Antonio Brunetti

Insulin Resistance and Cancer Risk: An Overview of the Pathogenetic Mechanisms

Insulin resistance is common in individuals with obesity or type 2 diabetes (T2D), in which circulating insulin levels are frequently increased. Recent epidemiological and clinical evidence points to a link between insulin resistance and cancer. The mechanisms for this association are unknown, but hyperinsulinaemia (a hallmark of insulin resistance) and the increase in bioavailable insulin-like growth factor I (IGF-I) appear to have a role in tumor initiation and progression in insulin-resistant patients. Insulin and IGF-I inhibit the hepatic synthesis of sex-hormone binding globulin (SHBG), whereas both hormones stimulate the ovarian synthesis of sex steroids, whose effects, in breast epithelium and endometrium, can promote cellular proliferation and inhibit apoptosis. Furthermore, an increased risk of cancer among insulin-resistant patients can be due to overproduction of reactive oxygen species (ROS) that can damage DNA contributing to mutagenesis and carcinogenesis. On the other hand, it is possible that the abundance of inflammatory cells in adipose tissue of obese and diabetic patients may promote systemic inflammation which can result in a protumorigenic environment. Here, we summarize recent progress on insulin resistance and cancer, focusing on various implicated mechanisms that have been described recently, and discuss how these mechanisms may contribute to cancer initiation and progression.

A nucleoprotein complex containing Sp1, C/EBP beta, and HMGI-Y controls human insulin receptor gene transcription.

ABSTRACT HMGI-Y is an architectural transcription factor that regulates gene expression in vivo by controlling the formation of stereospecific multiprotein complexes on the AT-rich regions of certain gene promoters. Recently, we demonstrated that HMGI-Y is required for proper transcription of the insulin receptor (IR) gene. Here we provide evidence that transcriptional activation of the human IR promoter requires the assembly of a transcriptionally active multiprotein-DNA complex which includes, in addition to HMGI-Y, the ubiquitously expressed transcription factor Sp1 and the CCAAT-enhancer binding protein β (C/EBPβ). Functional integrity of this nucleoprotein complex is required for full transactivation of the IR gene by Sp1 and C/EBPβ in cells readily expressing IRs. We show that HMGI-Y physically interacts with Sp1 and C/EBPβ and facilitates the binding of both factors to the IR promoter in vitro. Furthermore, HMGI-Y is needed for transcriptional synergism between these factors in vivo. Repression of HMGI-Y function adversely affects both Sp1- and C/EBPβ-induced transactivation of the IR promoter. Together, these findings demonstrate that HMGI-Y plays significant molecular roles in the transcriptional activities of these factors in the context of the IR gene and provide concordant support for the hypothesis that, in affected individuals, a putative defect in these nuclear proteins may cause decreased IR expression with subsequent impairment of insulin signaling and action.

Increased expression of AP2 and Sp1 transcription factors in human thyroid tumors: a role in NIS expression regulation?

BackgroundSodium/iodide symporter (NIS) is a key protein in iodide transport by thyroid cells and this activity is a prerequisite for effective radioiodide treatment of thyroid cancer. In the majority of thyroid cancers, however, iodide uptake is reduced, probably as a result of decreased NIS protein expression.MethodsTo identify the mechanisms that negatively affect NIS expression in thyroid tumors, we performed electrophoresis mobility shift assays and immunoblot analysis of nuclear protein extracts from normal and tumoral thyroid tissues from 14 unrelated patients.ResultsTwo proteins closely related to the transcription factors AP2 and Sp1 were identified in the nuclear extracts. Expression of both AP2 and Sp1 in nuclear extracts from thyroid tumors was significantly higher than that observed in corresponding normal tissues.ConclusionThese observations raise the possibility that NIS expression, and subsequently iodide transport, are reduced in thyroid tumors at least in part owing to alterations in the binding activity of AP2 and Sp1 transcription factors to NIS promoter.

