Differential expression and signaling activation of insulin receptor isoforms A and B: A link between breast cancer and diabetes

Insulin Receptor Isoforms Differently Regulate Cell Proliferation and Apoptosis in the Ligand-Occupied and Unoccupied State

The insulin receptor (IR) presents two isoforms (IR-A and IR-B) that differ for the α-subunit C-terminal. Both isoforms are expressed in all human cells albeit in different proportions, yet their functional properties-when bound or unbound to insulin-are not well characterized. From a cell model deprived of the Insulin-like Growth Factor 1 Receptor (IGF1-R) we therefore generated cells exhibiting no IR (R-shIR cells), or only human IR-A (R-shIR-A), or exclusively human IR-B (R-shIR-B) and we studied the specific effect of the two isoforms on cell proliferation and cell apoptosis. In the absence of insulin both IR-A and IR-B similarly inhibited proliferation but IR-B was 2–3 fold more effective than IR-A in reducing resistance to etoposide-induced DNA damage. In the presence of insulin, IR-A and IR-B promoted proliferation with the former significantly more effective than the latter at increasing insulin concentrations. Moreover, only insulin-bound IR-A, but not IR-B, protected cells from etoposide-induced cytotoxicity. In conclusion, IR isoforms have different effects on cell proliferation and survival. When unoccupied, IR-A, which is predominantly expressed in undifferentiated and neoplastic cells, is less effective than IR-B in protecting cells from DNA damage. In the presence of insulin, particularly when present at high levels, IR-A provides a selective growth advantage.

Rbfox2 mediates exon 11 inclusion in insulin receptor pre-mRNA splicing in hepatoma cells

Insulin receptor (IR) pre-mRNA undergoes alternative splicing that produces two isoforms, IR-A and IR-B. The ratio of IR-A to IR-B varies among tissues, which strongly suggests that IR mRNA alternative splicing is regulated in a tissue-specific manner. However, the precise molecular mechanism for IR alternative splicing remains to be elucidated, especially in liver. In this study, we have analyzed IR alternative splicing mechanism by preparing a mini-gene splicing reporter with rat genomic DNA. The splicing reporter that contains exon 11 and its flanking intronic sequences could reproduce alternative splicing pattern in rat hepatoma H4IIE cells. Introducing several deletions in introns of the reporter revealed that intron 11 contains the region near exon 11 essential to promote exon 11 inclusion. This region contains an UGCAUG sequence, a specific binding site for the Rbfox splicing regulator, and mutation in this sequence results in exon 11 skipping. Furthermore, RbFox2 knockdown in H4IIE cells enhanced exon 11 skipping of endogenous IR pre-mRNA. Lastly mutations in the SRSF3 binding site of exon11 together with the Rbfox2 binding site completely abolished exon 11 inclusion with a mini-gene reporter pre-mRNA. Our results indicate that RbFox2 and SRSF3 proteins mediate exon 11 inclusion in rat hepatoma cells.

Insulin receptor is implicated in triple-negative breast cancer by decreasing cell mobility

Triple-negative breast cancer (TNBC) has a poor prognosis and typically earlier onset of metastasis in comparison with other breast cancer subtypes. It has been reported that insulin receptor (INSR) is downregulated in TNBC, however, its clinical significance and functions in TNBC remain to be elucidated. In this study, we found that INSR expression was significantly downregulated in TNBC, and overexpression of INSR suppressed cell migration and invasion in TNBC. In addition, the survival rate of breast cancer patients with low INSR expression was lower than that of patients with high INSR expression. INSR expression was significantly correlated with lymph node metastasis, clinical tumor stages, ER status, PR status, and the proliferation index Ki-67 expression. In summary, our study suggests that INSR may serve as a biomarker for breast cancer prognosis and it may be a potential target for TNBC treatment.

