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

The protein kinase B/Akt signalling pathway in human malignancy

Protein kinase B or Akt (PKB/Akt) is a serine/threonine kinase, which in mammals comprises three highly homologous members known as PKBalpha (Akt1), PKBbeta (Akt2), and PKBgamma (Akt3). PKB/Akt is activated in cells exposed to diverse stimuli such as hormones, growth factors, and extracellular matrix components. The activation mechanism remains to be fully characterised but occurs downstream of phosphoinositide 3-kinase (PI-3K). PI-3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), a lipid second messenger essential for the translocation of PKB/Akt to the plasma membrane where it is phosphorylated and activated by phosphoinositide-dependent kinase-1 (PDK-1) and possibly other kinases. PKB/Akt phosphorylates and regulates the function of many cellular proteins involved in processes that include metabolism, apoptosis, and proliferation. Recent evidence indicates that PKB/Akt is frequently constitutively active in many types of human cancer. Constitutive PKB/Akt activation can occur due to amplification of PKB/Akt genes or as a result of mutations in components of the signalling pathway that activates PKB/Akt. Although the mechanisms have not yet been fully characterised, constitutive PKB/Akt signalling is believed to promote proliferation and increased cell survival and thereby contributing to cancer progression. This review surveys recent developments in understanding the mechanisms and consequences of PKB/Akt activation in human malignancy.

Two novel phosphorylation sites on FKHR that are critical for its nuclear exclusion

FKHR is phosphorylated by protein kinase B (PKB) at Thr24, Ser256 and Ser319 in response to growth factors, stimulating the nuclear exit and inactivation of this transcription factor. Here we identify two further residues, Ser322 and Ser325, that become phosphorylated in insulin-like growth factor-1 (IGF-1)-stimulated cells and which are mediated by the phosphatidylinositol 3-kinase-dependent PKB-catalysed phosphorylation of Ser319. Phosphorylation of Ser319 forms a consensus sequence for phosphorylation by CK1, allowing it to phosphorylate Ser322, which in turn primes the CK1-catalysed phosphorylation of Ser325. IGF-1 stimulates the phosphorylation of Thr24, Ser256, Ser319, Ser322 and Ser325 in embryonic stem (ES) cells, but not in PDK1-/- ES cells, providing genetic evidence that PDK1 (the upstream activator of PKB) is required for the phosphorylation of FKHR in mammalian cells. In contrast, the phosphorylation of Ser329 is unaffected by IGF-1 and the phosphorylation of this site is not decreased in PDK1-/- ES cells. The cluster of phosphorylation sites at Ser319, Ser322, Ser325 and Ser329 appears to accelerate nuclear export by controlling the interaction of FKHR with the Ran-containing protein complex that mediates this process.

Activation of SGK1 by HGF, Rac1 and integrin-mediated cell adhesion in MDCK cells: PI-3K-dependent and -independent pathways

The SGK1 protein belongs to the AGC gene family of kinases that are regulated by phosphorylation mediated by PDK1. SGK1 regulation is accomplished by several pathways including growth-factor and stress-mediated signaling. We have expanded the analysis of SGK1 regulation in epithelial cells. We used HA-tagged SGK1 to transiently transfect MDCK cells and study the regulation of SGK1 upon stimulation with HGF, cAMP or upon adhesion of the cells to immobilized fibronectin. In addition, we studied the regulation of SGK1 activity by small GTP-binding proteins of the Rho family.

Treatment of MDCK cells with HGF leads to a time-dependent activation of SGK1 that is blocked by wortmanin. This activation requires the conserved phosphorylation site present in the activation loop of the kinase (T256 in SGK1) and the phosphorylation site present in a hydrophobic domain at its C-terminus (S422 in SGK1), which are targets for PDK1/PDK2-mediated regulation of SGK1. We tested whether SGK1 could be activated by cAMP as it contains a putative PKA site. We were unable to demonstrate a significant activation of HA-SGK1 by cAMP stimulation under conditions where we detect cAMP-mediated phosphorylation of the transcription factor CREB.

