IGF-I receptor-induced cell-cell adhesion of MCF-7 breast cancer cells requires the expression of junction protein ZO-1

IGF-1R targeting in cancer - does sub-cellular localization matter?

The insulin-like growth factor receptor (IGF-1R) was among the most intensively pursued kinase targets in oncology. However, even after a slew of small-molecule and antibody therapeutics reached clinical trials for a range of solid tumors, the initial promise remains unfulfilled. Mechanisms of resistance to, and toxicities resulting from, IGF-1R-targeted drugs are well-catalogued, and there is general appreciation of the fact that a lack of biomarker-based patient stratification was a limitation of previous clinical trials. But no next-generation therapeutic strategies have yet successfully exploited this understanding in the clinic.Currently there is emerging interest in re-visiting IGF-1R targeted therapeutics in combination-treatment protocols with predictive biomarker-driven patient-stratification. One such biomarker that emerged from early clinical trials is the sub-cellular localization of IGF-1R. After providing some background on IGF-1R, its drugging history, and the trials that led to the termination of drug development for this target, we look more deeply into the correlation between sub-cellular localization of IGF-1R and susceptibility to various classes of IGF-1R - targeted agents.

Cannabidiol Antiproliferative Effect in Triple-Negative Breast Cancer MDA-MB-231 Cells Is Modulated by Its Physical State and by IGF-1

Cannabidiol (CBD) is a non-psychoactive phytocannabinoid that has been discussed for its safety and efficacy in cancer treatments. For this reason, we have inquired into its use on triple-negative human breast cancer. Analyzing the biological effects of CBD on MDA-MB-231, we have demonstrated that both CBD dosage and serum concentrations in the culture medium influence its outcomes; furthermore, light scattering studies demonstrated that serum impacts the CBD aggregation state by acting as a surfactant agent. Pharmacological studies on CBD in combination with chemotherapeutic agents reveal that CBD possesses a protective action against the cytotoxic effect exerted by cisplatin on MDA-MB-231 grown in standard conditions. Furthermore, in a low serum condition (0.5%), starting from a threshold concentration (5 µM), CBD forms aggregates, exerts cytostatic antiproliferative outcomes, and promotes cell cycle arrest activating autophagy. At doses above the threshold, CBD exerts a highly cytotoxic effect inducing bubbling cell death. Finally, IGF-1 and EGF antagonize the antiproliferative effect of CBD protecting cells from harmful consequences of CBD aggregates. In conclusion, CBD effect is strongly associated with the physical state and concentration that reaches the treated cells, parameters not taken into account in most of the research papers.

Comparative Analysis of Cell-Cell Contact Abundance in Ovarian Carcinoma Cells Cultured in Two- and Three-Dimensional In Vitro Models

Simple Summary

Tumor resistance to therapy is a crucial problem of today’s oncology. The emerging data indicate that tumor microenvironment is the key participant in the resistance development. One of the most basic aspect of tumor microenvironment is intercellular adhesion. Our data obtained using monolayer culture, matrix-free and matrix-based three-dimensional in vitro models indicate that the abundance of cell-cell contact proteins is varying depending on the microenvironment. These differences coincided with the degree of the resistance to therapeutics. The importance of adhesion proteins in tumor resistance may provide the fundamental basis for improving cancer treatment approaches and must be taken into account when screening candidate drugs.

Abstract

Tumor resistance to therapy is associated with the 3D organization and peculiarities of the tumor microenvironment, of which intercellular adhesion is a key participant. In this work, the abundance of contact proteins was compared in SKOV-3 and SKOV-3.ip human ovarian adenocarcinoma cell lines, cultivated in monolayers, tumor spheroids and collagen hydrogels. Three-dimensional models were characterized by extremely low expression of basic molecules of adherens junctions E-cadherin and demonstrated a simultaneous decrease in desmosomal protein desmoglein-2, gap junction protein connexin-43 and tight junction proteins occludin and ZO-1. The reduction in the level of contact proteins was most pronounced in collagen hydrogel, accompanied by significantly increased resistance to treatment with doxorubicin and targeted anticancer toxin DARPin-LoPE. Thus, we suggest that 3D models of ovarian cancer, especially matrix-based models, tend to recapitulate tumor microenvironment and treatment responsiveness to a greater extent than monolayer culture, so they can be used as a highly relevant platform for drug efficiency evaluation.

Decreased expression of ZO-1 is associated with tumor metastases in liver cancer

Certain types of cancer exhibit downregulated expression of zonula occludens-1 (ZO-1), which serves an important function in tumor progression; however, the underlying molecular mechanisms that lead to this downregulation in cancer remain unclear. In the present study, the expression of ZO-1 in liver cancer (LC) tissues was investigated. Western blot and reverse transcription-quantitative polymerase chain reaction assays were used to detect the expression of ZO-1 protein and mRNA in LC tissues and paired adjacent non-tumorous tissues. The results indicated that, compared with non-tumorous tissues, the expression of ZO-1 was significantly downregulated at the protein (P<0.001) and mRNA (P=0.006) levels in LC tissue samples. In addition, various cellular and molecular methods were applied, including MTT, colony formation, flow cytometry and Transwell assays. The results indicated that overexpression of ZO-1 inhibited cell viability, proliferation and migration, and induced G₀/G₁ phase arrest in vitro.

Expression of insulin-like growth factor-1 receptor in circulating tumor cells of patients with breast cancer is associated with patient outcomes

In patients with breast cancer, markers of aggressiveness such as dysregulation of the insulin-like growth factor receptor (IGF1R) system and E-cadherin loss are commonly observed. Reduced IGF1R expression is correlated with decreased E-cadherin levels and increased cell motility. We assessed IGF1R and E-cadherin expression in circulating tumor cells (CTCs) in patients with breast cancer. Peripheral blood mononuclear cells of early (n = 87)- and metastatic (n = 126)-stage breast cancer patients (obtained prior to adjuvant and first-line chemotherapy) were evaluated using double immunofluorescence (IF) staining for cytokeratin (CK) and IGF1R. Triple IF using CK, IGF1R, and E-cadherin antibodies was performed in selected CTC(+) patients. IGF1R(+) CTCs were more frequently observed in early disease than in metastatic disease (86% vs 68% of CTCs, P = 0.04) stage, whereas IGF1R(-) CTCs were more common in metastatic than in early disease (32% vs 14% of CTCs, P = 0.002). 100% of CTC(+) patients with early disease, compared to 79% of those with metastatic disease, harbored IGF1R(+) CTCs (P = 0.007). Patients with early disease and exclusively IGF1R(+) CTCs had longer disease-free (P = 0.02) and overall survival (P = 0.001) compared to patients with both IGF1R(+) and IGF1R(-) CTC populations. 67% of early-stage CTC(+) patients evaluated had exclusively IGF1R(+)/E-cadherin(+) CTCs, 33% also had IGF1R(-)/E-cadherin(-) CTCs, and none had exclusively IGF1R(-)/E-cadherin(-) CTCs compared to 17%, 75%, and 8% of metastatic patients, respectively (P = 0.027). Similarly, in paired samples of patients with early disease that progressed to metastatic disease, the proportion of IGF1R(+)/E-cadherin(+) CTCs was reduced and IGF1R(-)/E-cadherin(-) CTCs were increased in the metastatic stage compared to early disease stage. IGF1R(+) CTCs are commonly detected in breast cancer, and their frequency decreases in the metastatic disease stage. IGF1R(+)/E-cadherin(+) CTCs also decrease in metastatic patients. IGF1R(+) CTCs are associated with favorable outcomes in early disease stage, suggesting that IGF1R expression is correlated with reduced metastatic potential in breast cancer.

