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Although members of the Slfn family have been implicated in the regulation of type I interferon (IFN) responses, the mechanisms by which they mediate their effects remain unknown. In the present study, we provide evidence that targeted disruption of the Slfn2 gene leads to increased transcription of IFN-stimulated genes (ISGs) and enhanced type I IFN-mediated antiviral responses. We demonstrate that Slfn2 interacts with protein phosphatase 6 regulatory subunit 1 (PPP6R1), leading to reduced type I IFN-induced activation of nuclear factor kappa B (NF-κB) signaling, resulting in reduced expression of ISGs. Altogether, these data suggest a novel mechanism by which Slfn2 controls ISG expression and provide evidence for a critical role for Slfn2 in the regulation of IFN-mediated biological responses. [ABSTRACT FROM AUTHOR]

We provide evidence that human SLFN5, an interferon (IFN)-inducible member of the Schlafen (SLFN) family of proteins, exhibits key roles in controlling motility and invasiveness of renal cell carcinoma (RCC) cells. Our studies define the mechanism by which this occurs, demonstrating that SLFN5 negatively controls expression of the matrix metalloproteinase 1 gene (MMP-1), MMP-13, and several other genes involved in the control of malignant cell motility. Importantly, our data establish that SLFN5 expression correlates with a better overall survival in a large cohort of patients with RCC. The inverse relationship between SLFN5 expression and RCC aggressiveness raises the possibility of developing unique therapeutic approaches in the treatment of RCC, by modulating SLFN5 expression. [ABSTRACT FROM AUTHOR]

Statins are 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors that act on the mevalonate pathway and inhibit synthesis of cholesterol, geranylgeranyl pyrophosphate (GGPP) and farnesyl pyrophosphate (FPP). In preclinical studies, these agents have been shown to inhibit proliferation, trigger apoptosis and promote cell differentiation of leukemia. Proposed mechanisms include cholesterol deprivation and inhibition of isoprenylation of important signaling molecules. Case reports and early clinical studies suggest a therapeutic potential for statins in acute myeloid leukemia (AML). In the other leukemias there are limited clinical data, but in vitro studies provide a strong rationale for future studies involving statins. The effects of statins on the immune system may lend these agents to a role in allogeneic stem cell transplant. While many of the studies are early, statins have the future potential to be integrated into conventional chemotherapy regimens with limited side effects. [ABSTRACT FROM AUTHOR]

Statins are HMG-CoA (3-hydroxy-3-methyl-glutaryl-coenzyme A) reductase inhibitors, which block the conversion of HMG-CoA to mevalonate and have potent cholesterol lowering properties. Beyond their importance in the generation of lipid lowering effects, the regulatory effects of statins on the mevalonate pathway have a significant impact on multiple other cellular functions. There is now extensive evidence that statins have anti-inflammatory and anti-neoplastic properties, but the precise mechanisms by which such responses are generated are not well understood. In the present study we demonstrate that statins engage a member of the protein kinase C (PKC) family of proteins, PKCδ, in acute promyelocytic leukemia (APL) cells. Our study shows that atorvastatin and fluvastatin induce proteolytic activation of PKCδ in the APL NB4 cell line, which expresses the t(15;17) translocation. Such engagement of PKCδ results in induction of its kinase domain and downstream regulation of pathways important for statin-dependent leukemia cell differentiation. Our research shows that the function of PKCδ is essential for statin-induced leukemic cell differentiation, as demonstrated by studies involving selective targeting of PKCδ using siRNAs. We also demonstrate that the potent enhancing effects of statins on all- trans retinoic acid (ATRA)-induced gene expression for CCL3 and CCL4 requires the function of PKCδ, suggesting a mechanism by which statins may promote ATRA-induced antileukemic responses. Altogether, our data establish a novel function for PKCδ as a mediator of statin-induced differentiation of APL cells and antileukemic effects. [ABSTRACT FROM AUTHOR]

mTOR-generated signals play critical roles in growth of leukemic cells by controlling mRNA translation of genes that promote mitogenic responses. Despite extensive work on the functional relevance of rapamycin-sensitive mTORC1 complexes, much less is known on the roles of rapamycin-insensitive (RI) complexes, including mTORC2 and Rl-mTORC1, in BCR-ABL-leukemogenesis. We provide evidence for the presence of mTORC2 complexes in BCR-ABL-transformed cells and identify phosphorylation of 4E- BP1 on Thr37/46 and Ser65 as RI-mTORC1 signals in primary chronic myelogenous leukemia (CML) cells. Our studies establish that a unique dual mTORC2/mTORC1 inhibitor, 051-027, induces potent suppressive effects on primitive leukemic progenitors from CML patients and generates antileukemic responses in cells expressing the T3151-BCR-ABL mutation, which is refractory to all BCR-ABL kinase inhibitors currently in clinical use. Induction of apoptosis by 051-027 appears to negatively correlate with induction of autophagy in some types of BCR-ABL transformed cells, as shown by the induction of autophagy during OSI-027-treatment and the potentiation of apoptosis by concomitant inhibition of such autophagy. Altogether, our studies establish critical roles for mTORC2 and Rl-mTORC1 complexes in survival and growth of BCR-ABL cells and suggest that dual therapeutic targeting of such complexes may provide an approach to overcome leukemic cell resistance in CML and Ph+ ALL. [ABSTRACT FROM AUTHOR]

