Overexpression of Grb2 in inflammatory lesions and preneoplastic foci and tumors induced by N-nitrosodimethylamine in helicobacter hepaticus-infectedand -noninfected A/J mice

Growth factors bind to membrane receptor tyrosine kinases, resulting in aut ophosphorylation and subsequent binding to proteins with SH2 domains, inclu ding 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 Pas and mitogen -activated protein kinase (MAPK). Overexpression of Grb2 has been demonstra ted in several types of neoplasia but has not been investigated in liver tu morigenesis. Here we investigated Grb2 expression in liver lesions in N-nit rosodimethylamine (NDMA)-treated Helicobacter hepaticus-infected and -nonin fected A/J mice at 1 year of age. Previously, we reported (6) that infectio n promotes the development of these NDMA-initiated tumors. In controls, Grb 2 immunostaining was absent from normal hepatic tissues, whereas the inflam matory lesions in infected livers were positive for cytoplasmic Grb2 in bot h hepatocytes and infiltrating leukocytes. All preneoplastic foci (7 of 7), 15 of 27 adenomas, and 3 of 7 carcinomas were positive for Grb2 by immunos taining 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 i nfected livers (p < 0.05 compared with uninfected controls) as well as in p reneoplastic foci, adenomas, and carcinomas. These livers also showed a 2.5 - to 2.8-fold increase in total Ras protein. The results suggest that upreg ulation of Grb2 is an early event in liver carcinogenesis, whether caused b y the bacterial infection or by NDMA. Concomitant upregulation of Ras p21 w ould ensure transmission of amplified signal from growth factors via Grb2.