Diabetes and periodontitis are two of the most prevalent diseases worldwide that negatively impact the quality of life of the individual suffering from them. They are part of the chronic inflammatory disease group or, as recently mentioned, non-communicable diseases, with inflammation being the meeting point among them. Inflammation hitherto includes vascular and tissue changes, but new technologies provide data at the intracellular level that could explain how the cells respond to the aggression more clearly.
1. Introduction
Diabetes is a chronic metabolic disease characterized by hyperglycemia and is one of the leading causes of death worldwide among non-communicable diseases
[1]. Periodontitis is the advanced form of periodontal disease and is one of the most prevalent diseases in the world. It is mostly caused by oral microbiota dysbiosis
[2][3], but some risk factors also impact its development such as diabetes, smoking and genetic predisposition
[4]. Diabetes is an important grade modifier used as an indicator of the rate of periodontitis progression
[5].
Inflammation marks the link between diabetes and periodontal disease
[6]. However, some authors consider that there is still scarce information based on research with representative samples and prospective longitudinal studies
[7]. It is important to note that it is difficult to perform a longitudinal study and to define whether the pathological associations are causal in nature in chronic inflammatory diseases studies.
Hitherto inflammation is described as vascular and tissue alterations. An aggression produces some cytokines that induces the extravasation of plasma and blood cells that try to control and restore the damage. Periodontitis and T2DM, independently, have elevated inflammatory markers. However, when present at the same time, there is an exacerbation of this immunoinflammatory response. M1-type macrophage
[8][9][10], neutrophils
[11][12][13][14], and polymorphonuclear cells (PMN)
[15] usually have their function upregulated, and dendritic cells are reduced or immature
[16]. Consequently, pro-inflammatory cytokines such as IL-1β, IL-17, IL-6, TNFα, INFγ used to be higher and IL-10 reduced
[17][18][19][20][21]. This scenario favors the great tissue destruction observed in periodontal tissues, micro and macrovascular lesions, lipid profile alterations (high low-density lipoprotein (LDL) and triglycerides) and difficulty in glycemic control
[22][23][24][25].
All these aforementioned mechanisms explain the pathogenesis at the tissue level. Tissues are made up of cells that mediate the immunity and suffer the aggression. New technological advances allow us to study deeply the cell mechanisms involved. Therefore, it is essential to understand not only the pathological alterations at the tissue level, but also the intracellular molecular mechanisms involved in this process that usually occur under subclinical conditions even before a state of complete inflammatory disease is established. Preclinical studies have been helpful in understanding the basic mechanisms involved in the onset of diabetes, periodontitis, and their systemic effects
[26].
2. Biological Membrane Alteration
Plasma membrane is a highly dynamic structure composed of phospholipid bilayer and lipid rafts, being one of the main structures of all living systems, delimiting cells and organelles such as lysosomes and mitochondria. Lipid rafts are composed of cholesterol, glycosphingolipids, and specific proteins, which are associated and dissociated in the second scale
[27][28][29].
These microdomains are involved in cellular signaling and membrane permeability, such as endo- and exocytosis during bacterial or toxin aggression
[30][31], immune cell activation
[32], redox signaling
[33][34], osteoclastogenesis induction
[28][35], and insulin secretion and sensitivity
[32][36]. The type and amount of lipids vary in each cell membrane according to their function and the individual’s diet and are influenced by lipid metabolism in health and disease condition. Disruption of this structure may alter several physiological cellular functions
[27][28][29].
Fatty acids (FA) are important membrane structural components and signaling molecules, and any change in their length or degree of saturation can directly impact membrane plasticity. High concentration of saturated FA (SFA) induces negative effects on the plasma membrane by increasing its fluidity and activity, increasing toll-like receptor (TLR) signaling translocation and RANKL activation
[28][37][38].
