Recycling Met

Abstract

Hammond et al. investigated the effects of pharmacological manipulation of endosomal sorting of the Met receptor tyrosine kinase (RTK) and discovered differential effects on various aspects of Met signaling, which suggested that receptor interactions within the sorting endosome may play a role in determining the consequences of Met activation. Stimulation of Met, the receptor for hepatocyte growth factor (HGF), elicits "invasive growth," a programmed response that includes mitogenesis, morphogenesis, and increased cell motility, as well as receptor down-regulation through internalization and degradation. Aberrant Met signaling has been implicated in tumor progression and metastasis. Hammond et al. used the proteasome inhibitor lactacystin, which interferes with Met trafficking and thereby endosomal degradation, to investigate the effects of endosomal sorting on Met signaling in HeLa cells. The authors used Western analysis of biotinylated Met, in combination with biotin stripping at the cell surface, to show that lactacystin promoted Met recycling to the cell membrane rather than inhibiting receptor internalization. Ligand-induced Met ubiquitination still occurred, although there were subtle changes in the ubiquitination pattern revealed by Western analysis. Lactacystin had different effects on HGF-stimulated signaling. The time course of HGF-dependent Met autophosphorylation was prolonged, whereas activation of mitogen-activated protein kinase (MAPK) signaling pathways was unchanged. HGF-dependent phosphorylation of Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate), a protein localized in sorting endosomes that has been implicated in RTK trafficking, was inhibited. Hrs knockdown with small, interfering RNA also prolonged Met phosphorylation and inhibited Met degradation. The authors concluded that receptor interactions within the sorting endosome are critical determinants of Met signaling. D. E. Hammond, S. Carter, J. McCullough, S. Urbé, G. Vande Woude, M. J. Clague, Endosomal dynamics of Met determine signaling output. Mol. Biol. Cell 14 , 1346-1354 (2003). [Abstract] [Full Text]