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Molecular Aspects of Medicine Apr 2015To maintain copious insulin granule stores in the face of ongoing metabolic demand, pancreatic beta cells must produce large quantities of proinsulin, the insulin... (Review)
Review
To maintain copious insulin granule stores in the face of ongoing metabolic demand, pancreatic beta cells must produce large quantities of proinsulin, the insulin precursor. Proinsulin biosynthesis can account for up to 30-50% of total cellular protein synthesis of beta cells. This puts pressure on the beta cell secretory pathway, especially the endoplasmic reticulum (ER), where proinsulin undergoes its initial folding, including the formation of three evolutionarily conserved disulfide bonds. In normal beta cells, up to 20% of newly synthesized proinsulin may fail to reach its native conformation, suggesting that proinsulin is a misfolding-prone protein. Misfolded proinsulin molecules can either be refolded to their native structure or degraded through ER associated degradation (ERAD) and autophagy. These degraded molecules decrease proinsulin yield but do not otherwise compromise beta cell function. However, under certain pathological conditions, proinsulin misfolding increases, exceeding the genetically determined threshold of beta cells to handle the misfolded protein load. This results in accumulation of misfolded proinsulin in the ER - a causal factor leading to beta cell failure and diabetes. In patients with Mutant INS-gene induced diabetes of Youth (MIDY), increased proinsulin misfolding due to insulin gene mutations is the primary defect operating as a "first hit" to beta cells. Additionally, increased proinsulin misfolding can be secondary to an unfavorable ER folding environment due to genetic and/or environmental factors. Under these conditions, increased wild-type proinsulin misfolding becomes a "second hit" to the ER and beta cells, aggravating beta cell failure and diabetes. In this article, we describe our current understanding of the normal proinsulin folding pathway in the ER, and then review existing links between proinsulin misfolding, ER dysfunction, and beta cell failure in the development and progression of type 2, type 1, and some monogenic forms of diabetes.
Topics: Diabetes Mellitus; Diabetes Mellitus, Type 1; Diabetes Mellitus, Type 2; Endoplasmic Reticulum; Endoplasmic Reticulum Stress; Humans; Insulin-Secreting Cells; Oxidoreductases; Proinsulin; Protein Folding; Unfolded Protein Response
PubMed: 25579745
DOI: 10.1016/j.mam.2015.01.001 -
Genome Biology Sep 2022Non-coding genetic variants that influence gene transcription in pancreatic islets play a major role in the susceptibility to type 2 diabetes (T2D), and likely also...
BACKGROUND
Non-coding genetic variants that influence gene transcription in pancreatic islets play a major role in the susceptibility to type 2 diabetes (T2D), and likely also contribute to type 1 diabetes (T1D) risk. For many loci, however, the mechanisms through which non-coding variants influence diabetes susceptibility are unknown.
RESULTS
We examine splicing QTLs (sQTLs) in pancreatic islets from 399 human donors and observe that common genetic variation has a widespread influence on the splicing of genes with established roles in islet biology and diabetes. In parallel, we profile expression QTLs (eQTLs) and use transcriptome-wide association as well as genetic co-localization studies to assign islet sQTLs or eQTLs to T2D and T1D susceptibility signals, many of which lack candidate effector genes. This analysis reveals biologically plausible mechanisms, including the association of T2D with an sQTL that creates a nonsense isoform in ERO1B, a regulator of ER-stress and proinsulin biosynthesis. The expanded list of T2D risk effector genes reveals overrepresented pathways, including regulators of G-protein-mediated cAMP production. The analysis of sQTLs also reveals candidate effector genes for T1D susceptibility such as DCLRE1B, a senescence regulator, and lncRNA MEG3.
CONCLUSIONS
These data expose widespread effects of common genetic variants on RNA splicing in pancreatic islets. The results support a role for splicing variation in diabetes susceptibility, and offer a new set of genetic targets with potential therapeutic benefit.
