Interleukin 3-dependent survival by the Akt protein kinase

The oncogenic TEL/PDGFR beta fusion protein induces cell death through JNK/SAPK pathway

The TEL/PDGFR beta (T/P) fusion protein isolated from patients bearing a t(5;12) translocation is transforming when expressed in haematopoietic cells. To examine the signal transduction events activated by this protein, we measured the effect of T/P on activation of the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) in mouse bone marrow-derived Ba/F3 cells. Significant increase in the activity of JNK/SAPK1 was observed in transient transfection as well as in Ba/F3 cells stably expressing T/P. This activation was abrogated when the T/P-expressing cells were treated with a specific inhibitor of the PDGFR beta tyrosine kinase, indicating that the activity of the PDGFR beta part of the fusion protein was involved in JNK/SAPK activation. Expression of a dominant negative mutant of mitogen-activated protein kinase kinase 4 (MKK4), a direct activator of JNK/SAPK, prevented T/P-induced JNK/SAPK activation. In addition, inhibition of phosphoinositide-3 OH kinase (PI-3 kinase), a promoting survival factor, potentiated the effect of T/P on JNK/SAPK activation. Interestingly, expression of T/P was shown to initiate an apoptotic response that was enhanced by treatment of cells with the PI-3 kinase inhibitor LY294002, suggesting that T/P mediated cell death through activation of JNK/SAPK signalling pathway. Consistent with this hypothesis, expression of the dominant negative mutant of MKK4 decreased T/P-mediated apoptosis, while a dominant-negative mutant of PI-3 kinase enhances cell death. These findings indicate that activation of JNK/SAPK by T/P is related to apoptosis rather than cell proliferation and transformation.

JNK activation is not required for Fas-mediated apoptosis

Fas ligation in the presence of cycloheximide induced Jun N-terminal kinase 1 (JNK1) and JNK2 phosphorylation, caspase activation and cell death in the IL-3-dependent cell line BAF3. Fas-mediated apoptosis was prevented by expression of dominant negative FADD but not inhibited by IL-3. To investigate the role of JNK activation in this process, we examined cells over-expressing a JNK-specific phosphatase M3/6. M3/6 prevented Fas stimulation of JNK, but did not affect Fas-mediated caspase activation or cell death, demonstrating that JNK activation is not required for these processes.

Cytokine-Induced Protein Kinase B Activation and Bad Phosphorylation Do Not Correlate with Cell Survival of Hemopoietic Cells

Activation of phosphoinositide-3 kinases (PI3Ks), their downstream target protein kinase B (PKB), and phosphorylation of Bad have all been implicated in survival signaling in many systems. However, it is not known whether these events are sufficient or necessary to universally prevent apoptosis. To address this issue, we have used three different factor-dependent hemopoietic cell lines, MC/9, BaF/3, and factor-dependent (FD)-6, which respond to a range of cytokines, to investigate the relationship between PI3K, PKB, and Bad activity with survival. The cytokines IL-3, IL-4, stem cell factor (SCF), GM-CSF, and insulin all induced the rapid and transient activation of PKB in responsive cell lines. In all cases, cytokine-induced PKB activation was sensitive to inhibition by the PI3K inhibitor, LY294002. However, dual phosphorylation of the proapoptotic protein Bad was found not to correlate with PKB activation. In addition, we observed cell-type-specific differences in the ability of the same cytokine to induce Bad phosphorylation. Whereas IL-4 induced low levels of dual phosphorylation of Bad in FD-6, it was unable to in MC/9 or BaF/3. Insulin, which was the most potent inducer of PKB in FD-6, induced barely detectable Bad phosphorylation. In addition, the ability of a particular cytokine to induce PKB activity did not correlate with its ability to promote cell survival and/or proliferation. These data demonstrate that, in hemopoietic cells, activation of PKB does not automatically confer a survival signal or result in phosphorylation of Bad, implying that other survival pathways must be involved.

Phosphorylation of serine 256 by protein kinase B disrupts transactivation by FKHR and mediates effects of insulin on insulin-like growth factor-binding protein-1 promoter activity through a conserved insulin response sequence

Insulin inhibits the expression of multiple genes in the liver containing an insulin response sequence (IRS) (CAAAA(C/T)AA), and we have reported that protein kinase B (PKB) mediates this effect of insulin. Genetic studies in Caenorhabditis elegans indicate that daf-16, a forkhead/winged-helix transcription factor, is a major target of the insulin receptor-PKB signaling pathway. FKHR, a human homologue of daf-16, contains three PKB sites and is expressed in the liver. Reporter gene studies in HepG2 hepatoma cells show that FKHR stimulates insulin-like growth factor-binding protein-1 promoter activity through an IRS, and introduction of IRSs confers this effect on a heterologous promoter. Insulin disrupts IRS-dependent transactivation by FKHR, and phosphorylation of Ser-256 by PKB is necessary and sufficient to mediate this effect. Antisense studies indicate that FKHR contributes to basal promoter function and is required to mediate effects of insulin and PKB on promoter activity via an IRS. To our knowledge, these results provide the first report that FKHR stimulates promoter activity through an IRS and that phosphorylation of FKHR by PKB mediates effects of insulin on gene expression. Signaling to FKHR-related forkhead proteins via PKB may provide an evolutionarily conserved mechanism by which insulin and related factors regulate gene expression.

Akt mediates cytoprotection of endothelial cells by vascular endothelial growth factor in an anchorage-dependent manner

Regulation of endothelial cell apoptosis is a critical modulator of normal and pathological angiogenesis. In this study, we examined the role of the protein kinase Akt/PKB in endothelial cell survival in response to growth factor and matrix attachment signals. Vascular endothelial growth factor(VEGF)-induced cytoprotection of endothelial cell monolayers correlated with the wortmannin-sensitive induction of Akt activity. Transfection of an adenovirus expressing a dominant-negative Akt mutant decreased endothelial cell viability in the presence of VEGF. Conversely, adenoviral transduction of wild-type Akt facilitated the cell survival effects of VEGF, whereas transduction of constitutively active Akt conferred endothelial cell survival in the absence of VEGF. Constitutively active Akt also conferred survival to endothelial cells in suspension culture, whereas stimulation with VEGF did not. In suspension cultures, VEGF stimulation was unable to activate Akt, and Akt protein levels were repressed in cells undergoing anoikis. These data suggest that cross-talk between growth factor- and anchorage-dependent signaling pathways are essential for Akt activation and endothelial cell survival.

