Caspase-mediated cleavage of hematopoietic progenitor kinase 1 (HPK1) converts an activator of NFkappaB into an inhibitor of NFkappaB

HPK1 citron homology domain regulates phosphorylation of SLP76 and modulates kinase domain interaction dynamics

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of T-cell receptor signaling and as such is an attractive target for cancer immunotherapy. Although the role of the HPK1 kinase domain (KD) has been extensively characterized, the function of its citron homology domain (CHD) remains elusive. Through a combination of structural, biochemical, and mechanistic studies, we characterize the structure-function of CHD in relationship to KD. Crystallography and hydrogen-deuterium exchange mass spectrometry reveal that CHD adopts a seven-bladed β-propellor fold that binds to KD. Mutagenesis associated with binding and functional studies show a direct correlation between domain-domain interaction and negative regulation of kinase activity. We further demonstrate that the CHD provides stability to HPK1 protein in cells as well as contributes to the docking of its substrate SLP76. Altogether, this study highlights the importance of the CHD in the direct and indirect regulation of HPK1 function.

Current strategies for targeting HPK1 in cancer and the barriers to preclinical progress

INTRODUCTION

Hematopoietic progenitor kinase 1 (HPK1), a 97-kDa serine/threonine Ste20-related protein kinase, functions as an intracellular negative regulator, primarily in hematopoietic lineage cells, where it regulates T cells, B cells, dendritic cells, and other immune cells. Loss of HPK1 kinase activity results in exacerbated cytokine secretion, enhanced T cell signaling, improved viral clearance, and thus increased restraint of tumor growth. These findings highlight HPK1 as a promising target for immuno-oncology treatments, culminating in the advancement of candidate compounds targeting HPK1 to clinical trials by several biotech enterprises.

AREAS COVERED

Through searching PubMed, Espacenet-patent search, and clinicaltrials.gov, this review provides a comprehensive analysis of HPK1, encompassing its structure and roles in various downstream signaling pathways, the consequences of constitutive activation of HPK1, and potential therapeutic strategies to treat HPK1-driven malignancies. Moreover, the review outlines the patents issued for small molecule inhibitors and clinical investigations of HPK1.

EXPERT OPINION

To enhance the success of tumor immunotherapy in clinical trials, it is important to develop protein degraders, allosteric inhibitors, and antibody-drug conjugates based on the crystal structure of HPK1, and to explore combination therapy approaches. Although several challenges remain, the development of HPK1 inhibitors display promising in preclinical and clinical studies.

Hematopoietic Progenitor Kinase 1 in Tumor Immunology: A Medicinal Chemistry Perspective

Hematopoietic progenitor kinase 1 (HPK1), a hematopoietic cell-restricted member of the serine/threonine Ste20-related protein kinases, is a negative regulator of the T cell receptor, B cell receptor, and dendritic cells. Loss of HPK1 kinase function increases cytokine secretion and enhances T cell signaling, virus clearance, and tumor growth inhibition. Therefore, HPK1 is considered a promising target for tumor immunotherapy. Several HPK1 inhibitors have been reported to regulate T cell function. In addition, HPK1-targeting PROTACs, which can induce the degradation of HPK1, have also been developed. Here, we provide an overview of research concerning HPK1 protein structure, function, and inhibitors and propose perspectives and insights for the future development of agents targeting HPK1.

Reduced expression of hematopoietic progenitor kinase 1 in T follicular helper cells causes autoimmunity of systemic lupus erythematosus

BACKGROUD

T follicular helper (Tfh) cells have been discovered to be the main CD4⁺ T cells assisting B cells to produce antibody. They are over activated in patients with systemic lupus erythematosus (SLE) and consequently lead to excessive immunity. Hematopoietic progenitor kinase 1 (HPK1) negatively regulates T cell-mediated immune responses and TCR signal. This study aimed to investigate the roles of HPK1 in SLE Tfh cells.

METHODS

HPK1 mRNA and protein levels in Tfh cells were measured by real-time quantitative PCR and western blot analysis, respectively. The production of IL-21, B cell-activating factor (BAFF), interferon γ (IFNγ), IL-17A, IgM, IgG1, IgG2, and IgG3 were analyzed using enzyme linked immunosorbent assay. Tfh cells proliferation was evaluated with 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

HPK1 mRNA and protein levels were significantly reduced in SLE Tfh cells, and negatively correlated with SLE disease activity index (SLEDAI) and Systemic Lupus International Collaborating Clinics/American College of Rheumatology (SLICC/ACR) Damage Index for SLE (SDI). Knocking down HPK1 with siRNA in normal Tfh cells greatly elevated Tfh cells proliferation and secretions of IL-21, BAFF, IFNγ, IgG1, IgG2, and IgG3. There were no marked alterations in IL-17A and IgM productions. The opposite effects were observed in SLE Tfh cells transfected with HPK1 overexpressing plasmid: Tfh cells proliferation and productions of IL-21, BAFF, IFNγ, IgG1, IgG2, and IgG3 were all alleviated. And there were no significant changes in IL-17A and IgM levels.

CONCLUSION

Our results suggest for the first time that inhibited expression of HPK1 in SLE Tfh cells leading to Tfh cells overactivation and B cells overstimulation, subsequently, the onset and progression of SLE.

The Kinase MAP4K1 Inhibits Cytosolic RNA-Induced Antiviral Signaling by Promoting Proteasomal Degradation of TBK1/IKKε

TANK-binding kinase 1 (TBK1)/IκB kinase-ε (IKKε) mediates robust production of type I interferons (IFN-I) and proinflammatory cytokines in response to acute viral infection. However, excessive or prolonged production of IFN-I is harmful and even fatal to the host by causing autoimmune disorders. In this study, we identified mitogen-activated protein kinase kinase kinase kinase 1 (MAP4K1) as a negative regulator in the RIG-I-like receptor (RLR) signaling pathway. MAP4K1, a member of Ste20-like serine/threonine kinases, was previously known as a prominent regulator in adaptive immunity by downregulating T-cell receptor (TCR) signaling and B-cell receptor (BCR) signaling. However, its role in regulating antiviral innate immune signaling is still unclear. This study reports an undiscovered role of MAP4K1, which inhibits RLR signaling by targeting TBK1/IKKε for proteasomal degradation via the ubiquitin ligase DTX4. We initially identify MAP4K1 as an interacting partner of TBK1 by yeast two-hybrid screens and subsequently investigate its function in RLR-mediated antiviral signaling pathways. Overexpression of MAP4K1 significantly inhibits RNA virus-triggered activation of IFN-β and the production of proinflammatory cytokines. Consistently, knockdown or knockout experiments show opposite effects. Furthermore, MAP4K1 promotes the degradation of TBK1/IKKε by K48-linked ubiquitination via DTX4. Knockdown of DTX4 abrogated the ubiquitination and degradation of TBK1/IKKε. Collectively, our results identify that MAP4K1 acts as a negative regulator in antiviral innate immunity by targeting TBK1/IKKε, discover a novel TBK1 inhibitor, and extend a novel functional role of MAP4K1 in immunity. IMPORTANCE TANK-binding kinase 1 (TBK1)/IκB kinase-ε (IKKε) mediates robust production of type I interferons (IFN-I) and proinflammatory cytokines to restrict the spread of invading viruses. However, excessive or prolonged production of IFN-I is harmful to the host by causing autoimmune disorders. In this study, we identified that mitogen-activated protein kinase kinase kinase kinase 1 (MAP4K1) is a negative regulator in the RLR signaling pathway. Notably, MAP4K1 promotes the degradation of TBK1/IKKε by K48-linked ubiquitination via the ubiquitin ligase DTX4, leading to the negative regulation of the IFN signaling pathway. Previous studies showed that MAP4K1 has a pivotal function in adaptive immune responses. This study identifies that MAP4K1 also plays a vital role in innate immunity and outlines a novel mechanism by which the IFN signaling pathway is tightly controlled to avoid excessive inflammation. Our study documents a novel TBK1 inhibitor, which serves as a potential therapeutic target for autoimmune diseases, and elucidated a significant function for MAP4K1 linked to innate immunity in addition to subsequent adaptive immunity.

