Selective modulation of type 1 insulin-like growth factor receptor signaling and functions by beta1 integrins

β1-integrin controls IGF-1R internalization and intracellular signaling

Cell adhesion-dependent phosphorylation of insulin-like growth factor 1 receptor (IGF-1R) on its C-terminal tail (CT) at Tyr1250/1251 promotes receptor internalization and Golgi accumulation. We previously proposed that this phosphorylation is associated with cell migration and cancer aggressiveness, distinguishing IGF-1R activity from that of insulin receptor, which lacks these tyrosines. Here, we further investigated how adhesion signaling influences IGF-1R location and activity in migratory cancer cells and R- fibroblasts. We observed that IGF-1R, in triple-negative breast cancer tissues, is predominantly intracellular and dispersed from the plasma membrane compared with nontumor tissue. Datasets from basal-like breast cancer patients indicated a strong, positive correlation between IGF-1R protein expression and that of β1-integrin (ITGB1). In triple-negative breast cancer cells with high ITGB1 expression, suppressing ITGB1 enhanced IGF-1R stability and its retention at the plasma membrane, and reduced IGF-1R internalization during cell adhesion. In R- fibroblasts, we observed reduced IGF-1R autophosphorylation and Golgi accumulation when ITGB1 was suppressed. The stability of a Tyr1250/1251Phe (FF) IGF-1R mutant was less affected by ITGB1 suppression, indicating that Tyr1250/1251 phosphorylation is required for ITGB1-enhanced receptor internalization. Furthermore, a Tyr1250/1251Glu (EE) IGF-1R mutant exhibited a gain of cell migration and colony formation potential compared to WT IGF-1R or FF mutant. Tyr1250/1251 resides within the CT 1248SFYYS1252 motif, which engages the IGF-1R kinase domain. In silico, we investigated how mutation of these tyrosines may alter 1248SFYYS1252 conformation, dictating trajectory of the distal CT. We conclude that Tyr1250/1251 phosphorylation confers IGF-1R with unique protumorigenic signaling in a manner that is enhanced by ITGB1.

Insulin-like growth factor family and prostate cancer: new insights and emerging opportunities

Prostate cancer is the second most commonly diagnosed cancer in men. The mammalian insulin-like growth factor (IGF) family is made up of three ligands (IGF-I, IGF-II, and insulin), three receptors (IGF-I receptor (IGF-1R), insulin receptor (IR), and IGF-II receptor (IGF-2R)), and six IGF-binding proteins (IGFBPs). IGF-I and IGF-II were identified as potent mitogens and were previously associated with an increased risk of cancer development including prostate cancer. Several reports showed controversy about the expression of the IGF family and their connection to prostate cancer risk due to the high degree of heterogeneity among prostate tumors, sampling bias, and evaluation techniques. Despite that, it is clear that several IGF family members play a role in prostate cancer development, metastasis, and androgen-independent progression. In this review, we aim to expand our understanding of prostate tumorigenesis and regulation through the IGF system. Further understanding of the role of IGF signaling in PCa shows promise and needs to be considered in the context of a comprehensive treatment strategy.

Functional Redundancy between β1 and β3 Integrin in Activating the IR/Akt/mTORC1 Signaling Axis to Promote ErbB2-Driven Breast Cancer

Integrin receptors coordinate cell adhesion to the extracellular matrix (ECM) to facilitate many cellular processes during malignant transformation. Despite their pro-tumorigenic roles, therapies targeting integrins remain limited. Here, we provide genetic evidence supporting a functional redundancy between β1 and β3 integrin during breast cancer progression. Although ablation of β1 or β3 integrin alone has limited effects on ErbB2-driven mammary tumorigenesis, deletion of both receptors resulted in a significant delay in tumor onset with a corresponding impairment in lung metastasis. Mechanistically, stiff ECM cooperates with integrin receptors to recruit insulin receptors (IRs) to focal adhesion through the formation of integrin/IR complexes, thereby preventing their lysosomal degradation. β1/β3 integrin-deficient tumors that eventually emerged exhibit impaired Akt/mTORC1 activity. Murine and human breast cancers exhibiting enhanced integrin-dependent activity also display elevated IR/Akt/mTORC1 signaling activity. Together, these observations argue that integrin/IR crosstalk transduces mechanical cues from the tumor microenvironment to promote ErbB2-dependent breast cancer progression.

Myoblasts With Higher IRS-1 Levels Are Eliminated From the Normal Cell Layer During Differentiation

Insulin receptor substrate (IRS)-1 is a major substrate of insulin-like growth factor (IGF)-I receptors. It is well-known that IGF-I and II play essential roles in myogenesis progression. Herein, we report an unexpected phenomenon that IRS-1-overexpressing L6 myoblasts are eliminated from normal cell layers at the beginning of differentiation. Initially, the IRS protein level and apoptosis were examined during myogenic differentiation in L6 myoblasts. We found that the IRS-1 protein level decreased, whereas active caspase 3 increased around 1 day after induction of differentiation. The addition of a pan-caspase inhibitor, Z-VAD-FMK, inhibited differentiation-induced suppression of the IRS-1 protein level. Apoptosis was not enhanced in L6 myoblasts stably expressing high levels of IRS-1 (L6-IRS-1). However, when L6-IRS-1 was cultured with control cells (L6-mock), we observed that L6-IRS-1 was eliminated from the cell layer. We have recently reported that, in L6-IRS-1, internalization of the IGF-I receptor was delayed and IGF signal activation was sustained for a longer period than in L6-mock. When cells stably expressing IRS-1 3YA mutant, which could not maintain the IGF signals, were cultured with normal cells, elimination from the cell layer was not detected. These data suggested that the high level of IRS-1 in myoblasts induces elimination from the cell layer due to abnormal sustainment of IGF-I receptor activation.