Regulation and function of protein kinase B and MAP kinase activation by the IL-5/GM-CSF/IL-3 receptor

Interleukin (IL)-3, IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) regulate proliferation, differentiation and apoptosis of target cells. Receptors for these cytokines consist of a cytokine-specific alpha subunit and a common shared beta c subunit. Tyrosine phosphorylation of the beta c is thought to play a critical role in mediating signal transduction events. We have examined the effect of mutation of beta c tyrosines on the activation of multiple signal transduction pathways. Activation of protein kinase B (PKB) required JAK2 and was inhibited by dominant-negative phosphatidylinositol 3-kinase (P13K). Overexpression of JAK2 was sufficient to activate both protein kinase B (PKB) and extracellular regulated kinase-1 (ERK1). Tyrosine 577 and 612 were found to be critical for the activation of PKB and ERK1, but not activation of STAT transcription factors. Activation of both PKB and ERK have been implicated in the regulation of proliferation and apoptosis. We generated GM-CSFR stable cell lines expressing receptor mutants to evaluate their effect on these processes. Activation of both PKB and ERK was perturbed, while STAT activation remained unaffected. Tyrosines 577 and 612 were necessary for optimal proliferation, however, mutation of these tyrosine residues did not affect GM-CSF mediated rescue from apoptosis. These data demonstrate that while phosphorylation of beta c tyrosine residues 577 and 612 are important for optimal cell proliferation, rescue from apoptosis can be mediated by alternative signalling routes apparently independent of PKB or ERK activation.

FcgammaRIIb modulation of surface immunoglobulin-induced Akt activation in murine B cells

We examined activation of the serine/threonine kinase Akt in the murine B cell line A20. Akt is activated in a phosphoinositide 3-kinase (PtdIns 3-kinase)-dependent manner upon stimulation of the antigen receptor, surface immunoglobulin (sIg). In contrast, Akt induction is reduced upon co-clustering of sIg with the B cell IgG receptor, FcgammaRIIb. Co-clustering of sIg-FcgammaRIIb transmits a dominant negative signal and is associated with reduced accumulation of the PtdIns 3-kinase product phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P3), known to be a potent activator of Akt. PtdIns 3-kinase is activated to the same extent with and without FcgammaRIIb co-ligation, indicating conditions supporting the generation of PtdIns 3,4,5-P3. We hypothesized that the decreased Akt activity arises from the consumption of PtdIns 3,4,5-P3 by the inositol-5-phosphatase Src homology 2-containing inositol 5-phosphatase (SHIP), which has been shown by us to be tyrosine-phosphorylated and associated with FcgammaRIIb when the latter is co-ligated. In direct support of this hypothesis, we report here that Akt induction is greatly reduced in fibroblasts expressing catalytically active but not inactive SHIP. Likewise, the reduction in Akt activity upon sIg-FcgammaRIIb co-clustering is absent from avian B cells lacking expression of SHIP. These findings indicate that SHIP acts as a negative regulator of Akt activation.

Mechanism of protein kinase B activation by insulin/insulin-like growth factor-1 revealed by specific inhibitors of phosphoinositide 3-kinase--significance for diabetes and cancer

Protein kinase B (PKB) is a member of the second messenger subfamily of protein kinases. The three isoforms of PKB identified have an amino-terminal pleckstrin homology domain, a central kinase domain, and a carboxy-terminal regulatory domain. PKB is the major downstream target of receptor tyrosine kinases that signal via the phosphoinositide (PI) 3-kinase. The crucial role of lipid second messengers in PKB activation has been dissected through the use of the PI 3-kinase-specific inhibitors wortmannin and LY294002. Receptor-activated PI 3-kinase synthesises the lipid second messenger PI-3,4,5-trisphosphate, leading to the recruitment of PKB to the membrane. Membrane attachment of PKB is mediated by its pleckstrin homology domain binding to PI-3,4,5-trisphosphate or PI-3,4-bisphosphate with high affinity. Activation of PKB alpha and beta is then achieved at the plasma membrane by phosphorylation of Thr308/309 in the A-loop of the kinase domain and Ser473/474 in the carboxy-terminal regulatory region, respectively. The upstream kinase that phosphorylates PKB on Thr308, termed PI-dependent protein kinase-1, has been identified and extensively characterised. A candidate for the Ser473/474 kinase, termed the integrin-linked kinase, has been identified recently. Activated PKB is implicated in glucose metabolism, transcriptional control, and in the regulation of apoptosis in many different cell types. Stimulation of PKB activity protects cells from apoptosis by phosphorylation and inactivation of the pro-apoptotic protein BAD. These results could explain why PKB is overexpressed in some ovarian, breast, and pancreatic carcinomas.

Distinct roles of the phosphatidylinositol 3-kinase and STAT5 pathways in IL-7-mediated development of human thymocyte precursors

Here, we define the IL-7R-activated signal that promotes survival and proliferation of T cell progenitors and demonstrate that it is distinct from the signals that induce differentiation. We show that IL-7 activates PKB and STAT5 in human thymocytes. Into T cell precursors we introduced chimeric receptors with a cytoplasmic domain of the IL-7R that is no longer able to activate PI-3K/PKB and STAT5 and tested the transduced cells in a fetal thymic organ culture. We also examined the T cell precursor activity of progenitors expressing dominant-negative forms of PI-3K or STAT5B. These experiments revealed that PI-3K/PKB activation is essential for the survival and proliferation of T cell precursors and suggest that STAT5 activated by IL-7 mediates T cell differentiation.

IL-3 and IL-4 Activate Cyclic Nucleotide Phosphodiesterases 3 (PDE3) and 4 (PDE4) by Different Mechanisms in FDCP2 Myeloid Cells

