STAT3 signaling is induced by intercellular adhesion in squamous cell carcinoma cells

The acetylation of STAT3 at K685 attenuates NPM-ALK-induced tumorigenesis

Nucleophosmin-anaplastic lymphoma kinase (NPM-ALK), a fusion protein generated by a chromosomal translocation, is a causative gene product of anaplastic large cell lymphoma (ALCL). It induces cell proliferation and tumorigenesis by activating the transcription factor, signal transducer and activator of transcription factor 3 (STAT3). We herein demonstrated that STAT3 underwent acetylation at K685 in a manner that was dependent on the kinase activity of NPM-ALK. To investigate the role of STAT3 acetylation in NPM-ALK-induced oncogenesis, we generated Ba/F3 cells expressing NPM-ALK in which STAT3 was silenced by shRNA, named STAT3-KD cells, and then reconstituted wild-type STAT3 or the STAT3 K685R mutant into these cells. The phosphorylation level of the K685R mutant at Y705 and S727 was significantly higher than that of wild-type STAT3 in STAT3-KD cells. The expression of STAT3 target genes, such as IL-6, Pim1, Pim2, and Socs3, was more strongly induced by the reconstitution of the K685R mutant than wild-type STAT3. In addition, the proliferative ability of STAT3-KD cells reconstituted with the K685R mutant was slightly higher than that of STAT3-KD cells reconstituted with wild-type STAT3. In comparisons with the inoculation of STAT3-KD cells reconstituted with wild-type STAT3, the inoculation of STAT3-KD cells reconstituted with the K685R mutant significantly enhanced tumorigenesis and hepatosplenomegaly in nude mice. Collectively, these results revealed for the first time that the acetylation of STAT3 at K685 attenuated NPM-ALK-induced oncogenesis.

E-cadherin acts as a positive regulator of the JAK-STAT signaling pathway during Drosophila oogenesis

The JAK-STAT pathway is evolutionary conserved. The simplicity of this signaling in Drosophila, due to the limited redundancy between pathway components, makes it an ideal model for investigation. In the Drosophila follicular epithelium, highly stereotyped functions of JAK-STAT signaling have been well characterized, but how signaling activity is regulated precisely to allow the different outcomes is not well understood. In this tissue, the ligand is secreted by the polar cells positioned at each follicle extremity, thus generating a gradient of JAK-STAT activity in adjacent cells. One way to control the delivered quantity of ligand is by regulating the number of polar cells, which is reduced by apoptosis to exactly two at each pole by mid-oogenesis. Hence, JAK-STAT activity is described as symmetrical between follicle anterior and posterior regions. Here, we show that JAK-STAT signaling activity is actually highly dynamic, resulting in asymmetry between poles by mid-oogenesis. Interestingly, we found similar temporal dynamics at follicle poles in the accumulation of the adherens junction E-cadherin protein. Remarkably, E-cadherin and JAK-STAT signaling not only display patterning overlaps but also share functions during oogenesis. In particular, we show that E-cadherin, like JAK-STAT signaling, regulates polar cell apoptosis non-cell-autonomously from follicle cells. Finally, our work reveals that E-cadherin is required for optimal JAK-STAT activity throughout oogenesis and that E-cadherin and Stat92E, the transcription factor of the pathway, form part of a physical complex in follicle cells. Taken together, our study establishes E-cadherin as a new positive regulator of JAK-STAT signaling during oogenesis.

Roads to Stat3 Paved with Cadherins

The engagement of cadherins, cell-to-cell adhesion proteins, triggers a dramatic increase in the levels and activity of the Rac/Cdc42 GTPases, through the inhibition of proteasomal degradation. This leads to an increase in transcription and secretion of IL6 family cytokines, activation of their common receptor, gp130, in an autocrine manner and phosphorylation of the signal transducer and activator of transcription-3 (Stat3) on tyrosine-705 by the Jak kinases. Stat3 subsequently dimerizes, migrates to the nucleus and activates the transcription of genes involved in cell division and survival. The Src oncogene also increases Rac levels, leading to secretion of IL6 family cytokines and gp130 activation, which triggers a Stat3-ptyr705 increase. Interestingly, at the same time, Src downregulates cadherins in a quantitative manner, while cadherins are required to preserve gp130 levels for IL6 family signalling. Therefore, a fine balance between Src527F/Rac/IL6 and Src527F/cadherin/gp130 levels is in existence, which is required for Stat3 activation. This further demonstrates the important role of cadherins in the activation of Stat3, through preservation of gp130 function. Conversely, the absence of cadherin engagement correlates with low Stat3 activity: In sparsely growing cells, both gp130 and Stat3-ptyr705 levels are very low, despite the fact that cSrc is active in the FAK (focal adhesion kinase)/cSrc complex, which further indicates that the engagement of cadherins is important for Stat3 activation, not just their presence. Furthermore, the caveolin-1 protein downregulates Stat3 through binding and sequestration of cadherins to the scaffolding domain of caveolin-1. We hypothesize that the cadherins/Rac/gp130 axis may be a conserved pathway to Stat3 activation in a number of systems. This fact could have significant implications in Stat3 biology, as well as in drug testing and development.

