Phosphoproteomics Analysis Reveals a Potential Role of CHK1 in Regulation of Innate Immunity through IRF3

Checkpoint Kinase 1 (Chk1) inhibition fails to activate the Stimulator of Interferon Genes (STING) innate immune signalling in a human coculture cancer system

Utilising Checkpoint Kinase 1 (Chk1) inhibitors to increase cytoplasmic DNA may be a potential strategy to increase the sensitivity of tumours to immune checkpoint modulators. The appearance of DNA in the cytoplasm can drive Cyclic GMP-AMP Synthase-2',3'-Cyclic Guanosine Monophosphate-Adenosine Monophosphate-Stimulator of Interferon Genes (cGAS-cGAMP-STING) inflammatory, anti-tumour T-cell activity via a type I interferon (IFN) and nuclear factor-κB response. In the THP1-Dual reporter cell line, the STING agonist cGAMP activated both reporters, and increased phosphorylation of the innate immune pathway signallers Tank Binding Kinase 1 (TBK1) and Interferon Regulatory Factor (IRF) 3. Inhibition of Chk1 increased TBK1 but not IRF3 phosphorylation and did not induce IRF or NF-κB reporter activation. cGAMP induced a Type I IFN response in THP1 cells whereas inhibition of Chk1 did not. HT29 or HCC1937 cell treatment with a Chk1 inhibitor increased cytoplasmic dsDNA in treated HCC1937 but not HT29 cells and increased IRF reporter activation in cocultured THP1-Dual cells. HT29 cells pre-treated with gemcitabine or camptothecin had elevated cytoplasmic dsDNA and IRF reporter activation in cocultured THP1-Dual cells. Camptothecin or gemcitabine plus a Chk1 inhibitor increased cytoplasmic dsDNA but Chk1 inhibition suppressed IRF reporter activation in cocultured THP1 cells. In THP1-Dual cells treated with cGAMP, Chk1 inhibition suppressed the activation of the IRF reporter compared to cGAMP alone. These results suggest that, in some cellular models, there is little evidence to support the combination of Chk1 inhibitors with immune checkpoint modulators and, in some combination regimes, may even prove deleterious.

Grass carp (Ctenopharyngodon idellus) Cdc25a down-regulates IFN 1 expression by reducing TBK1 phosphorylation

In vertebrates, TANK Binding Kinase 1 (TBK1) plays an important role in innate immunity, mainly because it can mediate production of interferon to resist the invasion of pathogens. In mammals, cell division cycle-25a (Cdc25a) is a member of the Cdc25 family of cell division cycle proteins. It is a phosphatase that plays an important role in cell cycle regulation by dephosphorylating its substrate proteins. Currently, many phosphatases are reported to play a role in innate immunity. This is because the phosphatases can shut down or reduce immune signaling pathways by down-regulating phosphorylation signals. However, there are no reports on fish Cdc25a in innate immunity. In this paper, we conducted a preliminary study on the involvement of grass carp Cdc25a in innate immunity. First, we cloned the full-length cDNA of grass carp Cdc25a (CiCdc25a), and found that it shares the highest genetic relationship with that of Anabarilius grahami through phylogenetic tree comparison. In grass carp tissues and CIK cells, the expression of CiCdc25a mRNA was up-regulated under poly (I:C) stimulation. Therefore, CiCdc25a can respond to poly (I:C). The subcellular localization results showed that CiCdc25a is distributed both in the cytoplasm and nucleus. We also found that CiCdc25a can down-regulate the expression of IFN 1 with or without poly (I:C) stimulation. In other words, the down-regulation of IFN1 by CiCdc25a is independent of poly (I:C) stimulation. Further functional studies have shown that the inhibition of IFN1 expression by CiCdc25a may be related to decrease of TBK1 activity. We also confirmed that the phosphorylation of TBK1 at Ser¹⁷² is essential for production of IFN 1. In short, CiCdc25a can interact with TBK1 and subsequently inhibits the phosphorylation of TBK1, thereby weakens TBK1 activity. These results indicated that grass carp Cdc25a down-regulates IFN 1 expression by reducing TBK1 phosphorylation.