Signaling via the interferon-alpha receptor in chronic myelogenous leukemia cells

Inflammatory Cytokines Shape an Altered Immune Response During Myeloid Malignancies

The immune microenvironment is a critical driver and regulator of leukemic progression and hematological disease. Recent investigations have demonstrated that multiple immune components play a central role in regulating hematopoiesis, and dysfunction at the immune cell level significantly contributes to neoplastic disease. Immune cells are acutely sensitive to remodeling by leukemic inflammatory cytokine exposure. Importantly, immune cells are the principal cytokine producers in the hematopoietic system, representing an untapped frontier for clinical interventions. Due to a proinflammatory cytokine environment, dysregulation of immune cell states is a hallmark of hematological disease and neoplasia. Malignant immune adaptations have profound effects on leukemic blast proliferation, disease propagation, and drug-resistance. Conversely, targeting the immune landscape to restore hematopoietic function and limit leukemic expansion may have significant therapeutic value. Despite the fundamental role of the immune microenvironment during the initiation, progression, and treatment response of hematological disease, a detailed examination of how leukemic cytokines alter immune cells to permit, promote, or inhibit leukemia growth is lacking. Here we outline an immune-based model of leukemic transformation and highlight how the profound effect of immune alterations on the trajectory of malignancy. The focus of this review is to summarize current knowledge about the impacts of pro- and anti-inflammatory cytokines on immune cells subsets, their modes of action, and immunotherapeutic approaches with the potential to improve clinical outcomes for patients suffering from hematological myeloid malignancies.

Inflammatory Signaling Pathways in Preleukemic and Leukemic Stem Cells

Hematopoietic stem cells (HSCs) are a rare subset of bone marrow cells that usually exist in a quiescent state, only entering the cell cycle to replenish the blood compartment, thereby limiting the potential for errors in replication. Inflammatory signals that are released in response to environmental stressors, such as infection, trigger active cycling of HSCs. These inflammatory signals can also directly induce HSCs to release cytokines into the bone marrow environment, promoting myeloid differentiation. After stress myelopoiesis is triggered, HSCs require intracellular signaling programs to deactivate this response and return to steady state. Prolonged or excessive exposure to inflammatory cytokines, such as in prolonged infection or in chronic rheumatologic conditions, can lead to continued HSC cycling and eventual HSC loss. This promotes bone marrow failure, and can precipitate preleukemic states or leukemia through the acquisition of genetic and epigenetic changes in HSCs. This can occur through the initiation of clonal hematopoiesis, followed by the emergence preleukemic stem cells (pre-LSCs). In this review, we describe the roles of multiple inflammatory signaling pathways in the generation of pre-LSCs and in progression to myelodysplastic syndrome (MDS), myeloproliferative neoplasms, and acute myeloid leukemia (AML). In AML, activation of some inflammatory signaling pathways can promote the cycling and differentiation of LSCs, and this can be exploited therapeutically. We also discuss the therapeutic potential of modulating inflammatory signaling for the treatment of myeloid malignancies.

Deregulation of innate immune and inflammatory signaling in myelodysplastic syndromes

Myelodysplastic syndromes (MDSs) are a group of heterogeneous clonal hematologic malignancies that are characterized by defective bone marrow (BM) hematopoiesis and by the occurrence of intramedullary apoptosis. During the past decade, the identification of key genetic and epigenetic alterations in patients has improved our understanding of the pathophysiology of this disease. However, the specific molecular mechanisms leading to the pathogenesis of MDS have largely remained obscure. Recently, essential evidence supporting the direct role of innate immune abnormalities in MDS has been obtained, including the identification of multiple key regulators that are overexpressed or constitutively activated in BM hematopoietic stem and progenitor cells. Mounting experimental results indicate that the dysregulation of these molecules leads to abnormal hematopoiesis, unbalanced cell death and proliferation in patients' BM, and has an important role in the pathogenesis of MDS. Furthermore, there is compelling evidence that the deregulation of innate immune and inflammatory signaling also affects other cells from the immune system and the BM microenvironment, which establish aberrant associations with hematopoietic precursors and contribute to the MDS phenotype. Therefore, the deregulation of innate immune and inflammatory signaling should be considered as one of the driving forces in the pathogenesis of MDS. In this article, we review and update the advances in this field, summarizing the results from the most recent studies and discussing their clinical implications.