Transcriptional regulation of human insulin receptor gene by the high-mobility group protein HMGI(Y)

We have previously identified two closely related nuclear binding proteins that specifically interact with two unique functional AT‐rich sequences of the 5′ regulatory region of the human insulin receptor gene. Expression of these nuclear binding proteins increases during myocyte and adipocyte differentiation, and in other tissues appears to correlate with insulin receptor content. We have hypothesized, therefore, that insulin receptor expression in the insulin target tissues is regulated at least in part by these nuclear proteins. Here we show data on purification and biochemical characterization of these DNA binding proteins. Using a conventional chromatographic purification procedure combined with electrophoresis mobility shift assay and immunoblot analyses, a unique ~15 kDa protein, either identical to or highly related to the architectural transcription factor HMGI(Y), has now been identified, suggesting an essential role for HMGI(Y) in regulating insulin receptor gene transcription. Direct evidence of HMGI(Y) insulin receptor promoter interactions is provided by functional analysis with the CAT reporter gene and by hormone binding studies in cells expressing HMGI(Y) antisense RNA. In these experiments, antisense HMGI(Y) specifically inhibits insulin receptor promoter function and insulin receptor protein expression, indicating that HMGI(Y) is required for proper transcription of insulin receptor gene. Moreover, our data consistently support the hypothesis that a putative defect in this nuclear binding protein may cause insulin receptor dysfunction with subsequent impairment of insulin signaling and action.—Brunetti, A., Manfioletti, G., Chiefari, E., Goldfine, I. D., Foti, D. Transcriptional regulation of human insulin receptor gene by the high‐mobility group protein HMGI(Y). FASEB J. 15, 492‐500 (2001)

Pseudogene-mediated posttranscriptional silencing of HMGA1 can result in insulin resistance and type 2 diabetes

Processed pseudogenes are non-functional copies of normal genes that arise by a process of mRNA retrotransposition. The human genome contains thousands of pseudogenes; however, knowledge regarding their biological role is limited. Previously, we demonstrated that high mobility group A1 (HMGA1) protein regulates the insulin receptor (INSR) gene and that two diabetic patients demonstrated a marked destabilization of HMGA1 mRNA. In this paper we report that this destabilization of HMGA1 mRNA is triggered by enhanced expression of RNA from an HMGA1 pseudogene, HMGA1-p. Targeted knockdown of HMGA1-p mRNA in patient cells results in a reciprocal increase in HMGA1 mRNA stability and expression levels with a parallel correction in cell-surface INSR expression and insulin binding. These data provide evidence for a regulatory role of an expressed pseudogene in humans and establishes a novel mechanistic linkage between pseudogene HMGA1-p expression and type 2 diabetes mellitus.

Increased O-glycosylation of insulin signaling proteins results in their impaired activation and enhanced susceptibility to apoptosis in pancreatic beta-cells

Because adverse effects of glucose were attributed to its increased routing through the hexosamine pathway (HBP), we inquired whether HBP activation affects pancreatic β‐cell survival. Exposure of human islets to high glucose resulted in increased apoptosis of β‐cells upon serum deprivation that was reversed by azaserine. Also, glucosamine, a direct precursor of the downstream product of the HBP, increased human β‐cells apoptosis upon serum deprivation, which was reversed by benzyl‐2‐acetamido‐2‐deoxy‐α‐D‐galactopyranoside (BADGP), an inhibitor of protein O‐glycosylation. These results were reproduced in RIN rat β‐cells. Glucosamine treatment resulted in inhibition of tyrosine‐phosphorylation of the insulin receptor (IR), IRS‐1, and IRS‐2, which was associated with increased O‐glycosylation. These changes caused impaired activation of the PI 3‐kinase/Akt survival signaling that resulted in reduced GSK‐3 and FOXO1a inactivation. BADGP reversed the glucosamine‐induced reduction in insulin‐stimulated phosphorylation of IR, IRS‐1, IRS‐2, Akt, GSK‐3, and FOXO1a. Impaired FOXO1a inactivation sustained expression of the pro‐apoptotic protein Bim, without affecting Bad, Bcl‐XL, or Bcl‐2 expression. These results indicate that hyperglycemia may increase susceptibility to apoptosis of human and rat β‐cell through activation of the HBP. Increased routing of glucose through this metabolic pathway results in impaired activation of the IR/IRSs/PI3‐kinase/Akt survival pathway by induction of O‐glycosylation of signaling molecules.

