PKB/Akt

The PDK1–PKB/Akt axis represents one of the most actively researched cell signaling pathways. This protein kinase cascade is known to play a central role in mediating the actions of a range of stimuli including insulin, growth factors, integrins and GPCRs in addition to being involved in the regulation of cell survival, cellular metabolism (including insulin-stimulated glucose transport and glycogen synthesis), gene expression, cell cycle entry and protein synthesis.

All the kinases associated with this pathway lie in the protein serine/threonine kinase family and form a single highly branching protein kinase cascade (hence their grouping together). Several of these kinases contain pleckstrin homology (PH) domains that bind specific phosphoinositide lipids (e.g. phosphoinositide-3,4,5-trisphosphate; PIP₃) which are generated in the plasma membrane in response to agonist activity. As a result, the kinases are activated in a phosphoinositide 3-OH-kinase (PI3-kinase)- dependent manner.

3-Phosphoinositide-dependent protein kinase-1 (PDK1) stands at the head of this important signaling pathway. Whether extracellular stimuli directly activate PDK1 (perhaps via the generation of plasma membrane-localized PIP₃), or whether they simply induce the translocation of PDK1 to its substrate proteins within the plasma membrane, is not known. PDK1 activates a number of AGC-family protein kinases (named after their homology to protein kinases A, G and C) by phosphorylation, including protein kinase B (PKB or Akt) via phosphorylation of the T-loop residue Thr 308. The full activation of PKB/Akt also involves the binding of PIP₃ to the PH domain of PKB/Akt and the phosphorylation of an additional residue, Ser 473, by an as yet unidentified kinase called “PDK2” (proposed to be DNA-dependent protein kinase: DNA-PK). There is a great deal of functional overlap between PKB/Akt isoforms; all phosphorylate the same RXRXXS/T motif and all are capable of transforming a cell when rendered constitutively active by the introduction of a myristoylation signal sequence.

Thr 308 and Ser 473 lie within regions of PKB/Akt that are conserved throughout the AGC family kinases. Hence, PDK1 also phosphorylates and activates several other AGC-family kinases, including the serum and glucocorticoid-induced kinases (SGK), atypical forms of protein kinase C (e.g. PKCζ and PKCι/λ), p70S6-kinase and p90RSK. PDK1 is therefore a central controller of multiple cell signaling pathways.

Once phosphorylated and activated, PKB/Akt phosphorylates and inhibits glycogen synthase kinase 3 (GSK3) leading to a decreased phosphorylation and activation of glycogen synthase. PKB/Akt also phosphorylates the mammalian target of rapamycin (mTOR, also known as FRAP and RAFT) although the role of this action is not yet known. GSK3 continues to grow in importance as it also plays a role in the regulation of β-catenin stability and thus gene expression.

mTOR, which can be phosphorylated by PKB/Akt, is unusual in that it possesses both serine/threonine protein kinase as well as lipid kinase activities. It is a large complex molecule that is a receptor for the immunosuppressant, rapamycin. mTOR, along with PDK1, then plays an as yet ill-defined role in the activation of p70S6K which is important in the control of protein synthesis, development and growth control. Thus, at least in part, the immunosuppressive activity of rapamycin is due to its actions on mTOR.

Other than rapamycin, there are few if any highly specific pharmacological inhibitors of this collection of protein kinases. This has made understanding the role of these protein kinases in mediating the effects of extracellular stimuli very difficult to ascertain. Furthermore, while in some cases kinase-dead derivatives of these kinases have been reported to act as dominant-negatives, this has often been highly controversial, largely because of their complex domain structures and abilities to interact with other proteins and signaling lipids. However, given the central importance of these protein kinases in numerous disease states (e.g. cancer and diabetes), the identification of specific inhibitors remains a very important goal.

Footnotes

a) Inhibitors in parentheses are non-selective or yet unproven to be highly specific.

Abbreviations

Brk: Breast tumor-related kinase
CTMP: C-terminal modulatory protein
DLP: Dynamin-like protein
DNA-PK: DNA-dependent protein kinase
FKBP12: FK502 binding protein
FRAP: FKBP12-rapamycin-associated protein
GSK3: Glycogen synthase kinase 3
H89: N-(2-[p-Bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide
IMPDH: Inosine-5’ monophosphate dehydrogenase
MSK: Mitogen- and stress-activated kinase
MTOR: Mammalian target of rapamycin
PIKfyve: FYVE domain containing phosphatidylinositol 3-phosphate 5-kinase
PKA: Protein kinase A
PKC: Protein Kinase C
PKG: Protein kinase G
POSH: Plenty of SH3 domains
PtdIns(3,4)P2: Phosphatidylinositol 3,4-bisphosphate
PtdIns(3,4,5)P3: Phosphatidylinositol 3,4,5-trisphosphate
RAFT: Rapamycin and FKBP12 target
Ro 31-8220: 2-{1-[3-Amidinothio)propyl]-1H-indol-3-yl}-3-(1-methylindol-3-yl)-maleimide
SB-216763: 3-(2,4-Dichlorophenyl)-4-(1-methyl-1H-indole-3-yl)-1H-pyrrole-2,5-dione
SB-415286: 3-(3-Chloro-4-hydroxyphenylamino)-4-(2-nitrophenyl)-1H-pyrrole-2,5-dione
SGK: Serum and glucocorticoid regulated kinase
TCL: T-cell leukemia
TSC2: Tuberous Sclerosis Complex-2
UCN-1: 7-Hydroxystaurosporine