Essential role for the Mnk pathway in the inhibitory effects of type I interferons on myeloproliferative neoplasm (MPN) precursors

The mechanisms of generation of the antineoplastic effects of interferons (IFNs) in malignant hematopoietic cells remain to be precisely defined. We examined the activation of type I IFN-dependent signaling pathways in malignant cells transformed by Jak2V617F, a critical pathogenic mutation in myeloproliferative neoplasms (MPNs). Our studies demonstrate that during engagement of the type I IFN receptor (IFNAR), there is activation of Jak-Stat pathways and also engagement of Mnk kinases. Activation of Mnk kinases is regulated by the Mek/Erk pathway and is required for the generation of IFN-induced growth inhibitory responses, but Mnk kinase activation does not modulate IFN-regulated Jak-Stat signals. We demonstrate that for type I IFNs to exert suppressive effects in malignant hematopoietic progenitors from patients with polycythemia vera, induction of Mnk kinase activity is required, as evidenced by studies involving pharmacological inhibition of Mnk or siRNA-mediated Mnk knockdown. Altogether, these findings provide evidence for key and essential roles of the Mnk kinase pathway in the generation of the antineoplastic effects of type I IFNs in Jak2V617F-dependent MPNs.

Ubiquitination-dependent regulation of signaling receptors in cancer

Ubiquitination of signaling cell surface receptors is a key mechanism regulating the availability of these receptors to interact with extracellular ligands. Accordingly, this regulation determines the sensitivity of cells to the humoral and locally secreted regulators of cell function, proliferation, and viability. Alterations in receptor ubiquitination and degradation are often encountered in cancers. Malignant cells utilize modified ubiquitination of signaling receptors to augment or attenuate signaling pathways on the basis of whether the outcome of this signaling is conducive or not for tumor growth and survival. These mechanisms as well as their significance for the treatment of human cancers are discussed.

Deregulation of Interferon Signaling in Malignant Cells

Interferons (IFNs) are a family of cytokines with potent antiproliferative, antiviral, and immunomodulatory properties. Much has been learned about IFNs and IFN-activated signaling cascades over the last 50 years. Due to their potent antitumor effects in vitro and in vivo, recombinant IFNs have been used extensively over the years, alone or in combination with other drugs, for the treatment of various malignancies. This review summarizes the current knowledge on IFN signaling components and pathways that are deregulated in human malignancies. The relevance of deregulation of IFN signaling pathways in defective innate immune surveillance and tumorigenesis are discussed.

Suppression of interferon (IFN)-inducible genes and IFN-mediated functional responses in BCR-ABL-expressing cells

The interferons (IFNs) are cytokines that play key roles in host defense against viral infections and immune surveillance against cancer. We report that BCR-ABL transformation of hematopoietic cells results in suppression of IFN-dependent responses, including transcription of IFN-inducible genes and generation of IFN-mediated antiviral effects. BCR-ABL transformation suppresses expression of several IFN-regulated genes containing IFN-sensitive response element (ISRE) or GAS elements in their promoters, including Isg15, Irf1, Irf9, and Ifit2 (interferon-induced protein with tetratricopeptide repeats 2). Suppression of transcription of ISRE-containing genes is also seen in cells expressing various BCR-ABL kinase domain mutants, including T315I, H396P, Y253F, and E255K, but not kinase-defective BCR-ABL. Such effects are associated with impaired IFN-dependent phosphorylation of Stat1 on Tyr(701) and Stat3 on Tyr(705) and defective binding of Stat complexes to ISRE or GAS elements. Beyond suppression of Stat activities, BCR-ABL inhibits IFN-inducible phosphorylation/activation of the p38 MAPK, suggesting a dual mechanism by which this abnormal fusion protein blocks IFN transcriptional responses. The inhibitory activities of BCR-ABL ultimately result in impaired IFNalpha-mediated protection against encephalomyocarditis virus infection and reversal of IFN-dependent growth suppression. Altogether, our data provide evidence for a novel mechanism by which BCR-ABL impairs host defenses and promotes malignant transformation, involving dual suppression of IFN-activated signaling pathways.

Suppression of programmed cell death 4 (PDCD4) protein expression by BCR-ABL-regulated engagement of the mTOR/p70 S6 kinase pathway

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.

