Familial hyperproinsulinemia due to a proposed defect in conversion of proinsulin to insulin

Discovery of insulin 100 years on

The discovery of insulin 100 years ago ranks among the greatest medical achievements ever. This sparked a revolution of scientific discovery and therapeutic intervention to treat people suffering with diabetes. A light was shone for other areas of medicine to illuminate what was possible with detailed scientific endeavour. There followed a range of firsts leading to the current time in which we now know more about this peptide hormone than almost any other protein in existence. This has allowed therapeutic advancement from a positon of knowledge leading to stunning innovation. This innovation is likely to lead to more physiological insulin replacement reducing the disease burden to individuals and society as whole.

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

Pathophysiology of diabetes: An overview

Diabetes mellitus is a chronic heterogeneous metabolic disorder with complex pathogenesis. It is characterized by elevated blood glucose levels or hyperglycemia, which results from abnormalities in either insulin secretion or insulin action or both. Hyperglycemia manifests in various forms with a varied presentation and results in carbohydrate, fat, and protein metabolic dysfunctions. Long-term hyperglycemia often leads to various microvascular and macrovascular diabetic complications, which are mainly responsible for diabetes-associated morbidity and mortality. Hyperglycemia serves as the primary biomarker for the diagnosis of diabetes as well. In this review, we would be focusing on the classification of diabetes and its pathophysiology including that of its various types.

Lessons for modern insulin development

There have been many advances in insulin with a realistic possibility of mimicking nature to improve insulin replacement, with a view to achieving improved metabolic control. Lessons can be learnt from the evolution of insulin, insulin development, and new advances in technology. This may lead to fewer side effects of therapy resulting in a lower risk of hypoglycaemia and less weight gain, which could in turn could reduce long-term complications for people with diabetes.

THE EFFECT OF DIABETES ON CARDIOVASCULAR SYSTEM

Diabetes mellitus (DM) is a critical and long-term disorder due to the insufficient production of insulin by the pancreas or ineffective use of insulin by the body. Importantly, cardiovascular disease (CVD) has long been thought to be linked with diabetes. Despite more diabetic individuals surviving from better medications and treatments, there has been significant rise in the morbidity and mortality from CVD. Indeed, the classification of DM based on the electrocardiogram signals of the heart will be an advantageous system. Further, computer-aided classification of DM with integrated algorithms may enhance the execution of the system. In this paper, we have reviewed various studies using heart rate variability signals for automated classification of diabetes. Furthermore, the different techniques used to extract the features and the efficiency of the classification systems are discussed.

INS-gene mutations: from genetics and beta cell biology to clinical disease

A growing list of insulin gene mutations causing a new form of monogenic diabetes has drawn increasing attention over the past seven years. The mutations have been identified in the untranslated regions of the insulin gene as well as the coding sequence of preproinsulin including within the signal peptide, insulin B-chain, C-peptide, insulin A-chain, and the proteolytic cleavage sites both for signal peptidase and the prohormone convertases. These mutations affect a variety of different steps of insulin biosynthesis in pancreatic beta cells. Importantly, although many of these mutations cause proinsulin misfolding with early onset autosomal dominant diabetes, some of the mutant alleles appear to engage different cellular and molecular mechanisms that underlie beta cell failure and diabetes. In this article, we review the most recent advances in the field and discuss challenges as well as potential strategies to prevent/delay the development and progression of autosomal dominant diabetes caused by INS-gene mutations. It is worth noting that although diabetes caused by INS gene mutations is rare, increasing evidence suggests that defects in the pathway of insulin biosynthesis may also be involved in the progression of more common types of diabetes. Collectively, the (pre)proinsulin mutants provide insightful molecular models to better understand the pathogenesis of all forms of diabetes in which preproinsulin processing defects, proinsulin misfolding, and ER stress are involved.

Zinc transporters and their role in the pancreatic β-cell

Zinc is an essential nutrient with tremendous importance for human health, and zinc deficiency is a severe risk factor for increased mortality and morbidity. As abnormal zinc homeostasis causes diabetes, and because the pancreatic β-cell contains the highest zinc content of any known cell type, it is of interest to know how zinc fluxes are controlled in β-cells. The understanding of zinc homeostasis has been boosted by the discovery of multiprotein families of zinc transporters, and one of them - zinc transporter 8 (ZnT8) - is abundantly and specifically expressed in the pancreatic islets of Langerhans. In this review, we discuss the evidence for a physiological role of ZnT8 in the formation of zinc-insulin crystals, the physical form in which most insulin is stored in secretory granules. In addition, we cross-examine this information, collected in genetically modified mouse strains, to the knowledge that genetic variants of the human ZnT8 gene predispose to the onset of type 2 diabetes and that epitopes on the ZnT8 protein trigger autoimmunity in patients with type 1 diabetes. The overall conclusion is that we are still at the dawn of a complete understanding of how zinc homeostasis operates in normal β-cells and how abnormalities lead to β-cell dysfunction and diabetes. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2012.00199.x, 2012).

