Insulin-like growth factor binding proteins in the human adrenal gland

The role of insulin-like growth factor system in the adrenocortical tumors

INTRODUCTION

The different presentation of adrenocortical tumors in benign adenoma (ACA) or adrenocortical carcinoma (ACC) is related to the variability at the molecular level. The insulin-like growth factor (IGF) system is one of the most frequently altered pathways in ACC. In this review we will critically analyze the evidence regarding the pathogenic role of the IGF system in adrenal tumorigenesis, focusing on ACC. We will also examine the preclinical and clinical studies which investigated the targeting of the IGF system as a therapeutic approach in ACC.

EVIDENCE ACQUISITION

The IGF system plays a crucial role in the embryogenesis of adrenal glands. No significant alterations of the IGF system were observed in ACA. In ACC, the IGF2 overexpression is one of the most frequent molecular change presented in more than 85% of cases. However, IGF2 seems to be only a tumor progression factor which requires additional hits to trigger adrenal tumorigenesis. Also, the IGF1 receptor (IGF1R) appears to be higher expressed in ACC. Many IGF1R target-drugs have been developed to inhibit the activation of the IGF system.

EVIDENCE SYNTHESIS

Preclinical studies using antibody or tyrosine kinase which target the IGF1R, or the dual-targeting of IGF1R and insulin receptor (IR) reduced ACC cells proliferation both in vitro and in vivo in mouse xenograft model. However, these promising results were not confirmed in clinical trials.

CONCLUSIONS

Nowadays, predictive markers for the response of target-IGF therapy are missing and further studies which investigate new molecular markers and evaluate the entire IGF receptors, including the IR, are urgently needed.

The role of epithelial growth factors and insulin growth factors in the adrenal neoplasms

Human fetal and adult adrenal gland express both insulin growth factor-1 (IGF-1) and IGF-2, their receptors (IGF-Rs) and a variety of specific IGF binding proteins suggesting their potential role in the regulation of adrenal growth and function. IGF-2 overexpression is essential for the growth of monoclonal lesions, such as large benign adenomas (ACA) and adrenocortical carcinomas (ACC) and has been found to contribute to tumorigenesis in Beckwith-Wiedemann syndrome. IGF-2 is the most highly expressed gene observed in more than 85% of ACCs. However, no significant differences in clinical, biological and transcriptomic traits were found between tumors with high and low expression of IGF-2. On the contrary, the expression of IGF-1R, mediating the IGF-2 effects in vivo, was more discriminant between malignant (overexpression) and benign tumors. Data on the role of epithelial growth factor (EGF) and its receptor (EGF-R) in adrenocortical tumorigenesis are controversial. Several studies have shown EGF-R overexpression in ACCs but not in benign ACAs, suggesting that EGF-R could potentially be used as a marker for the differential diagnosis of ACAs and ACCs. Although, in vitro and animal studies provide promising results in the therapeutic role of IGF and EGF pathway inhibitors, the available data in humans are still not encouraging. Herein, we aim to present recent data on the role of IGF and EGF pathways in adrenal development and tumorigenesis and their potential implication in the treatment of the ACC, a rare malignancy with very poor prognosis.

Adrenocortical growth and cancer

The adrenal gland consists of two distinct parts, the cortex and the medulla. Molecular mechanisms controlling differentiation and growth of the adrenal gland have been studied in detail using mouse models. Knowledge also came from investigations of genetic disorders altering adrenal development and/or function. During embryonic development, the adrenal cortex acquires a structural and functional zonation in which the adrenal cortex is divided into three different steroidogenic zones. Significant progress has been made in understanding adrenal zonation. Recent lineage tracing experiments have accumulated evidence for a centripetal differentiation of adrenocortical cells from the subcapsular area to the inner part of the adrenal cortex. Understanding of the mechanism of adrenocortical cancer (ACC) development was stimulated by knowledge of adrenal gland development. ACC is a rare cancer with a very poor overall prognosis. Abnormal activation of the Wnt/β-catenin as well as the IGF2 signaling plays an important role in ACC development. Studies examining rare genetic syndromes responsible for familial ACT have played an important role in identifying genetic alterations in these tumors (like TP53 or CTNNB1 mutations as well as IGF2 overexpression). Recently, genomic analyses of ACT have shown gene expression profiles associated with malignancy as well as chromosomal and methylation alterations in ACT and exome sequencing allowed to describe the mutational landscape of these tumors. This progress leads to a new classification of these tumors, opening new perspectives for the diagnosis and prognostication of ACT. This review summarizes current knowledge of adrenocortical development, growth, and tumorigenesis.