Ubp43 regulates BCR-ABL leukemogenesis via the type 1 interferon receptor signaling

Interferon (IFN) signaling induces the expression of interferon-responsive genes and leads to the activation of pathways that are involved in the innate immune response. Ubp43 is an ISG15-specific isopeptidase, the expression of which is activated by IFN. Ubp43 knock-out mice are hypersensitive to IFN-alpha/beta and have enhanced resistance to lethal viral and bacterial infections. Here we show that in addition to protection against foreign pathogens, Ubp43 deficiency increases the resistance to oncogenic transformation by BCR-ABL. BCR-ABL viral transduction/transplantation of wild-type bone marrow cells results in the rapid development of a chronic myeloid leukemia (CML)-like myeloproliferative disease; in contrast, a significantly increased latency of disease development is observed following BCR-ABL viral transduction/transplantation of Ubp43-deficient bone marrow cells. This resistance to leukemic development is dependent on type 1 IFN (IFN-alpha/beta) signaling in Ubp43-deficient cells. Increased levels of type 1 IFN are also detected in the serum of CML mice. These results suggest that inhibition of Ubp43-negative effect on IFN signaling can potentiate the response to increased endogenous IFN levels in innate immune responses against cancer development, indicating that pharmacological inhibition of Ubp43 may be of benefit in cancers and others diseases in which interferon is currently prescribed.

The PI3' Kinase Pathway in Interferon Signaling

Interferons (IFNs) are pleiotropic cytokines with important immunomodulatory, antitumor, antiviral, and growth inhibitory properties. Consistent with the multiplicity of their effects, the IFNs activate several cellular cascades after they bind to their receptors. Although the Jak-Stat pathway is the most studied pathway, it appears that additional signaling cascades play roles in IFN signaling. The phosphatidylinositol 3'-kinase (PI3'K) pathway has emerged as one of the critical players in IFN signal transduction and is the focus of this review. This pathway is activated distinctively from the Stat pathway but plays important roles in the regulation of IFN-dependent gene transcription and initiation of mRNA translation.

Role of protein kinase C-δ (PKC-δ) in the generation of the effects of IFN-α in chronic myelogenous leukemia cells

OBJECTIVE

The mechanisms by which interferon alpha (IFN-alpha) 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-delta, is activated during treatment of BCR-ABL cells with IFN-alpha 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-delta in response to IFN-alpha in CML-derived cell lines. The effects of pharmacological inhibition of PKC-delta on the suppressive effects of IFN-alpha on leukemic CFU-GM progenitors from CML patients were assessed by clonogenic assays in methylcellulose.

RESULTS

IFN-alpha treatment of the sensitive CML-derived KT-1 cell line resulted in phosphorylation of PKC-delta and activation of its kinase domain. Such phosphorylation/activation of PKC-delta was required for phosphorylation of Stat1 on serine 727, as inhibition of PKC-delta activity blocked the IFN-alpha-dependent serine phosphorylation of Stat1 and IFN-alpha-inducible gene transcription. IFN-alpha treatment strongly inhibited leukemic CFU-GM progenitor colony formation from bone marrow or peripheral blood of patients with CML, and such inhibition was reversed by concomitant treatment of the cells with the PKC-delta pharmacologic inhibitor rottlerin.

CONCLUSION

Taken altogether, our data demonstrate that PKC-delta 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.

Raf and VEGF: emerging therapeutic targets in Kaposi's sarcoma-associated herpesvirus infection and angiogenesis in hematopoietic and nonhematopoietic tumors

Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically associated with several cancers including Kaposi's sarcoma (KS), primary effusion lymphoma, and multicentric Castleman's disease. KSHV-mediated pathogenesis is dependent mainly on KSHV infection as well as on the microenvironment provided by the growth factors (GFs)/inflammatory cytokines (ICs). Recently, we determined that oncoprotein Raf enhances KSHV infection of target cells. Interestingly, Raf regulates the expression of a variety of GFs/ICs including those involved in angiogenesis such as vascular endothelial growth factor (VEGF). In this review, we discuss the effect of the Raf-GF/IC autocrine/paracrine loop on KSHV infection of both hematopoietic and nonhematopietic cells, and associated disease conditions.

Activation of mitogen-activated protein kinase kinase (MKK) 3 and MKK6 by type I interferons

There 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 IFNalpha 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, MAPKAPK-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. IFNalpha- and IFNbeta-dependent transcription via either interferon-stimulated response element or IFNgamma 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.