αvβ3 Integrin Mediates Radioresistance of Prostate Cancer Cells through Regulation of Survivin

The α_vβ₃ integrin is involved in various physiologic and pathologic processes such as wound healing, angiogenesis, tumor growth, and metastasis. The impact of α_vβ₃ integrin on the radiosensitivity of prostate cancer cells and the molecular mechanism controlling cell survival in response to ionizing radiation (IR) was investigated. Both LNCaP cells stably transfected with α_vβ₃ integrin and PC-3 cells that contain endogenous β₃ integrin were used. This study demonstrated that α_vβ₃ integrin increases survival of α_vβ₃-LNCaP cells upon IR while small hairpin RNA (shRNA)-mediated knockdown of α_vβ₃ integrin in PC-3 cells sensitizes to radiation. Expression of α_vβ₃ integrin in LNCaP cells also enhances anchorage-independent cell growth while knockdown of α_vβ₃ integrin in PC-3 cells inhibits anchorage-independent cell growth. The α_vβ₃ antagonist, cRGD, significantly increases radiosensitivity in both α_vβ₃-LNCaP and PC-3 cells. Moreover, α_vβ₃ integrin prevents radiation-induced downregulation of survivin. Inhibition of survivin expression by siRNA or shRNA enhances IR-induced inhibition of anchorage-independent cell growth. Overexpression of wild-type survivin in PC-3 cells treated with α_vβ₃ integrin shRNA increases survival of cells upon IR. These findings reveal that α_vβ₃ integrin promotes radioresistance and regulates survivin levels in response to IR. IMPLICATIONS: Future translational research on targeting α_vβ₃ integrin and survivin may reveal novel approaches as an adjunct to radiotherapy for patients with prostate cancer.

Therapeutic Implications for Overcoming Radiation Resistance in Cancer Therapy

Ionizing radiation (IR), such as X-rays and gamma (γ)-rays, mediates various forms of cancer cell death such as apoptosis, necrosis, autophagy, mitotic catastrophe, and senescence. Among them, apoptosis and mitotic catastrophe are the main mechanisms of IR action. DNA damage and genomic instability contribute to IR-induced cancer cell death. Although IR therapy may be curative in a number of cancer types, the resistance of cancer cells to radiation remains a major therapeutic problem. In this review, we describe the morphological and molecular aspects of various IR-induced types of cell death. We also discuss cytogenetic variations representative of IR-induced DNA damage and genomic instability. Most importantly, we focus on several pathways and their associated marker proteins responsible for cancer resistance and its therapeutic implications in terms of cancer cell death of various types and characteristics. Finally, we propose radiation-sensitization strategies, such as the modification of fractionation, inflammation, and hypoxia and the combined treatment, that can counteract the resistance of tumors to IR.

αvβ6 integrin is required for TGFβ1-mediated matrix metalloproteinase2 expression

Transforming growth factor (TGF) β1 activity depends on a complex signalling cascade that controls expression of several genes. Among others, TGFβ1 regulates expression of matrix metalloproteinases (MMPs) through activation of Smads. In the present study, we demonstrate for the first time that the αvβ6 integrin interacts with TGFβ receptor II (TβRII) through the β6 cytoplasmic domain and promotes Smad3 activation in prostate cancer (PrCa) cells. Another related αv integrin, αvβ5, as well as the αvβ6/3 integrin, which contains a chimeric form of β6 with a β3 cytoplasmic domain, do not associate with TβRII and fail to show similar responses. We provide evidence that αvβ6 is required for up-regulation of MMP2 by TGFβ1 through a Smad3-mediated transcriptional programme in PrCa cells. The functional relevance of these results is underscored by the finding that αvβ6 modulates cell migration in an MMP2-dependent manner on an αvβ6-specific ligand, latency-associated peptide (LAP)-TGFβ. Overall, these mechanistic studies establish that expression of a single integrin, αvβ6, is sufficient to promote activation of Smad3, regulation of MMP2 levels and consequent catalytic activity, as well as cell migration. Our study describes a new TGFβ1-αvβ6-MMP2 signalling pathway that, given TGFβ1 pro-metastatic activity, may have profound implications for PrCa therapy.

Calreticulin affects cell adhesiveness through differential phosphorylation of insulin receptor substrate-1

Cellular adhesion to the underlying substratum is regulated through numerous signaling pathways. It has been suggested that insulin receptor substrate 1 (IRS-1) is involved in some of these pathways, via association with and activation of transmembrane integrins. Calreticulin, as an important endoplasmic reticulum-resident, calcium-binding protein with a chaperone function, plays an obvious role in proteomic expression. Our previous work showed that calreticulin mediates cell adhesion not only by affecting protein expression but also by affecting the state of regulatory protein phosphorylation, such as that of c-src. Here, we demonstrate that calreticulin affects the abundance of IRS-1 such that the absence of calreticulin is paralleled by a decrease in IRS-1 levels and the unregulated overexpression of calreticulin is accompanied by an increase in IRS-1 levels. These changes in the abundance of calreticulin and IRS-1 are accompanied by changes in cell-substratum adhesiveness and phosphorylation, such that increases in the expression of calreticulin and IRS-1 are paralleled by an increase in focal contact-based cell-substratum adhesiveness, and a decrease in the expression of these proteins brings about a decrease in cell-substratum adhesiveness. Wild type and calreticulin-null mouse embryonic fibroblasts (MEFs) were cultured and the IRS-1 isoform profile was assessed. Differences in morphology and motility were also quantified. While no substantial differences in the speed of locomotion were found, the directionality of cell movement was greatly promoted by the presence of calreticulin. Calreticulin expression was also found to have a dramatic effect on the phosphorylation state of serine 636 of IRS-1, such that phosphorylation of IRS-1 on serine 636 increased radically in the absence of calreticulin. Most importantly, treatment of cells with the RhoA/ROCK inhibitor, Y-27632, which among its many effects also inhibited serine 636 phosphorylation of IRS-1, had profound effects on cell-substratum adhesion, in that it suppressed focal contacts, induced extensive close contacts, and increased the strength of adhesion. The latter effect, while counterintuitive, can be explained by the close contacts comprising labile bonds but in large numbers. In addition, the lability of bonds in close contacts would permit fast locomotion. An interesting and novel finding is that Y-27632 treatment of MEFs releases them from contact inhibition of locomotion, as evidenced by the invasion of a cell's underside by the thin lamellae and filopodia of a cell in close apposition.

Integrin αvβ6 promotes an osteolytic program in cancer cells by upregulating MMP2

The molecular circuitries controlling osseous prostate metastasis are known to depend on the activity of multiple pathways, including integrin signaling. Here, we demonstrate that the αvβ6 integrin is upregulated in human prostate cancer bone metastasis. In prostate cancer cells, this integrin is a functionally active receptor for fibronectin and latency-associated peptide-TGF-β1; it mediates attachment and migration upon ligand binding and is localized in focal contacts. Given the propensity of prostate cancer cells to form bone metastatic lesions, we investigated whether the αvβ6 integrin promotes this type of metastasis. We show for the first time that αvβ6 selectively induces matrix metalloproteinase 2 (MMP2) in vitro in multiple prostate cancer cells and promotes osteolysis in vivo in an immunodeficient mouse model of bone metastasis through upregulation of MMP2, but not MMP9. The effect of αvβ6 on MMP2 expression and activity is independent of androgen receptor in the analyzed prostate cancer cells. Increased levels of parathyroid hormone-related protein (PTHrP), known to induce osteoclastogenesis, were also observed in αvβ6-expressing cells. However, by using MMP2 short hairpin RNA, we demonstrate that the αvβ6 effect on bone loss is due to upregulation of soluble MMP2 by the cancer cells, not due to changes in tumor growth rate. Another related αv-containing integrin, αvβ5, fails to show similar responses, underscoring the significance of αvβ6 activity. Overall, these mechanistic studies establish that expression of a single integrin, αvβ6, contributes to the cancer cell-mediated program of osteolysis by inducing matrix degradation through MMP2. Our results open new prospects for molecular therapy for metastatic bone disease.

