Cathelicidin CATH-2 suppresses the NF-κB/ROS/NLRP3 signaling pathway via regulating mTOR-dependent autophagy during Streptococcus suis infection

Cathelicidin CATH-2 has been reported to exert potent anti-inflammatory activity in different species though neutralizing stimuli such as lipopolysaccharide (LPS) and lipoteichoic acid (LTA). CATH-2 has been shown to inhibit Streptococcus suis (S. suis)-induced activation of dendritic cells and macrophages by binding to LTA. However, the exact mechanism of this prophylactically anti-inflammatory activity remains unclear. Therefore, we investigated the anti-inflammatory activity and mechanism of CATH-2 in mice peritoneal macrophages pretreated with CATH-2 followed by S. suis infection. The results showed that CATH-2 pretreatment significantly reduced S. suis-induced transcription and secretion of interleukin (IL)-1β, IL-6, and IL-12. CATH-2 also downregulated NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) expression and apoptosis-associated speck-like protein containing a CARD (ASC) oligomerization, and inhibited the maturation of IL-1β, suggesting that CATH-2 inhibits NLRP3 activation. In addition, CATH-2 significantly inhibited S. suis-induced phosphorylation of p65 and extracellular signal-regulated kinase (ERK). Further study showed that CATH-2 inhibited S. suis-induced reactive oxygen species (ROS) by upregulating the expression of ROS scavenging genes including catalase (CAT) and superoxide dismutase 1 (SOD1). Mechanistically, transcriptome analysis revealed that CATH-2 regulated the protein kinase B (ATK)/mammalian target of rapamycin (mTOR) pathway, which was evident by the downregulation of phosphorylated (p)-ATK and p-mTOR induced by CATH-2. Notably, CATH-2 induced autophagy and autophagic flux. Inhibition of mTOR using rapamycin enhanced the CATH-2-induced autophagic efficacy, demonstrating that CATH-2 induces mTOR-dependent autophagy. However, inhibition of autophagy using 3-methyladenine (3-MA) reversed the reduction in the expression of p-p65, p-ERK, and IL-1β induced by CATH-2. Our study reveals that CATH-2 inhibits the nuclear factor kappa-B (NF-κB)/NLRP3-mediated inflammatory response through the induction of mTOR-dependent autophagy during S. suis infection, which provides new insight into the anti-inflammatory pathways of antimicrobial peptides.