Cotransfection of SGK1 with activated small GTP-binding proteins revealed that Rac1, but not Rho or Rap1, induces activation of SGK1. However, this activation was wortmanin insensitive and dominant-negative Rac1 did not inhibit the HGF-mediated activation of SGK1. Adhesion of MDCK cells to immobilized fibronectin also leads to activation of SGK1. However, it appears that the integrin-mediated activation of HA-SGK1 differs from AKT activation in the fact that AKT phosphorylation was blocked by wortmanin (or LY294002)whereas HA-SGK1 was not. The adhesion-dependent activation, however, requires the intact phosphorylation sites of SGK1. Co-transfection of HA-SGK1 with RacV12 results in increased activity in adherent cells compared with HA-SGK1 alone. Since RacN17 failed to inhibit adhesion dependent-activation of SGK1,it suggests that integrin activation is achieved by a parallel Rac-independent pathway.

The activation of SGK1 by HGF and integrin provides a link between HGF-mediated protection of MDCK from de-attachment induced apoptosis(anoikis). We demonstrate that dephosphorylation of the transcription factor FKRHL1 induced by cell de-attachment is prevented by activated SGK1,suggesting that SGK1 regulates cell survival pathways.

In summary, we demonstrate that SGK1 activation could be achieved through signaling pathways involved in the regulation of cell survival, cell-cell and cell-matrix interactions. SGK1 activation can be accomplished via HGF,PI-3K-dependent pathways and by integrin-mediated, PI-3K independent pathways. In addition, activation of SGK1 by the small GTP-binding protein Rac1 has been observed.

The phosphatidylinositol 3-kinase (PI3K)-Akt pathway suppresses Bax translocation to mitochondria

Bax, a proapoptotic member of the Bcl-2 family, localizes largely in the cytoplasm but redistributes to mitochondria in response to apoptotic stimuli, where it induces cytochrome c release. In this study, we show that the phosphatidylinositol 3-OH kinase (PI3K)-Akt pathway plays an important role in the regulation of Bax subcellular localization. We found that LY294002, a PI3K inhibitor, blocked the effects of serum to prevent Bax translocation to mitochondria and that expression of an active form of PI3K suppressed staurosporine-induced Bax translocation, suggesting that PI3K activity is essential for retaining Bax in the cytoplasm. In contrast, both U0126, a MEK inhibitor, and active MEK had little effect on Bax localization. In respect to downstream effectors of PI3K, we found that expression of active Akt, but not serum and glucocorticoid-induced protein kinase (SGK), suppressed staurosporine-induced translocation of Bax, whereas dominant negative Akt moderately promoted Bax translocation. Expression of Akt did not alter the levels of Bax, Bcl-2, Bcl-X(L), or phosphorylated JNK under the conditions used, suggesting that there were alternative mechanisms for Akt in the suppression of Bax translocation. Collectively, these results suggest that the PI3K-Akt pathway inhibits Bax translocation from cytoplasm to mitochondria and have revealed a novel mechanism by which the PI3K-Akt pathway promotes survival.

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.

Control of cell cycle exit and entry by protein kinase B-regulated forkhead transcription factors

AFX-like Forkhead transcription factors, which are controlled by phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, are involved in regulating cell cycle progression and cell death. Both cell cycle arrest and induction of apoptosis are mediated in part by transcriptional regulation of p27(kip1). Here we show that the Forkheads AFX (FOXO4) and FKHR-L1 (FOXO3a) also directly control transcription of the retinoblastoma-like p130 protein and cause upregulation of p130 protein expression. Detailed analysis of p130 regulation demonstrates that following Forkhead-induced cell cycle arrest, cells enter G(0) and become quiescent. This is shown by a change in phosphorylation of p130 to G(0)-specific forms and increased p130/E2F-4 complex formation. Most importantly, long-term Forkhead activation causes a sustained but reversible inhibition of proliferation without a marked increase in apoptosis. As for the activity of the Forkheads, we also show that protein levels of p130 are controlled by endogenous PI3K/PKB signaling upon cell cycle reentry. Surprisingly, not only nontransformed cells, but also cancer cells such as human colon carcinoma cells, are forced into quiescence by Forkhead activation. We therefore propose that Forkhead inactivation by PKB signaling in quiescent cells is a crucial step in cell cycle reentry and contributes to the processes of transformation and regeneration.

