Proteins phosphorylated on tyrosine in human breast cancer

Proteins phosphorylated on tyrosine are detectable by antibodies against phosphotyrosine (P-Tyr) in cells transformed by oncogene-encoded tyrosine kinases. In this work, P-Tyr antibodies were used to investigate the existence of abnormal levels of tyrosine phosphoproteins in human breast cancers. Three human breast cancer cell lines and 37 human breast cancer specimens were examined by Western blot analysis and in vitro kinase assays. In the SK-BR-3 cell line three major phosphoproteins of approximate Mr of 185,000 (p185), 135,000 (p135) and 110,000 (p110) were detected. P185 was identified as the HER-2 gene-encoded protein. In the MCF-7 and CG-5 cell lines, a protein of approximate Mr of 170,000 (p170), together with a p135 and a p110, were found to be phosphorylated on tyrosine. P135 and p110, were also found to be abnormally phosphorylated on tyrosine in 50% of the breast cancer samples tested. In all samples the HER-2 protein was detectable at various levels but was not found to be phosphorylated.