Macrophages Require Constitutive NF-kB Activation To Maintain A1 Expression and Mitochondrial Homeostasis

NF-kB is a critical mediator of macrophage inflammatory responses, but its role in regulating macrophage survival has yet to be elucidated. Here, we demonstrate that constitutive NF-kB activation is essential for macrophage survival. Blocking the constitutive activation of NF-kB with pyrrolidine dithiocarbamate or expression of IkBa induced apoptosis in macrophagelike RAW 264.7 cells and primary human macrophages. This apoptosis was independent of additional death-inducing stimuli, including Fas ligation. Suppression of NF-kB activation induced a time-dependent loss of mitochondrial transmembrane potential (DCm) and DNA fragmentation. Examination of initiator caspases revealed the cleavage of caspase 9 but not caspase 8 or the effector caspase 3. Addition of a general caspase inhibitor, z-VAD.fmk, or a specific caspase 9 inhibitor reduced DNA fragmentation but had no effect on DCm collapse, indicating this event was caspase independent. To determine the pathway leading to mitochondrial dysfunction, analysis of Bcl-2 family members established that only A1 mRNA levels were reduced prior to DCm loss and that ectopic expression of A1 protected against cell death following inactivation of NF-kB. These data suggest that inhibition of NF-kB in macrophages initiates caspase 3independent apoptosis through reduced A1 expression and mitochondrial dysfunction. Thus, constitutive NFkB activation preserves macrophage viability by maintaining A1 expression and mitochondrial homeostasis.