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Alex Toker

Publications and source records attributed to Alex Toker.

25 records · Page 2Linked to original sources
PubMed2002

Regulation of novel protein kinase C epsilon by phosphorylation.

The activity and intracellular localization of protein kinase C (PKC) family members are controlled by phosphorylation at three highly conserved sites in the catalytic kinase domain. In the case of the novel PKCepsilon isoform, these are Thr(566) in the activation loop, Thr(710) in the turn motif and Ser(729) in the C-terminal hydrophobic motif. In the present study, we analysed the contribution of the phosphoinositide-dependent kinase 1 (PDK-1) and PKCepsilon kinase activity in controlling the phosphorylation of Thr(566) and Ser(729). In NIH 3T3 fibroblasts, PKCepsilon migrated as a single band, and stimulation with platelet-derived growth factor resulted in the appearance of a second band with a slower electrophoretic mobility, concomitant with an increase in phosphorylation of Thr(566) and Ser(729). Cells transfected with an active PDK-1 allele also resulted in increased PKCepsilon Thr(566) and Ser(729) phosphorylation, whereas an active myristoylated PKCepsilon mutant was constitutively phosphorylated at these sites. Protein kinase-inactive mutants of PKCepsilon were not phosphorylated at Ser(729) in cells, and phosphorylation of this site leads to dephosphorylation of the activation-loop Thr(566), an effect which can be reversed with either okadaic acid or co-transfection with active PDK-1. In vitro, PDK-1 catalysed the phosphorylation of purified PKCepsilon in the presence of mixed micelles containing either diacylglycerol or PtdIns(3,4,5)P(3), concomitant with an increase in Ser(729) phosphorylation. These studies reveal that the mechanism of phosphorylation of a novel PKC is the same as that for conventional PKCs: PDK-1 phosphorylation of the activation loop triggers autophosphorylation of the hydrophobic motif. However, the regulation of this phosphorylation is different for novel and conventional PKCs. Specifically, the phosphorylation of novel PKCs is regulated rather than constitutive.

3-Phosphoinositide-Dependent Protein Kinases↗
PubMed2002

3'-phosphoinositide-dependent kinase-1 (PDK-1) in PI 3-kinase signaling.

The recently discovered 3'-phosphoinositide-dependent kinase-1 (PDK-1) is a serine/threonine protein kinase which phosphorylates several members of the conserved AGC kinase superfamily (comprising the prototypes protein kinases A (PKA), G (PKG) and C (PKC)). Phosphorylation of a threonine or serine residue in the activation loop (also known as the T-loop) of these kinases is a critical step in their activation, and is typically accompanied by additional phosphorylations elsewhere in the molecule. Phosphorylation of the activation loop is a common regulatory mechanism shared by most serine/threonine as well as tyrosine kinases as it facilitates alignment of amino acid residues in the active site which are involved in the phosphotransferase reaction. Therefore the discovery of PDK-1 as the enzyme which mediates this event in many protein kinases introduced a new and important step in signaling pathways which regulate numerous important cellular processes including cellular survival, glucose transport and metabolism, tumor progression as well as protein translation. Moreover, the finding that PDK-1 function is mediated in part by the phosphoinositide 3'-OH-kinase (PI 3-K) pathway also provided an explanation as to how the lipid products of PI 3-K, namely phosphatidylinositol-3,4-bisphosphate (PtdIns-3,4-P2) and phosphatidylinositol-3,4-5-trisphosphate (PtdIns-3,4,5-P3) stimulate the activation of protein kinase-dependent signaling pathways. These initial landmark observations were followed by many important studies which provided additional mechanistic insight into both PDK-1 regulation as well as the role of this kinase in cellular function. This review will focus on the regulation of PDK-1 and the various mechanisms which it uses to contribute to the activation of target kinases.

3-Phosphoinositide-Dependent Protein Kinases↗
PubMed2002

The role of NFAT transcription factors in integrin-mediated carcinoma invasion.

Integrins, receptors for extracellular matrix ligands, are critical regulators of the invasive phenotype. Specifically, the alpha(6)beta(4) integrin has been linked with epithelial cell motility, cellular survival and carcinoma invasion, hallmarks of metastatic tumours. Previous studies have also shown that antagonists of the NFAT (nuclear factor of activated T-cells) family of transcription factors exhibit strong anti-tumour-promoting activity. This suggests that NFAT may function in tumour metastasis. Here, we investigate the involvement of NFAT in promoting carcinoma invasion downstream of the alpha(6)beta(4) integrin. We provide evidence that both NFAT1, and the recently identified NFAT5 isoform, are expressed in invasive human ductal breast carcinomas and participate in promoting carcinoma invasion using cell lines derived from human breast and colon carcinomas. NFAT1 and NFAT5 activity correlates with the expression of the alpha(6)beta(4) integrin. In addition, the transcriptional activity of NFAT5 is induced by alpha(6)beta(4) clustering in the presence of chemo-attractants, resulting in enhanced cell migration. These observations show that NFATs are targets of alpha(6)beta(4) integrin signalling and are involved in promoting carcinoma invasion, highlighting a novel function for this family of transcription factors in human cancer.

Antigens, Surface↗