Isoform- and Paralog-Switching in IR-Signaling: When Diabetes Opens the Gates to Cancer

Insulin receptor (IR) and IR-related signaling defects have been shown to trigger insulin-resistance in insulin-dependent cells and ultimately to give rise to type 2 diabetes in mammalian organisms. IR expression is ubiquitous in mammalian tissues, and its over-expression is also a common finding in cancerous cells. This latter finding has been shown to associate with both a relative and absolute increase in IR isoform-A (IR-A) expression, missing 12 aa in its EC subunit corresponding to exon 11. Since IR-A is a high-affinity transducer of Insulin-like Growth Factor-II (IGF-II) signals, a growth factor is often secreted by cancer cells; such event offers a direct molecular link between IR-A/IR-B increased ratio in insulin resistance states (obesity and type 2 diabetes) and the malignant advantage provided by IGF-II to solid tumors. Nonetheless, recent findings on the biological role of isoforms for cellular signaling components suggest that the preferential expression of IR isoform-A may be part of a wider contextual isoform-expression switch in downstream regulatory factors, potentially enhancing IR-dependent oncogenic effects. The present review focuses on the role of isoform- and paralog-dependent variability in the IR and downstream cellular components playing a potential role in the modulation of the IR-A signaling related to the changes induced by insulin-resistance-linked conditions as well as to their relationship with the benign versus malignant transition in underlying solid tumors.

Enhanced TGF-β Signaling Contributes to the Insulin-Induced Angiogenic Responses of Endothelial Cells

Angiogenesis, the development of new blood vessels, is a key process in disease. We reported that insulin promotes translocation of transforming growth factor β (TGF-β) receptors to the plasma membrane of epithelial and fibroblast cells, thus enhancing TGF-β responsiveness. Since insulin promotes angiogenesis, we addressed whether increased autocrine TGF-β signaling participates in endothelial cell responses to insulin. We show that insulin enhances TGF-β responsiveness and autocrine TGF-β signaling in primary human endothelial cells, by inducing a rapid increase in cell surface TGF-β receptor levels. Autocrine TGF-β/Smad signaling contributed substantially to insulin-induced gene expression associated with angiogenesis, including TGF-β target genes encoding angiogenic mediators; was essential for endothelial cell migration; and participated in endothelial cell invasion and network formation. Blocking TGF-β signaling impaired insulin-induced microvessel outgrowth from neonatal aortic rings and modified insulin-stimulated blood vessel formation in zebrafish. We conclude that enhanced autocrine TGF-β signaling is integral to endothelial cell and angiogenic responses to insulin.

Pro-angiogenic Role of Insulin: From Physiology to Pathology

The underlying molecular mechanisms involve in the regulation of the angiogenic process by insulin are not well understood. In this review article, we aim to describe the role of insulin and insulin receptor activation on the control of angiogenesis and how these mechanisms can be deregulated in human diseases. Functional expression of insulin receptors and their signaling pathways has been described on endothelial cells and pericytes, both of the main cells involved in vessel formation and maturation. Consequently, insulin has been shown to regulate endothelial cell migration, proliferation, and in vitro tubular structure formation through binding to its receptors and activation of intracellular phosphorylation cascades. Furthermore, insulin-mediated pro-angiogenic state is potentiated by generation of vascular growth factors, such as the vascular endothelial growth factor, produced by endothelial cells. Additionally, diseases such as insulin resistance, obesity, diabetes, and cancer may be associated with the deregulation of insulin-mediated angiogenesis. Despite this knowledge, the underlying molecular mechanisms need to be elucidated in order to provide new insights into the role of insulin on angiogenesis.

Metabolic syndrome and pre-diabetes contribute to racial disparities in breast cancer outcomes: hypothesis and proposed pathways

BACKGROUND

Women with obesity and type 2 diabetes (T2D) are at greater risk of dying from breast cancer than women without these conditions. Obesity and T2D are associated with insulin resistance and endogenous hyperinsulinemia and are more common in Black women. There is increasing disparity in breast cancer mortality between Black and White women in the USA. We hypothesize that insulin resistance and endogenous hyperinsulinemia in Black women with breast cancer contribute to their greater breast cancer mortality and are associated with increased insulin receptor signalling in tumours.

METHODS

We will recruit 350 Black women and 936 White women with newly diagnosed breast cancer. We will determine the presence or absence of the metabolic syndrome/pre-diabetes and insulin resistance by measuring body mass index, waist circumference, lipids, blood pressure, glucose, insulin-like growth factor binding protein 1 and insulin. Breast cancer prognosis will be determined by a Nottingham Prognostic Index (NPI), with poor prognosis being defined as NPI >4.4. Tumour insulin receptor signalling will be determined by immunohistochemistry. Insulin receptor subtype expression will be measured using Nanostring. Analysis of these factors will determine whether endogenous hyperinsulinemia is associated with a worse prognosis in Black women than White women and increased tumour insulin receptor signalling.