Overview of Candida albicans and Human Papillomavirus (HPV) Infection Agents and their Biomolecular Mechanisms in Promoting Oral Cancer in Pediatric Patients

Oral carcinoma represents one of the most common malignancies worldwide. Oral squamous cell carcinomas (OSCCs) account over 90% of all oral malignant tumors and are characterized by high mortality in the advanced stages. Early diagnosis is often a challenge for its ambiguous appearance in early stages. Mucosal infection by the human papillomavirus (HPV) is responsible for a growing number of malignancies, particularly cervical cancer and oropharyngeal carcinomas. In addition, Candida albicans (C. albicans), which is the principal fungi involved in the oral cancer development, may induce carcinogenesis through several mechanisms, mainly promoting inflammation. Medical knowledge and research on adolescent/pediatric patients' management and prevention are in continuous evolution. Besides, microbiota can play an important role in maintaining oral health and therefore all human health. The aim of this review is to evaluate epidemiological and pathophysiological characteristics of the several biochemical pathways involved during HPV and C. albicans infections in pediatric dentistry.

Control of Intestinal Epithelial Permeability by Lysophosphatidic Acid Receptor 5

BACKGROUND & AIMS

Epithelial cells form a monolayer at mucosal surface that functions as a highly selective barrier. Lysophosphatidic acid (LPA) is a bioactive lipid that elicits a broad range of biological effects via cognate G protein-coupled receptors. LPA receptor 5 (LPA₅) is highly expressed in intestinal epithelial cells, but its role in the intestine is not well-known. Here we determined the role of LPA₅ in regulation of intestinal epithelial barrier.

METHODS

Epithelial barrier integrity was determined in mice with intestinal epithelial cell (IEC)-specific LPA₅ deletion, Lpar5^ΔIEC. LPA was orally administered to mice, and intestinal permeability was measured. Dextran sulfate sodium (DSS) was used to induce colitis. Human colonic epithelial cell lines were used to determine the LPA₅-mediated signaling pathways that regulate epithelial barrier.

RESULTS

We observed increased epithelial permeability in Lpar5^ΔIEC mice with reduced claudin-4 expression. Oral administration of LPA decreased intestinal permeability in wild-type mice, but the effect was greatly mitigated in Lpar5^ΔIEC mice. Serum lipopolysaccharide level and bacterial loads in the intestine and liver were elevated in Lpar5^ΔIEC mice. Lpar5^ΔIEC mice developed more severe colitis induced with DSS. LPA₅ transcriptionally regulated claudin-4, and this regulation was dependent on transactivation of the epidermal growth factor receptor, which induced localization of Rac1 at the cell membrane. LPA induced the translocation of Stat3 to the cell membrane and promoted the interaction between Rac1 and Stat3. Inhibition of Stat3 ablated LPA-mediated regulation of claudin-4.

CONCLUSIONS

This study identifies LPA₅ as a regulator of the intestinal barrier. LPA₅ promotes claudin-4 expression in IECs through activation of Rac1 and Stat3.

The E-Cadherin and N-Cadherin Switch in Epithelial-to-Mesenchymal Transition: Signaling, Therapeutic Implications, and Challenges

Epithelial-to-Mesenchymal Transition (EMT) has been shown to be crucial in tumorigenesis where the EMT program enhances metastasis, chemoresistance and tumor stemness. Due to its emerging role as a pivotal driver of tumorigenesis, targeting EMT is of great therapeutic interest in counteracting metastasis and chemoresistance in cancer patients. The hallmark of EMT is the upregulation of N-cadherin followed by the downregulation of E-cadherin, and this process is regulated by a complex network of signaling pathways and transcription factors. In this review, we summarized the recent understanding of the roles of E- and N-cadherins in cancer invasion and metastasis as well as the crosstalk with other signaling pathways involved in EMT. We also highlighted a few natural compounds with potential anti-EMT property and outlined the future directions in the development of novel intervention in human cancer treatments. We have reviewed 287 published papers related to this topic and identified some of the challenges faced in translating the discovery work from bench to bedside.

