CREB is a regulatory target for the protein kinase Akt/PKB

The regulation and activities of the multifunctional serine/threonine kinase Akt/PKB

The serine/threonine kinase Akt, or protein kinase B (PKB), has recently been a focus of intense research. It appears that Akt/PKB lies in the crossroads of multiple cellular signaling pathways and acts as a transducer of many functions initiated by growth factor receptors that activate phosphatidylinositol 3-kinase (PI 3-kinase). Akt/PKB is particularly important in mediating several metabolic actions of insulin. Another major activity of Akt/PKB is to mediate cell survival. In addition, the recent discovery of the tumor suppressor PTEN as an antagonist of PI 3-kinase and Akt/PKB kinase activity suggests that Akt/PKB is a critical factor in the genesis of cancer. Thus, elucidation of the mechanisms of Akt/PKB regulation and its physiological functions should be important for the understanding of cellular metabolism, apoptosis, and cancer.

Syk and Bruton's tyrosine kinase are required for B cell antigen receptor-mediated activation of the kinase Akt

Activation of Akt by multiple stimuli including B cell antigen receptor (BCR) engagement requires phosphatidylinositol 3-kinase and regulates processes including cell survival, proliferation, and metabolism. BCR cross-linking activates three families of non-receptor protein tyrosine kinases (PTKs) and these are transducers of signaling events including phospholipase C and mitogen-activated protein kinase activation; however, the relative roles of PTKs in BCR-mediated Akt activation are unknown. We examined Akt activation in Lyn-, Syk- and Btk-deficient DT40 cells and B cells from Lyn(-/-) mice. BCR-mediated Akt activation required Syk and was partially dependent upon Btk. Increased BCR-induced Akt phosphorylation was observed in Lyn-deficient DT40 cells and Lyn(-/-) mice compared with wild-type cells suggesting that Lyn may negatively regulate Akt function. BCR-induced tyrosine phosphorylation of the phosphatidylinositol 3-kinase catalytic subunit was abolished in Syk-deficient cells consistent with a receptor-proximal role for Syk in BCR-mediated phosphatidylinositol 3-kinase activation; in contrast, it was maintained in Btk-deficient cells, suggesting Btk functions downstream of phosphatidylinositol 3-kinase. Calcium depletion did not influence BCR-induced Akt phosphorylation/activation, showing that neither Syk nor Btk mediates its effects via changes in calcium levels. Thus, BCR-mediated Akt stimulation is regulated by multiple non-receptor PTK families which regulate Akt both proximal and distal to phosphatidylinositol 3-kinase activation.

Insulin-like growth factor-I induces bcl-2 promoter through the transcription factor cAMP-response element-binding protein

Insulin-like growth factor-I (IGF-I) is known to prevent apoptosis induced by diverse stimuli. The present study examined the effect of IGF-I on the promoter activity of bcl-2, a gene with antiapoptotic function. A luciferase reporter driven by the promoter region of bcl-2 from -1640 to -1287 base pairs upstream of the translation start site containing a cAMP-response element was used in transient transfection assays. Treatment of PC12 cells with IGF-I enhanced the bcl-2 promoter activity by 2.3-fold, which was inhibited significantly (p < 0.01) by SB203580, an inhibitor of p38 mitogen-activated protein kinase (MAPK). Cotransfection of the bcl-2 promoter with MAPK kinase 6 and the beta isozyme of p38 MAPK resulted in 2-3-fold increase in the reporter activity. The dominant negative form of MAPKAP-K3, a downstream kinase activated by p38 MAPK, and the dominant negative form of cAMP-response element-binding protein, inhibited the reporter gene activation by IGF-I and p38beta MAPK significantly (p < 0.01). IGF-I increased the activity of p38beta MAPK introduced into the cells by adenoviral infection. Thus, we have characterized a novel signaling pathway (MAPK kinase 6/p38beta MAPK/MAPKAP-K3) that defines a transcriptional mechanism for the induction of the antiapoptotic protein Bcl-2 by IGF-I through the nuclear transcription factor cAMP-response element-binding protein in PC12 cells.

