Abstract
Programmed cell death (PCD) can play a crucial role in tuning the immune response to microbial infection. Although PCD can occur in different forms, all are mediated by a family of proteases called caspases. Caspase-2 is the most conserved caspase, however, its function in cell death is ill-defined. Previously we demonstrated that live attenuated cattle vaccine strain Brucella abortus RB51 induces caspase-2-mediated and caspase-1-independent PCD of infected macrophages. We also discovered that rough attenuated B. suis strain VTRS1 induces a caspase-2-mediated and caspase-1-independent proinflammatory cell death in infected macrophages, which was tentatively coined “caspase-2-mediated pyroptosis”. However, the mechanism of caspase-2-mediated cell death pathway remained unclear. In this study, we found that caspase-2 mediated proinflammatory cell death of RB51-infected macrophages and regulated many genes in different PCD pathways. We show that the activation of proapoptotic caspases-3 and -8 was dependent upon caspase-2. Caspase-2 regulated mitochondrial cytochrome c release and TNFα production, both of which are known to activate caspase-3 and caspase-8, respectively. In addition to TNFα, RB51-induced caspase-1 and IL-1β production was also driven by caspase-2-mediated mitochondrial dysfunction. Interestingly, pore formation, a phenomenon commonly associated with caspase-1-mediated pyroptosis, occurred; however, unlike its role in S. typhimurium-induced pyroptosis, pore formation did not contribute to RB51-induced proinflammatory cell death. Our data suggest that caspase-2 acts as an initiator caspase that mediates a novel RB51-induced hybrid cell death that simulates but differs from typical non-proinflammatory apoptosis and caspase-1-mediated proinflammatory pyroptosis. The initiator role of the caspase-2-mediated cell death was also conserved in cellular stress-induced cell death of macrophages treated with etoposide, naphthalene, or anti-Fas. Caspase-2 also regulated caspase-3 and -8 activation, as well as cell death in macrophages treated with each of the three reagents. Taken together, our data has demonstrated that caspase-2 can play an important role in mediating a proinflammatory response and a hybrid cell death that demonstrates features of both apoptosis and pyroptosis.
Highlights
Programmed cell death (PCD) is a crucial process initiated by the host in response to cellular stress and microbial infections
We found that RB51-infected macrophages exhibited mitochondrial dysfunction, activation of the caspase cascade, IL-1β and TNFα secretion, and pore formation in the plasma membrane—all of which were dependent upon caspase-2
To confirm the caspase-2 mediated cell death, wild type (WT) and caspase-2 deficient bone-marrow derived macrophages (BMDMs) were infected with RB51, and cell death was assessed by Annexin V/propidium iodide (PI) staining and lactate dehydrogenase (LDH) release
Summary
Programmed cell death (PCD) is a crucial process initiated by the host in response to cellular stress and microbial infections. Pyroptosis, and necroptosis are three pathways of PCD that can occur during microbial infections with substantially different outcomes. Pyroptosis or “death by fire,” is inflammatory PCD typically mediated by caspase-1 (Bergsbaken et al, 2009). Caspase-1 processes proinflammatory cytokines (IL-1β and IL-18), and secretion of these cytokines requires pore formation in the plasma membrane, which leads to cell swelling and eventually lysis. Necroptosis is a newly identified type of PCD that includes a proinflammatory response as well as loss of plasma membrane integrity (Vandenabeele et al, 2010). In contrast to apoptosis and pyroptosis, serine/threonine kinases, RIP1 and RIP3, mediate necroptosis. Necroptosis leads to the release of intracellular contents; mostly damage associated molecular patterns (DAMPs)
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