New opportunities: harnessing induced pluripotency for discovery in diabetes and metabolism

The landmark discovery of induced pluripotent stem cells (iPSCs) by Shinya Yamanaka has transformed regenerative biology. Previously, insights into the pathogenesis of chronic human diseases have been hindered by the inaccessibility of patient samples. However, scientists are now able to convert patient fibroblasts into iPSCs and differentiate them into disease-relevant cell types. This ability opens new avenues for investigating disease pathogenesis and designing novel treatments. In this review, we highlight the uses of human iPSCs to uncover the underlying causes and pathological consequences of diabetes and metabolic syndromes, multifactorial diseases whose etiologies have been difficult to unravel using traditional methodologies.

The regulated secretory pathway and human disease: insights from gene variants and single nucleotide polymorphisms

The regulated secretory pathway provides critical control of peptide, growth factor, and hormone release from neuroendocrine and endocrine cells, and neurons, maintaining physiological homeostasis. Propeptides and prohormones are packaged into dense core granules (DCGs), where they frequently undergo tissue-specific processing as the DCG matures. Proteins of the granin family are DCG components, and although their function is not fully understood, data suggest they are involved in DCG formation and regulated protein/peptide secretion, in addition to their role as precursors of bioactive peptides. Association of gene variation, including single nucleotide polymorphisms (SNPs), with neuropsychiatric, endocrine, and metabolic diseases, has implicated specific secreted proteins and peptides in disease pathogenesis. For example, a SNP at position 196 (G/A) of the human brain-derived neurotrophic factor gene dysregulates protein processing and secretion and leads to cognitive impairment. This suggests more generally that variants identified in genes encoding secreted growth factors, peptides, hormones, and proteins involved in DCG biogenesis, protein processing, and the secretory apparatus, could provide insight into the process of regulated secretion as well as disorders that result when it is impaired.

Insulin gene mutations and diabetes

Some mutations of the insulin gene cause hyperinsulinemia or hyperproinsulinemia. Replacement of biologically important amino acid leads to defective receptor binding, longer half-life and hyperinsulinemia. Three mutant insulins have been identified: (i) insulin Chicago (F49L or PheB25Leu); (ii) insulin Los Angeles (F48S or PheB24Ser); (iii) and insulin Wakayama (V92L or ValA3Leu). Replacement of amino acid is necessary for proinsulin processing results in hyperproinsulinemia. Four types have been identified: (i) proinsulin Providence (H34D); (ii) proinsulin Tokyo (R89H); (iii) proinsulin Kyoto (R89L); and (iv) proinsulin Oxford (R89P). Three of these are processing site mutations. The mutation of proinsulin Providence, in contrast, is thought to cause sorting abnormality. Compared with normal proinsulin, a significant amount of proinsulin Providence enters the constitutive pathway where processing does not occur. These insulin gene mutations with hyper(pro)insulinemia were very rare, showed only mild diabetes or glucose intolerance, and hyper(pro)insulinemia was the key for their diagnosis. However, this situation changed dramatically after the identification of insulin gene mutations as a cause of neonatal diabetes. This class of insulin gene mutations does not show hyper(pro)insulinemia. Mutations at the cysteine residue or creating a new cysteine will disturb the correct disulfide bonding and proper conformation, and finally will lead to misfolded proinsulin accumulation, endoplasmic reticulum stress and apoptosis of pancreatic β-cells. Maturity-onset diabetes of the young (MODY) or an autoantibody-negative type 1-like phenotype has also been reported. Very recently, recessive mutations with reduced insulin biosynthesis have been reported. The importance of insulin gene mutation in the pathogenesis of diabetes will increase a great deal and give us a new understanding of β-cell biology and diabetes. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2011.00100.x, 2011).

Clinical and molecular genetics of neonatal diabetes due to mutations in the insulin gene

Over the last decade our insight into the causes of neonatal diabetes has greatly expanded. Neonatal diabetes was once considered a variant of type 1 diabetes that presented early in life. Recent advances in our understanding of this disorder have established that neonatal diabetes is not an autoimmune disease, but rather is a monogenic form of diabetes resulting from mutations in a number of different genes encoding proteins that play a key role in the normal function of the pancreatic beta-cell. Moreover, a correct genetic diagnosis can affect treatment and clinical outcome. This is especially true for patients with mutations in the genes KCNJ11 or ABCC8 that encode the two protein subunits (Kir6.2 and SUR1, respectively) of the ATP-sensitive potassium channel. These patients can be treated with oral sulfonylurea drugs with better glycemic control and quality of life. Recently, mutations in the insulin gene (INS) itself have been identified as another cause of neonatal diabetes. In this article, we review the role of INS mutations in the pathophysiology of neonatal diabetes.

