Review
Structure, regulation and function of PKB/AKT—a major therapeutic target

https://doi.org/10.1016/j.bbapap.2003.11.009Get rights and content

Abstract

Protein phosphorylation and dephosphorylation play a major role in intracellular signal transduction activated by extracellular stimuli. Protein kinase B (PKB/Akt) is a central player in the signal transduction pathways activated in response to growth factors or insulin and is thought to contribute to several cellular functions including nutrient metabolism, cell growth and apoptosis. Recently, several significant publications have described novel mechanisms used to regulate PKB. Since the alteration of PKB activity is associated with several human diseases, including cancer and diabetes, understanding PKB regulation is an important task if we are to develop successful therapeutics.

Introduction

Protein kinase Bα (PKBα) was initially identified by homology cloning [1]. The kinase domain is similar to that within protein kinase A (PKA) and protein kinase C (PKC), therefore it was termed Related to A and C-Protein Kinase (RAC-PK but later changed to PKB) [1], [2]. Soon after the product of a murine oncogene, v-Akt (AKT8 retrovirus), turned out to be a cellular homologue of PKB, termed c-Akt [3]. Two additional PKB family members have also been identified, PKBβ/c-Akt2 and PKBγ/c-Akt3 [4], [5]. The tissue distribution of PKB isoforms was recently determined using quantitative RT-PCR [6]. In mouse tissues, both the α and β isoforms are ubiquitously expressed, whereas the γ isoform is not detected in several tissues where α and β isoforms are highly expressed, but is relatively highly expressed in brain and testis. PKBβ is expressed predominantly in insulin target tissues, such as fat cells, liver and skeletal muscle. The PKB family of kinases is evolutionarily conserved in eukaryotes ranging from C. elegans to man (except yeast). The amino acid identity between C. elegans and human PKB is around 60%, whereas that between mouse, rat and human it is more than 95%. The three PKB isoforms share a similarity in their catalytic domain with a group of kinases from the AGC family that consists of more than 80 kinases. Most of these protein kinases are regulated by second messengers such as cyclic mononucleotides, Ca2+ or phosphoinositides and many of them are thought to be transducers of cell growth signalling (Fig. 1).

To date, most evidence suggests that PKB is a downstream target of several receptor tyrosine kinases that are regulated by physiological important cell stimuli, such as growth factors and insulin, and therefore plays a major role in metabolism, cell growth and cell survival. Identification of its downstream targets is an important task and will provide further evidence of the wide functional range of this kinase. Furthermore, several reports have described the contribution of PKB isoforms to human diseases, including cancer and diabetes (reviewed in [7], [8]), indicating that a precise understanding of the regulation and function of PKB would be of benefit to the management of these diseases. In this review, we describe molecular details of structure, regulation and function of PKB.

Section snippets

Domain structure of PKB

All three PKB isoforms consist of a conserved domain structure: an amino terminal pleckstrin homology (PH) domain, a central kinase domain and a carboxyl-terminal regulatory domain that contains the hydrophobic motif, a characteristic of AGC kinase (Fig. 1). The PH domain was originally found in pleckstrin, the major phosphorylation substrate for PKC in platelets [9]. The PH domain interacts with membrane lipid products such as phosphatidylinositol(3,4,5)trisphosphate [PtdIns(3,4,5)P3] produced

Upstream of PKB

Full activation of PKB is a multi-step process and several proteins responsible for each step have been identified and characterized [22] (Fig. 3). A number of stimuli can promote activation of PKB through the activation of receptor tyrosine kinases. One of the most significant findings in the early stages of PKB research was the PI3-kinase-dependent activation of PKB [23], [24], [25], [26]. PKB is activated by receptor tyrosine kinases such as platelet derived growth factor receptor (PDGF-R),