Although the roles of Jak-Stat pathways in type I and II interferon (IFN)-dependent transcriptional regulation are well established, the precise mechanisms of mRNA translation for IFN-sensitive genes remain to be defined. We examined the effects of IFNs on the phosphorylation/activation of eukaryotic translation initiation factor 4B (eIF4B). Our data show that eIF4B is phosphorylated on Ser422 during treatment of sensitive cells with alpha IFN (IFN-α) or IFN-γ. Such phosphorylation is regulated, in a cell type-specific manner, by either the p70 S6 kinase (S6K) or the p90 ribosomal protein S6K (RSK) and results in enhanced interaction of the protein with eIF3A (p170/eIF3A) and increased associated ATPase activity. Our data also demonstrate that IFN-inducible eIF4B activity and IFN-stimulated gene 15 protein (ISG15) or IFN-γ-inducible chemokine CXCL-10 protein expression are diminished in S6k1/S6k2 double-knockout mouse embryonic fibroblasts. In addition, IFN-α-inducible ISG15 protein expression is blocked by eIF4B or eIF3A knockdown, establishing a requirement for these proteins in mRNA translation/protein expression by IFNs. Importantly, the generation of IFN-dependent growth inhibitory effects on primitive leukemic progenitors is dependent on activation of the S6K/eIF4B or RSK/eIF4B pathway. Taken together, our findings establish critical roles for S6K and RSK in the induction of IFN-dependent biological effects and define a key regulatory role for eIF4B as a common mediator and integrator of IFN-generated signals from these kinases. [ABSTRACT FROM AUTHOR]

Type I interferons (IFNs) are cytokines with diverse biological properties, including antiviral, growth inhibitory, and immunomodulatory effects. Although several signaling pathways are activated during engagement of the type I IFN receptor and participate in the induction of lFN responses, the mechanisms of generation of specific signals for distinct biological effects remain to be elucidated. We provide evidence that a novel member of the protein kinase C (PKC) family of proteins is rapidly phosphorylated and activated during engagement of the type I IFN receptor. In contrast to other members of the PKC family that are also regulated by lFN receptors, PKCη does not regulate IFN-inducible transcription of interferon-stimulated genes or generation of antiviral responses. However, its function promotes cell cycle arrest and is essential for the generation of the suppressive effects of IFNα on normal and leukemic human myeloid (colony-forming unit-granulocyte macrophage) bone marrow progenitors. Altogether, our studies establish PKCη as a unique element in IFN signaling that plays a key and essential role in the generation of the regulatory effects of type I IFNs on normal and leukemic hematopoiesis. [ABSTRACT FROM AUTHOR]

Uteroglobin (UG) or Clara Cell 10 kDa protein (CC10) is a small, stable, epithelial secretory anti-inflammatory protein. Uteroglobin has been shown to inhibit neointimal formation in vivo after balloon angioplasty through an unknown mechanism. An interaction between UG and plasma fibronectin (Fn) has been demonstrated in mice. Since Fn plays a key role in endothelial cell (EC) migration and angiogenesis, we investigated whether recombinant human UG (rhUG) affects EC migration via Fn binding. In this report, we show a saturable binding of rhUG to Fn depending on Fn conformation and that rhUG is covalently cross-linked to Fn by transglutaminase (TGase). Additionally, our study highlights that rhUG can also bind to exogenously added or self-secreted Fn on the membrane of human primary microvascular endothelial cells (HMVEC), although these complexes are weakly associated with the plasmalemma. Upon the interaction with Fn in solid phase, rhUG strongly inhibits HMVEC attachment on Fn, but not on other ECM proteins. Consequently, rhUG also inhibits cell migration in a dose dependent fashion (I.C.50 = 65 nM) and hinders the “wound healing” in vitro. The small size, stability and human tolerability of rhUG suggest that rhUG in slow-release form or genetically delivered could be used in humans to modulate cell/Fn interactions in the context of tumor microenvironment or in the context of inflammation and fibrosis. J. Cell. Physiol. 207: 553–561, 2006. © 2006 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Acute promyelocytic leukemia is a form of acute myelogenous leukemia, characterized by the t(15;17) chromososmal translocation and the presence of the abnormal PML-RAR α fusion protein. All- trans- retinoic acid is a potent agent for the treatment of this fatal subtype of AML, and is particularly effective when combined with cytotoxic chemotherapy. The important biological activities of all- trans- retinoic acid in vitro and in vivo have provoked extensive studies over the years, aimed to define the mechanisms by which it induces its antileukemic effects. It is now well established that all- trans- retinoic acid when administered at pharmacological doses can reverse the dominant-negative effects that the PML-RAR α oncoprotein exhibits on the functions of the wild type PML and RARα proteins. All- trans- retinoic acid induces gene transcription via retinoic acid responsive elements (RARE) that are present in the promoters of retinoid-responsive genes that ultimately result in the production of protein products that regulate leukemic cell differentiation and induce cell-cycle arrest. There is now accumulating evidence that additional signalling pathways are activated during all- trans- retinoic acid-treatment of cells, involving Stat-proteins, tyrosine kinases and mitogen-activated protein (Map) kinases. This review summarizes the current knowledge on the signalling cascades activated by all- trans- retinoic acid in APL cells. The clinical implications and potential translational applications from the accumulating knowledge in the field are also discussed. [ABSTRACT FROM AUTHOR]

