Phosphotyrosine antibodies as probes for activated oncogene products endowed with tyrosine kinase activity

The usefulness of phosphotyrosine antibodies for the detection of physiologically regulated or deregulated kinases is shown in this paper. This rather rare enzymatic activity is shared by receptors for some polypeptide growth factors and by the products of class 1 oncogenes. The antibodies are able to detect proteins phosphorylated on tyrosine in fibroblasts stimulated with growth factors, such as EGF and PDGF. The major phosphorylated protein species are the receptors themselves, which undergo phosphorylation only following the addition of the exogenous factor and only transiently. Phosphotyrosine antibodies were able to detect the products of the retroviral class 1 oncogenes, which are endowed of deregulated tyrosine kinase activity. In fact, in these cases a constitutive phosphorylation of the relevant proteins was observed, which occurred continuously and independently of the presence or lack of the growth factor. A tyrosine kinase constitutively activated in human gastric carcinoma cells was detected by P-Tyr antibodies. This molecule has been characterized at molecular level and the mechanisms responsible for its enzymatic activation has been investigated. The question of whether the tyrosine kinase identified is responsible for the induction and the maintenance of the transformed phenotype in gastric carcinomas remains to be answered. It is reasonable to suggest that this might be the case by analogy with other known pathologies, such as class 1 oncogenes activated by transduction by retroviruses, abnormal expression of EGF receptors or deregulated activity of c-abl encoded proteins in CML and ALL. Thus, the search for deregulated kinases by means of phosphotyrosine antibodies seems to be useful for identifying new activated oncogenes in clinical oncology.