Association of SP1 rs1353058818 and STAT3 rs1053004 gene polymorphisms with human tongue squamous cell carcinoma

Objective: To study the association between SP1 rs1353058818 and STAT3 rs1053004 gene polymorphisms and risk of human tongue squamous cell carcinoma (TSCC).Methods: Sanger sequencing was used to determine the genotypes of SP1 rs1353058818 and STAT3 rs1053004 loci in 240 TSCC patients and 240 controls. Levels of hsa-miR-149-5p and hsa-miR-21-5p and expression levels of SP1 and STAT3 proteins in tumor tissues and adjacent normal tissues of TSCC patients were ascertained.Results: Carrying the SP1 rs1353058818 locus deletion allele was a high risk factor for TSCC (OR = 2.997, 95% CI: 1.389-6.466, P = 0.003). The STAT3 rs1053004 locus A allele was a protective factor for TSCC (OR = 0.604, 95% CI: 0.460-0.793, P < 0.001). There was a negative correlation between SP1 mRNA and hsa-miR-149-5p in tumor and adjacent normal tissues (r = -0.81, -0.77). The expression of SP1 protein in tumor tissues of the SP1 rs1353058818 locus DD genotype was significantly higher than in tissues of the ID type, and in tissues of type II it was the lowest. STAT3 mRNA was positively correlated with hsa-miR-21-5p in tumor and adjacent normal tissues (r = 0.75, 0.78). The expression level of STAT3 protein in tumor tissues of patients with STAT3 rs1053004 locus GG genotype was significantly higher than in patients with type GA, and it was the lowest in patients with type AA.Conclusion: Polymorphisms in the SP1 rs1353058818 and STAT3 rs1053004 loci are associated with the risk of human TSCC.

PI3k and Stat3: Oncogenes that are Required for Gap Junctional, Intercellular Communication

Gap junctional, intercellular communication (GJIC) is interrupted in cells transformed by oncogenes such as activated Src. The Src effector, Ras, is required for this effect, so that Ras inhibition restores GJIC in Src-transformed cells. Interestingly, the inhibition of the Src effector phosphatidyl-inositol-3 kinase (PI3k) or Signal Transducer and Activator of Transcription-3 (Stat3) pathways does not restore GJIC. In the contrary, inhibition of PI3k or Stat3 in non-transformed rodent fibroblasts or epithelial cells or certain human lung carcinoma lines with extensive GJIC inhibits communication, while mutational activation of PI3k or Stat3 increases GJIC. Therefore, it appears that oncogenes such as activated Src have a dual role upon GJIC; acting as inhibitors of communication through the Ras pathway, and as activators through activation of PI3k or Stat3. In the presence of high Src activity the inhibitory functions prevail so that the net effect is gap junction closure. PI3k and Stat3 constitute potent survival signals, so that their inhibition in non-transformed cells triggers apoptosis which, in turn, has been independently demonstrated to suppress GJIC. The interruption of gap junctional communication would confine the apoptotic event to single cells and this might be essential for the maintenance of tissue integrity. We hypothesize that the GJIC activation by PI3k or Stat3 may be linked to their survival function.

Elevated Src family kinase activity stabilizes E-cadherin-based junctions and collective movement of head and neck squamous cell carcinomas

EGF receptor (EGFR) overexpression is thought to drive head and neck carcinogenesis however clinical responses to EGFR-targeting agents have been modest and alternate targets are actively sought to improve results. Src family kinases (SFKs), reported to act downstream of EGFR are among the alternative targets for which increased expression or activity in epithelial tumors is commonly associated to the dissolution of E-cadherin-based junctions and acquisition of a mesenchymal-like phenotype. Robust expression of total and activated Src was observed in advanced stage head and neck tumors (N=60) and in head and neck squamous cell carcinoma lines. In cultured cancer cells Src co-localized with E-cadherin in cell-cell junctions and its phosphorylation on Y419 was both constitutive and independent of EGFR activation. Selective inhibition of SFKs with SU6656 delocalized E-cadherin and disrupted cellular junctions without affecting E-cadherin expression and this effect was phenocopied by knockdown of Src or Yes. These findings reveal an EGFR-independent role for SFKs in the maintenance of intercellular junctions, which likely contributes to the cohesive invasion E-cadherin-positive cells in advanced tumors. Further, they highlight the need for a deeper comprehension of molecular pathways that drive collective cell invasion, in absence of mesenchymal transition, in order to combat tumor spread.