Redox regulation of forkhead proteins through a p66shc-dependent signaling pathway

Genetic determinants of longevity include the forkhead-related transcription factor DAF-16 in the worm Caenorhabditis elegans and the p66shc locus in mice. We demonstrate that p66shc regulates intracellular oxidant levels in mammalian cells and that hydrogen peroxide can negatively regulate forkhead activity. In p66shc-/- cells, the activity of the mammalian forkhead homolog FKHRL1 is increased and redox-dependent forkhead inactivation is reduced. In addition, expression of FKHRL1 results in an increase in both hydrogen peroxide scavenging and oxidative stress resistance. These results demonstrate an important functional relation between three distinct elements linked to aging: forkhead proteins, p66shc, and intracellular oxidants.

Insulin regulation of insulin-like growth factor-binding protein-1 gene expression is dependent on the mammalian target of rapamycin, but independent of ribosomal S6 kinase activity

Insulin inhibits the expression of the hepatic insulin-like growth factor-binding protein-1 (IGFBP-1) and glucose-6-phosphatase (G6Pase) genes. The signaling pathway that mediates these events requires the activation of phosphatidylinositol 3-kinase, whereas transfection studies have suggested an involvement of Akt (protein kinase B) and FKHR, a transcription factor regulated by Akt. We now demonstrate that insulin repression of endogenous IGFBP-1 gene transcription was blocked by rapamycin or by amino acid starvation. Rapamycin inhibited the mammalian target of rapamycin (mTOR) and the subsequent activation of p70/p85 S6 protein kinase-1 (S6K1) by insulin, whereas amino acid depletion prevented insulin induction of these signaling molecules. Importantly, we demonstrate that insulin regulation of the thymine-rich insulin response element of the IGFBP-1 promoter was also inhibited by rapamycin. However, sustained activation of S6K1 did not repress this promoter. In addition, rapamycin did not affect insulin regulation of G6Pase expression or Akt activation. We propose that these observations indicate that an mTOR-dependent, but S6K-independent mechanism regulates the suppression of IGFBP-1 (but not G6Pase) gene expression by insulin. Therefore, although the insulin-responsive sequence of the G6Pase gene promoter is related to that of the IGFBP-1 promoter, the signaling pathways that mediate suppression of these genes are distinct.

Stay mellow, stay young

Mutations in single genes can extend the life-span of several model organisms, but the picture of whether and how those genes contribute to longevity in mammals remains muddy. New work links proteins involved in worm and mammalian aging and also ties in reactive oxygen species, destructive molecules that are thought to contribute to aging in a variety of creatures. Although the growing number of similar aging themes among diverse organisms is clarifying some general ideas, the details--and variations--are turning out to be a complex business.

Expression of FKHR, FKHRL1, and AFX genes in the rodent ovary: evidence for regulation by IGF-I, estrogen, and the gonadotropins