Initial expression of interferon alpha receptor 2 (IFNAR2) on CD34-positive cells and its down-regulation correlate with clinical response to interferon therapy in chronic myelogenous leukemia

In order to investigate the mechanism of interferon-alpha (IFNalpha) action in the treatment of chronic myelogenous leukemia (CML), we examined surface expressions of both type I interferon receptor 1 (IFNAR1) and 2 (IFNAR2) subunits on CD34-positive cells in bone marrow (BM) in a total of 57 CML patients. Initial cell-surface IFNAR2 expression at diagnosis assessed by flow cytometry widely distributed but showed overall significantly higher expression in CML patients when compared with normal controls. In 15 fresh patients who subsequently received IFNalpha therapy, IFNAR2 expression at diagnosis was significantly higher in cytogenetic good responders than in poor responders. Down-regulation of IFNAR2 expression during IFNalpha therapy was observed only in good responders but not in poor responders. In addition to protein level, both initial high IFNAR2c mRNA expression level and its down-regulation during IFNalpha therapy, in purified CD34-positive cells, were also observed only in good responders. In contrast to IFNAR2, cell-surface IFNAR1 expression was generally lower than IFNAR2, and correlation between either the pretreatment level or down-regulation of IFNAR1 and clinical response was not evident. With in vitro IFNalpha stimulation, CD34-positive cells showed down-regulations of cell-surface IFNAR2, and IFNAR1 to a lesser extent, in one good-responder patient, but not in one poor-responder patient. Serum soluble interferon receptor (sIFNR) was higher in untreated CML patients than in normal controls, without any correlation with clinical response to IFNalpha. Thus, the pretreatment protein and mRNA expression levels of IFNAR2 and their down-regulations during IFNalpha therapy correlate well with IFNalpha response in CML patients.

Identification of genes differentially regulated by the P210 BCR/ABL1 fusion oncogene using cDNA microarrays

OBJECTIVE

The t(9;22) translocation is associated with more than 95% of cases of chronic myeloid leukemia. The resulting fusion of the BCR and ABL1 loci produces the constitutively active BCR/ABL1 tyrosine kinase. A wide range of signal transduction molecules are activated by BCR/ABL1, including MYC, PI-3 kinase, and different STAT molecules. In contrast, relatively few genes are known to be regulated by BCR/ABL1 at the level of transcription.

MATERIALS AND METHODS

In an effort to better understand the transcriptional program activated by BCR/ABL1, we used cDNA microarrays to evaluate the relative expression of approximately 6450 human genes in U937 myelomonocytic cells expressing P210 BCR/ABL1 via a tetracycline-inducible promoter.

RESULTS

We confirmed the previously reported up-regulation of the PIM1 and JUN oncogenes by BCR/ABL1. In addition, we identified 59 more genes up-regulated by BCR/ABL1. Interestingly, roughly one third of these were genes previously reported to be interferon (IFN)-responsive, including the OAS1, IFIT1, IFI16, ISGF3G, and STAT1 genes. An additional seven BCR/ABL1-regulated genes were found to be IFN-responsive in U937 cells. The expression profile also included genes encoding transcription factors, kinases, and signal transduction molecules, as well as genes regulating cell growth, differentiation, apoptosis, and cell adhesion, features previously suggested to be affected by BCR/ABL1.

CONCLUSION

These observations shed novel insight into the mechanism of BCR/ABL1 action and provide a range of targets for further investigation.

Role of the p38 mitogen-activated protein kinase pathway in the generation of the effects of imatinib mesylate (STI571) in BCR-ABL-expressing cells

