STAT3 exerts two-way regulation in the biological effects of IL-6 in M1 leukemia cells

The NEDD8-activating enzyme inhibitor MLN4924 induces G2 arrest and apoptosis in T-cell acute lymphoblastic leukemia

The first-in-class compound MLN4924 is a small molecule inhibitor that selectively inactivates NEDD8-activating enzyme (NAE). The anticancer effects of MLN4924 have been attributed to impaired neddylation of Cullin proteins. Here, we show that treatment of T-cell acute lymphoblastic leukemia (T-ALL) cells with MLN4924 potently suppressed the neddylation of Cullins and the oncogenic growth of T-ALL cells in-vitro. Moreover, MLN4924 induced disease regression in an in vivo xenograft model. MLN4924 also induced cell cycle arrest at G2 phase and apoptosis in T-ALL cells. However, inhibition of the neddylation of Cullins alone could not explain the effects of MLN4924 in T-ALL cells. Gene expression profiling indicated ribosome function, steroid biosynthesis, and hematopoietic cell lineage pathways were affected by MLN4924 treatment. MLN4924 also induced nucleolar disruption, suggesting nucleolar stress signaling might contribute to the anticancer effects of MLN4924 in T-ALL cells. In addition, MLN4924 treatment reduced 14-3-3ξ\δ protein levels in T-ALL cells. Thus, MLN4924 may inhibit T-ALL cell proliferation via several pathways.

EGFR-mediated apoptosis via STAT3

The Epidermal Growth Factor Receptor (EGFR) is a cell surface receptor with primary implications in cell growth in both normal and malignant tissue. Paradoxically, cell lines that hyperexpress the EGFR have been documented to undergo receptor-mediated apoptosis. The underlying mechanism by which EGF-induced apoptosis occurs however remains inexplicit. In an attempt to identify this mechanism, we assessed downstream effectors of EGFR in MDA-MB-468 cells during conditions of EGF-induced apoptosis. The effector assessment revealed STAT3 as a potential mediator of EGF-induced apoptosis. Alternative strategies for activating STAT3, independent of EGFR stimulation, resulted in the induction of the apoptotic pathways. A reduction in STAT3 expression via RNAi resulted in a significant attenuation of EGF-induced PARP cleavage. Our findings support STAT3 as a positive mediator of EGF-induced apoptosis in MDA-MB-468 cells.

RACK1 promotes the proliferation of THP1 acute myeloid leukemia cells

The receptor for activated C kinase 1 (RACK1), an adaptor protein implicated in the regulation of multiple signaling pathways, has been reported to contribute to the survival of leukemic progenitor cells by enhancing the activity of glycogen synthase kinase 3β (GSK3β). However, it remains unknown whether RACK1 also contributes to the oncogenic growth of acute myeloid leukemia (AML) cells. Here, we report that transient or stable silencing of endogenous RACK1 expression by RACK1 short hairpin RNAs (shRNAs) led to impaired proliferation of THP1 AML cells without inducing terminal differentiation. Further exploration revealed that RACK1 loss-of-function resulted in reduced GSK3β activity. GSK3β shRNA treatment showed similar effects to RACK1 loss-of-function. Our data collectively suggest that RACK1 contributes to THP1 cell proliferation through, at least partially, enhancing GSK3β activity. Thus, targeting RACK1 may have some important therapeutic implications in the treatment of AML.

Knockdown of stat3 expression by RNAi inhibits in vitro growth of human ovarian cancer

Background

The aim of the study was to investigate the suppressive effects of pSilencer2.1-U6-siRNA-stat3 recombinant plasmids on the growth of ovarian cancer in vitro.

Material and methods.

Three pairs of DNA template (stat3-1, stat3-2, stat3-3) specific for different target sites on stat3 mRNA were synthesized to reconstruct pSilencer2.1-U6-siRNA-stat3s, which were transfected into SKOV3 cells. The expressions of STAT3, BcL-2, cyclin D1 and C-myc in these cells were detected by Western blot and Northern blot. The cell cycle and the growth were determined by flow cytometry (FCM) and MTT assay, respectively. Cell apoptosis was determined by TUNEL staining.