A perspective on HPK1 as a novel immuno-oncology drug target

In this perspective review, the role Hematopoietic Progenitor Kinase 1 (HPK1) in tumor immunity will be reviewed, with special emphasis on how T cells are negatively-regulated at different junctures of cancer-immunity cycle by this regulatory kinase. The review will highlight the strengths and weaknesses of HPK1 as a candidate target for novel immuno-oncology (IO) drug development that is centered on the use of small molecule kinase inhibitor to modulate the immune response against cancer. Such a therapeutic approach, if proven successful, could supplement the cancer cell-centric standard of care therapies in order to fully meet the therapeutic needs of cancer patients.

The Crosstalk Between Hippo-YAP Pathway and Innate Immunity

Recognition of pathogen-associated molecular patterns (PAMPs) triggers expression of antiviral interferons and proinflammatory cytokines, which functions as the frontier of host defense against microbial pathogen invasion. Hippo-YAP pathway regulates cell proliferation, survival, differentiation and is involved in diverse life processes, including tissue homeostasis and tumor suppression. Emerging discoveries elucidated that the components of Hippo-YAP pathway, such as MST1/2, NDR1/2, and YAP/TAZ played crucial regulatory roles in innate immunity. Meanwhile the innate immune signaling also exhibited regulatory effect on Hippo-YAP pathway. As for the importance of these two pathways, it would be interesting to figure out the deeper biological implications of their interplays. This review focuses on the regulation between Hippo-YAP pathway and innate immune signaling. We also propose the possible contribution of these interplays to tumor development.

Hematopoietic progenitor kinase 1 down-regulates the oncogenic receptor tyrosine kinase AXL in pancreatic cancer

The oncogenic receptor tyrosine kinase AXL is overexpressed in cancer and plays an important role in carcinomas of multiple organs. However, the mechanisms of AXL overexpression in cancer remain unclear. In this study, using HEK293T, Panc-1, and Panc-28 cells and samples of human pancreatic intraepithelial neoplasia (PanIN), along with several biochemical approaches and immunofluorescence microscopy analyses, we sought to investigate the mechanisms that regulate AXL over-expression in pancreatic ductal adenocarcinoma (PDAC). We found that AXL interacts with hematopoietic progenitor kinase 1 (HPK1) and demonstrate that HPK1 down-regulates AXL and decreases its half-life. The HPK1-mediated AXL degradation was inhibited by the endocytic pathway inhibitors leupeptin, bafilomycin A1, and monensin. HPK1 accelerated the movement of AXL from the plasma membrane to endosomes in pancreatic cancer cells treated with the AXL ligand growth arrest-specific 6 (GAS6). Moreover, HPK1 increased the binding of AXL to the Cbl proto-oncogene (c-Cbl); promoted AXL ubiquitination; decreased AXL-mediated signaling, including phospho-AKT and phospho-ERK signaling; and decreased the invasion capability of PDAC cells. Importantly, we show that AXL expression inversely correlates with HPK1 expression in human PanINs and that patients whose tumors have low HPK1 and high AXL expression levels have shorter survival than those with low AXL or high HPK1 expression (p < 0.001). Our results suggest that HPK1 is a tumor suppressor that targets AXL for degradation via the endocytic pathway. HPK1 loss of function may contribute to AXL overexpression and thereby enhance AXL-dependent downstream signaling and tumor invasion in PDAC.

Multiple conformational states of the HPK1 kinase domain in complex with sunitinib reveal the structural changes accompanying HPK1 trans-regulation

Hematopoietic progenitor kinase 1 (HPK1 or MAP4K1) is a Ser/Thr kinase that operates via the c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) signaling pathways to dampen the T-cell response and antitumor immunity. Accordingly, selective HPK1 inhibition is considered a means to enhance antitumor immunity. Sunitinib, a multi-receptor tyrosine kinase (RTK) inhibitor approved for the management of gastrointestinal stromal tumors (GISTs), renal cell carcinoma (RCC), and pancreatic cancer, has been reported to inhibit HPK1 in vitro In this report, we describe the crystal structures of the native HPK1 kinase domain in both nonphosphorylated and doubly phosphorylated states, in addition to a double phosphomimetic mutant (T165E,S171E), each complexed with sunitinib at 2.17-3.00-Å resolutions. The native nonphosphorylated cocrystal structure revealed an inactive dimer in which the activation loop of each monomer partially occupies the ATP- and substrate-binding sites of the partner monomer. In contrast, the structure of the protein with a doubly phosphorylated activation loop exhibited an active kinase conformation with a greatly reduced monomer-monomer interface. Conversely, the phosphomimetic mutant cocrystal structure disclosed an alternative arrangement in which the activation loops are in an extended domain-swapped configuration. These structural results indicate that HPK1 is a highly dynamic kinase that undergoes trans-regulation via dimer formation and extensive intramolecular and intermolecular remodeling of the activation segment.

Hematopoietic Progenitor Kinase-1 Structure in a Domain-Swapped Dimer

Enhancement of antigen-specific T cell immunity has shown significant therapeutic benefit in infectious diseases and cancer. Hematopoietic progenitor kinase-1 (HPK1) is a negative-feedback regulator of T cell receptor signaling, which dampens T cell proliferation and effector function. A recent report showed that a catalytic dead mutant of HPK1 phenocopies augmented T cell responses observed in HPK1-knockout mice, indicating that kinase activity is critical for function. We evaluated active and inactive mutants and determined crystal structures of HPK1 kinase domain (HPK1-KD) in apo and ligand bound forms. In all structures HPK1-KD displays a rare domain-swapped dimer, in which the activation segment comprises a well-conserved dimer interface. Biophysical measurements show formation of dimer in solution. The activation segment adopts an α-helical structure which exhibits distinct orientations in active and inactive states. This face-to-face configuration suggests that the domain-swapped dimer may possess alternative selectivity for certain substrates of HPK1 under relevant cellular context.

