The Shc gene encodes three overlapping
proteins which all contain a carboxy-terminal SH2 domain. Shc
proteins are ubiquitously expressed and are downstream targets and effectors of activated
tyrosine kinases (TK). We investigated
tyrosine-phosphorylation of Shc
proteins in normal and transformed cells. In
tumor cells with known TK gene alterations Shc
proteins were constitutively phosphorylated and complexed with the activated TK. No constitutive Shc phosphorylation was found in primary cell cultures and normal tissues. In 14 of 27 tumor cell lines with no reported TK alterations, Shc
proteins were constitutively phosphorylated and formed stable complexes with novel
tyrosine-phosphorylated
polypeptides. Ten distinct Shc-associated
phosphoproteins were identified with molecular weights ranging from 30 to 200 kDa. In a subset of
carcinoma cell lines, phosphorylated Shc
proteins complexed with a p175
phosphoprotein that was identified as the constitutively activated EGFR. In one
glioblastoma cell line, a Shc-associated p190 was identified as the activated PDGFR. In 13 of 14 acute
leukemia samples phosphorylated Shc
proteins were constitutively complexed with a p140
phosphoprotein. Some of the Shc-associated
phosphoproteins (EGFR, PDGFR, erbB-2, Met, bcr-abl, H4-ret) bound both the Shc- and Grb2-SH2 domains in vitro; others (p175; p70-p80) only the Shc-SH2 domain and yet others (p140) only the Grb2-SH3 domains. These results indicate that Shc
proteins are common substrates of constitutively activated TKs and that the analysis of Shc phosphorylation allow the identification of
tumors with constitutive TK activation.