CARMA3 deficiency abrogates G protein-coupled receptor-induced NF-{kappa}B activation

Lysophospholipid signaling in the function and pathology of the reproductive system

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two prominent signaling lysophospholipids (LPs) exerting their functions through a group of G protein-coupled receptors (GPCRs). This review covers current knowledge of the LP signaling in the function and pathology of the reproductive system.

METHODS

PubMed was searched up to May 2008 for papers on lysophospholipids/LPA/S1P/LPC/SPC in combination with each part of the reproductive system, such as testis/ovary/uterus.

RESULTS

LPA and SIP are found in significant amounts in serum and other biological fluids. To date, 10 LP receptors have been identified, including LPA(1-5) and S1P(1-5). In vitro and in vivo studies from the past three decades have demonstrated or suggested the physiological functions of LP signaling in reproduction, such as spermatogenesis, male sexual function, ovarian function, fertilization, early embryo development, embryo spacing, implantation, decidualization, pregnancy maintenance and parturition, as well as pathological roles in ovary, cervix, mammary gland and prostate cancers.

CONCLUSIONS

Receptor knock-out and other studies indicate tissue-specific and receptor-specific functions of LP signaling in reproduction. More comprehensive studies are required to define mechanisms of LP signaling and explore the potential use as a therapeutic target.

Essential role for epidermal growth factor receptor in glutamate receptor signaling to NF-kappaB

Glutamate is a critical neurotransmitter of the central nervous system (CNS) and also an important regulator of cell survival and proliferation. The binding of glutamate to metabotropic glutamate receptors induces signal transduction cascades that lead to gene-specific transcription. The transcription factor NF-kappaB, which regulates cell proliferation and survival, is activated by glutamate; however, the glutamate receptor-induced signaling pathways that lead to this activation are not clearly defined. Here we investigate the glutamate-induced activation of NF-kappaB in glial cells of the CNS, including primary astrocytes. We show that glutamate induces phosphorylation, nuclear accumulation, DNA binding, and transcriptional activation function of glial p65. The glutamate-induced activation of NF-kappaB requires calcium-dependent IkappaB kinase alpha (IKKalpha) and IKKbeta activation and induces p65-IkappaBalpha dissociation in the absence of IkappaBalpha phosphorylation or degradation. Moreover, glutamate-induced IKK preferentially targets the phosphorylation of p65 but not IkappaBalpha. Finally, we show that the ability of glutamate to activate NF-kappaB requires cross-coupled signaling with the epidermal growth factor receptor. Our results provide insight into a glutamate-induced regulatory pathway distinct from that described for cytokine-induced NF-kappaB activation and have important implications with regard to both normal glial cell physiology and pathogenesis.

A20 is a negative regulator of BCL10- and CARMA3-mediated activation of NF-κB

The molecular complex containing CARMA proteins, BCL10 and TRAF6 has been identified recently as a key component in the signal transduction pathways that regulate activation of the nuclear factor κB (NF-κB) transcription factor. Here, we report that the inducible protein A20 negatively regulates these signaling cascades by means of its deubiquitylation activity. We show that A20 perturbs assembly of the complex containing CARMA3, BCL10 and IKKγ/NEMO, thereby suppressing activation of NF-κB. Together, our results further define the molecular mechanisms that control activation of NF-κB and reveal a function for A20 in the regulation of CARMA and BCL10 activity in lymphoid and non-lymphoid cells.

The signaling adaptor p62 is an important NF-kappaB mediator in tumorigenesis

The balance between cell death and survival, two critical aspects of oncogenic transformation, determines the outcome of tumorigenesis. Nuclear factor-kappaB (NF-kappaB) is a critical regulator of survival; it is induced by the oncogene Ras and, when inhibited, accounts for the cell death response of Ras-transformed cells. Here, we show that the signaling adaptor p62 is induced by Ras, its levels are increased in human tumors, and it is required for Ras-induced survival and transformation. p62-/- mice are resistant to Ras-induced lung adenocarcinomas. p62 is necessary for Ras to trigger IkappaB kinase (IKK) through the polyubiquitination of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), and its deficiency produces increased reactive oxygen species (ROS) levels, which account for the enhanced cell death and reduced tumorigenicity of Ras in the absence of p62.

