| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
EDITORIAL FOCUS
Milhauser Laboratories, New York University School of Medicine, New York, New York
THE AKT SERINE/THREONINE KINASE (also called protein kinase B or PKB) has emerged as a critical signaling molecule within eukaryotic cells. In addition to work clarifying its regulation by upstream kinases and phosphatases, significant efforts have been applied to the identification of potential binding partners, which may alter its biological function. Carboxy-terminal modulator protein (CTMP) is such an Akt-interacting protein. Ono et al. (26) now present evidence that CTMP, originally identified as a suppressor of Akt phosphorylation, may also act as an activator of Akt under certain experimental conditions. This editorial focus evaluates the different models for the role of CTMP in Akt regulation while also discussing possible physiological approaches to harness the complexity of Akt protein-protein interaction in view of their therapeutic potential.
Over the past decade, the Akt serine/threonine kinase has emerged as a critical signaling component within all cells of higher eukaryotes. Akt activation is regulated by phosphatidylinositol 3-kinase (PI3K)-dependent second messenger molecules (4, 14, 24). As a target of PI3K (17), Akt regulates a wide range of biological responses that include cell survival, proliferation, transcription, protein synthesis, and nutrient metabolism (4, 15, 27, 30). All three Akt isoforms (Akt1, Akt2, and Akt3) in mammals share a common structure that consists of an NH2-terminal regulatory domain including a pleckstrin homology (PH) domain (16), a hinge region connecting the PH domain to a kinase domain with serine/threonine specificity (1), and a COOH-terminal region required for the induction and maintenance of its kinase activity (7). Its kinase domain shares significant homology with other members of the AGC kinase family, which share phosphoinositide-dependent kinase (PDK-1) as a common upstream regulator (29).
In animals, Akt (Akt1) is ubiquitously expressed at high levels, with the exception of kidney, liver, and spleen (2, 11, 20). Akt2 expression varies between different tissues, with higher expression levels in muscle, intestinal organs, and reproductive tissues (19, 22). Akt3, in turn, is expressed the highest in brain and testis (25). All Akt isoforms are regulated similarly and in vitro phosphorylate substrates with equal specificity and efficiency. While it is possible that varying implications of different Akt isoforms in human disease result from different patterns of expression, recent studies have found nonredundancy in physiological functions (3). Furthermore, experiments selectively disrupting the Akt genes in the mouse germ line have resulted in mutant mice with specific phenotypes. All mice deficient for a single Akt isoform are viable, but depending on which Akt isoform has been deleted, they either show significant retardation of growth and reduction of body weight (after Akt1 deletion) (8, 10), defects in the regulation of blood glucose concentrations following insulin stimulation (after knocking out the Akt2 gene) (9), or reduced brain sizes (after Akt3 deletion) (13, 30). The sole fact that knockout mutants deficient of single isoforms are viable with only a subtle difference in phenotype may indicate the capacity of the three Akt isoforms to compensate for one another.
Given the functional importance of Akt as a key modulator of intracellular signaling, significant efforts have been applied to the identification of potential binding partners, mainly by using yeast two-hybrid analysis. Comprehensive reviews of several Akt interacting molecules and their mechanisms of action are given by Brazil et al. (5) and Du and Tsichlis (12). Among these are proteins with enzymatic activity that interact with Akt and modify its phosphorylation state. Examples for this mode of regulation include other enzymes, such as PDK-1. Other Akt-interacting proteins lack intrinsic activity, but they bind to one or more regions of the Akt molecule and modify its activity and/or intracellular localization.
