Introduction
Chronic kidney disease (CKD) has been recognized as an important public health problem worldwide, and the global estimated prevalence of CKD is approximately 13.4%(Lv and Zhang 2019). The overall prevalence of CKD is more than 10% in China, and CKD has become an important public health problem(Wang et al. 2023). Although CKD risk factors such as poorly controlled diabetes and hypertension are well established, efforts to reduce these risk factors alone have not yet resulted in a decrease in the prevalence of CKD(Coresh et al. 2007). Therefore, identifying nontraditional modifiable risk factors for CKD has become an important public health issue worldwide.
Periodontitis is a chronic inflammatory disease with a high prevalence of 45%–50% and is the sixth most common human disease(Rehnqvist 1981). Almost 90% of Chinese adults suffer from periodontal disease of various severities, and over 30% have severe periodontitis(Jiao et al. 2021). The progression of periodontitis leads to tooth loss, impaired quality of life and significant systemic inflammation(Tonetti, Van Dyke and Working group 1 of the joint 2013). Several studies have shown an association between periodontitis and systemic diseases, including diabetes mellitus(Genco and Borgnakke 2013), cardiovascular disease(Bui et al. 2019), and CKD(Deschamps-Lenhardt, Martin-Cabezas, Hannedouche and Huck 2019, Kapellas, Singh, Bertotti, Nascimento, Jamieson and Perio 2019). Interestingly, periodontitis and CKD share common risk factors, such as age, smoking and poorly controlled diabetes(Fisher, Taylor, Papapanou, Rahman and Debanne 2008). Studies have suggested a bidirectional relationship between periodontitis and CKD based on biological research. Macrophage polarization plays a key role in inflammatory processes, including periodontitis and CKD(Navarrete et al. 2014). Macrophage polarization toward the M1 phenotype and activation byPorphyromonas gingivalis  (P.g ) play proinflammatory roles through the secretion of the cytokines IL-1β and IL-6 (Lew et al. 2018), which are closely associated with the destructive phase of periodontitis(Wang et al. 2021). An increased proportion of M1 macrophages positively correlates with progressive periodontitis(Yang et al. 2018). More importantly, M1 macrophage polarization has been reported to be associated with the progression of renal lesions in animal models(Anders and Ryu 2011, Lech et al. 2014). Macrophage polarization to the proinflammatory M1 phenotype is characterized by the production of certain inflammatory factors (IL-1β, IL-6 and IL-17) in damaged kidney tissue(Kon, Linton and Fazio 2011, Ricardo, van Goor and Eddy 2008). Some data suggest that uncontrolled macrophage polarization might be an important mechanism for the chronic inflammation observed in CKD(Li et al. 2015) and periodontitis. Therefore, inflammatory cytokines produced by macrophage polarization may play a role in periodontitis and CKD.
Nuclear factor kappa B (NF-κB) is a transcription factor with numerous biological functions, including regulating the expression of various proinflammatory factors(Mussbacher et al. 2019). CKD is associated with inflammation and immune responses, which activate the NF-кB signaling pathway and upregulate the expression of many downstream factors, including IL-6, ICAM-1 and MCP-1(Sucajtys-Szulc, Debska-Slizien, Rutkowski, Szolkiewicz, Swierczynski and Smolenski 2022). The microbiota plays important roles in metabolism, nutritional intake, physiology and maturation of the immune system, and it aids the progression of CKD(Bhargava, Merckelbach, Noels, Vohra and Jankowski 2022). Some bacteria, including P.gTreponema denticola  (T.d ), and Tonerella forsythia  (T.f ), were examined, and the results indicated their role in promoting the initiation and progression of inflammation in periodontitis and CKD(Mahendra et al. 2022). The gram-negative bacteria P.g is one of the most important periodontal pathogens that is frequently observed in the subgingival biofilm(Baek, Ji, Kim and Choi 2015, Palm, Demirel, Bengtsson and Khalaf 2015). P.glipopolysaccharide (LPS) can induce periodontal diseases by increasing the number of M1 phenotype macrophages via the NF-кB signaling pathway(Huang, Tian, Li and Xu 2019). Moreover, M1 macrophages produce many proinflammatory factors in response to P.gstimulation(Holden, Attard, Laughton, Mansell, O’Brien-Simpson and Reynolds 2014, Lam, O’Brien-Simpson, Holden, Lenzo, Fong and Reynolds 2016), and these effects were associated with NF-кB pathway activation(Liu et al. 2020). Studies have suggested that elevated serum antibodies against P.g are associated with decreased renal function(Iwasaki et al. 2012). More importantly, it has been shown that LPS derived from P.g damages glomerular endothelial cells to promote kidney disease in mice, suggesting that P.g may promote the pathological process of CKD. NLRP3, which is the downstream transcription factor of NF-κB signaling, is a pattern recognition receptor with a key role in host defense against pathogens(Agostini, Martinon, Burns, McDermott, Hawkins and Tschopp 2004, Goncalves et al. 2017). NLRP3, which consists of apoptosis-associated speck-like protein (ASC) and caspase-1, assembles the NLRP3 inflammasome(Agostini, Martinon, Burns, McDermott, Hawkins and Tschopp 2004), which plays key roles in CKD and periodontitis. P.g is also considered to be a crucial factor that modulates inflammation by activating NF-κB/NLRP3 signaling(Lv, Fan, Jiang, Wang, Qiu and Ji 2021). Therefore, we hypothesized that P.g may promote the progression of CKD via the NF-κB/NLRP3 signaling pathway in patients with periodontitis.
Ferroptosis is a regulatory cell death mode induced by the small molecule erastin that is different from autophagy, apoptosis, and necrosis(Dixon et al. 2012). Our previous study showed that P.gcould directly cause ferroptosis in the liver and trigger NAFLD in vivo and in vitro (Yao, Lan, Li, Wang, Qi and Liu 2023). Ferroptosis is involved in inflammatory processes in HGFs in response to P.g products. Ferroptosis is observed in the gingival tissue of rats with periodontitis(Qiao et al. 2022). In recent years, ferroptosis has emerged as a major area of focus in the molecular mechanisms of CKD. The expression of SLC7A11 (a cystine-glutamic acid anti-transporter) reduces the level of glutathione (GSH) and leads to ferroptosis in vascular smooth muscle cells, which regulates the occurrence of CKD(Ye et al. 2022). Zhou et al. found that the expression of glutathione peroxidase 4 (GPX4) in renal tubular epithelial cells was decreased, and inhibiting iron death notably decreased renal injury and inflammatory cell accumulation in kidney disease patients(Zhou, Xue, Hou and Dai 2022). Moreover, the NF‑κB pathway is linked to ferroptosis; NF‑κB p65 phosphorylation suppresses ferroptosis, and p65 deletion contributes to intestinal epithelial cell ferroptosis(Xu et al. 2020). Our previous work showed that ferroptosis and the NF-κB signaling pathway formed a positive feedback loop in hepatocyte inflammation during the progression of NAFLD(Yao, Lan, Li, Wang and Qi 2023). We hypothesize that ferroptosis in the kidney depends on the NF‑κB/NLRP3 pathway via P.g stimulation of M1 macrophage polarization during the progression of CKD.
In the present study, a periodontitis model was constructed byP.g gavage, and the occurrence of periodontitis and CKD, M1 macrophage polarization, the NF‑κB pathway and ferroptosis in kidney tissue were analyzed in vivo. Glomerular mesangial cells (GMCs) stimulated with P.g were examined in vitro, and GMC ferroptosis was activated through the NF‑κB/NLRP3 pathway by stimulating M1 macrophage polarization with P.g supernatant.