Topics: Diabetes Mellitus, Type 1; Diabetes Mellitus, Type 2; Exodeoxyribonucleases; Humans; Islets of Langerhans; Proinsulin; Protein Isoforms; RNA Splicing; RNA, Long Noncoding
PubMed: 36109769
DOI: 10.1186/s13059-022-02757-0 -
Annals of the New York Academy of... Apr 2018The endoplasmic reticulum (ER) is broadly distributed throughout the cytoplasm of pancreatic beta cells, and this is where all proinsulin is initially made. Healthy beta... (Review)
Review
The endoplasmic reticulum (ER) is broadly distributed throughout the cytoplasm of pancreatic beta cells, and this is where all proinsulin is initially made. Healthy beta cells can synthesize 6000 proinsulin molecules per second. Ordinarily, nascent proinsulin entering the ER rapidly folds via the formation of three evolutionarily conserved disulfide bonds (B7-A7, B19-A20, and A6-A11). A modest amount of proinsulin misfolding, including both intramolecular disulfide mispairing and intermolecular disulfide-linked protein complexes, is a natural by-product of proinsulin biosynthesis, as is the case for many proteins. The steady-state level of misfolded proinsulin-a potential ER stressor-is linked to (1) production rate, (2) ER environment, (3) presence or absence of naturally occurring (mutational) defects in proinsulin, and (4) clearance of misfolded proinsulin molecules. Accumulation of misfolded proinsulin beyond a certain threshold begins to interfere with the normal intracellular transport of bystander proinsulin, leading to diminished insulin production and hyperglycemia, as well as exacerbating ER stress. This is most obvious in mutant INS gene-induced Diabetes of Youth (MIDY; an autosomal dominant disease) but also likely to occur in type 2 diabetes owing to dysregulation in proinsulin synthesis, ER folding environment, or clearance.
Topics: Animals; Diabetes Mellitus; Endoplasmic Reticulum; Endoplasmic Reticulum Stress; Humans; Insulin-Secreting Cells; Mutation; Proinsulin; Protein Folding; Protein Transport
PubMed: 29377149
DOI: 10.1111/nyas.13531 -
Journal of Diabetes Science and... May 2022The ability to measure insulin secretion from pancreatic beta cells and monitor glucose-insulin physiology is vital to current health needs. C-peptide has been used...
BACKGROUND
The ability to measure insulin secretion from pancreatic beta cells and monitor glucose-insulin physiology is vital to current health needs. C-peptide has been used successfully as a surrogate for plasma insulin concentration. Quantifying the expected variability of modelled insulin secretion will improve confidence in model estimates.
METHODS
Forty-three healthy adult males of Māori or Pacific peoples ancestry living in New Zealand participated in an frequently sampled, intravenous glucose tolerance test (FS-IVGTT) with an average age of 29 years and a BMI of 33 kg/m. A 2-compartment model framework and standardized kinetic parameters were used to estimate endogenous pancreatic insulin secretion from plasma C-peptide measurements. Monte Carlo analysis (N = 10 000) was then used to independently vary parameters within ±2 standard deviations of the mean of each variable and the 5th and 95th percentiles determined the bounds of the expected range of insulin secretion. Cumulative distribution functions (CDFs) were calculated for each subject for area under the curve (AUC) total, AUC Phase 1, and AUC Phase 2. Normalizing each AUC by the participant's median value over all N = 10 000 iterations quantifies the expected model-based variability in AUC.
RESULTS
Larger variation is found in subjects with a BMI > 30 kg/m, where the interquartile range is 34.3% compared to subjects with a BMI ≤ 30 kg/m where the interquartile range is 24.7%.
CONCLUSIONS
Use of C-peptide measurements using a 2-compartment model and standardized kinetic parameters, one can expect ~±15% variation in modelled insulin secretion estimates. The variation should be considered when applying this insulin secretion estimation method to clinical diagnostic thresholds and interpretation of model-based analyses such as insulin sensitivity.
Topics: Adult; Blood Glucose; C-Peptide; Diabetes Mellitus, Type 2; Glucose Tolerance Test; Humans; Insulin; Insulin Resistance; Insulin Secretion; Male
PubMed: 33588609
DOI: 10.1177/1932296821991120 -
Cells May 2023Type 1 diabetes (T1D) is a chronic autoimmune disorder that damages beta cells in the pancreatic islets of Langerhans and results in hyperglycemia due to the loss of... (Review)
Review
Type 1 diabetes (T1D) is a chronic autoimmune disorder that damages beta cells in the pancreatic islets of Langerhans and results in hyperglycemia due to the loss of insulin. Exogenous insulin therapy can save lives but does not halt disease progression. Thus, an effective therapy may require beta-cell restoration and suppression of the autoimmune response. However, currently, there are no treatment options available that can halt T1D. Within the National Clinical Trial (NCT) database, a vast majority of over 3000 trials to treat T1D are devoted to insulin therapy. This review focuses on non-insulin pharmacological therapies. Many investigational new drugs fall under the category of immunomodulators, such as the recently FDA-approved CD-3 monoclonal antibody teplizumab. Four intriguing candidate drugs fall outside the category of immunomodulators, which are the focus of this review. Specifically, we discuss several non-immunomodulators that may have more direct action on beta cells, such as verapamil (a voltage-dependent calcium channel blocker), gamma aminobutyric acid (GABA, a major neurotransmitter with effects on beta cells), tauroursodeoxycholic acid (TUDCA, an endoplasmic reticulum chaperone), and volagidemab (a glucagon receptor antagonist). These emerging anti-diabetic drugs are expected to provide promising results in both beta-cell restoration and in suppressing cytokine-derived inflammation.