The protein phosphatase 2A regulatory subunit B56γ mediates suppression of T cell receptor (TCR)-induced nuclear factor-κB (NF-κB) activity

NF-κB is an important transcription factor in the immune system, and aberrant NF-κB activity contributes to malignant diseases and autoimmunity. In T cells, NF-κB is activated upon TCR stimulation, and signal transduction to NF-κB activation is triggered by a cascade of phosphorylation events. However, fine-tuning and termination of TCR signaling are only partially understood. Phosphatases oppose the role of kinases by removing phosphate moieties. The catalytic activity of the protein phosphatase PP2A has been implicated in the regulation of NF-κB. PP2A acts in trimeric complexes in which the catalytic subunit is promiscuous and the regulatory subunit confers substrate specificity. To understand and eventually target NF-κB-specific PP2A functions it is essential to define the regulatory PP2A subunit involved. So far, the regulatory PP2A subunit that mediates NF-κB suppression in T cells remained undefined. By performing a siRNA screen in Jurkat T cells harboring a NF-κB-responsive luciferase reporter, we identified the PP2A regulatory subunit B56γ as negative regulator of NF-κB in TCR signaling. B56γ was strongly up-regulated upon primary human T cell activation, and B56γ silencing induced increased IκB kinase (IKK) and IκBα phosphorylation upon TCR stimulation. B56γ silencing enhanced NF-κB activity, resulting in increased NF-κB target gene expression including the T cell cytokine IL-2. In addition, T cell proliferation was increased upon B56γ silencing. These data help to understand the physiology of PP2A function in T cells and the pathophysiology of diseases involving PP2A and NF-κB.

The CUL7/F-box and WD repeat domain containing 8 (CUL7/Fbxw8) ubiquitin ligase promotes degradation of hematopoietic progenitor kinase 1

HPK1, a member of mammalian Ste20-like serine/threonine kinases, is lost in >95% pancreatic cancer through proteasome-mediated degradation. However, the mechanism of HPK1 loss has not been defined. The aims of this study are to identify the ubiquitin ligase and to examine the mechanisms that targets HPK1 degradation. We found that the CUL7/Fbxw8 ubiquitin ligase targeted HPK1 for degradation via the 26 S proteasome. The ubiquitination of HPK1 required its kinase activity and autophosphorylation. Wild-type protein phosphatase 4 (PP4), but not the phosphatase-dead PP4 mutant, PP4-RL, inhibits the interaction of Fbxw8 with HPK1 and Fbxw8-mediated ubiquitination of HPK1. In addition, we showed that Thr-355 of HPK1 is a key PP4 dephosphorylation site, through which CUL7/Fbxw8 ubiquitin ligase and PP4 regulates HPK1 stability. Knockdown of Fbxw8 restores endogenous HPK1 protein expression and inhibits cell proliferation of pancreatic cancer cells. Our study demonstrated that targeted degradation of HPK1 by the CUL7/Fbxw8 ubiquitin ligase constitutes a negative-feedback loop to restrain the activity of HPK1 and that CUL7/Fbxw8 ubiquitin ligase promotes pancreatic cancer cell proliferation. CUL7/Fbxw8 ubiquitin ligase-mediated HPK1 degradation revealed a direct link and novel role of CUL7/Fbxw8 ubiquitin ligase in the MAPK pathway, which plays a critical role in cell proliferation and differentiation.

Hematopoietic progenitor kinase 1 (HPK1) is required for LFA-1–mediated neutrophil recruitment during the acute inflammatory response

Hematopoietic progenitor kinase 1 (HPK1) regulates LFA-1 affinity and thereby controls adhesion and postadhesion functions of neutrophils. Hematopoietic progenitor kinase 1 (HPK1) is critically involved in neutrophil trafficking during acute inflammation.

A PP4 holoenzyme balances physiological and oncogenic nuclear factor-kappa B signaling in T lymphocytes

Signal transduction to nuclear factor-kappa B (NF-κB) involves multiple kinases and phosphorylated target proteins, but little is known about signal termination by dephosphorylation. By RNAi screening, we have identified protein phosphatase 4 regulatory subunit 1 (PP4R1) as a negative regulator of NF-κB activity in T lymphocytes. PP4R1 formed part of a distinct PP4 holoenzyme and bridged the inhibitor of NF-κB kinase (IKK) complex and the phosphatase PP4c, thereby directing PP4c activity to dephosphorylate and inactivate the IKK complex. PP4R1 expression was triggered upon activation and proliferation of primary human T lymphocytes and deficiency for PP4R1 caused sustained and increased IKK activity, T cell hyperactivation, and aberrant NF-κB signaling in NF-κB-addicted T cell lymphomas. Collectively, our results unravel PP4R1 as a previously unknown activation-associated negative regulator of IKK activity in lymphocytes whose downregulation promotes oncogenic NF-κB signaling in a subgroup of T cell lymphomas.

Augmented generation of protein fragments during wakefulness as the molecular cause of sleep: a hypothesis

Despite extensive understanding of sleep regulation, the molecular-level cause and function of sleep are unknown. I suggest that they originate in individual neurons and stem from increased production of protein fragments during wakefulness. These fragments are transient parts of protein complexes in which the fragments were generated. Neuronal Ca²⁺ fluxes are higher during wakefulness than during sleep. Subunits of transmembrane channels and other proteins are cleaved by Ca²⁺-activated calpains and by other nonprocessive proteases, including caspases and secretases. In the proposed concept, termed the fragment generation (FG) hypothesis, sleep is a state during which the production of fragments is decreased (owing to lower Ca²⁺ transients) while fragment-destroying pathways are upregulated. These changes facilitate the elimination of fragments and the remodeling of protein complexes in which the fragments resided. The FG hypothesis posits that a proteolytic cleavage, which produces two fragments, can have both deleterious effects and fitness-increasing functions. This (previously not considered) dichotomy can explain both the conservation of cleavage sites in proteins and the evolutionary persistence of sleep, because sleep would counteract deleterious aspects of protein fragments. The FG hypothesis leads to new explanations of sleep phenomena, including a longer sleep after sleep deprivation. Studies in the 1970s showed that ethanol-induced sleep in mice can be strikingly prolonged by intracerebroventricular injections of either Ca²⁺ alone or Ca²⁺ and its ionophore (Erickson et al., Science 1978;199:1219-1221; Harris, Pharmacol Biochem Behav 1979;10:527-534; Erickson et al., Pharmacol Biochem Behav 1980;12:651-656). These results, which were never interpreted in connection to protein fragments or the function of sleep, may be accounted for by the FG hypothesis about molecular causation of sleep.

