ACTIVATION OF SRC FAMILY KINASES IN HUMAN PRE-B CELLS BY IL-7

lL-7 was identified originally as a specific pre-B cell growth factor. We have investigated its signal transduction mechanism by using the h uman pre-B cell line Nalm-6, and have found that it stimulates tyrosin e phosphorylation of various proteins: pp27, pp43, pp46, pp54, pp64, p p78, pp90, pp105, and pp120. Antiphosphotyrosine immunoprecipitates fr om IL-7-stimulated Nalm-6 showed two major proteins of M(r) = 60,000 a nd 55,000, capable of autophosphorylation. Autophosphorylation was max imal 10 min after the cells were challenged with the cytokine. Antipho sphotyrosine immunoprecipitates from IL-7-stimulated cells also increa sed tyrosine phosphorylation of the exogenously added substrate histon e H2B. Furthermore, by using a polyclonal anti-IL-7 receptor (IL-7R) A b in Western blotting analysis, we observed that antiphosphotyrosine i mmunoprecipitates were associated with the IL-7R in a transient manner . These data indicate that the IL-7R associates with tyrosine-phosphor ylated proteins as its amino acid sequence is devoid of a putative sit e of tyrosine phosphorylation. These results were confirmed as several P-32-labeled proteins were visualized after immunoprecipitation by us ing anti-IL-7R Ab. Anti-IL-7R immunoprecipitates from IL-7-stimulated cells revealed a unique band of M(r) = 60,000 associated with the rece ptor able to autophosphorylate in the presence of ATP and Mn2+. Hence, we identified p59(fyn) and p53/56(lyn) to be stimulated by IL-7. In c ontrast to p53/56(lyn), p59(fyn) was found to be associated constituti vely with the cloned IL-7R. These data emphasize the role of the src f amily in hematopoiesis.