Insulin-like growth factor system on adrenocortical tumorigenesis

The insulin-like growth factor (IGF) signaling pathway has many important roles in normal cell growth and development. Remarkably, all of the components of this system (IGFs, receptors, and binding proteins) are expressed in human fetal adrenals. Beckwith-Wiedemann syndrome, a congenital overgrowth disorder characterized by a high risk of development of childhood tumors, is also distinguished by a high incidence of adrenocortical carcinomas. This disease has been associated with structural abnormalities at the 11p15 locus, which harbors the IGF2 gene as well as the genes coding for insulin, H19, and p57kip2. Notably, rearrangements at the 11p15 locus and overexpression of IGF2 were also described in sporadic adrenocortical tumors. In addition, the IGF2 overexpression was exclusively demonstrated in adults with adrenocortical tumors as a frequent feature of the malignant state. More recent studies demonstrated that the interaction of IGF-2 with IGF receptor type 1 (IGF-1R) plays also a pivotal role in adrenocortical tumorigenesis. IGF1R expression levels were significantly higher in pediatric adrenocortical carcinomas, suggesting that IGF1R expression represents a potential prognostic marker in this group of patients. These findings indicate that the IGF system is an important pathway for autonomous growth of adrenocortical cells and potential inhibitors of this system could be a rational therapeutic target for adrenocortical tumors.

Differences in the molecular mechanisms of adrenocortical tumorigenesis between children and adults

Adrenocortical carcinoma (ACC) is a rare malignancy with poor prognosis. The incidence of pediatric adrenocortical tumors (ACT) is remarkably high in Southern Brazil, where it is estimated to be 15 times greater than the world occurrence, due to a high frequency of a germline mutation (p.R337H) of the TP53 gene. Differently from adults, pediatric adrenocortical neoplasms with apparently poor prognosis based on histopathological features have often a good clinical outcome. A high Weiss score is definitely not a good predictor of survival in children, but it is much more discriminative of a poor outcome in adult tumors. Besides important differences in prognosis, adrenocortical tumorigenesis has distinct patterns between children and adults. In this review, we summarize recent data from ours and other Institutions, showing that the prognostic importance of molecular markers is striking different between pediatric and adult ACT. Although the majority of pediatric ACT are associated with p.R337H germline mutation, it is not a predictor of poor outcome in children and adolescents with ACT. On the other side, TP53 somatic mutations define a subgroup of adult ACC with different tumorigenesis and unfavorable prognosis. IGF system has a central role in the malignant phenotype of ACT, but in adult tumors it is mediated by IGF2 over-expression and in pediatric tumors by IGF1R over-expression. Finally, SF1 over-expression is associated with decreased overall survival and recurrence-free survival in adult ACC, but not in the pediatric group. In conclusion, discriminating benign and malignant behavior is more challenging in pediatric ACT than in adult tumors.

The IGF system in in-vitro human decidualization

Decidualization of endometrial stromal cells (ESCs) is critical for a successful pregnancy but the molecular mechanisms of the process are poorly understood. In this study, we investigated whether the insulin-like growth factor (IGF) network is involved in this cellular process. Expression kinetics of members of the IGF system was examined at both mRNA and protein levels during in-vitro decidualization of cultured human ESCs. We found a significant up-regulation of IGF-II as well as of IGF-I receptor and the A and B insulin receptor (InsR) isoforms. In addition, levels of the key adaptor proteins insulin receptor substrate 1 (IRS-1) and IRS-2 increased, suggesting a potential involvement of the IGF signalling pathway in the decidualization process. Expression of two IGF binding proteins, IGFBP-1 and IGFBP-4, which can inhibit IGF action, also increased. In order to determine whether IGF signalling was activated during decidualization, the phosphorylation status of the receptors and the adaptor proteins was estimated. Only IRS-2 was slightly phosphorylated in decidualized cells and was further activated by the addition of exogenous IGF-II. These results suggest that the IGF signalling pathway could play a crucial role in the functions of decidualized endometrial cells.

