On the biosynthesis, intracellular transport and mechanism of conversion of proinsulin to insulin and C-peptide

Amino acid polymorphisms in human histocompatibility leukocyte antigen class II and proinsulin epitope have impacts on type 1 diabetes mellitus induced by immune-checkpoint inhibitors

Introduction

Immune-checkpoint inhibitors are effective in various advanced cancers. Type 1 diabetes mellitus induced by them (ICI-T1DM) is a serious complication requiring prompt insulin treatment, but the immunological mechanism behind it is unclear.

Methods

We examined amino acid polymorphisms in human histocompatibility leukocyte antigen (HLA) molecules and investigated proinsulin epitope binding affinities to HLA molecules.

Results and Discussion

Twelve patients with ICI-T1DM and 35 patients in a control group without ICI-T1DM were enrolled in the study. Allele and haplotype frequencies of HLA-DRB1*04:05, DQB1*04:01, and most importantly DPB1*05:01 were significantly increased in patients with ICI-T1DM. In addition, novel amino acid polymorphisms in HLA-DR (4 polymorphisms), in DQ (12 polymorphisms), and in DP molecules (9 polymorphisms) were identified. These amino acid polymorphisms might be associated with the development of ICI-T1DM. Moreover, novel human proinsulin epitope clusters in insulin A and B chains were discovered in silico and in vitro peptide binding assays to HLA-DP5. In conclusion, significant amino acid polymorphisms in HLA-class II molecules, and conformational alterations in the peptide-binding groove of the HLA-DP molecules were considered likely to influence the immunogenicity of proinsulin epitopes in ICI-T1DM. These amino acid polymorphisms and HLA-DP5 may be predictive genetic factors for ICI-T1DM.

Biophysical alteration of the secretory track in β-cells due to molecular overcrowding: the relevance for diabetes

Recent data demonstrate that accumulation of misfolded proteins within the early part of the secretory track of β-cells causes impaired insulin synthesis and development of diabetes. The molecular mechanism of this cellular dysfunction remains largely unknown. Using basic molecular principles and computer simulations, we suggested recently that hyperglycemic conditions can generate substantial molecular crowding effects in the secretory track of β-cells leading to significant alterations of the insulin biosynthesis capabilities. Here, we review the major molecular mechanisms that may be implicated in the alteration of insulin synthesis in susceptible β-cells. Steric repulsions and volume exclusion in the endoplasmic reticulum (ER) increase the propensity of misfolding of proinsulin (the precursor molecule of insulin). In addition, similar forces might act in the next secretory compartments (Golgi and vesicles) leading to (i) altered packaging of proinsulin in vesicles (ii) entrapment of proinsulin convertases and/or restricted accessibility for these convertases to the cleavage sites on the surface of the proinsulin and (iii) depressed kinetic rate of the transformation of the native proinsulin in active insulin and C-peptide. These concepts are expressed in simple mathematical terms relating the kinetic coefficient of proinsulin to insulin conversion to the levels of proinsulin misfolding and hyperglycemic stress. The present approach is useful for understanding molecular phenomena associated with the pathogenesis of diabetes. It also offers practical means for predicting the state of pancreatic β-cells from measurements of the insulin to proinsulin ratio in the blood. This is of immediate clinical relevance and may improve the diagnosis of diabetes.

β-Cell dysfunction under hyperglycemic stress: a molecular model

BACKGROUND

Pancreatic β cells respond to chronic hyperglycemia by increasing the synthesis of proinsulin (the precursor molecule of insulin). Prolonged stimulations lead to accumulation of misfolded proinsulin in the secretory track, delayed insulin secretion, and release of unprocessed proinsulin in the blood. The molecular mechanisms connecting the state of endoplasmic reticulum overloading with the efficiency of proinsulin to insulin conversion remain largely unknown.

METHODS

Computer simulations can help us to understand mechanistic features of the β-cell secretory defect and to design experiments that may reveal the molecular basis of this dysfunction. We used molecular crowding concepts and statistical thermodynamics to dissect possible biophysical mechanisms underlying the alteration of the secretory track of β cells and to elucidate the chemistry aspects of the secretory dysfunction. We then used numerical algorithms to relate the degree of biophysical alteration of these secretory compartments with the change of proinsulin to insulin conversion rate.

RESULTS

Our computer simulations suggest that overloading the endoplasmic reticulum initiates downstream molecular crowding effects that affect protein translational mechanisms, including proinsulin misfolding, delayed packing of proinsulin in secretory vesicles, and low kinetic coefficient of proinsulin to insulin conversion.