One interacting molecule that regulates Akt activity by binding is the 27-kDA protein CTMP. In the original study describing CMTP published in 2001, Maira et al. (23) showed that CTMP binds to the COOH terminus of Akt in a region including the hydrophobic motif (HM) and COOH-terminal serine phosphorylation site (23). The authors showed that Akt and CTMP formed an endogenous complex at the plasma membrane, and that the effect of CTMP binding was to reduce Akt phosphorylation on pSer473, resulting in a decrease in Akt activity. Furthermore, when expressed in mink lung cells transformed with the viral oncogene v-Akt (AKT8 cells), CTMP overexpression resulted in a phenotypic regression of these transformed cells to wild type. Maira et al. also applied genetic approaches to investigate the effect of altered CTMP expression on soft agar colony formation and tumorigenic potential of AKT8 cells. Their findings strongly suggested that CTMP indeed acted as a negative regulator of Akt. As a model for CTMP action, Maira et al. propose a model in which binding of CTMP inhibited phosphorylation of Akt by upstream kinases (23) (Fig. 1A).
|
In their recent article, Ono et al. (26) now provide evidence that CTMP may also act as a positive regulator of Akt signaling. While both Maira et al. (23) and Ono et al. present comparable results to show that the interaction of Akt and CTMP occurs at the plasma membrane, their studies differ diametrically regarding the consequences of this interaction on Akt phosphorylation and downstream biological responses. Specifically, using various cell lines, including Cos-1, HepG2, HeLa, and NIH3T3 cells, Ono et al. show that CTMP overexpression induces Akt phosphorylation on critical residues, leading to increased phosphorylation of downstream substrates, facilitating antiapoptosis and glucose metabolism. In contrast, downregulation of endogenous CTMP expression by using small interfering RNA blocked Akt phosphorylation. From these findings, the authors extract a different model for CTMP function, in which CTMP acts as an Akt activator by facilitating its translocation to the plasma membrane, where Akt then is an accessible substrate for phosphorylation by upstream kinases (26) (Fig. 1B).
When comparing the studies by Maira et al. (23) and Ono et al. (26) side by side, one is left with an irreconcilable disagreement. In support of Maira et al., concerns remain about the relative amplitude of the effects observed by Ono et al. In the Ono et al. study, stimulation of cells with various activators of Akt triggers an increase in phosphorylation/activity of only 1–3 fold, whereas Maira et al. report changes at least 1 magnitude larger. Still, these technical arguments aside, both articles have used comparable approaches to come to rather disparate findings. Looking at these differences, though, it may not be simply a question of which study may be right or how both could fit into a common model. The difference could be the result of dose-dependent effects and biological responses between different cell lines or sublines thereof, which may be further complicated by the existence of a nuclear pool of CTMP in certain cell types (18).
Nevertheless, both articles make a strong case for the involvement of CTMP in modulating Akt activity and downstream response. An interesting molecule such as CTMP deserves further efforts directed at clarifying its role in animal physiology, especially given its potential as a therapeutic target to alter Akt-dependent responses. As the field of Akt signaling has learned, such insights may come from knockout approaches. Thus it should be only a matter of time until one of the many groups working on this pathway will step forward with a mutant mouse deficient for CTMP or one of Akt's other interacting proteins. CTMP is an excellent example for how the identification of Akt modulators has enriched our understanding of pathway function.
In the grander scheme of progress, it will be important to determine which of these molecules is the most relevant and how they affect Akt physiology on an organismal or tissue-specific level. As it stands now, it appears that Akt activity and function are subject to regulation by multiple modifiers, not yet including the possibility that several of these interacting proteins may compete for regulating Akt in parallel. How cells use the complexity of these possible interactions to fine-tune Akt-dependent responses and to compartmentalize specificity within the cellular context remains an important question to be answered by future studies.
