14-3-3 proteins and survival kinases cooperate to inactivate BAD by BH3 domain phosphorylation

Methylpyridinium (MPP(+))- and nerve growth factor-induced changes in pro- and anti-apoptotic signaling pathways in SH-SY5Y neuroblastoma cells

The parkinsonian neurotoxin methylpyridinium (MPP(+)) mimics the neuropathology of Parkinson's disease (PD) and likely kills neurons by inhibiting complex I of the electron transport chain and increasing oxidative stress. We examined the time course of activation/inactivation of multiple pro- and anti-apoptotic signaling pathways in MPP(+)-induced apoptotic death of SH-SY5Y neuroblastoma cells. We found an early increase and later decrease of transcriptional activity of the generally anti-apoptotic nuclear factor kappa-beta (NF-kappa B) and early increases in activating phosphorylation of the anti-apoptotic upstream kinase protein kinase B (PKB, also known as AKT). Sequestration-inducing phosphorylation of pro-apoptotic BAD protein increased early then declined. A small biphasic increase in the generally pro-apoptotic p38 kinase activity paralleled the biphasic rise in NF-kappa B-mediated transcription. Inhibition of p38 kinase with 5 micro M SB203540, inhibition of MEK-ERK with 50 micro M U0126, or inhibition of phosphatidylinositol-3-kinase (PI3K) with 10 micro M LY294002 reduced cell viability by 4, 18 or 37%, respectively, after 24 h. All three kinase inhibitors increased cell death in response to 24 h of MPP(+), with the greatest effect shown by LY294002. Nerve growth factor (NGF) caused an early increase in activating phosphorylation of PKB/AKT and MEK-ERK and increased cell survival during MPP(+) exposure. We found that acute MPP(+) exposure activates multiple interacting death- and survival-promoting pathways. Survival-promoting MEK-ERK and PI3K pathways contribute to viability during MPP(+) exposure, both are activated by NGF, and loss of PI3K-mediated signaling and NF-kappa B-mediated transcription may commit cells irreversibly to apoptosis in this model. It remains unknown to what extent these signaling pathways modulate dopamine neuronal death in PD.

Activation of BAD by therapeutic inhibition of epidermal growth factor receptor and transactivation by insulin-like growth factor receptor

Novel cancer chemotherapeutics are required to induce apoptosis by activating pro-apoptotic proteins. Both epidermal growth factor (EGF) and insulin-like growth factor (IGF) provide potent survival stimuli in many epithelia, and activation of their receptors is commonly observed in solid human tumors. Here we demonstrate that blockade of the EGF receptor by a new drug in phase III clinical trails for cancer, ZD1839, potently induces apoptosis in mammary epithelial cell lines and primary cultures, as well as in a primary pleural effusion from a breast cancer patient. We identified the mechanism of apoptosis induction by ZD1839. We showed that it prevents cell survival by activating the pro-apoptotic protein BAD. Moreover, we demonstrate that IGF transactivates the EGF receptor and that ZD1839 blocks IGF-mediated phosphorylation of MAPK and BAD. Many cancer therapies kill tumor cells by inducing apoptosis as a consequence of targeting DNA; however, the threshold at which apoptosis can be triggered through DNA damage is often different from that in normal cells. Our results indicate that by targeting a growth factor-mediated survival signaling pathway, BAD phosphorylation can be manipulated therapeutically to induce apoptosis.

Integrins engage mitochondrial function for signal transduction by a mechanism dependent on Rho GTPases

We show here the transient activation of the small GTPase Rac, followed by a rise in reactive oxygen species (ROS), as necessary early steps in a signal transduction cascade that lead to NFkappaB activation and collagenase-1 (CL-1)/matrix metalloproteinase-1 production after integrin-mediated cell shape changes. We show evidence indicating that this constitutes a new mechanism for ROS production mediated by small GTPases. Activated RhoA also induced ROS production and up-regulated CL-1 expression. A Rac mutant (L37) that prevents reorganization of the actin cytoskeleton prevented integrin-induced CL-1 expression, whereas mutations that abrogate Rac binding to the neutrophil NADPH membrane oxidase in vitro (H26 and N130) did not. Instead, ROS were produced by integrin-induced changes in mitochondrial function, which were inhibited by Bcl-2 and involved transient membrane potential loss. The cells showing this transient decrease in mitochondrial membrane potential were already committed to CL-1 expression. These results unveil a new molecular mechanism of signal transduction triggered by integrin engagement where a global mitochondrial metabolic response leads to gene expression rather than apoptosis.

Glucagon-like peptide-2 receptor activation engages bad and glycogen synthase kinase-3 in a protein kinase A-dependent manner and prevents apoptosis following inhibition of phosphatidylinositol 3-kinase

Activation of glucagon-like peptide-2 receptor (GLP-2R) signaling promotes expansion of the mucosal epithelium indirectly via activation of growth and anti-apoptotic pathways; however, the cellular mechanisms coupling direct GLP-2R activation to cell survival remain poorly understood. We now demonstrate that GLP-2, in a cycloheximide-insensitive manner, enhanced survival in baby hamster kidney cells stably transfected with the rat GLP-2R; reduced mitochondrial cytochrome c efflux; and attenuated the caspase-dependent cleavage of Akt, poly(ADP-ribose) polymerase, and beta-catenin following inhibition of phosphatidylinositol 3-kinase (PI3K) by LY294002. The prosurvival effects of GLP-2 on LY294002-induced cell death were independent of Akt, p90(Rsk), or p70 S6 kinase activation; were mimicked by forskolin; and were abrogated by inhibition of protein kinase A (PKA) activity. GLP-2 inhibited activation of glycogen synthase kinase-3 (GSK-3) through phosphorylation at Ser(21) in GSK-3alpha and at Ser(9) in GSK-3beta in a PI3K-independent, PKA-dependent manner. GLP-2 reduced LY294002-induced mitochondrial association of endogenous Bad and Bax and stimulated phosphorylation of a transfected Bad fusion protein at Ser(155) in a PI3K-independent, but H89-sensitive manner, a modification known to suppress Bad pro-apoptotic activity. These results suggest that GLP-2R signaling enhances cell survival independently of PI3K/Akt by inhibiting the activity of a subset of pro-apoptotic downstream targets of Akt in a PKA-dependent manner.