In FDCP2 myeloid cells, IL-4 activated cyclic nucleotide phosphodiesterases PDE3 and PDE4, whereas IL-3, granulocyte-macrophage CSF (GM-CSF), and phorbol ester (PMA) selectively activated PDE4. IL-4 (not IL-3 or GM-CSF) induced tyrosine phosphorylation of insulin-receptor substrate-2 (IRS-2) and its association with phosphatidylinositol 3-kinase (PI3-K). TNF-α, AG-490 (Janus kinase inhibitor), and wortmannin (PI3-K inhibitor) inhibited activation of PDE3 and PDE4 by IL-4. TNF-α also blocked IL-4-induced tyrosine phosphorylation of IRS-2, but not of STAT6. AG-490 and wortmannin, not TNF-α, inhibited activation of PDE4 by IL-3. These results suggested that IL-4-induced activation of PDE3 and PDE4 was downstream of IRS-2/PI3-K, not STAT6, and that inhibition of tyrosine phosphorylation of IRS molecules might be one mechnism whereby TNF-α could selectively regulate activities of cytokines that utilized IRS proteins as signal transducers. RO31-7549 (protein kinase C (PKC) inhibitor) inhibited activation of PDE4 by PMA. IL-4, IL-3, and GM-CSF activated mitogen-activated protein (MAP) kinase and protein kinase B via PI3-K signals; PMA activated only MAP kinase via PKC signals. The MAP kinase kinase (MEK-1) inhibitor PD98059 inhibited IL-4-, IL-3-, and PMA-induced activation of MAP kinase and PDE4, but not IL-4-induced activation of PDE3. In FDCP2 cells transfected with constitutively activated MEK, MAP kinase and PDE4, not PDE3, were activated. Thus, in FDCP2 cells, PDE4 can be activated by overlapping MAP kinase-dependent pathways involving PI3-K (IL-4, IL-3, GM-CSF) or PKC (PMA), but selective activation of PDE3 by IL-4 is MAP kinase independent (but perhaps IRS-2/PI3-K dependent).

Ca2+-induced apoptosis through calcineurin dephosphorylation of BAD

The Ca2+-activated protein phosphatase calcineurin induces apoptosis, but the mechanism is unknown. Calcineurin was found to dephosphorylate BAD, a pro-apoptotic member of the Bcl-2 family, thus enhancing BAD heterodimerization with Bcl-xL and promoting apoptosis. The Ca2+-induced dephosphorylation of BAD correlated with its dissociation from 14-3-3 in the cytosol and translocation to mitochondria where Bcl-xL resides. In hippocampal neurons, L-glutamate, an inducer of Ca2+ influx and calcineurin activation, triggered mitochondrial targeting of BAD and apoptosis, which were both suppressible by coexpression of a dominant-inhibitory mutant of calcineurin or pharmacological inhibitors of this phosphatase. Thus, a Ca2+-inducible mechanism for apoptosis induction operates by regulating BAD phosphorylation and localization in cells.

Dissociation of apoptosis from proliferation, protein kinase B activation, and BAD phosphorylation in interleukin-3-mediated phosphoinositide 3-kinase signaling

Interleukin-3 (IL-3) acts as both a growth and survival factor for many hemopoietic cells. IL-3 treatment of responsive cells leads to the rapid and transient activation of Class IA phosphoinositide-3-kinases (PI3Ks) and the serine/threonine kinase Akt/protein kinase B (PKB) and phosphorylation of BAD. Each of these molecules has been implicated in anti-apoptotic signaling in a wide range of cells. Using regulated expression of dominant-negative p85 (Deltap85) in stably transfected IL-3-dependent BaF/3 cells, we have specifically investigated the role of class IA PI3K in IL-3 signaling. The major functional consequence of Deltap85 expression in these cells is a highly reproducible, dramatic reduction in IL-3-induced proliferation. Expression of Deltap85 reduces IL-3-induced PKB phosphorylation and activation and phosphorylation of BAD dramatically, to levels seen in unstimulated cells. Despite these reductions, the levels of apoptosis observed in the same cells are very low and do not account for the reduction in IL-3-dependent proliferation we observe. These results show that Deltap85 inhibits both PKB activity and BAD phosphorylation without significantly affecting levels of apoptosis, suggesting that there are targets other than PKB and BAD that can transmit survival signals in these cells. Our data indicate that the prime target for PI3K action in IL-3 signaling is at the level of regulation of proliferation.

Two distinct interleukin-3-mediated signal pathways, Ras-NFIL3 (E4BP4) and Bcl-xL, regulate the survival of murine pro-B lymphocytes

Hematopoietic cells require cytokine-initiated signals for survival as well as proliferation. The pathways that transduce these signals, ensuring timely regulation of cell fate genes, remain largely undefined. The NFIL3 (E4BP4) transcription factor, Bcl-xL, and constitutively active mutants of components in Ras signal transduction pathways have been identified as key regulation proteins affecting murine interleukin-3 (IL-3)-dependent cell survival. Here we show that expression of NFIL3 is regulated by oncogenic Ras mutants through both the Raf-mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways. NFIL3 inhibits apoptosis without affecting Bcl-xL expression. By contrast, Bcl-xL levels are regulated through the membrane proximal portion in the cytoplasmic domain of the receptor (betac chain), which is shared by IL-3 and granulocyte-macrophage colony-stimulating factor. Activation of either pathway alone is insufficient to ensure cell survival, indicating that multiple independent signal transduction pathways mediate the survival of developing B-lymphoid cells.

Phosphorylation and inactivation of BAD by mitochondria-anchored protein kinase A

Signaling pathways between cell surface receptors and the BCL-2 family of proteins regulate cell death. Survival factors induce the phosphorylation and inactivation of BAD, a proapoptotic member. Purification of BAD kinase(s) identified membrane-based cAMP-dependent protein kinase (PKA) as a BAD Ser-112 (S112) site-specific kinase. PKA-specific inhibitors blocked the IL-3-induced phosphorylation on S112 of endogenous BAD as well as mitochondria-based BAD S112 kinase activity. A blocking peptide that disrupts type II PKA holoenzyme association with A-kinase-anchoring proteins (AKAPs) also inhibited BAD phosphorylation and eliminated the BAD S112 kinase activity at mitochondria. Thus, the anchoring of PKA to mitochondria represents a focused subcellular kinase/substrate interaction that inactivates BAD at its target organelle in response to a survival factor.

Adhesion-mediated signaling in the regulation of mammary epithelial cell survival

Tissue architecture in multicellular organisms is maintained through adhesive interactions between cells and their neighbors, and between cells and the underlying extracellular matrix. These interactions are important in the dynamic regulation of tissue organization as well as the control of cell proliferation, differentiation and apoptosis. The ultimate goal of this regulation is to promote cell growth and differentiation only when the cell is in the correct location, and to delete cells that have become displaced from their proper environment. It therefore plays an important role in development and tissue remodeling. In this review we consider the molecular mechanisms by which cell-matrix interactions contribute to cell survival, and discuss their role in mammary gland development and function.