Results

Of the three siRNAs, only siRNA targeting stat3-3 markedly suppressed the protein expression of stat3 in SKOV3 cells; MTT assay and FCM showed that transfection of stat3-3 siRNA could significantly suppress the growth of SKOV3 cells and arrest the cell cycle in vitro. TUNEL staining also showed massive apoptosis in SKOV3 cells transfected with stat3-3 siRNA.

Conclusions

pSilencer2.1-U6-siRNA-stat3-3 can significantly inhibit the STAT3 expression in human ovarian cancer cells resulting in the inhibition of the cancer growth and the increase of apoptosis of cancer cells.

Blockade of Janus kinase-2 signaling ameliorates mouse liver damage due to ischemia and reperfusion

Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling is one of the major pathways for cytokine signal transduction. However, the role of the JAK/STAT pathway in liver ischemia/reperfusion is not clear. This study focuses on Janus kinase-2 (JAK2), which functions upstream of signal transducer and activator of transcription 1 (STAT1) in JAK/STAT, and its role in the mechanism of liver ischemia/reperfusion injury (IRI). Partial warm ischemia was produced in the hepatic lobes of C57BL/6 mice for 90 minutes, and this was followed by 6 hours of reperfusion. Mice were treated with a JAK2 inhibitor (tyrphostin AG490; 40 mg/kg intraperitoneally) or vehicle 60 minutes prior to ischemic insult. JAK2 blockade resulted in a significant reduction of hepatocyte apoptosis and liver injury. Macrophage and neutrophil infiltration, as assessed by immunohistochemistry, was markedly decreased in AG490-treated livers in comparison with controls. The expression of pro-inflammatory cytokines [tumor necrosis factor alpha, interleukin 6 (IL-6), and IL-1beta] and chemokines [chemokine (C-X-C motif) ligand 10 (CXCL-10) and CXCL-2] was also significantly reduced in the AG490-treated group in comparison with controls. AG490-treated livers showed fewer cells positive for terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling and reduced cleaved caspase-3 protein expression in parallel with increased B-cell lymphoma extra large expression. We employed AG490 (75 mM) in primary bone marrow-derived macrophage (BMM) and hepatoma cell (CRL1830) cultures, which were both stimulated with lipopolysaccharide (LPS; 10 ng/mL). In BMM cultures, AG490 depressed otherwise LPS-induced pro-inflammatory gene expression programs (IL-6, IL-12p40, IL-1beta, CXCL-10, and inducible nitric oxide synthase). In hepatoma cells, AG490 reduced cleaved caspase-3 expression. Moreover, JAK2 blockade inhibited STAT1 and STAT3 phosphorylation. This is the first report documenting that JAK2 signaling is essential in the pathophysiology of liver IRI, as its selective blockage ameliorated the disease process and protected livers from inflammation and apoptosis.

Basal c-Jun NH2-terminal protein kinase activity is essential for survival and proliferation of T-cell acute lymphoblastic leukemia cells

Hyperactivation of c-Jun NH2-terminal protein kinase (JNK) has been found in various malignant lymphocytes and inhibition of JNK activity leads to cell cycle arrest and apoptosis. However, the role of JNK activity in the oncogenic growth of T-cell acute lymphoblastic leukemia (T-ALL) cells remains largely unknown. Here, we report that treatment of T-ALL cells with JNK inhibitors led to cell cycle arrest and apoptosis and increased sensitivity to Fas-mediated apoptosis, whereas weak ectopic expression of MKK7-JNK1 fusion protein, which shows constitutive JNK activity, in T-ALL cells resulted in accelerated cell cycle progression and resistance to Fas-mediated apoptosis. The protein levels of c-Myc and Bcl-2 were reduced in the presence of JNK inhibitors but were enhanced with MKK7-JNK1. Small interfering RNA against JNK1, but not JNK2, exhibited similar effects to JNK inhibitors. These findings suggest that targeting JNK, especially JNK1 isoform, may have some important therapeutic implications in the treatment of T-ALL. Further exploration revealed that JNK protein and basal JNK activity in T-ALL cells showed aberrant subcellular localization, but no hyperactivation of JNK was observed. Thus, our work suggests that there might be novel mechanism(s) other than hyperactivation underlying the protumorigenic role of JNK activity.