14-3-3 antagonizes Ras-mediated Raf-1 recruitment to the plasma membrane to maintain signaling fidelity

We have investigated the role that S259 phosphorylation, S621 phosphorylation, and 14-3-3 binding play in regulating Raf-1 activity. We show that 14-3-3 binding, rather than Raf-1 phosphorylation, is required for the correct regulation of kinase activity. Phosphorylation of S621 is not required for activity, but 14-3-3 binding is essential. When 14-3-3 binding to conserved region 2 (CR2) was disrupted, Raf-1 basal kinase activity was elevated and it could be further activated by (V12,G37)Ras, (V23)TC21, and (V38)R-Ras. Disruption of 14-3-3 binding at CR2 did not recover binding of Raf-1 to (V12,G37)Ras but allowed more efficient recruitment of Raf-1 to the plasma membrane and stimulated its phosphorylation on S338. Finally, (V12)Ras, but not (V12,G37)Ras, displaced 14-3-3 from full-length Raf-1 and the Raf-1 bound to Ras. GTP was still phosphorylated on S259. Our data suggest that stable association of Raf-1 with the plasma membrane requires Ras-mediated displacement of 14-3-3 from CR2. Small G proteins that cannot displace 14-3-3 fail to recruit Raf-1 to the membrane efficiently and so fail to stimulate kinase activity.

Phenylephrine promotes phosphorylation of Bad in cardiac myocytes through the extracellular signal-regulated kinases 1/2 and protein kinase A

Studies in non-cardiomyocytic cells have shown that phosphorylation of the Bcl-2 family protein Bad on Ser-112, Ser-136 and Ser-155 decreases its pro-apoptotic activity. Both phenylephrine (100 microM) and the cell membrane-permeating cAMP analog, 8-(4-chlorophenylthio)-cAMP (100 microM), protected against 2-deoxy-D-glucose-induced apoptosis in neonatal rat cardiac myocytes as assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). In cardiac myocytes, phenylephrine primarily stimulates the alpha-adrenoceptor, but, at high concentrations (100 microM), it also increases the activity of the cAMP-dependent protein kinase, protein kinase A (PKA) through the beta-adrenoceptor. Phenylephrine (100 microM) promoted rapid phosphorylation of Bad(Ser-112) and Bad(Ser-155), though we were unable to detect phosphorylation of Bad(Ser-136). Phosphorylation of Bad(Ser-112) was antagonized by either prazosin or propranolol, indicating that this phosphorylation required stimulation of both alpha(1)- and beta-adrenoceptors. Phosphorylation of Bad(Ser-155) was antagonized only by propranolol and was thus mediated through the beta-adrenoceptor. Inhibitor studies and partial purification of candidate kinases by fast protein liquid chromatography showed that the p90 ribosomal S6 kinases, p90RSK2/3 [which are activated by the extracellular signal-regulated kinases 1 and 2 (ERK1/2)] directly phosphorylated Bad(Ser-112), whereas the PKA catalytic subunit directly phosphorylated Bad(Ser-155). However, efficient phosphorylation of Bad(Ser-112) also required PKA activity. These data suggest that, although p90RSK2/3 phosphorylate Bad(Ser-112) directly, phosphorylation of this site is enhanced by phosphorylation of Bad(Ser-155). These phosphorylations potentially diminish the pro-apoptotic activity of Bad and contribute to the cytoprotective effects of phenylephrine and 8-(4-chlorophenylthio)-cAMP.

Keeping killers on a tight leash: transcriptional and post-translational control of the pro-apoptotic activity of BH3-only proteins

BH3-only proteins are structurally distant members of the Bcl-2 protein family that trigger apoptosis. Genetic experiments have shown that these proteins are essential initiators of programmed cell death in species as distantly related as mice and C. elegans. BH3-only proteins share with each other and with the remainder of the Bcl-2 family only a nine amino acid BH3 (Bcl-2 Homology) region. Mutational analyses have demonstrated that this domain is required for their ability to bind to Bcl-2-like pro-survival proteins and to initiate apoptosis. So far only one BH3-only protein, EGL-1, has been identified in C. elegans and it is required for all developmentally programmed death of somatic cells in this species. In contrast, mammals have at least 10 BH3-only proteins that differ in their expression pattern and mode of activation. Studies in gene targeted mice have indicated that different BH3-only proteins are required for the initiation of distinct apoptotic stimuli. The pro-apoptotic activities of BH3-only proteins are stringently controlled by a variety of mechanisms. C. elegans egl-1 as well as mammalian hrk/dp5, noxa, puma/bbc3 and bim/bod are regulated by a diverse range of transcription factors. Certain BH3-only proteins, including Bad, Bik/Nbk, Bid, Bim/Bod and Bmf, are restrained by post-translational modifications that cause their sequestration from pro-survival Bcl-2 family members. In this review we describe current knowledge of the functions and transcriptional as well as post-translational control mechanisms of BH3-only proteins.

Structural insight into the role of the second intracellular loop of the bradykinin 2 receptor in signaling and internalization

The second cytoplasmic loop (IC2) of the bradykinin B2 receptor plays a vital role in its dynamic life cycle including the activation, internalization, desensitization, and resensitization of this receptor. Here, we probe the structure and function of the IC2, with particular emphasis on threonine-137, which is crucial for signal transduction and internalization. Mutation of this threonine to proline (T137P) produces wild type (WT) signaling and complete inhibition of internalization. Incorporation of aspartate (T137D) leads to a marked reduction in receptor signaling but with WT receptor uptake. The T137D mutation coupled with serine to alanine substitution of S335 and S341 within the distal C-terminus recovers signaling, leading to an actually enhanced arachidonic acid release and phosphoinositide turnover compared to WT bradykinin B2 receptor (BKB2R). To provide a structural basis for the actions of this mutant, the conformational features of IC2 (both WT and mutant) were investigated by high-resolution NMR. The NMR analysis illustrated two prominent alpha-helices at the N- (L123-M138) and C-termini (A149-I156) of the IC2 receptor domain. Incorporating these structural characteristics into a model of BKB2R, we determined that the entire N-terminal helix of IC2 is incorporated as TM3, placing Y131 1.5 helical turns into TM3 and T137 at the membrane surface. The NMR data indicated no structural changes upon substitution of T137D. These results suggest that the altered signaling of the T137D mutant can be attributed to the introduction of a negative charge, indicating that phosphorylation of this residue takes place and participates in the life cycle of this receptor. Additionally, the return to WT signal capacity of the mutation T137D/S335A/S341A, to overcome the deleterious T137D substitution points to a functional interaction between the IC2 and the C-terminus.