Akt promotes cell survival by phosphorylating and inhibiting a Forkhead transcription factor

Survival factors can suppress apoptosis in a transcription-independent manner by activating the serine/ threonine kinase Akt, which then phosphorylates and inactivates components of the apoptotic machinery, including BAD and Caspase 9. In this study, we demonstrate that Akt also regulates the activity of FKHRL1, a member of the Forkhead family of transcription factors. In the presence of survival factors, Akt phosphorylates FKHRL1, leading to FKHRL1's association with 14-3-3 proteins and FKHRL1's retention in the cytoplasm. Survival factor withdrawal leads to FKHRL1 dephosphorylation, nuclear translocation, and target gene activation. Within the nucleus, FKHRL1 triggers apoptosis most likely by inducing the expression of genes that are critical for cell death, such as the Fas ligand gene.

Myogenic signaling of phosphatidylinositol 3-kinase requires the serine-threonine kinase Akt/protein kinase B

The oncogene p3k, coding for a constitutively active form of phosphatidylinositol 3-kinase (PI 3-kinase), strongly activates myogenic differentiation. Inhibition of endogenous PI 3-kinase activity with the specific inhibitor LY294002, or with dominant-negative mutants of PI 3-kinase, interferes with myotube formation and with the expression of muscle-specific proteins. Here we demonstrate that a downstream target of PI 3-kinase, serine-threonine kinase Akt, plays an important role in myogenic differentiation. Expression of constitutively active forms of Akt dramatically enhances myotube formation and expression of the muscle-specific proteins MyoD, creatine kinase, myosin heavy chain, and desmin. Transdominant negative forms of Akt inhibit myotube formation and the expression of muscle-specific proteins. The inhibition of myotube formation and the reduced expression of muscle-specific proteins caused by the PI 3-kinase inhibitor LY294002 are completely reversed by constitutively active forms of Akt. Wild-type cellular Akt effects a partial reversal of LY294002-induced inhibition of myogenic differentiation. This result suggests that Akt can substitute for PI 3-kinase in the stimulation of myogenesis; Akt may be an essential downstream component of PI 3-kinase-induced muscle differentiation.

Affinity Modulation of Very Late Antigen-5 Through Phosphatidylinositol 3-Kinase in Mast Cells

Adhesiveness of integrins is up-regulated rapidly by a number of molecules, including growth factors, cytokines, chemokines, and other cell surface receptors, through a mechanism termed inside-out signaling. The inside-out signaling pathways are thought to alter integrin affinity for ligand, or cell surface distribution of integrin by diffusion/clustering. However, it remains to be clarified whether any physiologically relevant agonists induce a rapid change in the affinity of β1 integrins and how ligand-binding affinity is modulated upon stimulation. In this study, we reported that affinity of β1 integrin very late Ag-5 (VLA-5) for fibronectin was rapidly increased in bone marrow-derived mast cells by Ag cross-linking of FcεRI. Ligand-binding affinity of VLA-5 was also augmented by receptor tyrosine kinases when the phospholipase Cγ-1/protein kinase C pathway was inhibited. Wortmannin suppressed induction of the high affinity state VLA-5 in either case. Conversely, introduction of a constitutively active p110 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) increased the binding affinity for fibronectin. Failure of a constitutively active Akt to stimulate adhesion suggested that the affinity modulation mechanisms mediated by PI 3-kinase are distinct from the mechanisms to control growth and apoptosis by PI 3-kinase. Taken together, our findings demonstrated that the increase of affinity of VLA-5 was induced by physiologically relevant stimuli and PI 3-kinase was a critical affinity modulator of VLA-5.

Anti-apoptotic function of Rac in hematopoietic cells

The small GTP-binding protein Rac plays a pivotal role in the regulation of diverse physiological events including reorganization of the actin cytoskeleton, cell cycle progression, and transformation. Here we show an anti-apoptotic effect of Rac in interleukin-3-dependent murine hematopoietic BaF3 cells. Activated Rac(G12V), when ectopically expressed in BaF3 cells, rendered the cells resistant to apoptosis upon interleukin-3 deprivation, while activated mutants of Rho and Cdc42 displayed no significant anti-apoptotic effect. In contrast to activated Ras, which also supports cell survival in the absence of interleukin-3, Rac required fetal bovine serum for the prevention of cell death. The involvement of phosphatidylinositol 3-kinase downstream of Rac was demonstrated by the inhibition of Rac-induced cell survival by wortmannin and LY294002 and the presence of phosphatidylinositol kinase activity in the Rac immunoprecipitate. Furthermore, the serine/threonine kinase Akt was stimulated by activated Rac and fetal bovine serum in a synergistic manner. Rac-induced Akt activation was mediated by phosphorylation of threonine-308 and serine-473. In addition to the phosphatidylinositol 3-kinase/Akt pathway, the p38 mitogen-activated protein kinase pathway was crucial for Rac-dependent survival, whereas p38 mitogen-activated protein kinase nas not implicated in Ras-induced anti-apoptotic signaling. These findings provide evidence for the involvement of Rac in survival signaling of hematopoietic cells.

Cellular protection mechanisms against extracellular heme. heme-hemopexin, but not free heme, activates the N-terminal c-jun kinase

Hemopexin protects cells lacking hemopexin receptors by tightly binding heme abrogating its deleterious effects and preventing nonspecific heme uptake, whereas cells with hemopexin receptors undergo a series of cellular events upon encountering heme-hemopexin. The biochemical responses to heme-hemopexin depend on its extracellular concentration and range from stimulation of cell growth at low levels to cell survival at otherwise toxic levels of heme. High (2-10 microM) but not low (0.01-1 microM) concentrations of heme-hemopexin increase, albeit transiently, the protein carbonyl content of mouse hepatoma (Hepa) cells. This is due to events associated with heme transport since cobalt-protoporphyrin IX-hemopexin, which binds to the receptor and activates signaling pathways without tetrapyrrole transport, does not increase carbonyl content. The N-terminal c-Jun kinase (JNK) is rapidly activated by 2-10 microM heme-hemopexin, yet the increased intracellular heme levels are neither toxic nor apoptotic. After 24 h exposure to 10 microM heme-hemopexin, Hepa cells become refractory to the growth stimulation seen with 0.1-0.75 microM heme-hemopexin but HO-1 remains responsive to induction by heme-hemopexin. Since free heme does not induce JNK, the signaling events, like phosphorylation of c-Jun via activation of JNK as well as the nuclear translocation of NFkappaB, G2/M arrest, and increased expression of p53 and of the cell cycle inhibitor p21(WAF1/CIP1/SDI1) generated by heme-hemopexin appear to be of paramount importance in cellular protection by heme-hemopexin.

