Insulinomas and expression of an insulin splice variant

A Rare Case of Insulinoma in a Thin, Lean Adult Male: A Case Report

Insulinoma is an insulin-secreting tumor that causes hypoglycemia due to inappropriately high insulin secretion. The Whipple's triad, which comprises indications of hypoglycemia (tremor, sweating, irritability, uneasiness, and weakness), plasma glucose concentration <55 mg/dL (3.0 mmol/L), and resolution of symptoms after administration of glucose, is utilized for the determination of insulinoma. In this report, we present the case of a thin, lean, adult male with a BMI of 22, who presented with repetitive episodes of tremor, sweating, weariness, and perplexity that occurred amid fasting and settled with meals, fulfilling Whipple's triad criteria for the determination of insulinoma. The episodes frequently led to seizures. Supervised fasting was carried out, which revealed raised C-peptide levels, low blood glucose, and negative sulfonylurea screen. A computed tomography (CT) scan localized the tumor, and surgical resection was planned.

Phaeochromocytomas overexpress insulin transcript and produce insulin

INTRODUCTION

Phaeochromocytomas are tumours originating in the medulla of the adrenal gland. They produce catecholamines, and some tumours also produce ectopic hormones. Two types of glucose imbalances occur in phaeochromocytoma patients, hyperglycaemia and hypoglycaemic attacks. Therefore, we tested whether insulin transcript (INS), insulin, and a hybrid read-through transcript between exons from insulin and insulin-like growth factor 2 (INS-IGF2) were expressed in phaeochromocytomas.

METHODS

We measured the expression of insulin using immunohistochemistry. The expression of INS-IGF2 was determined by qRT-PCR in formalin-fixed and paraffin-embedded tissue from 20 phaeochromocytomas. The expression of INS and INS-IGF2 transcriptswas also analysed in 182 phaeochromocytomas and paragangliomas using publicly available datasets in The Cancer Genome Atlas (TCGA) Database.

RESULTS

Of 20 phaeochromocytomas, 16 stained positive for insulin. The distribution of positive cells was mostly scattered, with some focal expression indicating clonal expansion. Nineteen tumours expressed high levels of INS and INS-IGF2 transcripts. The expression of the two transcripts corresponded closely. In the TCGA dataset, phaeochromocytoma expresses higher levels of INS and INS-IGF2 transcripts compared to the normal non-tumour adrenal glands. Thus, the expression of INS and INS-IGF2 seems to be a general phenomenon in phaeochromocytoma.

CONCLUSION

Most phaeochromocytomas contain cells that overexpress INS and INS-IGF2 transcripts. Most tumours also display heterogeneous expression of polypeptides immunoreactive to monoclonal anti-insulin antibodies. Clinically this may relate to both hyperglycaemia and hypoglycaemic attacks seen in patients with phaeochromocytoma as well as autocrine tumour growth.

Peptide Hormones as Tumor Markers in Clinical Practice

Peptide hormones represent a major class of hormones that are made from amino acids by specialized endocrine glands. The maturation of bioactive hormones take place in the rough endoplasmic reticulum and Golgi apparatus, where preprohormones are proteolytically cleaved into prohormones, and subsequently into mature peptide hormones. Once the bioactive hormones are released into the circulation, they interact with receptors located on the plasma membrane of target cells, and initiate intracellular signaling pathways to regulate physiological processes including energy metabolism, growth, stress, and reproduction. However, excessive amount of circulating peptide hormones often associates with the presence of tumors. Section 2 discusses 10 peptide hormones as tumor markers and their clinical application in aiding the diagnosis of tumors as well as monitoring the disease process.