Insulin-like growth factor-1 signaling is responsible for cathepsin G-induced aggregation of breast cancer MCF-7 cells

Cathepsin G (CG), a neutrophil serine protease, induces cell migration and multicellular aggregation of human breast cancer MCF-7 cells in a process that is dependent on E-cadherin and CG enzymatic activity. While these tumor cell aggregates can cause tumor emboli that could represent intravascular growth and extravasation into the surrounding tissues, resulting in metastasis, the molecular mechanism underlying this process remains poorly characterized. In this study, we aimed to identify the signaling pathway that is triggered during CG-mediated stimulation of cell aggregation. Screening of a library of compounds containing approximately 90 molecular-targeting drugs revealed that this process was suppressed by the insulin-like growth factor-1 (IGF-1) receptor (IGF-1R)-specific kinase inhibitor OSI-906, as well as the multikinase inhibitors axitinib and sunitinib. Antibody array analysis, which is capable of detecting tyrosine phosphorylation of 49 distinct receptor tyrosine kinases, and the results of immunoprecipitation studies indicated that IGF-1R is phosphorylated in response to CG treatment. Notably, IGF-1R neutralization via treatment with a specific antibody or silencing of IGF-1R expression through siRNA transfection suppressed cell aggregation. Furthermore, CG treatment of MCF-7 cells resulted in increased release of IGF-1 into the medium for 24 h, while antibody-mediated IGF-1 neutralization partially prevented CG-induced cell aggregation. These results demonstrate that autocrine IGF-1 signaling is partly responsible for the cell aggregation induced by CG.

Cross-Talk between Adiponectin and IGF-IR in Breast Cancer

Obesity is a chronic and multifactorial disorder that is reaching epidemic proportions. It is characterized by an enlarged mass of adipose tissue caused by a combination of size increase of preexisting adipocytes (hypertrophy) and de novo adipocyte differentiation (hyperplasia). Obesity is related to many metabolic disorders like hypertension, type 2 diabetes, metabolic syndrome, and cardiovascular disease, and it is associated with an increased risk of cancer development in different tissues including breast. Adipose tissue is now regarded as not just a storage reservoir for excess energy, but rather as an endocrine organ, secreting a large number of bioactive molecules called adipokines. Among these, adiponectin represents the most abundant adipose tissue-excreted protein, which exhibits insulin sensitizing, anti-inflammatory, and antiatherogenic properties. The serum concentrations of adiponectin are inversely correlated with body mass index. Recently, low levels of plasma adiponectin have been associated with an increased risk for obesity-related cancers and development of more aggressive phenotype, concomitantly with alterations in the bioavailability of insulin-like growth factor-I (IGF-I) and IGF-I receptor (IGF-IR) signaling pathways. In this review, we discuss the cross-talk between adiponectin/AdipoR1 and IGF-I/IGF-IR in breast cancer.

NADPH Oxidase Activity in Cerebral Arterioles Is a Key Mediator of Cerebral Small Vessel Disease-Implications for Prevention

Cerebral small vessel disease (SVD), a common feature of brain aging, is characterized by lacunar infarcts, microbleeds, leukoaraiosis, and a leaky blood-brain barrier. Functionally, it is associated with cognitive decline, dementia, depression, gait abnormalities, and increased risk for stroke. Cerebral arterioles in this syndrome tend to hypertrophy and lose their capacity for adaptive vasodilation. Rodent studies strongly suggest that activation of Nox2-dependent NADPH oxidase activity is a crucial driver of these structural and functional derangements of cerebral arterioles, in part owing to impairment of endothelial nitric oxide synthase (eNOS) activity. This oxidative stress may also contribute to the breakdown of the blood-brain barrier seen in SVD. Hypertension, aging, metabolic syndrome, smoking, hyperglycemia, and elevated homocysteine may promote activation of NADPH oxidase in cerebral arterioles. Inhibition of NADPH oxidase with phycocyanobilin from spirulina, as well as high-dose statin therapy, may have potential for prevention and control of SVD, and high-potassium diets merit study in this regard. Measures which support effective eNOS activity in other ways-exercise training, supplemental citrulline, certain dietary flavonoids (as in cocoa and green tea), and capsaicin, may also improve the function of cerebral arterioles. Asian epidemiology suggests that increased protein intakes may decrease risk for SVD; conceivably, arginine and/or cysteine-which boosts tissue glutathione synthesis, and can be administered as N-acetylcysteine-mediate this benefit. Ameliorating the risk factors for SVD-including hypertension, metabolic syndrome, hyperglycemia, smoking, and elevated homocysteine-also may help to prevent and control this syndrome, although few clinical trials have addressed this issue to date.

IGF-1 Receptor and Adhesion Signaling: An Important Axis in Determining Cancer Cell Phenotype and Therapy Resistance

IGF-1R expression and activation levels generally cannot be correlated in cancer cells, suggesting that cellular proteins may modulate IGF-1R activity. Strong candidates for such modulation are found in cell-matrix and cell-cell adhesion signaling complexes. Activated IGF-1R is present at focal adhesions, where it can stabilize β1 integrin and participate in signaling complexes that promote invasiveness associated with epithelial mesenchymal transition (EMT) and resistance to therapy. Whether IGF-1R contributes to EMT or to non-invasive tumor growth may be strongly influenced by the degree of extracellular matrix engagement and the presence or absence of key proteins in IGF-1R-cell adhesion complexes. One such protein is PDLIM2, which promotes both cell polarization and EMT by regulating the stability of transcription factors including NFκB, STATs, and beta catenin. PDLIM2 exhibits tumor suppressor activity, but is also highly expressed in certain invasive cancers. It is likely that distinct adhesion complex proteins modulate IGF-1R signaling during cancer progression or adaptive responses to therapy. Thus, identifying the key modulators will be important for developing effective therapeutic strategies and predictive biomarkers.

