The PKR-T2DM-Apoptosis-Inflammation Axis: An Insightful Review Supporting the Role of the Axis in Periodontal Disease

Protein kinase R (PKR) has emerged as a critical regulator in the pathogenesis of both diabetes and periodontal diseases. PKR, a serine/threonine protein kinase, is activated in response to various stress signals, including viral infections, metabolic disturbances, and inflammatory cytokines. Understanding the role of PKR in the context of type 2 Diabetes Mellitus (T2DM), apoptosis, and inflammation in periodontal diseases can provide insights into periodontal disease pathophysiology and help formulate novel therapeutic strategies. This review aims to provide a comprehensive overview of the PKR-T2DMapoptosis-inflammation axis in periodontal diseases, detailing the structure and function of PKR, the impact of diabetes on periodontal health, the mechanisms of apoptosis and inflammation in periodontal tissues, and the potential therapeutic implications.

Introduction

Periodontal diseases encompass a spectrum of inflammatory conditions affecting the supporting structures of the teeth, including the gingiva, periodontal ligament, cementum, and alveolar bone [1], [2]. These diseases progress from gingivitis, characterized by inflammation confined to the gingiva, to periodontitis, where inflammation leads to the destruction of the periodontal ligament and alveolar bone, potentially resulting in tooth loss [1], [2]. Periodontal diseases are prevalent worldwide, affecting millions of individuals and posing significant public health challenges [3], [4]. The prevalence of severe periodontitis is estimated to be around 10%–15% globally, making it one of the most common chronic diseases [5]–[7]. Notably, the impact of periodontal diseases extends beyond oral health, influencing systemic conditions such as cardiovascular disease [8], diabetes [9]–[15] and adverse pregnancy outcomes. In recent years, the bidirectional relationship between type 2 diabetes mellitus (T2DM) and periodontal diseases has been extensively documented [16]. Diabetes has been shown to increase the risk and severity of periodontal diseases, while periodontal infections can negatively affect glycemic control. This complex interplay is mediated by various biological mechanisms, including inflammation, immune responses, and cellular stress [17]. In the subsequent sections, this complex interrelationship has been discussed in detail, placing PKR at the centre of DM, apoptosis, and inflammation.

PKR: Structure, Activation, and Role in Defence Mechanisms

PKR is encoded by the EIF2AK2 gene and is an integral component of the innate immune response. It is primarily recognized for its role in antiviral defence mechanisms [18], [19]. PKR is typically activated by double-stranded RNA (dsRNA), a common byproduct of viral replication. Upon activation, PKR undergoes autophosphorylation, which enables it to phosphorylate various substrates, including the eukaryotic initiation factor 2 alpha (eIF2α). The binding of dsRNA to PKR induces a conformational change, leading to its dimerization and autophosphorylation. This activation mechanism is crucial for PKR’s antiviral functions, as the phosphorylation of eIF2α results in the inhibition of protein synthesis, thereby limiting viral replication and serving as a defence mechanism to prevent the production of viral proteins during infection [20]–[23]. However, this process also affects cellular proteins, contributing to the regulation of cell growth and apoptosis [24], [25]. PKR interacts with various cellular proteins, influencing multiple signaling pathways. For instance, PKR can interact with the inhibitor of κB kinase (IKK) complex, leading to the activation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway. This interaction is crucial for the induction of inflammatory responses [26]. Additionally, PKR can activate the c-Jun N-terminal kinase (JNK) pathway, which is involved in stress responses and apoptosis [26].

Apart from PKR, the NF-κB pathway is a central regulator of the immune response, controlling the transcription of various pro-inflammatory cytokines, chemokines, and adhesion molecules [27]. PKR enhances NF-κB activation by phosphorylating IKK, leading to the degradation of IκB, an inhibitor of NF-κB. The activation of NF-κB results in the transcription of genes involved in inflammation and immune responses. Additionally, PKR modulates the JNK pathway; PKR-mediated activation of JNK leads to the phosphorylation of c-Jun, a component of the activator protein-1 (AP-1) transcription factor that, in turn, regulates the expression of genes involved in apoptosis and cell proliferation. Thus, PKR activation of the JNK pathway contributes to cellular stress responses and apoptosis. PKR also interacts with other signaling pathways, including the p38 mitogen-activated protein kinase (MAPK) pathway and the interferon regulatory factor (IRF) pathway. These interactions highlight the multifaceted role of PKR in regulating cellular responses to stress and inflammation [28]–[31].

DM and Periodontal Diseases

Chronic hyperglycemia in diabetes results in the formation of advanced glycation end-products (AGEs), which are proteins or lipids that undergo glycation after exposure to sugars. AGEs accumulate in various tissues, including the vasculature, kidneys, and periodontal tissues. The interaction of AGEs with their receptors (RAGE) induces oxidative stress, inflammation, and tissue damage [32]–[35]. Additionally, diabetes is associated with increased production of reactive oxygen species (ROS), leading to oxidative stress. ROS can damage cellular components, including DNA, proteins, and lipids. Oxidative stress in diabetes is further exacerbated by the activation of various pro-inflammatory pathways, including NF-κB and JNK, contributing to chronic inflammation and tissue damage [36]–[38]. Such responses lead to the destruction of periodontal tissues, including the gingiva, periodontal ligament, and alveolar bone. Thus, diabetes impairs the function of various components of the immune system and promotes periodontal disease development.