Functional Variants of the HMGA1 Gene and Type 2 Diabetes Mellitus

CONTEXT High-mobility group A1 (HMGA1) protein is a key regulator of insulin receptor (INSR) gene expression. We previously identified a functional HMGA1 gene variant in 2 insulin-resistant patients with decreased INSR expression and type 2 diabetes mellitus (DM). OBJECTIVE To examine the association of HMGA1 gene variants with type 2 DM. DESIGN, SETTINGS, AND PARTICIPANTS Case-control study that analyzed the HMGA1 gene in patients with type 2 DM and controls from 3 populations of white European ancestry. Italian patients with type 2 DM (n = 3278) and 2 groups of controls (n = 3328) were attending the University of Catanzaro outpatient clinics and other health care sites in Calabria, Italy, during 2003-2009; US patients with type 2 DM (n = 970) were recruited in Northern California clinics between 1994 and 2005 and controls (n = 958) were senior athletes without DM collected in 2004 and 2009; and French patients with type 2 DM (n = 354) and healthy controls (n = 50) were enrolled at the University of Reims in 1992. Genomic DNA was either directly sequenced or analyzed for specific HMGA1 mutations. Messenger RNA and protein expression for HMGA1 and INSR were measured in both peripheral lymphomonocytes and cultured Epstein-Barr virus-transformed lymphoblasts from patients with type 2 DM and controls. MAIN OUTCOME MEASURES The frequency of HMGA1 gene variants among cases and controls. Odds ratios (ORs) for type 2 DM were estimated by logistic regression analysis. RESULTS The most frequent functional HMGA1 variant, IVS5-13insC, was present in 7% to 8% of patients with type 2 DM in all 3 populations. The prevalence of IVS5-13insC variant was higher among patients with type 2 DM than among controls in the Italian population (7.23% vs 0.43% in one control group; OR, 15.77 [95% confidence interval {CI}, 8.57-29.03]; P < .001 and 7.23% vs 3.32% in the other control group; OR, 2.03 [95% CI, 1.51-3.43]; P < .001). In the US population, the prevalence of IVS5-13insC variant was 7.7% among patients with type 2 DM vs 4.7% among controls (OR, 1.64 [95% CI, 1.05-2.57]; P = .03). In the French population, the prevalence of IVS5-13insC variant was 7.6% among patients with type 2 DM and 0% among controls (P = .046). In the Italian population, 3 other functional variants were observed. When all 4 variants were analyzed, HMGA1 defects were present in 9.8% of Italian patients with type 2 DM and 0.6% of controls. In addition to the IVS5 C-insertion, the c.310G>T (p.E104X) variant was found in 14 patients and no controls (Bonferroni-adjusted P = .01); the c.*82G>A variant (rs2780219) was found in 46 patients and 5 controls (Bonferroni-adjusted P < .001); the c.*369del variant was found in 24 patients and no controls (Bonferroni-adjusted P < .001). In circulating monocytes and Epstein-Barr virus-transformed lymphoblasts from patients with type 2 DM and the IVS5-13insC variant, the messenger RNA levels and protein content of both HMGA1 and the INSR were decreased by 40% to 50%, and these defects were corrected by transfection with HMGA1 complementary DNA. CONCLUSIONS Compared with healthy controls, the presence of functional HMGA1 gene variants in individuals of white European ancestry was associated with type 2 DM.

Chronic hyperglycemia impairs insulin secretion by affecting insulin receptor expression, splicing, and signaling in RIN beta cell line and human islets of Langerhans.

