Direct relation between BCR-ABL tyrosine kinase activity and cyclin D2 expression in lymphoblasts

Leukemia cells bearing the Philadelphia (Ph) chromosome express a Bcr-Abl fusion protein with deregulated protein tyrosine kinase (PTK) activity, which plays a central role in the malignant transformation. Many different signal transduction pathways are activated by Bcr-Abl, but little is known about their downstream targets in specific cell lineages. We show here that Ph-positive cell lines as well as primary cells derived from chronic myeloid leukemia (CML) in lymphoid blast crisis or from acute lymphoblastic leukemia (ALL) consistently express high levels of cyclin D2, whereas expression of this protein is low or absent in comparable Ph-negative lines and Ph-positive myeloid lines. Inhibition of Bcr-Abl with STI571 resulted in down-regulation of cyclin D2 and reduction of the number of cells in S phase, although complete G1 arrest was not induced. The expression of cyclin D2 in Ph-positive lymphoblasts was mediated via the phosphatidyl-inositol-3 kinase pathway. Analogous results were seen in murine BaF/3 cells transfected with a BCR-ABL expression vector. In contrast to the human cell lines, murine Baf/BCR-ABL cells exposed to STI571 inhibitor were all arrested in G1. This arrest could be abrogated by exogenous expression of cyclin D2 from a transfected cDNA construct. We conclude that a direct connection exists between Bcr-Abl PTK activity and cell cycle progression in which cyclin D2 plays a critical role. However, cell cycle progression in human Ph-positive lymphoid cells is not entirely dependent on Bcr-Abl PTK, and additional genetic lesions must be present.

BCR-ABL and interleukin 3 promote haematopoietic cell proliferation and survival through modulation of cyclin D2 and p27Kip1 expression

Although it is evident that BCR-ABL can rescue cytokine-deprived hematopoietic progenitor cells from cell cycle arrest and apoptosis, the exact mechanism of action of BCR/ABL and interleukin (IL)-3 to promote proliferation and survival has not been established. Using the pro-B cell line BaF3 and a BaF3 cell line stably overexpressing BCR-ABL (BaF3-p210), we investigated the proliferative signals derived from BCR-ABL and IL-3. The results indicate that both IL-3 and BCR-ABL target the expression of cyclin Ds and down-regulation of p27(Kip1) to mediate pRB-related pocket protein phosphorylation, E2F activation, and thus S phase progression. These findings were further confirmed in a BaF3 cell line (TonB.210) where the BCR-ABL expression is inducible by doxycyclin and by using the drug STI571 to inactivate BCR-ABL activity in BaF3-p210. To establish the functional significance of cyclin D2 and p27(Kip1) expression in response to IL-3 and BCR-ABL expression, we studied the effects of ectopic expression of cyclin D2 and p27(Kip1) on cell proliferation and survival. Our results demonstrate that both cyclin D2 and p27(Kip1) have a role in BaF3 cell proliferation and survival, as ectopic expression of cyclin D2 is sufficient to abolish the cell cycle arrest and apoptosis induced by IL-3 withdrawal or by BCR-ABL inactivation, while overexpression of p27(Kip1) can cause cell cycle arrest and apoptosis in the BaF3 cells. Furthermore, our data also suggest that cyclin D2 functions upstream of p27(Kip1), cyclin E, and cyclin D3, and therefore, plays an essential part in integrating the signals from IL-3 and BCR-ABL with the pRB/E2F pathway.