptulose inhibitory actions. We conclude that beneficial effects of moderate exercise on metabolism are not mediated by effects on pancreatic beta cells. Diets elevated in fat decrease the beneficial effects of exercise on metabolic indices in vivo.

Animals↗

PubMed2002

Uncoupling protein 2 knockout mice have enhanced insulin secretory capacity after a high-fat diet.

Uncoupling protein 2 (UCP2) may act as an important regulator of insulin secretion. In this study, beta-cell function in UCP2-deficient mice was examined after a 45% high-fat diet (HFD) to assess its role during the development of diet-induced type 2 diabetes. HFD-fed UCP2 (-/-) mice have lower fasting blood glucose and elevated insulin levels when compared with wild-type (WT) mice. UCP2 (-/-) mice also have enhanced beta-cell glucose sensitivity compared with WT mice after HFD, a result that is due in part to the deterioration of glucose responsiveness in WT mice. HFD-fed UCP2 (-/-) mice have increased insulin secretory capacity as a result of increased pancreatic beta-cell mass and insulin content per islet. Islets from WT mice exposed to 0.5 mmol/l palmitate for 48 h have significantly reduced mitochondrial membrane potential, ATP concentrations, and glucose responsiveness compared with UCP2 (-/-) islets, suggesting that elevated UCP2 in WT mice increases proton leak and decreases mitochondrial ATP production. Highly increased carnitine palmitoyl transferase-1 gene expression in UCP2 (-/-) mice is suggestive of enhanced fatty acid oxidizing capacity, particularly after HFD stress. These results further establish UCP2 as a component in glucose sensing and suggest a possible new aspect of UCP2 function during the progression of type 2 diabetes.

Animals↗

PubMed2002

Endogenous regulation of insulin secretion by UCP2.

Uncoupling protein 2 (UCP2) expression is more-or-less ubiquitous, in tissues of diverse function. Its presence in adipose and muscle is postulated to be involved in regulation of energy expenditure and nutrient partitioning, particularly that of fats. In pancreatic islet beta cells, induction of UCP2 is shown to inhibit glucose-stimulated insulin secretion. Thus, insufficient insulin secretion in models of type 2 diabetes is associated with elevated UCP2 expression in islets. The evidence for such a role for UCP2 in the regulation of insulin secretion in islets is reviewed.

Animals↗