Describes the role of p38α map kinase in type I interferon signaling. Transcriptional factors; Activation of downstream effectors; Gene transcription; Antiviral properties; Phosphorylation; Biological responses.

We sought to determine the functional role of the CrkL adapter protein and downstream pathways in interferon signaling. In experiments using CrkL−/− mouse embryonic fibroblasts, we found that CrkL is required for IFNα-dependent gene transcription via GAS elements, apparently via the formation of DNA-binding complexes with Stat5. On the other hand, gene transcription via ISRE elements is intact in the absence of CrkL, indicating that the regulatory effects on gene transcription are mediated only via the formation of CrkL:Stat5 complexes. Our studies also indicate that activation of the small GTPase Rap1 by IFNα is defective in cells lacking CrkL, indicating that the protein plays a critical role in regulating activation of the growth inhibitory C3G/Rap1 pathway. The IFNα-inducible activation of the small GTPase Rap1 requires a functional N-terminus SH3 domain in the CrkL protein, while the C-terminus SH3 domain does not appear to play a role in such a CrkL-function. We also demonstrate that both the Tyk-2 and Jak-1 kinases are required for activation of the CrkL/Rap1 pathway, as the Type I IFN-dependent GTP-bound form of Rap1 is inhibited by overexpression of dominant-negative Tyk-2 or Jak-1 mutants and is defective in cells lacking Tyk-2 or Jak-1. Taken altogether, these findings indicate that CrkL provides an important link between Jak-kinases and downstream cascades that play critical roles in IFN-dependent transcriptional regulation and induction of growth inhibitory responses. [Copyright &y& Elsevier]

There is emerging evidence that the IFN-inducible family of Slfn genes and proteins play important roles in cell cycle progression and control of cellular proliferation, but the precise functional roles of different Slfn members in the regulation of tumorigenesis remain unclear. In the present study, we undertook a systematic analysis on the expression and functional relevance of different mouse Slfn genes in malignant melanoma and renal cell carcinoma cells. Our studies demonstrate that several mouse Slfn genes are up-regulated in response to IFN treatment of mouse melanoma and renal cell carcinoma cells, including Slfn1, Slfn2, Slfn4, Slfn5, and Slfn8. Our data show that Slfn2 and Slfn3 play essential roles in the control of mouse malignant melanoma cell proliferation and/or anchorage-independent growth, suggesting key and non-overlapping roles for these genes in the control of malignant melanoma tumorigenesis. In renal cell carcinoma cells, in addition to Slfn2 and Slfn3, Slfn5 also exhibits important antineoplastic effects. Altogether, our findings indicate important functions for distinct mouse Slfn genes in the control of tumorigenesis and provide evidence for differential involvement of distinct members of this gene family in controlling tumorigenesis. They also raise the potential of future therapeutic approaches involving modulation of expression of members of this family of genes in malignant melanoma and renal cell carcinoma. The family of Schlafen (Slfn) genes was first described in mouse cells, in studies that demonstrated up-regulation of members of this family during thymocyte maturation and T cell development (1). At that time, it was shown that Slfn expression in thymoma cells and fibroblasts suppresses cell proliferation (1). It is now well established that members of the Slfn family are widely expressed in mammals (2). There are currently many [ABSTRACT FROM AUTHOR]

Interferons (IFNs) have important antiviral and antineoplastic properties, but the precise mechanisms required for generation of these responses remain to be defined. We provide evidence that during engagement of the Type I IFN receptor (IFNR), there is up-regulation of expression of Sprouty (Spry) proteins 1, 2, and 4. Our studies demonstrate that IFN-inducible up-regulation of Spry proteins is Mnk kinase-dependent and results in suppressive effects on the IFN-activated p38 MAP kinase (MAPK), the function of which is required for transcription of interferon-stimulated genes (ISGs). Our data establish that ISG15 mRNA expression and IFN-dependent antiviral responses are enhanced in Spry1,2,4 triple knock-out mouse embryonic fibroblasts, consistent with negative feedback regulatory roles for Spry proteins in IFN-mediated signaling. In other studies, we found that siRNA-mediated knockdown of Spry1, Spry2, or Spry4 promotes IFN-inducible antileukemic effects in vitro and results in enhanced suppressive effects on malignant hematopoietic progenitors from patients with polycythemia vera. Altogether, our findings demonstrate that Spry proteins are potent regulators of Type I IFN signaling and negatively control induction of Type I IFN-mediated biological responses. [ABSTRACT FROM AUTHOR]

