Mcl-1 is a common target of stem cell factor and interleukin-5 for apoptosis prevention activity via MEK/MAPK and PI-3K/Akt pathways

Mad2 is required for optimal hematopoiesis: Mad2 associates with c-Kit in MO7e cells

Mitotic arrest deficiency 2 (Mad2) is a component of mitotic spindle checkpoint proteins and is essential for accurate chromosome segregation. We investigated a role for Mad2 in hematopoiesis using Mad2-haploinsufficient (Mad2+/-) mice. Mad2+/- bone marrow (BM) and spleen manifested decreased absolute numbers and cycling status of immature, but not mature, hematopoietic progenitor cells. Mad2+/- BM granulocyte-macrophage colony-forming units (CFU-GMs) did not manifest synergistic proliferation in response to stem cell factor (SCF) plus GM-CSF. The percentage of annexin V+ cells was higher in Mad2+/- than Mad2+/+c-Kit+lin- BM after culture with SCF and GM-CSF. However, no significant difference in phosphorylation of extracellular signal-related kinase (Erk1/2) at Thr202/Tyr204 and Akt at Ser473 between Mad2+/- and Mad2+/+BM c-Kit+lin- cells was observed. Immunoprecipitation assays performed in human MO7e cells demonstrated physical association of c-Kit with Mad2. Moreover, stimulation with SCF plus GM-CSF led to dissociation of Mad2 from c-Kit. Confocal microscopy demonstrated that Mad2 colocalized with c-Kit in the cytoplasm of MO7e cells. These results suggest that Mad2 is involved in synergistic growth of immature hematopoietic progenitor cells in response to SCF plus GM-CSF, effects that may be mediated via physical association of Mad2 with c-Kit.

IL-6-independent expression of Mcl-1 in human multiple myeloma

Mcl-1 is a critical antiapoptotic survival factor for human multiple myeloma (MM). We examined the importance of IL-6 for Mcl-1 expression in five MM cell lines and in primary MM cells from 14 patients. While culture of MM.1S cells in IL-6 did induce Mcl-1 expression, four other MM cell lines exhibited high levels of Mcl-1 expression that were unaffected by IL-6. Similarly, Mcl-1 expression in 10 of 14 primary MM isolates was found to be IL-6-independent. An analysis of the mechanisms responsible for IL-6-independent Mcl-1 expression was undertaken. ERK1/2 activity did not influence Mcl-1 expression, distinct from Mcl-1 regulation that occurs during myeloid differentiation from hematopoietic progenitor cells. Inhibition of the PI3K pathway led to growth inhibition of 8226 and ANBL-6 cells without reduction of Mcl-1 levels, and high level Mcl-1 expression was maintained in the absence of activated STAT3. Analysis of the transcriptional activity of 5'-regulatory sequences from the human Mcl-1 gene in MM cells demonstrated high levels of IL-6-independent indicator gene activation as predicted. These data demonstrate that the mechanisms regulating Mcl-1 levels in MM cells are heterogeneous, and are often independent from IL-6 signaling pathways.