Antigen Receptor Signaling in Activation and Survival of Lymphocytes
Table of Contents
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Introduction
The transcription factor nuclear factor κB (NF-κB) plays a key role in the immune system by controlling lymphocyte activation, proliferation and differentiation. Aberrant NF-κB activity has been implicated in several lymphoid malignancies and contributes to lymphoma cell survival. Signal transduction to NF-κB from engaged cell surface receptors involves multiple kinases and phosphorylated target proteins, but little is known about regulation and signal termination by dephosphorylation.
RNAi-based screening for novel negative regulators of NF-κB activity
To identify phosphatases involved in the regulation of NF-κB signaling upon T cell receptor (TCR) stimulation, we performed an RNA interference (RNAi)-based screen and found several novel NF-κB regulating phosphatases. We found that one of our candidates, protein phosphatase 4 regulatory subunit 1 (PP4R1), was only expressed upon activation and expansion of primary human T lymphocytes. Upon TCR stimulation PP4R1 binds to inhibitor of NF-κB kinase (IKK) α and β and channels PP4c phosphatase activity to dephosphorylate and inactivate the IKK complex. In the absence of PP4R1 we found increased IKK activity, T cell hyperactivation and increased survival of cutaneous T cell lymphoma (CTCL) cells. Thus, PP4R1 is a gatekeeper of NF-κB activation and a suppressor of T cell lymphoma survival.
PP4R1 is a suppressor of lymphoma cell survival. Protein phosphatase 4 regulatory subunit 1 (PP4R1) is expressed in activated primary human T lymphocytes and directs the phosphatase PP4c to the IKK complex, thereby facilitating its dephosphorylation and inactivation. Deficiency for PP4R1 causes sustained and increased IKK activity, T cell hyperactivation and survival of NF-κB-dependent lymphoma cells. | © Dr. Rüdiger Arnold
Hematopoietic progenitor kinase 1 (HPK1) as modulator of NF-κB signaling
Hematopoietic progenitor kinase (HPK1) is activated upon TCR stimulation and induces NF-κB signaling, while RNAi-mediated knock-down of HPK1 blunts TCR-induced NF-κB activation. Yet, the precise molecular mechanism of this process has remained elusive. We have shown that HPK1-mediated NF-κB 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 at sites different from known PKCθ consensus sites. Mutation of these sites completely abrogated phosphorylation of CARMA1 by HPK1, and mutated CARMA1 failed to restore TCR-mediated NF-κB activation and IL-2 expression in CARMA1-deficient T cells. Thus, we identified HPK1-mediated phosphorylation of CARMA1 as a regulatory mechanism of the NF‑κB response in T cells (Brenner et al. 2007 and Brenner et al. 2009).
Key References
- Arnold, R., Frey, C. R., Muller, W., Brenner, D., Krammer, P. H., and Kiefer, F. (2007). Sustained JNK signaling by proteolytically processed HPK1 mediates IL-3 independent survival during monocytic differentiation. Cell Death Differ 14, 568-575.
- Brenner, D., Golks, A., Becker, M., Frey, C.R., Novak, R., Melamed, D., Kiefer, F., Krammer, P.H. and Arnold, R. (2007).Caspase-cleaved HPK1 induces CD95L-independent activation-induced cell death in T and B lymphocytes. Blood 110: 3968-3977.
- Brenner, D., Brechmann, M., Röhling, S., Tapernoux, M., Mock, T., Winter, D., Lehmann, W.D., Kiefer, F., Thome, M., Krammer, P.H., and Arnold, R. (2009). Phosphorylation of CARMA1 by HPK1 is critical for NF-kappaB activation in T cells. Proc Nat Acad Sci 109: 14508-14513.
- Brenner, D., Krammer, P. H., and Arnold, R. (2008). Concepts of activated T cell death. Crit Rev Oncol Hematol 66, 52-64.
- Krammer, P. H., Arnold, R., and Lavrik, I. N. (2007). Life and death in peripheral T cells. Nat Rev Immunol 7, 532-542.