Patients with diabetes have high levels of SFA and overexpression of TLR4/CD36-mediated pathway in gingival fibroblasts
[30][39]. Palmitate (saturated) is enhanced in hyperglycemia, and it is even higher in the presence of
P. gingivalis [39], which is in agreement with the increase of FA uptake by lipid rafts after periodontal lipopolysaccharides (LPS) stimulation
[40], suggesting an exacerbation of inflammation in individuals with T2DM and periodontitis.
Polyunsaturated FA (PUFA) play a role in modulating mitochondrial function, inflammatory response, improving hormone sensitivity, especially insulin, and enhancing membrane fluidity and responsiveness. PUFA may present a pro-inflammatory profile (in case omega-6 prevail) in the initial phases of inflammatory response or an anti-inflammatory profile (with omega-3 being the most represented) during the resolution of inflammation. Omega-3 seems to inhibit factor nuclear kappa B (NF-κβ) activation and TLR dimerization, which reduces SFA pro-inflammatory stimulus
[29][37][39][41][42], and it has been also related to clinical and immunological benefits for patients with T2DM after daily supplementation and periodontal debridement
[43].
Integrity of membrane properties have been associated with a diet rich in unsaturated fats such as olive oil
[44], while high-fat diet (rich in saturated fat and cholesterol) seems to alter cellular properties and exacerbate the inflammatory response and increase hyperlipidemia and alveolar bone loss in periodontitis models
[45]. The inhibition of specific glycosphingolipids of lipid rafts improved glucose control, insulin sensitivity in T2DM patients
[36], and prevent RANKL-osteoclast induction
[33][35].
Lipid peroxidation (LPO) is an oxidative degradation of membrane lipids, which is increased in diabetes owing to the alteration of oxidative metabolism and the overproduction of reactive oxygen species (ROS)
[46]. This reaction produces lipid peroxides that bind to proteins and create unstable lipid radicals. Repeated cycles of LPO can activate the NF-κβ pathway, inducing a pro-inflammatory response, contributing to maintaining oxidative stress and causing serious damage to cell membranes
[46].
The intensity of LPO strongly depends on the degree of lipid unsaturation and this reaction is amplified as long as oxygen and unoxidized PUFA are available
[47]. Lipid marker alterations have been associated with the severity of periodontitis and uncontrolled T2DM. Several LPO markers are used to monitor ROS production, and they are positively associated with cytokines’ local and systemic expression in patients with T2DM and periodontitis with dyslipidemia, which is even worse in poorly controlled T2DM
[46] (
Figure 1).
Figure 1. The role of plasma membrane and its influence on different cellular responses. LPS, lipopolysaccharides; FFA, free fatty acid; oxLDL, oxidized LDL; LPO, lipid peroxidation; mtROS, mitochondrial reactive oxygen species; CD14/36, cluster of differentiation 14/36; PRR, pattern recognition receptors; RANKL, receptor activator of nuclear factor-kappa beta ligand; TLR, toll-like receptor; IKK, inhibitor of kappa B kinase; IL-1R, interleukin-1 receptor; MyD88, myeloid differentiation primary response 88; NF-κB, nuclear factor kappa B; TAB1/2/3, transforming growth factor beta (TGF-β) activated kinase 1-binding protein 1/2/3; TAK1, TGF-β activated kinase 1; NLRP3, NLR family pyrin domain-containing protein 3 inflammasome; Casp-1/-5, caspase-1/5; MAPK, mitogen-activated protein kinases; AP-1, Activator protein 1; MMPs, matrix metalloproteinases; ω-3, ômega-3 polyunsaturated fatty acid; PUFA, polyunsaturated fatty acid; SFA, saturated fatty acid; HG, high glucose.