Diabetes and cancer

Diabetes is a worldwide health problem that has been increasingly associated with various types of cancers. Epidemiologic studies have shown an increased risk of cancer as well as a higher mortality rate in patients with type 2 diabetes (T2D). The biologic mechanisms driving the link between T2D and cancer are not well understood. In this review, various proposed mechanisms are addressed to explain the relationship between T2D and cancer. Understanding the precise mechanisms that link T2D, obesity, and the metabolic syndrome with cancer will aid in developing treatments that will reduce mortality in individuals with T2D and cancer.

Inhibition of type I insulin-like growth factor receptor tyrosine kinase by picropodophyllin induces apoptosis and cell cycle arrest in T lymphoblastic leukemia/lymphoma

It has been recently shown that the type I insulin-like growth factor receptor (IGF-IR) contributes significantly to the survival of T lymphoblastic leukemia/lymphoma (T-LBL) cells, and it was therefore suggested that IGF-IR could represent a legitimate therapeutic target in this aggressive disease. Picropodophyllin (PPP) is a potent, selective inhibitor of IGF-IR that is currently used with notable success in clinical trials that include patients with aggressive types of epithelial tumors. In the present study, we tested the effects of PPP on Jurkat and Molt-3 cells; two prototype T-LBL cell lines. Our results demonstrate that PPP efficiently induced apoptotic cell death and cell cycle arrest of these two cells. These effects were attributable to alterations of downstream target proteins. By using proteomic analysis, seven different proteins were found to be affected by PPP treatment of Jurkat cells. These proteins are involved in various aspects of cellular metabolism, cytoskeleton organization and signal transduction pathways. The results suggest that PPP affects multiple signaling molecules and inhibits fundamental pathways that control cell growth and survival. Our study also provides novel evidence that PPP could be potentially utilized for the treatment of aggressive T-LBL.

Putative tumor metastasis-associated genes in human gastric cancer

Gastric cancer is one of the leading causes of cancer mortality and its malignancy, resulting from disseminated cancer cells of diffuse type, is clinically manifested as metastases to the liver and peritoneum. The aim of the present study was to identify putative tumor metastasis-associated genes in human gastric cancer cells of diffuse type. An MKN45 cell line constitutively expressing green fluorescent protein (MKN45-GFP) was established and selected using the Transwell® system for invasive sublines MKN45-GFP-4, MKN45-GFP-10 and MKN45-GFP-12. MKN45-GFP-10 and MKN45-GFP-12 are highly invasive compared to the others. The mRNA levels were measured with cDNA microarrays and correlated with their invasion abilities in these sublines. Many of the genes identified with a positive or negative correlation are associated with angiogenesis, cell cycle, cytoskeleton and cell motility, protease and cell adhesion, as well as cellular signal transduction. In particular, novel genes without known functions were also noted. RT-PCR and western blot analyses were applied to verify the expression of selective genes. Following orthotopical intraperitoneal implantation, MKN45-GFP-12 demonstrated significantly higher in vivo tumor malignancies than parental MKN45-GFP in ascites induction and liver -invasion in mice. We have identified putative gastric tumor metastasis-associated, as well as novel genes. These genes and their protein products are to be further explored for their functional roles associated with tumor metastasis. The molecular profiles of these identified genes, gene transcripts and proteins in the patient specimens are likely to be useful biomarkers for diagnostic, therapeutic and/or prognostics. Most importantly, they may be used as molecular targets for the discovery of antitumor drugs against human gastric cancer metastasis.

Targeting the insulin-like growth factor 1 receptor (IGF1R) signaling pathway for cancer therapy

INTRODUCTION

The IGF system controls growth, differentiation, and development at the cellular, organ and organismal levels. IGF1 receptor (IGF1R) signaling is dysregulated in many cancers. Numerous clinical trials are currently assessing therapies that inhibit either growth factor binding or IGF1R itself. Therapeutic benefit, often in the form of stable disease, has been reported for many different cancer types.

AREAS COVERED

Canonical IGF signaling and non-canonical pathways involved in carcinogenesis. Three recent insights into IGF1R signaling, namely hybrid receptor formation with insulin receptor (INSR), insulin receptor substrate 1 nuclear translocation, and evidence for IGF1R/INSR as dependence receptors. Different approaches to targeting IGF1R and mechanisms of acquired resistance. Possible mechanisms by which IGF1R signaling supports carcinogenesis and specific examples in different human tumors.

EXPERT OPINION

Pre-clinical data justifies IGF1R as a target and early clinical trials have shown modest efficacy in selected tumor types. Future work will focus upon assessing the usefulness or disadvantages of simultaneously targeting the IGF1R and INSR, biomarker development to identify potentially responsive patients, and the use of IGF1R inhibitors in combination therapies or as an adjunct to conventional chemotherapy.

IGF-1 can either protect against or increase LPS-induced damage in the developing rat brain

Periventricular leukomalacia (PVL) is a major form of brain damage in premature infants. This study was to test whether IGF-1 can prevent PVL-like brain damage induced by lipopolysaccharide (LPS) in the neonatal rat. Intraventricular delivery of LPS resulted in an acute brain inflammatory response, i.e., rapid recruitment of polymorphonuclear leukocytes (PMNs), activation of microglia and astrocytes, and induction of IL-1beta (IL1beta) expression. Brain inflammation was associated with the loss of O4+ preoligodendrocytes (preOLs), a decrease of myelin basic protein (MBP) in the white matter and an increase of pyknotic cells in the cortex. IGF-1 at a low dose significantly prevented LPS-induced deleterious effects without alteration of IL-1beta expression and microglia/astrocytes activation. On the other hand, the low dose of IGF-1 enhanced LPS-induced PMNs recruitment and blood-brain barrier (BBB) permeability, and caused intracerebral hemorrhage. At higher doses, co-application of IGF-1 with LPS resulted in a high mortality rate. Brains from the surviving rats showed massive PMN infiltration and intracerebral hemorrhage. However, these adverse effects were not found in rats treated with IGF-1 alone. This study provides the alarming evidence that in an acute inflammatory condition, IGF-1 may have severe, harmful effects on the developing brain.

