Diana Saleiro

Intersection of mTOR and STAT signaling in immunity

Optimal regulation of immune networks is essential for the generation of effective immune responses, and defects in such networks can lead to immunodeficiency while uncontrolled responses can result in autoimmune disorders. mTOR and STAT signaling cascades are key regulators of the differentiation and function of cells of the immune system. Both pathways act as sensors and transducers of environmental stimuli, and recent evidence has revealed points of crosstalk between these pathways, highlighting synergistic regulation of immune cell differentiation and function. We review here the current understanding of mTOR and STAT interactions in T cells and innate immune cells, and discuss potential mechanisms underlying these events. We further outline models for the intersection of these pathways in the regulation of immunity and highlight important areas for future research.

Targeting novel signaling pathways for resistant acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematologic malignancy that is the most common type of acute leukemia diagnosed in adults and the second most common type in children. The overall survival is poor and treatment is associated with significant complications and even death. In addition, a significant number of patients will not respond to therapy or relapse. In this review, several new signaling proteins aberrantly regulated in AML are described, including CREB, Triad1, Bcl-2 family members, Stat3, and mTOR/MEK. Identifying more effective and less toxic agents will provide novel approaches to treat AML.

Central Role of ULK1 in Type I Interferon Signaling

We provide evidence that the Unc-51-like kinase 1 (ULK1) is activated during engagement of the type I interferon (IFN) receptor (IFNR). Our studies demonstrate that the function of ULK1 is required for gene transcription mediated via IFN-stimulated response elements (ISRE) and IFNγ activation site (GAS) elements and controls expression of key IFN-stimulated genes (ISGs). We identify ULK1 as an upstream regulator of p38α mitogen-activated protein kinase (MAPK) and establish that the regulatory effects of ULK1 on ISG expression are mediated possibly by engagement of the p38 MAPK pathway. Importantly, we demonstrate that ULK1 is essential for antiproliferative responses and type I IFN-induced antineoplastic effects against malignant erythroid precursors from patients with myeloproliferative neoplasms. Together, these data reveal a role for ULK1 as a key mediator of type I IFNR-generated signals that control gene transcription and induction of antineoplastic responses.

Merestinib blocks Mnk kinase activity in acute myeloid leukemia progenitors and exhibits antileukemic effects in vitro and in vivo

Mitogen-activated protein kinase interacting protein kinases (Mnks) play important roles in the development and progression of acute myeloid leukemia (AML) by regulating eukaryotic translation initiation factor 4E (eIF4E) activation. Inhibiting Mnk1/2-induced phosphorylation of eIF4E may represent a unique approach for the treatment of AML. We provide evidence for antileukemic effects of merestinib, an orally bioavailable multikinase inhibitor with suppressive effects on Mnk activity. Our studies show that merestinib effectively blocks eIF4E phosphorylation in AML cells and suppresses primitive leukemic progenitors from AML patients in vitro and in an AML xenograft model in vivo. Our findings provide evidence for potent preclinical antileukemic properties of merestinib and support its clinical development for the treatment of patients with AML.

Interferon γ (IFNγ) Signaling via Mechanistic Target of Rapamycin Complex 2 (mTORC2) and Regulatory Effects in the Generation of Type II Interferon Biological Responses.

We provide evidence for a unique pathway engaged by the type II IFN receptor, involving mTORC2/AKT-mediated downstream regulation of mTORC1 and effectors. These events are required for formation of the eukaryotic translation initiation factor 4F complex (eIF4F) and initiation of mRNA translation of type II interferon-stimulated genes. Our studies establish that Rictor is essential for the generation of type II IFN-dependent antiviral and antiproliferative responses and that it controls the generation of type II IFN-suppressive effects on normal and malignant hematopoiesis. Together, our findings establish a central role for mTORC2 in IFNγ signaling and type II IFN responses.