Something old, something new and something borrowed: emerging paradigm of insulin-like growth factor type 1 receptor (IGF-1R) signaling regulation

The insulin-like growth factor type 1 receptor (IGF-1R) plays a key role in the development and progression of cancer; however, therapeutics targeting it have had disappointing results in the clinic. As a receptor tyrosine kinase (RTK), IGF-1R is traditionally described as an ON/OFF system, with ligand stabilizing the ON state and exclusive kinase-dependent signaling activation. Newly added to the traditional model, ubiquitin-mediated receptor downregulation and degradation was originally described as a response to ligand/receptor interaction and thus inseparable from kinase signaling activation. Yet, the classical model has proven over-simplified and insufficient to explain experimental evidence accumulated over the last decade, including kinase-independent signaling, unbalanced signaling, or dissociation between signaling and receptor downregulation. Based on the recent findings that IGF-1R “borrows” components of G-protein coupled receptor (GPCR) signaling, including β-arrestins and G-protein-related kinases, we discuss the emerging paradigm for the IGF-1R as a functional RTK/GPCR hybrid, which integrates the kinase signaling with the IGF-1R canonical GPCR characteristics. The contradictions to the classical IGF-1R signaling concept as well as the design of anti-IGF-1R therapeutics treatment are considered in the light of this paradigm shift and we advocate recognition of IGF-1R as a valid target for cancer treatment.

β1 integrins mediate resistance to ionizing radiation in vivo by inhibiting c-Jun amino terminal kinase 1

This study was carried out to dissect the mechanism by which β1 integrins promote resistance to radiation. For this purpose, we conditionally ablated β1 integrins in the prostatic epithelium of transgenic adenocarcinoma of mouse prostate (TRAMP) mice. The ability of β1 to promote resistance to radiation was also analyzed by using an inhibitory antibody to β1 , AIIB2, in a xenograft model. The role of β1 integrins and of a β1 downstream target, c-Jun amino-terminal kinase 1 (JNK1), in regulating radiation-induced apoptosis in vivo and in vitro was studied. We show that β1 integrins promote prostate cancer (PrCa) progression and resistance to radiation in vivo. Mechanistically, β1 integrins are shown here to suppress activation of JNK1 and, consequently apoptosis, in response to irradiation. Downregulation of JNK1 is necessary to preserve the effect of β1 on resistance to radiation in vitro and in vivo. Finally, given the established crosstalk between β1 integrins and type1 insulin-like growth factor receptor (IGF-IR), we analyzed the ability of IGF-IR to modulate β1 integrin levels. We report that IGF-IR regulates the expression of β1 integrins, which in turn confer resistance to radiation in PrCa cells. In conclusion, this study demonstrates that β1 integrins mediate resistance to ionizing radiation through inhibition of JNK1 activation.

IGF-IR promotes prostate cancer growth by stabilizing α5β1 integrin protein levels

Dynamic crosstalk between growth factor receptors, cell adhesion molecules and extracellular matrix is essential for cancer cell migration and invasion. Integrins are transmembrane receptors that bind extracellular matrix proteins and enable cell adhesion and cytoskeletal organization. They also mediate signal transduction to regulate cell proliferation and survival. The type 1 insulin-like growth factor receptor (IGF-IR) mediates tumor cell growth, adhesion and inhibition of apoptosis in several types of cancer. We have previously demonstrated that β1 integrins regulate anchorage-independent growth of prostate cancer (PrCa) cells by regulating IGF-IR expression and androgen receptor-mediated transcriptional functions. Furthermore, we have recently reported that IGF-IR regulates the expression of β1 integrins in PrCa cells. We have dissected the mechanism through which IGF-IR regulates β1 integrin expression in PrCa. Here we report that IGF-IR is crucial for PrCa cell growth and that β1 integrins contribute to the regulation of proliferation by IGF-IR. We demonstrate that β1 integrin regulation by IGF-IR does not occur at the mRNA level. Exogenous expression of a CD4 - β1 integrin cytoplasmic domain chimera does not interfere with such regulation and fails to stabilize β1 integrin expression in the absence of IGF-IR. This appears to be due to the lack of interaction between the β1 cytoplasmic domain and IGF-IR. We demonstrate that IGF-IR stabilizes the β1 subunit by protecting it from proteasomal degradation. The α5 subunit, one of the binding partners of β1, is also downregulated along with β1 upon IGF-IR knockdown while no change is observed in the expression of the α2, α3, α4, α6 and α7 subunits. Our results reveal a crucial mechanistic role for the α5β1 integrin, downstream of IGF-IR, in regulating cancer growth.

The tyrosine phosphatase SHP2 regulates Sertoli cell junction complexes

The blood-testis barrier (BTB) is a large junctional complex composed of tight junctions, adherens junctions, and gap junctions between adjacent Sertoli cells in the seminiferous tubules of the testis. Maintenance of the BTB as well as the controlled disruption and reformation of the barrier is essential for spermatogenesis and male fertility. Tyrosine phosphorylation of BTB proteins is known to regulate the integrity of adherens and tight junctions found at the BTB. SHP2 is a nonreceptor protein tyrosine phosphatase (PTP) and a key regulator of growth factor-mediated tyrosine kinase signaling pathways. We found that SHP2 is localized to Sertoli-Sertoli cell junctions in rat testis. The overexpression of a constitutive active SHP2 mutant, SHP2 Q79R, up-regulated the BTB disruptor ERK1/2 via Src kinase in primary rat Sertoli cells in culture. Furthermore, focal adhesion kinase (FAK), which also supports BTB integrity, was found to interact with SHP2 and constitutive activation of SHP2 decreased FAK tyrosine phosphorylation. Expression of the SHP2 Q79R mutant in primary cultured Sertoli cells also resulted in the loss of tight junction and adherens junction integrity that corresponded with the disruption of the actin cytoskeleton and mislocalization of adherens junction and tight junction proteins N-cadherin, β-catenin, and ZO-1 away from the plasma membrane. These results suggest that SHP2 is a key regulator of BTB integrity and Sertoli cell support of spermatogenesis and fertility.

