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.

Sprouty proteins are negative regulators of interferon (IFN) signaling and IFN-inducible biological responses.

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.

Essential role for Mnk kinases in type II interferon (IFNgamma) signaling and its suppressive effects on normal hematopoiesis.

IFNγ exhibits potent antitumor effects and plays important roles in the innate immunity against cancer. However, the mechanisms accounting for the antiproliferative effects of IFNγ still remain to be elucidated. We examined the role of Mnk1 (MAPK-interacting protein kinase 1) in IFNγ signaling. Our data demonstrate that IFNγ treatment of sensitive cells results in engagement of Mnk1, activation of its kinase domain, and downstream phosphorylation of the cap-binding protein eIF4E on Ser-209. Such engagement of Mnk1 plays an important role in IFNγ-induced IRF-1 (IFN regulatory factor 1) gene mRNA translation/protein expression and is essential for generation of antiproliferative responses. In studies aimed to determine the role of Mnk1 in the induction of the suppressive effects of IFNs on primitive hematopoietic progenitors, we found that siRNA-mediated Mnk1/2 knockdown results in partial reversal of the suppressive effects of IFNγ on human CD34+-derived myeloid (CFU-GM) and erythroid (BFU-E) progenitors. These findings establish a key role for the Mnk/eIF4E pathway in the regulatory effects of IFNγ on normal hematopoiesis and identify Mnk kinases as important elements in the control of IFNγ-inducible ISG mRNA translation.

Role of protein kinase C-delta (PKC-delta) in the generation of the effects of IFN-alpha 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.

Differential regulation of the p70 S6 kinase pathway by interferon alpha (IFNalpha) and imatinib mesylate (STI571) in chronic myelogenous leukemia cells.

The precise mechanisms by which imatinib mesylate (STI571) and interferon alpha (IFNalpha) exhibit antileukemic effects are not known. We examined the effects of IFNs or imatinib mesylate on signaling pathways regulating initiation of mRNA translation in BCR-ABL-expressing cells. Treatment of IFN-sensitive KT-1 cells with IFNalpha resulted in phosphorylation/activation of mammalian target of rapamycin (mTOR) and downstream activation of p70 S6 kinase. The IFN-activated p70 S6 kinase was found to regulate phosphorylation of S6 ribosomal protein, which regulates translation of mRNAs with oligopyrimidine tracts in the 5'-untranslated region. In addition, IFNalpha treatment resulted in an mTOR- and/or phosphatidyl-inositol 3'(PI 3') kinase-dependent phosphorylation of 4E-BP1 repressor of mRNA translation on sites that are required for its deactivation and dissociation from the eukaryotic initiation factor-4E (eIF4E) complex. In contrast to the effects of IFNs, imatinib mesylate suppressed p70 S6 kinase activity, consistent with inhibition of BCR-ABL-mediated activation of the mTOR/p70 S6 kinase pathway. Moreover, the mTOR inhibitor rapamycin enhanced the suppressive effects of imatinib mesylate on primary leukemic granulocyte macrophage-colony-forming unit (CFU-GM) progenitors from patients with chronic myelogenous leukemia (CML). Taken altogether, our data demonstrate that IFNs and imatinib mesylate differentially regulate PI 3' kinase/mTOR-dependent signaling cascades in BCR-ABL-transformed cells, consistent with distinct effects of these agents on pathways regulating mRNA translation. They also support the concept that combined use of imatinib mesylate with mTOR inhibitors may be an appropriate future therapeutic strategy for the treatment of CML.

Engagement of protein kinase C-theta in interferon signaling in T-cells.

Protein kinase C-theta (PKC-theta) 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-theta is engaged in interferon (INF) signaling in T-cells. Both Type I (alpha, beta) and Type II (gamma) IFNs induced phosphorylation of PKC-theta in human T-cell lines and primary human T-lymphocytes. Such phosphorylation of PKC-theta resulted in activation of its kinase domain, suggesting that this kinase plays a functional role in interferon signaling. Consistent with this, inhibition of PKC-theta protein expression using small interfering RNAs (siRNA) abrogated IFN-alpha- and IFN-gamma-dependent gene transcription via GAS elements. Similarly, blocking of PKC-theta kinase activity by overexpression of a dominant-negative PKC-theta mutant also blocked GAS-driven transcription, further demonstrating a requirement for PKC-theta in IFN-dependent transcriptional activation. The effects of PKC-theta on IFN-dependent gene transcription were not mediated by regulation of the IFN-activated STAT pathway, as siRNA-mediated PKC-theta knockdown had no effects on STAT1 phosphorylation and binding of STAT1-containing complexes to SIE/GAS elements. On the other hand, siRNA-mediated PKC-theta inhibition blocked phosphorylation/activation of MKK4, suggesting that interferon-dependent PKC-theta activation regulates downstream engagement of MAP kinase pathways. Altogether, these findings demonstrate that PKC-theta is an interferon-inducible kinase and strongly suggest that it plays an important role in the generation of interferon-responses in T-cells.

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.

Interferon-gamma engages the p70 S6 kinase to regulate phosphorylation of the 40S S6 ribosomal protein.

