The death domain kinase RIP1 links the immunoregulatory CD40 receptor to apoptotic signaling in carcinomas

RIP1 is a component of a TRAF2 complex, required for caspase-8 activation and tumor cell killing in response to ligand binding of CD40

The lectin concanavalin-A signals MT1-MMP catalytic independent induction of COX-2 through an IKKγ/NF-κB-dependent pathway

The lectin from Canavalia ensiformis (Concanavalin-A, ConA), one of the most abundant lectins known, enables one to mimic biological lectin/carbohydrate interactions that regulate extracellular matrix protein recognition. As such, ConA is known to induce membrane type-1 matrix metalloproteinase (MT1-MMP) which expression is increased in brain cancer. Given that MT1-MMP correlated to high expression of cyclooxygenase (COX)-2 in gliomas with increasing histological grade, we specifically assessed the early proinflammatory cellular signaling processes triggered by ConA in the regulation of COX-2. We found that treatment with ConA or direct overexpression of a recombinant MT1-MMP resulted in the induction of COX-2 expression. This increase in COX-2 was correlated with a concomitant decrease in phosphorylated AKT suggestive of cell death induction, and was independent of MT1-MMP’s catalytic function. ConA- and MT1-MMP-mediated intracellular signaling of COX-2 was also confirmed in wild-type and in Nuclear Factor-kappaB (NF-κB) p65−/− mutant mouse embryonic fibroblasts (MEF), but was abrogated in NF-κB1 (p50)−/− and in I kappaB kinase (IKK) γ−/− mutant MEF cells. Collectively, our results highlight an IKK/NF-κB-dependent pathway linking MT1-MMP-mediated intracellular signaling to the induction of COX-2. That signaling pathway could account for the inflammatory balance responsible for the therapy resistance phenotype of glioblastoma cells, and prompts for the design of new therapeutic strategies that target cell surface carbohydrate structures and MT1-MMP-mediated signaling. Concise summary Concanavalin-A (ConA) mimics biological lectin/carbohydrate interactions that regulate the proinflammatory phenotype of cancer cells through yet undefined signaling. Here we highlight an IKK/NF-κB-dependent pathway linking MT1-MMP-mediated intracellular signaling to the induction of cyclooxygenase-2, and that could be responsible for the therapy resistance phenotype of glioblastoma cells

Negative regulation of T cell receptor signaling by NFkappaB2/p100 (87.4)

Abstract The positive regulation of the NFκB signaling pathway in response to T cell receptor (TCR) stimulation has been well studied. However, little is known about the negative regulation of this pathway in T cells. This negative regulation is crucial in controlling the duration of TCR signaling and preventing abnormal lymphocyte activation and proliferation. Therefore, understanding the negative regulation of TCR-mediated NFκB signaling is essential in understanding the mechanisms involved in T cell function and homeostasis. TCR stimulation of human CD4+ T cells resulted in an increase in NFκB2/p100 expression with minimal increase in p52, its cleavage product. Due to the presence of inhibitory ankyrin-repeats in the unprocessed p100, this observation suggests that p100 may function as a negative regulator of the NFκB pathway. Consistent with this hypothesis, ectopic expression of p100 inhibited TCR-mediated NFκB activity and IL-2 production in Jurkat T cells. Conversely, knockdown of p100 expression enhanced NFκB transcriptional activity and IL-2 production upon TCR activation. p100 inhibited the pathway by binding and sequestering Rel transcription factors in the cytoplasm without affecting the activity of the upstream IκB kinase (IKK). Taken together, this study indicates that NFκB2/p100 can act as an inhibitory molecule in the TCR-mediated NFκB signaling pathway. This work was supported by a grant from the National Institutes of Health to A.T.T.</jats:p

Ubiquitination of RIP1 regulates an NF-κB-independent cell death switch in TNF signaling. (87.8)