Recent evidence suggests that insulin signaling through the insulin receptor A type (Ex11−), regulates insulin gene transcription. Because chronic hyperglycemia negatively affects insulin receptor function and regulates alternative splicing of the insulin receptor, we inquired whether chronic exposure of pancreatic β‐cells to high glucose results in alterations in insulin signaling due to changes in insulin receptor expression and relative abundance of its spliced isoforms. Our results demonstrate that the insulin receptor is localized in insulin secretory vescicles in human pancreatic β‐cells. Furthermore, we find that alterations in insulin expression and secretion caused by chronic exposure to high glucose are paralleled by decreased insulin receptor expression and increased relative abundance of the Ex11+ isoform in both human islets and RIN β‐cells. PDX‐1 and HMGI(Y) transcription factors are down‐regulated by high glucose. These changes are associated with defects in insulin signaling involving insulin receptor‐associated PI 3‐kinase/Akt/PHAS‐I pathway in RIN β‐cells. Re‐expression in RIN β‐cells chronically exposed to high glucose of the Ex11−, but not the Ex11+, isoform restored insulin mRNA expression. These data suggest that changes in early steps of insulin receptor signaling may play a role in determining β‐cell dysfunction caused by chronic hyperglycemia.

Defects in Insulin-Receptor Internalization and Processing in Monocytes of Obese Subjects and Obese NIDDM Patients

We investigated intracellular processing of the insulin-receptor complex in monocytes from 12 healthy control subjects, 11 obese nondiabetic subjects, and 13 obese patients with non-insulin-dependent diabetes mellitus (NIDDM) by measuring receptor internalization, recovery of cell-surface insulin binding after receptor internalization, and the release of intracellular intact insulin (insulin retroendocytosis). When monocytes from the three groups of subjects were exposed to 100 nM unlabeled insulin for 30 min at 37°C, the subsequent cell-surface 125I-labeled insulin binding was reduced, but the total number of insulin receptors, measured by radioimmunoassay, was not changed. These findings indicate a redistribution of insulin receptors from the surface to the cell interior. Insulin-receptor internalization was significantly lower in monocytes of obese NIDDM patients (mean ± SE 17.8 ± 4.7%) than in obese subjects and healthy control subjects (33.5 ± 4.5%, P < .05, and 34.4 ± 3.7%, P < .02, respectively). Moreover, in downregulated cells, a complete recovery of the initial insulin binding was observed in control subjects but not in obese NIDDM patients or obese nondiabetic subjects. The release of internalized insulin was also reduced in obese NIDDM patients and obese subjects (f1/2 = 49.0 ± 2.4 min, P < .02; 47.4 ± 5.7 min, P < .05; and 32.9 ± 3.8 in NIDDM patients, obese subjects, and control subjects, respectively). In the radioactivity released from monocytes of obese subjects and obese NIDDM patients, the percentage of intact insulin was higher (P < .05) than in control subjects, suggesting reduced intracellular insulin degradation in obese subjects and obese NIDDM patients. This study indicates that insulin-resistant obese subjects and obese NIDDM patients have multiple postbinding defects of the insulin-receptor intracellular processing. Among these defects, decreased insulin-receptor internalization is specifically associated with diabetic patients.

Activator Protein-2 Overexpression Accounts for Increased Insulin Receptor Expression in Human Breast Cancer

Various studies have shown that the insulin receptor (IR) is increased in most human breast cancers, and both ligand-dependent malignant transformation and increased cell growth occur in cultured breast cells overexpressing the IR. However, although numerous in vivo and in vitro observations have indicated an important contributory role for the IR in breast cancer cell biology, the molecular mechanisms accounting for increased IR expression in breast tumors have not previously been elucidated. Herein, we did immunoblot analyses of nuclear protein from cultured breast cancer cells and normal and tumoral tissues from breast cancer patients combined with promoter studies by using a series of human wild-type and mutant IR promoter constructs. We provide evidence that IR overexpression in breast cancer is dependent on the assembly of a transcriptionally active multiprotein-DNA complex, which includes the high-mobility group A1 (HMGA1) protein, the developmentally regulated activator protein-2 (AP-2) transcription factor and the ubiquitously expressed transcription factor Sp1. In cultured breast cancer cells and human breast cancer specimens, the expression of AP-2 was significantly higher than that observed in cells and tissues derived from normal breast, and this overexpression paralleled the increase in IR expression. However, AP-2 DNA-binding activity was undetectable with the IR gene promoter, suggesting that transactivation of this gene by AP-2 might occur indirectly through physical and functional cooperation with HMGA1 and Sp1. Our findings support this hypothesis and suggest that in affected individuals, hyperactivation of the AP-2 gene through the overexpression of IR may play a key role in breast carcinogenesis.