p38 MAPK contributes to CD54 expression and the enhancement of phagocytic activity during macrophage development

p38 is a subfamily of the mitogen-activated protein kinase (MAPK) superfamily with four isoforms. It has been well established that p38 plays a central role in the production of inflammatory molecules and is therefore required for the activation of macrophages in response to inflammatory stimuli. However, little is known about the roles of p38 in macrophage development. The difficulty to get mice deficient in multiple p38 isoforms complicates the study of p38 in macrophage development. With the model of bone marrow-derived murine macrophages and highly selective p38alpha/beta inhibitors SB203580 and SB239063, here we report that macrophage colony-stimulating factor (M-CSF) induces p38 activation during macrophage development. Inhibition of p38 activity showed minor effects on macrophage proliferation or survival, and did not block CD14, F4/80 expression. However, p38 inhibitors resulted in a significant reduction in CD54 expression and impaired phagocytic activity. Taken together, our data suggest that p38 contributes to macrophage development.

Inhibitory effect of linomide on lipopolysaccharide-induced proinflammatory cytokine tumor necrosis factor-alpha production in RAW264.7 macrophages through suppression of NF-kappaB, p38, and JNK activation

Linomide is an immunomodulator that can effectively inhibit the development of several autoimmune diseases in animal models. Previously, linomide was shown to influence macrophage function, although the mechanism was elusive. In this study, we investigated the effect of linomide on the macrophage inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha), production induced by lipopolysaccharide (LPS) in vitro on the murine macrophage cell line, RAW264.7. Linomide exposure reduced LPS-evoked TNF-alpha production in a dose-dependent manner. Gel shift and reporter gene analyses revealed linomide inhibited LPS-induced NF-kappaB binding to the NF-kappaB consensus oligonucleotide and NF-kappaB-mediated reporter gene expression. Immunoblot analysis showed that linomide inhibited phosphorylation of p38 kinase and c-jun N terminal kinase (JNK) in LPS-stimulated RAW264.7 cells. Taken together, these results suggest that linomide inhibits TNF-alpha production by suppressing the activation of NF-kappaB and mitogen-activated protein kinase (MAPK), which might, at least in part, contribute to the beneficial effects of linomide in the treatment of autoimmune diseases.

Drug discovery using the regulation of gene expression

The expression of a disease-relevant protein is controlled by a transcriptional program specifically regulated at all stages of normal development and during the adult life. Thus, regulation of gene expression as an approach to drug discovery is conceptually appealing because it provides a rational basis for molecular strategies aimed at modulating gene expression in given cell types and/or at a given time. Indeed, numerous pharmacologic agents have been identified that can either restore or suppress disease-relevant protein expression. In this review, the author critically examines new strategies and methodologies that are being used and developed to identify and validate new therapeutic targets by taking advantage of our knowledge on mechanisms regulating their expression at the transcriptional and post-transcriptional levels. The author also examines the impact of genome-wide approaches and methods aimed at controlling epigenetic mechanisms of gene regulation and concludes by extrapolating on future trends.