Upregulation of INS-IGF2 read-through expression and identification of a novel INS-IGF2 splice variant in insulinomas

Fusion transcripts arising from the combination of exons residing on neighboring genes on the same chromosome may give rise to chimeric or novel proteins. Such read-through transcripts have been detected in different cancers where they may be of pathogenetic interest. In this study, we describe for the first time the expression of a read-through transcript in insulinomas, a functioning neuroendocrine pancreatic neoplasm. The read-through transcript INS-IGF2, composed of exons from the two genes proinsulin precursor (INS) and insulin‑like growth factor 2 (IGF2), both mapping to chromosomal subband 11p15.5, was highly expressed in the two insulinomas analyzed. More precisely, version 2 of the INS-IGF2 transcript was expressed, indicating possible expression of the chimeric INS-IGF2 protein. We further identified a novel splice variant of the INS-IGF2 read-through transcript in one of the insulinomas, composed of exon 1 of INS3 and exons of IGF2. In the same tumor, we found high expression of INS3 and the presence of the A allele at SNP rs689. SNP rs689 has been previously described to regulate splicing of the INS transcript, indicating that this regulatory mechanism also affects splicing of INS-IGF2. The identification of the INS-IGF2 read-through transcript specifically in tumor tissue but not in normal pancreatic tissue suggests that high expression of INS-IGF2 could be neoplasia‑specific. These results may have potential clinical applications given that the read-through transcript could be used as a biomarker in insulinoma patients.

Mass spectrometric immunoassay of intact insulin and related variants for population proteomics studies

PURPOSE

The purpose of the work presented herein was to develop a high-throughput assay for the quantification of human insulin in plasma samples while simultaneously detecting, with high mass accuracy, any additional variant forms of insulin that might be present in each sample.

EXPERIMENTAL DESIGN

A mass spectrometric immunoassay (MSIA) was designed in which anti-human insulin antibodies were immobilized to commercially available mass spectrometric immunoassay pipette tips and used to capture insulin and related protein variants from human plasma.

RESULTS

Standard curves for insulin exhibited linearity (average R(2) for three days of analysis=0.99) and assay concentration limits of detection and limits of quantification for insulin were found to be 1 and 15 pM, respectively. Estimated coefficient of variations for inter-day experiments (n=3 days) were <8%. Simultaneously, the assay was shown to detect and identify insulin metabolites and synthetic insulin analogs (e.g. Lantus). Notably, insulin variants not known to exist in plasma were detected in diabetics.

CONCLUSIONS AND CLINICAL RELEVANCE

This introductory study sets a foundation toward the screening of large populations to investigate insulin isoforms, isoform frequencies, and their quantification.

Insulin secretion and insulin-producing tumors

Insulinomas are rare neuroendocrine tumors of pancreatic islet cells that retain the ability to produce and secrete insulin. In contrast to normally differentiated β-cells, insulinoma cells continue to secrete insulin and proinsulin at low blood glucose. This deregulated insulin secretion manifests clinically as fasting hypoglycemia. The molecular pathways that characterize normal insulin secretion and β-cell growth are reviewed and contrasted to the biology of insulinomas. The second half of this review summarizes the clinical approach to the disorder. The diagnosis of insulinoma is established by demonstrating inappropriately high insulin levels with coincident hypoglycemia at the time of a supervised fast. Localization of insulinomas is challenging owing to their small size but should be attempted to maximize the chance for successful surgical resection and avoid risks associated with reoperation. In the majority of cases, successful surgical resection leads to lifelong cure.

Comprehensive gene expression analysis of 5'-end of mRNA identified novel intronic transcripts associated with hepatocellular carcinoma

To elucidate the molecular feature of human hepatocellular carcinoma (HCC), we performed 5'-end serial analysis of gene expression (5'SAGE), which allows genome-wide identification of transcription start sites in addition to quantification of mRNA transcripts. Three 5'SAGE libraries were generated from normal human liver (NL), non-B, non-C HCC tumor (T), and background non-tumor tissues (NT). We obtained 226,834 tags from these libraries and mapped them to the genomic sequences of a total of 8,410 genes using RefSeq database. We identified several novel transcripts specifically expressed in HCC including those mapped to the intronic regions. Among them, we confirmed the transcripts initiated from the introns of a gene encoding acyl-coenzyme A oxidase 2 (ACOX2). The expression of these transcript variants were up-regulated in HCC and showed a different pattern compared with that of ordinary ACOX2 mRNA. The present results indicate that the transcription initiation of a subset of genes may be distinctively altered in HCC, which may suggest the utility of intronic RNAs as surrogate tumor markers.

