Met acts on Mdm2 via mTOR to signal cell survival during development

Abstract

Coordination of cell death and survival is crucial during embryogenesis and adulthood, and alteration of this balance can result in degeneration or cancer. Growth factor receptors such as Met can activate phosphatidyl-inositol-3Ј kinase (PI3K), a major intracellular mediator of growth and survival. PI3K can then antagonize p53-triggered cell death, but the underlying mechanisms are not fully understood. We used genetic and pharmacological approaches to uncover Met-triggered signaling pathways that regulate hepatocyte survival during embryogenesis. Here, we show that PI3K acts via mTOR (Frap1) to regulate p53 activity both in vitro and in vivo. mTOR inhibits p53 by promoting the translation of Mdm2, a negative regulator of p53. We also demonstrate that the PI3K effector Akt is required for Met-triggered Mdm2 upregulation, in addition to being necessary for the nuclear translocation of Mdm2. Inhibition of either mTOR or Mdm2 is sufficient to block cell survival induced by Hgf-Met in vitro. Moreover, in vivo inhibition of mTOR downregulates Mdm2 protein levels and induces p53-dependent apoptosis. Our studies identify a novel mechanism for Met-triggered cell survival during embryogenesis, involving translational regulation of Mdm2 by mTOR. Moreover, they reinforce mTOR as a potential drug target in cancer.

Met signals hepatocyte survival by preventing Fas-triggered FLIP degradation in a PI3k-Akt-dependent manner

“…3,5,8 By studying cell survival mechanisms in hepatocytes induced by Met, we have recently provided the first direct link between Met and p53 through a novel signaling mechanism involving Mdm2 regulation via the P13K-Akt-mTOR pathway. 35 Interestingly, knocking out the p53 gene did not perturb liver development, although the expression of Fas was reduced in these mutants. 36 Whether during liver development, alteration of survival signals leads to upregulation of Fas by p53 to accelerate death remains to be determined.…”

Abl interconnects oncogenic Met and p53 core pathways in cancer cells

“…For biochemical studies, cell extracts, immunoprecipitations, and western blots were performed as described previously. 14,15,44 For quantitative real-time PCR, total RNA was extracted with Trizol (Invitrogen), according to the manufacturer's instructions. Reverse transcription was performed with Superscript Reverse Transciptase II (Invitrogen), according to the manufacturer's instructions.…”

“…Survival assays and anchorage-independent growth assays were performed as previously described. 14,15,18 Data on biological assays are representative of three independent experiments performed in duplicate.…”

“…As p53 functions are highly modulated by post-translational modifications, 26,31 we evaluated whether c-Abl inhibition would affect p53 phosphorylation. Basal low levels of p53 phosphorylation on Ser 15 and Ser 46 residues in the N-terminal region, which are usually phosphorylated in response to stress stimuli, were found in GTL-16 cells. No significant changes were observed in the presence of c-Abl antagonists (Supplementary Figure S4a).…”

“…The Met-RTK and its ligand HGF have essential functions during embryogenesis and regenerative processes by regulating cell migration, survival, proliferation, and differentiation, similar to Met functions during the oncogenic processes, 4,[12][13][14][15][16][17] and effective Met antagonists have been identified. 4,18,19 Moreover, Met confers resistance to ErbB antagonists, as cancer cells use Met to maintain the RTK-driven oncogenic pathways active.…”