Histochemically demonstrable phosphotyrosyl-protein phosphatase in normal human breast, in benign breast diseases and in breast cancer.

The activity of phosphotyrosyl-protein phosphatase enzyme was investigated by a histochemical method in the normal human breast and in breast diseases in order to evaluate its possible significance in the genesis and in the growth of benign and malignant epithelial proliferative. In normal human breast tissue only a weak enzyme activity was present. The activity was elevated in benign disease in actively proliferative lesions and in 71% of the cases of breast cancers. When enzyme activity of breast cancers was compared with the content of receptors for epidermal growth factor and insulin-like growth factor-I, no association was found. It is concluded that phosphotyrosyl-protein phosphatase is increased in actively proliferating human breast diseases. Thus the putative increase in phosphotyrosyl-proteins mediating benign and malignant epithelial proliferations is rather caused by an increase in protein-tyrosine kinase activity than by a decrease in phosphotyrosyl-protein phosphatase activity.