Trends in Biochemical Sciences
ReviewTen years of protein kinase B signalling: a hard Akt to follow
Section snippets
Background and early observations
The origins of PKB/Akt research can be traced back to the discovery in 1977, by Staal and co-workers, of a transforming murine leukaemia virus from mice with a high incidence of spontaneous lymphoma [4]. This virus, termed AKT8, produced foci of malignant transformation in the mink lung epithelial cell line CCL-64 [5]. A unique feature of AKT8 was its inability to induce focus formation in other cell lines such as NIH3T3 fibroblasts, which suggested that the virus contained a previously
Initial characterization of PKB/Akt signalling
Experiments from Staal and co-workers had demonstrated that amplification of PKBα/AKT1 was detected in gastric adenocarcinoma [5]. With the cloning of the genes encoding PKB/Akt, the role of these genes in human cancers began to be examined more closely. Cheng and colleagues showed that the gene encoding Akt2 was amplified in two ovarian carcinoma cell lines and, similar to Staal and co-workers, the authors concluded that amplification of AKT2 contributed to the pathogenesis of the disease [11]
1995: a watershed year for PKB/Akt
The pace of PKB/Akt research began to pick up in 1995 and researchers started to focus on PKB/Akt molecules in other species. Experiments on the PKB/Akt homologue in Drosophila melanogaster, DRAC-PK, indicated that this protein was differentially expressed, and that the kinase activity was differentially regulated in embryo and adult flies [19]. Others reported that the PH domain of PKB/Akt played a role in mediating protein–protein interactions, nucleating the formation of PKB/Akt protein
The mechanism of PKB/Akt regulation: the role of lipid binding and phosphorylation
Early protein kinase assays using PKB/Akt had suggested that the kinase autophosphorylated when activated [30]. This suggested that, in addition to the binding of lipid to the PH domain of PKB/Akt, phosphorylation played a role in PKB/Akt regulation. This concept was supported by data showing that both serum stimulation, and protein phosphatase inhibitors such as pervanadate, induced PKB/Akt activation [34]. Incubation of PKB/Akt with protein phosphatase 2A (PP2A) in vitro inactivated the
The search for the upstream kinases
The discovery that phosphorylation on serine and threonine residues was crucial for PKB/Akt regulation precipitated an intensive search for the upstream kinase(s) responsible for this modification. Within one year, papers detailing the purification of a kinase that phosphorylated PKB/Akt on Thr308, and activated the kinase, were published 39, 40. This novel kinase was purified from rabbit skeletal muscle [39] and rat brain [40] and, consistent with the previous observation that
Cellular survival: a new role of PKB/Akt
Around the time that the pathway responsible for PKB/Akt activation was being described, others were searching for new downstream readouts of PKB/Akt activity. Up to this point, evidence for PKB/Akt function in glucose uptake and metabolism had been presented 32, 38, and the first cellular substrate for PKB/Akt, GSK-3, had been described [31]. Experiments using cerebellar granule neurons had demonstrated that withdrawal of serum or growth factors such as IGF-1 induced a rapid apoptosis of these
PDK1: a promiscuous upstream protein kinase
As the role of PKB/Akt in cell survival was being elucidated, others were striving to more fully characterize the enzyme responsible for PKB/Akt phosphorylation on residue Thr308 in the catalytic domain of the kinase. PKB/Akt is a member of a family of protein kinases that originally contained protein kinase A̱, cG̱MP-dependent protein kinase and protein kinase C̱, termed the AGC family. Proteins in this family contain regions of high homology in their kinase domains. More recently discovered
PTEN: a novel tumour suppressor protein involved in PKB/Akt regulation
In 1997, three independent groups identified a candidate tumour suppressor gene located at 10q23, a chromosomal region deleted in multiple tumour types, including glioblastoma, prostate and endometrial carcinoma, and melanoma (reviewed in Ref. [56]). This gene was termed PTEN (p̱hosphatase and sin homologue deleted on chromosome ), and subsequent studies indicated that mutations in PTEN occurred in a large fraction of glioblastoma and melanoma cell lines, advanced prostate cancers and
PKB/Akt ten years on: prospects for the future
The results of the past ten years of research have firmly established PKB/Akt as a key player in numerous physiological and pathophysiological processes (Fig. 4). Research into PKB/Akt has helped construct a map of growth factor signalling from the tyrosine kinase receptors at the plasma membrane, to enzymes responsible for generating second messengers (PI3K), to upstream kinases (PDK1), to effector kinases (PKB/Akt, p70S6K), to downstream targets such as BAD, GSK3-β and Forkhead family
Note added in proof
During the final stages of this review, a paper describing a novel protein that binds and inhibits PKB/Akt was published, further expanding our knowledge of PKB/Akt signalling [67].
Acknowledgements
The Friedrich Miescher Institute is part of the Novartis Research Foundation. We are very grateful to Jim Woodgett for helpful comments and insights on the manuscript, Michael Greenberg for permission to adapt Box 1 and Jianhua Feng for assistance with Table 1. Because of space restrictions, we could not include all pertinent references and we apologize to those researchers whose papers were not referenced, especially in Table 1 and Box 1.
References (67)
Mechanism of protein kinase B activation by insulin/insulin-like growth factor revealed by specific inhibitors of phosphoinositide 3-kinase – significance for diabetes and cancer
Pharmacol. Ther.
(1999)- et al.
The regulation and activities of the multifunctional serine/threonine kinase Akt/PKB
Exp. Cell Res.
(1999) A putative modular domain present in diverse signaling proteins
Cell
(1993)High affinity binding of inositol phosphates and phosphoinositides to the pleckstrin homology domain of RAC/protein kinase B and their influence on kinase activity
J. Biol. Chem.
(1997)Developmental regulation of expression and activity of multiple forms of the Drosophila RAC protein kinase
J. Biol. Chem.
(1995)Purification and characterization of phosphoinositide 3-kinase from rat liver
J. Biol. Chem.
(1990)PDGF-dependent tyrosine phosphorylation stimulates production of novel polyphosphoinositides in intact cells
Cell
(1989)The protein kinase encoded by the akt proto-oncogene is a target of the PDGF-activated phosphatidylinositol 3-kinase
Cell
(1995)Molecular basis for the substrate specificity of protein kinase B; comparison with MAPKAP kinase-1 and p70 S6 kinase
FEBS Lett.
(1996)Constitutive activation of protein kinase B and phosphorylation of p47phox by a membrane-targeted phosphoinositide 3-kinase
Curr. Biol.
(1996)