Beyond autophagy: New roles for ULK1 in immune signaling and interferon responses

The human serine/threonine kinase ULK1 is the human homolog of the Caenorhabditis elegans Unc-51 kinase and of the Saccharomyces cerevisiae autophagy-related protein kinase Atg1. As Unc-51 and Atg1, ULK1 regulates both axon growth and autophagy, respectively, in mammalian cells. However, a novel immunoregulatory role of ULK1 has been recently described. This kinase was shown to be required for regulation of both type I interferon (IFN) production and induction of type I IFN signaling. Optimal regulation of IFN production is crucial for generation of effective IFN-immune responses, and defects in such networks can be detrimental for the host leading to uncontrolled pathogen infection, tumor growth, or autoimmune diseases. Thus, ULK1 plays a central role in IFN-dependent immunity. Here we review the diverse roles of ULK1, with special focus on its importance to type I IFN signaling, and highlight important future study questions.

Dual targeting of eIF4E by blocking MNK and mTOR pathways in leukemia

ABSTRACT Dysregulation of mRNA translation leads to aberrant activation of cellular pathways that promote expansion and survival of leukemic clones. A key element of the initiation translation complex is eIF4E (eukaryotic translation initiation factor 4E). The mitogen‐activated protein kinase (MAPK) and mammalian target of rapamycin (mTOR) pathways play important roles in the regulation of eIF4E expression and downstream functional outcomes. Mitogen‐activated protein kinase interacting protein kinases (Mnks) control translation by phosphorylation of eIF4E, whereas the mTOR kinase phosphorylates/de‐activates the eIF4E inhibitor, 4E‐BP1, to release translational repression. Both pathways are often abnormally activated in leukemia cells and promote cell survival events by controlling expression of oncogenic proteins. Targeting these pathways may provide approaches to avoid aberrant proliferation and neoplastic transformation.

ULK1 in type I interferon response

Type I interferon (IFN) signaling leads to transcription and translation of key IFN-stimulated genes (ISGs), whose protein products exhibit anti-tumorigenic, anti-viral, and immunomodulatory functions [1–3]. These responses are triggered by the interaction of type I IFNs (IFNα, IFNβ, IFNω) with a unique cell surface receptor composed by two subunits: IFNα receptor 1 (IFNAR1) and IFNAR2 [1–3]. After the Type I IFN receptor is targeted by different members of this cytokine family, there is induction of the kinase activities of two known kinases; tyrosine kinase 2 (TYK2) and janus kinase 1 (JAK1), which activate several downstream signaling pathways, including STAT (signal transducer and activator of transcription), MAPKKK (mitogen-activated protein kinase kinase kinase), PI3K (phosphatidylinositol 3-kinase)-AKT, mTORC1 (mammalian target of rapamycin complex 1), and mTORC2 signaling cascades [1–3]. There has been rapid accumulation of information in the IFN-signaling field for over two decades. A detailed map of signaling events and elements that ultimately control IFN-responses has emerged [1–3]. Yet, unique mechanisms accounting for activation and coordination of distinct IFN signaling pathways still need to be defined. Similarly, the elements that account for optimized cellular responses and adjust the balance between negative and positive cellular control in the IFN-response, remain to be established. In a recently published report, we identified Unc-51-like kinase 1 (ULK1) as a novel positive regulator of type I IFN signaling, which is essential for both type I IFN-induced anti-viral and anti-proliferative responses [4]. Additionally, we presented evidence showing that engagement of type I IFN receptor activates a novel pathway, possibly involving both PI3K-AKT-dependent and independent meditators, culminating in activation of ULK1 [4]. Importantly, ULK1 kinase activity seems to cooperate, either directly or through intermediate kinases, with the MAPKKK-MKK3/6 signaling cascade for optimal IFN-induced activation of p38 MAPK [4]. Thus, ULK1 activation appears to mediate transcription of ISGs, in part, via regulation of the p38 MAPK pathway [4]. In a recent report, ULK1 was shown to localize to the nucleus and promote cell death by increasing poly (ADP-ribose) polymerase 1 (PARP1) activity in response to reactive oxygen species-induced cellular damage [5]. These new findings, taken together with our recently published report [4], raise the intriguing possibility that ULK1 translocates into the nucleus in response to type I IFNs, and could directly or indirectly control transcription of ISGs, but this remains to be explored in future studies. Type I IFNs can be produced by all nucleated cell types in response to pathogen infection, physiological signals, and other inducers and stimuli [6, 7]. However, the induction of an IFN response involves expression and activation of both positive and negative regulators of Type I IFN production and signaling [6, 7]. Many of these negative regulators are ISGs and IFN-regulated proteins, which control the time and magnitude of an IFN response, preventing inflammation and tissue damage [6, 7]. Interestingly, this seems to be the case also for ULK1 [8]. The stimulator of IFN genes (STING) is activated by cyclic dinucleotides, leading to activation of TBK1 (TRAF family member associated NF-κB activator-binding kinase 1) and phosphorylation of IRF3 [6, 7]. Active IRF3 translocates into the nucleus and induces expression of IFN genes, initiating an IFN response [6, 7]. Our data suggests that ULK1 is activated downstream of the type I IFN receptor, leading to transcription of key ISGs and positively mediating IFN responses [4]. However, based on other recent work, at a later stage ULK1 is activated by cyclic dinucleotides and acts as a negative regulator of STING, resulting in inhibition of IRF3, and subsequent blockage of type I IFN production, thus shutting down the IFN response [8]. Thus, taking together the evidence presented by Kono et al. [8] and our findings [4], one can propose a model in which ULK1 may initially promote IFN responses against pathogens, but it later inhibits STING activity and IFN production, thus limiting/optimizing the IFN response (Figure ​(Figure1).1). Further studies are warranted to fully understand this dual role of ULK1 in the IFN system. This has important implications in the IFN-signaling field and may provoke other questions in the future. For example, it would be interesting to identify putative phosphorylation sites of ULK1 that may correlate with negative versus positive regulatory effects on Type I IFN responses. A complete understanding of the mechanisms of action of ULK1 during IFN-signaling may also prove fundamental for the development of better therapies designed to either promote or inhibit type I IFN-dependent responses. Figure 1 Regulatory roles of ULK1 in Type I IFN responses

