Protein kinase B (c-Akt) : a multifunctional mediator of phosphatidylinositol 3-kinase activation

While a plethora of extracellular molecules exist that modulate cellular functions via binding to membrane receptors inside the cell, their actions are mediated by relatively few signalling mechanisms. One of these is activation of phosphatidylinositol 3- kinase (PI-3K), which results in the generation of a membrane- restricted second messenger, polyphosphatidylinositides contain- ing a 3`-phosphate. How these molecules transduced the effects of agonists of PI-3K was unclear until the recent discovery that several protein kinases become activated upon exposure to 3`- phosphorylated inositol lipids. These enzymes include protein kinase B (PKB)/AKT and PtdIns(3,4,5)P3 -dependent kinases 1 and 2, the first two of which interact with 3`-phosphorylated phosphoinositides via pleckstrin homology domains. Once targeted to the membrane by this motif, PKB becomes phosphorylated at two residues, which relieves intermolecular inhibition, allowing the activated complex to dissociate and modify its targets. Identification of these substrates is the subject of intensive research, since at least one must play a key role in suppressing apoptosis, as demonstrated by expression of activated alleles of PKB. The generation of effective trans- dominant mutants, coupled with genetic analysis of the protein kinase in simpler organisms, should help in elucidating out- standing questions in the functions, targets and regulation of this important mediator of PI-3K signalling

Intracellular control of developmental and regenerative axon growth

Axon growth is a highly regulated process that requires stimulating signals from extracellular factors. The extracellular signals are then transduced to regulate coordinately gene expression and local axon assembly. Growth factors, especially neurotrophins that act via receptor tyrosine kinases, have been heavily studied as extracellular factors that stimulate axon growth. Downstream of receptor tyrosine kinases, recent studies have suggested that phosphatidylinositol-3 kinase (PI3K) regulates local assembly of axonal cytoskeleton, especially microtubules, via glycogen synthase kinase 3β (GSK-3β) and multiple microtubule binding proteins. The role of extracellular signal regulated kinase (ERK) signalling in regulation of local axon assembly is less clear, but may involve the regulation of local protein translation. Gene expression during axon growth is regulated by transcription factors, among which cyclic AMP response element binding protein and nuclear factors of activated T-cells (NFATs) are known to be required for neurotrophin (NT)-induced axon extension. In addition to growth factors, extracellular matrix molecules and neuronal activity contribute importantly to control axon growth. Increasingly, evidence suggests that these influences act to enhance growth via coordinating with growth factor signalling. Finally, evidence is emerging that developmental versus regenerative axon growth may be mediated by distinct signalling pathways, both at the level of gene transcription and at the level of local axon assembly

An oncogenic mutation uncouples the v-Jun oncoprotein from positive regulation by the SAPK/JNK pathway in vivo

Stimulation of c-Jun transcriptional activity via phosphorylation mediated by the stress-activated or c-Jun amino-terminal (SAPK/JNK) subgroup of mitogen-activated protein kinases (MAP kinases) is thought to depend on a kinase-docking site (the delta region) within the amino-terminal activation domain, which is deleted from the oncogenic derivative, v-Jun [1] [2] [3]. This mutation markedly enhances v-Jun oncogenicity [4] [5]; however, its transcriptional consequences have not been resolved. In part, this reflects uncertainty as to whether binding of SAPK/JNK inhibits c-Jun function directly [6] [7] or, alternatively, serves to facilitate and maintain the specificity of positive regulatory phosphorylation [8]. Using a two-hybrid approach, we show that SAPK/JNK stimulates c-Jun transactivation in yeast and that this depends on both catalytic activity and physical interaction between the kinase and its substrate. Furthermore, c-Jun is active when tethered to DNA via SAPK/JNK, demonstrating that kinase binding does not preclude transactivation. Taken together, these results suggest that SAPK/JNK acts primarily as a positive regulator of c-Jun transactivation in situ, and that loss of the docking site physically uncouples v-Jun from this control. This loss-of-function model accounts for the deficit of v-Jun regulatory phosphorylation and repression of TPA response element (TRE)-dependent transcription observed in v-Jun-transformed cells and predicts that an important property of the oncoprotein is to antagonise SAPK/JNK-dependent gene expression

Activation of SAPK/JNK by TNF receptor 1 through a noncytotoxic TRAF2-dependent pathway

Interaction oi the p55 tumor necrosis factor receptor 1 (TNF-R1)-associated signal transducer TRADD with FADD signals apoptosis, whereas the TNF receptor-associated factor 2 protein (TRAF2) is required for activation of the nuclear transcription factor nuclear factor kappa B. TNF-induced activation of the stress-activated protein kinase (SAPK) was shown to occur through a noncytotoxic TRAF2-dependent pathway. TRAF2 was both sufficient and necessary for activation of SAPK by TNF-R1; conversely, expression of a dominant-negative FADD mutant, which blocks apoptosis, did not interfere with SAPK activation. Therefore, SAPK activation occurs through a pathway that is not required for TNF-R1-induced apoptosis