There is accumulating evidence that mammalian target of rapamycin (mTOR)-activated pathways play important roles in cell growth and survival of BCR-ABL-transformed cells. We have previously shown that the mTOR/p70 S6 kinase (p70 S6K) pathway is constitutively activated in BCR-ABL transformed cells and that inhibition of BCR-ABL kinase activity by imatinib mesylate abrogates such activation. We now provide evidence for the existence of a novel regulatory mechanism by which BCR-ABL promotes cell proliferation, involving p70 S6K-mediated suppression of expression of programmed cell death 4 (PDCD4), a tumor suppressor protein that acts as an inhibitor of cap-dependent translation by blocking the translation initiation factor eIF4A. Our data also establish that second generation BCR-ABL kinase inhibitors block activation of p70 S6K and downstream engagement of the S6 ribosomal protein in BCR- ABL transformed cells. Moreover, PDCD4 protein expression is up-regulated by inhibition of the BCR-ABL kinase in K562 cells and BaF3/BCR-ABL transfectants, suggesting a mechanism for the generation of the proapoptotic effects of such inhibitors. Knockdown of PDCD4 expression results in reversal of the suppressive effects of nilotinib and imatinib mesylate on leukemic progenitor colony formation, suggesting an important role for this protein in the generation of antileukemic responses. Altogether, our studies identify a novel mechanism by which BCR-ABL may promote leukemic cell growth, involving sequential engagement of the mTOR/p70 S6K pathway and downstream suppression of PDCD4 expression. [ABSTRACT FROM AUTHOR]

There is accumulating evidence that the p38 MAP kinase is accumulating evidence that the p38 MAP kinase pathway plays important roles in Type I interferon (IFN) signaling, but the mechanisms regulating p38 activation during engagement of the Type I IFN receptor remain to be defined. We sought to identify the events that lead to activation of the p38 MAP kinase in response to Type I IFNs. Our data demonstrate that treatment of sensitive cell lines with IFNα results in activation of both MAP kinase kinase 3 (MKK3) and MAP kinase kinase 6 (MKK6). Such IFN-inducible activation of MKK3 and MKK6 is essential for downstream phosphorylation and activation of the p38 MAP kinase, as shown by studies using mouse embryonic fibroblasts (MEFs) with targeted disruption of the Mkk3 and Mkk6 genes (MKK3-/- MKK6-/-). Similarly, IFN-dependent activation of the downstream effectors of p38, MAP-KAPK-2 and MAPKAPK-3, is not detectable in cells lacking Mkk3 and Mkk6, demonstrating that the function of these MAP kinase kinases is required for full activation of the p38 pathway. To define the functional relevance of MKK3/6 engagement in Type I IFN signaling, IFN-inducible gene transcription was evaluated in the MKK3/MKK6 double knock-out cells. IFNα- and IFNβ-dependent transcription via either interferon-stimulated response element or IFNy activated site elements was defective in MKK3 -I-IMKK6 -I- MEFs in luciferase reporter assays. In addition, IFN-dependent induction of two genes known to be of importance in the generation of IFN responses, Isgl5 and Irf-9, was diminished in the absence of Mkk3 and Mkk6. The effects of Mkk3 and Mkk6 on IFN-dependent transcription were unrelated to any effects on the phosphorylation and activation of STAT proteins, indicating the presence of a STAT-independent mechanism. Altogether, our findings demonstrate that MKK3 and MKK6 are rapidly activated during engagement of the Type I IFN receptor and play important roles in Type I IFN signaling and the generation of IFN responses. nase pathway plays important roles in Type I interferon (IFN) signaling, but the mechanisms regulating p38 activation during engagement of the Type I IFN receptor remain to be defined. We sought to identify the events that lead to activation of the p38 MAP kinase in response to Type I IFNs. Our data demonstrate that treatment of sensitive cell lines with IFNa results in activation of both MAP kinase kinase 3 (MKK3) and MAP kinase kinase 6 (MKK6). Such IFN-inducible activation of MKK3 and MKK6 is essential for downstream phosphorylation and activation of the p38 MAP kinase, as shown by studies using mouse embryonic fibroblasts (MEFs) with targeted disruption of the Mkk3 and Mkk6 genes (MKK3-I- MKK6-I-). Similarly, IFN-dependent activation of the downstream effectors of p38, MAP-KAPK-2 and MAPKAPK-3, is not detectable in cells lacking Mkk3 and Mkk6, demonstrating that the function of these MAP kinase kinases is required for full activation of the p38 pathway. To define the functional relevance of MKK3/6 engagement in Type I IFN signaling, IFN-inducible gene transcription was evaluated in the MKK3/MKK6 double knock-out cells. IFNα- and IFNβ-dependent transcription via either interferon-stimulated response element or IFNγ activated site elements was defective in MKK3 -/-/MKK6 -/- MEFs in luciferase reporter assays. In addition, IFN-dependent induction of two genes known to be of importance in the generation of IFN responses, Isg15 and Irf-9, was diminished in the absence of Mkk3 and Mkk6. The effects of Mkk3 and Mkk6 on IFN-dependent transcription were unrelated to any effects on the phosphorylation and activation of STAT proteins, indicating the presence of a STAT-independent mechanism. Altogether, our findings demonstrate that MKK3 and MKK6 are rapidly activated during engagement of the Type I IFN receptor and play important roles in Type I IFN signaling and the generation of IFN responses. [ABSTRACT FROM AUTHOR]

