Live-cell fluorescence imaging reveals high stoichiometry of Grb2 binding to the EGF receptor sustained during endocytosis.
Growth factors bind to membrane receptor tyrosine kinases, resulting in autophosphorylation and subsequent binding to proteins with SH2 domains, including growth factor receptor-bound protein 2 (Grb2). Grb2 bridges receptors to tyrosine kinase substrates such as SHC and SOS, which in turn facilitate the activation of downstream signaling pathways, including Ras and mitogen-activated protein kinase (MAPK). Overexpression of Grb2 has been demonstrated in several types of neoplasia but has not been investigated in liver tumorigenesis. Here we investigated Grb2 expression in liver lesions in N-nitrosodimethylamine (NDMA)-treated Helicobacter hepaticus-infected and -noninfected A/J mice at 1 year of age. Previously, we reported (6) that infection promotes the development of these NDMA-initiated tumors. In controls, Grb2 immunostaining was absent from normal hepatic tissues, whereas the inflammatory lesions in infected livers were positive for cytoplasmic Grb2 in both hepatocytes and infiltrating leukocytes. All preneoplastic foci (7 of 7), 15 of 27 adenomas, and 3 of 7 carcinomas were positive for Grb2 by immunostaining in both infected and noninfected NDMA-initiated livers. Involvement of Grb2 was confirmed by immunoblotting of similarly infected mice at 9 to 18 months of age, showing a 2.5- to 3.3-fold increase in Grb2 protein in infected livers (p < 0.05 compared with uninfected controls) as well as in preneoplastic foci, adenomas, and carcinomas. These livers also showed a 2.5- to 2.8-fold increase in total Ras protein. The results suggest that upregulation of Grb2 is an early event in liver carcinogenesis, whether caused by the bacterial infection or by NDMA. Concomitant upregulation of Ras p21 would ensure transmission of amplified signal from growth factors via Grb2.