The main product of specialized tissues regulates cell life and may cause neoplastic transformation

Many tissues and cells in vertebrates are highly specialized and devoted to a single function through the action of a single molecule, that we call the "main product" (MP) of the cell. The hypothesis here proposed is that these MPs control all aspects of the cell life, namely activity, division, differentiation and apoptosis. Evidences supporting this hypothesis are reported for the immune system, pancreatic beta-cells, melanocytes, connective tissues, thyroid cells, skin and erythroid cells. In all cases cell division and differentiation is promoted by a normal activity of the MP, while hyperactivity leads to cell apoptosis. Evidences are also provided that alterations of the activity of the MP may elicit pathological disorders; in particular mutations altering the structure of the MP may elicit tumoural transformation.

Generation and characterization of human insulin-releasing cell lines

Background

The in vitro culture of insulinomas provides an attractive tool to study cell proliferation and insulin synthesis and secretion. However, only a few human beta cell lines have been described, with long-term passage resulting in loss of insulin secretion. Therefore, we set out to establish and characterize human insulin-releasing cell lines.

Results

We generated ex-vivo primary cultures from two independent human insulinomas and from a human nesidioblastosis, all of which were cultured up to passage number 20. All cell lines secreted human insulin and C-peptide. These cell lines expressed neuroendocrine and islets markers, confirming the expression profile found in the biopsies. Although all beta cell lineages survived an anchorage independent culture, none of them were able to invade an extracellular matrix substrate.

Conclusion

We have established three human insulin-releasing cell lines which maintain antigenic characteristics and insulin secretion profiles of the original tumors. These cell lines represent valuable tools for the study of molecular events underlying beta cell function and dysfunction.

Evaluation of polymorphic splicing in the mechanism of the association of the insulin gene with diabetes

The association of type 1 diabetes with the insulin gene (IDDM2 locus) has been mapped to a short haplotype encompassing two single nucleotide polymorphisms (SNPs) in perfect linkage disequilibrium (r(2) = 1) with each other and with the two allele classes at the variable number of tandem repeats (VNTR) polymorphism upstream of the transcription site. Although it is believed that the genetic effect is mediated through transcriptional effects of the VNTR, an alternative mechanism has been recently proposed: In transfected cells, the common A allele at one of the SNPs (-23A-->T, in relation to the translation-initiation codon) weakens the splicing of intron 1, resulting in a minor ( approximately 15% of total RNA) transcript with a longer 5' untranslated region and sixfold enhanced translational efficiency. The purpose of our study was to confirm these findings in RNA from normal human pancreas and thymus. We report that pancreas does contain the alternative transcript in an allele-dependent manner but at a very low proportion (<5% of total INS mRNA). We believe that this level would have a minor, if any, biological effect involved in the mechanism of the IDDM2 locus.

Complex splicing control of the human Thrombopoietin gene by intronic G runs

The human thrombopoietin (THPO) gene displays a series of alternative splicing events that provide valuable models for studying splicing mechanisms. The THPO region spanning exon 1–4 presents both alternative splicing of exon 2 and partial intron 2 (IVS2) retention following the activation of a cryptic 3′ splice site 85 nt upstream of the authentic acceptor site. IVS2 is particularly rich in stretches of 3–5 guanosines (namely, G1–G10) and we have characterized the role of these elements in the processing of this intron. In vivo studies show that runs G7–G10 work in a combinatorial way to control the selection of the proper 3′ splice site. In particular, the G7 element behaves as the splicing hub of intron 2 and its interaction with hnRNP H1 is critical for the splicing process. Removal of hnRNP H1 by RNA interference promoted the usage of the cryptic 3′ splice site so providing functional evidence that this factor is involved in the selection of the authentic 3′ splice site of THPO IVS2.

The insulin gene in type 1 diabetes

Insulin is a key autoantigen in the autoimmune process leading to the development of type 1 diabetes. Recent studies in both humans and mice have shown that variation in the expression of the insulin gene, in the thymus rather than the pancreas, contributes to disease susceptibility by affecting self-tolerance to insulin. These findings have brought about a paradigm-shift in our understanding of self-tolerance and autoimmunity to molecules with tissue-restricted expression, which are often the target of autoimmune disease.