The antiapoptotic gene mcl-1 is up-regulated by the phosphatidylinositol 3-kinase/Akt signaling pathway through a transcription factor complex containing CREB

mcl-1 is an immediate-early gene activated by the granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 3 (IL-3) signaling pathways and plays an important role in the viability response of these cytokines. In this study, we demonstrated that cytokine stimulation of mcl-1 mRNA and protein expression were attenuated by pretreatment of cells with phosphatidylinositol 3-kinase (PI3-K) inhibitors. Reporter gene assays further showed that the PI3-K/Akt signaling pathway was involved in IL-3 activation of mcl-1 gene transcription. Analysis of the mcl-1 promoter revealed that both promoter elements, SIE at position -87 and CRE-2 at -70, contribute to IL-3 stimulation of mcl-1 gene expression. Although either the SIE site or the CRE-2 site alone was sufficient to confer IL-3 inducibility on a heterologous promoter, only IL-3 activation of the CRE-2 reporter was mediated via the PI3-K/Akt pathway. The SIE binding activity was constitutively high in cells deprived of or stimulated by IL-3. In contrast, the CRE-2 binding activity was low in cytokine-starved cells and was strongly induced within 1 h following cytokine treatment of cells. In addition, cytokine induction of the CRE-2 but not of the SIE binding activity was dependent on activation of the PI3-K/Akt signaling pathway. Lastly, we showed that CREB was one component of the CRE-2 binding complex and played a role in IL-3 activation of the mcl-1 reporter gene. Taken together, our results suggest that both PI3-K/Akt-dependent and -independent pathways contribute to the IL-3 activation of mcl-1 gene expression. Activation of mcl-1 by the PI3-K/Akt-dependent pathway is through a transcription factor complex containing CREB.

Characterization of Fractalkine in Rat Brain Cells: Migratory and Activation Signals for CX3CR-1-Expressing Microglia

Molecular analyses of the chemokine fractalkine and its receptor CX3C-R1 in the rat brain have revealed a striking polarization: fractalkine is expressed constitutively in neurons and is up-regulated by TNF-α and IL-1β in astrocytes. Expression of its specific receptor, CX3C-R1, is restricted to astrocytes and microglia. We have analyzed the functional correlates of this expression and demonstrate that fractalkine induces microglial cell migration and activation. However, the activity of this chemokine on astrocytes may also be highly relevant in inducing astrocyte-microglia cell interactions through cytokine/mediator release leading to microglial activation.

Adenovirus-directed expression of a nonphosphorylatable mutant of CREB (cAMP response element-binding protein) adversely affects the survival, but not the differentiation, of rat granulosa cells

Although usually considered to be a constitutively expressed protein, in the primate ovary the expression of CREB (cAMP response element-binding protein) is extinguished after ovulation, and its loss is temporally associated with the cessation of proliferation of luteal cells and the ultimate commitment of the corpus luteum to undergo regression. To determine the cellular consequences of the loss of CREB expression, we expressed a nonphosphorylatable mutant of CREB (CREB M1) in primary cultures of rat granulosa cells using a replication-defective adenovirus vector. Expression of CREB M1 did not block granulosa cell differentiation as assessed by acquisition of the ability to produce estrogen and progesterone in response to FSH or forskolin. However, granulosa cells expressing CREB M1, but not adenovirus-directed beta-galactosidase or enhanced green fluorescent protein, exhibited a 35% reduction in viability that was further reduced to 65% after stimulation with 10 microM forskolin. These results demonstrate that the trophic effects of cAMP (proliferation and survival) on ovarian granulosa cells are functionally separate from the effects of cAMP on differentiation and provide novel evidence that CREB may function as a cell survival factor in the ovary. The separation of signaling pathways that govern differentiation and survival in the ovary thereby provides a mechanism by which progesterone production, which is absolutely essential for the maintenance of pregnancy, can continue despite the cessation of proliferation of luteal cells and their commitment to cell death (luteolysis).