Germinal center kinases in immune regulation

Germinal center kinases (GCKs) participate in a variety of signaling pathways needed to regulate cellular functions including apoptosis, cell proliferation, polarity and migration. Recent studies have shown that GCKs are participants in both adaptive and innate immune regulation. However, the differential activation and regulatory mechanisms of GCKs, as well as upstream and downstream signaling molecules, remain to be fully defined. It remains unresolved whether and how GCKs may cross-talk with existing signaling pathways. This review stresses the progresses in research of GCKs relevant to the immune system.

Dual role of hematopoietic progenitor kinase 1 (HPK1) as a positive regulator of 1α,25-dihydroxyvitamin D-induced differentiation and cell cycle arrest of AML cells and as a mediator of vitamin D resistance

Recent clinical trials aimed at improved treatment of AML by administration of vitamin D derivatives showed unremarkable results, suggesting development of vitamin D resistance in patients' AML blasts. Since mechanisms of vitamin D resistance are not clear, we studied 40AF cells, a subline of HL60 cells that can proliferate in the presence of 1α,25-dihydroxyvitamin D₃ (1,25D). We found that mRNA and protein levels of HPK1, an upstream MAP4 kinase, are dramatically increased in 40AF cells, and HPK1 protein is further increased when the 1,25D resistance of 40AF cells is partially reversed by the addition of carnosic acid and p38MAPK inhibitor SB202190 (DCS cocktail). Knockdown of HPK1 reduces 1,25D/DCS-induced differentiation of both 1,25D-sensitive HL60 and U937 cells and 1,25D-resistant 40AF cells, but the effect of HPK1 knockdown on differentiation-associated G 1 arrest is more apparent in the resistant than the sensitive cells. To explain why 40AF and the intrinsically vitamin D-resistant KG-1a cells can proliferate in the presence of vitamin D, we found that the cleaved HPK1 fragment (HPK1-C) level is high in 40AF and KG-1a cells, but when differentiation is induced by DCS, HPK1-C decreases while full-length (FL)-HPK1 increases. Accordingly, inhibition of proteolysis with the pan-caspase inhibitor Q-VD-OPh reduced HPK1 cleavage and enhanced DCS-induced differentiation of 40AF cells. The results indicate that FL-HPK1 is a positive regulator of vitamin D-induced differentiation in AML cells, but the cleaved HPK1 fragment inhibits differentiation. Thus, high HPK1 cleavage activity contributes to vitamin D resistance, and HPK1 has a dual role in AML cell differentiation.

Down-regulation of B cell receptor signaling by hematopoietic progenitor kinase 1 (HPK1)-mediated phosphorylation and ubiquitination of activated B cell linker protein (BLNK)

Hematopoietic progenitor kinase 1 (HPK1) is a Ste20-like serine/threonine kinase that suppresses immune responses and autoimmunity. B cell receptor (BCR) signaling activates HPK1 by inducing BLNK/HPK1 interaction. Whether HPK1 can reciprocally regulate BLNK during BCR signaling is unknown. Here, we show that HPK1-deficient B cells display hyper-proliferation and hyper-activation of IκB kinase and MAPKs (ERK, p38, and JNK) upon the ligation of BCR. HPK1 attenuates BCR-induced cell activation via inducing BLNK threonine 152 phosphorylation, which mediates BLNK/14-3-3 binding. Furthermore, threonine 152-phosphorylated BLNK is ubiquitinated at lysine residues 37, 38, and 42, leading to attenuation of MAPK and IκB kinase activation in B cells during BCR signaling. These results reveal a novel negative feedback regulation of BCR signaling by HPK1-mediated phosphorylation, ubiquitination, and subsequent degradation of the activated BLNK.

Inhibited expression of hematopoietic progenitor kinase 1 associated with loss of jumonji domain containing 3 promoter binding contributes to autoimmunity in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by T cell overactivation and B cell hyper-stimulation. Hematopoietic progenitor kinase 1 (HPK1, also called MAP4K1) negatively regulates T cell-mediated immune responses. However, the role of HPK1 and the mechanisms that regulate HPK1 expression in SLE remain poorly understood. Using chromatin immunoprecipitation (ChIP) microarray data, we identified markedly increased histone H3 lysine 27 trimethylation (H3K27me3) enrichment at the HPK1 promoter of SLE CD4+ T cells relative to controls, and confirmed this observation using ChIP and real-time PCR experiments. We further found that HPK1 mRNA and protein levels were significantly decreased in CD4+ T cells of patients with SLE, and that this decrease was not caused by exposure to standard SLE medications. Down-regulating HPK1 in healthy CD4+ T cells significantly accelerated T cell proliferation and production of IFNγ and IgG. Consistent with these findings, overexpressing HPK1 in SLE CD4+ T cells caused a significant decrease in T cell reactivity. In addition, we observed a striking decrease in jumonji domain containing 3 (JMJD3) binding, but no marked change in enhancer of zeste homolog 2 (EZH2) binding, at the HPK1 promoter region in SLE CD4+ T cells compared to healthy controls. SiRNA knock down of JMJD3 in healthy CD4+ T cells led to decreased JMJD3 binding and increased H3K27me3 enrichment at the HPK1 promoter region, thus inhibiting the expression of HPK1. Concordantly, plasmid-induced overexpression of JMJD3 in SLE CD4+ T cells led to increased JMJD3 binding, decreased H3K27me3 enrichment, and up-regulated HPK1 expression. Our results show for the first time that inhibited HPK1 expression in SLE CD4+ T cells is associated with loss of JMJD3 binding and increased H3K27me3 enrichment at the HPK1 promoter, contributing to T cell overactivation and B cell overstimulation in SLE. These findings suggest that HPK1 may serve as a novel target for effective SLE therapy.

The SLP-76 Src homology 2 domain is required for T cell development and activation

The adapter protein Src homology 2 (SH2) domain-containing leukocyte protein of 76 kDa (SLP-76) is critical for multiple aspects of T cell development and function. Through its protein-binding domains, SLP-76 serves as a platform for the assembly of multiple enzymes and adapter proteins that function together to activate second messengers required for TCR signal propagation. The N terminus of SLP-76, which contains three tyrosines that serve as docking sites for SH2 domain-containing proteins, and the central proline-rich region of SLP-76 have been well studied and are known to be important for both thymocyte selection and activation of peripheral T cells. Less is known about the function of the C-terminal SH2 domain of SLP-76. This region inducibly associates with ADAP and HPK1. Combining regulated deletion of endogenous SLP-76 with transgenic expression of a SLP-76 SH2 domain mutant, we demonstrate that the SLP-76 SH2 domain is required for peripheral T cell activation and positive selection of thymocytes, a function not previously attributed to this region. This domain is also important for T cell proliferation, IL-2 production, and phosphorylation of protein kinase D and IκB. ADAP-deficient T cells display similar, but in some cases less severe, defects despite phosphorylation of a negative regulatory site on SLP-76 by HPK1, a function that is lost in SLP-76 SH2 domain mutant T cells.