Inhibition of IGF-IR tyrosine kinase induces apoptosis and cell cycle arrest in imatinib-resistant chronic myeloid leukaemia cells

Although signalling through the type I insulin-like growth factor receptor (IGF-IR) maintains the survival of haematopoietic cells, a specific role of IGF-IR in haematological neoplasms remains largely unknown. Chronic myeloid leukaemia (CML) is the most common subtype of chronic myeloproliferative diseases. Typically, CML evolves as a chronic phase (CP) disease that progresses into accelerated (AP) and blast phase (BP) stages. In this study, we show that IGF-IR is universally expressed in four CML cell lines. IGF-IR was expressed in only 30% and 25% of CP and AP patients, respectively, but its frequency of expression increased to 73% of BP patients. Increased expression levels of IGF-IR with CML progression was supported by quantitative real-time PCR that demonstrated significantly higher levels of IGF-IR mRNA in BP patients. Inhibition of IGF-IR decreased the viability and proliferation of CML cell lines and abrogated their growth in soft agar. Importantly, inhibition of IGF-IR decreased the viability of cells resistant to imatinib mesylate including BaF3 cells transfected with p210 BCR-ABL mutants, CML cell lines and primary neoplastic cells from patients. The negative effects of inhibition of IGF-IR were attributable to apoptosis and cell cycle arrest due to alterations of downstream target proteins. Our findings suggest that IGF-IR could represent a potential molecular target particularly for advanced stage or imatinib-resistant cases.

IGF-IR tyrosine kinase interacts with NPM-ALK oncogene to induce survival of T-cell ALK+ anaplastic large-cell lymphoma cells

Type I insulin-like growth factor receptor (IGF-IR) tyrosine kinase plays important roles in the pathogenesis of several malignancies. Although it promotes the growth of stimulated hematopoietic cells, a direct role of IGF-IR in malignant lymphoma has not been identified. Anaplastic lymphoma kinase-positive anaplastic large-cell lymphoma (ALK(+) ALCL) is a unique type of T-cell lymphoma. Approximately 85% of ALK(+) ALCL cases harbor the translocation t(2;5)(p23;q35), which generates the chimeric oncogene NPM-ALK. In the present study, we explored a possible role of IGF-IR in ALK(+) ALCL. Our results demonstrate that IGF-IR and IGF-I are widely expressed in ALK(+) ALCL cell lines and primary tumors. Importantly, we identified novel reciprocal functional interactions between IGF-IR and NPM-ALK. Antagonism of IGF-IR decreased the viability, induced apoptosis and cell-cycle arrest, and decreased proliferation and colony formation of ALK(+) ALCL cell lines. These effects could be explained by alterations of cell survival regulatory proteins downstream of IGF-IR signaling. Our findings improve current understanding of the biology of IGF-IR and NPM-ALK and have significant therapeutic implications as they identify IGF-IR signaling as a potential therapeutic target in ALK(+) ALCL and possibly other types of malignant lymphoma.

Different expression of occludin and ZO-1 in primary and metastatic liver tumors

Tight junction (TJ) components were found to be correlated with carcinogenesis and tumor development. TJs are composed of three main integral membrane proteins; occludin, claudins and JAMs. Alteration of the TJ protein expression may play an important role in the process of cell dissociation, which is among the first steps of tumor invasion and metastasis. Reduced expression of ZO-1 has been reported to be associated with invasion of several tumors. The aim of the present study was to detect differences between occludin and ZO-1 expression in normal liver samples, HCCs and colorectal liver metastases. Expression of occludin and ZO-1 was analysed in 25 surgically removed human hepatocellular carcinomas (HCC) and 25 human colorectal liver metastases. Gene expression levels were measured by real-time RT PCR, protein expression was determined by immunohistochemistry, comparing tumors with the surrounding nontumorous parenchyma and with seven normal liver samples. Occludin and ZO-1 mRNAs showed significant downregulation in HCCs in comparison with normal liver and were also downregulated in the metastases when compared with normal liver. Occludin and ZO-1 proteins were weakly expressed on hepatocytes in normal liver, while strong expression was found on bile canaliculi. In HCCs occludin and ZO-1 did not show immunopositivity on tumor cells, while colorectal metastatic tumors revealed high levels of these molecules. HCCs and metastases are characterized by markedly different protein expression pattern of occludin and ZO-1, which phenomenon might be attributed to the different histogenesis of these tumors.

Insulin-like growth factor-I receptor, E-cadherin and alpha v integrin form a dynamic complex under the control of alpha-catenin

Dynamic crosstalk between cell adhesion molecules, extracellular matrix and soluble informative factors is essential for cancer cell migration and invasion. Here, we investigated the mechanisms by which the E-cadherin/catenin complex and alpha v integrin can modulate insulin-like growth factor-I (IGF-I)-induced cell migration. Human colon mucosa, human colon cancer cell lines, HT29-D4 and HCT-8 derivatives that differ in their expression of alpha-catenin, were used as models. Interactions between E-cadherin, alpha v integrin and IGF-I receptor (IGF-IR) were analyzed by coimmunoprecipitation and immunolocalization experiments. The impact of these interactions on cell mobility was determined by haptotaxis assays. We report that alpha v integrin, E-cadherin and IGF-IR form a ternary complex in both cultured cancer cells and human normal colonic mucosa. alpha-Catenin regulates the scaffolding of this complex. IGF-IR ligation by IGF-I induces the disruption of the complex and the relocalization of alpha v integrin from cell-cell contacts to focal contact sites. This perturbation is correlated with the observed increase in cell migration. These results suggest that regulation of the alpha v integrin/E-cadherin/IGF-IR scaffolding is essential for the modulation of cell mobility. Its alteration could be of major importance to sustain alterations in cell adhesion that occur during cancer cell invasion and metastasis.

Insulin-like growth factor system gene expression in cervical scrapes from women with squamous intraepithelial lesions and cervical cancer

BACKGROUND

There is ample evidence that the insulin-like growth factors (IGF) system is involved in the development of several types of cancer. The aim of this study was to evaluate the expression levels of IGF-I, IGF-II, IGF binding protein 3 (IGFBP-3) and IGF-I receptor (IGF-IR) in exfoliated cervical cells in cervical carcinogenesis.

METHODS

mRNA levels of IGF-I, IGF-II, IGFBP-3 and IGF-IR were assessed by real-time PCR in 105 cervical scrapes obtained from 16 patients diagnosed with low-grade squamous intraepithelial lesions (LSIL), 24 with high-grade SIL (HSIL), 23 with cervical cancer, and 42 from controls with normal Papanicolau (Pap) test.

