Phytohemagglutinin-induced changes in tyrosine protein kinase and its endogenous substrates in human lymphocytes

We have measured changes in tyrosine protein kinase activity in the particulate and soluble fractions of phytohemagglutinin (PHA)-stimulated human lymphocytes using a synthetic tyrosine-containing peptide as substrate. Following PHA stimulation, the activity of this enzyme remained stable for 24 h and then fell sharply in both fractions. Phosphorylation of endogenous substrates was also studied by gel electrophoresis of the same subcellular preparations which had been incubated with [gamma-32P]ATP. Gels were treated with alkali to hydrolyse phosphoserine, dried and autoradiographed. In the particulate fraction from resting lymphocytes, two prominent bands were labelled, one of 55-60 kD and another of 38 kD. Direct analysis showed that these proteins were labelled largely on tyrosine residues. The 38 kD band became less intensely labelled 24 h following PHA addition, and had disappeared by 72 h. Two new bands appeared at this time: a 42 kD band which was phosphorylated mostly on threonine and a 32 kD band phosphorylated on tyrosine. The 55-60 kD band remained unchanged up to 96 h following PHA addition. The final band pattern in stimulated lymphocytes resembled that of the malignant T lymphoblastic cell line JM1. Our data suggest that tyrosine protein kinases in resting lymphocytes are part of a mechanism which transduces external growth signals to the cell interior, and that this mechanism is inactivated once the signal has been transmitted. The pattern of endogenous substrates which are phosphorylated on tyrosine and threonine residues in the malignant T lymphoblastic cell lines JM1 is likely to be characteristic of proliferating T lymphoid cells rather than of the malignant state, since similar bands appear in normal lymphocytes upon PHA stimulation.