RECK controls breast cancer metastasis by modulating a convergent, STAT3-dependent neoangiogenic switch

Metastasis is the primary cause of cancer-related death in oncology patients. A comprehensive understanding of the molecular mechanisms that cancer cells usurp to promote metastatic dissemination is critical for the development and implementation of novel diagnostic and treatment strategies. Here we show that the membrane protein RECK (Reversion-inducing cysteine-rich protein with kazal motifs) controls breast cancer metastasis by modulating a novel, non-canonical and convergent signal transducer and activator of transcription factor 3 (STAT3)-dependent angiogenic program. Neoangiogenesis and STAT3 hyperactivation are known to be fundamentally important for metastasis, but the root molecular initiators of these phenotypes are poorly understood. Our study identifies loss of RECK as a critical and previously unknown trigger for these hallmarks of metastasis. Using multiple xenograft mouse models, we comprehensively show that RECK inhibits metastasis, concomitant with a suppression of neoangiogenesis at secondary sites, while leaving primary tumor growth unaffected. Further, with functional genomics and biochemical dissection we demonstrate that RECK controls this angiogenic rheostat through a novel complex with cell surface receptors to regulate STAT3 activation, cytokine signaling, and the induction of both vascular endothelial growth factor and urokinase plasminogen activator. In accordance with these findings, inhibition of STAT3 can rescue this phenotype both in vitro and in vivo. Taken together, our study uncovers, for the first time, that RECK is a novel regulator of multiple well-established and robust mediators of metastasis; thus, RECK is a keystone protein that may be exploited in a clinical setting to target metastatic disease from multiple angles.

New basic approach to treat non-small cell lung cancer based on RNA-interference

BACKGROUND

To date the therapy for non-small cell lung cancer (NSCLC) is associated with severe side effects, frustrating outcomes, and does not consider different tumor characteristics. The RNA-interference (RNAi) pathway represents a potential new approach to treat NSCLC. With small interfering ribonucleic acids (siRNAs), it is possible to reduce the expression of proliferation-dependent proteins in tumor cells, leading to their apoptosis. We propose that siRNAs could be adapted to the tumor type and may cause fewer side effects than current therapy.

METHODS

Four NSCLC cell lines were cultured under standard conditions and transfected with three different concentrations of siRNAs targeted against the hypoxia-inducible factors 1α and 2α (HIF1α and HIF2α) and signal transducer and activator of transcription 3 (STAT3). The expression was observed by quantitative real-time polymerase chain reaction and western blots. For the analysis of cell growth three days after transfection, the cell number was detected using a CASY cell counter system.

RESULTS

The results of the silencing of the analyzed factors differ in each cell line. Cell growth was significantly reduced in all cell lines after transfection with HIF1α- and STAT3-siRNA. The silencing of HIF2α resulted in a significant effect on cell growth in squamous, and large-cell lung cancer.

CONCLUSIONS

This study shows that the knockdown and viability to siRNA transfection differ in each tumor type according to the used siRNA. This implies that the tumor types differ among themselves and should be treated differently. Therefore, the authors suggest a possible approach to a more personalized treatment of NSCLC.

Engaged for survival: From cadherin ligation to STAT3 activation

In normal tissues or tumors, cells have extensive opportunities for adhesion to their neighbors. This state is mimicked by dense cell cultures. In this review, we integrate some recent findings on a key signal transducer, STAT3 (signal transducer and activator of transcription-3), whose activity is dramatically increased following cadherin-mediated cell to cell adhesion. Cadherin engagement, favored in dense cell cultures, causes a dramatic increase in total Rac/Cdc42 protein levels through inhibition of proteasomal degradation, which is followed by activation of IL-6 and STAT3. The cadherin/Rac/IL-6/STAT3 axis offers a potent survival signal that is a prerequisite for neoplastic transformation, as well as normal tissue function.

