Apoptosis is a physiological process of cell death that plays a critical role in normal development, as well as in the pathophysiology of a variety of diseases. The fundamental cellular mechanism behind apoptosis is due to a balance between anti-apoptotic and pro-apoptotic factors, which may be shifted by extracellular signals. 14-3-3 proteins play an important suppressing role in several apoptotic pathways in animals. These are a family of abundant, widely expressed 28-33-kDa acidic polypeptides that spontaneously self-assemble as dimers. The 14-3-3 family of proteins mediates signal transduction by binding to phosphoserine-containing proteins. There are at least seven distinct genes for 14-3-3 in vertebrates, giving rise to nine isotypes (Alpha, Beta, Gamma, Delta, Epsilon, Eta, Sigma, Tau, and Zeta, with Alpha and Delta being phosphorylated forms of Beta and Zeta, respectively) (Ref.1).
Apoptosis can be initiated by ligand binding to members of the TNF (Tumor Necrosis Factor) family of membrane bound receptors, e.g., TNFR1 and CD95 (Fas). Activation of TNFR1 leads to recruitment of TRADD (TNF Receptor Associated Death Domain), which subsequently recruits the FADD (Fas-Associated Death Domain) and Caspase 8 (Ref.2). Caspase 8 directly activates Caspase 3 and BID (BH3-Interacting Domain death agonist), which activates BCL2 (B-Cell CLL/Lymphoma-2) in the mitochondria. Caspase 3 removes the N-terminal regulatory domain of MEKK1 (MAPK/ERK kinase kinase-1) from the catalytic domain, which ultimately results in apoptosis. TNF binding to TNFR1 also activates the recruitment of other TRADD, which via NIK (NF-KappaB-Inducing Kinase) ultimately activates the anti-apoptotic transcription factor NF-kappaB (Nuclear Factor-kappaB) complexed with inhibitory I-kappaB-Alpha (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells). The apoptosis-suppressing activity of NF-kappaB includes regulation of expression of the zinc finger protein A20 and anti-apoptotic genes (Ref.3). In addition, the binding of 14-3-3 to A20 mediates interaction with the c-Raf kinase, implicating a role for A20 in signal transduction and providing another 14-3-3-involving link between signal transduction and apoptosis (Ref.4). Intriguingly, 14-3-3 isoforms Eta and Zeta bind to the zinc finger domain of A20, which results in blocking other A20-binding inhibitor of NF-kappaB, ABIN1 (Nef-associated Factor-1) proteins from interacting with this domain. Moreover, A20 inhibits IKK (I-kappaB kinase) activation of NF-kappaB after apoptosis is prevented. With its nuclear localization signal, exposed NF-kappaB translocates to the nucleus, where it binds to kappaB sites in the promoter region of NF-kappaB -responsive genes. In addition, IL-1 (Interleukin-1) and LPS (Lipopolysaccharides) up-regulate A20 in endothelial cells, via a pathway similar to the TNF pathway, converging at the IKK complex (Ref.3). This involves the binding of IL-1/LPS to IL-1R/TLR4 (Toll-Like Receptor-4), which triggers a common intracellular signaling cascade involving the adapter proteins MyD88, TRAF6 (TNF Receptor-Associated Factor-6) and IRAK (IL-1R-Associated Kinase).
The kinase PI3K (Phosphatidylinositol-3 Kinase) and Akt1 exert their anti-apoptotic roles at several points in the apoptotic machinery. Besides phosphorylating BAD (BCL2 Associated Death Promoter) and FKHRL1 (Forkhead), Akt1 also inactivates Caspase 9 by phosphorylation on the Ser196 residue. ASK1 (Apoptosis Signal-regulating Kinase-1) associates with TRAF2 (TNF-R-Associating Factor-2), and mediates activation of c-Jun and c-Fos via the MKK4 (MAP Kinase Kinase-4)-JNK (Jun N-terminal kinase) pathway, resulting in TNF expression. Phosphorylation and 14-3-3 binding to ASK1 result in inhibition of the pathway (Ref.5). ASK1 appears to be a general mediator of cell death because it is responsive to a variety of additional death signals, including oxidative stress and treatment with the chemotherapeutic drugs cisplatin and paclitaxel. BAD inhibits the anti-apoptotic functions of BCL2 and BCL-XL but as a consequence of consecutive phosphorylation by Akt1 and PKA (Protein Kinase-A), BAD is released from BCL2 or BCL-XL and loses its pro-apoptotic effect. Conversely, pro-apoptotic phosphatases such as Calcineurin and PP2A (Protein Phosphatase 2A) dephosphorylate BAD, causing release from 14-3-3 proteins (Ref.6). Once phosphorylated, BAD can be complexed by 14-3-3 proteins in the cytoplasm, preventing the association of BAD with the mitochondrially localized BCL-XL and BCL2 and therefore inhibiting apoptosis. Similarly, phosphorylated FKHRL1 associates with 14-3-3 proteins and is therefore retained in the cytoplasm. On deprivation from survival signals, however, dephosphorylation of FKHRL1 leads to its dissociation from 14-3-3 and translocation to the nucleus, resulting in the subsequent activation of apoptotic genes like FasL (Ref.7).
14-3-3 proteins are abundantly expressed in the brain and have been detected in the cerebrospinal fluid of patients with different neurological disorders. By their interaction with more than 100 binding partners, 14-3-3 proteins modulate the action of proteins that are involved in regulation of cell cycle arrest in response to DNA damage, cell cycle timing, intracellular trafficking, regulation of ion channels, and intracellular signaling in response to stress, mating pheromone in yeast, photoreceptor development and learning in Drosophila, cellular response to stress and survival factors in mammals, and the Ras/Raf signaling pathway in various organisms (Ref.8).
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