Inflammasomes play a critical role in the innate cellular immune response to pathogen infection. The inflammasome, a cytosolic multiprotein complex, recognizes substrates produced during infection or tissue damage, and triggers an inflammatory response by releasing the proinflammatory cytokine IL-1β and IL-18. Researchers have recently identified bromodomain-containing protein-4 (Brd4) as a key epigenetic regulator of the genes involved in inflammasome activation. Researchers from the University of Illinois School of Molecular & Cellular Biology have determined that inhibition or deletion of Brd4 in myeloid cells suppresses the inflammasome activation and the associated immune response in response to Salmonella infection.
Lin-Feng Chen, a professor of biochemistry, studied NLRC4 inflammasome activation and the mechanistic role of Brd4 in maintaining the expression of Naips. Naips (nucleotide-domain and NLR family apoptosis inhibitory proteins) are major components of NLRC4 inflammasome, which recognizes bacterial flagellin and type III secretion system (T3SS) apparatus proteins during infection. Chen’s groundbreaking research, “Brd4 regulates NLRC4 inflammasome activation by facilitating IRF8-mediated transcription of Naips,” was published in the Journal of Cell Biology in collaboration with James Slauch, a professor of microbiology.
Brd4’s function in regulating the expression of inflammatory genes is well recognized. From his past work on Brd4, Chen has identified transcriptional regulation as a critical mechanism for Brd4-mediated inflammatory gene expression, via its binding to their promoters or enhancers to activate RNA polymerase II-dependent transcription elongation. In addition to transcription regulation, another major mechanism for inflammatory gene expression is at the post-transcription level, namely inflammasome-mediated processing of pro-IL-1β and IL-18 to their mature forms.
“This raised the question of whether Brd4 can contribute to the second mechanism for the production of cytokines, such as IL-1β and IL-18,” Chen said.
In this project, Chen’s lab aimed to uncover the function of Brd4 in inflammasome activation. He studied how Brd4 affects the transcription of the inflammasome components for inflammasome activation. In addition, “very few researchers are trying to understand how inflammasomes are activated on a transcriptional level,” he said.
Transcription factors can regulate various components of an inflammasome to activate the multiprotein complex optimally. IRF8 and lineage-determining factor PU.1 were found to control transcription of Naips in resting macrophages. Chen’s lab found that Brd4 forms a complex with IRF8 and PU.1 to facilitate IRF8/PU.1-mediated Naip transcription, making it essential for optimal expression of NLRC4 inflammasome components and the subsequent inflammasome-mediated cytokine release and pyroptosis, an inflammasome-mediated inflammatory cell death. Though the regulation of NLCR4 is not well understood, these findings provide more insight into its mechanism for NLRC4 inflammasome activation.
“Inflammasome activation has been implicated in a variety of human diseases. Currently researchers are developing different inhibitors that can block activation of inflammasome,” Chen said.
These inhibitors can have great therapeutic effects. Blocking inflammasome activation in tumor microenvironments, for example, can prevent cancer cell proliferation. As a key inflammatory gene regulator, Brd4 presents itself as a promising drug target. The Chen lab’s discoveries on the biological role of Brd4 may have significant clinical implications.