IKK antagonizes CD95 ligation-mediated apoptosis by regulating NF-κB activity

The CD95 (Apo1/Fas)/CD95 ligand system plays pivotal roles in various aspects of immune regulation and function by triggering apoptosis. Besides the apoptosis signaling pathway, CD95 ligation also induces the activation of NF-kappaB. Previous studies suggest that IkappaB kinase (IKK) may be a key player in cell survival by mediating NF-kappaB activation. However, the roles of IKK in CD95 ligation-mediated apoptosis and NF-kappaB activation are still not clear. In this report, we show that expression of the caspase-resistant uncleavable IKKbeta (UCIKKbeta) mutant suppressed CD95 ligation-mediated cell death in HeLa cells. Furthermore, CD95 ligation induced much more cell death in IKKbeta-/- murine embryonic fibroblasts (MEFs) than in wild type MEFs, despite that IKK was only marginally activated upon CD95 ligation. Pretreatment of HeLa cells with a specific IKK inhibitor NEMO-binding domain (NBD) peptide blocked CD95 ligation-induced NF-kappaB transcriptional activity. And UCIKKbeta enhanced the basal NF-kappaB activity, and consequently led to higher NF-kappaB activity upon CD95 ligation in HeLa cells. Therefore, IKK antagonizes CD95 ligation-mediated apoptosis by regulating NF-kappaB activity.

Cytokines in the differentiation therapy of leukemia: from laboratory investigations to clinical applications

Differentiation therapy of leukemia is the treatment of leukemia cells with biological or chemical agents that induce the terminal differentiation of the cancer cells. It is regarded as a novel and targeted approach to leukemia treatment, based on our better understanding of the hematopoietic process and the mechanisms of its deregulation during leukemogenesis. Clinically, differentiation therapy has been most successful in acute promyelocytic leukemia using all-trans-retinoic acid as the inducer, either alone or in combination with chemotherapy. This review presents evidence that a number of hematopoietic cytokines play important roles in both normal and aberrant hematopoietic processes. In vitro laboratory investigations in the past two decades using well-characterized myeloid leukemic cell lines and primary blast cells from leukemia patients have revealed that many hematopoietic cytokines can trigger lineage-specific differentiation of leukemia cells, which may have important implications in the clinical setting. Moreover, our current understanding of cytokine interactions and the molecular mechanisms of cytokine-induced leukemic cell differentiation will be discussed in the light of recent findings. Finally, ways in which laboratory research on cytokines in the differentiation therapy of leukemia can lead to the improved design of protocols for future clinical applications to leukemia therapy will also be addressed.

Atiprimod blocks phosphorylation of JAK-STAT and inhibits proliferation of acute myeloid leukemia (AML) cells

In studies of multiple myeloma cells, atiprimod was shown to block Stat3 activation and inhibited colony-forming cell proliferation. We hypothesized that atiprimod may also inhibit activation of intracellular signaling pathways in AML cells resulting in apoptosis and growth inhibition. We demonstrate that atiprimod inhibited clonogenic growth of AML cell lines and fresh AML marrow cells whereas it did not significantly affect growth of normal hematopoietic progenitors from marrow samples of healthy controls. Atiprimod decreased phosphorylation of Stat3 and Stat5, and protein levels of Jak2, whereas gene expression of Jak2 was not affected. Atiprimod further induced apoptosis by cleavage of caspase 3 and PARP. In summary, our data suggest that atiprimod has a significant antiproliferative and proapoptotic effect on AML cells. This effect may be facilitated by inhibition of the Jak-Stat signaling pathway. Further evaluation of atiprimod in clinical trials of AML should be considered.

The rational designed antagonist derived from the complex structure of interleukin-6 and its receptor affectively blocking interleukin-6 might be a promising treatment in multiple myeloma