Apoptosis signaling kinases: from stress response to health outcomes

Apoptosis is a highly regulated process essential for the development and homeostasis of multicellular organisms. Whereas caspases, a large family of intracellular cysteine proteases, play central roles in the execution of apoptosis, other proapoptotic and antiapoptotic regulators such as the members of the Bcl-2 family are also critically involved in the regulation of apoptosis. A large body of evidence has revealed that a number of protein kinases are among such regulators and regulate cellular sensitivity to various proapoptotic signals at multiple steps in apoptosis. However, recent progress in the analysis of these apoptosis signaling kinases demonstrates that they generally act as crucial regulators of diverse cellular responses to a wide variety of stressors, beyond their roles in apoptosis regulation. In this review, we have cataloged apoptosis signaling kinases involved in cellular stress responses on the basis of their ability to induce apoptosis and discuss their roles in stress responses with particular emphasis on health outcomes upon their dysregulation.

Pro- and anti-apoptotic CD95 signaling in T cells

The TNF receptor superfamily member CD95 (Fas, APO-1, TNFRSF6) is known as the prototypic death receptor in and outside the immune system. In fact, many mechanisms involved in apoptotic signaling cascades were solved by addressing consequences and pathways initiated by CD95 ligation in activated T cells or other "CD95-sensitive" cell populations. As an example, the binding of the inducible CD95 ligand (CD95L) to CD95 on activated T lymphocytes results in apoptotic cell death. This activation-induced cell death was implicated in the control of immune cell homeostasis and immune response termination. Over the past years, however, it became evident that CD95 acts as a dual function receptor that also exerts anti-apoptotic effects depending on the cellular context. Early observations of a potential non-apoptotic role of CD95 in the growth control of resting T cells were recently reconsidered and revealed quite unexpected findings regarding the costimulatory capacity of CD95 for primary T cell activation. It turned out that CD95 engagement modulates TCR/CD3-driven signal initiation in a dose-dependent manner. High doses of immobilized CD95 agonists or cellular CD95L almost completely silence T cells by blocking early TCR-induced signaling events. In contrast, under otherwise unchanged conditions, lower amounts of the same agonists dramatically augment TCR/CD3-driven activation and proliferation. In the present overview, we summarize these recent findings with a focus on the costimulatory capacity of CD95 in primary T cells and discuss potential implications for the T cell compartment and the interplay between T cells and CD95L-expressing cells including antigen-presenting cells.

Dynamics within the CD95 death-inducing signaling complex decide life and death of cells

This study explores the dilemma in cellular signaling that triggering of CD95 (Fas/APO-1) in some situations results in cell death and in others leads to the activation of NF-kappaB. We established an integrated kinetic mathematical model for CD95-mediated apoptotic and NF-kappaB signaling. Systematic model reduction resulted in a surprisingly simple model well approximating experimentally observed dynamics. The model postulates a new link between c-FLIP(L) cleavage in the death-inducing signaling complex (DISC) and the NF-kappaB pathway. We validated experimentally that CD95 stimulation resulted in an interaction of p43-FLIP with the IKK complex followed by its activation. Furthermore, we showed that the apoptotic and NF-kappaB pathways diverge already at the DISC. Model and experimental analysis of DISC formation showed that a subtle balance of c-FLIP(L) and procaspase-8 determines life/death decisions in a nonlinear manner. We present an integrated model describing the complex dynamics of CD95-mediated apoptosis and NF-kappaB signaling.

Caspases and kinases in a death grip

The complex process of apoptosis is orchestrated by caspases, a family of cysteine proteases with unique substrate specificities. Accumulating evidence suggests that cell death pathways are finely tuned by multiple signaling events, including direct phosphorylation of caspases, whereas kinases are often substrates of active caspases. Importantly, caspase-mediated cleavage of kinases can terminate prosurvival signaling or generate proapoptotic peptide fragments that help to execute the death program and facilitate packaging of the dying cells. Here, we review caspases as kinase substrates and kinases as caspase substrates and discuss how the balance between cell survival and cell death can be shifted through crosstalk between these two enzyme families.

Phosphorylation of CARMA1 by HPK1 is critical for NF-kappaB activation in T cells

Activation of the NF-kappaB pathway in T cells is required for induction of an adaptive immune response. Hematopoietic progenitor kinase (HPK1) is an important proximal mediator of T-cell receptor (TCR)-induced NF-kappaB activation. Knock-down of HPK1 abrogates TCR-induced IKKbeta and NF-kappaB activation, whereas active HPK1 leads to increased IKKbeta activity in T cells. Yet, the precise molecular mechanism of this process remains elusive. Here, we show that HPK1-mediated NF-kappaB activation is dependent on the adaptor protein CARMA1. HPK1 interacts with CARMA1 in a TCR stimulation-dependent manner and phosphorylates the linker region of CARMA1. Interestingly, the putative HPK1 phosphorylation sites in CARMA1 are different from known PKC consensus sites. Mutations of residues S549, S551, and S552 in CARMA1 abrogated phosphorylation of a CARMA1-linker construct by HPK1 in vitro. In addition, CARMA1 S551A or S5549A/S551A point mutants failed to restore HPK1-mediated and TCR-mediated NF-kappaB activation and IL-2 expression in CARMA1-deficient T cells. Thus, we identify HPK1 as a kinase specific for CARMA1 and suggest HPK1-mediated phosphorylation of CARMA1 as an additional regulatory mechanism tuning the NF-kappaB response upon TCR stimulation.