TFAR19, a novel apoptosis-related gene cloned from human leukemia cell line TF-1, could enhance apoptosis of some tumor cells induced by growth factor withdrawal

Using the cDNA-representative differences analysis (cDNA-RDA) approach, we identified a novel gene, TFAR19 (TF-1 cell apoptosis related gene-19), from TF-1 cells undergoing apoptosis. The human TFAR19 encodes a protein which shares significant homology to the corresponding proteins of species ranging from yeast to mice. TFAR19 exhibits a ubiquitous expression pattern and its expression is upregulated in the tumor cells undergoing apoptosis. Overexpression of TFAR19 in tumor cells enhances apoptosis triggered by growth factor or serum deprivation. We propose that TFAR19 may play a general role in the apoptotic process.

The PTEN/MMAC1 tumor suppressor phosphatase functions as a negative regulator of the phosphoinositide 3-kinase/Akt pathway

The PTEN/MMAC1 phosphatase is a tumor suppressor gene implicated in a wide range of human cancers. Here we provide biochemical and functional evidence that PTEN/MMAC1 acts a negative regulator of the phosphoinositide 3-kinase (PI3-kinase)/Akt pathway. PTEN/MMAC1 impairs activation of endogenous Akt in cells and inhibits phosphorylation of 4E-BP1, a downstream target of the PI3-kinase/Akt pathway involved in protein translation, whereas a catalytically inactive, dominant negative PTEN/MMAC1 mutant enhances 4E-BP1 phosphorylation. In addition, PTEN/MMAC1 represses gene expression in a manner that is rescued by Akt but not PI3-kinase. Finally, higher levels of Akt activation are observed in human prostate cancer cell lines and xenografts lacking PTEN/MMAC1 expression when compared with PTEN/MMAC1-positive prostate tumors or normal prostate tissue. Because constitutive activation of either PI3-kinase or Akt is known to induce cellular transformation, an increase in the activation of this pathway caused by mutations in PTEN/MMAC1 provides a potential mechanism for its tumor suppressor function.

Role of cytokines and extracellular matrix in the regulation of haemopoietic stem cells

The understanding of molecular mechanisms regulating the formation, growth and differentiation of haemopoietic stem cells has advanced considerably recently. Particular progress has been made in defining the cytokines, chemokines and extracellular matrix components which retain and maintain primitive haemopoietic cell populations in bone marrow. Furthermore, signal transduction pathways that are critical for haemopoiesis, both in vivo and in vitro, and that are activated by cytokines have also been identified and further characterised. The importance of these processes has, this year, been exemplified by the phenotypes of mice deficient in key signal transduction proteins and the discovery that mutations in the component proteins of some signalling pathways are linked to human diseases. Significant advances in understanding the molecular mechanisms for mobilisation of stem cells from bone marrow have also been made this year; this has potential importance for bone marrow transplantation.

An essential role of phosphatidylinositol 3-kinase in myogenic differentiation

The oncogene p3k, coding for a constitutively active form of phosphatidylinositol 3-kinase (PI 3-kinase; EC 2.7.1.137), strongly enhances myogenic differentiation in cultures of chicken-embryo myoblasts. It increases the size of the myotubes and induces elevated levels of the muscle-specific proteins MyoD, myosin heavy chain, creatine kinase, and desmin. Inhibition of PI 3-kinase activity with LY294002 or with dominant-negative mutants of PI 3-kinase interferes with myogenic differentiation and with the induction of muscle-specific genes. PI 3-kinase is therefore an upstream mediator for the expression of the muscle-specific genes and is both necessary and rate-limiting for the process of myogenesis.

Vascular endothelial growth factor regulates endothelial cell survival through the phosphatidylinositol 3'-kinase/Akt signal transduction pathway. Requirement for Flk-1/KDR activation

Vascular endothelial growth factor (VEGF) has been found to have various functions on endothelial cells, the most prominent of which is the induction of proliferation and differentiation. In this report we demonstrate that VEGF or a mutant, selectively binding to the Flk-1/KDR receptor, displayed high levels of survival activity, whereas Flt-1-specific ligands failed to promote survival of serum-starved primary human endothelial cells. This activity was blocked by the phosphatidylinositol 3'-kinase (PI3-kinase)-specific inhibitors wortmannin and LY294002. Endothelial cells cultured in the presence of VEGF and the Flk-1/KDR-selective VEGF mutant induced phosphorylation of the serine-threonine kinase Akt in a PI3-kinase-dependent manner. Akt activation was not detected in response to stimulation with placenta growth factor or an Flt-1-selective VEGF mutant. Furthermore, a constitutively active Akt was sufficient to promote survival of serum-starved endothelial cells in transient transfection experiments. In contrast, overexpression of a dominant-negative form of Akt blocked the survival effect of VEGF. These findings identify the Flk-1/KDR receptor and the PI3-kinase/Akt signal transduction pathway as crucial elements in the processes leading to endothelial cell survival induced by VEGF. Inhibition of apoptosis may represent a major aspect of the regulatory activity of VEGF on the vascular endothelium.

Regulation of cell death protease caspase-9 by phosphorylation

Caspases are intracellular proteases that function as initiators and effectors of apoptosis. The kinase Akt and p21-Ras, an Akt activator, induced phosphorylation of pro-caspase-9 (pro-Casp9) in cells. Cytochrome c-induced proteolytic processing of pro-Casp9 was defective in cytosolic extracts from cells expressing either active Ras or Akt. Akt phosphorylated recombinant Casp9 in vitro on serine-196 and inhibited its protease activity. Mutant pro-Casp9(Ser196Ala) was resistant to Akt-mediated phosphorylation and inhibition in vitro and in cells, resulting in Akt-resistant induction of apoptosis. Thus, caspases can be directly regulated by protein phosphorylation.

Involvement of the pleckstrin homology domain in the insulin-stimulated activation of protein kinase B

Involvement of the pleckstrin homology (PH) domain in the insulin-stimulated activation of protein kinase B (PKB) was investigated in human embryonic kidney 293 cells. Different PKB constructs that contain mutations or deletions in the PH domain were transfected into cells, and the results on the basal and insulin-induced kinase activities were analyzed. Deletion of the entire PH domain (DeltaPH-PKB) did not impair the kinase activity; in contrast, the basal activity was elevated with respect to wild-type PKB. In addition, DeltaPH-PKB was responsive to insulin, and as for wild-type PKB, this was dependent on phosphoinositide 3-kinase. By contrast, a point mutation within the PH domain that impairs phospholipid binding (R25C) resulted in a construct that was not responsive to insulin. However, this defect was overcome by mutations that mimic the phosphorylation state of the active kinase. The increase in the basal activity of DeltaPH-PKB was shown to be due to an elevation in the level of phosphorylation of this construct. In addition, the subcellular localization of DeltaPH-PKB, as determined by both immunofluorescence and fractionation, was predominately cytosolic, and DeltaPH-PKB was present in the plasma membrane at much lower levels compared with wild-type PKB. These data show that phosphorylation is the major factor regulating the activity of PKB and that either removal of the PH domain or binding of phospholipids is required to permit this phosphorylation. In addition, membrane localization does not appear to be required for the activation process, but instead, binding of PKB to membrane phospholipids permits a conformational change in the molecule that allows for phosphorylation.