Seven mutations in the human insulin gene linked to permanent neonatal/infancy-onset diabetes mellitus

Permanent neonatal diabetes mellitus (PNDM) is a rare disorder usually presenting within 6 months of birth. Although several genes have been linked to this disorder, in almost half the cases documented in Italy, the genetic cause remains unknown. Because the Akita mouse bearing a mutation in the Ins2 gene exhibits PNDM associated with pancreatic beta cell apoptosis, we sequenced the human insulin gene in PNDM subjects with unidentified mutations. We discovered 7 heterozygous mutations in 10 unrelated probands. In 8 of these patients, insulin secretion was detectable at diabetes onset, but rapidly declined over time. When these mutant proinsulins were expressed in HEK293 cells, we observed defects in insulin protein folding and secretion. In these experiments, expression of the mutant proinsulins was also associated with increased Grp78 protein expression and XBP1 mRNA splicing, 2 markers of endoplasmic reticulum stress, and with increased apoptosis. Similarly transfected INS-1E insulinoma cells had diminished viability compared with those expressing WT proinsulin. In conclusion, we find that mutations in the insulin gene that promote proinsulin misfolding may cause PNDM.

Hypoglycemia associated with the production of insulin-like growth factor II in a pancreatic islet cell tumor: a case report

An insulinoma is characterized by endogenous hyperinsulinemia and hypoglycemia. However, it has been reported that insulinomas with normal levels of plasma insulin and a normal insulin to glucose ratio occur in patients with hypoglycemia. Although overproduction of Insulin-like growth factor II (IGF-II) by non-islet cell tumors such as large mesenchymal tumors, can cause hypoglycemia, no cases of circulating plasma IGF-II from an islet cell tumor contributing to hypoglycemia have been reported. We report here a rare case of a pancreatic islet tumor in a patient with hypoglycemia that was associated with increased plasma IGF-II, which returned to normal after tumor resection.

Effect of carbohydrate distribution on postprandial glucose peaks with the use of continuous glucose monitoring in type 2 diabetes

BACKGROUND

Large postprandial glucose peaks are associated with increased risk of diabetic complications and cardiovascular disease.

OBJECTIVE

We investigated the effect of carbohydrate distribution on postprandial glucose peaks with continuous blood glucose monitoring (CGMS), when consuming a moderate carbohydrate diet in energy balance in subjects with type 2 diabetes.

DESIGN

Twenty-three subjects with type 2 diabetes were randomly assigned to each of four 3-d interventions in a crossover design with a 4-d washout period. Identical foods were provided for each treatment with a ratio of total carbohydrate to protein to fat of 40%:34%:26% but differing in carbohydrate content at each meal: even distribution (CARB-E; approximately 70 g carbohydrate), breakfast (CARB-B), lunch (CARB-L), and dinner(CARB-D), each providing approximately 125 g carbohydrate in the loaded meal in a 9-MJ diet. Glucose concentrations were continuously measured with CGMS. Outcomes were assessed by postprandial peak glucose (G(max)), time spent > 12 mmol/L (T > 12), and total area under the glucose curve (AUC(20)).

RESULTS

Daily G(max) differed between treatments (P = 0.003) with CARB-L (14.2 +/- 1.0 mmol/L), CARB-E (14.5 +/- 0.9 mmol/L), and CARB-D (14.6 +/- 0.8 mmol/L) being similar but lower than CARB-B (16.5 +/- 0.8 mmol/L). Meal G(max) was weakly related to carbohydrate amount and glycemic load (r = 0.40-0.44). T > 12 differed between treatments (P = 0.014), and a treatment x fasting blood glucose (FBG) interaction (P = 0.003) was observed with CARB-L (184 +/- 74 min) < CARB-B (190 +/- 49 min) < CARB-D (234 +/- 87 min) < CARB-E (262 +/- 91 min). Total AUC(20) was not significantly different between treatments. After adjustment for FBG, treatment became significant (P = 0.006); CARB-L (10 049 +/- 718 mmol/L x 20 h) < CARB-E (10 493 +/- 706 mmol/L x 20 h) < CARB-B (10 603 +/- 642 mmol/L x 20 h) < CARB-D (10 717 +/- 638 mmol/L x 20 h).

CONCLUSION

CARB-E did not optimize blood glucose control as assessed by postprandial peaks, whereas CARB-L provided the most favorable postprandial profile.

Insulin structure and function

Throughout much of the last century insulin served a central role in the advancement of peptide chemistry, pharmacology, cell signaling and structural biology. These discoveries have provided a steadily improved quantity and quality of life for those afflicted with diabetes. The collective work serves as a foundation for the development of insulin analogs and mimetics capable of providing more tailored therapy. Advancements in patient care have been paced by breakthroughs in core technologies, such as semisynthesis, high performance chromatography, rDNA-biosynthesis and formulation sciences. How the structural and conformational dynamics of this endocrine hormone elicit its biological response remains a vigorous area of study. Numerous insulin analogs have served to coordinate structural biology and biochemical signaling to provide a first level understanding of insulin action. The introduction of broad chemical diversity to the study of insulin has been limited by the inefficiency in total chemical synthesis, and the inherent limitations in rDNA-biosynthesis and semisynthetic approaches. The goals of continued investigation remain the delivery of insulin therapy where glycemic control is more precise and hypoglycemic liability is minimized. Additional objectives for medicinal chemists are the identification of superagonists and insulins more suitable for non-injectable delivery. The historical advancements in the synthesis of insulin analogs by multiple methods is reviewed with the specific structural elements of critical importance being highlighted. The functional refinement of this hormone as directed to improved patient care with insulin analogs of more precise pharmacology is reported.