Proteasome-mediated degradation and functions of hematopoietic progenitor kinase 1 in pancreatic cancer

Hematopoietic progenitor kinase 1 (HPK1) regulates stress responses, proliferation, and apoptosis in hematopoietic cells. In this study, we examined the expression, regulation, and functions of HPK1 in pancreatic ductal adenocarcinomas (PDA). We found that loss of HPK1 protein expression correlated significantly with the progression of pancreatic intraepithelial neoplasias (P = 0.001) and development of invasive PDA. Similarly, HPK1 protein was not expressed in any of eight PDA cell lines examined but was expressed in immortalized human pancreatic duct epithelial (HPDE) cells. There was no difference in HPK1 mRNA levels in PDA cell lines or primary PDA compared with those in HPDE cells or ductal epithelium in chronic pancreatitis and normal pancreas, respectively. Treatment of Panc-1 cells with a proteasome inhibitor, MG132, increased the HPK1 protein levels in a dose-dependent manner, suggesting that alteration in proteasome activity contributes to the loss of HPK1 protein expression in pancreatic cancer. Like the endogenous HPK1, both wild-type HPK1 and its kinase-dead mutant, HPK1-M46, overexpressed in Panc-1 cells, were also targeted by proteasome-mediated degradation. After MG132 withdrawal, wild-type HPK1 protein expression was markedly decreased within 24 hours, but kinase-dead HPK1 mutant protein expression was sustained for up to 96 hours. Therefore, HPK1 kinase activities were required for the loss of HPK1 protein in PDAs. Furthermore, restoring wild-type HPK1 protein in PDA cells led to the increase in p21 and p27 protein expression and cell cycle arrest. Thus, HPK1 may function as a novel tumor suppressor and its loss plays a critical role in pancreatic cancer.

Neuroprotection against ischemic brain injury by knockdown of hematopietic progenitor kinase 1 expression

In this study, we investigated the role of hematopietic progenitor kinase 1 (HPK1) in delayed neuronal damage after cerebral ischemia and the possible regulatory mechanisms of this event. Our data show that tyrosine phosphorylation of HPK1 was significantly increased at 6 h of ischemic-reperfusion compared with sham control. The increase in p-HPK1, p-MLK3, p-MKK7, and p-JNK3 was attenuated by HPK1 antisense oligodeoxynucleotides intra-cerebroventricular infusion, but not MS-ODNs or vehicle. Intracerebroventricular infusion of antisense oligodeoxynucleotides also increased the number of surviving pyramidal neurons, whereas MS-ODNs or vehicle (TE) groups had no effects. These results indicate that knockdown of HPK1 expression provides neuroprotection through downregulation activation of the MLK3-MKK7-JNK3 pathway following cerebral ischemia in the rat hippocampus CA1 subfield.

Proteome-wide changes induced by the Hsp90 inhibitor, geldanamycin in anaplastic large cell lymphoma cells

The molecular chaperone heat shock protein 90 (Hsp90) affects the function of many oncogenic signaling proteins including nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) expressed in anaplastic large cell lymphoma (ALCL). While ALK-positive ALCL cells are sensitive to the Hsp90 inhibitor and the geldanamycin (GA) analog, 17-allylamino-17-demethoxygeldanamycin (17-AAG), the proteomic effects of these drugs on ALK-positive ALCL cells are unpublished. In this study, we investigated the cellular, biologic, and proteomic changes occurring in ALK-positive ALCL cells in response to GA treatment. GA induced G2/M cell cycle arrest and caspase-3-mediated apoptosis. Furthermore, quantitative proteomic changes analyzed by cleavable isotope-coded affinity tag-LC-MS/MS (cICAT-LC-MS/MS) identified 176 differentially expressed proteins. Out of these, 49 were upregulated 1.5-fold or greater and 70 were downregulated 1.5-fold or greater in GA-treated cells. Analysis of biological functions of differentially expressed proteins revealed diverse changes, including induction of proteins involved in the 26S proteasome as well as downregulation of proteins involved in signal transduction and protein and nucleic acid metabolism. Pathway analysis revealed changes in MAPK, WNT, NF-kappaB, TGFbeta, PPAR, and integrin signaling components. Our studies reveal some of the molecular and proteomic consequences of Hsp90 inhibition in ALK-positive ALCL cells and provide novel insights into the mechanisms of its diverse cellular effects.

Functional implications of caspase-mediated RhoGDI2 processing during apoptosis of HL60 and K562 leukemia cells

RhoGDI2, a cytosolic regulator of Rho GTPase, is cleaved during apoptosis in a caspase-3 dependent fashion. By using 2D-gel electrophoresis, mass spectrometry and Western blotting we investigate in this paper the functional consequences of RhoGDI2 processing. We can show that loss of the N-terminal 19 amino acids results in a shift of the isoelectric point of the truncated RhoGDI2 (NDelta19) to a more basic value due to the removal of 9 acidic amino acids from the N-terminus, which may be responsible for enhanced retention of the N-terminally truncated protein within the nuclear compartment. Fusion of the p53 nuclear export signaling sequence MFRELNEALELK to NDelta19 (NDelta19NES) abolished its apoptosis promoting properties, while overexpression of NDelta19 significantly increased the susceptibility to apoptosis induction by the proteasome inhibitor PSI and by staurosporine. These results suggest that cleavage of RhoGDI2 by caspase-3 is not a functionally irrelevant bystander effect of caspase activation during apoptosis, but rather expedites progression of the apoptotic process.

Caspase-cleaved HPK1 induces CD95L-independent activation-induced cell death in T and B lymphocytes

Life and death of peripheral lymphocytes is strictly controlled to maintain physiologic levels of T and B cells. Activation-induced cell death (AICD) is one mechanism to delete superfluous lymphocytes by restimulation of their immunoreceptors and it depends partially on the CD95/CD95L system. Recently, we have shown that hematopoietic progenitor kinase 1 (HPK1) determines T-cell fate. While full-length HPK1 is essential for NF-kappaB activation in T cells, the C-terminal fragment of HPK1, HPK1-C, suppresses NF-kappaB and sensitizes toward AICD by a yet undefined cell death pathway. Here we show that upon IL-2-driven expansion of primary T cells, HPK1 is converted to HPK1-C by a caspase-3 activity below the threshold of apoptosis induction. HPK1-C selectively blocks induction of NF-kappaB-dependent antiapoptotic Bcl-2 family members but not of the proapoptotic Bcl-2 family member Bim. Interestingly, T and B lymphocytes from HPK1-C transgenic mice undergo AICD independently of the CD95/CD95L system but involving caspase-9. Knock down of HPK1/HPK1-C or Bim by small interfering RNA shows that CD95L-dependent and HPK1/HPK1-C-dependent cell death pathways complement each other in AICD of primary T cells. Our results define HPK1-C as a suppressor of antiapoptotic Bcl-2 proteins and provide a molecular basis for our understanding of CD95L-independent AICD of lymphocytes.

Life and death in peripheral T cells

During the course of an immune response, antigen-reactive T cells clonally expand and then are removed by apoptosis to maintain immune homeostasis. Life and death of T cells is determined by multiple factors, such as T-cell receptor triggering, co-stimulation or cytokine signalling, and by molecules, such as caspase-8 (FLICE)-like inhibitory protein (FLIP) and haematopoietic progenitor kinase 1 (HPK1), which regulate the nuclear factor-kappaB (NF-kappaB) pathway. Here, we discuss the concepts of activation-induced cell death (AICD) and activated cell-autonomous death (ACAD) in the regulation of life and death in T cells.