Molecular mechanisms involved in CD43-mediated apoptosis of TF-1 cells. Roles of transcription Daxx expression, and adhesion molecules

CD43 (leukosialin, sialophorin), an abundant leukocyte surface sialoglycoprotein, regulates leukocyte adhesion and transmits activating signals in T cells and dendritic cells. Immobilized anti-CD43 monoclonal antibody (mAb) MEM-59 has been previously shown to induce apoptosis of hematopoietic progenitors. In this study we show that it also triggers apoptosis of the myeloid progenitor-derived cell line TF-1. The kinetics of the MEM-59-induced apoptosis were unusually slow, with the first apoptotic cells appearing 36-48 h after their contact with the immobilized antibody; in 5 days, 90% of the cells were dead. CD43-mediated apoptosis was enhanced by coimmobilized anti-CD45 mAb and partly suppressed by coimmobilized anti-CD50 (ICAM-3) or anti-CD99 mAb. The MEM-59-triggered apoptosis of TF-1 cells was also inhibited by the overexpression of an apoptotic regulator, Daxx. CD43-mediated apoptosis was preceded by the repression of the DNA binding activity of the transcription factor AP-1. DNA array screening revealed that the expression of several genes encoding apoptosis-regulating proteins, including 14-3-3 proteins and the granulocyte macrophage colony-stimulating factor (GM-CSF) receptor beta-subunit, was repressed in TF-1 cells bound to immobilized MEM-59. The down-regulation of 14-3-3 proteins and GM-CSF receptor beta was accompanied by translocation of the proapoptotic protein Bad to the mitochondria. These results suggest that engagement of CD43 may, presumably through the repressing transcription, initiate a Bad-dependent apoptotic pathway.

Epstein-Barr virus BALF1 is a BCL-2-like antagonist of the herpesvirus antiapoptotic BCL-2 proteins

Cellular BCL-2 family proteins can inhibit or induce programmed cell death in part by counteracting the activity of other BCL-2 family members. All sequenced gammaherpesviruses encode a BCL-2 homologue that potently inhibits apoptosis and apparently escapes some of the regulatory mechanisms that govern the functions of their cellular counterparts. Examples of these protective proteins include BHRF1 of Epstein-Barr virus (EBV) and KSBcl-2 of Kaposi's sarcoma-associated herpesvirus, also known as human herpesvirus 8. The gamma-1 subgroup of these viruses, such as EBV, encodes a second BCL-2 homologue. We have now found that this second BCL-2 homologue encoded by EBV, BALF1, inhibits the antiapoptotic activity of EBV BHRF1 and of KSBcl-2 in several transfected cell lines. However, BALF1 failed to inhibit the cellular BCL-2 family member, BCL-x(L). Thus, BALF1 acts as a negative regulator of the survival function of BHRF1, similar to the counterbalance observed between cellular BCL-2 family members. Unlike the cellular BCL-2 family antagonists, BALF1 lacked proapoptotic activity and could not be converted into a proapoptotic factor in a manner similar to cellular BCL-2 proteins by caspase cleavage or truncation of the N terminus. Coimmunoprecipitation experiments and immunofluorescence assays suggest that a minimal amount, if any, of the BHRF1 and BALF1 proteins colocalizes inside cells, suggesting that mechanisms other than direct interaction explain the suppressive function of BALF1.

Identification of a novel phosphorylation site, Ser-170, as a regulator of bad pro-apoptotic activity

Bad is a pro-apoptotic member of the Bcl-2 family of proteins that is thought to exert a death-promoting effect by heterodimerization with Bcl-X(L), nullifying its anti-apoptotic activity. Growth factors may promote cell survival at least partially through phosphorylation of Bad at one or more of Ser-112, -136, or -155. Our previous work showed that Bad is also phosphorylated in response to cytokines at another site, which we now identify as Ser-170. The functional role of this novel phosphorylation site was assessed by site-directed mutagenesis and analysis of the pro-apoptotic function of Bad in transiently transfected HEK293 and COS-7 cells or by stable expression in the cytokine-dependent cell line, MC/9. In general, mutation of Ser-170 to Ala results in a protein with increased ability to induce apoptosis, similar to the S112A mutant. Mutation of Ser-170 to Asp, mimicking a constitutively phosphorylated site, results in a protein that is virtually unable to induce apoptosis. Similarly, the S112A/S170D double mutant does not cause apoptosis in HEK293 and MC/9 cell lines. These data strongly suggest that phosphorylation of Bad at Ser-170 is a critical event in blocking the pro-apoptotic activity of Bad.

Induction of apoptotic death in cells via Bad gene expression by infectious pancreatic necrosis virus infection

A Bcl-2 related family member, Bad, promotes cell death, and its function is regulated by phosphorylation. In this study, we show how the IPNV elicits the induction of Bad gene expression and promotes host apoptotic death. Anti-IPNV-E1S polyclonal and anti-VP3 monoclonal antibodies are used to neutralize the virus that blocks the prime death signal via the virus receptor. In the viability assay, each antibody could also enhance cell viability during IPNV infection. We tested tyrosine kinase inhibitors on IPNV-infected cells in order to assess their effect on blocking the death signal. With 100 microg/ml genistein treatment, Bad-like gene expression was blocked, either by rescuing the IPNV-infected CHSE-214 cells or by blocking internucleosomal DNA cleavage; but the tyrphostin group did not block Bad expression. For CHSE-214 cells, treatment with the protein synthesis-inhibitor, cycloheximide (1microg/ml), blocked new protein synthesis via activated tyrosine kinase during IPNV infection. We found that Bad protein expression could be blocked, and apoptotic death prevented. Together, these results demonstrate that the IPNV exerts up-regulation of a pro-apoptotic death gene (Bad), the expression of which serves to trigger apoptotic cell death. Our data also suggests that the IPNV induces apoptotic death via a viral receptor which triggers death effector Bad gene expression, possibly through a tyrosine kinase-dependent pathway.