Anti-epidermal growth factor receptor therapy in head and neck squamous cell carcinoma: focus on potential molecular mechanisms of drug resistance

Targeted therapy against the epidermal growth factor receptor (EGFR) is one of the most promising molecular therapeutics for head and neck squamous cell carcinoma (HNSCC). EGFR is overexpressed in a wide range of malignancies, including HNSCC, and initiates important signal transduction pathways in HNSCC carcinogenesis. However, primary and acquired resistance are serious problems and are responsible for low single-agent response rate and tumor recurrence. Therefore, an improved understanding of the molecular mechanisms of resistance to EGFR inhibitors may provide valuable indications to identify biomarkers that can be used clinically to predict response to EGFR blockade and to establish new treatment options to overcome resistance. To date, no predictive biomarker for HNSCC is available in the clinic. Therapeutic resistance to anti-EGFR therapy may arise from mechanisms that can compensate for reduced EGFR signaling and/or mechanisms that can modulate EGFR-dependent signaling. In this review, we will summarize some of these molecular mechanisms and describe strategies to overcome that resistance.

Effect of ouabain on myocardial ultrastructure and cytoskeleton during the development of ventricular hypertrophy

The aim of this work is to study cytoskeletal impairment during the development of ouabain-induced ventricular hypertrophy. Male Sprague-Dawley rats were treated with either ouabain or saline. Systolic blood pressure (SBP) was recorded weekly. At the end of the 3rd and 6th week, the rats were killed and cardiac mass index were measured. Hematoxylin-eosin and Sirius red staining were carried out and cardiac ultrastructure were studied using transmission electron microscopy. The mRNA level of Profilin-1, Desmin, PCNA, TGF-β(1) and ET-1 in the left ventricle were measured using real-time quantitative PCR while their protein levels were examined by Western blot or immunohistochemistry. After 3 weeks, there was no significant difference in the mean SBP, cardiac mass index, mRNA and protein expression of PCNA, TGF-β(1) and ET-1 between the two groups. However, ouabain-treated rats showed disorganized cardiac cytoskeleton with abnormal expression of Profilin-1 and Desmin. After 6 weeks, the cardiac mass index remained the same in the two groups while PCNA, TGF-β(1), and ET-1 have been upregulated in ouabain-treated rats. The cardiac cytoskeletal impairment was more severe in ouabain-treated rats with further changes of Profilin-1 and Desmin. Cytoskeletal abnormality is an ultra-early change during ouabain-induced ventricular hypertrophy, before the release of hypertrophic factors. Therapy for prevention of ouabain-induced hypertrophy should start at the early stage by preventing the cytoskeleton from disorganization.

The R(h)oads to Stat3: Stat3 activation by the Rho GTPases

The signal transducer and activator of transcription-3 (Stat3) is a member of the STAT family of cytoplasmic transcription factors. Overactivation of Stat3 is detected with high frequency in human cancer and is considered a molecular abnormality that supports the tumor phenotype. Despite concerted investigative efforts, the molecular mechanisms leading to the aberrant Stat3 activation and Stat3-mediated transformation and tumorigenesis are still not clearly defined. Recent evidence reveals a crosstalk close relationship between Stat3 signaling and members of the Rho family of small GTPases, including Rac1, Cdc42 and RhoA. Specifically, Rac1, acting in a complex with the MgcRacGAP (male germ cell RacGAP), promotes tyrosine phosphorylation of Stat3 by the IL6-receptor family/Jak kinase complex, as well as its translocation to the nucleus. Studies have further revealed that the mutational activation of Rac1 and Cdc42 results in Stat3 activation, which occurs in part through the upregulation of IL6 family cytokines that in turn stimulates Stat3 through the Jak kinases. Interestingly, evidence also shows that the engagement of cadherins, cell to cell adhesion molecules, specifically induces a striking increase in Rac1 and Cdc42 protein levels and activity, which in turn results in Stat3 activation. In this review we integrate recent findings clarifying the role of the Rho family GTPases in Stat3 activation in the context of malignant progression.

Biological signalling activity measurements using mass spectrometry

MS (mass spectrometry) techniques are rapidly evolving to high levels of performance and robustness. This is allowing the application of these methods to the interrogation of signalling networks with unprecedented depth and accuracy. In the present review we discuss how MS-based multiplex quantification of kinase activities and phosphoproteomics provide complementary means to assess biological signalling activity. In addition, we discuss how a wider application of these analytical concepts to quantify kinase signalling will result in a more comprehensive understanding of normal and disease biology at the system level.