CONCLUSIONS

Together with previous experimental data, the present study can help us to better understand chemistry aspects related to the secondary translational mechanisms in β cells and how hyperglycemic stress can alter secretory function. This can give a further impetus to the development of novel software to be used in a clinical setup for prediction and assessment of diabetic states in susceptible patients.

Specific cell surface binding sites shared by human Pro-IGF-I Eb-peptides and rainbow trout Pro-IGF-I Ea-4-peptide

Human pro-IGF-I Eb-peptide (hEb) and rainbow trout pro-IGF-I Ea-4-peptide (rtEa-4) have recently been shown to share unique biological activities [Gen. Comp. Endocrinol. 126 (2002) 342; Cell. Exp. Cell. Res. 280 (2002) 75]. To further understand the action mechanism of these proteins, we studied the binding properties of hEb-peptide and rtEa-4-peptide to intact human neuroblastoma cells (SK-N-F1) and membrane preparations. Human Eb-peptide and rtEa-4-peptide bind to a high-affinity binding site with an apparent dissociation constant of 3.2+/-1.9 x 10(-11) and 2.9+/-1.8 x 10(-11)M, respectively. Homologous displacement assay demonstrated the presence of a second binding site with an IC(50) of 4.8+/-2.6 x 10(-6)M for hEb-peptide and 2.1+/-0.6 x 10(-6)M for rtEa-4-peptide, respectively. Competition assays showed that hEb-peptide and rtEa-4-peptide shared common binding sites, distinct from those for IGF-I and insulin. In addition, chemical cross-linking studies revealed two specific binding complexes. Our findings support the notion that the initial step of pro-IGF-I E-peptide action is mediated through the interaction with conserved and specific putative membrane receptors on neuroblastoma cells.

Effect of chloroquine on biosynthesis, release and degradation of insulin in isolated islets of rat pancreas

Insulin has been reported to degrade inside the islets and islet lysosomal proteases have been thought to take part. As chloroquine is regarded as a potent lysosomotropic agent, an attempt has been made to see whether chloroquine has an influence on intrainsular degradation of insulin. After preculture of collagenase-isolated rat islets at 11 mM glucose together with [3H]leucine for 3 days for labelling newly synthesized insulin, islets were cultured for 1 day at 2.2 or 22 mM glucose with or without 0.02 mM chloroquine. Afterwards, radioactivity was measured in the proinsulin/insulin fraction. For control, the influence of chloroquine during 3-h incubation of both freshly isolated and precultured islets was also studied. During the 1-day culture at 2.2 mM glucose, prelabelled insulin was degraded significantly and addition of chloroquine did not alter the amount of insulin degraded. At 22 mM glucose, no significant amount of insulin had been degraded. During the 3-h incubation of freshly isolated as well as precultured islets, chloroquine was found to inhibit significantly glucose-induced biosynthesis of insulin. Glucose-induced release of insulin, however, was not influenced by chloroquine. It is concluded that chloroquine does not influence glucose-induced release or intra-insular degradation of insulin, but it interferes with the biosynthesis of insulin.

Calcium calmodulin and hormone secretion

As long ago as 1970, it was proposed that Ca2+ can act as a 'second messenger' like cAMP (Rasmussen & Nagata, 1979). The recognition that calmodulin is a major Ca2+ binding protein in non-muscle cells has prompted the suggestion that calmodulin may serve an analogous role for Ca2+ to that served by protein kinase for cAMP (Wang & Waisman, 1979), or at least to the regulatory subunit of the cyclic nucleotide-dependent kinases. It is becoming clear that calmodulin probably does play a role in stimulus secretion coupling in endocrine cells. Nevertheless, some of the experimental approaches which have led to this rather tentative conclusion do induce some doubts, as we have attempted to indicate. Many of the pharmacological agents used in the studies cited in this review are not specific in their interaction with calmodulin. For example, the phenothiazines also inhibit phospholipid-sensitive protein kinase. The introduction of more specific drugs, such as the naphthalene sulphonamides, may lead to a clearer picture of the role of calmodulin in hormone secretion. Relationships probably exist between cyclic nucleotides, calcium, calmodulin, phosphatidylinositol (PI) turnover and phospholipids in the overall control of the secretory process (see Fig. 1). There is considerable evidence that calcium is the primary internal signal initiating exocytosis of hormone from many glands. However, it appears that cyclic nucleotides can modulate the calcium signal either positively or negatively and it is possible that cAMP and calcium can separately activate secretion. The presence of both calmodulin-activated adenylate cyclase and cyclic nucleotide phosphodiesterase in the same tissue would appear to suggest either spatial or temporal control mechanisms or that (diagram; see text) the calcium requirement for calmodulin activation differs between the two enzymes. The true explanation is probably far more complex and involves perhaps as yet unknown factors that can differentially influence the activity of calmodulin itself in membranes and in cytosol. Berridge (1982) and Rasmussen (1980) give detailed accounts and review current hypotheses regarding relationships between the cyclic nucleotide and calcium second messenger systems. The various possible interrelationships of the putative messengers have been encompassed by the term 'Synarchic regulation' (Rasmussen, 1980). These concepts and the elucidation of the mechanisms by which cyclic AMP and calcium are involved in the control of secretion from particular cell types will make fascinating reading over the next few years.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin biosynthesis and diabetes mellitus