Topics: Humans; Diabetes Mellitus, Type 1; Cytoprotection; Insulin-Secreting Cells; Islets of Langerhans; Insulin; Immunologic Factors; Insulin, Regular, Human
PubMed: 37296593
DOI: 10.3390/cells12111472 -
Cellular and Molecular Life Sciences :... Feb 2021Proinsulin C-peptide (C-peptide) has drawn much research attention. Even if the peptide has turned out not to be important in the treatment of diabetes, every phase of...
Proinsulin C-peptide (C-peptide) has drawn much research attention. Even if the peptide has turned out not to be important in the treatment of diabetes, every phase of C-peptide research has changed our view on insulin and peptide hormone biology. The first phase revealed that peptide hormones can be subject to processing, and that their pro-forms may involve regulatory stages. The second phase revealed the possibility that one prohormone could harbor more than one activity, and that the additional activities should be taken into account in the development of hormone-based therapies. In the third phase, a combined view of the evolutionary patterns in hormone biology allowed an assessment of C-peptide´s role in physiology, and of how biological activities and physiological functions are shaped by evolutionary processes. In addition to this distinction, C-peptide research has produced further advances. For example, C-peptide fragments are successfully administered in immunotherapy of type I diabetes, and plasma C-peptide levels remain a standard for measurement of beta cell activity in patients. Even if the concept of C-peptide as a hormone is presently not supported, some of its bioactivities continue to influence our understanding of evolutionary changes of also other peptides.
Topics: C-Peptide; Diabetes Mellitus, Type 1; Evolution, Molecular; Humans; Protein Aggregates; Protein Conformation
PubMed: 32959070
DOI: 10.1007/s00018-020-03636-2 -
Diabetologia Oct 2020The discovery of insulin in 1921 has been one of greatest scientific achievements of the 20th century. Since then, the availability of insulin has shifted the focus of... (Review)
Review
The discovery of insulin in 1921 has been one of greatest scientific achievements of the 20th century. Since then, the availability of insulin has shifted the focus of diabetes treatment from trying to keep patients alive to saving and improving the life of millions. Throughout this time, basic and clinical research has advanced our understanding of insulin synthesis and action, both in healthy and pathological conditions. Yet, multiple aspects of insulin production remain unknown. In this review, we focus on the most recent findings on insulin synthesis, highlighting their relevance in diabetes. Graphical abstract.
Topics: Crystallization; Diabetes Mellitus, Type 1; Diabetes Mellitus, Type 2; Gene Expression Regulation; Glucose; Humans; Insulin; Insulin Secretion; Proinsulin; Protein Biosynthesis; Protein Folding; Protein Precursors; RNA Processing, Post-Transcriptional; RNA, Messenger; Secretory Vesicles
PubMed: 32894308
DOI: 10.1007/s00125-020-05192-7 -
The Journal of Clinical Endocrinology... Nov 2022Aberrant biosynthesis and secretion of the insulin precursor proinsulin occurs in both type I and type II diabetes. Inflammatory cytokines are implicated in pancreatic...
CONTEXT
Aberrant biosynthesis and secretion of the insulin precursor proinsulin occurs in both type I and type II diabetes. Inflammatory cytokines are implicated in pancreatic islet stress and dysfunction in both forms of diabetes, but the mechanisms remain unclear.
OBJECTIVE
We sought to determine the effect of the diabetes-associated cytokines on proinsulin folding, trafficking, secretion, and β-cell function.
METHODS
Human islets were treated with interleukin-1β and interferon-γ for 48 hours, followed by analysis of interleukin-6, nitrite, proinsulin and insulin release, RNA sequencing, and unbiased profiling of the proinsulin interactome by affinity purification-mass spectrometry.