The role of the c-Jun N-terminal kinase 2-α-isoform in non-small cell lung carcinoma tumorigenesis

The c-Jun N-terminal kinases (JNKs) are members of the mitogen-activated protein kinase family and have been implicated in tumorigenesis. One isoform in particular, JNK2α, has been shown to be frequently activated in primary brain tumors, to enhance several tumorigenic phenotypes and to increase tumor formation in mice. As JNK is frequently activated in non-small cell lung carcinoma (NSCLC), we investigated the role of the JNK2α isoform in NSCLC formation by examining its expression in primary tumors and by modulating its expression in cultured cell lines. We discovered that 60% of the tested primary NSCLC tumors had three-fold higher JNK2 protein and two- to three-fold higher JNK2α mRNA expression than normal lung control tissue. To determine the importance of JNK2α in NSCLC progression, we reduced JNK2α expression in multiple NSCLC cell lines using short hairpin RNA. Cell lines deficient in JNK2α had decreased cellular growth and anchorage-independent growth, and the tumors were four-fold smaller in mass. To elucidate the mechanism by which JNK2α induces NSCLC growth, we analyzed the JNK substrate, signal transducer and activator of transcription 3 (STAT3). Our data demonstrates for the first time that JNK2α can regulate the transcriptional activity of STAT3 by phosphorylating the Ser727 residue, thereby regulating the expression of oncogenic genes, such as c-Myc. Furthermore, reintroduction of JNK2α2 or STAT3 restored the tumorigenicity of the NSCLC cells, demonstrating that JNK2α is important for NSCLC progression. Our studies reveal a novel mechanism in which phosphorylation of STAT3 is mediated by a constitutively active JNK2 isoform, JNK2α.

Activation of Src protein tyrosine kinase plays an essential role in urocortin-mediated cardioprotection

Urocortin is a 40 amino acid peptide of the corticotrophin-releasing factor (CRF) family that is synthesized and released by cardiac myocytes. Endogenous urocortin expression is increased during ischemia/reperfusion (I/R) and addition of exogenous urocortin reduces cell death caused by I/R injury. Studies have also showed that the protective action of urocortin is mediated by the activation of ERK1/2. We discovered that a non-receptor tyrosine kinase, Src, is involved in the urocortin-induced activation of ERK1/2 in mouse atrial HL-1 myocytes. The selective Src family kinase inhibitor, PP2, reduced the urocortin-induced phosphorylation of ERK1/2, and so did the expression of a dominant-negative mutant of Src in transfected HL-1 cells. Inhibition of Src by PP2 also reduced urocortin's protective effects in HL-1 cells after hypoxia/reoxygenation (H/R), as assessed by flow cytometry and caspase-3 activation assay. Titration studies indicated that as little as 10(-8)M urocortin was sufficient to induce Src activation. Maximal phosphorylation/activation of Src and ERK1/2 were both detected after 5 min incubation with urocortin. These effects of urocortin were largely mediated by CRF receptor-1, although a minor contribution of CRF receptor-2 cannot be excluded. Here we report for the first time that short-term treatment with urocortin causes rapid phosphorylation of Src, and that the urocortin-activated Src kinase serves as an upstream modulator of ERK1/2 activation, playing an essential role in urocortin-mediated cardioprotection.

Beyond structure, to survival: activation of Stat3 by cadherin engagement

Cells in normal tissues or in tumors have extensive opportunities for adhesion to their neighbors and the importance of cell to cell contact in the study of fundamental cellular processes is beginning to emerge. In this review, we discuss recent evidence of dramatic changes in the activity of an important signal transducer found to be profoundly affected by cell to cell adhesion, the signal transducer and activator of transcription-3 (Stat3). Direct cadherin engagement, growth of cells to postconfluence, or formation of multicellular aggregates were found to induce a striking increase in the levels of Stat3 activity, Rac1/Cdc42, and members of the IL6 receptor family in different settings. This activation was specific to Stat3, in that the levels of the extracellular signal regulated kinase (Erk1/2), a signal transducer often coordinately activated with Stat3 by a number of growth factors or oncogenes, remained unaffected by cell density. Density-dependent Stat3 activation may play a key role in survival, and could contribute to the establishment of cell polarity. It is clear that at any given time the total Stat3 activity levels in a cell are the sum of the effects of cell to cell adhesion plus the conventional Stat3 activating factors present.