Akt1 promotes focal adhesion disassembly and cell motility through phosphorylation of FAK in growth factor-stimulated cells

The crosstalk between spatial adhesion signals and temporal soluble signals is key in regulating cellular responses such as cell migration. Here we show that soluble growth factors enhance integrin signaling through Akt phosphorylation of FAK at Ser695 and Thr700. PDGF treatment or overexpression of active Akt1 in fibroblasts increased autophosphorylation of FAK at Tyr397, an essential event for integrin turnover and cell migration. Phosphorylation-defective mutants of FAK (S695A and T700A) underwent autophosphorylation at Tyr397 and promoted cell migration in response to the integrin ligand fibronectin, but importantly, not in response to PDGF. This study has unveiled a novel function of Akt as an 'ignition kinase' of FAK in growth factor signaling and may shed light on the mechanism by which growth factors regulate integrin signaling.

Trop-2 inhibits prostate cancer cell adhesion to fibronectin through the β1 integrin-RACK1 axis

Trop-2 is a transmembrane glycoprotein upregulated in several human carcinomas, including prostate cancer (PrCa). Trop-2 has been suggested to regulate cell-cell adhesion, given its high homology with the other member of the Trop family, Trop-1/EpCAM, and its ability to bind the tight junction proteins claudin-1 and claudin-7. However, a role for Trop-2 in cell adhesion to the extracellular matrix has never been postulated. Here, we show for the first time that Trop-2 expression in PrCa cells correlates with their aggressiveness. Using either shRNA-mediated silencing of Trop-2 in cells that endogenously express it, or ectopic expression of Trop-2 in cells that do not express it, we show that Trop-2 inhibits PrCa cell adhesion to fibronectin (FN). In contrast, expression of another transmembrane receptor, α(v) β(5) integrin, does not affect cell adhesion to this ligand. We find that Trop-2 does not modulate either protein or activation levels of the prominent FN receptors, β(1) integrins, but acts through increasing β(1) association with the adaptor molecule RACK1 and redistribution of RACK1 to the cell membrane. As a result of Trop-2 expression, we also observe activation of Src and FAK, known to occur upon β(1) -RACK1 interaction. These enhanced Src and FAK activities are not mediated by changes in either the activity of IGF-IR, which is known to bind RACK1, or IGF-IR's ability to associate with β(1) integrins. In summary, our data demonstrate that the transmembrane receptor Trop-2 is a regulator of PrCa cell adhesion to FN through activation of the β(1) integrin-RACK1-FAK-Src signaling axis.

β1 integrin deletion enhances progression of prostate cancer in the TRAMP mouse model

β1 integrin regulates the response of both normal and cancer cells to their local environment. Although mis-localised in prostate cancer, the role β1 integrin plays in prostate development and carcinogenesis remains unknown. To assess the role of β1 integrin in vivo, we conditionally deleted β1 integrin from prostate epithelium and subsequently crossed these mice to the TRAMP prostate carcinogenesis model. Deletion of β1 integrin following castration and subsequent androgen supplementation resulted in an expansion of the p63-positive basal cell population and decreased differentiation. Consistent with these findings, deletion of β1 integrin in TRAMP mice decreased animal survival, decreased retention of normal prostate morphology, increased the percentage of tissue with poorly differentiated carcinoma, and increased cell proliferation. This study demonstrates that β1 integrin regulates several aspects of normal prostate development and in contrast to its role in several other tissues, its loss is associated with increased rates of prostate tumour progression.

PSA regulates androgen receptor expression in prostate cancer cells

BACKGROUND

Prostate-specific antigen (PSA) is a pivotal downstream target gene of the androgen receptor (AR), and a serum biomarker to monitor prostate cancer (PrCa) progression. It has been reported that PSA transactivates AR, but the mechanistic requirements of this response have not been investigated.

METHODS

We studied the localization of PSA, AR, and Src in intracellular compartments of synthetic androgen (R1881)-stimulated LNCaP and C4-2B PrCa cells, using immunofluorescence and subcellular fractionation approaches. We also investigated the effect of downregulation of PSA on AR expression by immunoblotting and real-time PCR using short hairpin RNA (shRNA) and small interfering RNA (siRNA). Src activity was analyzed by immunoblotting.

RESULTS

R1881 stimulation induced nuclear localization of both PSA and AR in LNCaP and C4-2B PrCa cells as well as increased phosphorylation of Src. Stable shRNA or transient siRNA knockdown of PSA resulted in reduced AR protein levels as well as AR mRNA levels in C4-2B cells. Similar to C4-2B cells, ablation of AR levels upon silencing of PSA was also confirmed in VCaP cells, another androgen-independent cell line. Silencing of PSA did not cause significant changes in Src activation; besides, Src regulation by integrins did not appear to affect AR transcriptional activity.

CONCLUSIONS

PSA localizes to nuclei of androgen-stimulated PrCa cells, and controls AR mRNA and protein levels. This regulatory loop is specific for PSA, does not involve known AR activators such as Src and AKT, and may contribute to AR signaling under conditions of increasing PSA levels in patients.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

Insulin-like growth factor 1 stimulation of androgen receptor activity requires β(1A) integrins

Despite the findings that β1 integrins play a vital role in the regulation of cell proliferation and survival, the mechanisms through which they operate and lead to cancer progression remain elusive. Previously, our laboratory has shown that β(1A) integrins support insulin-like growth factor 1 (IGFI)-mediated mitogenic and transforming activities. Here, we report that β(1A) integrins regulate basal levels of IGF-IR, although they are not critical for maintaining cancer cell morphology. Upon transfection of β(1A) siRNA and consequent downregulation of IGF-IR, we show inhibition of anchorage-independent growth of prostate cancer cells, a function which is dependent on IGF-IR expression. In addition, we demonstrate that IGFI-mediated activation of androgen receptor (AR), known to occur in prostate cancer cells, requires expression of β(1A) integrins as evaluated by luciferase reporter assays and immunoblotting analysis. Since β(1A) integrin levels are increased by R1881 or dihydrotestosterone (DHT), our results imply that β(1A) integrins support an androgen-enhanced feedback loop that regulates the expression of IGF-IR. β(1A) integrins also regulate inducible levels of IGF-IR in cells stimulated by androgen or by a combination of androgen and IGFI, as evaluated by flow cytometric analysis and immunoblotting. Furthermore, upon transfection of β(1A) siRNA and consequent downregulation of IGF-IR, neither activation of AKT, an effector of IGF-IR, nor AR levels are affected. We conclude that β(1A) integrin expression is critical for maintaining the regulatory crosstalk between IGF-IR and AR.

