A1 is a constitutive and inducible Bcl-2 homologue in mature human neutrophils

During the process of terminal differentiation toward mature neutrophils, the anti-apoptotic proteins Bcl-2 and Bcl-x become down-regulated and eventually cease to be expressed, whereas the death-promoting Bcl-2 homologue, Bax, persists. Thus, the disappearance of anti-apoptotic homologues was thought to account for the early demise of mature neutrophils. However, although the survival of mature human neutrophils can be prolonged by a variety of factors, no anti-apoptotic Bcl-2 homologues have previously been identified. Human A1 is a Bcl-2 homologue previously shown to be present in endothelial cells and to convey anti-apoptotic function in vitro. We describe here that human A1 mRNA is constitutively expressed in mature neutrophils and is up-regulated by G-CSF and LPS, agonists that promote neutrophil survival. In addition, we show progressive A1 mRNA accumulation in HL-60 cells during all-trans retinoic acid-driven neutrophilic differentiation. Our findings suggest that A1 may have an important role in neutrophilic development and in modulating mature neutrophil survival.