Concomitant activation of the PI3K-Akt and the Ras-ERK signaling pathways is essential for transformation by the V-SEA tyrosine kinase oncogene

V-SEA is the transforming component of S13 Avian Erythroblastosis Retrovirus that causes erythroblastosis and anemia in chicken. Like all members in the family (MET, RON, SEA), its cytosolic domain possesses two tyrosine autophosphorylation sites in the tandemly arranged bidentate motif that serve as docking sites for SH2 domain-containing proteins. Here, we investigated phosphotyrosine-dependent activation of signaling pathways and their significance in V-SEA-induced transformation and/or proliferation. We demonstrated that V-SEA activates the PI3K-Akt signaling pathway primarily in Y557- and secondarily in Y564-dependent manner. V-SEA was also shown to induce the tyrosine phosphorylation of the Gab2 protein, leading to PI3K association and thus providing an alternative route for PI3K activation. On the other hand, activation of the Ras-ERK pathway is primarily via Y564 and secondarily via Y557. A dominant-negative form of Ras inhibited V-SEA-induced ERK phosphorylation in concentration dependent manner suggesting the importance of the Grb2-Ras signaling axis in V-SEA-induced ERK activation. The biological significance of activation of the PI3K-Akt and the Ras-ERK pathways in V-SEA-induced transformation was analysed in the V-SEA-RAT1 and V-SEA-3T3 cell lines by employing specific inhibitors, LY294002 and PD98059 compounds. Both the PD and LY compounds inhibited cell growth, but only the PD compound caused reversion of the transformed phenotype. In addition, both compounds inhibited focal colony formation by the transformants in soft agar. Thus, transformation by the V-SEA oncogene is a function of the concomitant activation of, at least, the PI3K-Akt and Ras-ERK signaling pathways that regulate cell growth and morphology.

Induction of Bad-mediated apoptosis by Sindbis virus infection: involvement of pro-survival members of the Bcl-2 family

It is known that infection with Sindbis virus (SNV) induces apoptosis, which is inhibited by two pro-survival members of the Bcl-2 family, Bcl-2 and Bcl-xL. However, the mechanism of involvement of the other members of the Bcl-2 family in SNV-induced apoptosis remains unclear. In this study we report that Bad protein, one of the pro-apoptotic Bcl-2 family members, mediates apoptosis in the mammalian cells infected with SNV. Expression of Bad was shown to promote SNV-induced apoptosis in human embryonic kidney 293T and baby hamster kidney cells. SNV infection also induced translocation of endogenous Bad into mitochondria and heterodimerization of Bad with Bcl-xL. On the other hand, the structurally most similar pro-survival members, Bcl-2, Bcl-xL, and Bcl-w, suppressed SNV-induced apoptosis in the absence of Bad, whereas Mcl-1 and A1 did not. Bcl-w could inhibit SNV-induced apoptosis in the presence of Bad, but Bcl-xL could not. Bad could be coimmunoprecipitated with Bcl-xL or Bcl-2, but not with Bcl-w. Two viral Bcl-2 homologs, E1B19K and BHRF1, also suppressed SNV-induced apoptosis irrespective of the presence of Bad and no physical association with Bad was observed. These results suggest that direct interaction of Bad with pro-survival members of the Bcl-2 family contributes to the progress of SNV-induced apoptosis and that nonbinding members restrain SNV-induced apoptosis irrespective of Bad expression.

Delayed mammary gland involution in MMTV-AKT1 transgenic mice

AKT1/protein kinase Balpha is a protein-serine/threonine kinase that regulates multiple targets involved in cell survival and cell cycle progression in a variety of cell types including breast cancer cells. To explore the role of Akt1 in mammary gland function and tumorigenesis, transgenic mice were generated that express human AKT1 under the control of the MMTV promoter. Virgin transgenic mice did not exhibit a dominant phenotype, but upon cessation of lactation, a notable delay in involution occurred compared to age-matched non-transgenic mice. The delay in involution coincided with increased hyperplasia as evidenced by an increased number of binucleated epithelial cells and a marked elevation in cyclin D1 expression in mammary epithelium. The delayed involution phenotype corresponded to increased phosphorylation of Thr308 in AKT1 and Ser136 in BAD, but not phosphorylation of Ser21 in GSK-3alpha. There was no evidence of mammary dysplasia or neoplasia during the lifespan of multiparous transgenic mice. These data suggest that AKT1 is involved in cell survival in the lactating and involuting mammary gland, but that overexpression of AKT1 alone is insufficient to induce transformation.

The G2/M regulator 14-3-3sigma prevents apoptosis through sequestration of Bax

In response to DNA damage and genotoxic stress, the p53 tumor suppressor triggers either cell cycle arrest or apoptosis. The G(2) arrest after damage is, in part, mediated by the p53 target, 14-3-3final sigma (final sigma). Colorectal tumor cells lacking final sigma are exquisitely sensitive to DNA damage. Here we analyzed the mechanism of this sensitivity in final sigma(-/-) as compared with final sigma(+/+) human colorectal tumor cells. Exposure to adriamycin resulted in rapid apoptosis only in final sigma(-/-) cells. This was further characterized by caspase-3 activation, p21(CIP1) cleavage, and CDK2 activation. Moreover, Bax was rapidly translocated out of the cytoplasm, and cytochrome c was released in final sigma(-/-) cells. Transient adenovirus-mediated reconstitution of final sigma in the final sigma(-/-) cells led to effective rescue of this phenotype and protected cells against apoptosis. The association of final sigma, Bax, and CDK1 in protein complexes may be the basis for this antiapoptotic mechanism. In conclusion, final sigma not only enforces the p53-dependent G(2) arrest but also delays the apoptotic signal transduction.