Malt1 ubiquitination triggers NF-kappaB signaling upon T-cell activation

Triggering of antigen receptors on lymphocytes is critical for initiating adaptive immune response against pathogens. T-cell receptor (TCR) engagement induces the formation of the Carma1-Bcl10-Malt1 (CBM) complex that is essential for activation of the IkappaB kinase (IKK)/NF-kappaB pathway. However, the molecular mechanisms that link CBM complex formation to IKK activation remain unclear. Here we report that Malt1 is polyubiquitinated upon T-cell activation. Ubiquitin chains on Malt1 provide a docking surface for the recruitment of the IKK regulatory subunit NEMO/IKKgamma. TRAF6 associates with Malt1 in response to T-cell activation and can function as an E3 ligase for Malt1 in vitro and in vivo, mediating lysine 63-linked ubiquitination of Malt1. Multiple lysine residues in the C-terminus of Malt1 serve as acceptor sites for the assembly of polyubiquitin chains. Malt1 mutants that lack C-terminal ubiquitin acceptor lysines are impaired in rescuing NF-kappaB signaling and IL-2 production in Malt1-/- T cells. Thus, our data demonstrate that induced Malt1 ubiquitination is critical for the engagement of CBM and IKK complexes, thereby directing TCR signals to the canonical NF-kappaB pathway.

Identification of TRAF6-dependent NEMO polyubiquitination sites through analysis of a new NEMO mutation causing incontinentia pigmenti

The regulatory subunit NEMO is involved in the mechanism of activation of IkappaB kinase (IKK), the kinase complex that controls the NF-kappaB signaling pathway. During this process, NEMO is modified post-translationally through K63-linked polyubiquitination. We report the molecular characterization of a new missense mutation of NEMO (A323P) which causes a severe form of incontinentia pigmenti (OMIM#308300), an inherited disease characterized predominantly by skin inflammation. The A323P mutation was found to impair TNF-, IL-1-, LPS- and PMA/ionomycin-induced NF-kappaB activation, as well as to disrupt TRAF6-dependent NEMO polyubiquitination, due to a defective NEMO/TRAF6 interaction. Mutagenesis identified the affected ubiquitination sites as three lysine residues located in the vicinity of A323. Unexpectedly, these lysines were ubiquitinated together with two previously identified lysines not connected to TRAF6. Mutation of all these ubiquitination sites severely impaired NF-kappaB activation induced by stimulation with IL-1, LPS, Nod2/RICK or serum/LPA. In contrast, mutation at all of these sites had only a limited effect on stimulation by TNF. These findings indicate that post-translational modification of NEMO through K63-linked polyubiquitination is a key event in IKK activation and that perturbation of this step may cause human pathophysiology.

Protein kinase C alpha-CARMA3 signaling axis links Ras to NF-kappa B for lysophosphatidic acid-induced urokinase plasminogen activator expression in ovarian cancer cells

We reported previously that a signaling pathway consisting of G(i)-Ras-NF-kappaB mediates lysophosphatidic acid (LPA)-induced urokinase plasminogen activator (uPA) upregulation in ovarian cancer cells. However, it is not clear what signaling components link Ras to nuclear factor (NF)-kappaB for this LPA-induced event. In the present study, we found that treatment of protein kinase C (PKC) inhibitors including conventional PKC (cPKC) inhibitor Gö6976 abolished LPA-induced uPA upregulation in ovarian cancer cell lines tested, indicating the importance of cPKC activity in this LPA-induced event. Indeed, LPA stimulation led to the activation of PKCalpha and Ras-PKCalpha interaction. Although constitutively active mutants of PKCalpha (a cPKC), PKCtheta (a novel PKC (nPKC)) and PKCzeta (an atypical PKC (aPKC)) were all able to activate NF-kappaB and upregulate uPA expression, only dominant-negative PKCalpha mutant attenuated LPA-induced NF-kappaB activation and uPA upregulation. These results suggest that PKCalpha, rather than PKC isoforms in other PKC classes, participates in LPA-induced NF-kappaB activation and uPA upregulation in ovarian cancer cells. To determine the signaling components downstream of PKCalpha mediating LPA-induced uPA upregulation, we showed that forced expression of dominant-negative CARMA3 or silencing CARMA3, Bcl10 and MALT1 with specific siRNAs diminished these LPA-induced events. Furthermore, we demonstrated that PKCalpha/CARMA3 signaling axis is important in LPA-induced ovarian cancer cell in vitro invasion.