Follicular development is dependent on both intraovarian growth regulatory factors, such as IGF-I and estrogen, as well as the pituitary gonadotropins, FSH and LH. Recently, we have shown that FSH impacts the IGF-I pathway via stimulation of the PI3K cascade leading to phosphorylation of protein kinase B (PKB)/Akt and the PKB-related kinase, Sgk. This study was undertaken to determine if during ovarian follicular development FSH regulates putative targets of PKB and Sgk, namely specific Forkhead transcription factor family members. Using in vivo and in vitro mouse and rat models, we show 1) that FKHR [Forkhead homolog of rhabdomysarcoma = Forkhead box binding protein (Foxo1), FKHRL1 (Forkhead-like protein-1 = Foxo3), and AFX (a Forkhead transcription factor = Foxo4); all defined according to the Human and Mouse Gene Nomenclature Committee) are expressed in the rodent ovary and 2) that FSH regulates transcription of the FKHR gene as well as phosphorylation of FKHR protein. Specifically, FSH/PMSG (primarily via E2) enhance expression of the FKHR gene in granulosa cells of developing follicles. Furthermore, E2 enhances expression of other IGF-I pathway components (IGF-1Rbeta and Glut-1), and IGF-I enhances expression of ERbeta, indicating that these two hormones comprise an autocrine regulatory network within growing follicles. In contrast, FSH and LH/human CG (via cAMP, PKA, and PI3K pathways) terminate FKHR expression as granulosa cells differentiate to luteal cells. In naïve granulosa cells, both FSH and IGF-I stimulate rapid phosphorylation of FKHR at multiple sites causing its redistribution from the nucleus to the cytoplasm in a PI3K-dependent manner. However, the effects of FSH and IGF-I differ markedly in differentiated granulosa cells in which FSH (but not IGF-I) induces Sgk and enhances phosphorylation of FKHR, PKB, and Sgk. The elevated expression of FKHR in granulosa cells of growing follicles indicates that FKHR may be linked to the proliferation of granulosa cells and that its phosphorylation by FSH, IGF-I, and other factors may impact its functional activity in this process. Thus, as a target of FSH (cAMP), E2 and IGF-I signaling in granulosa cells, FKHR likely coordinates numerous cell survival mechanisms.

Yeast protein kinases and the RHO1 exchange factor TUS1 are novel components of the cell integrity pathway in yeast

The PKC1-associated mitogen-activated protein (MAP) kinase pathway of Saccharomyces cerevisiae regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. Activation of PKC1 occurs via the GTPase RHO1 and the kinase pair PKH1 and PKH2. Here we report that YPK1 and YPK2, an essential pair of homologous kinases and proposed downstream effectors of PKH and sphingolipids, are also regulators of the PKC1-controlled MAP kinase cascade. ypk mutants display random distribution of the actin cytoskeleton and severely reduced activation of the MAP kinase MPK1. Upregulation of the RHO1 GTPase switch or the PKC1 effector MAP kinase pathway suppresses the growth and actin defects of ypk cells. ypk lethality is also suppressed by overexpression of an uncharacterized gene termed TUS1. TUS1 is a novel RHO1 exchange factor that contributes to cell wall integrity-mediated modulation of RHO1 activity. Thus, TUS1 and the YPKs add to the growing complexity of RHO1 and PKC1 regulation in the cell integrity signaling pathway. Furthermore, our findings suggest that the YPKs are a missing link between sphingolipid signaling and the cell integrity pathway.

FKHR-L1 can act as a critical effector of cell death induced by cytokine withdrawal: protein kinase B-enhanced cell survival through maintenance of mitochondrial integrity

Survival signals elicited by cytokines include the activation of phosphatidylinositol 3-kinase (PI3K), which in turn promotes the activation of protein kinase B (PKB). Recently, PKB has been demonstrated to phosphorylate and inactivate forkhead transcription factor FKHR-L1, a potent inducer of apoptosis. To explore the mechanisms underlying the induction of apoptosis after cytokine withdrawal or FKHR-L1 activation, we used a cell line in which FKHR-L1 activity could be specifically induced. Both cytokine withdrawal and FKHR-L1 activation induced apoptosis, which was preceded by an upregulation in p27KIP1 and a concomitant decrease in cells entering the cell cycle. Induction of apoptosis by both cytokine withdrawal and activation of FKHR-L1 correlated with the disruption of mitochondrial membrane integrity and cytochrome c release. This was preceded by upregulation of the pro-apoptotic Bcl-2 family member Bim. Ectopic expression of an inhibitory mutant of FKHR-L1 substantially reduced the levels of apoptosis observed after cytokine withdrawal. Activation of PKB alone was sufficient to promote cell survival, as measured by maintenance of mitochondrial integrity and the resultant inhibition of effector caspases. Furthermore, hematopoietic stem cells isolated from Bim-/- mice exhibited reduced levels of apoptosis upon inhibition of PI3K/PKB signaling. These data demonstrate that activation of FKHR-L1 alone can recapitulate all known elements of the apoptotic program normally induced by cytokine withdrawal. Thus PI3K/PKB--mediated inhibition of this transcription factor likely provides an important mechanism by which survival factors act to prevent programmed cell death.