 |
GRANTS |
|---|
 TOP GRANTS REFERENCES |
|---|
|
FOOTNOTES |
|---|
|
REFERENCES |
|---|
|
TOPGRANTSREFERENCES |
|---|
2. Bellacosa A, Franke TF, Gonzalez-Portal ME, Datta K, Taguchi T, Gardner J, Cheng JQ, Testa JR, Tsichlis PN. Structure, expression and chromosomal mapping of c-akt: relationship to v-akt and its implications. Oncogene 8: 745–754, 1993.[Web of Science][Medline]
3. Bellacosa A, Kumar CC, Di Cristofano A, Testa JR. Activation of AKT kinases in cancer: implications for therapeutic targeting. Adv Cancer Res 94: 29–86, 2005.[CrossRef][Web of Science][Medline]
4. Brazil DP, Hemmings BA. Ten years of protein kinase B signalling: a hard Akt to follow. Trends Biochem Sci 26: 657–664, 2001.[CrossRef][Web of Science][Medline]
5. Brazil DP, Park J, Hemmings BA. PKB binding proteins. Getting in on the Akt. Cell 111: 293–303, 2002.[CrossRef][Web of Science][Medline]
6. Chae KS, Martin-Caraballo M, Anderson M, Dryer SE. Akt activation is necessary for growth factor-induced trafficking of functional KCa channels in developing parasympathetic neurons. J Neurophysiol 93: 1174–1182, 2005.
7. Chan TO, Rittenhouse SE, Tsichlis PN. AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation. Annu Rev Biochem 68: 965–1014, 1999.[CrossRef][Web of Science][Medline]
8. Chen WS, Xu PZ, Gottlob K, Chen ML, Sokol K, Shiyanova T, Roninson I, Weng W, Suzuki R, Tobe K, Kadowaki T, Hay N. Growth retardation and increased apoptosis in mice with homozygous disruption of the Akt1 gene. Genes Dev 15: 2203–2208, 2001.
9. Cho H, Mu J, Kim JK, Thorvaldsen JL, Chu Q, Crenshaw EB 3rd, Kaestner KH, Bartolomei MS, Shulman GI, Birnbaum MJ. Insulin resistance and a diabetes mellitus-like syndrome in mice lacking the protein kinase Akt2 (PKB beta). Science 292: 1728–1731, 2001.
10. Cho H, Thorvaldsen JL, Chu Q, Feng F, Birnbaum MJ. Akt1/PKBalpha is required for normal growth but dispensable for maintenance of glucose homeostasis in mice. J Biol Chem 276: 38349–38352, 2001.
11. Coffer PJ, Woodgett JR. Molecular cloning and characterisation of a novel putative protein-serine kinase related to the cAMP-dependent and protein kinase C families. Eur J Biochem 201: 475–481, 1991.[Web of Science][Medline]
12. Du K, Tsichlis PN. Regulation of the Akt kinase by interacting proteins. Oncogene 24: 7401–7409, 2005.[CrossRef][Web of Science][Medline]
13. Easton RM, Cho H, Roovers K, Shineman DW, Mizrahi M, Forman MS, Lee VM, Szabolcs M, de Jong R, Oltersdorf T, Ludwig T, Efstratiadis A, Birnbaum MJ. Role for Akt3/protein kinase Bgamma in attainment of normal brain size. Mol Cell Biol 25: 1869–1878, 2005.
14. Franke TF, Cantley Apoptosis LC. A Bad kinase makes good. Nature 390: 116–117, 1997.[CrossRef][Medline]
15. Franke TF, Hornik CP, Segev L, Shostak GA, Sugimoto C. PI3K/Akt and apoptosis: size matters. Oncogene 22: 8983–8998, 2003.[CrossRef][Web of Science][Medline]
16. Franke TF, Tartof KD, Tsichlis PN. The SH2-like Akt homology (AH) domain of c-akt is present in multiple copies in the genome of vertebrate and invertebrate eucaryotes. Cloning and characterization of the Drosophila melanogaster c-akt homolog Dakt1. Oncogene 9: 141–148, 1994.[Web of Science][Medline]
17. Franke TF, Yang SI, Chan TO, Datta K, Kazlauskas A, Morrison DK, Kaplan DR, Tsichlis PN. The protein kinase encoded by the Akt proto-oncogene is a target of the PDGF-activated phosphatidylinositol 3-kinase. Cell 81: 727–736, 1995.[CrossRef][Web of Science][Medline]
18. Gutierrez-Barrera AM, Menter DG, Abbruzzese JL, Reddy SA. Establishment of three-dimensional cultures of human pancreatic duct epithelial cells. Biochem Biophys Res Commun 358: 698–703, 2007.[CrossRef][Web of Science][Medline]
19. Jones PF, Jakubowicz T, Hemmings BA. Molecular cloning of a second form of RAC protein kinase. Cell Regul 2: 1001–1009, 1991.[Web of Science][Medline]
20. Jones PF, Jakubowicz T, Pitossi FJ, Maurer F, Hemmings BA. Molecular cloning and identification of a serine/threonine protein kinase of the second-messenger subfamily. Proc Natl Acad Sci USA 88: 4171–4175, 1991.