Lithium activates the serine/threonine kinase Akt-1 and suppresses glutamate-induced inhibition of Akt-1 activity in neurons

This report describes a modulatory action of lithium and glutamate on the activity of serine/threonine kinase Akt-1. Lithium is most commonly used to treat bipolar disorder, but the mechanism of its therapeutic action remains unknown. We have recently demonstrated that lithium protects against glutamate-induced excitotoxicity in cultured brain neurons and in an animal model of cerebral ischemia. This study was undertaken to investigate the role of Akt-1, activated by the phosphatidylinositol 3-kinase (PI 3-K) signaling pathway, in mediating glutamate excitotoxicity and lithium protection in cerebellar granule cells. High levels of phosphorylation and activity of Akt-1 were detected in cerebellar neurons cultured in the presence of serum. Protracted treatment with selective PI 3-K inhibitors, wortmannin and LY294002, abolished Akt-1 activity and induced neuronal death that could be reduced by long-term lithium pretreatment. Exposure of cells to glutamate induced a rapid and reversible loss of Akt-1 phosphorylation and kinase activity. These effects were closely correlated with excitotoxicity and caspase 3 activation and were prevented by phosphatase inhibitors, okadaic acid and caliculin A. Long-term lithium pretreatment suppressed glutamate-induced loss of Akt-1 activity and accelerated its recovery toward the control levels. Lithium treatment alone induced rapid increase in PI 3-K activity, and Akt-1 phosphorylation with accompanying kinase activation, which was blocked by PI 3-K inhibitors. Lithium also increased the phosphorylation of glycogen synthase kinase-3 (GSK-3), a downstream physiological target of Akt. Thus, modulation of Akt-1 activity appears to play a key role in the mechanism of glutamate excitotoxicity and lithium neuroprotection.

Regulation of GLUT1 gene transcription by the serine/threonine kinase Akt1

We used mouse hepatoma (Hepa1c1c7) cells to study the role of the serine/threonine kinase Akt in the induction of GLUT1 gene expression. In order to selectively turn on the Akt kinase cascade, we expressed a hydroxytamoxifen-regulatable form of Akt (myristoylated Akt1 estrogen receptor chimera (MER-Akt1)) in the Hepa1c1c7 cells; we verified that hydroxytamoxifen stimulates MER-Akt1 activity to a similar extent as the activation of endogenous Akt by insulin. Our studies reveal that stimulation of MER-Akt1 by hydroxytamoxifen induces GLUT1 mRNA and protein accumulation to levels comparable to that induced by insulin; therefore, activation of the Akt cascade suffices to induce GLUT1 gene expression in this cell system. Furthermore, expression of a kinase-inactive Akt mutant partially inhibits the response of the GLUT1 gene to insulin. Additional studies reveal that the induction of GLUT1 mRNA by Akt and by insulin reflects increased mRNA synthesis and not decreased mRNA degradation. Our findings imply that the GLUT1 gene responds to insulin at the transcriptional level and that Akt mediates a step in the activation of GLUT1 gene expression in this system.