Human interleukin-6 is involved in the maintenance and progression of several diseases such as multiple myeloma (MM), rheumatoid arthritis, or osteoporosis. Our previous work demonstrated that an interleukin-6 antagonist peptide (named PT) possessed potential bioactivity to antagonize the function of hIL-6 and could efficiently induce the growth arrest and apoptosis of XG-7 and M1 cells in a dose-dependent manner. In this study, the theoretical interaction of the peptide PT with its receptor was analyzed further more with molecular docking and molecular dynamics methods. The theoretical studies showed that PT possessed very high affinity to interleukin-6R and offered a practical means of imposing long-term blockade of interleukin-6 activity in vivo. According to the theoretical results, the biological evaluation of PT was researched on two different cells models with more sensitive approaches: (1) The antagonist activity of PT was studied on the interleukin-6 dependent MM cells (XG-7) cultured with interleukin-6. In the other interleukin-6 dependent MM cells (SKO-007), they survived themselves by auto/paracrine without the exogenous interleukin-6, and also could be antagonized by PT. The therapeutic value of PT only limited on the interleukin-6 dependent category in MM. (2) Myeloid leukemia M1 cells were induced for growth arrest and apoptosis in response to interleukin-6. The results supported our previous findings and showed that PT could be evaluated by protecting the cells from interleukin-6 induced apoptosis. In conclusion, PT could induce interleukin-6-dependent XG-7 and SKO-007 cells to apoptosis while inhibit interleukin-6-stimulated apoptosis in M1 cells.

CD40 mediated human cholangiocyte apoptosis requires JAK2 dependent activation of STAT3 in addition to activation of JNK1/2 and ERK1/2

CD40 is critically involved in Fas-mediated cholangiocyte apoptosis during liver inflammation, but the underlying signalling events are poorly understood. Our recent work implicated AP-1 in CD40-induced cholangiocyte apoptosis, but suggested involvement of other signalling pathways. Because STAT3 has been implicated in liver regeneration we investigated this signalling pathway during CD40 mediated cholangiocyte apoptosis. Western immunoblotting, electrophoretic mobility gel shift assays, In situ DNA end labelling and caspase-3 activity were used to investigate intracellular signalling and apoptosis in primary human cholangiocytes following CD40 activation. CD40-activation induced caspase-3 dependent cholangiocyte apoptosis and 3-fold increases in JNK/ERK phosphorylation (concomitant with increased AP-1 binding activity) and 4-fold increases in pSTAT3, which were sustained for up to 24 h. Protein levels of c-Jun, c-Fos and pSTAT3 confirmed the upregulation. Phosphorylation of p38 remained unchanged suggesting that this MAP kinase was not involved in CD40 mediated apoptosis. Increased JAK2 phosphorylation accompanied increased STAT3 phosphorylation after CD40 ligation. Cholangiocytes were also shown to express JAK1 and 3 which was phosphorylated following control stimulation with TNFalpha or IL2 respectively but not after CD40 ligation. JNK, ERK and JAK2 inhibitors partially abrogated apoptosis and when used in combination reduced it to basal levels. In conclusion, induction of CD40-mediated cholangiocyte apoptosis requires JAK2-mediated phosphorylation of STAT3 as well as sustained JNK1/2, ERK1/2 activation. This study demonstrates that STAT3 can function as a proapoptotic factor in primary human liver epithelial cells.

Proteomic analysis of interleukin 6-induced differentiation in mouse myeloid leukemia cells

Cytokine-induced differentiation of myeloid leukemia cells has important therapeutic implications, but the mechanism remains to be clarified. M1 cell, a mouse acute myeloid leukemia cell line, which underwent growth inhibition, terminal differentiation and apoptosis in response to IL-6, was selected as an experimental model to study on the molecular mechanisms of myeloid cell differentiation on a proteome-wide scale. Cell differentiation was evaluated by cell morphology and CD11b expression. With two-dimensional (2D) gel analyses, 17 protein spots showed obvious changes in quantity during the process of differentiation were found. With matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF-MS) or/and nano-electrospray ionization MS/MS (ESI-MS/MS) analysis, 15 protein spots were identified. The mRNA levels of these 15 proteins during differentiation were also examined using a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis. Except two proteins, the mRNA levels demonstrated similar expression patterns to what the proteomic analysis revealed. The identified proteins were known to be involved in different cellular functions, including protein synthesis, transcription, signal transduction, cell cycle control, cell rescue and defense, cellular organization, and metabolism. Notably, seven proteins were not described before to be involved in differentiation. Our data provide novel information for a better understanding of the mechanisms by which terminal differentiation of acute myeloid leukemia cells induced by IL-6.

