BCR-ABL induces opposite phenotypes in murine ES cells according to STAT3 activation levels.

The mechanisms by which p210-BCR-ABL determines hematopoietic stem cells fate remain poorly understood. To better understand the behavior of BCR-ABL in pluripotent stem cells, we previously developed a murine embryonic stem (ES) cell model transformed by p210-BCR-ABL and reported that BCR-ABL activates STAT3, a major protein involved in ES cells self-renewal, which leads specifically to inhibition of ES cells differentiation. We show here that BCR-ABL either inhibits differentiation or, unexpectedly, induces a rapid commitment to differentiation of murine ES cells, according to the intracellular levels of activated STAT3. We show that inhibition of endogenous STAT3 activation with an inducible STAT3 protein with dominant-negative activity (STAT3F) results in an early, rapid and complete differentiation of BCR-ABL-expressing ES cells, whereas control ES cells retain a more undifferentiated phenotype. This phenomenon could be totally abrogated by PD98059, a specific MEK1 inhibitor, suggesting the involvement of mitogen-activated protein kinase (MAP-Kinase)/ERK1/2 pathway, which was found constitutively phosphorylated in BCR-ABL-expressing cells. In addition, BCR-ABL-expressing ES cells harboring low levels of activated STAT3 committed more rapidly through hematopoietic differentiation, since embryoid bodies (EBs) derived from these cells were able to generate numerous hematopoietic progenitors 2 days early. Moreover, BCR-ABL-expressing ES cells cultured first with low levels of activated STAT3 before EBs derivation displayed a more rapid loss of pluripotency than controls and failed to generate hematopoietic progenitors. This phenomenon was partially abrogated when ES cells were first exposed to PD98059 or to the tyrosine kinase inhibitor imatinib mesylate. From this predictive model, we suggest that variations of the activation levels in BCR-ABL substrates such as STAT3 may represent "instructive" secondary cooperating events involved in the transformation of the leukemic cell phenotype during the course of CML.