Induction of resistance or enhancement to a transplantable murine plasmacytoma by transfer of non-immune leucocytes.

Newborn mice have a lower spontaneous resistance to the growth of a syngeneic plasmacytoma (MOPC-460) as compared to adult mice. The transfer of different leucocyte populations from non-immunized adult donors to newborn mice influence in a dual way the resistance to MOPC-460 growth, depending on the number of cells transferred. The transfer of a low number of neutrophils, thymus or spleen cells enhances the MOPC-460 takes. Higher numbers of neutrophils, thymus or bone marrow cells induce an effective protenction. By contrast, macrophages over a dose of 1 X 10(4) constantly produce a reduction of tumour growth

β4 integrin activates a Shp2–Src signaling pathway that sustains HGF-induced anchorage-independent growth

Despite being a cell–matrix adhesion molecule, β4 integrin can prompt the multiplication of neoplastic cells dislodged from their substrates (anchorage-independent growth). However, the molecular events underlying this atypical behavior remain partly unexplored. We found that activation of the Met receptor for hepatocyte growth factor results in the tyrosine phosphorylation of β4, which is instrumental for integrin-mediated recruitment of the tyrosine phosphatase Shp2. Shp2 binding to β4 enhances the activation of Src, which, in turn, phosphorylates the multiadaptor Gab1 predominantly on consensus sites for Grb2 association, leading to privileged stimulation of the Ras–extracellular signal-regulated kinase (ERK) cascade. This signaling axis can be inhibited by small interfering RNA–mediated β4 depletion, by a β4 mutant unable to bind Shp2, and by pharmacological and genetic inhibition of Shp2 or Src. Preservation of the β4 docking sites for Shp2 as well as the integrity of Shp2, Src, or ERK activity are required for the β4-mediated induction of anchorage-independent growth. These results unravel a novel pathway whereby β4 directs tyrosine kinase–based signals toward adhesion-unrelated outcomes

Mutations in the met Oncogene Unveil a "Dual Switch" Mechanism Controlling Tyrosine Kinase Activity *

The met oncogene, encoding the high affinity hepatocyte growth factor receptor, is the only known gene inherited in human cancer that is invariably associated with somatic duplication of the mutant locus. Intriguingly, mutated Met requires ligand stimulation in order to unleash its transforming potential. Furthermore, individuals bearing a germ line met mutation develop cancer only late in life and with incomplete penetrance. To date, there is no molecular explanation for this unique behavior, which is unusual for a dominant oncogene. Here we investigate the molecular mechanisms underlying met oncogenic conversion by generating antibodies specific for the differently phosphorylated forms of the Met protein. Using these antibodies, we show that activation of wild-type Met is achieved through sequential phosphorylation of Tyr1235 and Tyr1234 in the activation loop and that mutagenesis of either tyrosine dramatically impairs kinase function. Surprisingly, oncogenic Met mutants never become phosphorylated on Tyr1234 despite their high enzymatic activity, and mutagenesis of Tyr1234 does not affect their biochemical or biological function. By analyzing the enzymatic properties of the mutant proteins in different conditions, we demonstrate that oncogenic mutations do not elicit constitutive kinase activation but simply overcome the requirement for the second phosphorylation step, thus reducing the threshold for activation. In the presence of activating signals, these mutations result therefore in a dynamic imbalance toward the active conformation of the kinase. This explains why mutant met provides an oncogenic predisposition but needs a second activating "hit," provided by sustained ligand stimulation or receptor overexpression, to achieve a fully transformed phenotype

Hepatocyte growth factor/scatter factor stimulates the Ras-guanine nucleotide exchanger

Hepatocyte growth factor/scatter factor (HGF/SF) induces mitogenesis and cell dissociation upon binding to the protein-tyrosine kinase receptor encoded by the MET proto-oncogene (p190MET). The signal transduction pathways downstream from the receptor activation are largely unknown. We show that HGF/SF activates Ras protein. HGF/SF stimulation of metabolically labeled A549 cells raised the amount of Ras-bound radiolabeled guanine nucleotides by over 5-fold. Furthermore, following HGF/SF stimulation of these cells, 50% of Ras was in the GTP-bound active state. The uptake by Ras of radiolabeled GTP was also increased by 5-fold following HGF/SF stimulation in digitonin-permeabilized A549 cells. Moreover, HGF/SF treatment of A549 cells leads to stimulation of the cytosolic Ras-guanine nucleotide exchange activity, measured as accelerated release of [3H]GDP from purified recombinant Ras protein in vitro, in a dose- and time-dependent manner. Likewise, treatment with the protein-tyrosine kinase inhibitor 3-(1',4'-dihydroxytetralyl)methylene-2-oxindole of GTL-16 cells (featuring a p190MET receptor constitutively active) significantly decreased the cytosolic Ras-guanine nucleotide exchange activity. These data demonstrate that HGF/SF activates Ras protein by shifting the equilibrium toward the GTP-bound state and increases the uptake of guanine nucleotides by Ras, through mechanism(s) including the activation of a Ras-guanine nucleotide exchanger