RESULTS

IGF-I mRNA levels were very low and no significant differences were seen between control and other groups. IGF-II mRNA levels were significantly lower in LSIL than in control group (median [arbitrary units]: 0.38 vs. 2.42, P=0.006) but its expression in HSIL and cervical cancer was similar to the one observed in controls. IGFBP-3 mRNA levels were significantly lower in cancer than in controls (median [arbitrary units]: 0.43 vs. 0.73, P=0.03). We observed a decrease in IGF-IR gene expression as the SIL degree increased (median for controls, LSIL, HSIL, and cervical carcinoma [arbitrary units]: 31.24, 9.08, 8.95, and 3.56, respectively). IGF-IR mRNA levels were significantly lower in HSIL and cervical cancer in comparison with controls (P=0.043 and P<0.001, respectively).

CONCLUSIONS

The present observations suggest that a reduced expression of IGFBP-3 and IGF-IR can be associated with progression to cervical cancer; the specific role played by the IGF-IR in this process remains unclear.

Autocrine production of insulin-like growth factor-I (IGF-I) affects paracellular transport across epithelial cells in vitro

Autocrine production of growth factors can have significant effects on cell activity. We report for the first time that autocrine production of insulin-like growth factor-I (IGF-I) alters paracellular transport across bovine mammary epithelial cells in vitro. Paracellular transport was assessed by measuring phenol red transport across mammary alveolar cells-large T antigen (MAC-T cells) derived from parental mammary epithelial cells, cultured on porous membranes and compared with two different transfected MAC-T cell lines that constitutively secrete IGF-I. Phenol red transport was essentially blocked in parental cell culture after six days, while IGF-I secreting cells provided essentially no barrier. Surprisingly, neither co-culture studies between parental and IGF-I-secreting cells nor addition of exogenous IGF-I or IGF-binding protein-3 reversed the phenol red transport properties. IGF-I-secreting cells did however express lower levels of the junction components occludin and E-cadherin than parental cells, suggesting that localized autocrine IGF-I activity might lead to increased permeability via changes in both the tight and adherens junction protein levels.

Insulin-like Growth Factor-1 (IGF-1) Induces WISP-2/CCN5 via Multiple Molecular Cross-talks and Is Essential for Mitogenic Switch by IGF-1 Axis in Estrogen Receptor–Positive Breast Tumor Cells

Previously, we have shown that the expression of Wnt-1-induced signaling protein-2 (WISP-2), also known as CCN5, can be regulated by multiple stimulants in estrogen receptor (ER)-positive breast tumor cells to exert their mitogenic action in these cells. Here, we show that insulin-like growth factor-1 (IGF-1), a strong mitogen, enhanced the expression of the WISP-2/CCN5 gene parallel with the induction of proliferation of ER-positive breast tumor cells. An additive effect was also seen in combination with estrogen. Perturbation of IGF-1-induced WISP-2/CCN5 expression by WISP-2-specific RNA interference impaired the mitogenic action of IGF-1 on ER-positive breast tumor cells. Furthermore, the studies have shown that the multiple molecular cross-talks and side-talks among IGF-1R, ER-alpha, and phosphatidylinositol 3-kinase (PI3K)/Akt signaling molecules are required to induce WISP-2/CCN5 mRNA by IGF-1 in ER-positive, noninvasive breast tumor cells. Because a pure anti-ER ICI 182,780 is not only able to suppress the up-regulation of WISP-2/CCN5 mRNA expression by IGF-1, it also suppresses the PI3K/Akt activity induced by IGF-1 in MCF-7 cells; we anticipate that the membrane ER receptor may participate in this event. Collectively, these studies propose for the first time that WISP-2/CCN5 is an integral signaling molecule in mitogenic action of IGF-1 axis in ER-positive human breast tumor cells.

The role of the IGF system in cancer growth and metastasis: overview and recent insights

IGF-I receptor (IGF-IR) signaling and functions are mediated through the activities of a complex molecular network of positive (e.g., type I IGF) and negative (e.g., the type II IGF receptor, IGF-IIR) effectors. Under normal physiological conditions, the balance between the expression and activities of these molecules is tightly controlled. Changes in this delicate balance (e.g., overexpression of one effector) may trigger a cascade of molecular events that can ultimately lead to malignancy. In recent years, evidence has been mounting that the IGF axis may be involved in human cancer progression and can be targeted for therapeutic intervention. Here we review old and more recent evidence on the role the IGF system in malignancy and highlight experimental and clinical studies that provide novel insights into the complex mechanisms that contribute to its oncogenic potential. Controversies arising from conflicting evidence on the relevance of IGF-IR and its ligands to human cancer are discussed. Our review highlights the importance of viewing the IGF axis as a complex multifactorial system and shows that changes in the expression levels of any one component of the axis, in a given malignancy, should be interpreted with caution and viewed in a wider context that takes into account the expression levels, state of activation, accessibility, and functionality of other interacting components. Because IGF targeting for anticancer therapy is rapidly becoming a clinical reality, an understanding of this complexity is timely because it is likely to have an impact on the design, mode of action, and clinical outcomes of newly developed drugs.

Sodium/potasium ATPase (Na+, K+-ATPase) and ouabain/related cardiac glycosides: a new paradigm for development of anti- breast cancer drugs?

Prolonged exposure to 17beta-estradiol (E2) is a key etiological factor for human breast cancer. The biological effects and carcinogenic effects of E2 are mediated via estrogen receptors (ERs), ERalpha and ERbeta. Anti-estrogens, e.g. tamoxifen, and aromatase inhibitors have been used to treat ER-positive breast cancer. While anti-estrogen therapy is initially successful, a major problem is that most tumors develop resistance and the disease ultimately progresses, pointing to the need of developing alternative drugs targeting to other critical targets in breast cancer cells. We have identified that Na+, K+-ATPase, a plasma membrane ion pump, has unique/valuable properties that could be used as a potentially important target for breast cancer treatment: (a) it is a key player of cell adhesion and is involved in cancer progression; (b) it serves as a versatile signal transducer and is a target for a number of hormones including estrogens and (d) its aberrant expression and activity are implicated in the development and progression of breast cancer. There are several lines of evidence indicating that ouabain and related digitalis (the potent inhibitors of Na+, K+-ATPase) possess potent anti-breast cancer activity. While it is not clear how the suggested anti-cancer activity of these drugs work, several observations point to ouabain and digitalis as being potential ER antagonists. We critically reviewed many lines of evidence and postulated a novel concept that Na+, K+-ATPase in combination with ERs could be important targets of anti-breast cancer drugs. Modulators, e.g. ouabain and related digitalis could be useful to develop valuable anti-breast cancer drugs as both Na+, K+-ATPase inhibitors and ER antagonists.