Protein kinase Cotheta (PKC-θ) plays important roles in the activation and survival of lymphocytes and is the predominant PKC isoform expressed in T-cells. Interferons regulate T-cell function and activation, but the precise signaling mechanisms by which they mediate such effects have not been elucidated. We determined whether PKC-θ is engaged in interferon (INF) signaling in T-cells. Both Type I (α, β) and Type II (γ) IFNs induced phosphorylation of PKC-θ in human T-cell lines and primary human T-lymphocytes. Such phosphorylation of PKC-θ resulted in activation of its kinase domain, suggesting that this kinase plays a functional role in interferon signaling. Consistent with this, inhibition of PKC-θ protein expression using small interfering RNAs (siRNA) abrogated IFN-α- and IFN-γ-dependent gene transcription via GAS elements. Similarly, blocking of PKC-θ kinase activity by overexpression of a dominantnegative PKC-θ mutant also blocked GAS-driven transcription, further demonstrating a requirement for PKC-θ in IFN-dependent transcriptional activation. The effects of PKC-θ on IFN-dependent gene transcription were not mediated by regulation of the IFN-activated STAT pathway, as siRNA-mediated PKC-θ knockdown had no effects on STAT1 phosphorylation and binding of STAT1-containing complexes to SIE/GAS elements. On the other hand, siRNA-mediated PKC-θ inhibition blocked phosphorylation/activation of MKK4, suggesting that interferon-dependent PKC-θ activation regulates downstream engagement of MAP kinase pathways. Altogether, these findings demonstrate that PKC-θ is an interferon-inducible kinase and strongly suggest that it plays an important role in the generation of interferonresponses in T-cells. [ABSTRACT FROM AUTHOR]

Type I interferons are pleiotropic cytokines that transduce signals via activation of multiple downstream signaling cascades, including the Jak–Stat pathway. Although the roles of Stat1 and Stat2 in Type I interferon signaling are well established, the roles that other Stat-family members play in the induction of IFN-responses remain to be defined. In previous studies, we have shown that Stat5 associates with the CrkL adapter and forms a signaling complex that binds DNA. In the present study, we provide evidence that Stat5 is phosphorylated on serines 725/730 in an IFNα- and IFNβ-dependent manner, providing direct evidence that serine phosphorylation of the protein is a component of an interferon signaling cascade. Such serine phosphorylation of Stat5 is Map kinase- and PI 3′-kinase independent, while the activation of the serine kinase that phosphorylates Stat5 is regulated by upstream tyrosine kinase activity. Using mouse embryonic fibroblasts with targeted disruption of the Stat5a and Stat5b genes, we demonstrate that full activation of Stat5 is required for Type I interferon-dependent gene transcription via GAS elements. Altogether, our data provide evidence that Stat5 plays an important role in IFN-signaling and participates in the induction of Type I IFN-dependent responses. Furthermore, our results strongly suggest that, in addition to phosphorylation on tyrosine residues, phosphorylation on serine residues exhibits regulatory effects on the transcriptional capacity of Stat5. [Copyright &y& Elsevier]

The Type I IFN receptor-generated signals required for initiation of mRNA translation and, ultimately, induction of protein products that mediate IFN responses, remain unknown. We have previously shown that IFNα and IFNβ induce phosphorylation of insulin receptor substrate proteins and downstream engagement of the phosphatidylinositol (PI) 3'-kinase pathway. In the present study we provide evidence for the existence of a Type I IFN-dependent signaling cascade activated downstream of PI 3'-kinase, involving p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated on threonine 421 and serine 424 and is activated during treatment of cells with IFNα or IFNβ. Such activation of p70 S6K is blocked by pharmacological inhibitors of the PI 3'-kinase or the FKBP 12-rapamycin-associated protein/mammalian target of rapamycin (FRAP/ mTOR). Consistent with this, the Type I IFN-dependent phosphorylation/activation of p70 S6K is defective in embryonic fibroblasts from mice with targeted disruption of the p85α and p85β subunits of the PI 3'-kinase (p85α-/-β-/-). Treatment of sensitive cell lines with IFNα or IFNfi also results in phosphorylation/inactivation of the 4E-BP-1 repressor of mRNA translation. Such 4E-BP1 phosphorylation is also PI3'-kinase-dependent and rapamycin-sensitive, indicating that the Type I IFNinducible activation of PI3'-kinase and FRAP/mTOR resuits in dissociation of 4E-BP1 from the eukaryotic initiation factor-4E (eIF4E) complex. Altogether, our data establish that the Type I IFN receptor-activated PI 3'kinase pathway mediates activation of the p70 S6 kinase and inactivation of 4E-BP1, to regulate mRNA translation and induction of Type I IFN responses. [ABSTRACT FROM AUTHOR]