Hypoglycemia

Under physiologic conditions, glucose plays a critical role in providing energy to the central nervous system. A precipitous drop in the availability of this substrate results in dramatic symptoms that signal a medical emergency and warrant immediate therapy aimed at restoring plasma glucose to normal levels. A systemic approach to the differential diagnosis is useful in identifying the cause of hypoglycemia. Once established, a specific and/or definitive intervention that addresses that underlying problem can be implemented. In most cases, this systemic approach to diagnosis and therapy is rewarded with a good outcome for the patient.

Variants in the ghrelin gene are associated with metabolic syndrome in the Old Order Amish

CONTEXT

Mature ghrelin has been shown to stimulate eating and to participate in the regulation of insulin signaling and glucose homeostasis. Its gene, GHRL, is located on chromosome 3 in a region where we have shown linkage to eating behavior, percentage body fat, and total and low-density lipoprotein cholesterol levels in subjects of the Amish Family Diabetes Study.

OBJECTIVE

Our objective was to determine whether mutations in GHRL might influence eating behavior and risk for metabolic syndrome, obesity, diabetes, and related traits.

DESIGN

We genotyped 856 Amish samples for three missense polymorphisms in GHRL, Arg51Gln, Leu72Met (rs696217), and Gln90Leu (rs4684677) and performed association analyses with eating behavior traits and metabolic syndrome as defined by the National Cholesterol Education Program Adult Treatment Panel III guidelines.

SUBJECTS

Our subjects were adult participants in the Amish Family Diabetes Study.

RESULTS

The allele frequencies of these variants were 0.03, 0.04, and 0.03, respectively. The prevalence of metabolic syndrome was lower among those carrying the 51Gln allele (3.8 vs. 15.8%; age- and sex-adjusted odds ratio = 0.22; P = 0.031). Hunger scores tended to be lower among 51Gln allele carriers but did not reach statistical significance (P = 0.07). The Leu72Met variant was also associated with increased prevalence of metabolic syndrome (23.2 vs. 13.4%; age- and sex-adjusted odds ratio = 2.57; P = 0.02) as well as higher fasting glucose, lower high-density lipoprotein, and higher triglyceride levels (P = 0.02, P = 0.007, and P = 0.04, respectively). The two variants were not in linkage disequilibrium with each other, suggesting independent effects. We conclude that mutations in GHRL may confer risk for the metabolic syndrome.

Behavior in the eukaryotic secretory pathway of insulin-containing fusion proteins and single-chain insulins bearing various B-chain mutations

In the secretory pathway, endoproteolytic cleavage of the insulin precursor protein promotes a change in the biophysical properties of the processed insulin product, and this may be relevant for its intracellular trafficking. We have now studied several independent point mutants contained within the insulin B-chain, S9D, H10D, V12E (called B9D, B10D, and B12E), as well as the double point mutant P28K,K29P (B28K,B29P), that have been reported to inhibit insulin oligomerization. In yeast cells, the unprocessed precursor of each of these mutants is secreted, whereas >90% of the endoproteolytically released single-chain insulin moiety is retained intracellularly; a large portion of the B9D, B10D, and B12E single-chain insulins exhibit abnormally slow mobility upon nonreducing SDS-PAGE, despite normal mobility upon reducing SDS-PAGE. Although no free thiols can be detected, each of these mutants exhibits increased disulfide accessibility to dithiothreitol. After dithiothreitol treatment, a portion of the molecules can reoxidize to a form more compact than the original single-chain insulin mutants formed in vivo (indicating initial disulfide mispairing). Disulfide mispairing of a fraction of B9D, B10D, and B12E mutants also occurs in the context of single-chain insulin and even in authentic proinsulin expressed within the secretory pathway of mammalian cells. We conclude that analyses of the intracellular trafficking of certain oligomerization-defective insulin mutants is complicated by the formation of disulfide isomers in the secretory pathway.

Diabetes in adolescent patients: diagnostic dilemmas

The classification of diabetes mellitus by types (1 or 2), or by age of onset (juvenile or adult), helps to clarify many aspects of pathophysiology, prognosis, and therapy. However, less-commonly encountered patients, presenting in childhood or adolescence, may not fit neatly into one or the other group. These include teenagers who present with new-onset diabetes with ketoacidosis, but who are later able to be managed permanently as type 2 patients. Other adolescent patients present with only minimal glucose intolerance, then proceed to develop type 1 diabetes, with evidence of autoimmune etiology, after a variable number of years. Four patients are presented to illustrate these diagnostic dilemmas.