Currently, T2DM patients have dyslipidemia, an imbalance of body lipids characterized by high levels of triglycerides and LDL and low levels of high-density lipoprotein (HDL). This condition increases oxidative metabolism and LPO, thereby maintaining a vicious cycle of chronic pro-inflammatory condition
[46]. The disturbance of glycemic metabolism and the continued activation of the polyol pathway to metabolize the excess of glucose also causes membrane alterations, increases LPO, and coupled with reduced antioxidant system (AOX)
[48] upregulates immune cell responses and visceral adiposity
[49]. Transcriptome analysis has facilitated the evidence of deregulation of different inflammatory molecular pathways, by co-expressed genes, in association with the quality of adipose tissue and type 2 diabetes
[50].
3. Aggression Recognition
Humans are multicellular organisms, and it is essential to distinguish between our own cells and others that can be harmful, as well as physical and chemical factors, through membrane cell receptors that stimulate immunity. These are fundamental sensory elements for host defense that can be stimulated by hormones and inflammatory mediators.
The immune system recognizes aggression through the connection of pathogen-associated molecular patterns (
PAMPs) and damage-associated molecular patterns (
DAMPs) to pattern recognition receptors (PRRs). These PRRs can be TLRs, NOD-like receptors (NLRs), RAGE, C-type lectin receptors (CLRs), and complement receptors. The increased levels of interleukin (IL)-1β enhance the expression of some cell receptors such as TLR4 which are involved in the signaling and activation of NF-κβ and mitogen-activated protein kinase (MAPK) pathways
[51].
Inflammasomes, the key regulators of innate, adaptive, and host responses, are a cytoplasmic multi-protein complex composed of NLRs and different types of proteins. NIMA-related kinase 7 (Nek7) is an indispensable upstream factor involved in NLR family pyrin domain-containing protein 3 (NLRP3) inflammasome formation and regulates the release of pro-inflammatory cytokines. The inflammasome complex activates caspase-1 and -5, which consequently release the first cytokines IL-1β and IL-18 against
PAMP or
DAMP, producing a cascade of local and systemic responses
[52].
Inflammasomes are activated and modulated by different metabolic alterations, and it has been reported that
P. gingivalis infection induces an overexpression of PRRs and NLRP3 in T2DM-periodontitis patients, along with caspase-1 and IL-1β
[51][53]. The most recently discovered innate immune cells on the periodontal tissue of periodontitis patients and mice-models of periodontitis are the innate lymphoid cells (ILCs). They are activated by
PAMPs and
DAMPs and play a role on initiation, modulation, and resolution of inflammation through cytokine release
[54]. Adenosine monophosphate-activated protein kinase (AMPK) acts as a modulator of ILCs function
[54] and NLRP3
[55], reducing their negative effects.
Hyperglycemia, even in intermittent periods, exacerbates TLR4 and RAGE expression
[56]. The disease severity has been related to elevations in pro-inflammatory cytokine expression and their involvement in the increased expression of RAGE or TLR4 on the surface of epithelial cells, fibroblasts, and macrophages
[56]. A significant association between RAGE polymorphism and patients with periodontitis and T2DM exists, but no association was observed in patients with only periodontitis
[57]. However, it is difficult to establish whether this polymorphism can be considered a risk factor related to the development of periodontitis when associated with T2DM or if this genetic alteration is just linked to diabetes. Further investigations in patients with diabetes but without periodontitis are necessary to confirm this risk
[26].
Recently, polymorphisms of TNF-α, TNFR1, TNFR2 and lymphotoxin-α were evaluated: no SNP was found to be a cross-susceptibility factor between periodontitis and T2DM. Therefore, the development of periodontitis in T2DM may be related to pathological alterations in the periodontium caused by diabetes due to hyperglycemia, high AGE levels and oxidative stress. T2DM is suggested to mask the impact of periodontitis on systemic inflammation
[58].
The entire transcriptional profile of LPS of
P. gingivalis in PDL cells has been recently described, and 36 differentially expressed genes (DEGs) have been identified in PDLs cultured with LPS for 24 h and 72 h. It was possible to observe that different biological processes, molecular functions, and cellular components are involved in the initiation and progression of periodontitis
[59]. Additionally, dysregulation of immunoactivation mechanisms of neutrophils and B cells were evidenced by differentially expressed genes
[60][61].