Phosphorylated Forms of Activated Caspases Are Present in Cytosol From HL-60 Cells During Etoposide-Induced Apoptosis

Treatment of HL-60 human leukemia cells with etoposide induces apoptotic cell death and activation of at least 18 electrophoretically distinct cysteine-dependent aspartate-directed protease (caspase) isoforms, several of which differ only in their isoelectric points. The purpose of the present study was to determine whether activated caspases are phosphorylated. Phosphatase treatment of cytosolic extracts containing active caspases followed by affinity labeling with N-(N-benzyloxycarbonylglutamyl-N-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy] methyl ketone (Z-EK(bio)D-aomk) showed a mobility shift in several of the labeled species, suggesting that phosphorylated forms of these enzymes are present in the extracts. Metabolic labeling with 32P followed by etoposide treatment and subsequent affinity purification of affinity-labeled caspases confirmed that at least three caspase species were phosphorylated. To detect effects of the phosphorylation on enzymatic activity, caspase-mediated cleavage of aspartylglutamylvalinylaspartyl-7-amino-4-trifluoromethylcoumarin (DEVD-AFC) and poly(ADP-ribose) polymerase (PARP) by phosphorylated and dephosphorylated extracts was measured. No significant changes in Km or vmax were detected using DEVD-AFC. In contrast, a slight, but significant enhancement of PARP cleavage was observed in dephosphorylated extracts, suggesting that phosphorylation of active caspases could have an inhibitory effect on enzyme activity. These observations, which provide the first evidence that caspases are phosphoproteins, suggest that caspases may be targets for some of the growing list of protein kinases that are involved in apoptotic events.

© 1998 by The American Society of Hematology.

Emerging roles for SH2/PTB-containing Shc adaptor proteins in the developing mammalian brain

In mammalian systems, SH2-containing cytoplasmic signalling molecules are known to play an important role in determining cell responsiveness to the environment. In particular, following activation of a receptor protein tyrosine kinase (RPTK), proteins like Shc and Grb2 bind to phosphotyrosine residues of stimulated receptors, thereby activating downstream components of specific signalling pathways. The ShcA gene was identified in 1992 and was found to encode three proteins with properties of adaptor molecules coupling RPTKs to Ras. Early data obtained in non-neuronal cells have revealed that Shc and Grb2 proteins are highly expressed and activated in all cells. However, recent analyses of ShcA mRNA and protein in the developing brain revealed progressive downregulation of their expression during differentiation from neuroblasts to neurons. Conversely, the two newly identified Shc homologues (ShcB/Sli and ShcC/Rai) are highly expressed in the mature brain.Thus, variations in the intracellular levels of adaptor proteins might represent one of the mechanisms by which a differentiating cell changes its ability to respond to a given factor, allowing a cell to choose between proliferation and differentiation.

Role of Akt and c-Jun N-terminal kinase 2 in apoptosis induced by interleukin-4 deprivation

We have shown previously that interleukin-4 (IL-4) protects TS1alphabeta cells from apoptosis, but very little is known about the mechanism by which IL-4 exerts this effect. We found that Akt activity, which is dependent on phosphatidylinositol 3 kinase, is reduced in IL-4-deprived TS1alphabeta cells. Overexpression of wild-type Akt or a constitutively active Akt mutant protects cells from IL-4 deprivation-induced apoptosis. Readdition of IL-4 before the commitment point is able to restore Akt activity. We also show expression and c-Jun N-terminal kinase 2 activation after IL-4 deprivation. Overexpression of the constitutively activated Akt mutant in IL-4-deprived cells correlates with inhibition of c-Jun N-terminal kinase 2 activity. Finally, TS1alphabeta survival is independent of Bcl-2, Bcl-x, or Bax.

Phosphorylated Forms of Activated Caspases Are Present in Cytosol From HL-60 Cells During Etoposide-Induced Apoptosis

Treatment of HL-60 human leukemia cells with etoposide induces apoptotic cell death and activation of at least 18 electrophoretically distinct cysteine-dependent aspartate-directed protease (caspase) isoforms, several of which differ only in their isoelectric points. The purpose of the present study was to determine whether activated caspases are phosphorylated. Phosphatase treatment of cytosolic extracts containing active caspases followed by affinity labeling with N-(N-benzyloxycarbonylglutamyl-N-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy] methyl ketone (Z-EK(bio)D-aomk) showed a mobility shift in several of the labeled species, suggesting that phosphorylated forms of these enzymes are present in the extracts. Metabolic labeling with 32P followed by etoposide treatment and subsequent affinity purification of affinity-labeled caspases confirmed that at least three caspase species were phosphorylated. To detect effects of the phosphorylation on enzymatic activity, caspase-mediated cleavage of aspartylglutamylvalinylaspartyl-7-amino-4-trifluoromethylcoumarin (DEVD-AFC) and poly(ADP-ribose) polymerase (PARP) by phosphorylated and dephosphorylated extracts was measured. No significant changes in Km or vmax were detected using DEVD-AFC. In contrast, a slight, but significant enhancement of PARP cleavage was observed in dephosphorylated extracts, suggesting that phosphorylation of active caspases could have an inhibitory effect on enzyme activity. These observations, which provide the first evidence that caspases are phosphoproteins, suggest that caspases may be targets for some of the growing list of protein kinases that are involved in apoptotic events.

© 1998 by The American Society of Hematology.