A novel hIL-6 antagonist peptide from computer-aided design contributes to suppression of apoptosis in M1 cells

Based on the complex crystal structure of human interleukin-6 (hIL-6) and its receptor (hIL-6R), a novel hIL-6 antagonist peptide (named PT) was designed using computer-guided design method. Dealing with molecular docking and molecular dynamics methods, the interaction between PT and hIL-6R was analyzed. The theoretical studies showed that PT possessed very high affinity to hIL-6R and offered a practical means of imposing long-term blockade of hIL-6 activity in vivo. This effect was examined due to growth arrest and apoptosis induced by hIL-6 in myeloblastic cell line M1 cells in a dose-dependent manner. The findings demonstrate that PT could also act as an excellent antagonist candidate for the induction of growth arrest and apoptosis. Furthermore, murine M1 myeloid cell line, which was induced by the physiological inducer hIL-6 to undergo apoptosis and growth arrest, could be used as a subtle model system to test hIL-6 antagonist.

Tissue inhibitor of matrix metalloproteinase-1 overexpression in M1 myeloblasts impairs IL-6-induced differentiation

The balance between matrix metalloproteinase (MMP) and tissue inhibitor of matrix metalloproteinase (TIMP) is important for extracellular matrix interactions of hematopoietic cells. MMP-independent growth modulating activity for TIMP-1 on B lymphocytes and erythroid progenitors has also been described, but a role for TIMP-1 in myelomonocytic differentiation has not been previously reported. In this study, we demonstrate that TIMP-1 overexpression impairs differentiation of the myeloblastic M1 cell line following interleukin (IL)-6 stimulation. We generated retroviral vectors coexpressing human TIMP-1 and the green fluorescent protein (GFP) and stably transduced murine M1 myeloid cells. TIMP-1 expressing cells showed a large reduction in IL-6-induced macrophage differentiation in vitro that was reversible with a specific monoclonal antibody. The differentiation delay in M1/TIMP-1 cells was also specifically reversible by pharmacologic phosphatidylinositol-3 kinase (PI3-K) inhibition. Additionally, overexpression of a TIMP-1/GFP fusion protein also impaired M1 differentiation and this protein was localized to the cell surface, consistent with an autocrine receptor-mediated mechanism. Surprisingly, TIMP-1 transduced cells had a selective advantage for growth in IL-6, indicating that functional effects on growth and differentiation of M1 cells were primarily through an autocrine mechanism. Intrinsic TIMP-1 expression in myeloid leukemia cells might thus impact upon survival or differentiation.

Translocation of transcription regulators into the nucleus during granulocyte commitment of HL-60 cells

Expression of transcription factors required for lineage commitment of differentiating cells (C/EBPβ and c-Myb) and for survival of differentiated cells (STATs and NFκB) was examined in the HL-60 cell line. Differentiation was induced by treating the cells with retinoic acid. c-Myb expression in the nucleus restored at the precommitment stage (18 h) what concurred with the highest nuclear level of C/EBPβ, which suggests a combinatorial interaction of these transcription factors in the granulocytic signalling pathway. Expression of STAT5a and STAT5b varied during differentiation, whereas no significant changes were seen in STAT3 levels. Increased cytosolic level of NFκB p65 during precommitment and commitment stages of granulocytic differentiation coincided with augmentation of the STAT5a protein level, which could be evidence of their possible cooperation during granulocytic-lineage commitment of HL-60 cells. Our results suggest that the studied transcription factors cooperatively promote signalling in the differentiating promyelocytic HL-60 cell line in response to retinoic acid.Key words: C/EBP, Myb, STAT, NFκB, phosphorylation.

