3 | RELATIONSHIP BETWEEN CAP AND MAIN RELATED PATHWAYS

3.1 | JAK/STAT pathway

JAK/STAT pathway is involved in inflammation and immune responses with duality. Firstly, JAK/STAT1 is the transcription pathway of HMGB1, that is, LPS induces macrophages to release HMGB1 through the TLR4/IFN-β/STAT1 signal axis. Secondly, α7nAChR regulates JAK2/STAT3 after being activated. After vagus nerve stimulation or α7nAChR activation, JAK2 is recruited to α7nAChR to form a heterodimeric complex. By phosphorylating STAT3 to reduce the nuclear translocation of NF-κB (de Jonge et al., 2005), STAT3 acts as a dominant-negative inhibitor of NF-κB DNA binding activity and directly inhibits pro-inflammatory cytokines induction by interacting with NF-κB p65 (Yu, Zhang, & Kone, 2002). STAT3 is not directly connected to pro-inflammatory cytokines production, and attenuated cytokine expression mediated by α7nAChR stimulation is a synergy between NF-κB and JAK/STAT (shown in Figure 2C) (Tasaka et al., 2015). Cholinergic agonists (nicotine and GTS-21) inhibit IL-6-mediated MCP-1 production and ICAM-1 expression through JAK2/STAT3 pathway in a SHP1/2 phosphatase-dependent manner. It is associated with a reduction of JAK2 and STAT3 phosphorylation, STAT3-specific DNA binding, and reduction of suppressor of cytokine signaling 3 (SOCS3, a regulator of the JAK2/STAT3 pathway) levels (Chatterjee, Al-Abed, Sherry, & Metz, 2009). Especially in the anti-inflammatory and inhibitory cell apoptosis pathways, α7nAChR plays a key role in JAK2/STAT3 and NF-κB (Marrero & Bencherif, 2009). Normally, activation of α7nAChR and inhibition of JAK2 can attenuate STAT3 phosphorylation to exert subsequent responses, while α7nAChR signaling requires STAT3 protein rather than its tyrosine phosphorylation. Unphosphorylated STAT3 can also compete with an inhibitor of NF-κB α (IκBα) to bind NF-κB and inhibit TNF transcriptional activation to regulate the innate immune response during infection. However, the STAT3–NF-κB complex may enhance the production of other cytokines (Peña et al., 2010).

3.2 | NF-κB pathway

NF-κB inhibition can reduce inflammation and inhibit macrophages/monocytes to play a key role in innate immune response. While NF-κB is activated by microbial components (such as LPS, peptidoglycan, dsRNA, etc.) through TLR. The downstream signaling cascade includes IκB kinase (IKK) activation, IκB phosphorylation, NF-κB translocation, and subsequent gene transcription. Meanwhile, NF-κB is also regulated by JAK2/STAT3, as described in 3.1. α7nAChR activation-mediated CAP prevents NF-κB nuclear translocation in macrophages, while nicotine inhibits IKK phosphorylation and NF-κB transcriptional activity through α7nAChR signaling (Yoshikawa et al., 2006). In sepsis, α7nAChR-mediated anti-inflammatory and TLR4-mediated pro-inflammatory pathways are activated, and several pro-inflammatory cytokines are released. The classic anti-inflammatory drug dexamethasone can also inhibit proinflammatory cytokines by up-regulating α7nAChR and ACh, and inhibits TLR4/MyD88/NF-κB pathway, thereby inhibiting inflammation. The specific α7nAChR antagonist α-bungarotoxin eliminates the effects of dexmedetomidine (Zi et al., 2019).

3.3 | MAPK pathway

Mitogen-activated protein kinase (MAPK) signaling pathway includes c-Jun N-terminal kinase (JNK), extracellular regulated protein kinases (ERK), and P38 MAPK. These components can act both independently and synergistically. MAPK activation requires the phosphorylation of its specific amino acid sequence, combined with stimulation of other kinases, translocates to the nucleus and activates the transcription of proinflammatory genes. MAPK is closely related to the downstream NF-κB in CAP and plays an important role in inflammation. Regulation of α7nAChR activation by microglia on inflammatory mediators is achieved by inhibiting the P38 and ERK pathways in the MAPK pathway (Lei, 2009). In colitis model, the improvement of vagal activity may activate ERK1/2 and NF-κB translocation, and induce the transcription of proinflammatory genes through the interaction of α7nAChR and the peripheral release of ACh in the inflamed colonic mucosa (P. Sun et al., 2013). K opioid receptor agonists can activate α7nAChR through MAPK-ERK1/2 signaling pathway, regulate CAP, inhibit NF-κB expression, reduce inflammation, and postoperative cognitive dysfunction caused by cardiopulmonary bypass (Fan, Duan, & Sun, 2019). Ligation-induced acute pancreatitis in rats is related to the continuous increase of M3 mAChR expression in a time-dependent manner and is also related to the increase in the activation of stress-activated protein kinase JNK (Samuel, Zaheer, Fisher, & Zaheer, 2003). JNK is a major negative regulator of intestinal secretion induced by mAChR. mAChR stimulation increases the phosphorylation of MAPKs and is inhibited by MAPK inhibitors (Khan et al., 2015). Also, activation of α7nAChR (selective agonist PNU282987) significantly reduces apoptosis and intracellular oxidative stress levels and prevents oxidative stress-induced damage by inhibiting the vascular peroxidase-1 in endothelial cells in a JNK signaling pathway-dependent manner (D. J. Li et al., 2014). Consequently, MAPK signaling pathway can also play a role in CAP-mediated inflammation, and its relationship with JAK2/STAT3 and NF-κB in CAP is complex.

3.4 | PI3K/Akt pathway

PI3K pathway responds to the activation of pro-inflammatory signaling pathway in cells to some extent in the immune defense mechanism (Williams et al., 2004). Activation of α7nAChR results in formation of heterodimeric complexes between α7nAChR protein and JAK2, which can initiate signal transduction mediated by STAT3, followed by PI3K tyrosine phosphorylation and Akt serine phosphorylation (Shaw, Bencherif, & Marrero, 2002). The relationship between PI3K and TLR4 is still not completely clear and may be bidirectional. On the one hand, TLR4 activates PI3K to limit TNF-α production, and the pharmacological blockade of PI3K leads to enhanced activation of NF-κB (Chaurasia et al., 2010). At the same time, it is also possible that PI3K negatively regulates TLR4. PI3K/Akt activation can reduce apoptosis and TLR4 expression, and PI3K inhibitors can increase local inflammation (Ke et al., 2012). Studies have revealed that nicotine inhibits the overexpression of TLR4 by activating the α7nAChR/PI3K pathway, and PI3K/Akt plays a negative feedback role in it (T. H. Kim, Kim, & Lee, 2014).