Up-regulated expression of zonula occludens protein-1 in human melanoma associates with N-cadherin and contributes to invasion and adhesion

During the process of malignant transformation, nascent melanoma cells escape keratinocyte control through down-regulation of E-cadherin and instead communicate among themselves and with fibroblasts via N-cadherin-based cell-cell contacts. The zonula occludens (ZO) protein-1 is a membrane-associated component of both the tight and adherens junctions found at sites of cell-cell contact. In most cancers, levels of ZO-1 are typically down-regulated, leading to increased motility. Here we report the novel observation that ZO-1 expression is up-regulated in melanoma cells and is located at adherens junctions between melanoma cells and fibroblasts. Immunofluorescence and co-immunoprecipitation studies showed co-localization of ZO-1 with N-cadherin. Down-regulation of ZO-1 in melanoma cells through RNA interference produced marked changes in cell morphology--leading to a less-dendritic, more rounded phenotype. Consistent with a role in N-cadherin-based adhesion, RNAi-treated melanoma cells were less adherent and invasive when grown in a collagen gel. These data provide the first evidence that increased ZO-1 expression in melanoma contributes to the oncogenic behavior of this tumor and further illustrate that protein products of genes, such as ZO-1, can function in either a pro- or anti-oncogenic manner when expressed in different cellular contexts.

Gene expression profiles in cells transformed by overexpression of the IGF-I receptor

To identify genes associated with insulin-like growth factor-I receptor (IGF-IR)-mediated cellular transformation, we isolated genes that are differentially expressed in R- cells (derived from the IGF-IR knockout mouse) and R+ cells (R- cells that overexpress the IGF-IR). From these, 45 genes of known function were expressed at higher levels in R+ cells and 22 were expressed at higher levels in R- cells. Differential expression was confirmed by Northern blot analysis of R+ and R- cells. Genes expressed more abundantly in R+ cells are associated with (1) tumour growth and metastasis including, betaigH3, mts1, igfbp5 protease, and mystique; (2) cell division, including cyclin A1 and cdk1; (3) signal transduction, including pkcdeltabp and lmw-ptp; and (4) metabolism including ATPase H+ transporter and ferritin. In MCF-7 cells IGF-I induced expression of two genes, lasp-1 and mystique, which could contribute to metastasis. Lasp-1 expression required activity of the PI3-kinase signalling pathway. Mystique was highly expressed in metastatic but not in androgen-dependent prostate cancer cell lines and Mystique overexpression in MCF-7 cells promoted cell migration and invasion. We conclude that genes identified in this screen may mediate IGF-IR function in cancer progression.

Mystique is a new insulin-like growth factor-I-regulated PDZ-LIM domain protein that promotes cell attachment and migration and suppresses Anchorage-independent growth

By comparing differential gene expression in the insulin-like growth factor (IGF)-IR null cell fibroblast cell line (R- cells) with cells overexpressing the IGF-IR (R+ cells), we identified the Mystique gene expressed as alternatively spliced variants. The human homologue of Mystique is located on chromosome 8p21.2 and encodes a PDZ LIM domain protein (PDLIM2). GFP-Mystique was colocalized at cytoskeleton focal contacts with alpha-actinin and beta1-integrin. Only one isoform of endogenous human Mystique protein, Mystique 2, was detected in cell lines. Mystique 2 was more abundant in nontransformed MCF10A breast epithelial cells than in MCF-7 breast carcinoma cells and was induced by IGF-I and cell adhesion. Overexpression of Mystique 2 in MCF-7 cells suppressed colony formation in soft agarose and enhanced cell adhesion to collagen and fibronectin. Point mutation of either the PDZ or LIM domain was sufficient to reverse suppression of colony formation, but mutation of the PDZ domain alone was sufficient to abolish enhanced adhesion. Knockdown of Mystique 2 with small interfering RNA abrogated both adhesion and migration in MCF10A and MCF-7 cells. The data indicate that Mystique is an IGF-IR-regulated adapter protein located at the actin cytoskeleton that is necessary for the migratory capacity of epithelial cells.

IGF-I receptor, cell-cell adhesion, tumour development and progression

The insulin-like growth factor I receptor (IGF-IR) has been implicated in the development and progression of many common cancers and other neoplastic diseases. The tumorigenic potential of IGF-IR relies on its antiapoptotic and transforming activities. The molecular mechanisms by which IGF-IR controls the proliferation and survival of tumour cells have been extensively studied and many pathways have been delineated. However, the role of IGF-IR in the regulation of non-mitogenic cell functions is less well understood. Here we focus on IGF-IR-dependent cell-cell adhesion. Limited studies suggested that IGF-IR can regulate cell aggregation and intercellular adhesion mediated by cadherins and cadherin-associated proteins. We review the mechanisms of this process and discuss the impact of IGF-IR-dependent cell-cell adhesion on the phenotype of tumour cells.

Association of ARVCF with zonula occludens (ZO)-1 and ZO-2: binding to PDZ-domain proteins and cell-cell adhesion regulate plasma membrane and nuclear localization of ARVCF

ARVCF, an armadillo-repeat protein of the p120(ctn) family, associates with classical cadherins and is present in adherens junctions, but its function is poorly understood. Here, we show that ARVCF interacts via a C-terminal PDZ-binding motif with zonula occludens (ZO)-1 and ZO-2. ARVCF and ZO-1 partially colocalize in the vicinity of the apical adhesion complex in polarized epithelial Madin-Darby canine kidney cells. ARVCF, ZO-1, and E-cadherin form a complex and are recruited to sites of initial cell-cell contact in sparse cell cultures. E-cadherin binding and plasma membrane localization of ARVCF require the PDZ-binding motif. Disruption of cell-cell adhesion releases ARVCF from the plasma membrane and an increased fraction of the protein localizes to the nucleus. Nuclear localization of ARVCF also requires the PDZ-binding motif and can be mediated by the PDZ domains of ZO-2. Thus, the interaction of ARVCF with distinct PDZ-domain proteins determines its subcellular localization. Interactions with ZO-1 and ZO-2, in particular, may mediate recruitment of ARVCF to the plasma membrane and the nucleus, respectively, possibly in response to cell-cell adhesion cues.