CONCLUSIONS

The results of this study will determine if the metabolic syndrome and pre-diabetes contribute to racial disparities in breast cancer mortality. It may provide the basis for targeting systemic insulin resistance and/or tumour insulin receptor signalling to reduce racial disparities in breast cancer mortality. Copyright © 2016 John Wiley & Sons, Ltd.

Insulin receptor isoforms: an integrated view focused on gestational diabetes mellitus

The human insulin receptor (IR) exists in two isoforms that differ by the absence (IR-A) or the presence (IR-B) of a 12-amino acid segment encoded by exon 11. Both isoforms are functionally distinct regarding their binding affinities and intracellular signalling. However, the underlying mechanisms related to their cellular functions in several tissues are only partially understood. In this review, we summarize the current knowledge in this field regarding the alternative splicing of IR isoform, tissue-specific distribution and signalling both in physiology and disease, with an emphasis on the human placenta in gestational diabetes mellitus (GDM). Furthermore, we discuss the clinical relevance of IR isoforms highlighted by findings that show altered insulin signalling due to differential IR-A and IR-B expression in human placental endothelium in GDM pregnancies. Future research and clinical studies focused on the role of IR isoform signalling might provide novel therapeutic targets for treating GDM to improve the adverse maternal and neonatal outcomes.

Does Insulin Explain the Relation between Maternal Obesity and Poor Lactation Outcomes? An Overview of the Literature

It is well established that obese women are at increased risk of delayed lactogenesis and short breastfeeding duration, but the underlying causal contributors remain unclear. This review summarizes the literature examining the role of insulin in lactation outcomes. Maternal obesity is a strong risk factor for insulin resistance and prediabetes, but until recently a direct role for insulin in milk production had not been elucidated. Over the past 6 y, studies in both animal models and humans have shown insulin-sensitive gene expression to be dramatically upregulated specifically during the lactation cycle. Insulin is now considered to play a direct role in lactation, including essential roles in secretory differentiation, secretory activation, and mature milk production. At the same time, emerging clinical research suggests an important association between suboptimal glucose tolerance and lactation difficulty. To develop effective interventions to support lactation success in obese women further research is needed to identify how, when, and for whom maternal insulin secretion and sensitivity affect lactation ability.

Understanding the Key to Targeting the IGF Axis in Cancer: A Biomarker Assessment

Type 1 insulin like growth factor receptor (IGF-1R) targeted therapies showed compelling pre-clinical evidence; however, to date, this has failed to translate into patient benefit in Phase 2/3 trials in unselected patients. This was further complicated by the toxicity, including hyperglycemia, which largely results from the overlap between IGF and insulin signaling systems and associated feedback mechanisms. This has halted the clinical development of inhibitors targeting IGF signaling, which has limited the availability of biopsy samples for correlative studies to understand biomarkers of response. Indeed, a major factor contributing to lack of clinical benefit of IGF targeting agents has been difficulty in identifying patients with tumors driven by IGF signaling due to the lack of predictive biomarkers. In this review, we will describe the IGF system, rationale for targeting IGF signaling, the potential liabilities of targeting strategies, and potential biomarkers that may improve success.

Oncogenic functions of IGF1R and INSR in prostate cancer include enhanced tumor growth, cell migration and angiogenesis

We scrutinized the effect of insulin receptor (INSR) in addition to IGF1R in PCa using in vitro and in vivo models. In-vitro overexpression of IGF1R and INSRA, but not INSRB increased cell proliferation, colony formation, migration, invasion and resistance to apoptosis in prostate cancer cells (DU145, LNCaP, PC3). Opposite effects were induced by downregulation of IGF1R and total INSR, but not INSRB. In contrast to tumor cells, non-cancerous epithelial cells of the prostate (EP156T, RWPE-1) were inhibited on overexpression and stimulated by knockdown of receptors. In-vivo analyses using the chicken allantoic membrane assay confirmed the tumorigenic effects of IGF1R and INSR. Apart of promoting tumor growth, IGF1R and INSR overexpression also enhanced angiogenesis indicated by higher vessel density and increased number of desmin-immunoreactive pericytes. Our study underscores the oncogenic impact of IGF1R including significant effects on tumor growth, cell migration, sensitivity to apoptotic/chemotherapeutic agents and angiogenesis, and characterizes the INSR, in particular the isoform INSRA, as additional cancer-promoting receptor in prostate cancer. Both, the insulin-like growth factor receptor 1 and the insulin receptor exert oncogenic functions, thus proposing that both receptors need to be considered in therapeutic settings.