Differential epidermal growth factor receptor signaling regulates anchorage-independent growth by modulation of the PI3K/AKT pathway

Tumor cells are capable of surviving loss of nutrients and anchorage in hostile microenvironments. Under these conditions adapting to specific signaling pathways may shift the balance between growth and cellular dormancy. Here we report a mechanism by which EGFR differentially modulates the PI3K/AKT pathway in cellular stress conditions. When carcinoma cells were cultured as multicellular aggregates (MCA), cyclin D1 was induced through a serum-dependent EGFR activating pathway, triggering cell proliferation. The expression of cyclin D1 required both EGFR-mediated ERK and AKT activation. In serum-starved MCAs, EGFR activation was associated with active ERK1/2 but not AKT and failed to induce cyclin D1. Analysis revealed that, under serum-starved conditions, EGFR-Y1086 residue was poorly autophosphorylated and this correlated with failure to phosphorylate Gab1. Accordingly, the EGFR activation failed to induce EGFR/PI3K complex formation or AKT activation, preventing cyclin D1 induction. Furthermore, we show that in serum-starved MCA, expression of constitutively active AKT re-established cyclin D1 expression and induced proliferation in an EGFR-dependent manner. Thus, modulation of the PI3K/AKT pathway by context-dependent EGFR signaling may regulate tumor cell growth and dormancy.

Activated Rac1 requires gp130 for Stat3 activation, cell proliferation and migration

Rac1 (Rac) is a member of the Rho family of small GTPases which controls cell migration by regulating the organization of actin filaments. Previous results suggested that mutationally activated forms of the Rho GTPases can activate the Signal Transducer and Activator of Transcription-3 (Stat3), but the exact mechanism is a matter of controversy. We recently demonstrated that Stat3 activity of cultured cells increases dramatically following E-cadherin engagement. To better understand this pathway, we now compared Stat3 activity levels in mouse HC11 cells before and after expression of the mutationally activated Rac1 (Rac(V12)), at different cell densities. The results revealed for the first time a dramatic increase in protein levels and activity of both the endogenous Rac and Rac(V12) with cell density, which was due to inhibition of proteasomal degradation. In addition, Rac(V12)-expressing cells had higher Stat3, tyrosine-705 phosphorylation and activity levels at all densities, indicating that Rac(V12) is able to activate Stat3. Further examination of the mechanism of Stat3 activation showed that Rac(V12) expression caused a surge in mRNA of Interleukin-6 (IL6) family cytokines, known potent Stat3 activators. Knockdown of gp130, the common subunit of this family reduced Stat3 activity, indicating that these cytokines may be responsible for the Stat3 activation by Rac(V12). The upregulation of IL6 family cytokines was required for cell migration and proliferation induced by Rac(V12), as shown by gp130 knockdown experiments, thus demonstrating that the gp130/Stat3 axis represents an essential effector of activated Rac for the regulation of key cellular functions.

Cadherin-cadherin engagement promotes cell survival via Rac1/Cdc42 and signal transducer and activator of transcription-3

Signal transducer and activator of transcription-3 (Stat3) is activated by a number of receptor and nonreceptor tyrosine kinases, whereas a constitutively active form of Stat3 alone is sufficient to induce neoplastic transformation. In the present report, we show that Stat3 can also be activated through homophilic interactions by the epithelial (E)-cadherin. Indeed, by plating cells onto surfaces coated with fragments encompassing the two outermost domains of this cadherin, we clearly show that cadherin engagement can activate Stat3, even in the absence of direct cell-to-cell contact. Most importantly, our results also reveal for the first time an unexpected and dramatic surge in total Rac1 and Cdc42 protein levels triggered by cadherin engagement and an increase in Rac1 and Cdc42 activity, which is responsible for the Stat3 stimulation observed. Inhibition of cadherin interactions using a peptide, a soluble cadherin fragment, or genetic ablation induced apoptosis, points to a significant role of this pathway in cell survival signaling, a finding that could also have important therapeutic implications. (Mol Cancer Res 2009;7(8):1310-27).

Inhibition of STAT-3 results in radiosensitization of human squamous cell carcinoma

BACKGROUND

Signal transducer and activator of transcription-3 (STAT-3) is a downstream component of the Epidermal Growth Factor Receptor (EGFr) signaling process that may facilitate the resistance of tumor cells to conventional cancer treatments. Studies were performed to determine if inhibition of this downstream protein produces radiosensitization.

METHODS/RESULTS

A431 cells (human squamous cell carcinoma cells with EGFr overexpression) were found to be sensitized to radiation after treatment with STAT-3 small interfering RNA (siRNA). Therefore, a short hairpin RNA (shRNA) against STAT-3 was designed and cloned into a pBABE vector system modified for shRNA expression. Following transfection, clone 2.1 was selected for further study as it showed a dramatic reduction of STAT-3 protein (and mRNA) when compared to A431 parental cells or a negative control shRNA cell line (transfected with STAT-3 shRNA with 2 base pairs mutated). A431 2.1 showed doubling times of 25-31h as compared to 18-24h for the parental cell line. The A431 shRNA knockdown STAT-3 cells A431 were more sensitive to radiation than A431 parental or negative STAT-3 control cells.