Expression of insulin-like growth factor-II and its receptor in pediatric and adult adrenocortical tumors

BACKGROUND

Adrenocortical tumors are heterogeneous neoplasms with incompletely understood pathogenesis. IGF-II overexpression has been consistently demonstrated in adult adrenocortical carcinomas.

OBJECTIVES

The objective of the study was to analyze expression of IGF-II and its receptor (IGF-IR) in pediatric and adult adrenocortical tumors and the effects of a selective IGF-IR kinase inhibitor (NVP-AEW541) on adrenocortical tumor cells.

PATIENTS

Fifty-seven adrenocortical tumors (37 adenomas and 20 carcinomas) from 23 children and 34 adults were studied.

METHODS

Gene expression was determined by quantitative real-time PCR. Cell proliferation and apoptosis were analyzed in NCI H295 cells and a new cell line established from a pediatric adrenocortical adenoma.

RESULTS

IGF-II transcripts were overexpressed in both pediatric adrenocortical carcinomas and adenomas. Otherwise, IGF-II was mainly overexpressed in adult adrenocortical carcinomas (270.5 +/- 130.2 vs. 16.1 +/- 13.3; P = 0.0001). IGF-IR expression was significantly higher in pediatric adrenocortical carcinomas than adenomas (9.1 +/- 3.1 vs. 2.6 +/- 0.3; P = 0.0001), whereas its expression was similar in adult adrenocortical carcinomas and adenomas. IGF-IR expression was a predictor of metastases in pediatric adrenocortical tumors in univariate analysis (hazard ratio 1.84; 95% confidence interval 1.28-2.66; P = 0.01). Furthermore, NVP-AEW541 blocked cell proliferation in a dose- and time-dependent manner in both cell lines through a significant increase of apoptosis.

CONCLUSION

IGF-IR overexpression was a biomarker of pediatric adrenocortical carcinomas. Additionally, a selective IGF-IR kinase inhibitor had antitumor effects in adult and pediatric adrenocortical tumor cell lines, suggesting that IGF-IR inhibitors represent a promising therapy for human adrenocortical carcinoma.

Insulin-like growth factor binding protein 2 (IGFBP-2) separates hypertrophic and hyperplastic effects of growth hormone (GH)/IGF-I excess on adrenocortical cells in vivo

GH and IGF-I are capable of inducing cellular hypertrophy and/or hyperplasia. Chronic overexpression of GH in transgenic mice results in systemically and locally increased IGF-I levels and in disproportionate overgrowth, including adrenocortical enlargement and corticosterone hypersecretion. Using PEPCK-bovine GH transgenic (G) mice, we demonstrate that adrenal enlargement involves both hypertrophy (44%) and hyperplasia (50%) of zona fasciculata cells. To clarify whether IGFBP-2 affected cell volume and number, we crossed hemizygous G mice with hemizygous CMV-IGFBP-2 transgenic (B) mice, generating G mice, B mice, GB double transgenic mice, and nontransgenic controls (C). The absolute weight of the adrenal glands was significantly increased in 5-wk- and 4-month-old G mice vs. C and B mice. IGFBP-2 overexpression in GB mice reduced this effect of GH excess by 26% and 37% in 5-wk- and 4-month-old animals, respectively. GH-induced hypertrophy of zona fasciculata cells was completely abolished by IGFBP-2 overexpression in GB mice whereas hyperplasia was not affected. Basal and ACTH-induced plasma corticosterone levels of 4-month-old G mice, but not of GB mice, were two- to threefold increased compared with C mice. Plasma ACTH levels were similar in all groups. Our data show that IGFBP-2 potently separates hypertrophic and hyperplastic effects of GH/IGF-I excess on adrenocortical cells.