Dwarfism in mice lacking collagen-binding integrins α2β1 and α11β1 is caused by severely diminished IGF-1 levels

Mice with a combined deficiency in the α2β1 and α11β1 integrins lack the major receptors for collagen I. These mutants are born with inconspicuous differences in size but develop dwarfism within the first 4 weeks of life. Dwarfism correlates with shorter, less mineralized and functionally weaker bones that do not result from growth plate abnormalities or osteoblast dysfunction. Besides skeletal dwarfism, internal organs are correspondingly smaller, indicating proportional dwarfism and suggesting a systemic cause for the overall size reduction. In accordance with a critical role of insulin-like growth factor (IGF)-1 in growth control and bone mineralization, circulating IGF-1 levels in the sera of mice lacking either α2β1 or α11β1 or both integrins were sharply reduced by 39%, 64%, or 81% of normal levels, respectively. Low hepatic IGF-1 production resulted from diminished growth hormone-releasing hormone expression in the hypothalamus and, subsequently, reduced growth hormone expression in the pituitary glands of these mice. These findings point out a novel role of collagen-binding integrin receptors in the control of growth hormone/IGF-1-dependent biological activities. Thus, coupling hormone secretion to extracellular matrix signaling via integrins represents a novel concept in the control of endocrine homeostasis.

Skeletal unloading-induced insulin-like growth factor 1 (IGF-1) nonresponsiveness is not shared by platelet-derived growth factor: the selective role of integrins in IGF-1 signaling

Integrin receptors bind extracellular matrix proteins, and this link between the cell membrane and the surrounding matrix may translate skeletal loading to biologic activity in osteoprogenitor cells. The interaction between integrin and growth factor receptors allows for mechanically induced regulation of growth factor signaling. Skeletal unloading leads to decreased bone formation and osteoblast proliferation that can be explained in part by a failure of insulin-like growth factor 1 (IGF-1) to activate its signaling pathways in unloaded bone. The aim of this study is to determine whether unloading-induced resistance is specific for IGF-1 or common to other skeletal growth factors, and to examine the regulatory role of integrins in IGF-1 signaling. Bone marrow osteoprogenitor (BMOp) cells were isolated from control or hindlimb suspended rats. Unloaded BMOp cells treated with IGF-1 failed to respond with increased proliferation, receptor phosphorylation, or signaling activation in the setting of intact ligand binding, whereas the platelet-derived growth factor (PDGF) response was fully intact. Pretreatment of control BMOp cells with an integrin inhibitor, echistatin, failed to disrupt PDGF signaling but blocked IGF-1 signaling. Recovery of IGF-1 signaling in unloaded BMOp cells followed the recovery of marked reduction in integrin expression induced by skeletal unloading. Selective targeting of integrin subunits with siRNA oligonucleotides revealed that integrin β1 and β3 are required for normal IGF-1 receptor phosphorylation. We conclude that integrins, in particular integrin β3, are regulators of IGF-1, but not PDGF, signaling in osteoblasts, suggesting that PDGF could be considered for investigation in prevention and/or treatment of bone loss during immobilization and other forms of skeletal unloading.

Angiogenesis in cancer. Basic mechanisms and therapeutic advances

Etiological concepts on cancer development, malignant growth and tumour propagation have undergone a revolutionary development during recent years: Among other aspects, the discovery of angiogenesis - the growth of new blood vessels from pre-existing vasculature - as a key element in the pathogenesis of malignancy has opened an abundance of biologic insights and subsequent therapeutic options, which have led to improved prognosis in many cancers including those originating from colon, lung, breast and kidney. Thereby, targeting the major pro-angiogenic stimulus vascular endothelial growth factor (VEGF) became the focus for therapeutic interventions. However, the use of VEGF-targeting drugs has been shown to be of limited efficacy, which might lie in the fact that tumor angiogenesis is mediated by a variety of different subcellular systems. This review focuses on the basic mechanisms involved in angiogenesis, which potentially represent novel targets for pharmacological agents in the treatment of malignancies.

Cooperation between integrins and growth factor receptors in signaling and endocytosis

All multicellular animals express receptors for growth factors (GFs) and extracellular matrix (ECM) molecules. Integrin-type ECM receptors anchor cells to their surroundings and concomitantly activate intracellular signal transduction pathways. The same signaling mechanisms are regulated by GF receptors (GFRs). Recently, intensive research efforts have revealed novel mechanisms describing how the two receptor systems collaborate at many different levels. Integrins can directly bind to GFs and promote their activation. Adhesion receptors also organize signaling platforms and assist GFRs or even activate them via ligand-independent mechanisms. Furthermore, integrins can orchestrate endocytosis and recycling of GFRs. Here, we review the present knowledge about the interplay between integrins and GFRs and discuss recent ideas of how this collaboration may explain some previous controversies in integrin research.

Type IV collagen-initiated signals provide survival and growth cues required for liver metastasis

The liver is a major site of metastasis for human malignancies, yet the factors that regulate tumor cell survival and growth in this organ remain elusive. Previously, we reported that M-27(IGF-IR) murine lung carcinoma cells with ectopic insulin-like growth factor-1 (IGF-I) receptor overexpression acquired a site-specific, liver-metastasizing potential. Gene expression profiling and subsequent RNA and protein analyses revealed that this was associated with major changes to the expression of extracellular matrix (ECM) protein-encoding genes including type III, IV and XVIII collagen genes, and these changes were also observed in the respective tumors in vivo. Because type IV collagen was the most prominently altered ECM protein in this model, we further analyzed its functional relevance to liver metastasis. M-27 cells stably overexpressing type IV collagen α1 and α2 chains were generated and their growth and metastatic properties investigated. We found that these cells acquired a site-selective growth advantage in the liver and this was associated with cell rescue from anoikis in a collagen IV/α2 integrin/FAK-dependent manner and increased responsiveness to IGF-I. Conversely, collagen IV or focal adhesion kinase (FAK) silencing by small-interfering RNA in highly metastatic tumor cells enhanced anoikis and decreased liver metastases formation. Moreover, analysis of human surgical specimens revealed uniformly high collagen IV expression in 65/65 hepatic metastases analyzed, regardless of tissue of origin, whereas it was variable and generally low in 50/50 primary colorectal carcinoma specimens examined. The results suggest that collagen IV-conveyed signals are essential cues for liver metastasis in diverse tumor types and identify mediators of collagen IV signaling as potential therapeutic targets in the management of hepatic metastases.