Physiological functions of PKB

PKB isoforms contribute to a variety of cellular responses, including cell growth, cell survival and metabolism. This multiplicity of PKB functions might be due to the variation and specificity of its substrates. Up to now, more than 50 proteins have been identified as putative substrates for PKB. Peptide sequences that are preferentially phosphorylated by PKB were characterized by Alessi et al. [74] and Obata et al. [75]. The minimal substrate consensus sequence for PKB, RXRXXS/T, where X is

Conclusions

The detailed knowledge of upstream regulators, and downstream targets, of PKB will be important to the understanding of normal cellular functions, as well as the management of human diseases such as cancer and diabetes. In the past 10 years, the mechanism for PKB activation, lipid second messenger-mediated phosphorylation of PKB, has been well characterized, except for the identification of the kinase(s) responsible for Ser473 phosphorylation. This is the missing piece of the puzzle in terms of

Acknowledgements

We thank David Barford (ICR, London) for preparing Fig. 3. Swiss Cancer League supports part of the work in the author's laboratory. The Friedrich Miescher Institute is part of the Novartis Research Foundation.

References (149)

  • J. Yang et al.

    Molecular mechanism for the regulation of protein kinase B/Akt by hydrophobic motif phosphorylation

    Mol. Cell

    (2002)
  • A. Balendran et al.

    PDK1 acquires PDK2 activity in the presence of a synthetic peptide derived from the carboxyl terminus of PRK2

    Curr. Biol.

    (1999)
  • D.P. Brazil et al.

    Ten years of protein kinase B signalling: a hard Akt to follow

    Trends Biochem. Sci.

    (2001)
  • T.F. Franke et al.

    The protein kinase encoded by the Akt proto-oncogene is a target of the PDGF-activated phosphatidylinositol 3-kinase

    Cell

    (1995)
  • T. Maehama et al.

    The tumor suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate

    J. Biol. Chem.

    (1998)
  • V. Stambolic et al.

    Negative regulation of PKB/Akt-dependent cell survival by the tumor suppressor PTEN

    Cell

    (1998)
  • M. Huse et al.

    The conformational plasticity of protein kinases

    Cell

    (2002)
  • D.R. Alessi et al.

    Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Bα

    Curr. Biol.

    (1997)
  • K.E. Anderson et al.

    Translocation of PDK-1 to the plasma membrane is important in allowing PDK-1 to activate protein kinase B

    Curr. Biol.

    (1998)
  • M.R. Williams et al.

    The role of 3-phosphoinositide-dependent protein kinase 1 in activating AGC kinases defined in embryonic stem cells

    Curr. Biol.

    (2000)
  • M.M. Hill et al.

    Insulin-stimulated protein kinase B phosphorylation on Ser-473 is independent of its activity and occurs through a staurosporine-insensitive kinase

    J. Biol. Chem.

    (2001)
  • S. Persad et al.

    Regulation of protein kinase B/Akt-serine 473 phosphorylation by integrin-linked kinase

    J. Biol. Chem.

    (2001)
  • M.M. Hill et al.

    Identification of a plasma membrane raft-associated PKB ser473 kinase activity that is distinct from ILK and PDK1

    Curr. Biol.

    (2002)
  • A. Toker et al.

    Akt/Protein kinase B is regulated by autophosphorylation at the hypothetical PDK-2 site

    J. Biol. Chem.

    (2000)
  • J. Laine et al.

    The protooncogene TCL1 is an Akt kinase coactivator

    Mol. Cell

    (2000)
  • R. Chen et al.

    Regulation of Akt/PKB activation by tyrosine phosphorylation

    J. Biol. Chem.

    (2001)
  • T. Jiang et al.

    Interaction between Src and a C-terminal proline-rich motif of Akt is required for Akt activation

    J. Biol. Chem.

    (2003)
  • N.M. Conus et al.

    Direct identification of tyrosine 474 as a regulatory phosphorylation site for the Akt protein kinase

    J. Biol. Chem.

    (2002)
  • D.P. Brazil et al.