Activation of the protein kinase Akt/PKB by the formation of E-cadherin-mediated cell-cell junctions. Evidence for the association of phosphatidylinositol 3-kinase with the E-cadherin adhesion complex

E-cadherins are surface adhesion molecules localized at the level of adherens junctions, which play a major role in cell adhesiveness by mediating calcium-dependent homophylic interactions at sites of cell-cell contacts. Recently, E-cadherins have been also implicated in a number of biological processes, including cell growth and differentiation, cell recognition, and sorting during developmental morphogenesis, as well as in aggregation-dependent cell survival. As phosphatidylinositol (PI) 3-kinase and Akt play a critical role in survival pathways in response to both growth factors and extracellular stimuli, these observations prompted us to explore whether E-cadherins could affect intracellular molecules regulating the activity of the PI 3-kinase/Akt signaling cascade. Using Madin-Darby canine kidney cells as a model system, we show here that engagement of E-cadherins in homophylic calcium-dependent cell-cell interactions results in a rapid PI 3-kinase-dependent activation of Akt and the subsequent translocation of Akt to the nucleus. Moreover, we demonstrate that the activation of PI 3-kinase in response to cell-cell contact formation involves the phosphorylation of PI 3-kinase in tyrosine residues, and the concomitant recruitment of PI 3-kinase to E-cadherin-containing protein complexes. These findings indicate that E-cadherins can initiate outside-in signal transducing pathways that regulate the activity of PI 3-kinase and Akt, thus providing a novel molecular mechanism whereby the interaction among neighboring cells and their adhesion status may ultimately control the fate of epithelial cells.

Induction of NF-kappaB by the Akt/PKB kinase

The serine/threonine kinase Akt (also known as protein kinase B, PKB) is activated by numerous growth-factor and immune receptors through lipid products of phosphatidylinositol (PI) 3-kinase. Akt can couple to pathways that regulate glucose metabolism or cell survival [1]. Akt can also regulate several transcription factors, including E2F, CREB, and the Forkhead family member Daf-16 [2] [3] [4]. Here, we show that Akt can regulate signaling pathways that lead to induction of the NF-kappaB family of transcription factors in the Jurkat T-cell line. This induction occurs, at least in part, at the level of degradation of the NF-kappaB inhibitor IkappaB, and is specific for NF-kappaB, as other inducible transcription factors are not affected by Akt overexpression. Furthermore, the effect requires the kinase activity and pleckstrin homology (PH) domain of Akt. Also, Akt does not act alone to induce cytokine promoters and NF-kappaB reporters, because signals from other pathways are required to observe the effect. These studies uncover a previously unappreciated connection between Akt and NF-kappaB induction that could have implications for the control of T-cell growth and survival.

Role and regulation of 90 kDa ribosomal S6 kinase (RSK) in signal transduction

Extracellular signals activate mitogen-activated protein kinase (MAPK) cascades to execute complex cellular programs, like proliferation, differentiation and apoptosis. In mammalian cells, three MAPK families have been characterized: extracellular signal-regulated kinase (ERK), which is activated by growth factors, peptide hormones and neurotransmitters, and Jun kinase (JNK) and p38 MAPK, which are activated by cellular stress stimulus as well as growth factors. This review describes the family of 90 kDa ribosomal S6 kinases (RSK; also known as p90rsk or MAPK-activated protein kinase-1, MAPKAP-K1), which were among the first substrates of ERK to be discovered and which has proven to be a ubiquitous and versatile mediator of ERK signal transduction. RSK is composed of two functional kinase domains that are activated in a sequential manner by a series of phosphorylations. Recently, a family of RSK-related kinases that are activated by ERK as well as p38 MAPK were discovered and named mitogen- and stress-activated protein kinases (MSK). A number of cellular functions of RSK have been proposed. (1) Regulation of gene expression via association and phosphorylation of transcriptional regulators including c-Fos, estrogen receptor, NFkappaB/IkappaB alpha, cAMP-response element-binding protein (CREB) and CREB-binding protein; (2) RSK is implicated in cell cycle regulation in Xenopus laevis oocytes by inactivation of the Myt1 protein kinase leading to activation of the cyclin-dependent kinase p34cdc2; (3) RSK may regulate protein synthesis by phosphorylation of polyribosomal proteins and glycogen synthase kinase-3; and (4) RSK phosphorylates the Ras GTP/GDP-exchange factor, Sos leading to feedback inhibition of the Ras-ERK pathway.