Pathogenesis of non-insulin-dependent (type II) diabetes mellitus (NIDDM) - genetic predisposition and metabolic abnormalities

Non-insulin-dependent diabetes mellitus (NIDDM), also known as type II diabetes, is characterized by abnormal glucose homeostasis, resulting in hyperglycemia, and is associated with microvascular, macrovascular, and neuropathic complications. NIDDM is a complex disease with many causes. Both genetic and environmental factors play important roles in the pathogenesis of NIDDM. Cumulative evidence on the high prevalence of NIDDM in certain ethnic groups, the high concordance rate for the disease in monozygotic twins, familial aggregation, and familial transmission patterns suggests that the genetic component plays an important etiological role in the development of NIDDM. In genetically predisposed individuals, there is a slow progression from a normal state to hyperglycemia, largely due to a combination of insulin resistance and defects in insulin secretion. Although numerous candidate genes responsible for insulin resistance and for the defects in insulin secretion have been reported, no specific gene(s) accounting for the majority of cases of the common type of NIDDM has been identified. Considerable evidence indicates that environmental and other factors, including diet, stress, physical activity, obesity and aging, also play an important role in the development of the disease. In conclusion, the pathogenic process of NIDDM depends on a complex interaction between genetic and environmental factors.

First direct assay for intact human proinsulin

We describe a sensitive two-site sandwich enzyme-linked immunosorbent assay for the measurement of intact human proinsulin in 100 μL of serum or plasma. The assay is based on the use of two monoclonal antibodies specific for epitopes at the C-peptide/insulin A chain junction and at the insulin B chain/C-peptide junction, respectively. Cross-reactivities with insulin, C-peptide, and the four proinsulin conversion intermediates were negligible. The detection limit in buffer was 0.2 pmol/L (3 standard deviations from zero). The working range was 0.2–100 pmol/L. The mean intra- and interassay coefficients of variation were 2.4% and 8.9%, respectively. The mean recovery of added proinsulin was 103%. Dilution curves of 40 serum samples are parallel to the proinsulin calibration curve. Proinsulin concentrations in 20 fasting healthy subjects were all above the limit of detection: median (range), 2.7 pmol/L (1.1–6.9 pmol/L). Six fasting non-insulin-dependent diabetes mellitus and five insulinoma patients had proinsulin concentrations significantly higher than healthy subjects: median (range), 7.7 pmol/L (3.2–18 pmol/L) and 153 pmol/L (98–320 pmol/L), respectively.

Genetics of non-insulin-dependent (type-II) diabetes mellitus

Both genetic and environmental factors contribute to the etiology of non-insulin-dependent diabetes. The genetic component is heterogeneous and in some patients is probably complex, involving multiple genes. Specific genetic defects have been identified for rate monogenic forms of NIDDM: maturity-onset diabetes of the young, or MODY (which is due to glucokinase mutations in about 40% of families), syndromes of extreme insulin resistance (which often involve the insulin receptor), and diabetes-deafness syndromes (with defects in mitochondrial genes). In contrast, the genes involved in common forms of NIDDM are still uncertain. Mutations have been extensively searched in genes regulating insulin signaling and secretion. Some evidence of involvement has been produced for insulin-receptor substrate-1, glycogen synthase, the glucagon receptor, a ras-related protein (Rad), histocompatibility antigens, PC-1, and fatty acid binding protein, but the contributions of these genes to NIDDM is probably small. Other candidate genes (e.g. insulin, insulin receptor, glucose transporters) have been excluded as major diabetogenes. New insights are expected in the near future from the systematic scanning of the genome for linkage with NIDDM.

Intracellular transport of proinsulin in pancreatic beta-cells. Structural maturation probed by disulfide accessibility

In pancreatic islets, formation of beta-secretory granule cores involves early proinsulin homohexamerization and subsequent insulin condensation. We examined proinsulin conformational maturation by monitoring accessibility of protein disulfide bonds. Proinsulin disulfides are intact immediately upon synthesis, but are > or = 90% sensitive to in vivo reduction with 2 mM dithiothreitol; wash out of dithiothreitol leads to reoxidation, proinsulin transport, and conversion to insulin. With t1/2 approximately 10 min, newly synthesized proinsulin becomes resistant to disulfide reduction, correlating with endoplasmic reticulum (ER) export. However, inhibition of ER export with brefeldin A blocks acquisition of resistance to reduction, and once proinsulin arrives in the Golgi, it resists reduction despite brefeldin treatment. Moreover, in vivo, resistance of proinsulin disulfides is overcome after increasing [dithiothreitol] > 10-fold, or in vitro, in islets lysed in a zinc-free, but not a zinc-containing, medium. Employing 30 mM dithiothreitol in vivo, a further decrease in disulfide accessibility is observed following proinsulin conversion to insulin. Incubation of islets with chloroquine or zinc enhances and diminishes accessibility of insulin disulfides, respectively. We hypothesize that two major conformational changes culminating in granule core formation, proinsulin hexamerization and insulin condensation, are sensitive to zinc and occur upon ER exit and arrival in immature secretory granules, respectively.