Human SLFN5 is a transcriptional co-repressor of STAT1-mediated interferon responses and promotes the malignant phenotype in glioblastoma

We provide evidence that the IFN-regulated member of the Schlafen (SLFN) family of proteins, SLFN5, promotes the malignant phenotype in glioblastoma multiforme (GBM). Our studies indicate that SLFN5 expression promotes motility and invasiveness of GBM cells, and that high levels of SLFN5 expression correlate with high-grade gliomas and shorter overall survival in patients suffering from GBM. In efforts to uncover the mechanism by which SLFN5 promotes GBM tumorigenesis, we found that this protein is a transcriptional co-repressor of STAT1. Type-I IFN treatment triggers the interaction of STAT1 with SLFN5, and the resulting complex negatively controls STAT1-mediated gene transcription via interferon stimulated response elements. Thus, SLFN5 is both an IFN-stimulated response gene and a repressor of IFN-gene transcription, suggesting the existence of a negative-feedback regulatory loop that may account for suppression of antitumor immune responses in glioblastoma.

Central Regulatory Role for SIN1 in Interferon γ (IFNγ) Signaling and Generation of Biological Responses

The precise signaling mechanisms by which type II IFN receptors control expression of unique genes to induce biological responses remain to be established. We provide evidence that Sin1, a known element of the mammalian target of rapamycin complex 2 (mTORC2), is required for IFNγ-induced phosphorylation and activation of AKT and that such activation mediates downstream regulation of mTORC1 and its effectors. These events play important roles in the assembly of the eukaryotic translation initiation factor 4F (eIF4F) and mRNA translation of IFN-stimulated genes. Interestingly, IFNγ-induced tyrosine phosphorylation of STAT1 is reduced in cells with targeted disruption of Sin1, leading to decreased transcription of several IFNγ-inducible genes in an mTORC2-independent manner. Additionally, our studies establish that Sin1 is essential for generation of type II IFN-dependent antiviral effects and antiproliferative responses in normal and malignant hematopoiesis. Together, our findings establish an important role for Sin1 in both transcription and translation of IFN-stimulated genes and type II IFN-mediated biological responses, involving both mTORC2-dependent and -independent functions.