Hematopoietic progenitor kinase 1 negatively regulates T cell receptor signaling and T cell-mediated immune responses

HPK1 is a Ste20-related serine-threonine kinase that inducibly associates with the adaptors SLP-76 and Gads after T cell receptor (TCR) signaling. Here, HPK1 deficiency resulted in enhanced TCR-induced phosphorylation of SLP-76, phospholipase C-gamma1 and the kinase Erk, more-persistent calcium flux, and increased production of cytokines and antigen-specific antibodies. Furthermore, HPK1-deficient mice were more susceptible to experimental autoimmune encephalomyelitis. Although the interaction between SLP-76 and Gads was unaffected, the inducible association of SLP-76 with 14-3-3tau (a phosphorylated serine-binding protein and negative regulator of TCR signaling) was reduced in HPK1-deficient T cells after TCR stimulation. HPK1 phosphorylated SLP-76 and induced the interaction of SLP-76 with 14-3-3tau. Our results indicate that HPK1 negatively regulates TCR signaling and T cell-mediated immune responses.

Sustained JNK signaling by proteolytically processed HPK1 mediates IL-3 independent survival during monocytic differentiation

We studied monocytic differentiation of primary mouse progenitor cells to understand molecular mechanisms of differentiation. We found a tightly controlled non-apoptotic activation of caspase-3 that correlated with differentiation. Although caspase activity was already detected during monocytic differentiation, a caspase-3 target has not been identified yet. We show that hematopoietic progenitor kinase 1 (HPK1) is processed towards its N- and C-terminal fragments during monocytic differentiation. While HPK1 is an immunoreceptor-proximal kinase in T and B cells, its role in myeloid cells is elusive. Here, we show that the N-terminal cleavage product, HPK1-N, comprising the kinase domain, confers progenitor cell survival independent of the growth factor IL-3. Furthermore, HPK1-N causes differentiation of progenitor cells towards the monocytic lineage. In contrast to full-length kinase, HPK1-N is constitutively active causing sustained JNK activation, Bad phosphorylation and survival. Blocking of caspase activity during differentiation of primary mouse progenitor cells leads to reduced HPK1-N levels, suppressed JNK activity and attenuated monocytic differentiation. Our work explains growth factor-independent survival during monocytic differentiation by caspase-mediated processing of HPK1 towards HPK1-N.

How T lymphocytes switch between life and death

While insufficient cell death of activated T cells can result in autoimmune disorders, elimination of too many T cells can lead to immunodeficiency. Therefore, T lymphocyte fate is highly regulated and requires that cells can switch from an apoptosis-resistant towards an apoptosis-sensitive state. This switch is tightly controlled by various effector molecules. Basically, two separate pathways control the fate of antigen-activated T cells: activation-induced cell death (AICD) and activated T cell autonomous death (ACAD). Autoreactive T lymphocytes are eliminated by restimulation via their T cell receptor (TCR) and undergo AICD involving death receptors (extrinsic pathway). In contrast, ACAD can lead to T cell deletion without TCR restimulation, and is determined by the ratio between anti- and pro-apoptotic Bcl-2 family members at the mitochondria (intrinsic pathway). While the extrinsic and the intrinsic pathway lead to caspase activation, non-caspase proteases (e.g., cathepsins) can be released by the lysosomes and might contribute to AICD as well as to ACAD. Activated T cells poses cell death escape mechanisms which are needed for survival of (memory) T cells, but are deleterious for autoimmune disorders or progression of T cell lymphomas.

Activation or suppression of NFkappaB by HPK1 determines sensitivity to activation-induced cell death

Restimulation of the T-cell receptor (TCR) in activated T cells induces CD95 (Fas/Apo-1)-mediated activation-induced cell death (AICD). The TCR-proximal mechanisms leading to AICD are elusive. Here we characterize hematopoietic progenitor kinase 1 (HPK1) as a differentially regulated TCR-proximal signaling protein involved in AICD of primary T cells. We show that HPK1 is a functional component of the endogenous IkappaB kinase (IKK) complex and is crucial for TCR-mediated NFkappaB activation. While full-length HPK1 enhances IKKbeta phosphorylation, siRNA-mediated knockdown of HPK1 blunts TCR-mediated NFkappaB activation and increases cell death. We also demonstrate proteolytic processing of HPK1 into HPK1-C, specifically in AICD-sensitive primary T cells. The cleavage product HPK1-C sequesters the inactive IKK complex and suppresses NFkappaB upon TCR restimulation by binding to IKKalpha and IKKbeta. T cells of HPK1-C transgenic mice are sensitized towards TCR-mediated AICD. Consequently, preventing HPK1-C generation in primary T cells by siRNA-mediated knockdown results in decreased AICD. Thus, these results show a novel mechanism of sensitization of T lymphocytes towards AICD by suppression of NFkappaB, and propose that HPK1 is a life/death switch in T lymphocytes.

Enhanced JNK activation by NESK without kinase activity upon caspase-mediated cleavage during apoptosis

Nck-interacting kinase-like embryo-specific kinase (NESK) is a protein kinase that is predominantly expressed in skeletal muscle during the late stages of mouse embryogenesis. NESK belongs to the germinal center kinase (GCK) family and selectively activates the c-Jun N-terminal kinase (JNK) pathway when overexpressed in cultured cells. Some members of the GCK family have been shown to be proteolytically cleaved and activated during apoptosis. Here, we report that NESK is also proteolytically cleaved during apoptosis. Treatment of NESK-transfected HeLa cells with TNF-alpha in the presence of cycloheximide or with staurosporine induced proteolytic cleavage of NESK. The cleavage of NESK occurred at two sites, generating three fragments: an N-terminal fragment containing a kinase domain, an intermediate fragment and a C-terminal fragment containing a regulatory CNH domain. These two cleavages occurred in a stepwise manner and were dependent on a caspase activity. The cleavage sites were identified as aspartic acid residues at 868 and 1091. The N-terminal fragment had less kinase activity than the full-length NESK and did not activate the JNK pathway. In contrast, the C-terminal fragment activated the JNK pathway more strongly than the full-length NESK and promoted TNF-alpha-induced apoptotic cell death. These results implicate NESK in the JNK pathway-mediated promotion of apoptosis through its C-terminal regulatory domain generated by proteolytic cleavage during apoptosis, in a unique manner different from other GCK family kinases.