We provide evidence that type I IFN-induced STAT activation is diminished in cells with targeted disruption of the Rictor gene, whose protein product is a key element of mTOR complex 2. Our studies show that transient or stable knockdown of Rictor or Sin1 results in defects in activation of elements of the STAT pathway and reduced STAT-DNA binding complexes. This leads to decreased expression of several IFN-inducible genes that mediate important biological functions. Our studies also demonstrate that Rictor and Sin1 play essential roles in the generation of the suppressive effects of IFNα on malignant erythroid precursors from patients with myeloproliferative neoplasms. Altogether, these findings provide evidence for critical functions for Rictor/Sin1 complexes in type I IFN signaling and the generation of type I IFN antineoplastic responses. [ABSTRACT FROM AUTHOR]

There is emerging evidence that mammalian target of Rapamycin (mTOR) complexes play important roles in IFN-signaling and mRNA translation of interferon-stimulated genes (ISGs), by controlling the function of downstream effector pathways. We provide evidence that IFN-induced ISG expression is defective in cells with targeted disruption of the Rictor gene, a key component of the mTORC2 complex. Our studies demonstrate that there is defective expression of several IFN-inducible genes that mediate important biological functions, including anti-proliferative, antiviral and pro-apoptotic responses. Importantly, we also found that Rictor and Sin1 play essential roles in the generation of the suppressive effects of IFN alpha on malignant erythroid precursors from patients with myeloproliferative neoplasms (MPNs), suggesting unique roles for such complexes in the IFN-system. Altogether, our data provide evidence for critical and essential roles for Rictor/Sin1 complexes in IFN-signaling and the control of IFN-responses. [ABSTRACT FROM AUTHOR]

IFNα exerts potent inhibitory activities against malignant melanoma cells in vitro and in vivo, but the mechanisms by which it generates its antitumor effects remain unknown. We examined the effects of interferon α (IFNα) on the expression of human members of the Schlafen (SLFN) family of genes, a group of cell cycle regulators that mediate growth-inhibitory responses. Using quantitative RT-real time PCR, we found detectable basal expression of all the different human SLFN genes examined (SLFN5, SLFN11, SLFN12, SLFN13, and SLFN14), in malignant melanoma cells and primary normal human melanocytes, but SLFN5 basal expression was suppressed in all analyzed melanoma cell lines. Treatment of melanoma cells with IFNα resulted in induction of expression of SLFN5 in malignant cells, suggesting a potential involvement of this gene in the antitumor effects of IFNα. Importantly, stable knockdown of SLFN5 in malignant melanoma cells resulted in increased anchorage-independent growth, as evidenced by enhanced colony formation in soft agar assays. Moreover, SLFN5 knockdown also resulted in increased invasion in three-dimensional collagen, suggesting a dual role for SLFN5 in the regulation of invasion and anchorage-independent growth of melanoma cells. Altogether, our findings suggest an important role for the SLFN family of proteins in the generation of the anti-melanoma effects of IFNα and for the first time directly implicate a member of the human SLFN family in the regulation of cell invasion. [ABSTRACT FROM AUTHOR]

We have shown that the mammalian target of rapamycin (mTOR) down-regulates thrombin-induced ICAM-1 expression in endothelial cells by suppressing the activation of NP-κB. However, the mechanisms by which mTOR is activated to modulate these responses remain to be addressed. Here, we show that thrombin engages protein kinase C (PKC)-δ and phosphat- tidylinositol 3-kinase (PI3K)/Akt pathways to activate mTOR and thereby dampens NP-κB activation and intercellular adhesion molecule 1 (ICAM-1) expression. Stimulation of human vascular endothelial cells with thrombin induced the phosphorylation of mTOR and its downstream target p70 S6 kinase in a PKC-κ- and PI3K/Akt-dependent manner. Consistent with this, thrombin-induced phosphorylation of p70 S6 kinase was defective in embryonic fibroblasts from mice with targeted disruption of PKC-δ (Pkc-δ-/-), p85α and p85β subunits of the PI3K p85α-/-), or Akt1 and Akt2 (Akt1-/-2-/-). Furthermore, we observed that expression of the constitutively active form of PKC-δ or Akt was sufficient to induce NF-κB activation and ICAM-1 expression, and that co-expression of mTOR suppressed these responses. In reciprocal experiments, inhibition/depletion of mTOR augmented NP-κB activation and ICAM-1 expression induced by PKC-δ or Akt. In control experiments, increasing or impairing mTOR signaling by the above approaches produced similar effects on NP-κB activation and ICAM-1 expression induced by thrombin. Thus, these data reveal an important role of PKC-δ and PI3K/Akt pathways in activating mTOR as an endogenous modulator to ensure a tight regulation of NP-κB signaling of ICAm-1 expression in endothelial cells. [ABSTRACT FROM AUTHOR]