Activation of protein kinase B/Akt is sufficient to repress the glucocorticoid and cAMP induction of phosphoenolpyruvate carboxykinase gene

A rat hepatoma cell line, H4IIE, was stably transfected with a tamoxifen regulatable Akt-1 construct. Treatment of these cells with tamoxifen caused a rapid stimulation of Akt enzymatic activity that was comparable with the activity observed with the endogenous Akt after insulin stimulation. Prior studies have extensively documented that insulin can repress the glucocorticoid and cAMP-stimulated increase in phosphoenolpyruvate carboxykinase (PEPCK) gene transcription. Activation of this regulatable Akt with tamoxifen was found to mimic the dominant inhibitory effect of insulin on PEPCK gene transcription. Dose response curves to insulin and tamoxifen demonstrated that this response was very sensitive to Akt activation although the maximal response observed with tamoxifen activation was slightly less than that observed with insulin, indicating that the response to insulin may also involve other signaling cascades. The regulation of PEPCK transcription via Akt was, like that previously described for insulin, not dependent upon 70 kDa S6 kinase activity in that it was not inhibited by rapamycin. Finally, the expression of a kinase dead Akt was able to partially inhibit the ability of insulin to stimulate this response. In summary, the present results indicate that activation of Akt alone is sufficient to repress the glucocorticoid and cAMP-stimulated increase in PEPCK gene transcription.

Biology of the interleukin-2 receptor

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Regulation of proliferation, differentiation and survival by the IL-3/IL-5/GM-CSF receptor family

The receptors for the I1-3/IL-5/GM-CSF cytokine family are composed of a heterodimeric complex of a cytokine-specific alpha chain and a common beta chain (betac). Binding of IL-3/IL-5/GM-CSF to their respective receptors rapidly induces activation of multiple intracellular signalling pathways, including the Ras-Raf-ERK, the JAK/STAT, the phosphatidylinositol 3-kinase PKB, and the JNK/SAPK and p38 signalling pathways. This review focuses on recent advancements in understanding how these different signalling pathways are activated by IL-3/IL-5/GM-CSF receptors, and how the individual pathways contribute to the pleiotropic effects of IL-3/IL-5/GM-CSF on their target cells, including proliferation, differentiation, survival, and effector functions.

Protein kinase B (c-Akt): a multifunctional mediator of phosphatidylinositol 3-kinase activation

While a plethora of extracellular molecules exist that modulate cellular functions via binding to membrane receptors inside the cell, their actions are mediated by relatively few signalling mechanisms. One of these is activation of phosphatidylinositol 3-kinase (PI-3K), which results in the generation of a membrane-restricted second messenger, polyphosphatidylinositides containing a 3'-phosphate. How these molecules transduced the effects of agonists of PI-3K was unclear until the recent discovery that several protein kinases become activated upon exposure to 3'-phosphorylated inositol lipids. These enzymes include protein kinase B (PKB)/AKT and PtdIns(3,4, 5)P3-dependent kinases 1 and 2, the first two of which interact with 3'-phosphorylated phosphoinositides via pleckstrin homology domains. Once targeted to the membrane by this motif, PKB becomes phosphorylated at two residues, which relieves intermolecular inhibition, allowing the activated complex to dissociate and modify its targets. Identification of these substrates is the subject of intensive research, since at least one must play a key role in suppressing apoptosis, as demonstrated by expression of activated alleles of PKB. The generation of effective transdominant mutants, coupled with genetic analysis of the protein kinase in simpler organisms, should help in elucidating outstanding questions in the functions, targets and regulation of this important mediator of PI-3K signalling.

Linking extracellular survival signals and the apoptotic machinery

The survival of cells in multicellular organisms requires continuous stimulation from the extracellular environment. The phosphatidylinositol-3' kinase/Akt signaling cascade has been identified as a critical pathway for the transduction of extracellular survival signals. The finding that the pro-apoptotic protein BAD is a substrate of Akt/PKB has provided the first link between extracellular survival signals and the apoptotic machinery.

Selective regulation of Bcl-XL by a Jak kinase-dependent pathway is bypassed in murine hematopoietic malignancies

Bcl-2 family proteins are key regulators of apoptosis and function as cell death antagonists (e.g., Bcl-2, Bcl-XL, and Mcl-1) or agonists (e.g., Bax, Bad, and Bak). Here we report that among the Bcl-2 family of proteins tested (Bcl-2, Bcl-XL, Mcl-1, Bax, Bad, and Bak), Bcl-XL was unique in that its protein levels were tightly regulated by hemopoietins in both immortal and primary myeloid progenitors. Investigating signaling pathways utilized by cytokine receptors established that the regulation of Bcl-XL protein levels is mediated by the Jak kinase pathway and is independent of other signaling effectors including STATs, PI-3' kinase, and Ras. Moreover, we provide the first direct evidence that Bcl-X is altered in cancer, because bcl-X expression was activated selectively by retroviral insertions in murine myeloid and T-cell hemopoietic malignancies. Tumors harboring bcl-X insertions had altered bcl-X RNAs, expressed elevated levels of Bcl-XL protein, and lacked the requirements for cytokines normally essential for cell survival. Finally, overexpression of Bcl-XL effectively protected IL-3-dependent myeloid cells from apoptosis following removal of trophic factors. Therefore, Bcl-XL functions as a key cytokine regulated anti-apoptotic protein in myelopoiesis and contributes to leukemia cell survival.

The Hyperresponsiveness of Cells Expressing Truncated Erythropoietin Receptors Is Contingent on Insulin-Like Growth Factor-1 in Fetal Calf Serum

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70S6K. Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.

The Hyperresponsiveness of Cells Expressing Truncated Erythropoietin Receptors Is Contingent on Insulin-Like Growth Factor-1 in Fetal Calf Serum

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70S6K. Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.

Dissociation of cytokine-induced phosphorylation of Bad and activation of PKB/akt: involvement of MEK upstream of Bad phosphorylation

The phosphatidylinositol 3-kinase (PI3K)-signaling pathway has emerged as an important component of cytokine-mediated survival of hemopoietic cells. Recently, the protein kinase PKB/akt (referred to here as PKB) has been identified as a downstream target of PI3K necessary for survival. PKB has also been implicated in the phosphorylation of Bad, potentially linking the survival effects of cytokines with the Bcl-2 family. We have shown that granulocyte/macrophage colony-stimulating factor (GM-CSF) maintains survival in the absence of PI3K activity, and we now show that when PKB activation is also completely blocked, GM-CSF is still able to stimulate phosphorylation of Bad. Interleukin 3 (IL-3), on the other hand, requires PI3K for survival, and blocking PI3K partially inhibited Bad phosphorylation. IL-4, unique among the cytokines in that it lacks the ability to activate the p21ras-mitogen-activated protein kinase (MAPK) cascade, was found to activate PKB and promote cell survival, but it did not stimulate Bad phosphorylation. Finally, although our data suggest that the MAPK pathway is not required for inhibition of apoptosis, we provide evidence that phosphorylation of Bad may be occurring via a MAPK/ERK kinase (MEK)-dependent pathway. Together, these results demonstrate that although PI3K may contribute to phosphorylation of Bad in some instances, there is at least one other PI3K-independent pathway involved, possibly via activation of MEK. Our data also suggest that although phosphorylation of Bad may be one means by which cytokines can inhibit apoptosis, it may be neither sufficient nor necessary for the survival effect.