CONCLUSION

A431 cells stably transfected with shRNA against STAT-3 resulted in enhanced radiosensitivity. Further work will be necessary to determine whether the inhibition of STAT-3 phosphorylation is a necessary step for the radiosensitization that is induced by the inhibition of EGFr.

Polarized subcellular localization of Jak/STAT components is required for efficient signaling

Three protein complexes control polarization of epithelial cells: the apicolateral Crumbs and Par-3 complexes and the basolateral Lethal giant larvae complex. Polarization results in the specific localization of proteins and lipids to different membrane domains. The receptors of the Notch, Hedgehog, and WNT pathways are among the proteins that are polarized, with subcellular receptor localization representing an important aspect of signaling regulation. For example, in the WNT pathway, differential DFz2 receptor localization results in activation of either the canonical or the planar polarity pathway. Despite the large body of research on the vertebrate JAK/STAT pathway, there are no reports indicating polarized signaling. By using the conserved Drosophila JAK/STAT pathway as a system, we find that the receptor and its associated kinase are located in the apical membrane of epithelial cells. Unexpectedly, the transcription factor STAT is enriched in the apicolateral membrane domain of ectoderm epithelial cells in a Par-3-dependent manner. Our results indicate that preassembly of STAT and the Receptor/JAK complex to specific membrane domains is a key aspect for signaling efficiency. Our results also suggest that receptor polarization in the ectoderm cell membrane restricts the cell's response to ligands provided by neighboring cells.

STAT3 activation in pressure-overloaded feline myocardium: role for integrins and the tyrosine kinase BMX

Growth, survival and cytoskeletal rearrangement of cardiomyocytes are critical for cardiac hypertrophy. Signal transducer and activator of transcription-3 (STAT3) activation is an important cardioprotective factor associated with cardiac hypertrophy. Although STAT3 activation has been reported via signaling through Janus Kinase 2 (JAK2) in several cardiac models of hypertrophy, the importance of other nonreceptor tyrosine kinases (NTKs) has not been explored. Utilizing an in vivo feline right ventricular pressure-overload (RVPO) model of hypertrophy, we demonstrate that in 48 h pressure-overload (PO) myocardium, STAT3 becomes phosphorylated and redistributed to detergent-insoluble fractions with no accompanying JAK2 activation. PO also caused increased levels of phosphorylated STAT3 in both cytoplasmic and nuclear fractions. To investigate the role of other NTKs, we used our established in vitro cell culture model of hypertrophy where adult feline cardiomyocytes are embedded three-dimensionally (3D) in type-I collagen and stimulated with an integrin binding peptide containing an Arg-Gly-Asp (RGD) motif that we have previously shown to recapitulate the focal adhesion complex (FAC) formation of 48 h RVPO. RGD stimulation of adult cardiomyocytes in vitro caused both STAT3 redistribution and activation that were accompanied by the activation and redistribution of c-Src and the TEC family kinase, BMX, but not JAK2. However, infection with dominant negative c-Src adenovirus was unable to block RGD-stimulated changes on either STAT3 or BMX. Further analysis in vivo in 48 h PO myocardium showed the presence of both STAT3 and BMX in the detergent-insoluble fraction with their complex formation and phosphorylation. Therefore, these studies indicate a novel mechanism of BMX-mediated STAT3 activation within a PO model of cardiac hypertrophy that might contribute to cardiomyocyte growth and survival.

The function of Stat3 in tumor cells and their microenvironment

Stat3 was initially recognized as a transcription factor and mediates the nuclear action of many different cytokines and growth factors. In addition to its roles in normal cell function, the inappropriate activation of Stat3 in tumor cells has attracted the attention of tumor biologists and has led to the consideration of Stat3 as a drug target. The induction of Stat3 activity under physiological circumstances is transient and many different levels of activation and deactivation have been defined. In addition to kinases and phosphatases, the SOCS proteins and the PIAS proteins have been recognized as negatively regulating components, which fine-tune the extent and the duration of Stat3 function. Its nuclear cytoplasmic shuttling is exquisitely regulated and adds to the complexity of Stat3 action. Newly discovered associations with cytoplasmic molecules suggest functions outside the conventional transcriptional regulation context. High molecular weight transcription complexes suggest that Stat3 might assume roles in transcriptional induction as well as in transcriptional suppression. The aberrant activation in tumor cells and the central function of Stat3 in the communication between cells of the immune system and tumor cells are of great interest for translational research projects and innovative drug development.