Accelerated in vivo growth of prostate tumors that up-regulate interleukin-6 is associated with reduced retinoblastoma protein expression and activation of the mitogen-activated protein kinase pathway

Interleukin-6 (IL-6) is a multifunctional cytokine that activates the signaling pathways of Janus kinases-signal transducers and activators of transcription (STAT) and/or mitogen-activated protein kinases (MAPK) in various tumors. Thus, it modulates cell growth and apoptosis. IL-6 levels are elevated in tissues and sera from prostate cancer patients and IL-6 receptor expression has been detected in prostate cancer cell lines and clinical specimens. Continuous exposure of prostate cancer cells to IL-6 might alter their responsiveness to this cytokine. To gain more insight into the function of IL-6 in prostate carcinoma, we have inoculated LNCaP-IL-6+ cells, generated after prolonged treatment with IL-6, into nude mice (total n = 16, two independent experiments). Controls included animals bearing LNCaP-IL-6- cells, passaged at the same time as LNCaP-IL-6+ cells without supplementation of IL-6. LNCaP-IL-6+ tumor volumes were larger than those of their counterparts at all time points. There were no signs of cachexia in any of the experimental animals and all mice were free of metastases. To better understand the mechanisms responsible for accelerated growth of LNCaP-IL-6+ tumors, we have investigated the expression of cell-cycle regulatory molecules by Western blot analysis. The levels of cyclin-dependent kinase 2 were elevated in LNCaP-IL-6+ cells. There was a strong down-regulation of cyclins D1 and E in the LNCaP-IL-6+ subline. The cell-cycle inhibitor p27 was expressed at a low level in LNCaP-IL-6+ cells and could not be up-regulated by addition of IL-6. Most notably, LNCaP-IL-6+ cells exhibited a reduced expression of the hypophosphorylated form of the retinoblastoma protein (pRb). Accelerated tumor growth in our model system was also associated with alterations in IL-6-signaling pathways. The ability of IL-6 to induce tyrosine phosphorylation of STAT3 was abolished in the LNCaP-IL-6+ subline. In contrast, the levels of the MAPK extracellular signal-regulated kinases 1/2 increased in cells generated after long-term IL-6 treatment. The inhibitor of MAPK kinase PD 98059 retarded the proliferation of LNCaP-IL-6+ but not that of control cells. In summary, we show in the present study that chronic exposure of prostate cancer cells to IL-6 facilitates tumor growth in vivo by abolishment of the growth control by pRb and activation of the MAPK signaling pathway. These findings could be relevant to understand the role of IL-6 in prostate cancer progression.

Requirement of STAT3 activation for differentiation of mucosal stratified squamous epithelium

STAT3, a member of the signal transducers and activators of transcription (STAT) family, has been shown to play a key role in promoting proliferation, differentiation, or cell cycle progression, depending on cell type. A number of signaling pathways are altered in laryngeal papillomas, benign tumors induced by human papillomavirus 6/11. Papillomas overexpress the epidermal growth factor receptor and display enhanced MAP kinase and PI-3-kinase activity. They also show reduced activation of Akt and reduced levels of tyrosine-phosphorylated STAT3, due to overexpression of the tumor suppressor, PTEN. As papillomas show abnormalities in terminal differentiation, we examined the potential role of STAT3 in regulating epithelial differentiation. Laryngeal epithelial cells were suspended in supplemented serum-free medium. Differentiation was measured by Western blot analysis of keratin 13. Normal laryngeal epithelial cells were transfected with a constitutively active STAT3 or a dominant negative STAT3. Cells were transferred to suspension culture 24 h after transfection. Increased expression of keratin 13 was accompanied by the activation of STAT3 when differentiation was induced, and expression of a constitutively active STAT3 (STAT3C) enhanced the expression of keratin 13. In contrast, expression of a dominant negative STAT3 (Y705F) inhibited the expression of keratin 13. We conclude that activation of STAT3 is required for the differentiation of normal human stratified squamous epithelium.