Growth factor induction of cytosolic protein tyrosine kinase activity in human haemopoietic progenitor cells isolated by flow cytometry

We employed a highly sensitive method to assay protein tyrosine kinase activity in extracts of subpopulations of CD34⁺ bone marrow progenitor cells isolated by fluorescence activated cell sorting in an attempt to better define how growth-factor induction of enzymatic activity relates to progenitor cell maturation. FAGS analysis confirmed that, under the conditions employed, essentially all of the CD34⁺ cells in adult human marrow that lacked the CD38 antigen were devoid of the myeloid maturation marker CD33 as well as the lineage antigens: CD10, 13, 14, 15, 16, 19, 71 and glycophorin A. A variable portion (50-90%) of these CD34⁺, CD38^- progenitor cells expressed HLA-DR. CD34⁺, CD38^- cells that did not express HLA-DR were found to lack detectable levels of either membrane or cytosolic tyrosine kinase activity, HLA-DR⁺ progenitor cells that lacked CD38 possessed elevated levels of cytosolic tyrosine kinase activity but only low levels of plasma membrane activity. In contrast, CD34⁺ cells that expressed CD38 (and HLA-DR) possessed high levels of membrane-associated tyrosine kinase activity. A cocktail of haemopoietic growth factors that included IL-3, IL-6 and stem cell factor effectively induced tyrosine kinase activity in CD34⁺, CD38^-, HLA-DR^- progenitor cells. Growth factor induction of tyrosine kinase activity in these cells was not inhibited by actinomycin D or cyclohexamide. Most of the tyrosine kinase activity induced by these growth factors was recovered from the cytosolic fraction of disrupted cells. Thus, induction of cytosolic tyrosine kinase activity is an early event in the response of uncommitted haemopoietic cells to haemopoietic growth factors. Subsequent activation of membrane tyrosine kinases may initiate key transduction processes as these cells begin to differentiate.