2  Cellular and molecular inflammatory-related mechanisms
In physiological conditions, NF-κB (a heterotrimer with p65 and p50 subunits associated with the IκBα inhibitory subunit) is localized to the cytoplasm. Under the effect of inflammatory stimuli (for instance cytokines – IL-8, IL-1β, IL-6, or TNF-α, UV exposure, etc), the heterodimer dissociates, IκBα is degraded, and the two components, p50 and p65, translocate into the nucleus binding the promoter regions from different genes that are involved in initiating several cellular pathways linked to chronic diseases, tumorigenesis, angiogenesis and metastasis. Among the genes regulated by NF-κB are the ones for the expression of TNF-α, COX-2, MMP-9, nitric oxide synthase – the inducible form (iNOS), cytokines (IL-1, IL-6 and IL-8), 5-lipooxigenase (5-LOX), vascular endothelial growth factor (VEGF); all of these genes, when up-regulated fire-up the vicious circle constituted from oxidative stress and inflammation (Aggarwal, 2009; Kawabata et al., 2012; Kunnumakkara et al., 2018; Nimigean et al., 2018, 2019; Poll et al., 2018a, 2018b; Reuter et al., 2010). Interestingly, the NF-κB cascade is also activated by some factors that affect the circadian rhythm, such as aging or sleep deprivation. In animal models, high-fat diets as well as obesity – pro-inflammatory states, are directly correlated with a decrease of the amplitude of circadian activity and rhythmic gene expression, thus, suggesting that the inflammatory pathways are directly responsible for influencing the circadian clock (Gachon et al., 2018; Kohsaka et al., 2007; Osorio et al., 2016).
Another important inflammatory pathway involves STAT3, a cytoplasm protein that is phosphorylated by JAK 1, 2, and 3 (Janus-activated kinases), under the influence of inflammatory stimuli. STAT3 reaches the nucleus and functions as a transcriptional factor, stimulating the synthesis of inflammatory mediators (Pandurangan et al., 2015; Sung et al., 2012). The MAPK/ERK (mitogen-activated protein kinases) group, including stress activated protein-kinases p38, JNK (c-Jun N-terminal kinases) and ERK (extracellular signal-regulated kinases), is also involved in the inflammatory cascade as a response to a detrimental stimulus (Liang et al., 2016). Also, to the family of transcription factors we can comprise nuclear factor erythroid 2-related factor 2 (Nrf2), activator protein-1 (AP-1), nuclear factor of activated T cells (NFAT) or hypoxia-inducible factor-1α (HIF-1α), all being recognized for their role in stress response as well as in mediating inflammation (Panieri et al., 2020; Reuter et al., 2010) (Fig. 1 ).
Fig. 1 Regulation of cellular pathways under the influence of pro-inflammatory stimuli (Keap1 – Kelch-like ECH-associated protein 1; Cul3 – Cullin 3; Nrf2 – Nuclear factor erythroid 2-related factor 2; IKK – IκB kinase; NF-κB – Nuclear factor kappa-light-chain-enhancer of activated B cells heterodimer, consisting of p50, p65 and IkBα proteins; STAT3 – Signal transducer and activator of transcription 3; ERK/MAPK – mitogen-activated protein kinases; JNK – c-Jun N-terminal kinases; MEK – Mitogen-activated protein kinases kinase; HIFα – Hypoxia-inducible factor α; HIFβ – Hypoxia-inducible factor β; AP1 – Activator protein 1, with its associated proteins cFOS - and cJUN; Maf – Transcription factor Maf; ARE – antioxidant response element).
Pro-inflammatory stimuli activate several regulatory cell processes. The activation of IKK (IκB kinase) causes the cleavage of NF-kβ, associated with nuclear translocation for p50 and p65, while that of MAPK directs the phosphorylation of p38, ERK and JNK. Also notable are the dimerization of STAT3 and the association of HIFα with HIFβ, all resulting in the activation of several transcription factors or direct stimulation of gene expression, targeting pro-inflammatory molecules. However, an increased expression of antioxidant enzymes can be encountered due to decreased degradation of Nrf2 and its activation of ARE, post-nuclear translocation and association with Maf. The activation of all the inflammation-related pathways, along the cellular and metabolic disturbances that follow, contributes to the high occurrence of obesity and associated pathology (diabetes mellitus, metabolic syndrome, CVD, Alzheimer's disease, etc), especially under the influence of modern life factors (malnutrition, smoking, pollution, low physical activity) (De Lorenzo et al., 2016; Georgescu et al., 2014; Georgescu, 2014; Gustafson et al., 2007; Libby, 2012; Ungurianu et al., 2017, 2019a; Wang et al., 2004; Zanfirescu et al., 2019). The obese state is characterized by low-grade systemic inflammation, with impaired synthesis of adipokines and activation of pro-inflammatory signalling pathways leading to insulin resistance. Adipose tissue inflammation is one of the determining factors of obesity complications, impairing the adipocyte secretory function and hormonal balance.
Physiologically, adipose tissue resident macrophages, displaying both a pro-inflammatory and anti-inflammatory phenotype, are involved in several housekeeping processes (differentiation of preadipocytes, abstraction of necrotic and apoptotic cells, angiogenesis adjustment, etc). The expansion of adipose tissue, due to long-lasting overnutrition, induces prolonged hypoxia (the activation of HIF-1α), and consequent inflammation as monocytes infiltrate in the hypoxic region and become pro-inflammatory macrophages, process during which TNF-α, monocyte chemotactic protein (MCP-1), IL-6, MMPs, IL-8, VEGF are released. Further, the inflammatory response elicited by dysfunctional adipocytes, hypoxia, adipocyte expansion, and impaired fatty acids storage is accelerated by formation of reactive species, either oxygen (ROS) or nitrogen (RNS) (Crewe et al., 2017; Demaria et al., 2014; Fujisaka et al., 2013; Margina et al., 2012; Pasarica et al., 2009; Paun et al., 2015; Schipper et al., 2012; Trayhurn, 2014).
Metabolic impairments associated with obesity may contribute to the release of inflammatory mediators (IL-18, IL-1β) due to the stimulation of the NLR (nucleotide-binding oligomerization domain (NOD)-like receptors) family pyrin domain-containing 3 (NLRP3) which forms a cytoplasmic complex known as the NLRP3 inflammasome, with direct modulating role on the innate immune system. This pathway is involved in obesity as well as associated inflammation, and among the endogenous danger signals activating it we can find glucose and ROS. Also, literature data states that IL-18 as well as IL-1β are produced as an effect of TLR (Toll-like receptors)-mediated NF-κB activation, thus pointing out to the interaction of different signalling pathways (Fettelschoss et al., 2011; Tschopp and Schroder, 2010; Zhou et al., 2010).
In the light of these pathways, consolidated research is needed to point out in ability of some dietary components, based on the RLRS approach, to reduce the expression of inflammatory molecules and resolve the ROS-inflammation cycle, in order to diminish the risk for long-time comorbidities associated with obesity/metabolic impairments.