Cyclic adenosine monophosphate-mediated protection against bile acid-induced apoptosis in cultured rat hepatocytes

Cyclic adenosine monophosphate (cAMP) has been shown to modulate apoptosis. To evaluate the role of cAMP in bile acid-induced hepatocyte apoptosis, we studied the effect of agents that increase cAMP on the induction of apoptosis by glycochenodeoxycholate (GCDC) in cultured rat hepatocytes. GCDC induced apoptosis in 26.5%+/-1.1% of hepatocytes within 2 hours. Twenty-minute pretreatment of hepatocytes with 100 micromol/L 8-(4-chlorothiophenyl) cAMP (CP-cAMP) resulted in a reduction in the amount of apoptosis to 35.2%+/-3.8% of that seen in hepatocytes treated with GCDC alone. Other agents that increase intracellular cAMP, including dibutyryl cAMP (100 micromol/L), glucagon (200 nmol/L), and a combination of forskolin (20 micromol/L) and 3-isobutyl-1-methylxanthine (20 micromol/L), also inhibited GCDC-induced apoptosis to a similar extent. Pretreatment with the protein kinase A (PKA) inhibitor, KT5720, prevented the protective effect of CP-cAMP and inhibited CP-cAMP-induced activation of PKA activity. Inhibitors of phosphatidylinositol 3-kinase (PI3K), wortmannin (50 nmol/L), or Ly 294002 (20 micromol/L) also prevented the cytoprotective effect of cAMP. PI3K assays confirmed that wortmannin (50 nmol/L) inhibited PI3K activity, while CP-cAMP had no effect on the activity of this lipid kinase. GCDC increased mitogen-activated protein kinase (MAPK) activity, but had no effect on stress-activated protein kinase (SAPK) activity in hepatocytes. cAMP decreased basal and GCDC-induced MAPK activity and increased SAPK activity. The MAPK kinase inhibitor, PD 98059, inhibited both GCDC-mediated MAPK activation and GCDC-induced apoptosis.

IN CONCLUSION

1) agents that increase intracellular cAMP protect against hepatocyte apoptosis induced by hydrophobic bile acids; 2) activation of MAPK by GCDC may be involved in bile acid-induced apoptosis; and 3) cAMP-mediated cytoprotection against bile acid-induced apoptosis appears to involve PKA, MAPK, and PI3K.

Mechanisms and consequences of activation of protein kinase B/Akt

Protein kinase B (PKB)/Akt is a growth-factor-regulated serine/threonine kinase which contains a pleckstrin homology domain. Binding of phosphoinositide 3-OH kinase products to the pleckstrin homology domain results in translocation of PKB/Akt to the plasma membrane where it is activated by phosphorylation by upstream kinases including the phosphoinoside-dependent kinase 1 (PDK1). Activated PKB/Akt provides a survival signal that protects cells from apoptosis induced by various stresses, and also mediates a number of metabolic effects of insulin.

A requirement for NF-kappaB activation in Bcr-Abl-mediated transformation

Bcr-Abl is a chimeric oncoprotein that is strongly implicated in acute lymphoblastic (ALL) and chronic myelogenous leukemias (CML). This deregulated tyrosine kinase selectively causes hematopoietic disorders resembling human leukemias in animal models and transforms fibroblasts and hematopoietic cells in culture. Bcr-Abl also protects cells from death induced on cytokine deprivation or exposure to DNA damaging agents. In addition, the antiapoptotic function of Bcr-Abl is thought to play a necessary role in hematopoietic transformation and potentially in leukemogenesis. The transcription factor NF-kappaB has been identified recently as an inhibitor of apoptosis and as a potential regulator of cellular transformation. This study shows that expression of Bcr-Abl leads to activation of NF-kappaB-dependent transcription by causing nuclear translocation of NF-kappaB as well as by increasing the transactivation function of the RelA/p65 subunit of NF-kappaB. Importantly, this activation is dependent on the tyrosine kinase activity of Bcr-Abl and partially requires Ras. The ability of Bcr-Abl to protect cytokine-dependent 32D myeloid cells from death induced by cytokine deprivation or DNA damage does not, however, require functional NF-kappaB. However, using a super-repressor form of IkappaBalpha, we show that NF-kappaB is required for Bcr-Abl-mediated tumorigenicity in nude mice and for transformation of primary bone marrow cells. This study implicates NF-kappaB as an important component of Bcr-Abl signaling. NF-kappaB-regulated genes, therefore, likely play a role in transformation by Bcr-Abl and thus in Bcr-Abl-associated human leukemias.

Protein kinase B/Akt mediates effects of insulin on hepatic insulin-like growth factor-binding protein-1 gene expression through a conserved insulin response sequence

Insulin regulates the expression of multiple hepatic genes through a conserved insulin response sequence (IRS) (CAAAAC/TAA) by an as yet undetermined mechanism. Protein kinase B/Akt (PKB/Akt), a member of the PKA/PKC serine/threonine kinase family, functions downstream from phosphatidylinositol 3'-kinase (PI3K) in mediating effects of insulin on glucose transport and glycogen synthesis. We asked whether PKB/Akt mediates sequence-specific effects of insulin on hepatic gene expression using the model of the insulin-like growth factor binding protein-1 (IGFBP-1) promoter. Insulin lowers IGFBP-1 mRNA levels, inhibits IGFBP-1 promoter activity, and activates PKB/Akt in HepG2 hepatoma cells through a PI3K-dependent, rapamycin-insensitive mechanism. Constitutively active PI3K and PKB/Akt are each sufficient to mediate effects of insulin on the IGFBP-1 promoter in a nonadditive fashion. Dominant negative K179 PKB/Akt disrupts the ability of insulin and PI3K to activate PKB/Akt and to inhibit promoter activity. The IGFBP-1 promoter contains two IRSs each of which is sufficient to mediate sequence-specific effects of insulin, PI3K, and PKB/Akt on promoter activity. Highly related IRSs from the phosphoenolpyruvate carboxykinase and apolipoprotein CIII genes also are effective in this setting. These results indicate that PKB/Akt functions downstream from PI3K in mediating sequence-specific effects of insulin on the expression of IGFBP-1 and perhaps multiple hepatic genes through a conserved IRS.