Expression of Intratumoral IGF-II Is Regulated by the Gene Imprinting Status in Triple Negative Breast Cancer from Vietnamese Patients

African American women suffer higher incidence and mortality of triple negative breast cancer (TNBC) than Caucasian women. TNBC is very aggressive, causing the worst clinical outcome. We previously demonstrated that tumors from these patients express high IGF-II and exhibit high activation of the IGF signaling pathways. IGF-II gene expression is imprinted (monoallelic), promotes tumor progression, and metastasis and regulates Survivin, a TNBC prognostic marker. Since BC mortality has increased among young Vietnamese women, we analyzed 48 (paired) TNBC samples from Vietnamese patients to assess IGF-II expression. We analyzed all samples by qrtPCR for identification of IGF-II heterozygosity and to determine allelic expression of the IGF-II gene. We also analyzed the tissues for proIGF-II and Survivin by RT-PCR and Western blotting. A total of 28 samples displayed IGF-II heterozygosity of which 78% were biallelic. Tumors with biallelic IGF-II gene expression exhibited the highest levels of proIGF-II and Survivin. Although 100% of these tissues corresponding normal samples were biallelic, they expressed significantly lower levels of or no proIGF-II and Survivin. Thus, IGF-II biallelic gene expression is differentially regulated in normal versus tumor tissues. We propose that intratumoral proIGF-II is dependent on the IGF-II gene imprinting status and it will promote a more aggressive TNBC.

Elevated SRPK1 lessens apoptosis in breast cancer cells through RBM4-regulated splicing events

Imbalanced splicing of premessenger RNA is typical of tumorous malignancies, and the regulatory mechanisms involved in several tumorigenesis-associated splicing events are identified. Elevated expression of serine-arginine protein kinase 1 (SRPK1) may participate in the pathway responsible for the dysregulation of splicing events in malignant tumor cells. In this study, we observed a correlation between the cytoplasmic accumulation of RNA-binding motif protein 4 (RBM4) and up-regulated SRPK1 in breast cancer cells. The production of the IR-B and MCL-1S transcripts was induced separately by the overexpression of RBM4 and SRPK1 gene silencing. Overexpressed RBM4 simultaneously bound to the CU-rich elements within the MCL-1 exon2 and the downstream intron, which subsequently facilitated the exclusion of the regulated exon. Breast cancer cells are deprived of apoptotic resistance through the RBM4-mediated up-regulation of the IR-B and MCL-1S transcripts. These findings suggest that the splicing events regulated by the SRPK1-RMB4 network may contribute to tumorigenesis through altered sensitivity to apoptotic signals in breast cancer cells.

Insulin receptor isoform switching in intestinal stem cells, progenitors, differentiated lineages and tumors: evidence that IR-B limits proliferation

Despite evidence for the impact of insulin on intestinal epithelial physiology and pathophysiology, the expression patterns, roles, and regulation of insulin receptor (IR) and IR isoforms in the intestinal epithelium are not well characterized. IR-A is thought to mediate the proliferative effects of insulin or insulin growth factors (IGFs) in fetal or cancer cells. IR-B is considered to be the metabolic receptor for insulin in specialized tissues. This study used a novel Sox9-EGFP reporter mouse that permits isolation of intestinal epithelial stem cells (IESCs), progenitors, enteroendocrine cells and differentiated lineages, the Apc(Min/+) mouse model of precancerous adenoma and normal human intestinal and colorectal cancer (CRC) cell lines. We tested the hypothesis that there is differential expression of IR-A or IR-B in stem and tumor cells versus differentiated intestinal epithelial cells (IECs) and that IR-B impacts cell proliferation. Our findings provide evidence that IR-B expression is significantly lower in highly proliferative IESCs and progenitor cells versus post-mitotic, differentiated IECs and in subconfluent and undifferentiated versus differentiated Caco-2 cells. IR-B is also reduced in Apc(Min/+) tumors and highly tumorigenic CRC cells. These differences in IR-B were accompanied by altered levels of mRNAs encoding muscleblind-like 2 (MBNL2), a known regulator of IR alternative splicing. Forced IR-B expression in subconfluent and undifferentiated Caco-2 cells reduced proliferation and increased biomarkers of differentiation. Our findings indicate that the impact of insulin on different cell types in the intestinal epithelium might differ depending on relative IR-B IR-A expression levels and provide new evidence for the roles of IR-B to limit proliferation of CRC cells.