Abstract TNF receptor 1 (TNFR1) can trigger opposing responses within the same cell: one leads to a pro-survival response whereas the other leads to cell death. The pro-survival response is mediated primarily by activation of the NF-κB signaling pathway, which leads to the expression of pro-survival genes such as c-FLIP, c-IAPs, TRAFs and BCL-2 family members. On the other hand, TNF can also trigger activation of CASPASE 8 and 3 leading eventually to apoptosis. In most cell types, stimulation with TNF does not lead to apoptosis unless NF-κB signaling is blocked. Hence, NF-κB-mediated transcription of pro-survival genes acts as a cell death switch during TNF signaling. This current study demonstrates the existence of another cell death switch elsewhere in the pathway that is independent of NF-κB. Our results show that lysine 63-linked ubiquitination of RIP1 on lysine 377 inhibits TNF-induced apoptosis first through an NF-κB-independent mechanism, and subsequently, through an NF-κB-dependent mechanism. In contrast, in the absence of ubiquitination, RIP1 serves as a pro-apoptotic signaling molecule. Therefore, RIP1 is a dual-function molecule that can be either pro-survival or pro-death depending on its ubiquitination state and this serves as an NF-κB-independent cell death switch early in TNF signaling. These results provide an explanation for the conflicting reports on the role of RIP1 in cell death, which was previously implicated to be both pro-survival and pro-death. Since TRAF2 is the E3 ligase for RIP1, these observations also provide an explanation for the NF-κB-independent anti-apoptotic function of TRAF2 previously described.</jats:p

NEMO/IKKγ regulates an early NF-κB-independent cell-death checkpoint during TNF signaling

TNF receptor 1 (TNFR1) ligation can result in cell survival or cell death. What determines which of the two opposing responses is triggered is not fully understood. The current model suggests that it is the activation of the NF-κB pathway and its induction of pro-survival genes, or the lack thereof, which determines the outcome. NF-κB essential modifier (NEMO)/IκB kinase gamma (IKKγ)-deficient cells are highly sensitive to apoptosis and since NEMO is essential for NF-κB activation, it has been assumed that this is due to the lack of NF-κB. This study demonstrates that this assumption was incorrect and that NEMO has another anti-apoptotic function that is independent of its role in the NF-κB pathway. NEMO prevents receptor interacting protein-1 (RIP1) from engaging CASPASE-8 prior to NF-κB-mediated induction of anti-apoptotic genes. Without NEMO, RIP1 associates with CASPASE-8 resulting in rapid tumor necrosis factor (TNF)-induced apoptosis. These results suggest that there are two cell death checkpoints following TNF stimulation: an early transcription-independent checkpoint whereby NEMO restrains RIP1 from activating the caspase cascade, followed by a later checkpoint dependent on NF-κB-mediated transcription of pro-survival genes

Ubiquitination of RIP1 Regulates an NF-κB-Independent Cell-Death Switch in TNF Signaling

SummaryTNF receptor 1 (TNFR1) can trigger opposing responses within the same cell: a prosurvival response or a cell-death pathway [1, 2]. Cell survival requires NF-κB-mediated transcription of prosurvival genes [3–9]; apoptosis occurs if NF-κB signaling is blocked [5, 7–9]. Hence, activation of NF-κB acts as a cell-death switch during TNF signaling. This study demonstrates that the pathway includes another cell-death switch that is independent of NF-κB. We show that lysine 63-linked ubiquitination of RIP1 on lysine 377 inhibits TNF-induced apoptosis first through an NF-κB-independent mechanism and, subsequently, through an NF-κB-dependent mechanism. In contrast, in the absence of ubiquitination, RIP1 serves as a proapoptotic signaling molecule by engaging CASPASE-8. Therefore, RIP1 is a dual-function molecule that can be either prosurvival or prodeath depending on its ubiquitination state, and this serves as an NF-κB-independent cell-death switch early in TNF signaling. These results provide an explanation for the conflicting reports on the role of RIP1 in cell death; this role was previously suggested to be both prosurvival and prodeath [10–12]. Because TRAF2 is the E3 ligase for RIP1 [13], these observations provide an explanation for the NF-κB-independent antiapoptotic function previously described for TRAF2 [14–16]