A novel point mutation in the human insulin gene giving rise to hyperproinsulinemia (proinsulin Kyoto)

We have identified a 65-yr-old nonobese Japanese man with diabetes mellitus, fasting hyperinsulinemia (150-300 pM), and a reduced fasting C-peptide/insulin molar ratio of 2.5-3.0. Fasting hyperinsulinemia was also found in his son and daughter. Analysis of insulin isolated from the serum of the proband and his son by reverse-phase high performance liquid chromatography revealed a minor peak coeluting with human insulin and a major peak of proinsulin-like materials. The insulin gene of the patient was amplified by the polymerase chain reaction and the products were sequenced. A novel point mutation was identified in which guanine was replaced by thymine. The substitution gives rise to a new HindIII recognition site and results in the amino acid replacement of leucine for arginine at position 65. These results indicate that the amino-acid replacement prevents recognition of the C-peptide-A chain dibasic protease and results in an elevation of proinsulin-like materials in the circulation. Furthermore, in this family the proinsulin-like materials is due to a biosynthetic defect, inherited as an autosomal dominant trait. Rapid detection of this mutation can be accomplished by HindIII restriction enzyme mapping of polymerase chain reaction-generated DNA, which enables us to facilitate the diagnosis and screening.

Structural domains and molecular lifestyles of insulin and its precursors in the pancreatic beta cell

Insulin is both produced and degraded within the pancreatic Beta cell. Production involves the synthesis of the initial insulin precursor preproinsulin, which is converted to proinsulin shortly after (or during) translocation into the lumen of the rough endoplasmic reticulum. Proinsulin is then transported to the trans-cisternae of the Golgi complex where it is directed towards nascent secretory granules. Conversion of proinsulin to insulin and C-peptide arises within secretory granules, and is dependent upon their acidification. Granule contents are discharged by exocytosis in response to an appropriate stimulus. This represents the regulated secretory pathway to which more than 99% of proinsulin is directed in Beta cells of a healthy individual. An alternative route also exists in the Beta cell, the constitutive secretory pathway. It involves the rapid transfer of products from the Golgi complex to the plasma membrane for immediate release, with, it is supposed, little occasion for prohormone conversion. Even if delivered appropriately to secretory granules, not all insulin is released; some is degraded by fusion of granules with lysosomes (crinophagy). Each event in the molecular lifestyles of insulin and its precursors in the Beta cell will be seen to be governed by their own discrete functional domains. The identification and characterisation of these protein domains will help elucidate the steps responsible for delivery of proinsulin to secretory granules and conversion to insulin. Understanding the molecular mechanism of these steps may, in turn, help to explain defective insulin production in certain disease states including diabetes mellitus.

Use of DNA polymorphisms for genetic analysis of non-insulin dependent diabetes mellitus

Polymorphisms occur on the average of one out of every 500 base pairs of DNA, and these polymorphisms provide useful markers for genetic analysis. Hundreds of RFLP markers have been mapped at regular intervals throughout the human genome. Diabetes genes have not been mapped with these markers, however, only one MODY family has been partially evaluated. This type of analysis is further complicated if NIDDM is multigenic and/or polygenic. RFLPs have been used to evaluate specific candidate loci for NIDDM, e.g. the insulin, insulin receptor and glucose transporter genes. For these analyses, population and family studies (limited in number) have suggested that none of these loci are major contributors to the genetic susceptibility to NIDDM. In no case, however, could a contribution of 10% or less of these loci be confidently excluded, because of variable penetrance, different degrees of linkage disequilibrium between RFLPs and putative mutations, the frequencies of the RFLPs in non-diabetic populations, and inadequate sample size. The conclusions are clear: either (1) the correct candidate gene(s) has not been found, or (2) sample sizes need to be increased by at least an order of magnitude, or (3) newer methods of analysis must be adopted (e.g. use of extended haplotypes and associations with subphenotypes, or screening with allele specific oligonucleotide probes, denaturing gradient gel electrophoresis or direct genomic sequencing of polymerase chain reaction amplified DNA).

Intracellular transport and sorting of mutant human proinsulins that fail to form hexamers

Human proinsulin and insulin oligomerize to form dimers and hexamers. It has been suggested that the ability of prohormones to self associate and form aggregates may be responsible for the sorting process at the trans-Golgi. To examine whether insulin oligomerization is required for proper sorting into regulated storage granules, we have constructed point mutations in human insulin B chain that have been previously shown to prevent formation of insulin hexamers (Brange, J., U. Ribel, J. F. Hansen, G. Dodson, M. T. Hansen, S. Havelund, S. G. Melberg, F. Norris, K. Norris, L. Snel, A. R. Sorensen, and H. O. Voight. 1988. Nature [Lond.]. 333:679-682). One mutant (B10His----Asp) allows formation of dimers but not hexamers and the other (B9Ser----Asp) prevents formation of both dimers and hexamers. The mutants were transfected into the mouse pituitary AtT-20 cells, and their ability to be sorted into regulated secretory granules was compared to wild-type insulin. We found that while B10His----Asp is sorted somewhat less efficiently than wild-type insulin as reported previously (Carroll, R. J., R. E. Hammer, S. J. Chan, H. H. Swift, A. H. Rubenstein, and D. F. Steiner. 1988. Proc. Natl. Acad. Sci. USA. 85:8943-8947; Gross, D. J., P. A. Halban, C. R. Kahn, G. C. Weir, and L. Villa-Kumaroff. 1989. Proc. Natl. Acad. Sci. USA. 86:4107-4111). B9Ser----Asp is targeted to granules as efficiently as wild-type insulin. These results indicate that self association of proinsulin into hexamers is not required for its targeting to the regulated secretory pathway.