The signals generated by the IFNgamma receptor to initiate mRNA translation and generation of protein products that mediate IFNgamma responses are largely unknown. In the present study, we provide evidence for the existence of an IFNgamma-dependent signaling cascade activated downstream of the phosphatidylinositol (PI) 3'-kinase, involving the mammalian target of rapamycin (mTOR) and the p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated and activated during engagement of the IFNgamma receptor in sensitive cell lines. Such activation of p70 S6 kinase is blocked by pharmacological inhibitors of the PI 3' kinase and mTOR, and is abrogated in double-knockout mouse embryonic fibroblasts for the alpha and beta isoforms of the p85 regulatory subunit of the PI 3'-kinase. The IFNgamma-activated p70 S6 kinase subsequently phosphorylates the 40S S6 ribosomal protein on serines 235/236, to regulate IFNgamma-dependent mRNA translation. In addition to phosphorylation of 40S ribosomal protein, IFNgamma also induces phosphorylation of the 4E-BP1 repressor of mRNA translation on threonines 37/46, threonine 70, and serine 65, sites whose phosphorylation is required for the inactivation of 4E-BP1 and its dissociation from the eukaryotic initiation factor-4E (eIF4E) complex. Thus, engagement of the PI 3'-kinase and mTOR by the IFNgamma receptor results in the generation of two distinct signals that play roles in the initiation of mRNA translation, suggesting an important role for this pathway in IFNgamma signaling.

Role of p38alpha 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.

Activation of protein kinase C delta by all-trans-retinoic acid.

All-trans-retinoic acid (RA) is a potent inhibitor of leukemia cell proliferation and induces differentiation of acute promyelocytic leukemia cells in vitro and in vivo. For RA to induce its biological effects in target cells, binding to specific retinoic acid nuclear receptors is required. The resulting complexes bind to RA-responsive elements (RAREs) in the promoters of RA-inducible genes to initiate gene transcription and to generate protein products that mediate the biological effects of RA. In this report, we provide evidence that a member of the protein kinase C (PKC) family of proteins, PKC delta, is activated during RA treatment of the NB-4 and HL-60 acute myeloid leukemia cell lines as well as the MCF-7 breast cancer cell line. Such RA-dependent phosphorylation was also observed in primary acute promyelocytic leukemia cells and resulted in activation of the kinase domain of PKC delta. In studies aimed at understanding the functional relevance of PKC delta in the induction of RA responses, we found that pharmacological inhibition of PKC delta (but not of other PKC isoforms) diminished RA-dependent gene transcription via RAREs. On the other hand, overexpression of a constitutively active form of the kinase strongly enhanced RA-dependent gene transcription via RAREs. Gel shift assays and chromatin immunoprecipitation studies demonstrated that PKC delta associated with retinoic acid receptor-alpha and was present in an RA-inducible protein complex that bound to RAREs. Pharmacological inhibition of PKC delta activity abrogated the induction of cell differentiation and growth inhibition of NB-4 blast cells, demonstrating that its function is required for such effects. Altogether, our data provide strong evidence that PKC delta is activated in an RA-dependent manner and plays a critical role in the generation of the biological effects of RA in malignant cells.

Activation of protein kinase C delta by IFN-gamma.

Engagement of the type II IFN (IFN-gamma) receptor results in activation of the Janus kinase-Stat pathway and induction of gene transcription via IFN-gamma-activated site (GAS) elements in the promoters of IFN-gamma-inducible genes. An important event in IFN-gamma-dependent gene transcription is phosphorylation of Stat1 on Ser(727), which is regulated by a kinase activated downstream of the phosphatidylinositol 3'-kinase. Here we provide evidence that a member of the protein kinase C (PKC) family of proteins is activated downstream of the phosphatidylinositol 3'-kinase and is engaged in IFN-gamma signaling. Our data demonstrate that PKCdelta is rapidly phosphorylated during engagement of the type II IFNR and its kinase domain is induced. Subsequently, the activated PKCdelta associates with a member of the Stat family of proteins, Stat1, which acts as a substrate for its kinase activity and undergoes phosphorylation on Ser(727). Inhibition of PKCdelta activity diminishes phosphorylation of Stat1 on Ser(727) and IFN-gamma-dependent transcriptional regulation via IFN-gamma-activated site elements, without affecting the phosphorylation of the protein on Tyr(701). Thus, PKCdelta is activated during engagement of the IFN-gamma receptor and plays an important role in IFN-gamma signaling by mediating serine phosphorylation of Stat1 and facilitating transcription of IFN-gamma-stimulated genes.

Therapeutic implications for interferon-alpha in arthritis: a pilot study.

OBJECTIVE: To evaluate the therapeutic potential of interferon-a (IFN-a) in osteoarthritis (OA) and rheumatoid arthritis (RA) by examining regulation of cytokine antagonist expression. METHODS: Expression of interleukin 1 receptor antagonist (IL-1Ra) and soluble tumor necrosis factor receptor (sTNFR) was examined by ELISA in cells from freshly isolated synovial fluids (SF) and synovial tissues (ST) from patients with OA or RA, either left untreated or treated with IFN-a. Single (7) and paired (5) SF and ST cells from OA and RA patients were examined. As well, the ability of IFN-a to regulate gene expression levels for osteoprotegerin (OPG) and osteoprotegerin ligand (OPGL) was examined in freshly isolated SF cells from patients with RA, by reverse transcriptase polymerase chain reaction. RESULTS: IL-1Ra and sTNFR were found to be constitutively expressed in OA and RA SF and ST cells. IFN-a treatment resulted in an increase in both IL 1Ra and sTNFR production. Freshly isolated RA SF cells exhibited constitutive OPGL gene expression in both the non-T and T cell fractions of the SF. In contrast, OPG gene expression levels were undetectable or low. IFN-a treatment of RA SF cells resulted in upregulation of OPG gene expression in the T cell fraction of the RA SF cells, whereas OPGL gene expression remained unaffected. CONCLUSION: These in vitro data suggest a therapeutic role for IFN-a in the treatment of arthritis through upregulation of critical cytokine antagonists.

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.

The CrkL adapter protein is required for type I interferon-dependent gene transcription and activation of the small G-protein Rap1.

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 IFN alpha-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 IFN alpha 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 IFN alpha-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.