C-KIT LIGAND STIMULATES TYROSINE PHOSPHORYLATION OF A SIMILAR PATTERNOF PHOSPHOTYROSYL PROTEINS IN PRIMARY PRIMITIVE NORMAL HEMATOPOIETIC PROGENITORS THAT ARE CONSTITUTIVELY PHOSPHORYLATED IN COMPARABLE PRIMITIVE PROGENITORS IN CHRONIC PHASE CHRONIC MYELOGENOUS LEUKEMIA

Characteristic of Philadelphia (Ph)(+) chronic myelogenous leukemia (C ML) is the presence of the chimeric BCR/ABL (p210) protein possessing elevated protein tyrosine kinase activity relative to the normal c-abl tyrosine kinase. Our previous studies demonstrated subtle differences in the growth, phenotypic and morphologic characteristics of the most primitive subpopulations of primary lin(-) Ph(+) chronic phase CML bl asts and comparable primary lin(-) normal blasts. Recently, in compari ng proteins phosphorylated on tyrosine in these cell populations, we r eported a prominent 62 kDa phosphotyrosyl (P-tyr) protein constitutive ly present in primary primitive lin(-) CML chronic phase blasts which was virtually undetectable in primary primitive lin(-) normal blasts. In the present studies, we demonstrate that this P-tyr p62 from primar y primitive lin(-) chronic phase CML blasts co-immunoprecipitates with ras-GAP. Furthermore, in addition to the p210 protein, we show in who le cell lysates the presence of other clearly consistent but less prom inent P-tyr proteins with molecular weights of similar to 155, 140, 11 0, 55 and 45 kDa as well as more minor P-tyr proteins of similar to 19 0, 85, 52, 42 and 39 kDa constitutively present in primary primitive l in(-) chronic phase CML blasts. In analyzing proteins tyrosine phospho rylated in primary primitive lin(-) normal blasts in response to vario us hematopoietic growth factors, we found a striking similarity in the phosphorylation of four major (similar to 140, 110, 62 and 56 kDa) an d three minor (similar to 51, 45 and 42 kDa) P-tyr proteins after stim ulation with c-kit ligand and the P-tyr proteins constitutively phosph orylated in primary primitive lin(-) chronic phase CML blasts. Other g rowth factors tested (ie GM-CSF, G-CSF, IL-3, FLT3 ligand and EPO) wer e much less active or stimulated phosphorylation of other proteins. It is provocative that at least seven proteins rapidly and transiently p hosphorylated on tyrosine in the c-kit ligand signal transduction path way in lin(-) normal blasts may be constitutive substrates for the p21 0 activated tyrosine kinase in comparable lin(-) chronic phase CML bla sts. In addition, it is intriguing that some of the biological effects on hematopoietic progenitors attributed to the c-kit ligand may be si milar to some of the observed biological consequences of the p210 prot ein, including survival and expansion of a more mature stem cell popul ation, probably at the time of lineage commitment rather than at the l evel of the earliest self-renewing stem cell.