Distinct phosphorylation patterns underlie Akt activation by different survival factors in neurons

The survival of cultured cerebellar granule neurons can be maintained by depolarizing levels of potassium (high K(+), HK), insulin-like growth factor (IGF-1), cyclic AMP or lithium. We examined the possibility that the signaling pathways activated by these different factors converge and that Akt might represent such a point of convergence. Consistent with this possibility, we find that Akt is phosphorylated and activated by all four survival factors. The pattern of Akt phosphorylation induced by the four survival factors, however, shows differences. While IGF-1 induces phosphorylation of Akt at both Ser473 and Thr308, HK and cyclic AMP stimulate phosphorylation at Thr308 only. Lithium increases phosphorylation at Ser473 but not at Thr308. Our results are consistent with the possibility that Akt is a central component of different survival-promoting pathways in granule neurons. The different phosphorylation patterns, however, point to a previously unappreciated complexity in the regulation of Akt activity in neurons. Finally, we provide evidence indicating that SGK, a kinase that is structurally related to Akt, is also activated by the four survival factors.

Regulation and physiological roles of serum- and glucocorticoid-induced protein kinase isoforms

Serum- and glucocorticoid-induced protein kinase 1 (SGK1) was identified in 1993 as an immediate early gene whose mRNA levels increase dramatically within 30 minutes when cells are exposed to serum or glucocorticoids, or both. Subsequently, many other agonists, acting through a variety of signal transduction pathways, have been shown to induce SGK1 gene transcription in cells and tissues. SGK1 is a member of the "AGC" subfamily, which includes protein kinases A, G, and C, and its catalytic domain is most similar to protein kinase B (PKB). Like PKB, SGK1 is activated by phosphorylation in response to signals that stimulate phosphatidylinositol 3-kinase, and this is mediated by 3-phosphoinositide-dependent protein kinase 1 (PDK1) and another protein kinase that has yet to be identified. Thus, SGK1 is remarkable in being activated at both the transcriptional and posttranslational levels by a huge number of extracellular signals. In contrast, little is known about the transcriptional regulation of the two closely related isoforms SGK2 and SGK3, although they can be activated by phosphorylation. The substrate specificity of SGK isoforms superficially resembles that of PKB in that serine and threonine residues lying in Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr sequences (where Xaa is a variable amino acid) are phosphorylated. However, although they may have some substrates in common, evidence is emerging that SGK1 and PKB phosphorylate distinct proteins and have different functions in vivo. In particular, SGK1 plays an important role in activating certain potassium, sodium, and chloride channels, suggesting an involvement in the regulation of processes such as cell survival, neuronal excitability, and renal sodium excretion. Moreover, sustained high levels of SGK1 protein and activity may contribute to conditions such as hypertension and diabetic nephropathy. This raises the possibility that specific inhibitors of SGK1 may have therapeutic potential for the treatment of several diseases.

Transforming growth factor beta enhances epithelial cell survival via Akt-dependent regulation of FKHRL1

The Forkhead family of transcription factors participates in the induction of death-related genes. In NMuMG and 4T1 mammary epithelial cells, transforming growth factor beta (TGF beta) induced phosphorylation and cytoplasmic retention of the Forkhead factor FKHRL1, while reducing FHKRL1-dependent transcriptional activity. TGF beta-induced FKHRL1 phosphorylation and nuclear exclusion were inhibited by LY294002, an inhibitor of phosphatidylinositol-3 kinase. A triple mutant of FKHRL1, in which all three Akt phosphorylation sites have been mutated (TM-FKHRL1), did not translocate to the cytoplasm in response to TGF beta. In HaCaT keratinocytes, expression of dominant-negative Akt prevented TGF beta-induced 1) reduction of Forkhead-dependent transcription, 2) FKHRL1 phosphorylation, and 3) nuclear exclusion of FKRHL1. Forced expression of either wild-type (WT) or TM-FKHRL1, but not a FKHRL1 mutant with deletion of the transactivation domain, resulted in NMuMG mammary cell apoptosis. Evidence of nuclear fragmentation colocalized to cells with expression of WT- or TM-FKHRL1. The apoptotic effect of WT-FKHRL1 but not TM-FKHRL1 was prevented by exogenous TGF beta. Serum starvation-induced apoptosis was also inhibited by TGF beta in NMuMG and HaCaT cells. Finally, dominant-negative Akt abrogated the antiapoptotic effect of TGF beta. Taken together, these data suggest that TGF beta may play a role in epithelial cell survival via Akt-dependent regulation of FKHRL1.