    PKB binding proteins. Getting in on the Akt

    Cell

    (2002)
  • A.H. Kim et al.

    Akt1 regulates a JNK scaffold during excitotoxic apoptosis

    Neuron

    (2002)
  • M.K. Barthwal et al.

    Negative regulation of mixed lineage kinase 3 by protein kinase B/AKT leads to cell survival

    J. Biol. Chem.

    (2003)
  • C. Figueroa et al.

    Akt2 negatively regulates assembly of the POSH-MLK-JNK signaling complex

    J. Biol. Chem.

    (2003)
  • S. Fukumoto et al.

    Akt participation in the wnt signaling pathway through Dishevelled

    J. Biol. Chem.

    (2001)
  • H. Yuan et al.

    Suppression of glycogen synthase kinase activity is not sufficient for leukemia enhancer factor-1 activation

    J. Biol. Chem.

    (1999)
  • H. Konishi et al.

    Activation of protein kinase B (Akt/RAC-protein kinase) by cellular stress and its association with heat shock protein Hsp27

    FEBS Lett.

    (1997)
  • M.J. Rane et al.

    Heat shock protein 27 controls apoptosis by regulating Akt activation

    J. Biol. Chem.

    (2003)
  • A.D. Basso et al.

    Akt forms an intracellular complex with heat shock protein 90 (Hsp90) and Cdc37 and is destabilized by inhibitors of Hsp90 function

    J. Biol. Chem.

    (2002)
  • M.R. Gold

    Akt is TCL-ish: implications for B-cell lymphoma

    Trends Immunol.

    (2003)
  • D.R. Alessi et al.

    Molecular basis for the substrate specificity of protein kinase B: comparison with MAPKAP kinase-1 and p70 S6 kinase

    FEBS Lett.

    (1996)
  • T. Obata et al.

    Peptide and protein library screening defines optimal substrate motifs for AKT/PKB

    J. Biol. Chem.

    (2000)
  • J. Deprez et al.

    Phosphorylation and activation of heart 6-phosphofructo-2-kinase by protein kinase B and other protein kinases of the insulin signaling cascades

    J. Biol. Chem.

    (1997)
  • T. Schmelzle et al.

    TOR, a central controller of cell growth

    Cell

    (2000)
  • V.P. Krymskaya

    Tumour suppressors hamartin and tuberin: intracellular signalling

    Cell. Signal.

    (2003)
  • B.D. Manning et al.

    Identification of the tuberous sclerosis complex-2 tumor suppressor gene product tuberin as a target of the phosphoinositide 3-kinase/akt pathway

    Mol. Cell

    (2002)
  • H.C. Dan et al.

    Phosphatidylinositol 3-kinase/Akt pathway regulates tuberous sclerosis tumor suppressor complex by phosphorylation of tuberin

    J. Biol. Chem.

    (2002)
  • S.R. Datta et al.

    Akt phosphorylation of BAD couples survival signals to the cell-intrinsic death machinery

    Cell

    (1997)
  • M. Donepudi et al.

    Structure and zymogen activation of caspases

    Biophys. Chemist.

    (2002)
  • H.S. Park et al.

    Akt (protein kinase B) negatively regulates SEK1 by means of protein phosphorylation

    J. Biol. Chem.

    (2002)
  • G. Rena et al.

    Phosphorylation of the transcription factor forkhead family member FKHR by protein kinase B

    J. Biol. Chem.

    (1999)
  • P.F. Jones et al.

    Molecular cloning and identification of a serine/threonine protein kinase of the second-messenger subfamily

    Proc. Natl. Acad. Sci. U. S. A.

    (1991)
  • Cited by (642)

    • Roles of Trk receptors, tyrosine kinase receptors for neurotrophins, in the developing CNS

      2023, Receptor Tyrosine Kinases in Neurodegenerative and Psychiatric Disorders
    View all citing articles on Scopus
    View full text