CARMA1-mediated NF-kappaB and JNK activation in lymphocytes

Abstract

Activation of transcription factor nuclear factor-kappaB (NF-kappaB) and Jun N-terminal kinase (JNK) play the pivotal roles in regulation of lymphocyte activation and proliferation. Deregulation of these signaling pathways leads to inappropriate immune response and contributes to the development of leukemia/lymphoma. The scaffold protein CARMA1 [caspase-recruitment domain (CARD) membrane-associated guanylate kinase (MAGUK) protein 1] has a central role in regulation of NF-kappaB and the JNK2/c-Jun complex in both B and T lymphocytes. During last several years, tremendous work has been done to reveal the mechanism by which CARMA1 and its signaling partners, B cell CLL-lymphoma 10 and mucosa-associated lymphoid tissue 1, are activated and mediate NF-kappaB and JNK activation. In this review, we summarize our findings in revealing the roles of CARMA1 in the NF-kappaB and JNK signaling pathways in the context of recent advances in this field.

Fig. 1. Expression of a NF-κB-dependent EGFP gene in a mutant Jurkat T cell line, JPM50.6

(A-C) JGFP1 or (D-F) JPM50.6 cells were (A, D) unstimulated, (B, E) stimulated with PMA (10 ng/ml) plus CD28 monoclonal antibody (1 μg/ml), or (C, F) TNF-α (20 ng/ml) for 12 h. The cells were collected and analyzed by FACS. This figure is adapted from reference (7) with permission.

Fig. 2. Structures of CARMA1-3, Bcl10, and MALT1

CARD, caspase-recruitment domain; C-C, coiled-coil domain; MAGUK, membrane-associated guanylate kinase (GuK)-like domain; S/T rich, Ser/Thr rich domain; DD, death domain; Ig, immunoglobulin-like domain.

Fig. 3. Schematic presentation of the phosphorylation-dependent conformational change of CARMA1

PKC-dependent phosphorylation of the linker region of CARMA1 induces conformational change and enables CARMA1 to interact with Bcl10 (through their CARD domains).

Fig. 4. Schematic model of T-cell receptor-induced NF-κB activation

Stimulation of the antigen receptor initiates several proximal signaling events that lead to activation of PKCθ . The activated PKCθ phosphorylates CARMA1, which enables CARMA1 to associate with the downstream signaling components, Bcl10 and MALT1 (the CBM complex). CBM cooperates with TRAF6 to mediate ubiquitination of NEMO. PKCθ also induces activation of TAK1 in a CARMA1-independent manner, which in turn phosphorylates IKKα/β . The combined phosphorylation of IKKα/β and ubiquitination of NEMO activate the IKK complex, leading to phosphorylation and degradation of IκBα, and subsequent activation of NF-κB.

Fig. 5. CARMA1-deficient JPM50.6 cells are defective in TCR-induced JNK2 phosphorylation and c-Jun accumulation

(A, B) Jurkat or JPM50.6 (CARMA1-) cells (6×10⁶/sample) were stimulated with or without anti-CD3 plus anti-CD28 antibodies (6 and 3 μg/ml, respectively) for various time points. Whole cell lysates were subjected to SDS-PAGE and analyzed by immunoblotting with antibodies against phospho-JNK, JNK, CARMA1, phospho-ERK, or ERK (A) and c-Jun or tubulin (B). This figure is adapted from reference (13) with permission.

Fig. 6. The model of TCR-induced JNK phosphorylation

TCR and CD28 ligation activates various proximal signaling components leading to activation of PKCθ and CARMA1, which, in turn, recruits Bcl10 and induces its oligomerization. The oligomerized Bcl10 functions as a scaffold molecule for linking JNK2 and its upstream kinases TAK1 and MKK7. The activated JNK2 and independently activated JNK1 lead to accumulation of c-Jun and activation of AP-1, respectively.