The forkhead transcription factor Foxo1 (Fkhr) confers insulin sensitivity onto glucose-6-phosphatase expression

Type 2 diabetes is characterized by the inability of insulin to suppress glucose production in the liver and kidney. Insulin inhibits glucose production by indirect and direct mechanisms. The latter result in transcriptional suppression of key gluconeogenetic and glycogenolytic enzymes, phosphoenolpyruvate carboxykinase (Pepck) and glucose-6-phosphatase (G6p). The transcription factors required for this effect are incompletely characterized. We report that in glucogenetic kidney epithelial cells, Pepck and G6p expression are induced by dexamethasone (dex) and cAMP, but fail to be inhibited by insulin. The inability to respond to insulin is associated with reduced expression of the forkhead transcription factor Foxo1, a substrate of the Akt kinase that is inhibited by insulin through phosphorylation. Transduction of kidney cells with recombinant adenovirus encoding Foxo1 results in insulin inhibition of dex/cAMP-induced G6p expression. Moreover, expression of dominant negative Foxo1 mutant results in partial inhibition of dex/cAMP-induced G6p and Pepck expression in primary cultures of mouse hepatocyes and kidney LLC-PK1-FBPase(+) cells. These findings are consistent with the possibility that Foxo1 is involved in insulin regulation of glucose production by mediating the ability of insulin to decrease the glucocorticoid/cAMP response of G6p.

The forkhead transcription factor Foxo1 (Fkhr) confers insulin sensitivity onto glucose-6-phosphatase expression

Type 2 diabetes is characterized by the inability of insulin to suppress glucose production in the liver and kidney. Insulin inhibits glucose production by indirect and direct mechanisms. The latter result in transcriptional suppression of key gluconeogenetic and glycogenolytic enzymes, phosphoenolpyruvate carboxykinase (Pepck) and glucose-6-phosphatase (G6p). The transcription factors required for this effect are incompletely characterized. We report that in glucogenetic kidney epithelial cells, Pepck and G6p expression are induced by dexamethasone (dex) and cAMP, but fail to be inhibited by insulin. The inability to respond to insulin is associated with reduced expression of the forkhead transcription factor Foxo1, a substrate of the Akt kinase that is inhibited by insulin through phosphorylation. Transduction of kidney cells with recombinant adenovirus encoding Foxo1 results in insulin inhibition of dex/cAMP-induced G6p expression. Moreover, expression of dominant negative Foxo1 mutant results in partial inhibition of dex/cAMP-induced G6p and Pepck expression in primary cultures of mouse hepatocyes and kidney LLC-PK1-FBPase(+) cells. These findings are consistent with the possibility that Foxo1 is involved in insulin regulation of glucose production by mediating the ability of insulin to decrease the glucocorticoid/cAMP response of G6p.

Anoikis mechanisms

Anoikis is defined as apoptosis that is induced by inadequate or inappropriate cell-matrix interactions. It is involved in a wide diversity of tissue-homeostatic, developmental and oncogenic processes. The central problem of anoikis is to understand how integrin-mediated cell adhesion signals control the apoptotic machinery. In particular, the initiation of the caspase cascade in anoikis remains to be explained.