Significance of talin in cancer progression and metastasis

Upon detachment from the extracellular matrix, tumor epithelial cells and tumor-associated endothelial cells are capable of overcoming anoikis, gain survival benefits, and hence contribute to the process of metastasis. The focal-adhesion complex formation recruits the association of key adaptor proteins such as FAK (focal-adhesion kinase). Vimentin, paxillin, and talin are responsible for mediating the interaction between the actin cytoskeleton and integrins. Talin is an early-recruited focal-adhesion player that is of structural and functional significance in mediating interactions with integrin cytoplasmic tails leading to destabilization of the transmembrane complex and resulting in rearrangements in the extracellular integrin compartments that mediate integrin activation. Talin-mediated integrin activation plays a definitive role in integrin-mediated signaling and induction of downstream survival pathways leading to protection from anoikis and consequently resulting in cancer progression to metastasis. We recently reported that talin expression is significantly increased in prostate cancer compared with benign and normal prostate tissue and that this overexpression correlates with progression to metastatic disease implicating a prognostic value for talin during tumor progression. At the molecular level, talin is functionally associated with enhanced survival and proliferation pathways and confers anoikis resistance and metastatic spread of primary tumor cells via activation of the Akt survival pathway. In this review, we discuss the growing evidence surrounding the value of talin as a prognostic marker of cancer progression to metastasis and as therapeutic target in advanced prostate cancer, as well as the current understanding of mechanisms regulating its signaling activity in cancer.

The anti-angiogenic peptide, loop 6, binds insulin-like growth factor-1 receptor

Tissue inhibitors of metalloproteinases (TIMPs), the endogenous inhibitors of matrix metalloproteinases, have been shown to possess biological functions that are independent of their ability to inhibit matrix metalloproteinases. We have previously shown that the C-terminal domain of TIMP-2 and, in particular, Loop 6 inhibit capillary endothelial cell proliferation and angiogenesis both in vitro and in vivo. To elucidate the mechanism by which Loop 6 inhibits angiogenesis, we sought to determine whether its biological effects were the result of a known TIMP-2 protein-protein interaction or of a receptor-mediated event. In this study, we identify insulin-like growth factor-1 receptor as a binding partner of Loop 6/TIMP-2 and characterize this interaction on the endothelial cell surface and the consequences of this interaction on downstream receptor signaling.

Mutual interaction and reciprocal down-regulation between c-met and insulin receptor substrate-1

The insulin receptor substrate-1 (IRS-1) and c-met, the receptor for the hepatocyte growth factor (HGF) co-immuno-precipitate from lysates treated with the respective antibodies. The interaction between IRS-1 and c-met requires a tyrosyl phosphorylated IRS-1 and results in reciprocal down-regulation. IRS-1 inhibits cell motility, while the activated c-met promotes it. These and other results suggest an explanation for reports in the literature indicating that c-met levels are high and IRS-1 levels are low in human cancer metastases.

Cross-talk between the insulin-like growth factor (IGF) axis and membrane integrins to regulate cell physiology

The biology of cross-talk between activated growth factor receptors and cell-surface integrins is an area which has attracted much interest in recent years (Schwartz and Ginsberg, 2002). This review discusses the relationship between the insulin-like growth factor (IGF) axis and cell-surface integrin receptors in the regulation of various aspects of cell physiology. Key to these interactions are signals transmitted between integrins and the IGF-I receptor (IGF-IR) when either or both are bound to their cognate ligands and we will review the current state of knowledge in this area. The IGF axis comprises many molecular components and we will also discuss the potential role of these species in cross-talk with the integrin receptor. With respect to integrin ligands, we will mainly focus on the well-characterized interactions of the two extracellular matrix (ECM) glycoproteins fibronectin (FN) and vitronectin (VN) with cell-surface ligands, and, how this affects activity through the IGF axis. However, we will also highlight the importance of other integrin activation mechanisms and their impact on IGF activity.

Integrins and signal transduction

Integrin signaling has a critical function in organizing cells in tissues during both embryonic development and tissue repair. Following their binding to the extracellular ligands, the intracellular signaling pathways triggered by integrins are directed to two major functions: organization of the actin cytoskeleton and regulation of cell behaviour including survival, differentiation and growth. Basic research conducted in the past twelve years has lead to remarkable breakthroughs in this field. Integrins are catalytically inactive and translate positional cues into biochemical signals by direct and/or functional association with intracellular adaptors, cytosolic tyrosine kinases or growth factor and cytokine receptors. The purpose of this chapter is to highlight recent experimental and conceptual advances in integrin signaling with particular emphasis on the ability of integrins to regulate Fak/Src family kinases (SFKs) activation and the cross-talk with soluble growth factors receptors and cytokines.

Androgen and Src signaling regulate centrosome activity

Microtubules nucleated from gamma-tubulin ring complexes located at the centrosome regulate the localization of organelles, promote vesicular transport and direct cell migration. Although several signaling mechanisms have been identified that regulate microtubule dynamics during interphase, signaling pathways that promote microtubule nucleation remain elusive. We assayed microtubule regrowth following nocodazole washout in human fibroblasts and CHO-K1 cells adhered to fibronectin in either normal serum-free medium or the serum-free, growth-promoting medium, CCM1, which contains IGF1 and androgen, as well as other nutrients. The results indicate that integrin-mediated adhesion is not sufficient to promote rapid microtubule regrowth in either cell type. The addition of androgen, but not IGF1, for 5 minutes was sufficient to promote rapid regrowth and this occurred by a mechanism requiring the androgen receptor. Since Src is a component of the cytoplasmic androgen-receptor-signaling complex, we examined its role using Src siRNA, the Src kinase inhibitor SU6656, and the expression of a constitutively active Src mutant. The data show that Src signaling is both required and sufficient to promote rapid microtubule regrowth in cells adhered to fibronectin. Measurement of the density of microtubules close to the centrosome and the rates of GFP-EB1-labeled microtubules emanating from the centrosome indicated that Src signaling promotes microtubule nucleation. Furthermore, recovery of GFP-gamma-tubulin at the centrosome following photobleaching and measurements of endogenous gamma-tubulin levels at the centrosome showed that androgen and Src signaling regulate the levels of centrosomal gamma-tubulin. Thus, we propose that androgen and Src signaling regulate microtubule nucleation during interphase by promoting the centrosomal localization of gamma-tubulin.