4. Mitochondrial Dysfunction
Oxidative phosphorylation by mitochondria is responsible for most of the ATP produced and ROS production also appears
[62]. ROS release at early stages is adaptative, acting as important signaling molecules after an aggression, and is controlled by intracellular redox status through AOX
[52]. However, at high concentrations they cause cellular lesions
[50].
The excess of electron donors in the mitochondrial electron transport chain is one of the main factors responsible for NADH/NAD+ redox imbalance, because as more electrons are transported, the higher the ROS production
[63]. In metabolic disorders, positive feedback is established with the increased release of ROS which stimulates the neighboring mitochondria to control the excess of these molecules, resulting in more ROS production
[25][62].
Mitochondrial dysfunction is considered the major source of ROS causing damage to all cellular components and disrupting the normal signaling mechanisms. Altogether, these effects directly impaired the inflammatory response, inducing a pro-inflammatory state
[25][62]. Oxidative stress arises and is maintained due to the increase in mitochondrial ROS production and inefficient (or absence) of enough AOX levels, resulting in an imbalance of the cellular redox state
[64][65].
Advanced glycation end products (AGEs) arise from non-enzymatic glycation and oxidation of proteins and lipids
[66]. They cause cellular damage by modifying protein function and cellular interaction with the extracellular membrane, alter the intracellular Ca
2+ concentration and mitochondrial function, deregulate the inflammatory response, influence wound repair, and increase oxidative stress through the connection with its receptors, RAGE
[6][66]. It has been suggested that high levels of AGEs may modify collagen structure, making the periodontal tissues less soluble with less reparative tendency, and along with other altered cellular responses, making them more susceptible to periodontal breakdown
[67].
The degradation of AGEs occurs intracellularly by endocytosis and lysosomal activity, and galectin-3 have been discovered to be an essential molecule to AGEs removal
[68][69]. In addition, low levels of galectin-3 have been associated with deficiency in glucose uptake, endothelial dysfunction in a diabetic mice model
[70], and increased bone loss under high glucose condition and periodontal/LPS infection
[69], which are negatively regulated by micro-RNA-124-3p
[69]. Patients with diabetes used to have high levels of AGE and RAGE in human gingival fibroblast which may explain the accelerated periodontitis observed in these patients in accordance with the previous studies
[71][72].
Oxidative stress and the AGE-RAGE connection stimulate signaling pathways, such as MAPK and NF-κβ, with subsequent pro-inflammatory gene transcription and increased ROS production in endothelial cells, vascular smooth muscle cells, and macrophages. The high number and activity of immune cells, mostly by the excessive response of phagocytes during hyperinflammatory response, contribute to the overall cellular stress
[11][65][73]. Patients with T2DM and periodontitis showed higher levels of AGEs and ROS production than healthy individuals
[11]. This oxidative stress is induced even when both diseases are not present simultaneously; however, when they are together, it becomes more severe
[24].
Mitochondrial dysfunction and high mitochondrial ROS production
[25][62] result in cellular stress at the molecular level, causing a reduction in protein expression, loss of mitochondrial mass, and impaired membrane potential
[62], and these alterations are present in diabetes and periodontitis. Moreover, the accumulation of mitochondrial DNA (mtDNA) alterations such as mtDNA heteroplasmy and copy number, noncoding ribonucleic acid (RNA), epigenetic modification of the mitochondrial genome, epitranscriptomic regulation of the mtDNA-encoded mitochondrial transcriptome and mtDNA mutations and polymorphisms have been related to endothelial dysfunction, change in metabolism of the liver, adipose tissue, myocardium, and skeletal muscles, and poor metabolic control
[74][75][76][77][78]. These parameters could be used as markers to characterize the dysregulated immune-inflammatory response commonly detected in individuals with periodontitis and T2DM
[25][76].