Breast cancer metastatic potential: correlation with increased heterotypic gap junctional intercellular communication between breast cancer cells and osteoblastic cells

The breast cancer metastasis-suppressor gene BRMS1 is downregulated in metastatic breast cancer cells. Previous reports have shown restoration of gap junctional intercellular communication (GJIC) in the metastatic human breast carcinoma cell line MDA-MB-435 (435) transfected with BRMS1 cDNA. Metastasis, to a large extent in most breast cancers, occurs to bone. However, the reason for this preferential metastasis is not known. We explored cell-to-cell communication between 435 carcinoma cells and a human osteoblastic cell line, hFOB1.19, to determine whether carcinoma cells can form gap junctions with bone cells and to explore the role of these heterotypic gap junctions and the BRMS1 gene in breast cancer metastasis to bone. 435 cells displayed greater cell-to-cell communication with hFOB 1.19 cells than with themselves. Transfection of BRMS1 into 435 cells increased homotypic gap junctional communication but did not significantly affect heterotypic communication with hFOBs. However, heterotypic communication of BRMS1 transfectants with hFOB cells was reduced relative to homotypic communication. In contrast, parental 435 cells displayed greater heterotypic communication with hFOBs relative to homotypic communication. Our results suggest that there are differences in the relative homotypic and heterotypic GJIC of metastasis-capable and -suppressed cell lines.

Insulin-like growth factor-I plays a pathogenetic role in diabetic retinopathy

Diabetic retinopathy is a leading cause of blindness in the Western world. Aberrant intercellular adhesion molecule-1 expression and leukocyte adhesion have been implicated in its pathogenesis, raising the possibility of an underlying chronic inflammatory mechanism. In the current study, the role of insulin-like growth factor (IGF)-I in these processes was investigated. We found that systemic inhibition of IGF-I signaling with a receptor-neutralizing antibody, or with inhibitors of PI-3 kinase (PI-3K), c-Jun kinase (JNK), or Akt, suppressed retinal Akt, JNK, HIF-1alpha, nuclear factor (NF)-kappaB, and AP-1 activity, vascular endothelial growth factor (VEGF) expression, as well as intercellular adhesion molecule-1 levels, leukostasis, and blood-retinal barrier breakdown, in a relevant animal model. Intravitreous administration of IGF-I increased retinal Akt, JNK, HIF-1alpha, NF-kappaB, and AP-1 activity, and VEGF levels. IGF-I stimulated VEGF promoter activity in vitro, mainly via HIF-1alpha, and secondarily via NF-kappaB and AP-1. In conclusion, IGF-I participates in the pathophysiology of diabetic retinopathy by inducing retinal VEGF expression via PI-3K/Akt, HIF-1alpha, NF-kappaB, and secondarily, JNK/AP-1 activation. Taken together, these in vitro and in vivo signaling studies thus identify potential targets for pharmacological intervention to preserve vision in patients with diabetes.

Mice lacking insulin or insulin-like growth factor 1 receptors in vascular endothelial cells maintain normal blood–brain barrier

The blood-brain barrier (BBB) is created by a combination of endothelial cells with tight junctions and astrocytes. One of the key tight junction proteins, zona occludens-1 (ZO-1), has been reported to be stimulated in its expression by insulin and IGF-1. To assess the role of insulin and IGF-1 in endothelial cells in the BBB we have utilized mice with a vascular endothelial cell-specific knockout of the insulin receptor (VENIRKO) and IGF-1 receptor (VENIFARKO). Both of these mice show a normal BBB based on no increase in leakage of Evans blue dye in the brain of these mice basally or after cold injury. Furthermore, the structural integrity of the BBB and blood-retinal barrier (BRB) was intact using the vascular markers lectin B-4 and ZO-1, and both proteins were properly co-localized in both brain and retinal vascular tissue of these mice. These observations indicate that neither insulin nor IGF-1 signaling in vascular endothelial cells is required for development and maintenance of BBB or BRB.

Alpha-catenin is required for IGF-I-induced cellular migration but not invasion in human colonic cancer cells

The mechanisms by which growth factors cooperate with cell adhesion molecules to modulate epithelial cell motility remain poorly understood. Here, we investigated the role of the E-cadherin/catenin complex in insulin-like growth factor (IGF-I)-dependent cell migration and invasion. We used variants of the HCT-8 colon cancer family that differ in their expression of alphaE-catenin, an intracellular molecule that links the E-cadherin/catenin complex to the actin cytoskeleton. Migration was determined using a monolayer wound model and cell invasion by the penetration of the cells into type-I collagen gels. We showed that alpha-catenin-deficient cells were not able to migrate in cohort upon IGF-I stimulation. Transfection of these cells with alpha-catenin isoforms (alphaN- or alphaT-catenin) restored migratory response IGF-I. These results suggest that alpha-catenins are involved in the signal issued from the E-cadherin/catenin complex to regulate IGF-I-stimulated migration. In contrast, IGF-I promoted invasion of both alpha-catenin-deficient and alpha-catenin-expressing cells, indicating that alpha-catenin did not participate in the regulation of IGF-I-induced invasion. Inhibition of E-cadherin function by treatment with MB-2 monoclonal antibodies inhibited both IGF-I-dependent cell migration and invasion. Taken together, our results indicate that functional alpha-catenin is essential for migration but not for invasion, while E-cadherin is involved in both phenomena.

A Dominant Negative Type I Insulin-like Growth Factor Receptor Inhibits Metastasis of Human Cancer Cells

We have previously shown that LCC6 wild-type (WT) cells, a metastatic variant of MDA-MB-435 cancer cells originally derived from a breast cancer patient, exhibit enhanced motility in response to IGF-I compared with the parent MDA-MB-435 cells. To further understand the role of the type I insulin-like growth factor (IGF) receptor (IGF1R) in cancer metastasis we inhibited signaling via IGF1R using a C-terminal-truncated IGF1R. The truncated receptor retains the ligand binding domain but lacks the autophosphorylated tyrosine residues in the carboxyl terminus. Cells stably transfected with this truncated receptor (LCC6-DN cells) overexpressed the truncated IGF1R messenger RNA nearly 50-fold over endogenous receptor. The truncated receptor in the LCC6-DN cells behaved in a dominant negative manner to inhibit endogenous IGF1R activation by IGF-I. Compared with the LCC6-WT cells, LCC6-DN cells failed to phosphorylate the adaptor proteins insulin receptor substrate-1 and -2 in response to IGF-I and did not activate Akt after exposure to IGF-I. Unlike LCC6-WT cells, LCC6-DN cells did not show enhanced motility in response to IGF-I. To assay for metastasis, LCC6-WT and LCC6-DN cells were injected into the mammary fat pads of mice, and the primary xenograft tumors were removed after 21 days. Mice sacrificed 5 weeks later showed multiple lung metastases derived from LCC-WT xenografts, whereas mice harboring LCC6-DN xenografts showed no lung metastases. Our data show that IGF1R can regulate several aspects of the malignant phenotype. In these cells, metastasis but not proliferation requires IGF1R function.