Functional interactions of HPK1 with adaptor proteins

Hematopoietic progenitor kinase 1 (HPK1 or MAP4K1) is a hematopoietic-specific mammalian STE20-like protein serine/threonine kinase, comprised of a STE20-like kinase domain in its N-terminus, four proline-rich motifs, a caspase cleavage site, and a distal C-terminal Citron homology domain. HPK1 is involved in many cellular signaling cascades that include MAPK signaling, antigen receptor signaling, apoptosis, growth factor signaling, and cytokine signaling. HPK1 binds many adaptor proteins including members of the Grb2 family, Nck family, Crk family, SLP-76 family, and actin-binding adaptors like HIP-55. HPK1 tyrosine phosphorylation and kinase activation depend on the presence of adaptor proteins. Adaptor proteins are required not only for linking HPK1 to cell surface receptors like the EGFR, but also for downstream gene transcription like NFAT, AP-1 and IL-2. The HPK1 association with Crk, CrkL, and HIP-55 mediate HPK1-dependent c-Jun N-terminal kinase (JNK) activation, while the association of HPK1 with SLP-76, Gads, CrkL, Grb2, and Grap affect T- and B-cell dependent gene transcription. Interestingly, HPK1 has been implicated in both increasing and decreasing NFAT, AP-1, and IL-2 gene transcription in T-cells where adaptor proteins play a key role. Lastly, HPK1 will phosphorylate Crk and CrkL, in vitro, which presents a novel possibility for the regulation of adaptor proteins and downstream signaling events.

Activation of hematopoietic progenitor kinase 1 involves relocation, autophosphorylation, and transphosphorylation by protein kinase D1

Adaptive immune signaling can be coupled to stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK) and NF-kappaB activation by the hematopoietic progenitor kinase 1 (HPK1), a mammalian hematopoiesis-specific Ste20 kinase. To gain insight into the regulation of leukocyte signal transduction, we investigated the molecular details of HPK1 activation. Here we demonstrate the capacity of the Src family kinase Lck and the SLP-76 family adaptor protein Clnk (cytokine-dependent hematopoietic cell linker) to induce HPK1 tyrosine phosphorylation and relocation to the plasma membrane, which in lymphocytes results in recruitment of HPK1 to the contact site of antigen-presenting cell (APC)-T-cell conjugates. Relocation and clustering of HPK1 cause its enzymatic activation, which is accompanied by phosphorylation of regulatory sites in the HPK1 kinase activation loop. We show that full activation of HPK1 is dependent on autophosphorylation of threonine 165 and phosphorylation of serine 171, which is a target site for protein kinase D (PKD) in vitro. Upon T-cell receptor stimulation, PKD robustly augments HPK1 kinase activity in Jurkat T cells and enhances HPK1-driven SAPK/JNK and NF-kappaB activation; conversely, antisense down-regulation of PKD results in reduced HPK1 activity. Thus, activation of major lymphocyte signaling pathways via HPK1 involves (i) relocation, (ii) autophosphorylation, and (iii) transphosphorylation of HPK1 by PKD.

Human T lymphotropic virus type-1 p30II alters cellular gene expression to selectively enhance signaling pathways that activate T lymphocytes

Background

Human T-lymphotropic virus type-1 (HTLV-1) is a deltaretrovirus that causes adult T-cell leukemia/lymphoma and is implicated in a variety of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13^II and p30^II, which are incompletely defined in the virus life cycle or HTLV-1 pathogenesis. Proviral clones of the virus with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. Exogenous expression of p30^II differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and represses tax/rex RNA nuclear export.

Results

Herein, we further characterized the role of p30^II in regulation of cellular gene expression, using stable p30^II expression system employing lentiviral vectors to test cellular gene expression with Affymetrix U133A arrays, representing ~33,000 human genes. Reporter assays in Jurkat T cells and RT-PCR in Jurkat and primary CD4+ T-lymphocytes were used to confirm selected gene expression patterns. Our data reveals alterations of interrelated pathways of cell proliferation, T-cell signaling, apoptosis and cell cycle in p30^II expressing Jurkat T cells. In all categories, p30^II appeared to be an overall repressor of cellular gene expression, while selectively increasing the expression of certain key regulatory genes.

Conclusions

We are the first to demonstrate that p30^II, while repressing the expression of many genes, selectively activates key gene pathways involved in T-cell signaling/activation. Collectively, our data suggests that this complex retrovirus, associated with lymphoproliferative diseases, relies upon accessory gene products to modify cellular environment to promote clonal expansion of the virus genome and thus maintain proviral loads in vivo.

Protein Phosphatase 4 Is a Positive Regulator of Hematopoietic Progenitor Kinase 1

Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic specific mammalian Ste20-like protein kinase and has been implicated in many cellular signaling pathways including T cell receptor (TCR) signaling. However, little is known about the in vivo regulation of HPK1. We present evidence that HPK1 is positively regulated by protein phosphatase 4 (PP4; also called PPX and PPP4), a serine/threonine phosphatase. We found that PP4 interacted with HPK1 and that the proline-rich region of HPK1 was necessary and sufficient for this interaction. We also found that PP4 had phosphatase activity toward HPK1 in vivo and that co-transfection of PP4 with HPK1 resulted in specific kinase activation of HPK1. Moreover, we found that the PP4-induced HPK1 kinase activation was accompanied by an increase in protein expression of HPK1. Pulse-chase analysis showed that PP4 increased the half-life of HPK1. Further studies showed that HPK1 was subject to regulation by ubiquitination and ubiquitin-targeted degradation and that PP4 inhibited HPK1 ubiquitination. In addition, we found that TCR stimulation enhanced the PP4-HPK1 interaction and that wild-type PP4 enhanced, whereas a phosphatase-dead PP4 mutant inhibited, TCR-induced activation of HPK1 in Jurkat T cells. Combined with the observation that PP4 enhanced HPK1-induced JNK activation, our studies identify PP4 as a positive regulator for HPK1 and the HPK1-JNK signaling pathway.

Mona/Gads SH3C binding to hematopoietic progenitor kinase 1 (HPK1) combines an atypical SH3 binding motif, R/KXXK, with a classical PXXP motif embedded in a polyproline type II (PPII) helix

Hematopoietic progenitor kinase 1 (HPK1) is implicated in signaling downstream of the T cell receptor. Its non-catalytic, C-terminal half contains several prolinerich motifs, which have been shown to interact with different SH3 domain-containing adaptor proteins in vitro. One of these, Mona/Gads, was also shown to bind HPK1 in mouse T cells in vivo. The region of HPK1 that binds to the Mona/Gads C-terminal SH3 domain has been mapped and shows only very limited similarity to a recently identified high affinity binding motif in SLP-76, another T-cell adaptor. Using isothermal titration calorimetry and x-ray crystallography, the binding of the HPK1 motif to Mona/Gads SH3C has now been characterized in molecular detail. The results indicate that although charge interactions through an RXXK motif are essential for complex formation, a PXXP motif in HPK1 strongly complements binding. This unexpected binding mode therefore differs considerably from the previously described interaction of Mona/Gads SH3C with SLP-76. The crystal structure of the complex highlights the great versatility of SH3 domains, which allows interactions with very different proteins. This currently limits our ability to categorize SH3 binding properties by simple rules.