Long-term in vitro growth of human insulin-secreting insulinoma cells

OBJECTIVE

Long-term in vitro maintenance of human insulin-secreting insulinoma cells.

METHODS

(1) Cell culture of ex vivo-derived insulinoma cell suspensions from 8 individual human donors, using various cell culture medium supplementations; (2) determination of insulin synthesis and secretion using immunocytochemistry and insulin and pro-insulin radioimmunoassays; (3) nestin-immunostaining of long-term in vitro grown insulinoma cell suspensions, and (4) phase-contrast light microscopy for analyzing the in vitro growth characteristics of the insulinoma cells.

RESULTS

(1) Parallel persistence of in vitro insulinoma cell proliferation as well as insulin-synthesizing and -secreting capacity depended on both the co-culture of insulinoma cells with human fibroblasts and the supplementation of the cell culture medium with tissue culture supernatant derived from the rodent pituitary adenoma cell line GH-3; (2) immunostaining for insulin and secretagogin confirmed the neuroendocrine origin of the insulinoma cells grown in vitro; (3) insulin secretion capability persisted up to an observation period of 25 weeks; (4) insulin secretion rates after 6 weeks of in vitro growth ranged from 3.5 to 83.3 muU/ml/h/60,000 cells plated, and (5) after long-term in vitro growth of insulinoma-derived cell suspensions with persistent insulin-secreting capacity, nestin staining was observed predominantly in co-cultured fibroblasts.

CONCLUSION

Our data describe for the first time the long-term in vitro culture of insulin-secreting human insulinomas and highlight the importance of beta-cell trophic factors for insulinoma cell growth.

Variants in the human insulin gene that affect pre-mRNA splicing: is -23HphI a functional single nucleotide polymorphism at IDDM2?

Predisposition to type 1 diabetes and juvenile obesity is influenced by the susceptibility locus IDDM2 that includes the insulin gene (INS). Although the risk conferred by IDDM2 has been attributed to a minisatellite upstream of INS, intragenic variants have not been ruled out. We examined whether INS polymorphisms affect pre-mRNA splicing and proinsulin secretion using minigene reporter assays. We show that IVS1-6A/T (-23HphI+/-) is a key INS variant that influences alternative splicing of intron 1 through differential recognition of its 3' splice site. The A allele resulted in an increased production of mature transcripts with a long 5' leader in several cell lines, and the extended mRNAs generated more proinsulin in culture supernatants than natural transcripts. The longer mRNAs were significantly overrepresented among beta-cell-expressed sequenced tags containing the A allele as compared with those with T alleles. In addition, we show that a rare insertion/deletion polymorphism IVS1+5insTTGC (IVS-69), which is exclusively present in Africans, activated a downstream cryptic 5' splice site, extending the 5' leader by 30 bp. These results indicate that -23HphI and IVS-69 are the most important INS variants affecting pre-mRNA splicing and suggest that -23HphI+/- is a common functional single nucleotide polymorphism at IDDM2.

Extensive variation in the 5′-UTR of Dicer mRNAs influences translational efficiency

The Dicer enzyme is a key component of the RNA interference pathway and also responsible for the processing of micro RNAs, non-coding RNA molecules which regulate the activity of mRNAs by antisense base pairing. Little is known about the structure and regulation of human Dicer mRNA. A comprehensive characterization of Dicer 5'-untranslated region (5'-UTR) RNA structure revealed important diversity within human Dicer mRNA transcripts. Three exon 1 variants were defined, some of which exhibited very restricted patterns of tissue distribution. A number of alternatively spliced 5'-leader exons were also noted, revealing the potential for complex post-transcriptional regulation. Surprisingly, this diversity all occurred within the 5'-UTR of Dicer mRNAs and did not affect the coding region. The Dicer mRNA 5'-UTR variants had profound effects on translational efficiency both in vitro and in transiently transfected cells. A number of major Dicer RNA species are inefficient substrates for the translational machinery.