BH3-only Bcl-2 family members are coordinately regulated by the JNK pathway and require Bax to induce apoptosis in neurons

The Bcl-2 family of proteins are key regulators of programmed cell death. A distinct subfamily of BH3-only molecules has been identified, but their exact mechanism of action remains unclear. Here we show that the BH3-only Bcl-2 family members, Dp5/Hrk and Bim, are induced upstream of the Bax checkpoint in neuronal apoptosis in a manner that shows significant dependence on JNK signaling. We also show that Dp5 and other BH3-only proteins kill cerebellar granule neurons in a Bax-dependent manner. These studies demonstrate that BH3-only members do not act independently in their proapoptotic activities but rather require the action of multidomain proapoptotic Bcl-2 family members to produce cell death.

14-3-3 proteins; bringing new definitions to scaffolding

The 14-3-3 proteins are a part of an emerging family of proteins and protein domains that bind to serine/threonine-phosphorylated residues in a context specific manner, analogous to the Src homology 2 (SH2) and phospho-tyrosine binding (PTB) domains. 14-3-3 proteins bind and regulate key proteins involved in various physiological processes such as intracellular signaling (e.g. Raf, MLK, MEKK, PI-3 kinase, IRS-1), cell cycling (e.g. Cdc25, Wee1, CDK2, centrosome), apoptosis (e.g. BAD, ASK-1) and transcription regulation (e.g. FKHRL1, DAF-16, p53, TAZ, TLX-2, histone deacetylase). In contrast to SH2 and PTB domains, which serve mainly to mediate protein-protein interactions, 14-3-3 proteins in many cases alter the function of the target protein, thus allowing them to serve as direct regulators of their targets. This review focuses on the various mechanisms employed by the 14-3-3 proteins in the regulation of their diverse targets, the structural basis for 14-3-3-target protein interaction with emphasis on the role of 14-3-3 dimerization in target protein binding and regulation and provides an insight on 14-3-3 regulation itself.

14-3-3 proteins: key regulators of cell division, signalling and apoptosis

The 14-3-3 proteins constitute a family of conserved proteins present in all eukaryotic organisms so far investigated. These proteins have attracted interest because they are involved in important cellular processes such as signal transduction, cell-cycle control, apoptosis, stress response and malignant transformation and because at least 100 different binding partners for the 14-3-3 proteins have been reported. Although the exact function of 14-3-3 proteins is still unknown, they are known to (1) act as adaptor molecules stimulating protein-protein interactions, (2) regulate the subcellular localisation of proteins and (3) activate or inhibit enzymes. In this review, we discuss the role of the 14-3-3 proteins in three cellular processes: cell cycle control, signal transduction and apoptosis. These processes are regulated by the 14-3-3 proteins at multiple steps. The 14-3-3 proteins have an overall inhibitory effect on cell cycle progression and apoptosis, whereas in signal transduction they may act as stimulatory or inhibitory factors. This article contains supplementary material which may be viewed at the BioEssays website at http://www.interscience.wiley.com/jpages/0265-9247/Suppmat/23/v23_10.936.

A novel 14-3-3 gene is osmoregulated in gill epithelium of the euryhaline teleost Fundulus heteroclitus

We have cloned and analyzed the full-length coding sequence and 3′ untranslated region (UTR) of a unique 14-3-3 gene of the euryhaline teleost Fundulus heteroclitus, which we named 14-3-3.a. Phylogenetic analysis of the deduced amino acid sequence revealed that the 14-3-3.a gene product is most similar to vertebrate 14-3-3ζ and β, yet it displays considerable divergence to known classes of vertebrate 14-3-3 isoforms. The N and C termini of 14-3-3.a are the most unique regions, whereas the amino acid residues forming the amphipathic ligand-binding groove are highly conserved. F. heteroclitus 14-3-3.a mRNA expression is high in gill epithelium, moderate in intestine and brain, and low in gonads, white muscle and heart. Because 14-3-3 proteins are important molecular scaffolds and cofactors for phosphoproteins and signaling complexes, the high level of 14-3-3.a expression in gill epithelium of the euryhaline teleost F. heteroclitus suggests that it is crucial for signal transduction in gill epithelial cells. We provide evidence that 14-3-3.a is involved in osmosensory signal transduction by showing that its mRNA and protein levels in gill epithelium, but not in any other tissue analyzed, increase two- to fourfold within 24h of salinity transfer of fish from sea water to fresh water. These data are clear evidence for an important role of 14-3-3.a in the remodeling of gill epithelium during transition of euryhaline fish between plasma-hyperosmotic and plasma-hyposmotic environments.

Akt inhibits the orphan nuclear receptor Nur77 and T-cell apoptosis

Akt is a common mediator of cell survival in a variety of circumstances. Although some candidate Akt targets have been described, the function of Akt is not fully understood, particularly because of the cell type- and context-dependent apoptosis regulation. In this study, we demonstrate that one of the mechanisms by which Akt antagonizes apoptosis involves the inhibition of Nur77, a transcription factor implicated in T-cell receptor-mediated apoptosis. It has been suggested that Akt phosphorylates Nur77 directly, but whether Akt suppresses biological functions of Nur77 remains unknown. We found that Akt inhibited the DNA binding activity of Nur77 and stimulated its association with 14-3-3 in a phosphorylation site-dependent manner. Moreover, we found that expression of Akt suppressed Nur77-induced apoptosis in fibroblasts and activation-induced cell death of T-cell hybridomas. The inhibition of Nur77 by Akt suggests a mechanism that explains how T-cell receptor activation can promote survival in some instances even when Nur77 is induced. Collectively, these results may suggest that Akt is a negative regulator of Nur77 in T-cell apoptosis.