RESULTS
Cytokine treatment induced secretion of interleukin-6, nitrites, and insulin, as well as aberrant release of proinsulin. RNA sequencing showed that cytokines upregulated genes involved in endoplasmic reticulum stress, and, consistent with this, affinity purification-mass spectrometry revealed cytokine induced proinsulin binding to multiple endoplasmic reticulum chaperones and oxidoreductases. Moreover, increased binding to the chaperone immunoglobulin binding protein was required to maintain proper proinsulin folding in the inflammatory environment. Cytokines also regulated novel interactions between proinsulin and type 1 and type 2 diabetes genome-wide association studies candidate proteins not previously known to interact with proinsulin (eg, Ataxin-2). Finally, cytokines induced proinsulin interactions with a cluster of microtubule motor proteins and chemical destabilization of microtubules with Nocodazole exacerbated cytokine induced proinsulin secretion.
CONCLUSION
Together, the data shed new light on mechanisms by which diabetes-associated cytokines dysregulate β-cell function. For the first time, we show that even short-term exposure to an inflammatory environment reshapes proinsulin interactions with critical chaperones and regulators of the secretory pathway.
Topics: Humans; Proinsulin; Diabetes Mellitus, Type 2; Cytokines; Interleukin-6; Genome-Wide Association Study; Insulin; Islets of Langerhans; Insulin-Secreting Cells
PubMed: 36017587
DOI: 10.1210/clinem/dgac493 -
Microbial Cell Factories Oct 2014The rapid increase in the number of diabetic patients globally and exploration of alternate insulin delivery methods such as inhalation or oral route that rely on higher... (Review)
Review
The rapid increase in the number of diabetic patients globally and exploration of alternate insulin delivery methods such as inhalation or oral route that rely on higher doses, is bound to escalate the demand for recombinant insulin in near future. Current manufacturing technologies would be unable to meet the growing demand of affordable insulin due to limitation in production capacity and high production cost. Manufacturing of therapeutic recombinant proteins require an appropriate host organism with efficient machinery for posttranslational modifications and protein refolding. Recombinant human insulin has been produced predominantly using E. coli and Saccharomyces cerevisiae for therapeutic use in human. We would focus in this review, on various approaches that can be exploited to increase the production of a biologically active insulin and its analogues in E. coli and yeast. Transgenic plants are also very attractive expression system, which can be exploited to produce insulin in large quantities for therapeutic use in human. Plant-based expression system hold tremendous potential for high-capacity production of insulin in very cost-effective manner. Very high level of expression of biologically active proinsulin in seeds or leaves with long-term stability, offers a low-cost technology for both injectable as well as oral delivery of proinsulin.
Topics: Escherichia coli; Humans; Plants, Genetically Modified; Proinsulin; Recombinant Proteins; Saccharomyces cerevisiae
PubMed: 25270715
DOI: 10.1186/s12934-014-0141-0 -
Clinical Biochemistry Feb 2018Proinsulin, the precursor for insulin, is secreted in higher concentrations when β-cells are under stress and previous studies have shown that elevated proinsulin could...
Proinsulin, the precursor for insulin, is secreted in higher concentrations when β-cells are under stress and previous studies have shown that elevated proinsulin could be used as a marker for individuals in a pre-diabetic state. The aim of this study was to assess the stability of proinsulin across a wide concentration range (3-882 and 2-187pmol/L; total and intact proinsulin respectively) in whole blood to determine whether it could be used in routine clinical care. 51 subjects (26 normal glucose tolerance, 17 impaired glucose tolerance and 8 type 2 diabetes) had blood taken into EDTA tubes at 0, 60 & 120min following a glucose load. The samples were kept at room temperature (~20°C) with aliquots taken, centrifuged and frozen at 0, 24, 48 and 72h. Comparison of the combined data (pre and post-glucose load) of baseline with 72h as a percentage of baseline gave an average of 123% (95% CI: 119-127) and 107% (95% CI: 105-109) for total and intact proinsulin respectively. A small change in the stability of total proinsulin was observed whilst there was no clinical difference over the 72h period for intact proinsulin.
Topics: Adult; Blood Glucose; C-Peptide; Diabetes Mellitus, Type 2; Female; Glucose; Glucose Tolerance Test; Humans; Insulin; Male; Middle Aged; Prediabetic State; Proinsulin; Protein Stability; Reference Values
PubMed: 29051035
DOI: 10.1016/j.clinbiochem.2017.10.005