Abstract: The mechanisms by which retinoids regulate initiation of mRNA translation for proteins that mediate their biological effects are not known. We have previously shown that all-trans-retinoic acid (ATRA) induces mTOR-mediated activation of the p70 S6 kinase, suggesting the existence of a mechanism by which retinoids may regulate mRNA translation. We now demonstrate that treatment of acute promyelocytic leukemia (APL)-derived NB4 cells with ATRA results in dissociation of the translational repressor 4E-BP1 from the eukaryotic initiation factor eIF4E, and subsequent formation of eIF4G-eIF4E complexes. We also show that siRNA-mediated inhibition of 4E-BP1 expression enhances ATRA-dependent upregulation of p21Waf1/Cip1, a protein that plays a key role in the induction of retinoid-dependent responses. Our data also establish that ATRA- or cis-RA-dependent p21Waf1/Cip1 protein expression is enhanced in mouse embryonic fibroblasts with targeted disruption of the 4e-bp1 gene, in the absence of any effects on the transcriptional regulation of the p21Waf1/Cip1 gene. Moreover, generation of ATRA- or cis-retinoic acid (cis-RA)-antiproliferative responses is enhanced in 4E-BP1 knockout cells. Altogether, these findings strongly suggest a key regulatory role for the translational repressor 4E-BP1 in the generation of retinoid-dependent functional responses. [Copyright &y& Elsevier]

Arsenic trioxide (As2O3) is a potent inducer of apoptosis of leukemic cells in vitro and in vivo, but the mechanisms that mediate such effects are not well understood. We provide evidence that the Akt kinase is phosphorylated/activated during treatment of leukemia cells with As2O3, to regulate downstream engagement of mammalian target of rapamycin (mTOR) and its effectors. Using cells with targeted disruption of both the Akt1 and Akt2 genes, we found that induction of arsenic trioxide-dependent apoptosis is strongly enhanced in the absence of these kinases, suggesting that Akt1/Akt2 are activated in a negative feedback regulatory manner, to control generation of As2O3 responses. Consistent with this, As2O3-dependent pro-apoptotic effects are enhanced in double knock-out cells for both isoforms of the p70 S6 kinase (S6k1/S6k2), a downstream effector of Akt and mTOR. On the other hand, As2O3-dependent induction of apoptosis is diminished in cells with targeted disruption of TSC2, a negative upstream effector of mTOR. In studies using primary hematopoietic progenitors from patients with acute myeloid leukemia, we found that pharmacological inhibition of mTOR enhances the suppressive effects of arsenic trioxide on leukemic progenitor colony formation. Moreover, short interfering RNA-mediated inhibition of expression of the negative downstream effector, translational repressor 4E-BP1, partially reverses the effects of As2O3. Altogether, these data provide evidence for a key regulatory role of the Akt/mTOR pathway in the generation of the effects of As2O3, and suggest that targeting this signaling cascade may provide a novel therapeutic approach to enhance the anti-leukemic properties of As2O3. [ABSTRACT FROM AUTHOR]

Objective: The mechanisms by which interferon α (IFN-α) induces antileukemic responses in chronic myelogenous leukemia (CML) cells are not known. We examined whether a member of the protein kinase C (PKC) family of proteins, PKC-δ, is activated during treatment of BCR-ABL cells with IFN-α and participates in the induction of interferon responses. Methods: Immunoblots and immune complex kinase assays were performed to study the phosphorylation and activation of PKC-δ in response to IFN-α in CML-derived cell lines. The effects of pharmacological inhibition of PKC-δ on the suppressive effects of IFN-α on leukemic CFU-GM progenitors from CML patients were assessed by clonogenic assays in methylcellulose. Results: IFN-α treatment of the sensitive CML-derived KT-1 cell line resulted in phosphorylation of PKC-δ and activation of its kinase domain. Such phosphorylation/activation of PKC-δ was required for phosphorylation of Stat1 on serine 727, as inhibition of PKC-δ activity blocked the IFN-α–dependent serine phosphorylation of Stat1 and IFN-α–inducible gene transcription. IFN-α treatment strongly inhibited leukemic CFU-GM progenitor colony fromation from bone marrow or peripheral blood of patients with CML, and such inhibition was reversed by concomitant treatment of the cells with the PKC-δ pharmacologic inhibitor rottlerin. Conclusion: Taken altogether, our data demonstrate that PKC-δ plays a critical role in Type I IFN signaling in BCR-ABL expressing cells, acting as a serine kinase for Stat1, to regulate transcriptional activation of interferon-regulated genes and induction of antileukemic responses. [Copyright &y& Elsevier]