The US3 protein kinase of herpes simplex virus 1 mediates the posttranslational modification of BAD and prevents BAD-induced programmed cell death in the absence of other viral proteins

Earlier studies have shown that the d120 mutant of herpes simplex virus 1, which lacks both copies of the alpha4 gene, induces apoptosis in all cell lines tested. In some cell lines d120-induced apoptosis, manifested by the release of cytochrome c, activation of caspase 3, and fragmentation of cellular DNA, is blocked by the overexpression of Bcl-2. In these cells viral protein kinase U(S)3 delivered in trans blocks apoptosis induced by the mutant virus at a premitochondrial stage. We report that the U(S)3 protein kinase targets the pro-apoptotic BAD member of the Bcl-2 family. Specifically, the U(S)3 protein kinase mediates a posttranslational modification of BAD and blocks its cleavage, which is reported to activate apoptosis. Thus, U(S)3 protein kinase is the sole viral protein required to block activation of caspase 3, prevent cleavage of poly(ADP-ribose) polymerase, and block fragmentation of cellular DNA induced by BAD.

The growth-promoting activity of the Bad protein in chicken embryo fibroblasts requires binding to protein 14-3-3

Phosphorylation of the Bad protein is a key regulatory event in the prevention of apoptosis by survival factors. Phosphorylated Bad binds to the cytosolic 14-3-3 protein and is sequestered from the apoptotic machinery of the mitochondrial membrane. To examine the role of Bad in cell growth and apoptosis in primary cultures, we produced stable Bad transfectants of chicken embryo fibroblasts (CEF). As expected, serum starvation of Bad transfectants promoted apoptosis. However, Bad-transfected CEF maintained in media with a high serum concentration were capable of anchorage-independent growth and grew to a higher saturation density than control CEF transfected with the empty vector. High dilutions of the infectious retroviral vector RCAS expressing Bad led to the formation of multilayered cell foci. The growth-promoting effects of Bad were dependent on the serine 136 phosphorylation site and correlated directly with binding of Bad to 14-3-3. These results suggest that phosphorylated Bad promotes cell growth and in oncogenic transformation may contribute to the neoplastic phenotype of the cell.

p70S6 kinase signals cell survival as well as growth, inactivating the pro-apoptotic molecule BAD

Cytokines often deliver simultaneous, yet distinct, cell growth and cell survival signals. The 70-kDa ribosomal protein S6 kinase (p70S6K) is known to regulate cell growth by inducing protein synthesis components. We purified membrane-based p70S6K as a kinase responsible for site-specific phosphorylation of BAD, which inactivates this proapoptotic molecule. Rapamycin inhibited mitochondrial-based p70S6K, which prevented phosphorylation of Ser-136 on BAD and blocked cell survival induced by insulin-like growth factor 1 (IGF-1). Moreover, IGF-1-induced phosphorylation of BAD Ser-136 was abolished in p70S6K-deficient cells. Thus, p70S6K is itself a dual pathway kinase, signaling cell survival as well as growth through differential substrates which include mitochondrial BAD and the ribosomal subunit S6, respectively.

BAD/BCL-[X(L)] heterodimerization leads to bypass of G0/G1 arrest

The pro-apoptotic molecule BAD binds BCL-[X(L)] or BCL2 and inactivates their survival function. In addition to their anti-apoptotic function, BCL2 and BCL-[X(L)] also delay cell cycle entry from quiescence. We found that the BH3-only molecule BAD also exerted a cell cycle effect. BAD expression resulted in failure to cell cycle block in growth arrest conditions. In low serum and in confluence, fibroblasts constitutively or inducibly expressing BAD persisted in S phase, continued to incorporate BrdU, and exhibited sustained cyclin E/cdk2 activity. Mutation analysis indicated that the cell cycle effect of BAD was not dependent on its phosphorylation status or subcellular localization, but strictly co-segregated with BCL-[X(L)] binding. bclx(-/-) MEFs expressing BAD and bad(-/-) MEFs both arrested in G0/G1 in low serum similar to wild-type controls, suggesting that the ability to overcome the G0/G1 checkpoint resulted from the presence of BAD/BCL-x(L) heterodimers, rather than the absence of BCL-[X(L)] or BAD. These data provide evidence that in addition to regulating apoptosis, the BAD/BCL-[X(L)] heterodimer has a novel cell cycle function.

Phosphorylation of the protein kinase mutated in Peutz-Jeghers cancer syndrome, LKB1/STK11, at Ser431 by p90(RSK) and cAMP-dependent protein kinase, but not its farnesylation at Cys(433), is essential for LKB1 to suppress cell vrowth

Peutz-Jeghers syndrome is an inherited cancer syndrome that results in a greatly increased risk of developing tumors in those affected. The causative gene is a protein kinase termed LKB1, predicted to function as a tumor suppressor. The mechanism by which LKB1 is regulated in cells is not known. Here, we demonstrate that stimulation of Rat-2 or embryonic stem cells with activators of ERK1/2 or of cAMP-dependent protein kinase induced phosphorylation of endogenously expressed LKB1 at Ser(431). We present pharmacological and genetic evidence that p90(RSK) mediated this phosphorylation in response to agonists that activate ERK1/2 and that cAMP-dependent protein kinase mediated this phosphorylation in response to agonists that activate adenylate cyclase. Ser(431) of LKB1 lies adjacent to a putative prenylation motif, and we demonstrate that full-length LKB1 expressed in 293 cells was prenylated by addition of a farnesyl group to Cys(433). Our data suggest that phosphorylation of LKB1 at Ser(431) does not affect farnesylation and that farnesylation does not affect phosphorylation at Ser(431). Phosphorylation of LKB1 at Ser(431) did not alter the activity of LKB1 to phosphorylate itself or the tumor suppressor protein p53 or alter the amount of LKB1 associated with cell membranes. The reintroduction of wild-type LKB1 into a cancer cell line that lacks LKB1 suppressed growth, but mutants of LKB1 in which Ser(431) was mutated to Ala to prevent phosphorylation of LKB1 were ineffective in inhibiting growth. In contrast, a mutant of LKB1 that cannot be prenylated was still able to suppress the growth of cells.

