Anti-apoptotic Myeloid Cell Leukemia Research Articles

IgE Modulates Neutrophil Survival in Asthma: Role of Mitochondrial Pathway

The high-affinity IgE receptor (FcepsilonRI) has recently been reported to be expressed by neutrophils in atopic asthmatic individuals, leading to speculations that IgE could influence biological functions of these cells. In this study, we demonstrate that monomeric human IgE delayed spontaneous apoptosis of primary human neutrophils from atopic asthmatics in vitro. This effect was not dependent on FcepsilonRI cross-linking or autocrine release of soluble mediators; however, it was associated with increased expression of the antiapoptotic myeloid cell leukemia-1 protein, retention of the proapoptotic molecule Bax in the cytoplasm, decreased release of Smac from mitochondria, and reduced caspase-3 activity. Taken together, our results indicate that in vitro IgE can delay programmed cell death of neutrophils from allergic asthmatics and this may possibly contribute to neutrophilic inflammation in atopic asthma.

Two Waves of Mitochondrion Disruption in Apoptosis: Implications for the Design of Anti-Cancer Drugs.

Apoptosis plays an important role in preventing cancer development and promoting anti-cancer therapy. During mitochondria-mediated apoptosis, the disruption of mitochondria leads to the release of cytochrome c to form apoptosome with Apaf-1 and caspase-9, resulting in caspase-9 activation. However, the precise mechanisms controlling mitochondrion disruption remain elusive. Ablation of caspase-9 or other apoptosome components causes abnormal brain development and perinatal lethality in mice, making it difficult to assess the in vivo functions of caspase-9 signaling in many cell types and the role of caspase-9 in regulating mitochondria disruption. Through conditional knockout and inducible activation of caspase-9, we show that the loss of mitochondrion membrane potential (Δψm) during apoptosis proceeds in distinct caspase-9-dependent and independent steps. The initial loss of cells with high Δψm (Δψmh) was independent of caspase-9, but required BH3-only proteins of the Bcl-2 family, such as Bim and BNIP3L. Caspase-9 deficiency arrested the B cells at a stage with intermediate levels of Δψm (Δψmint). Apoptosis stimuli induced activation of effector caspases and cleavage of anti-apoptotic myeloid cell leukemia-1 (Mcl-1) in wild type but not caspase-9−/− B cells. Specific activation of caspase-9 by chemical-induced dimerization in human leukemia H9 cells also resulted in the loss of Δψm and cleavage of Mcl-1, but silencing of downstream effector caspases inhibited Mcl-1 cleavage and sequestered the cells at the Δψmint stage. Moreover, a cleavage-resistant Mcl-1 inhibited caspase-9-dependent mitochondrion disruption and apoptosis, indicating that caspase-9-mediated mitochondrion destruction involves Mcl-1 processing. Our data suggest that BH3-only proteins initiate the first wave of mitochondrion disruption during apoptosis with Δψmh to Δψmint transition, while caspase-9 signaling orchestrated the second wave of Δψmint to Δψm− transition through the cleavage of Mcl-1. Defects in mitochondria-mediated apoptosis pathway are associated with the development and treatment failures of many hematological malignancies. Identification of stepwise mitochondrial disruption may allow specific targeting of apoptotic machineries in various therapeutic settings. Chemical-induced dimerization of caspase-9 may be particularly useful for the design of anti-cancer drugs to treat those malignancies involving Mcl-1 over-expression.

MCL-1S, a Splicing Variant of the Antiapoptotic BCL-2 Family Member MCL-1, Encodes a Proapoptotic Protein Possessing Only the BH3 Domain

MCL-1 (myeloid cell leukemia-1) is an antiapoptotic BCL-2 family protein discovered as an early induction gene during myeloblastic leukemia cell differentiation. This survival protein has the BCL-2 homology (BH) domains 1, 2, and 3 and a C-terminal transmembrane region. We identified a short splicing variant of the MCL-1 mRNA in the human placenta encoding a protein, termed MCL-1 short (MCL-1S), with an altered C terminus as compared with the full-length MCL-1 long (MCL-1L), leading to the loss of BH1, BH2, and the transmembrane domains. Analysis of the human MCL-1 gene indicated that MCL-1S results from the splicing out of exon 2 during mRNA processing. MCL-1S, unlike MCL-1L, does not interact with diverse proapoptotic BCL-2-related proteins in the yeast two-hybrid system. In contrast, MCL-1S dimerizes with MCL-1L in the yeast assay and coprecipitates with MCL-1L in transfected mammalian cells. Overexpression of MCL-1S induces apoptosis in transfected Chinese hamster ovary cells, and the MCL-1S action was antagonized by the antiapoptotic MCL-1L. Thus, the naturally occurring MCL-1S variant represents a new proapoptotic BH3 domain-only protein capable of dimerizing with the antiapoptotic MCL-1L. The fate of MCL-1-expressing cells could be regulated through alternative splicing mechanisms and interactions of the resulting anti- and proapoptotic gene products.