EVI1 is an oncogene frequently associated with chronic and acute myeloid leukemia. In hematopoietic cells, EVI1 impairs several pathways including proliferation, differentiation, and apoptosis. Interferon-α (IFN-α) is a powerful cytokine that controls the immune response and limits the expansion of several tissues including bone marrow. These properties contribute to the effectiveness of IFN-α in the treatment of many neoplastic disorders especially chronic myeloid leukemia. We report here that in murine hematopoietic progenitors the expression of EVIl completely abrogates the antiproliferative and apoptotic effects of IFN-α. EVI1 does not repress the JAK/STAT signaling pathway or the activation of many IFN-responsive genes. On the contrary, EVI1 prolongs the phosphorylation of STAT1 and the activation of an IFN-dependent reporter gene. However, EVI1 specifically represses the IFN-dependent induction of the tumor suppressor PML and blocks the apoptotic pathways activated by PML. We show that the position of the ISRE, which is located within the first exon of PML, is critical to block PML induction by IFN-α. The relocation of the ISRE to a position upstream of the transcription start site is sufficient to re-establish the response to IFN in the presence of EVI1. Our data suggest that stabilized STAT1 phosphorylation and prolonged binding of the STAT1 complex to the first exon could impair PML transcription and inhibit the activation of PML-dependent apoptotic pathways resulting in loss of IFN response. These results point to a novel mechanism utilized by an oncogene to escape normal cell response to growth-controlling cytokines. [ABSTRACT FROM AUTHOR]

Acquisition of matrix metalloproteinase-2 (MMP-2) activity is temporally associated with increased migration and invasiveness of cancer cells. ProMMP-2 activation requires multimolecular complex assembly involving proMMP-2, membrane type 1-MMP (MT1-MMP, MMP14), and tissue inhibitor of metalloproteinases-2 (TIMP2). Because transforming growth factor-β1 (TGF-β1) promotes tumor invasion in advanced squamous cell carcinomas, the role of TGF-β1 in the regulation of MMP activity in a cellular model of invasive oral squamous cell carcinoma was examined. Treatment of oral squamous cell carcinoma cells with TGF-β1 promoted MMPdependent cell scattering and collagen invasion, increased expression of MMP-2 and MT1-MMP, and enhanced MMP-2 activation. TGF-β1 induced concomirant activation of ERK½ and p38 MAPK, and kinase inhibition studies revealed a negative regulatory role for ERK½ in modulating acquisition of MMP-2 activity. Thus, a reciprocal effect on proMMP-2 activation was observed whereupon blocking ERK½ phosphorylation promoted proMMP-2 activation and MT1-MMP activity, whereas inhibiting p38 MAPK activity decreased proteolytic potential. The cellular mechanism for the control of MT1-MMP catalytic activity involved concurrent reciprocal modulation of TIMP-2 expression by ERK½ and p38 MAPKs, such that inhibition of ERK½ phosphorylation decreased TIMP-2 production, and downregulation of p38 MAPK activity enhanced TIMP-2 synthesis. Further, p38 MAPK inhibition promoted ERK½ phosphorylation, providing additional evidence for cross-talk between MAPK pathways. These observations demonstrate the complex reciprocal effects of ERK½ and p38 MAPK in the regulation of MMP activity, which could complicate the use of MAPK-specific inhibitors as therapeutic agents to down-regulate the biologic effects of TGF-β1 on pericellular collagen degradation and tumot invasion. [ABSTRACT FROM AUTHOR]

Matrix metalloproteinases (MMPs) play an important role in the invasive behavior of a number of cancers including oral squamous cell cancer (OSCC), and increased expression of MMP-9 is correlated with invasive and metastatic OSCC. Because calcium is an important regulator of keratinocyte function, the effect of modulating extracellular calcium on MMP-9 expression in OSCC cell lines was evaluated. Increasing extracellular calcium induced a dose-dependent increase in MMP-9 expression in immortalized normal and premalignant oral keratinocytes, but not in two highly invasive OSCC cell lines. Differential activation of MAPK signaling was also induced by calcium, p38 MAPK activity was down-regulated, whereas ERK1/2 activity was enhanced. Pharmacologic inhibition of p38 MAPK activity or expression of a catalytically inactive mutant of the upstream kinase MAPK kinase 3 (MKK3) increased the calcium induced MMP-9 gene expression, demonstrating that p38 MAPK activity negatively regulated this process. Interestingly blocking p38 MAPK activity enhanced ERK1/2 phosphorylation, suggesting reciprocal regulation between the ERK1/2 and p38 MAPK pathways. Together these data support a model wherein calcium-induced MMP-9 expression is differentially regulated by the ERK1/2 and p38 MAPK pathways in oral keratinocytes, and the data suggest that a loss of this regulatory mechanism accompanies malignant transformation of the oral epithelium. [ABSTRACT FROM AUTHOR]