Beta1 integrins mediate cell proliferation in three-dimensional cultures by regulating expression of the sonic hedgehog effector protein, GLI1

The beta1 integrins play an important role in the modulation of cancer cell proliferation and tumor growth. We have previously shown that beta1 integrins associate with insulin-like growth factor type 1 receptor (IGF-IR) and regulate IGF-stimulated prostate cancer cell proliferation. In the present study, we find that downregulation of beta1 in prostate cancer cells inhibits IGF-IR and AKT activation. We also show that beta1 downregulation in two- and three-dimensional (3D) prostate cancer cell cultures significantly reduces expression of GLI1, a transcription factor known to be regulated by the IGF/AKT signaling pathway and to be a downstream effector of sonic hedgehog. Re-expression of GLI1 rescues the inhibitory effect of beta1 downregulation on prostate cancer cell proliferation in 3D cultures. We find that downregulation of beta1 significantly reduces surface expression of the associated alpha 5 integrin subunit. Our results indicate that the beta1/IGF-IR complex regulates expression of GLI1, which in turn promotes cancer cell proliferation in 3D cultures.

Angiogenesis as a strategic target for prostate cancer therapy

It is becoming increasingly clear that angiogenesis plays a crucial role in prostate cancer (CaP) survival, progression, and metastasis. Tumor angiogenesis is a hallmark of advanced cancers and an attractive treatment target in multiple solid tumors. By understanding the molecular basis of resistance to androgen withdrawal and chemotherapy in CaP, the rational design of targeted therapeutics is possible. This review summarizes the recent advancements that have improved our understanding of the role of angiogenesis in CaP metastasis and the potential therapeutic efficacy of inhibiting angiogenesis in this disease. Current therapeutic options for patients with metastatic hormone-refractory CaP are very limited. Targeting vasculature is a developing area, which shows promise for the control of late stage and recurrent CaP disease and for overcoming drug resistance. We discuss angiogenesis and its postulated mechanisms and focus on the regulation of angiogenesis in CaP progression and the therapeutic beneficial effects associated with targeting of the CaP vasculature to overcome the resistance to current treatments and CaP recurrence.

Candidate pathways and genes for prostate cancer: a meta-analysis of gene expression data

BACKGROUND

The genetic mechanisms of prostate tumorigenesis remain poorly understood, but with the advent of gene expression array capabilities, we can now produce a large amount of data that can be used to explore the molecular and genetic mechanisms of prostate tumorigenesis.

METHODS

We conducted a meta-analysis of gene expression data from 18 gene array datasets targeting transition from normal to localized prostate cancer and from localized to metastatic prostate cancer. We functionally annotated the top 500 differentially expressed genes and identified several candidate pathways associated with prostate tumorigeneses.

RESULTS

We found the top differentially expressed genes to be clustered in pathways involving integrin-based cell adhesion: integrin signaling, the actin cytoskeleton, cell death, and cell motility pathways. We also found integrins themselves to be downregulated in the transition from normal prostate tissue to primary localized prostate cancer. Based on the results of this study, we developed a collagen hypothesis of prostate tumorigenesis. According to this hypothesis, the initiating event in prostate tumorigenesis is the age-related decrease in the expression of collagen genes and other genes encoding integrin ligands. This concomitant depletion of integrin ligands leads to the accumulation of ligandless integrin and activation of integrin-associated cell death. To escape integrin-associated death, cells suppress the expression of integrins, which in turn alters the actin cytoskeleton, elevates cell motility and proliferation, and disorganizes prostate histology, contributing to the histologic progression of prostate cancer and its increased metastasizing potential.

CONCLUSION

The results of this study suggest that prostate tumor progression is associated with the suppression of integrin-based cell adhesion. Suppression of integrin expression driven by integrin-mediated cell death leads to increased cell proliferation and motility and increased tumor malignancy.

Beta1 integrin cytoplasmic variants differentially regulate expression of the antiangiogenic extracellular matrix protein thrombospondin 1

Beta(1) integrins play an important role in regulating cell proliferation and survival. Using small interfering RNA or an inhibitory antibody to beta(1), we show here that, in vivo, beta(1) integrins are essential for prostate cancer growth. Among the five known beta(1) integrin cytoplasmic variants, two have been shown to differentially affect prostate cell functions. The beta(1A) variant promotes normal and cancer cell proliferation, whereas the beta(1C) variant, which is down-regulated in prostate cancer, inhibits tumor growth and appears to have a dominant effect on beta(1A). To investigate the mechanism by which beta(1C) inhibits the tumorigenic potential of beta(1A), we analyzed changes in gene expression in cells transfected with either beta(1C) or beta(1A). The results show that beta(1C) expression increases the levels of an extracellular matrix protein, thrombospondin 1 (TSP1), an angiogenesis inhibitor. TSP1 protein levels are increased upon beta(1C) expression in prostate cancer cells as well as in beta(1)-null GD25 cells. We show that TSP1 does not affect proliferation, apoptosis, or anchorage-independent growth of prostate cancer cells. In contrast, the newly synthesized TSP1, secreted by prostate cancer cells expressing beta(1C), prevents proliferation of endothelial cells. In conclusion, our novel findings indicate that expression of the beta(1C) integrin variant in prostate glands prevents cancer progression by up-regulation of TSP1 levels and inhibition of angiogenesis.

Expression and function of the insulin receptor substrate proteins in cancer

The Insulin Receptor Substrate (IRS) proteins are cytoplasmic adaptor proteins that function as essential signaling intermediates downstream of activated cell surface receptors, many of which have been implicated in cancer. The IRS proteins do not contain any intrinsic kinase activity, but rather serve as scaffolds to organize signaling complexes and initiate intracellular signaling pathways. As common intermediates of multiple receptors that can influence tumor progression, the IRS proteins are positioned to play a pivotal role in regulating the response of tumor cells to many different microenvironmental stimuli. Limited studies on IRS expression in human tumors and studies on IRS function in human tumor cell lines and in mouse models have provided clues to the potential function of these adaptor proteins in human cancer. A general theme arises from these studies; IRS-1 and IRS-4 are most often associated with tumor growth and proliferation and IRS-2 is most often associated with tumor motility and invasion. In this review, we discuss the mechanisms by which IRS expression and function are regulated and how the IRS proteins contribute to tumor initiation and progression.

Integrin signaling aberrations in prostate cancer

Integrins are cell surface receptors for extracellular matrix proteins and play a key role in cell survival, proliferation, migration and gene expression. Integrin signaling has been shown to be deregulated in several types of cancer, including prostate cancer. This review is focused on integrin signaling pathways known to be deregulated in prostate cancer and known to promote prostate cancer progression.