Imatinib mesylate (STI571), a specific inhibitor of the BCR-ABL tyrosine kinase, exhibits potent antileukemic effects in vitro and in vivo. Despite the well established role of STI571 in the treatment of chronic myelogenous leukemia, the precise mechanisms by which inhibition of BCR-ABL tyrosine kinase activity results in generation of antileukemic responses remain unknown. In the present study we provide evidence that treatment of CML-derived BCR-ABL-expressing leukemia cells with STI571 results in activation of the p38 mitogen-activated protein (MAP) kinase signaling pathway. Our data indicate that STI571 induces phosphorylation of the p38 and activation of its kinase domain, in KT-1 cells and other BCR-ABL-expressing cell lines. We also identify the kinases MAP kinase-activated protein kinase-2 and Msk1 as two downstream effectors of p38, activated during inhibition of BCR-ABL activity by STI571. Importantly, pharmacological inhibition of p38 reverses the growth inhibitory effects of STI571 on primary leukemic colony-forming unit granulocyte/macrophage progenitors from patients with CML. Altogether, our data establish that activation of the p38 MAP kinase signaling cascade plays an important role in the generation of the effects of STI571 on BCR-ABL-expressing cells. They also suggest that, in addition to activation of mitogenic pathways, BCR-ABL promotes leukemogenesis by suppressing the function of growth inhibitory signaling cascades.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Role of p38α Map Kinase in Type I Interferon Signaling

Multiple signaling pathways are activated during engagement of the Type I interferon (IFN) receptor to mediate biological responses, including the Jak-Stat and Rac1/p38 Map kinase signaling cascades. In the present study we sought to determine the functional relevance of the p38alpha isoform in IFN signaling, using cells from mouse embryos with targeted disruption of the p38alpha gene. Our data demonstrate that p38alpha activation is essential for Type I IFN-dependent transcriptional regulation via ISRE or GAS elements. On the other hand, the function of p38alpha is not required for IFN-dependent Ser727 or Tyr701 phosphorylation of Stat1 and does not impact on the formation of ISGF3 or SIF nuclear binding complexes. In efforts to identify downstream effectors of p38 that may mediate IFN-dependent transcriptional responses, we found that IFNalpha activates the kinase Msk1, a known regulator of histone phosphorylation and chromatin remodeling. In other studies, we demonstrate that Type I IFN-dependent activation of the kinases MapKapK-2 and MapKapK-3 is defective in the absence of p38alpha, while Type I IFN-dependent antiviral properties are decreased in cells with targeted disruption of the MapKapK-2 gene. Altogether, our data establish that the p38alpha Map kinase pathway regulates activation of downstream effectors that participate in the induction of IFN-dependent gene transcription, to mediate IFN-responses.

Expression modes of interferon-α inducible genes in sensitive and resistant human melanoma cells stimulated with regular and pegylated interferon-α

Interferon-alpha with its antiproliferative activity is widely used for the treatment of viral infections and tumor therapy such as melanoma. Naturally occurring resistance to recombinant interferon alpha-2a (IFN-alpha) and severe side effects limit the therapeutic efficacy. Understanding of the molecular mechanisms involved in unresponsiveness may therefore lead to the development of novel formulations that overcome resistance. Here, we have applied oligonucleotide DNA microarrays with probe sets for about 11,400 human transcripts to study the expression of interferon-alpha inducible genes in a sensitive and resistant melanoma cell line over multiple time points and two interferon formulations. We identified two major groups of genes with termed interferon primary response genes (IPRGs) or interferon secondary response genes (ISRGs). IPRGs are upregulated early after interferon stimulation in both the sensitive and the resistant line and they contain IREs in the noncoding regulatory region. In contrast, ISRG expression occurs preferentially in the sensitive line ME15 at late time points, and this group of genes lacks typically IREs. In addition to these two major interferon response gene classes, we identified a relatively small number of genes with complex kinetic expression modes. In addition, we show for the first time that regular and pegylated recombinant interferons are equally potent activators of interferon (IFN) gene expression. Finally, we propose that the ISRGs are activated downstream of the primary response genes by a molecule or pathway, which awaits identification, and interferon inducible gene expression is thus more complicated than previously thought.

Role of Stat5 in type I interferon-signaling and transcriptional regulation

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 IFNalpha- and IFNbeta-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.

Activation of the p70 S6 kinase and phosphorylation of the 4E-BP1 repressor of mRNA translation by type I interferons

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 IFNalpha and IFNbeta 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 IFNalpha or IFNbeta. 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 p85alpha and p85beta subunits of the PI 3'-kinase (p85alpha-/-beta-/-). Treatment of sensitive cell lines with IFNalpha or IFNbeta 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 IFN-inducible activation of PI3'-kinase and FRAP/mTOR results 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.

