Chen lab reports a previously unrecognized function of G9a, a histone methyltransferase, in protein-specific translation during endotoxin tolerance (ET).
Muneer, A., Wang, L., Xie, L., Zhang, F., Wu, B., Mei, L., … & Chen, X. (2023). Non-canonical function of histone methyltransferase G9a in the translational regulation of chronic inflammation. Cell Chemical Biology.
Congrats to all authors.
Endotoxin-tolerant (ET) macrophage cells have similar molecular/immunopathological characteristics as chronic inflammation-associated complications including sepsis and ARDS. These extreme responses of host immune system to persistent infection involve (1) suppression of “tolerizeable” pro-inflammatory genes coupled with (2) upregulation of “non-tolerizeable” anti-microbial (anti-inflammatory) factors that, together, contribute to impaired adaptive immunity and susceptibility to secondary infection with an organ-damaging “cytokine storm”. However, despite significant clinical interest in ET, a unifying molecular mechanism that accounts for these two effects remains elusive.
Prior to this study, the best-understood function of G9a complex during ET was to suppress transcription of select pro-inflammatory genes. Here, we demonstrate that G9a interacts with translation regulators including METTL3, an N6-methyladenosine (m6A) RNA methyltransferase, and activates it to cooperatively upregulate the translation and/or expression of select m6A-modified mRNAs to promote proliferation of inflammatory cytokine-producing macrophage cells (hyperinflammation), reduce proliferation of CD8+ T cells and impair T cell function (lymphopenia); both major hallmarks of ET related complications. Mechanistically, G9a promotes methyltransferase activity of METTL3 at translational/post-translational level by regulating its expression, its methylation, and its cytosolic localization during ET.
Our translatome proteomics approach identified numerous “G9a-translated” proteins that unite the networks associated with inflammation dysregulation, T cell dysfunction, and systemic cytokine response. Interestingly, G9a-METTL3-m6A axis accounted for only a subset (~12%) of G9a-translated proteins in ET, suggesting that via interactions with distinct translation regulators other than METTL3, G9a coordinates additional, as yet unknown, mechanisms to facilitate gene-specific translation – necessitating further investigation.
Altogether, for the first time, we revealed a dual role of G9a in ET pathogenesis through its (a) canonical function in transcriptional repression of pro-inflammatory genes, and its (b) non-canonical/non-epigenetic function in promoting translation of anti-microbial/anti-inflammatory factors. More importantly, inhibition of G9a-regulated translation was shown to reverse hyperinflammation & lymphopenia and to hinder proteostasis (i.e., expression, secretion, and/or turnover) of numerous inflammatory disease-related factors involved in sepsis, ARDS, and COVID-19. Overall, our findings represent an attractive target for host-pathogenesis-dependent therapeutic intervention to treat ET-related chronic inflammatory diseases.