The role of SGK1 in hormone-regulated sodium transport

Ion transport in epithelia is regulated by a variety of hormonal and nonhormonal factors, including mineralocorticoids, insulin, shear stress and osmotic pressure. In mammals, the mineralocorticoid aldosterone is the principal regulator of sodium homeostasis and hence is central to the control of extracellular fluid volume and blood pressure. Aldosterone acts through a member of the nuclear receptor superfamily, the mineralocorticoid receptor (MR), to control the transcriptional activity of specific target genes. Recently, a serine/threonine kinase, SGK1 (serum and glucocorticoid-regulated kinase isoform 1) was identified as a candidate mediator of aldosterone action in the colon and distal nephron. The aldosterone-activated MR increases SGK1 gene transcription and SGK1, in turn, strongly stimulates the activity of the epithelial sodium channel (ENaC). Interestingly, other factors appear to regulate SGK1 gene expression and kinase activity. Insulin, for example, stimulates SGK1 activity (but not gene transcription) through its effects on phosphatidylinositol-3-kinase and osmotic shock appears to stimulate both SGK1 activity and gene transcription. Hence, SGK1 might integrate the effects of multiple hormonal and nonhormonal regulators of Na(+) transport in tight epithelia and thereby play a key role in volume homeostasis. It is interesting to speculate that SGK1 might be implicated in medical conditions, such as the insulin resistance syndrome, hypertension and congestive heart failure.

PKB/AKT: functional insights from genetic models

Since its discovery 10 years ago, the potential functions of protein kinase B (PKB)/AKT have been catalogued with increasing efficiency. The physiological relevance of some of the proposed mechanisms by which PKB/AKT mediates many of its effects has been questioned, and recent work using new reagents and approaches has revealed some cracks in our understanding of this important molecule, and also hinted that these effects may involve other players.

Constitutively active Akt is an important regulator of TRAIL sensitivity in prostate cancer

TRAIL/Apo-2L is a member of the tumor necrosis factor superfamily and has recently been shown to induce apoptosis in cancer cells, but not in normal cells. In nude mice injected with human tumors, TRAIL reduces the size of these tumors without side effects. Akt promotes cell survival and block apoptosis. Some prostate cancer cells express high levels of Akt due to lack of active lipid phosphatase PTEN, a negative regulator of PI-3 kinase pathway, which may be responsible for drug resistance. The objective of this paper is to investigate the intracellular molecules that regulate TRAIL resistance. We have examined caspase-8 activity, BID cleavage, Akt activity, mitochondrial membrane potential (DeltaPsi(m)) and apoptosis in prostate cancer (LNCap, PC-3, PC-3M and DU145) cells treated with or without TRAIL. PC-3, PC-3M and DU145 cells are sensitive to TRAIL, whereas LNCap cells are resistant. LNCap cells express the highest level of constitutively active Akt, which is directly correlated with TRAIL resistance. TRAIL activates caspase-8 in all the cell lines. Downregulation of constitutively active Akt by PI-3 kinase inhibitors (wortmannin and LY-294002), dominant negative Akt or PTEN, renders LNCap cells sensitive to TRAIL. Inhibition of TRAIL sensitivity occurs at the level of BID cleavage. Inhibition of protein synthesis by cycloheximide also causes LNCap cells sensitive to TRAIL. Overexpression of Bcl-2 or Bcl-X(L) inhibits TRAIL-induced DeltaPsi(m) and apoptosis. Overexpression of constitutively active Akt in PC-3M cells (express very low levels of constitutively active Akt) restores TRAIL resistance. These data suggest that elevated Akt activity protects LNCap cells from TRAIL-induced apoptosis, and the PI-3 kinase/Akt pathway may inhibit apoptotic signals by inhibiting processing of BID. Thus, constitutively active Akt is an important regulator of TRAIL sensitivity in prostate cancer.