Caspase-mediated cell death in hematological malignancies: theoretical considerations, methods of assessment, and clinical implications

Apoptosis, the caspase-mediated cell death, plays an important role in the etiology, pathogenesis and therapy of a variety of diseases. Abnormalities of apoptosis regulation, resulting in either its inhibition or enhancement, play a key role in the development of various malignant hematological disorders. Several routine and new therapeutic strategies in Oncohematology are based on apoptosis modulation. Cytotoxic effects of most antineoplastic drugs are based on induction of apoptosis. The accurate estimate of incidence of apoptosis, therefore, is of importance in Oncohematology. In this review we provide an overview of the methods designed to measure the incidence of apoptosis, including the recently developed assays that are based on detection of caspases activation. We also review recent findings on the role of caspase-mediated cell death in hematological malignancies and discuss their clinical implications, including new therapeutical strategies that evolve from these findings.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

Induction of CD69 activation molecule on human neutrophils by GM-CSF, IFN-gamma, and IFN-alpha

The CD69 glycoprotein is an early activation antigen of T and B lymphocytes but it expression is induced in vitro on cells of most hematopoietic lineages, including neutrophils after stimulation with PMA or fMLP. In this study, we investigated whether CD69 expression on human neutrophils could be modulated by inflammatory or anti-inflammatory cytokines (IL-1beta, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12, IL-18, G-CSF, GM-CSF, TNF-alpha, TGF-beta, IFN-alpha, IFN-gamma). Resting neutrophils from healthy subjects did not express CD69 on the cell surface; moreover, a preformed intracellular pool of CD69 was not evident in these cells. CD69 was barely detectable on these cells after overnight incubation in medium while overnight incubation with GM-CSF, IFN-gamma or IFN-alpha significantly induced CD69 expression on neutrophils with GM-CSF appearing to be the most potent inducer. This induction was dependent on a new protein synthesis as it was significantly inhibited by cycloheximide (about 50% inhibition). CD69 cross-linking on GM-CSF-primed neutrophils sinergized with LPS and increased TNF-alpha production and secretion suggesting a role for CD69-positive neutrophils in the pathogenesis and maintenance of different inflammatory diseases.

The p38 mitogen-activated protein kinase pathway and its role in interferon signaling

Interferons (IFNs) are pleiotropic cytokines that exhibit multiple biological effects on cells and tissues. IFN receptors are expressed widely in mammalian cells and virtually all different cell types express them on their surface. The Type I IFN receptor has a multichain structure, composed of at least two distinct receptor subunits, IFNalphaR1 and IFNalphaR2. Two Jak-kinases, Tyk-2 and Jak-1, associate with the different receptor subunits and are activated in response to IFNalpha or IFNbeta to regulate engagement of multiple downstream signaling cascades. These include the Stat-pathway, whose function is essential for transcriptional activation of IFN-sensitive genes, and the insulin receptor substrate pathway, which regulates downstream activation of the phosphatidyl-inositol-3' kinase. Members of the Map family of kinases are also activated by the Type I IFN receptor and participate in the generation of IFN signals. The p38 Map kinase pathway appears to play a very important role in the induction of IFN responses. p38 is rapidly activated during engagement of the Type I IFN receptor, and such an activation is regulated by the small G-protein Rac1, which functions as its upstream effector in a tyrosine kinase-dependent manner. The activated form of p38 regulates downstream activation of other serine kinases, notably MapKapK-2 and MapKapK-3, indicating the existence of Type I IFN-dependent signaling cascades activated downstream of p38. Extensive studies have shown that p38 plays a critical role in Type I IFN-dependent transcriptional regulation, without modifying activation of the Stat-pathway. It is now well established that the function of p38 is essential for gene transcription via ISRE or GAS elements, but has no effects on the phosphorylation of Stat-proteins, the formation of Stat-complexes, and their binding to the promoters of IFN-sensitive genes. As Type I IFNs regulate gene expression for proteins with antiviral properties, it is not surprising that pharmacological inhibition of the p38 pathway blocks induction of IFNalpha-antiviral responses. In addition, pharmacological inhibition of p38 abrogates the suppressive effects of Type I IFNs on normal human hematopoietic progenitors, indicating a critical role for this signaling cascade in the induction of the regulatory effects of Type I IFNs on hematopoiesis. p38 is also activated during IFNalpha-treatment of primary leukemia cells from patients with chronic myelogenous leukemia. Such activation is required for IFNalpha-dependent suppression of leukemic cell progenitor growth, indicating that this pathway plays a critical role in the induction of the antileukemic effects of IFNalpha.