The organization of tight junctions in epithelia: implications for mammary gland biology and breast tumorigenesis

Tight junctions (TJs), the most apical components of the cell-cell junctional complexes, play a crucial role in the establishment and maintenance of cell polarity within tissues. In secretory glandular tissues, such as the mammary gland, TJs are crucial for separating apical and basolateral domains. TJs also create the variable barrier regulating paracellular movement of molecules through epithelial sheets, thereby maintaining tissue homeostasis. Recent advances reveal that TJs exist as macromolecular complexes comprised of several types of membrane proteins, cytoskeletal proteins, and signaling molecules. Many of these components are regulated during mammary gland development and pregnancy cycles, and several have received much attention as possible "tumor suppressors" during progression to breast cancer.

Bcl-2 expression decreases cadherin-mediated cell-cell adhesion

Bcl-2, a member of the apoptosis-regulating family of proteins confers a survival advantage on cells by inhibiting apoptosis. Bcl-2 expression is estrogen-responsive and high in various tumors. Overexpression of Bcl-2 has been associated with the loss of contact inhibition, unregulated growth and foci formation in culture. In this study, we have examined the effects of bcl-2 overexpression and expression on cell-cell adhesion in MCF-7 and MDCK epithelial cell lines respectively. Overexpression of Bcl-2 in estrogen receptor-positive MCF-7 mammary carcinoma cells led to decreased cell surface E-cadherin and the disruption of junctional complexes concurrent with intracellular redistribution of their components. Particularly noticeable, was the partial nuclear localization of the tight junction-associated protein ZO-1 which coincided with upregulation of ErbB2. The expression of this EGF co-receptor is regulated by the ZO-1-associated transcription factor ZONAB. Growth in estrogen-depleted media led to downregulation of Bcl-2 expression and upregulation and membrane localization of all junctional proteins. Similar disruption in junctions, accompanied by decreased transepithelial resistance, was observed when Bcl-2 was expressed in MDCK cells. These results strongly suggest that Bcl-2 expression decreases the level of functional E-cadherin thereby interfering with junction formation. The inhibition of junction formation decreases cell-cell adhesion leading to the loss of contact inhibition, which, in vivo, can lead to unregulated growth and tumorigenesis.

The association between integrin-associated protein and SHPS-1 regulates insulin-like growth factor-I receptor signaling in vascular smooth muscle cells

Growth factor signaling is usually analyzed in isolation without considering the effect of ligand occupancy of transmembrane proteins other than the growth factor receptors themselves. In smooth muscle cells, the transmembrane protein Src homology 2 domain containing protein tyrosine phosphatase substrate-1 (SHPS-1) has been shown to be an important regulator of insulin-like growth factor-I (IGF-I) signaling. SHPS-1 is phosphorylated in response to IGF-I, leading to recruitment of Src homology 2 domain tyrosine phosphatase (SHP-2). Subsequently, SHP-2 is transferred to IGF-I receptor and regulates the duration of IGF-I receptor phosphorylation. Whether ligand occupancy of SHPS-1 influences SHPS-1 phosphorylation or SHP-2 recruitment, thereby altering growth factor signaling, is unknown. Previous studies have shown that integrin associated protein (IAP) associates with SHPS-1. We undertook these studies to determine whether this interaction controlled SHPS-1 phosphorylation and/or SHP-2 recruitment and thereby regulated IGF-I signaling. Disruption of IAP-SHPS-1 binding, by using an IAP monoclonal antibody or cells expressing mutant forms of IAP that did not bind to SHPS-1, inhibited IGF-I-stimulated SHPS-1 phosphorylation and SHP-2 recruitment. This was associated with a lack of SHP-2 transfer to IGF-I receptor and sustained receptor phosphorylation. This resulted in an inability of IGF-I to stimulate sustained mitogen-activated protein kinase activation, cell proliferation, and cell migration. The effect was specific for IGF-I because disruption of the IAP-SHPS-1 interaction had no effect on platelet-derived growth factor-stimulated SHPS-1 phosphorylation or cell migration. In summary, our results show that 1) ligand occupancy of SHPS-1 is a key determinant of its ability to be phosphorylated after IGF-I stimulation, and 2) the interaction between IAP and SHPS-1 is an important regulator of IGF-I signaling because disruption of the results in impaired SHP-2 recruitment and subsequent inhibition of IGF-I-stimulated cell proliferation and migration.

The GH-IGF-I axis and breast cancer

The growth hormone-insulin-like growth factor-I (GH-IGF-I) axis plays a fundamental role in the development of the breast. The maintenance of breast tissue architecture is aided by its effect on proliferation, differentiation and apoptosis. There has been increasing recognition of its role as a major determinant of breast cancer and, more recently, its involvement in the development of resistance to both tamoxifen and an important novel therapy for advanced disease, trastuzumab (Herceptin). Here, we discuss the influence of the GH-IGF-I axis in normal mammary development and homeostasis, its putative role in breast tumorigenesis and its interactions with estrogen signalling.

Tight junction proteins

A fundamental function of epithelia and endothelia is to separate different compartments within the organism and to regulate the exchange of substances between them. The tight junction (TJ) constitutes the barrier both to the passage of ions and molecules through the paracellular pathway and to the movement of proteins and lipids between the apical and the basolateral domains of the plasma membrane. In recent years more than 40 different proteins have been discovered to be located at the TJs of epithelia, endothelia and myelinated cells. This unprecedented expansion of information has changed our view of TJs from merely a paracellular barrier to a complex structure involved in signaling cascades that control cell growth and differentiation. Both cortical and transmembrane proteins integrate TJs. Among the former are scaffolding proteins containing PDZ domains, tumor suppressors, transcription factors and proteins involved in vesicle transport. To date two components of the TJ filaments have been identified: occludin and claudin. The latter is a protein family with more than 20 members. Both occludin and claudins are integral proteins capable of interacting adhesively with complementary molecules on adjacent cells and of co-polymerizing laterally. These advancements in the knowledge of the molecular structure of TJ support previous physiological models that exhibited TJ as dynamic structures that present distinct permeability and morphological characteristics in different tissues and in response to changing natural, pathological or experimental conditions.