Distribution of IGFBP-rP1 in normal human tissues

IGFBP-rP1/mac25 is a recently described member of the insulin-like growth factor binding protein (IGFBP) family. It has structural homology to the other members of the IGFBP family but has a lower affinity for insulin-like growth factors (IGFs). In previous studies using RNA blot hybridization, it was shown that the expression of IGFBP-rP1/mac25 was ubiquitous in normal human tissues. In this report we show by immunohistochemistry that the expression of IGFBP-rP1/mac25 is actually restricted to certain organs and specific cell types. We used an antibody raised against a decapeptide of the C-terminal part of the protein that recognizes a approximately 37-kD protein under reduced conditions. The immunohistochemistry performed on normal human tissues showed a ubiquitous intense staining of peripheral nerves and a variable degree of positive staining in smooth muscle cells, including those from blood vessel walls, gut, bladder, and prostate. Cilia from the respiratory system, epididymis, and fallopian tube showed intense immunoreactivity. Most endothelial cells showed some positivity, whereas fat cells, plasma cells, and lymphocytes were negative. There was specific expression limited to certain cell types in the kidney, adrenal gland, and skeletal muscle, indicating a possible specialized function of IGFBP-rP1/mac25 in these organs. We further noted an opposite pattern of staining in the lining epithelium of breast (typically positive) and prostate glands (largely negative). The specific localization of IGFBP-rP1/mac25 as described implies a function of the protein. However, its regulation within the IGF axis or a possible direct action of IGFBP-rP1/mac25 remains to be demonstrated.

Developmental and functional biology of the primate fetal adrenal cortex

The unique characteristics of the primate (particularly human) fetal adrenal were first realized in the early 1900s when its morphology was examined in detail and compared with that of other species. The unusual architecture of the human fetal adrenal cortex, with its unique and disproportionately enlarged fetal zone, its compact definitive zone, and its dramatic remodeling soon after birth captured the interest of developmental anatomists. Many detailed anatomical studies describing the morphology of the developing human fetal adrenal were reported between 1920 and 1960, and these morphological descriptions have not changed significantly. More recently, it has become clear that fetal adrenal cortical growth involves cellular hypertrophy, hyperplasia, apoptosis, and migration and is best described by the migration theory, i.e. cells proliferate in the periphery, migrate centripetally, differentiate during their migration to form the functional cortical zones, and then likely undergo apoptosis in the center of the cortex. Consistent with this model, cells of intermediate phenotype, arranged in columnar cords typical of migration, have been identified between the definitive and fetal zones. This cortical area has been referred to as the transitional zone and, based on the expression of steroidogenic enzymes, we consider it to be a functionally distinct cortical zone. Elegant experiments during the 1950s and 1960s demonstrated the central role of the primate fetal adrenal cortex in establishing the estrogenic milieu of pregnancy. Those findings were among the first indications of the function and physiological role of the human fetal adrenal cortex and led Diczfalusy and co-workers to propose the concept of the feto-placental unit, in which DHEA-S produced by the fetal adrenal cortex is used by the placenta for estrogen synthesis. Tissue and cell culture techniques, together with improved steroid assays, revealed that the fetal zone is the primary source of DHEA-S, and that its steroidogenic activity is regulated by ACTH. In recent years, function of the human and rhesus monkey fetal adrenal cortical zones has been reexamined by assessing the localization and ontogeny of steroidogenic enzyme expression. The primate fetal adrenal cortex is composed of three functionally distinct zones: 1) the fetal zone, which throughout gestation does not express 3 beta HSD but does express P450scc and P450c17 required for DHEA-S synthesis; 2) the transitional zone, which early in gestation is functionally identical to the fetal zone but late in gestation (after 25-30 weeks) expresses 3 beta HSD, P450scc, and P450c17, and therefore is the likely site of glucocorticoid synthesis, and 3) the definitive zone, which lacks P450c17 throughout gestation but late in gestation (after 22-24 weeks) expresses 3 beta HSD and P450scc, and therefore is the likely site of mineralocorticoid synthesis. Indirect evidence, based on effects of P450c21 deficiency and maternal estriol concentrations, indicate that the fetal adrenal cortex produces cortisol and DHEA-S early in gestation (6-12 weeks). However, controversy exists as to whether cortisol is produced de novo or derived from the metabolism of progesterone, as data regarding the expression of 3 beta HSD in the fetal adrenal cortex early in gestation are conflicting. During the 1960s, Liggins and colleagues demonstrated that in the sheep, cortisol secreted by the fetal adrenal cortex late in gestation regulates maturation of the fetus and initiates the cascade of events leading to parturition. Those pioneering discoveries provided insight into the mechanism underlying the timing of parturition and therefore were of particular interest to obstetricians and perinatologists confronted with the problems of preterm labor. However, although cortisol emanating from the fetal adrenal cortex promotes fetal maturation in primates as it does in sheep, its role in the regulation of primate parturition, unlike that in sheep