Mediators of aldosterone action in the renal tubule

The aldosterone-sensitive distal nephron extends from the second part of the distal convoluted tubule to the inner medullary collecting duct. As recently shown, aldosterone increases within two hours the abundance of the alpha-subunit of the epithelial sodium channel along the entire aldosterone-sensitive distal nephron, whereas it induces only in an initial portion of the aldosterone-sensitive distal nephron an apical translocation of all three epithelial sodium channel subunits. This suggests that another factor or factors determines the length of the aldosterone-sensitive distal nephron portion in which aldosterone controls epithelial sodium channel surface expression. Since the glucocorticoid-induced kinase SGK1 was identified as aldosterone-induced protein in 1999, it has been postulated to play a key regulatory role. The in-vivo localization of its induction to segment-specific cells of the aldosterone-sensitive distal nephron, and the in-vitro correlation of the amount of its hyperphosphorylated form with transepithelial sodium transport, support this hypothesis. Other recent studies unravel pathways other than those activated by aldosterone and insulin that impact on SGK1 expression and/or function, and thus shed some light onto the complex network that appears to control sodium transport. In view of the ongoing research, the question of how, and formally also whether, SGK1 acts on the epithelial sodium channel should be resolved in the near future.

Serum- and glucocorticoid-inducible kinase SGK phosphorylates and negatively regulates B-Raf

Phosphorylation can both positively and negatively regulate activity of the Raf kinases. Akt has been shown to phosphorylate and inhibit C-Raf activity. We have recently reported that Akt negatively regulates B-Raf kinase activation by phosphorylating multiple residues within its amino-terminal regulatory domain. Here we investigated the regulation of B-Raf by serum and glucocorticoid-inducible kinase, SGK, which shares close sequence identity with the catalytic domain of Akt but lacks the pleckstrin homology domain. We observed that SGK inhibits B-Raf activity. A comparison of substrate specificity between SGK and Akt indicates that SGK is a potent negative regulator of B-Raf. In contrast to Akt, SGK negatively regulates B-Raf kinase activity by phosphorylating only a single Akt consensus site, Ser(364). Under similar experimental conditions, SGK displays a measurably stronger inhibitory effect on B-Raf kinase activity than Akt, whereas Akt exhibits a more inhibitory effect on the forkhead transcription factor, FKHR. The selective substrate specificity is correlated with an enhanced association between Akt or SGK and their preferred substrates, FKHR and B-Raf, respectively. These results indicate that B-Raf kinase activity is negatively regulated by Akt and SGK, suggesting that the cross-talk between the B-Raf and other signaling pathways can be mediated by both Akt and SGK.

Regulation of cytokine-independent survival kinase (CISK) by the Phox homology domain and phosphoinositides

PKB/Akt and serum and glucocorticoid-regulated kinase (SGK) family kinases are important downstream targets of phosphatidylinositol 3 (PI-3) kinase and have been shown to mediate a variety of cellular processes, including cell growth and survival. Although regulation of Akt can be achieved through several mechanisms, including its phosphoinositide-binding Pleckstrin homology (PH) domain, how SGK kinases are targeted and regulated remains to be elucidated. Unlike Akt, cytokine-independent survival kinase (CISK)/SGK3 contains a Phox homology (PX) domain. PX domains have been implicated in several cellular events involving membrane trafficking. However, their precise function remains unknown. We demonstrate here that the PX domain of CISK interacts with phosphatidylinositol (PtdIns)(3,5)P2, PtdIns(3,4,5)P3, and to a lesser extent PtdIns(4,5)P2. The CISK PX domain is required for targeting CISK to the endosomal compartment. Mutation in the PX domain that abolished its phospholipid binding ability not only disrupted CISK localization, but also resulted in a decrease in CISK activity in vivo. These results suggest that the PX domain regulates CISK localization and function through its direct interaction with phosphoinositides. Therefore, CISK and Akt have evolved to utilize different lipid binding domains to accomplish a similar mechanism of activation in response to PI-3 kinase signaling.

Transcription-dependent and -independent control of neuronal survival by the PI3K-Akt signaling pathway

The PI3K-Akt signaling pathway plays a critical role in mediating survival signals in a wide range of neuronal cell types. The recent identification of a number of substrates for the serine/threonine kinase Akt suggests that it blocks cell death by both impinging on the cytoplasmic cell death machinery and by regulating the expression of genes involved in cell death and survival. In addition, recent experiments suggest that Akt may also use metabolic pathways to regulate cell survival.