14-3-3 is involved in p75 neurotrophin receptor-mediated signal transduction

The low affinity neurotrophin receptor (p75NTR) has been shown to mediate the apoptosis signaling to neural cells. However, the specific mechanisms of intracellular signal transduction of this process are largely unknown. To understand p75NTR-mediated signal transduction, we previously identified a protein that interacts with the intracellular domain of p75NTR, and we named it p75NTR-associated cell death executor (NADE). To elucidate further the signaling mechanisms utilized by p75NTR and NADE, we screened for NADE-binding protein(s) with the yeast two-hybrid method, and we identified 14-3-3epsilon as a NADE-binding protein in vivo. To examine whether 14-3-3epsilon affects the induction of p75NTR-mediated apoptosis, wild type or various deletion mutant forms of 14-3-3epsilon were co-expressed in HEK293, PC12nnr5, and oligodendrocytes. Interestingly, transient expression of the mutant form of 14-3-3epsilon lacking the 208-255 amino acid region blocked nerve growth factor-dependent p75NTR/NADE-mediated apoptosis, although this mutant form of 14-3-3epsilon continued to associate with NADE. These results suggest that 14-3-3epsilon plays an important role in the modulation of nerve growth factor-dependent p75NTR/NADE-mediated apoptosis.

Caspase cleavage enhances the apoptosis-inducing effects of BAD

The function of BAD, a proapoptotic member of the Bcl-2 family, is regulated primarily by rapid changes in phosphorylation that modulate its protein-protein interactions and subcellular localization. We show here that, during interleukin-3 (IL-3) deprivation-induced apoptosis of 32Dcl3 murine myeloid precursor cells, BAD is cleaved by a caspase(s) at its N terminus to generate a 15-kDa truncated protein. The 15-kDa truncated BAD is a more potent inducer of apoptosis than the wild-type protein, whereas a mutant BAD resistant to caspase 3 cleavage is a weak apoptosis inducer. Truncated BAD is detectable only in the mitochondrial fraction, interacts with BCL-X(L) at least as effectively as the wild-type protein, and is more potent than wild-type BAD in inducing cytochrome c release. Human BAD, which is 43 amino acids shorter than its mouse counterpart, is also cleaved by a caspase(s) upon exposure of Jurkat T cells to anti-FAS antibody, tumor necrosis factor alpha (TNF-alpha), or TRAIL. Moreover, a truncated form of human BAD lacking the N-terminal 28 amino acids is more potent than wild-type BAD in inducing apoptosis. The generation of truncated BAD was blocked by Bcl-2 in IL-3-deprived 32Dcl3 cells but not in Jurkat T cells exposed to anti-FAS antibody, TNF-alpha, or TRAIL. Together, these findings point to a novel and important role for BAD in maintaining the apoptotic phenotype in response to various apoptosis inducers.

Disruption of 3-phosphoinositide-dependent kinase 1 (PDK1) signaling by the anti-tumorigenic and anti-proliferative agent n-alpha-tosyl-l-phenylalanyl chloromethyl ketone

The anti-tumorigenic and anti-proliferative effects of N-alpha-tosyl-l-phenylalanyl chloromethyl ketone (TPCK) have been known for more than three decades. Yet little is known about the discrete cellular targets of TPCK controlling these effects. Previous work from our laboratory showed TPCK, like the immunosuppressant rapamycin, to be a potent inhibitor of the 70-kilodalton ribosomal S6 kinase 1 (S6K1), which mediates events involved in cell growth and proliferation. We show here that rapamycin and TPCK display distinct inhibitory mechanisms on S6K1 as a rapamycin-resistant form of S6K1 was TPCK-sensitive. Additionally, we show that TPCK inhibited the activation of the related kinase and proto-oncogene Akt. Upstream regulators of S6K1 and Akt include phosphoinositide 3-kinase (PI 3-K) and 3-phosphoinositide-dependent kinase 1 (PDK1). Whereas TPCK had no effect on either mitogen-regulated PI 3-K activity or total cellular PDK1 activity, TPCK prevented phosphorylation of the PDK1 regulatory sites in S6K1 and Akt. Furthermore, whereas both PDK1 and the mitogen-activated protein kinase (MAPK) are required for full activation of the 90-kilodalton ribosomal S6 kinase (RSK), TPCK inhibited RSK activation without inhibiting MAPK activation. Consistent with the capacity of RSK and Akt to mediate a cell survival signal, in part through phosphorylation of the pro-apoptotic protein BAD, TPCK reduced BAD phosphorylation and led to cell death in interleukin-3-dependent 32D cells. Finally, in agreement with results seen in embryonic stem cells lacking PDK1, protein kinase A activation was not inhibited by TPCK showing TPCK specificity for mitogen-regulated PDK1 signaling. TPCK inhibition of PDK1 signaling thus disables central kinase cascades governing diverse cellular processes including proliferation and survival and provides an explanation for its striking biological effects.

Protein phosphatase 2A activates the proapoptotic function of BAD in interleukin- 3-dependent lymphoid cells by a mechanism requiring 14-3-3 dissociation

BAD is a proapoptotic member of the BCL-2 family of proteins, which play a major role in regulating apoptosis in cytokine-dependent hematopoietic cells. The function of BAD is regulated by reversible phosphorylation. Deprivation of survival factors induces BAD dephosphorylation, resulting in apoptosis. Serine-threonine phosphatase activity dephosphorylated BAD in interleukin-3-dependent FL5.12 lymphoid cells. Inhibition of PP2A activity by treatment of cells with PP2A-selective inhibitors, okadaic acid and fostriecin, prevented BAD dephosphorylation in these cells. Conversely, BAD dephosphorylation was not inhibited by the PP1-selective inhibitor tautomycin. In cell-free extracts, BAD phosphatase activity was also inhibited by the PP2A-selective inhibitors okadaic acid and fostriecin, but not by the PP1-specific protein inhibitor I-2. Dissociation of 14-3-3 from BAD was a prerequisite for BAD dephosphorylation in vitro, suggesting a mechanism by which 14-3-3 can regulate the activation of the proapoptotic function of BAD in vivo. Significantly, the inhibition of BAD phosphatase activity rescued cell death induced by survival factor withdrawal in FL5.12 cells expressing wild-type BAD but not phosphorylation-defective mutant BAD. These data indicate that PP2A, or a PP2A-like enzyme, dephosphorylates BAD and, in conjunction with 14-3-3, modulates cytokine-mediated survival.