This review reports the use of recombinant DNA techniques in the study of the structure and regulation of expression of insulin genes in man and experimental animals. Insulin biosynthesis by pancreatic islet cells is predominantly regulated by change in plasma glucose concentration. Using a cell-free protein synthesizing system as an assay of functional proinsulin messenger RNA (mRNA), and hybridization analysis with a cloned DNA complementary to proinsulin mRNA, it has been determined that through changes in proinsulin mRNA levels. Insulin genes of the rat, chicken and human have been isolated and sequenced. The 5' ends of the genes have similar sequences suggesting areas important for regulation of transcription. There are two non-allelic insulin genes in the rat, but only one in chickens and humans. Intervening sequences, areas of DNA transcribed into precursor mRNA but which do not appear in mature mRNA, have been described within insulin genes. The insulin gene resides on chromosome 11 of humans as determined by DNA hybridization analysis of mouse human hybrid cells. The structure of the insulin gene in genomic DNA of humans has been analyzed in diabetics and non-diabetics. Insertions of DNA between 1500 and 3400 base pairs have been detected near the transcription initiation site in 65% of type II diabetics, and 25-30% of non-diabetics (this difference is significant at the p less than 0.001 level). Limitation of these insertions to this potential promotor region of the insulin gene suggests that they may alter gene expression in type II diabetes. These insertions of DNA may prove to be useful genetic markers for diabetes.

Endocrine pancreatic control of the release of gastric inhibitory polypeptide. A possible physiological role for C-peptide

Intravenous infusion in anaesthetized rats of rat II C-peptide at a dose which produced circulating levels of 22.8 +/- 1.8 nmol/l after 30 min, resulted in a significant reduction (141 +/- 7 to 50 +/- 4 pmol/l, p < 0.001, mean +/- SEM) in the immunoreactive gastric inhibitory polypeptide response to an intestinal perfusion with a fat emulsion. Immunoreactive insulin levels were unchanged from basal in this study. It is suggested that C-peptide must be considered as a candidate for the endocrine pancreatic factor which exerts a negative feedback upon gastric inhibitory polypeptide release.

Inhibition by kynurenine metabolites of proinsulin synthesis in isolated pancreatic islets

The effect of kynurenine metabolites on insulin biosynthesis was investigated in isolated pancreatic islets of the rat. Both quinaldic acid and 8-hydroxyquinaldic acid were found to produce significant inhibition of the proinsulin synthesis. However, the conversion process of proinsulin to insulin in the islet was not affected by these kynurenine metabolites. Furthermore, the inhibitory effect of these end-metabolites of dynurenine was characterized by preferential inhibition of proinsulin synthesis as distinct from non-insulin protein synthesis in the islet. In contrast to the significant inhibitory effect of quinaldic acid and 8-hydroxyquinaldic acid on proinsulin synthesis, xanthurenic acid and kynurenic acid were far less effective, and L-tryptophan, L-kynurenine, 3-hydroxyanthranilic acid and quinolinic acid showed little ability to inhibit proinsulin synthesis in islets.

In vitro conversion of proinsulin to insulin by cathepsin B and role of C-peptide

Cathepsin B, purified from isolated islets of Langerhans, when incubated with proinsulin under in vitro conditions could convert proinsulin to insulin and C-peptide, releasing free arginine and lysine. When C-peptide, prepared from rat pancreas, was added to the incubation system consisting of proinsulin and cathepsin B, it completely inhibited the conversion of proinsulin to insulin.

Endocrine pancreatic tumors

The pathology and cell biology of endocrine pancreatic tumors are reviewed. It is probable that all these tumors are "functioning" in the sense that they elaborate hormones that cause more or less conspicuous clinical syndromes. Identification of such secretory products is essential for an optimal diagnosis, localization, treatment, and follow-up. Recent data indicate that endocrine pancreatic tumors evolve from progenitor cells of ducts. This histogenetic mechanism may explain the occurrence not only of mixed or multihormonal tumors but also of tumors producing hormones that are absent from the adult human pancreas. In addition to their clinically apparent effects, many endocrine pancreatic tumors affect the surrounding endocrine pancreas in a characteristic way. The mechanisms behind and the potential diagnostic usefulness of these changes are discussed.