Insulin receptor phosphorylation by endogenous insulin or the insulin analog AspB10 promotes mammary tumor growth independent of the IGF-I receptor

Endogenous hyperinsulinemia and insulin receptor (IR)/IGF-I receptor (IGF-IR) phosphorylation in tumors are associated with a worse prognosis in women with breast cancer. In vitro, insulin stimulation of the IR increases proliferation of breast cancer cells. However, in vivo studies demonstrating that IR activation increases tumor growth, independently of IGF-IR activation, are lacking. We hypothesized that endogenous hyperinsulinemia increases mammary tumor growth by directly activating the IR rather than the IGF-IR or hybrid receptors. We aimed to determine whether stimulating the IR with the insulin analog AspB10 could increase tumor growth independently of IGF-IR signaling. We induced orthotopic mammary tumors in control FVB/n and hyperinsulinemic MKR mice, and treated them with the insulin analog AspB10, recombinant human IGF-I, or vehicle. Tumors from mice with endogenous hyperinsulinemia were larger and had greater IR phosphorylation, but not IGF-IR phosphorylation, than those from control mice. Chronic AspB10 administration also increased tumor growth and IR (but not IGF-IR) phosphorylation in tumors. IGF-I led to activation of both the IGF-IR and IR and probably hybrid receptors. Our results demonstrate that IR phosphorylation increases tumor growth, independently of IGF-IR/hybrid receptor phosphorylation, and warrant consideration when developing therapeutics targeting the IGF-IR, but not the IR.

A twenty-first century cancer epidemic caused by obesity: the involvement of insulin, diabetes, and insulin-like growth factors

Obesity has reached epidemic proportions in the developed world. The progression from obesity to diabetes mellitus type 2, via metabolic syndrome, is recognised, and the significant associated increase in the risk of major human cancers acknowledged. We review the molecular basis of the involvement of morbidly high concentrations of endogenous or therapeutic insulin and of insulin-like growth factors in the progression from obesity to diabetes and finally to cancer. Epidemiological and biochemical studies establish the role of insulin and hyperinsulinaemia in cancer risk and progression. Insulin-like growth factors, IGF-1 and IGF-2, secreted by visceral or mammary adipose tissue have significant paracrine and endocrine effects. These effects can be exacerbated by increased steroid hormone production. Structural studies elucidate how each of the three ligands, insulin, IGF-1, and IGF-2, interacts differently with isoforms A and B of the insulin receptor and with type I IGF receptor and explain how these protagonists contribute to diabetes-associated cancer. The above should inform appropriate treatment of cancers that arise in obese individuals and in those with diabetes mellitus type 2. Novel drugs that target the insulin and insulin-like growth factor signal transduction pathways are in clinical trial and should be effective if appropriate biomarker-informed patient stratification is implemented.

The epidemiology and molecular mechanisms linking obesity, diabetes, and cancer

The worldwide epidemic of obesity is associated with increasing rates of the metabolic syndrome and type 2 diabetes. Epidemiological studies have reported that these conditions are linked to increased rates of cancer incidence and mortality. Obesity, particularly abdominal obesity, is associated with insulin resistance and the development of dyslipidemia, hyperglycemia, and ultimately type 2 diabetes. Although many metabolic abnormalities occur with obesity and type 2 diabetes, insulin resistance and hyperinsulinemia appear to be central to these conditions and may contribute to dyslipidemia and altered levels of circulating estrogens and androgens. In this review, we will discuss the epidemiological and molecular links between obesity, type 2 diabetes, and cancer, and how hyperinsulinemia and dyslipidemia may contribute to cancer development. We will discuss how these metabolic abnormalities may interact with estrogen signaling in breast cancer growth. Finally, we will discuss the effects of type 2 diabetes medications on cancer risk.

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.