21. Knobbe CB, Trampe-Kieslich A, Reifenberger G. Genetic alteration and expression of the phosphoinositol-3-kinase/Akt pathway genes PIK3CA and PIKE in human glioblastomas. Neuropathol Appl Neurobiol 31: 486–490, 2005.[CrossRef][Web of Science][Medline]
22. Konishi H, Shinomura T, Kuroda S, Ono Y, Kikkawa U. Molecular cloning of rat RAC protein kinase 
and β and their association with protein kinase C 
. Biochem Biophys Res Commun 205: 817–825, 1994.[CrossRef][Web of Science][Medline]
23. Maira SM, Galetic I, Brazil DP, Kaech S, Ingley E, Thelen M, Hemmings BA. Carboxyl-terminal modulator protein (CTMP), a negative regulator of PKB/Akt and v-Akt at the plasma membrane. Science 294: 374–380, 2001.
24. Manning BD, Cantley LC. AKT/PKB signaling: navigating downstream. Cell 129: 1261–1274, 2007.[CrossRef][Web of Science][Medline]
25. Nakatani K, Sakaue H, Thompson DA, Weigel RJ, Roth RA. Identification of a human Akt3 (protein kinase B gamma) which contains the regulatory serine phosphorylation site. Biochem Biophys Res Commun 257: 906–910, 1999.[CrossRef][Web of Science][Medline]
26. Ono H, Sakoda H, Fujishiro M, Anai M, Kushiyama A, Fukushima Y, Katagiri H, Ogihara T, Oka Y, Kamata H, Horike N, Uchijima Y, Kurihara H, Asano T. Carboxy-terminal modulator protein induces Akt phosphorylation and activation, thereby enhancing antiapoptotic, glycogen synthetic, and glucose uptake pathways. Am J Physiol Cell Physiol 293: C1576–C1585, 2007.
27. Scheid MP, Woodgett JR. PKB/AKT: functional insights from genetic models. Nat Rev Mol Cell Biol 2: 760–768, 2001.[CrossRef][Web of Science][Medline]
28. Schick V, Majores M, Engels G, Spitoni S, Koch A, Elger CE, Simon M, Knobbe C, Blumcke I, Becker AJ. Activation of Akt independent of PTEN and CTMP tumor-suppressor gene mutations in epilepsy-associated Taylor-type focal cortical dysplasias. Acta Neuropathol (Berl) 112: 715–725, 2006.[CrossRef][Medline]
29. Toker A, Newton AC. Cellular signaling: pivoting around PDK-1. Cell 103: 185–188, 2000.[CrossRef][Web of Science][Medline]
30. Tschopp O, Yang ZZ, Brodbeck D, Dummler BA, Hemmings-Mieszczak M, Watanabe T, Michaelis T, Frahm J, Hemmings BA. Essential role of protein kinase B gamma (PKB gamma/Akt3) in postnatal brain development but not in glucose homeostasis. Development 132: 2943–2954, 2005.
This article has been cited by other articles:
|
|
 |
|
  T. F. Franke Intracellular Signaling by Akt: Bound to Be Specific Sci. Signal., June 17, 2008; 1(24): pe29 - pe29. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