Phosphorylation of the pro-apoptotic protein BIK: mapping of phosphorylation sites and effect on apoptosis

BIK is a pro-apoptotic BCL-2 family member and is the founding member of a subfamily of pro-apoptotic proteins known as "BH3-alone" proteins. Ectopic expression of BIK induces apoptosis in variety of mammalian cells. BIK complexes with various anti-apoptotic BCL-2 family proteins such as adenovirus E1B-19K and BCL-2 via the BH3 domain. However, the heterodimerization activity of BIK alone is insufficient for its apoptotic activity. Previous studies have shown that phosphorylation regulates the functional activity of both anti-apoptotic and pro-apoptotic members of the BCL-2 family. Here, we have examined phosphorylation of BIK and its effect on the apoptotic activity of BIK. We show that BIK exists as a phosphoprotein and is phosphorylated at residues 33 (threonine) and 35 (serine). Mutation of the phosphorylation sites, in which the Thr and Ser residues were changed to alanine residues, reduced the apoptotic activity of BIK without significantly affecting its ability to heterodimerize with BCL-2. Our results suggest that phosphorylation of BIK is required for eliciting efficient apoptotic activity. Partial purification of the protein kinase from HeLa cell cytoplasmic extracts suggest that BIK may be phosphorylated by a casein kinase II-related enzyme.

Protein kinase SGK mediates survival signals by phosphorylating the forkhead transcription factor FKHRL1 (FOXO3a)

Serum- and glucocorticoid-inducible kinases (SGKs) form a novel family of serine/threonine kinases that are activated in response to a variety of extracellular stimuli. SGKs are related to Akt (also called PKB), a serine/threonine kinase that plays a crucial role in promoting cell survival. Like Akt, SGKs are activated by the phosphoinositide-3 kinase (PI3K) and translocate to the nucleus upon growth factor stimulation. However the physiological substrates and cellular functions of SGKs remained to be identified. We hypothesized that SGKs regulate cellular functions in concert with Akt by phosphorylating common targets within the nucleus. The best-characterized nuclear substrates of Akt are transcription factors of the Forkhead family. Akt phosphorylates Forkhead transcription factors such as FKHRL1, leading to FKHRL1's exit from the nucleus and the consequent shutoff of FKHRL1 target genes. We show here that SGK1, like Akt, promotes cell survival and that it does so in part by phosphorylating and inactivating FKHRL1. However, SGK and Akt display differences with respect to the efficacy with which they phosphorylate the three regulatory sites on FKHRL1. While both kinases can phosphorylate Thr-32, SGK displays a marked preference for Ser-315 whereas Akt favors Ser-253. These findings suggest that SGK and Akt may coordinately regulate the function of FKHRL1 by phosphorylating this transcription factor at distinct sites. The efficient phosphorylation of these three sites on FKHRL1 by SGK and Akt appears to be critical to the ability of growth factors to suppress FKHRL1-dependent transcription, thereby preventing FKHRL1 from inducing cell cycle arrest and apoptosis. These findings indicate that SGK acts in concert with Akt to propagate the effects of PI3K activation within the nucleus and to mediate the biological outputs of PI3K signaling, including cell survival and cell cycle progression.

Wnt-1 signaling inhibits apoptosis by activating beta-catenin/T cell factor-mediated transcription

Wnt signaling plays a critical role in development and oncogenesis. Although significant progress has been made in understanding the downstream signaling cascade of Wnt signaling, little is known regarding Wnt signaling modification of the cell death machinery. Given that numerous oncogenes transform cells by providing cell survival function, we hypothesized that Wnt signaling may inhibit apoptosis. Here, we report that cells expressing Wnt-1 were resistant to cancer therapy-mediated apoptosis. Wnt-1 signaling inhibited the cytochrome c release and the subsequent caspase-9 activation induced by chemotherapeutic drugs, including both vincristine and vinblastine. Furthermore, we found that Wnt-1-mediated cell survival was dependent on the activation of beta-catenin/T cell factor (Tcf) transcription. Inhibition of beta-catenin/Tcf transcription by expression of the dominant-negative mutant of Tcf-4 blocked Wnt-1-mediated cell survival and rendered cells sensitive to apoptotic stimuli. These results provide the first demonstration that Wnt-1 inhibits cancer therapy-mediated apoptosis and suggests that Wnt-1 may exhibit its oncogenic potential through a mechanism of anti-apoptosis.

Lipoarabinomannan from Mycobacterium tuberculosis promotes macrophage survival by phosphorylating Bad through a phosphatidylinositol 3-kinase/Akt pathway

Efforts in prevention and control of tuberculosis suffer from the lack of detailed knowledge of the mechanisms used by pathogenic mycobacteria for survival within host cell macrophages. The exploitation of host cell signaling pathways to the benefit of the pathogen is a phenomenon that deserves to be looked into in detail. We have tested the hypothesis that lipoarabinomannan (LAM) from the virulent species of Mycobacterium tuberculosis possesses the ability to modulate signaling pathways linked to cell survival. The Bcl-2 family member Bad is a proapoptotic protein. Phosphorylation of Bad promotes cell survival in many cell types. We demonstrate that man-LAM stimulates Bad phosphorylation in a phosphatidylinositol 3-kinase (PI-3K)-dependent pathway in THP-1 cells. Man-LAM activated PI-3K. LAM-stimulated phosphorylation of Bad was abrogated in cells transfected with a dominant-negative mutant of PI-3K (Delta p85), indicating that activation of PI-3K is sufficient to trigger phosphorylation of Bad by LAM. Since phosphorylation of Bad occurred at serine 136, the target of the serine/threonine kinase Akt, the effect of LAM on Akt kinase activity was tested. Man-LAM could activate Akt as evidenced from phosphorylation of Akt at Thr(308) and by the phosphorylation of the exogenous substrate histone 2B. Akt activation was abrogated in cells transfected with Deltap85. The phosphorylation of Bad by man-LAM was abrogated in cells transfected with a kinase-dead mutant of Akt. These results establish that LAM-mediated Bad phosphorylation occurs in a PI-3K/Akt-dependent manner. It is therefore the first demonstration of the ability of a mycobacterial virulence factor to up-regulate a signaling pathway involved in cell survival. This is likely to be one of a number of virulence-associated mechanisms by which bacilli control host cell apoptosis.