Control of metabolism and hormone secretion from pancreatic alpha and beta cells
Author
Summary, in English
Type 2 Diabetes (T2D) is a multifactorial disease, which has made it difficult to resolve its pathophysiology. We investigated processes that could contribute to the development of T2D. In addition to well-known hallmarks of the disease, such as defective insulin secretion, insulin resistance and perturbed glucagon secretion, we aimed to find a possible role for CLOCK genes in the pathogenesis of T2D.
In paper 1, we showed that there is a link between the expression levels of core CLOCK genes in human islets and functional parameters important for T2D. This link to T2D was particularly strong for the core CLOCK components, PER2, PER3 and CRY2. We further investigated this possibility in paper 2, where we found that silencing of Per3 in a beta cell line disrupted insulin secretion in response to glucose and other secretagogues. Our observations suggest that exocytosis may be an underlying cause. In support of this assumption, we found down-regulation of crucial genes involved in the exocytotic machinery upon silencing of Per3. Together, our observations suggest that there is a link between the circadian clock machinery and beta cell function.
In paper, 3 we employed an alpha and a beta cell line, which were challenged with the same stimuli. We compared their responses in hormone release and metabolism in order to understand metabolic control of glucagon secretion. We found that two cell lines responded similarly to glucose: alpha cells increased glucagon secretion upon glucose stimulation while beta cells increased secretion of insulin. Differences, however, were primarily found in the coupling of glycolytic and mitochondrial metabolism. Moreover, inhibiting the malate-aspartate shuttle completely abolished glucagon secretion, while insulin secretion was largely preserved. This was likely due to a compensatory activity in the glycerolphosphate shuttle in beta cells.
So far, most observations seemed to involve mitochondrial metabolism in some way. Therefore we studied how mitochondria become molecularly equipped for metabolic coupling in beta cells. Characterization of a beta cell-specific knockout of Tfb1m (beta Tfb1m-/-), which encodes a protein controlling translation of mitochondrial proteins, showed mitochondrial dysfunction. This confirmed previous findings where TFB1M was identified as T2D risk gene in human islets. Islets from β-Tfb1m-/- mice showed impaired insulin secretion, contained less insulin in secretory granules and exhibited reduced beta cell mass. Mitochondria exhibited disrupted architecture. All measured metabolic parameters in mitochondria were impaired. Reactive oxygen species were increased, and signs of apoptosis and necrosis with accompanying inflammation were observed.
These studies have illustrated the complexity of the mechanisms involved in the pathogenesis of T2D. Thus, investigating different metabolic aspects of its pathogenesis, supported the multifactorial nature of the underlying mechanism of T2D development.
In paper 1, we showed that there is a link between the expression levels of core CLOCK genes in human islets and functional parameters important for T2D. This link to T2D was particularly strong for the core CLOCK components, PER2, PER3 and CRY2. We further investigated this possibility in paper 2, where we found that silencing of Per3 in a beta cell line disrupted insulin secretion in response to glucose and other secretagogues. Our observations suggest that exocytosis may be an underlying cause. In support of this assumption, we found down-regulation of crucial genes involved in the exocytotic machinery upon silencing of Per3. Together, our observations suggest that there is a link between the circadian clock machinery and beta cell function.
In paper, 3 we employed an alpha and a beta cell line, which were challenged with the same stimuli. We compared their responses in hormone release and metabolism in order to understand metabolic control of glucagon secretion. We found that two cell lines responded similarly to glucose: alpha cells increased glucagon secretion upon glucose stimulation while beta cells increased secretion of insulin. Differences, however, were primarily found in the coupling of glycolytic and mitochondrial metabolism. Moreover, inhibiting the malate-aspartate shuttle completely abolished glucagon secretion, while insulin secretion was largely preserved. This was likely due to a compensatory activity in the glycerolphosphate shuttle in beta cells.
So far, most observations seemed to involve mitochondrial metabolism in some way. Therefore we studied how mitochondria become molecularly equipped for metabolic coupling in beta cells. Characterization of a beta cell-specific knockout of Tfb1m (beta Tfb1m-/-), which encodes a protein controlling translation of mitochondrial proteins, showed mitochondrial dysfunction. This confirmed previous findings where TFB1M was identified as T2D risk gene in human islets. Islets from β-Tfb1m-/- mice showed impaired insulin secretion, contained less insulin in secretory granules and exhibited reduced beta cell mass. Mitochondria exhibited disrupted architecture. All measured metabolic parameters in mitochondria were impaired. Reactive oxygen species were increased, and signs of apoptosis and necrosis with accompanying inflammation were observed.
These studies have illustrated the complexity of the mechanisms involved in the pathogenesis of T2D. Thus, investigating different metabolic aspects of its pathogenesis, supported the multifactorial nature of the underlying mechanism of T2D development.
Publishing year
2014
Language
English
Publication/Series
Lund University Faculty of Medicine Doctoral Dissertation Series
Volume
2014:79
Document type
Dissertation
Publisher
Molecular Metabolism
Topic
- Endocrinology and Diabetes
Keywords
- CLOCK genes
- insulin
- glucagon
- metabolites
- mitochondria
- knock out
Status
Published
Research group
- Diabetes - Molecular Metabolism
- LUDC (Lund University Diabetes Centre)
Supervisor
ISBN/ISSN/Other
- ISSN: 1652-8220
- ISBN: 978-91-7619-008-1
Defence date
13 June 2014
Defence time
09:00
Defence place
"Lilla Aulan" Jan Waldenströms gata 5, Skånes universitetssjukhus i Malmö
Opponent
- Frans Schuit