Role of growth factors in the developmental regulation of the human fetal adrenal cortex

Development of the human fetal adrenals is characterized by rapid growth and high levels of steroidogenic activity during the latter two-thirds of pregnancy. By midgestation, the human fetal adrenals are composed of two distinct cortical zones: the predominant fetal zone, which occupies 80-90% of the cortical volume and produces large amounts of the delta 5-steroid dehydroepiandrosterone sulfate, and the narrow definitive zone, which surrounds the fetal zone. Late in gestation, the peripheral portion of the fetal zone develops into a third, functionally distinct compartment, the transitional zone, which is the likely site of cortisol synthesis. Soon after birth, the adrenal cortex is remodeled and the fetal zone disappears. The adult cortical zones are thought to arise from the definitive zone, which persists postnatally. Development of the human fetal adrenals is regulated primarily by corticortropin (ACTH) secreted from the fetal pituitary. However, as ACTH is not a mitogen per se, its proliferative actions on human fetal adrenal cortical cells are thought to be mediated by autocrine/paracrine growth factors produced by adrenal cortical cells in response to ACTH. In addition, these growth factors appear to modulate the functional response of fetal adrenal cortical cells to ACTH. The roles of several growth factors, including the insulin like growth factors I and II (IGF-I and IGF-II), epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), activin, inhibin, and the transforming growth factors alpha and beta (TGF-alpha and TGF-beta) have been examined. In cultured human fetal adrenal cortical cells, EGF, bFGF, and IGF-I and -II are mitogenic, whereas activin and TGF-beta inhibit proliferation. IGF-II, activin, and TGF-beta also modulate ACTH-stimulated steroidogenesis. Human fetal adrenal cortical cells express IGF-II, bFGF and the activin/inhibin subunits, and the abundance of mRNAs for each of these factors is up-regulated by ACTH, suggesting that these growth factors are autocrine/paracrine mediators of ACTH action. Thus, although human adrenal development is primarily regulated by ACTH, its actions appear to be mediated/modulated by a cohort of locally expressed growth factors, the net effect of which results in the unique growth and steroidogenic activity of the human fetal adrenal cortex.

The expression of insulin-like growth factor and its binding protein mRNA in the endometrium of postmenopausal patients with breast cancer receiving tamoxifen

BACKGROUND

Insulin-like growth factor I (IGF-I) is known to mediate estrogen effect in the uterus, whereas IGF binding proteins (IGFBPs) modulate the biologic effects of IGF-I. Tamoxifen may act via the IGF-IGFBP system in the postmenopausal endometrium.

METHODS

Endometrial samples were collected from 16 postmenopausal women with breast cancer of whom 9 received tamoxifen and the remaining 7 received no hormonal treatment. The expression of messenger RNA for IGF-I, IGF-II, and the IGFBPs 1-6 was studied using dot blot and Northern blot techniques.

RESULTS

Expression of mRNA for IGF-I, IGFBP-2, -3, -4, and -6 was present in endometrial specimens. The expression of IGF-I mRNA was similar in the tamoxifen-treated and control patients, whereas mRNA expression for IGF-II was not detected. The expression of IGFBP-2 and -4 mRNA predominated in the endometrium of patients who received tamoxifen.

CONCLUSIONS

These findings demonstrate that the IGF-IGFBP system is present in the postmenopausal endometrium and may be modulated by tamoxifen.