Partial diversion of a mutant proinsulin (B10 aspartic acid) from the regulated to the constitutive secretory pathway in transfected AtT-20 cells

A patient with type II diabetes associated with hyperproinsulinemia has been shown to have a point mutation in one insulin gene allele, resulting in replacement of histidine with aspartic acid at position 10 of the B-chain. To investigate the basis of the proinsulin processing defect, we introduced an identical mutation in the rat insulin II gene and expressed both the normal and the mutant genes in the AtT-20 pituitary corticotroph cell line. Cells expressing the mutant gene showed increased secretion of proinsulin relative to insulin and rapid release of newly synthesized proinsulin. Moreover, the mutant cell lines did not store the prohormone nor did they release it upon stimulation with secretagogues. These data indicate that a significant fraction of the mutant prohormone is released via the constitutive secretory pathway rather than the regulated pathway, thereby bypassing granule-related processing and regulated release.

A mutant human proinsulin is secreted from islets of Langerhans in increased amounts via an unregulated pathway

A coding mutation in the human insulin gene (His-B10----Asp) is associated with familial hyperproinsulinemia. To model this syndrome, we have produced transgenic mice that express high levels of the mutant prohormone in their islets of Langerhans. Strain 24-6 mice, containing about 100 copies of the mutant gene, were normoglycemic but had marked increases of serum human proinsulin immunoreactive components. Biosynthetic studies on isolated islets revealed that approximately 65% of the proinsulin synthesized in these mice was the human mutant form. Unlike the normal endogenous mouse proinsulin, which was almost exclusively handled via a regulated secretory pathway, up to 15% of the human [Asp10]proinsulin was rapidly secreted after synthesis via an unregulated or constitutive pathway, and approximately 20% was degraded within the islet cells. The secreted human [Asp10]proinsulin was not processed proteolytically. However, the processing of the normal mouse and human mutant proinsulins within the islets from transgenic mice was not significantly impaired. These findings suggest that the hyperproinsulinemia of the patients is the result of the continuous secretion of unprocessed mutant prohormone from the islets via this alternative unregulated pathway.

A superactive insulin: [B10-aspartic acid]insulin(human)

The genetic basis for a case of familial hyperproinsulinemia has been elucidated recently. It involves a single point mutation in the proinsulin gene resulting in the substitution of aspartic acid for histidine-10 of the B chain of insulin. We have synthesized a human insulin analogue, [AspB10]insulin, corresponding to the mutant proinsulin and evaluated its biological activity. [AspB10]Insulin displayed a binding affinity to insulin receptors in rat liver plasma membranes that was 534 +/- 146% relative to the natural hormone. In lipogenesis assays, the synthetic analogue exhibited a potency that was 435 +/- 144% relative to insulin, which is statistically not different from its binding affinity. Reversed-phase HPLC indicated that the synthetic analogue is more apolar than natural insulin. We suggest that the observed properties reflect changes in the conformation of the analogue relative to natural insulin, which result in a stronger interaction with the insulin receptor. Thus, a single substitution of an amino acid residue of human insulin has resulted in a superactive hormone.

A mutation in the B chain coding region is associated with impaired proinsulin conversion in a family with hyperproinsulinemia

Gruppuso et al. [Gruppuso, P.A., Gordon, P., Kahn, C. R., Cornblath, M., Zeller, W. P. & Schwartz, R. (1984) N. Engl. J. Med. 311, 629-634] have recently described a family in which hyperproinsulinemia is inherited in an autosomal dominant pattern, suggesting a structural abnormality in the proinsulin molecule as the basis for this disorder. However, unlike two previous kindreds with a similar syndrome, the serum proinsulin-like material in this family did not appear to be an intermediate conversion product but instead behaved like normal human proinsulin by several criteria. To further characterize this disorder we isolated and sequenced the insulin gene of the propositus. Leukocyte DNA was cloned into lambda-WES and recombinants containing the two insulin alleles, lambda MD41 and lambda MD51, were isolated by plaque hybridization. DNA sequencing of lambda MD51 showed that it contained the normal coding sequence for human preproinsulin. Sequence analysis of lambda MD41, however, revealed a single nucleotide substitution in the codon for residue 10 of proinsulin (CAC----GAC) that predicts the exchange of aspartic acid for histidine in the insulin B chain region. This mutation was also found in an insulin allele cloned from a second affected family member (propositus's father). These results, along with the linkage analysis of Elbein et al. [Elbein, S.C., Gruppuso, P., Schwartz, R., Skolnick, M. & Permutt, M.A. (1985) Diabetes 34, 821-824], strongly implicate this mutation as the cause of the hyperproinsulinemia in this family. Inhibition of the conversion of proinsulin to insulin may be related to altered folding and/or self-association properties of the [Asp10]proinsulin.