Genetic and cell biological analysis of integrin outside-in signaling

Integrins are cell surface transmembrane receptors that recognize and bind to extracellular matrix proteins and counter receptors. Binding of activated integrins to their ligands induces a vast number of structural and signaling changes within the cell. Large, multimolecular complexes assemble onto the cytoplasmic tails of activated integrins to engage and organize the cytoskeleton, and activate signaling pathways that ultimately lead to changes in gene expression. Additionally, integrin-mediated signaling intersects with growth factor-mediated signaling through various levels of cross-talk. This review discusses recent work that has tremendously broadened our understanding of the complexity of integrin-mediated signaling.

Insulin-like growth factor-1 receptor-targeted therapy for non-small cell lung cancer: a mini review

Lung cancer leads all other cancers in both incidence and mortality. Recent advances in underlying molecular pathogenesis have validated a panel of protein tyrosine kinases as new targets in lung cancer treatment. Insulin-like growth factor-1 receptor (IGF-1R) is an important tyrosine kinase receptor involved in cell proliferation, differentiation, metabolism, apoptosis, and angiogenesis. Aberrant activation of IGF-1R is frequently found in patients with lung cancer and contributes to malignant transformation and poor prognosis for patients with lung cancer. In this review, we focused on recent progress in the research of IGF-1R's role in lung cancer development and progression, including its structure and biological function, potential mechanisms of aberrant activation, and related oncogenic effects. We also discussed effective IGF-1R antagonists that are currently registered for clinic trials or are undergoing preclinical study with special emphasis on their antibodies and small molecule tyrosine kinase inhibitors.

Integrins in prostate cancer progression

Integrins, which are transmembrane receptors for extracellular matrix proteins, play a key role in cell survival, proliferation, migration, gene expression, and activation of growth factor receptors. Their functions and expression are deregulated in several types of cancer, including prostate cancer. In this article, we review the role of integrins in prostate cancer progression and their potential as therapeutic targets.

Prostate carcinoma and radiation therapy: therapeutic treatment resistance and strategies for targeted therapeutic intervention

Adenocarcinoma of the prostate remains a significant public health problem and a prevalent cancer in men. Prostate-specific antigen used as a biomarker has established a clear migration of patients towards earlier-stage disease at presentation. However, in spite of process improvements in traditional therapies including surgery, radiation therapy, and hormone management, there remains a significant cohort of patients with intermediate- to high-risk features for poor outcome in spite of optimal use of traditional management. This paper focuses on future treatment strategies integrating new therapeutic options with traditional management, specifically to pinpoint new radiation therapy strategies.

Integrin-linked kinase stabilizes myotendinous junctions and protects muscle from stress-induced damage

Skeletal muscle expresses high levels of integrin-linked kinase (ILK), predominantly at myotendinous junctions (MTJs) and costameres. ILK binds the cytoplasmic domain of β1 integrin and mediates phosphorylation of protein kinase B (PKB)/Akt, which in turn plays a central role during skeletal muscle regeneration. We show that mice with a skeletal muscle–restricted deletion of ILK develop a mild progressive muscular dystrophy mainly restricted to the MTJs with detachment of basement membranes and accumulation of extracellular matrix. Endurance exercise training enhances the defects at MTJs, leads to disturbed subsarcolemmal myofiber architecture, and abrogates phosphorylation of Ser473 as well as phosphorylation of Thr308 of PKB/Akt. The reduction in PKB/Akt activation is accompanied by an impaired insulin-like growth factor 1 receptor (IGF-1R) activation. Coimmunoprecipitation experiments reveal that the β1 integrin subunit is associated with the IGF-1R in muscle cells. Our data identify the β1 integrin–ILK complex as an important component of IGF-1R/insulin receptor substrate signaling to PKB/Akt during mechanical stress in skeletal muscle.

Substrate-bound insulin-like growth factor (IGF)-I-IGF binding protein-vitronectin-stimulated breast cell migration is enhanced by coactivation of the phosphatidylinositide 3-Kinase/AKT pathway by alphav-integrins and the IGF-I receptor

IGF-I can bind to the extracellular matrix protein vitronectin (VN) through the involvement of IGF-binding proteins-2, -3, -4, and -5. Because IGF-I and VN have established roles in tumor cell dissemination, we were keen to investigate the functional consequences of the interaction of IGF-I, IGF binding proteins (IGFBPs), and VN in tumor cell biology. Hence, functional responses of MCF-7 breast carcinoma cells and normal nontumorgenic MCF-10A mammary epithelial cells were investigated to allow side-by-side comparisons of these complexes in both cancerous and normal breast cells. We demonstrate that substrate-bound IGF-I-IGFBP-VN complexes stimulate synergistic increases in cellular migration in both cell types. Studies using IGF-I analogs determined this stimulation to be dependent on both heterotrimeric IGF-I-IGFBP-VN complex formation and the involvement of the IGF-I receptor (IGF-IR). Furthermore, the enhanced cellular migration was abolished on incubation of MCF-7 and MCF-10A cells with function blocking antibodies directed at VN-binding integrins and the IGF-IR. Analysis of the signal transduction pathways underlying the enhanced cell migration revealed that the complexes stimulate a transient activation of the ERK/MAPK signaling pathway while simultaneously producing a sustained activation of the phosphatidylinositide 3-kinase/AKT pathway. Experiments using pharmacological inhibitors of these pathways determined a requirement for phosphatidylinositide 3-kinase/AKT activation in the observed response. Overexpression of wild type and activated AKT further increases substrate-bound IGF-I-IGFBP-VN-stimulated migration. This study provides the first mechanistic insights into the action of IGF-I-IGFBP-VN complexes and adds further evidence to support the involvement of VN-binding integrins and their cooperativity with the IGF-IR in the promotion of tumor cell migration.

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.

Extracellular influences on tumour angiogenesis in the aged host

Whether tumours are epithelial or non-epithelial in origin, it is generally accepted that once they reach a certain size all solid tumours are dependent upon a vascular supply to provide nutrients. Accordingly, there is great interest in how the extracellular environment enhances or inhibits vascular growth. In this minireview, we will examine key extracellular components, their changes with ageing, and discuss how these alterations may influence the subsequent development of tumour vasculature in the aged host. Because of the tight correlation between advanced age and development of prostate cancer, we will use prostate cancer as the model throughout this review.

The integrin-growth factor receptor duet

Cell adhesion receptors, referred to as integrins, are recognized as key regulators of cellular processes including growth and differentiation. Integrins communicate with growth factor receptors (GFRs) to control specific cellular responses to stimuli originating in the extracellular environment. In this article, we review the role of integrins as molecular switches that modulate GFR activation and specificity. We also examine the reciprocal modulation of integrin functions by GFRs and the mechanisms through which those actions are fine-tuned.