cAMP prevents TNF-induced apoptosis through inhibiting DISC complex formation in rat hepatocytes

Abstract

Tumor necrosis factor α (TNF) is a pleiotropic proinflammatory cytokine that plays a role in immunity and the control of cell proliferation, cell differentiation, and apoptosis. The pleiotropic nature of TNF is due to the formation of different signaling complexes upon the binding of TNF to its receptor, TNF receptor type 1 (TNFR1). TNF induces apoptosis in various mammalian cells when the cells are co-treated with a transcription inhibitor like actinomycin D (ActD). When TNFR1 is activated, it recruits an adaptor protein, TNF receptor-associated protein with death domain (TRADD), through its cytoplasmic death effector domain (DED). TRADD, in turn, recruits other signaling proteins, including TNF receptor-associated protein 2 (TRAF2) and receptor-associated protein kinase (RIPK) 1, to form a complex. Subsequently, this complex combines with FADD and procaspase-8, converts into a death-inducing signaling complex (DISC) to induce apoptosis. Cyclic AMP (cAMP) is a second messenger that regulates various cellular processes such as cell proliferation, gene expression, and apoptosis. cAMP analogues are reported to act as anti-apoptotic agents in various cell types, including hepatocytes. We found that a cAMP analogue, dibutyryl cAMP (db-cAMP), inhibits TNF + ActD-induced apoptosis in rat hepatocytes. The protein kinase A (PKA) inhibitor KT-5720 reverses this inhibitory effect of cAMP on apoptosis. Cytoprotection by cAMP involves down-regulation of various apoptotic signal regulators like TRADD and FADD and inhibition of caspase-8 and caspase-3 cleavage. We also found that cAMP exerts its affect at the proximal level of TNF signaling by inhibiting the formation of the DISC complex upon the binding of TNF to TNFR1. In conclusion, our study shows that cAMP prevents TNF + ActD-induced apoptosis in rat hepatocytes by inhibiting DISC complex formation.

Phosphodiesterase 4 Inhibition as a Therapeutic Target for Alcoholic Liver Disease: From Bedside to Bench

“…Indeed, rolipram‐treated mice with adequate cAMP levels had much lower ER stress protein expression including a critical apoptotic protein, CHOP. Our in vitro findings further established that the cAMP pathway is a negative regulator of ER stress and TNFα‐induced toxicity induced by ethanol, which is in agreement with previous studies examining the role of cAMP signaling in TNFα and bile acid–induced toxicity …”

Role of cAMP and phosphodiesterase signaling in liver health and disease

“…EPAC activation also protects hepatocytes from bile acid induced mitochondrial apoptosis [113, 114]. Another pathway of cAMP-mediated protection involves PKA-mediated phosphorylation of CD95 (FasR) and preventing formation of deathinducing signal complex (DISC), activation of caspases and apoptosis [115, 116]. …”

A Role for cGMP in Inducible Nitric-oxide Synthase (iNOS)-induced Tumor Necrosis Factor (TNF) α-converting Enzyme (TACE/ADAM17) Activation, Translocation, and TNF Receptor 1 (TNFR1) Shedding in Hepatocytes

“…The cGMP-independent pathways involve nitrosylation and inhibition of caspase-3 and -8 (39,70,71), whereas the cGMP-dependent pathway leads to the down-regulation of the proapoptotic protein BNIP3 in hepatocytes (72). Cyclic nucleotides can also block death-inducing signaling complex formation in response to TNF␣ in hepatocytes (19,73). Results from this study suggest that rapid TNFR1 shedding through iNOS/ NO/cGMP/PKG-mediated TACE activation is one mechanism to limit cell death by blocking death-inducing signaling complex formation.…”