Diacylglycerol kinases are involved in cell signaling, either as regulators of diacylglycerol levels or as intracellular signal‐generating enzymes. However, neither their role in signal transduction nor their biochemical regulation has been elucidated. Hepatocyte growth factor (HGF), upon binding to its tyrosine kinase receptor, activates multiple signaling pathways stimulating cell motility, scattering, proliferation and branching morphogenesis. Herein we demonstrate that: (i) the enzymatic activity of α‐diacylglycerol kinase (αDgk) is stimulated by HGF in epithelial, endothelial and αDgk‐transfected COS cells; (ii) cellular expression of an αDgk kinase‐defective mutant inhibits activation of endogenous αDgk acting as dominant negative; (iii) specific inhibition of αDgk prevents HGF‐induced cell movement of endothelial cells; (iv) HGF induces the association of αDgk in a complex with Src, whose tyrosine kinase activity is required for αDgk activation by HGF; (v) Src wild type stimulates αDgk activity in vitro; and (vi) αDgk can be tyrosine phosphorylated in intact cells.

Activation of the HGF receptor, encoded by the c-MET protooncogene (Met receptor), triggers motility, matrixinvasion and branching morphogenesis in epithelial cells. It has recently been shown that the Met receptor interacts with Gab-1, an IRS-like adaptor protein, via the docking site (Y 1349 VHVNATY 1356 VNV) known to bind Grb2 and multiple SH2-containing signal transducers. Here we show that Gab1 is the major phosphorylation-substrate of the Met receptor and of its oncogenic variant Tpr-Met. A series of point mutations in the docking site established a direct correlation between the ability to recruit and phosphorylate Gab1 and the transforming potential. Interestingly, the mutations of either Y 1356 or N 1358 abolished the binding of both Grb2 and Gab1 in intact cells. Furthermore, peptides designed to block either the SH2 or the SH3 domains of Grb2 interfered with the receptor-Gab1 interaction. These data indicate that Gab1 coupling to the Met receptor requires binding of Grb2 and correlates with the transforming potential of Tpr-Met.

nef is a human immunodeficiency virus (HIV) gene encoding a 27-kDa myristoylated protein with structural features of a signal transducing molecule, but whose functions are largely unknown. We studied the interactions of Nef with the signal transduction pathways triggered by the platelet-derived growth factor (PDGF) receptor. The association of phosphatidylinositol (PI) 3-kinase with the activated receptor was severely impaired by nef expression. Conversely, PDGF-induced receptor tyrosine phosphorylation, binding to phospholipase C-gamma and to Ras-GAP were not modified. Microtubule-associated protein kinase activation and intracellular calcium influx in response to PDGF were either unaffected or only slightly enhanced. Nef significantly reduced the proliferative response to the growth factor, while the chemotactic response was unchanged. These data show that Nef affects selectively the PI 3-kinase signaling pathway and suggest that this interference results in some of the HIV adverse effects on host cell functions.

The assumption that genes encoding tyrosine kinase receptors could play a role in human cancers has been confirmed by the identification of oncogenic mutations in the kinase domain of RET and KIT. Recently, homologous residues were found mutated in MET, in papillary renal carcinomas (PRCs). The link coupling these genetic lesions to cellular transformation is still unclear. MET PRC mutations result in increased kinase activity and-in some instances, i.e., M1250T substitution-in changes in substrate specificity. A direct correlation occurs between the transforming potential of MET PRC mutants and their ability to constitutively associate with signal transducers through two phosphorylated tyrosines (Y 1349 VHVNATY 1356 VNV) located in the receptor tail. Substitution of these ''docking tyrosines'' with phenylalanines leaves unaffected the altered properties of the kinase but abrogates transformation and invasiveness in vitro. Uncoupling the receptor from signal transducers with a tyrosine-phosphorylated peptide derivative (Y p VNV) inhibits invasive growth induced by MET PRC mutants. These data indicate that constitutive receptor coupling to downstream signal transducers is a key mechanism in neoplastic transformation driven by mutated MET and suggest a therapeutic strategy to target neoplastic diseases associated with this oncogene.

Overexpressed or activated hepatocyte growth factor receptor, encoded by the MET proto-oncogene, was found in the majority of colorectal carcinomas (CRCs), whose stepwise progression to malignancy requires transcriptional activation of b-catenin. We here demonstrate that a functional crosstalk between Met and b-catenin signaling sustains and increases CRC cell invasive properties. Hepatocyte growth factor (HGF) stimulation prompts b-catenin tyrosine phosphorylation and dissociation from Met, and upregulates b-catenin expression via the phosphatidylinositol 3-kinase pathway in conditions that mimic those found by the invading and metastasizing cells. Additionally, a transcriptionally active form of b-catenin, known to be oncogenic, enhances Met expression. Furthermore, HGF treatment increases the activity of the b-catenin-regulated T-cell factor transcription factor in cells expressing the wild-type or the oncogenic b-catenin. In the mirror experiments, either Met or b-catenin knocking down also reduces their protein level. In biological assays, b-catenin knocking down abrogates the HGF-induced motile phenotype, whereas active b-catenin fosters ligand-independent cell scattering. Met and b-catenin also cooperate in promoting entry into the cell cycle and in protecting cells from apoptosis. In conclusion, Met and b-catenin pathways are mutually activated in CRC cells. This might generate a selfamplifying positive feedback loop resulting in the upregulation of the invasive growth properties of CRC cells.