Altered C-peptide/insulin molar ratios and glucose turnover rates after stimulation in nondiabetic offsprings of type II diabetic patients

We evaluated the serum glucose/insulin/C-peptide dynamics and C-peptide/insulin molar ratios during sequential standard meal and intravenous (IV) glucagon testing for 240 minutes in eight genetically predisposed but nondiabetic female offsprings of type II diabetic patients and seven weight-matched, normal female controls. Glucose turnover rates and metabolic clearance rates of glucose (MCRG) were also determined isotopically by the D-[3-3H]glucose infusion technique. All the subjects had normal fasting serum glucose and glycosylated hemoglobin (HbA1) values. After meal ingestion, mean serum glucose concentrations were not different except for 120 to 180 minute values, which were significantly higher in the offsprings v controls. After intravenous glucagon infusion, percent maximum increments of glucose were no different. Mean basal immunoreactive insulin (IRI) levels were significantly (P less than 0.02) higher in the nondiabetic offsprings v controls. Following meal ingestion, mean IRI rose to a peak at 40 minutes in both groups, but values were significantly (P less than 0.001) higher in the offsprings v controls. After glucagon administration, the percent maximum increment was significantly (P less than 0.05) lower in the offsprings v controls. Despite exaggerated IRI levels in the offsprings, the mean fasting and stimulated C-peptide levels were identical in both groups throughout the study period. Basal and stimulated C-peptide/IRI molar ratios were quantitatively lower but qualitatively no different in the nondiabetic offsprings v controls throughout the study period. Mean basal hepatic glucose output (HGO) was higher but not statistically different in the offsprings compared with the controls (2.10 +/- 0.28 v 1.65 +/- 0.15 mg/kg X min).(ABSTRACT TRUNCATED AT 250 WORDS)

Proinsulin radioimmunoassay in the evaluation of insulinomas and familial hyperproinsulinemia

Two new radioimmunoassays for human proinsulin (hPI) have been developed and used to study patients with islet cell tumors and familial hyperproinsulinemia. Both antisera were adsorbed against human C-peptide conjugated to Sepharose, following which cross-reactivity to insulin and C-peptide was less than 0.001%. Antiserum 18D recognized the junction between the insulin B-chain and C-peptide and provided fivefold greater sensitivity than our previously reported hPI assay. Antiserum 11E recognized a determinant which includes or is adjacent to the A-chain-C-peptide junction or which is specified by the tertiary structure. In all 20 patients studied with surgically confirmed islet cell tumors, fasting plasma proinsulinlike material (PLM) was abnormal (greater than 3 SD from the mean measured in either lean or obese subjects) in both assays. This provided better discrimination than has been reported for PLM measured by gel filtration (abnormal in 13 of 14 of the present samples) with a considerably less laborious procedure. Samples from two families in which a mutant proinsulin is present in the circulation have immunoreactivity in the two assays consistent with previous identification of the molecule as an A-chain-C-peptide-linked intermediate of proinsulin conversion. The immunoreactivity of a sample from another family in which large amounts of proinsulin circulate are consistent with an intact molecule being the predominant form. This assay will be useful for confirming the diagnosis of insulin-secreting tumor in patients suspected of recurrent fasting hypoglycemia and in physiologic studies of proinsulin secretion.

Human biochemical genetics of enzyme proteins in the new age of molecular genetics

Advances in protein biochemistry and immunology have had a major impact on the biochemical and genetical analysis of human proteins and have had applications in the analysis of the primary defects in metabolic disorders, as well as in cDNA cloning. The development and expansion of somatic cell genetic techniques has complemented conventional population and family study genetic methods. A large number of mammalian proteins undergo complex processing to achieve the synthesis of the biologically active protein. Much of this processing is under genetic control. Elucidation of these complexities requires a combination of biochemical, immunological and genetical approaches to determine the nature of the events involved.

Posttranslational cleavage of proinsulin is blocked by a point mutation in familial hyperproinsulinemia

Familial hyperproinsulinemia is characterized by the accumulation of proinsulin-like material (PLM) in the plasma of affected patients. This disorder is inherited in an autosomal dominant fashion. The accumulation of PLM is thought to be due to the impaired conversion of proinsulin to insulin. Although PLM has been suggested to have an amino acid substitution, it has been impossible to locate and identify a substituted amino acid, due to the difficulty in isolating sufficient amounts of PLM from plasma samples. Therefore, we analyzed leukocyte DNA from one member of a proinsulinemic family, and we found a point mutation that changed guanine to adenine in the insulin gene. This transition implies that a substitution of histidine for arginine has occurred at amino acid position 65. Furthermore, it indicates that arginine at 65 is essential for the conversion of proinsulin to insulin. Our results suggest a novel mechanism by which disease can be incurred: a heritable disorder can result from a posttranslational processing abnormality caused by a point mutation.

Pathophysiology, diagnosis, and management of glucose homeostasis in the neonate

The neonate appears to be in a transitional stage of glucose homeostasis. Maturation of neonatal glucose homeostasis requires coordination of opposing hormonal, neural, and enzymatic controls. The vulnerability of the neonate to carbohydrate disequilibrium has been described by tracing the maturation of carbohydrate homeostasis physiologically. The many examples of neonatal hypoglycemia and hyperglycemia have been enumerated. Much information in recent years has increased our understanding of the mechanism of these conditions in the newborn. Continued research of the biochemical and physiologic bases for alterations of carbohydrate metabolism should further enhance our ability to diagnose and treat the neonate effectively.