In addition to altered immunological pathways, diabetic patients exhibit reduce immune cell functions. Neutrophils, which are the first line of defense against bacterial infections, exhibit reduced chemotaxis, phagocytosis, and microbial killing in diabetic patients. Macrophages in diabetic patients show altered functions, including impaired cytokine production and phagocytosis. These defects in the immune response contribute to an increased susceptibility to periodontal infections and a diminished ability to resolve inflammation [39]–[41]. Furthermore, diabetes adversely affects wound healing and tissue regeneration in the periodontium. Notably, hyperglycemia impairs the proliferation and function of fibroblasts and osteoblasts, which are essential for the repair and regeneration of periodontal tissues. Additionally, the chronic inflammatory environment in diabetic patients inhibits the healing process, leading to persistent tissue destruction [42], [43].

Apoptosis in Periodontal Diseases

Gingival fibroblasts are essential for maintaining the structural integrity of the gingiva. In periodontal diseases, increased apoptosis of gingival fibroblasts contributes to the breakdown of the gingival connective tissue. Factors such as bacterial toxins, inflammatory cytokines, and oxidative stress can induce apoptosis in gingival fibroblasts, leading to tissue destruction. The periodontal ligament (PDL) is a specialized connective tissue that supports the teeth within the alveolar bone. Apoptosis of PDL cells in periodontal diseases results in the loss of attachment and destruction of the PDL. This process is mediated by various factors, including bacterial products, inflammatory mediators, and mechanical stress. Osteoblasts and osteoclasts are responsible for bone formation and resorption, respectively. In periodontal diseases, the balance between osteoblast and osteoclast activity is disrupted, leading to alveolar bone loss. Increased apoptosis of osteoblasts impairs bone formation, while enhanced survival of osteoclasts promotes bone resorption. Factors such as inflammatory cytokines, oxidative stress, and hyperglycemia can modulate the apoptosis of these cells, contributing to alveolar bone destruction [44], [45].

Increased osteoclast resorption and decreased osteoblast bone formation have become hallmarks in periodontal disease. Osteoblast bone samples obtained from alveolar bone of periodontitis patients have been shown to undergo apoptosis in the presence of TNF-related apoptosis-inducing ligand (TRAIL). In one study, the authors studied the intracellular apoptotic pathway induced by TRAIL, along with TRAIL death (DR4, DR5) and decoy (DcR1, DcR2) receptors in osteoclast bone samples. Additionally, serum TRAIL concentration was assessed. Results revealed DNA fragmentation and activation of caspase-8 and caspase-3, following TRAIL stimulation, in shorter duration. Furthermore, DcR2 downregulation and high serum TRAIL levels were detected. Through this study, the authors demonstrated the role of TRAIL, thus apoptosis, in periodontal disease development [46].

Inflammation in Periodontal Diseases

Cytokines and chemokines play a central role in the inflammatory response in periodontal diseases. Pro-inflammatory cytokines, such as interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6), are produced by various cells in the periodontium, including macrophages, fibroblasts, and epithelial cells. These cytokines promote the recruitment of immune cells to the site of infection and enhance the production of other inflammatory mediators [47], [48]. Additionally, matrix metalloproteinases (MMPs), a family of proteolytic enzymes that degrade extracellular matrix components, show increased expression in periodontal diseases. The expression of MMPs, such as MMP-1, MMP-8, and MMP-9, is upregulated in response to bacterial infection and inflammatory signals. The increased activity of MMPs contributes to the destruction of periodontal tissues, including the gingiva, periodontal ligament, and alveolar bone [49]. Along with pro-inflammatory cytokines and tissue-degrading enzymes, prostaglandins derived from arachidonic acid act as mediators through the cyclooxygenase (COX) pathway. Prostaglandin E2 (PGE2) is a major pro-inflammatory mediator in periodontal diseases. PGE2 is produced by various cells in the periodontium, including macrophages, fibroblasts, and osteoblasts. It promotes vasodilation, increases vascular permeability, and enhances the recruitment of immune cells to the site of infection. PGE2 also stimulates bone resorption by promoting osteoclast differentiation and activity [50], [51].

Existing evidence proves that innate and adaptive immune cells play a pivotal role in inflammation. Notably, both T and B cells contribute to the establishment of a chronic inflammatory environment in periodontal diseases [52]. Persistent inflammation leads to the continuous release of inflammatory mediators, which exacerbate tissue destruction and bone loss. The predominant cellular players include Th1, Th2, Th17, Tregs, neutrophils, and macrophages. Th1 cells produce pro-inflammatory cytokines such as interferon-gamma (IFN-γ) and interleukin-2 (IL-2), which activate macrophages and enhance their ability to phagocytose pathogens. Th1 responses are associated with the promotion of inflammation and tissue destruction in periodontal disease. Th2 cells secrete cytokines such as IL-4, IL-5, and IL-13, which are involved in B cell differentiation and antibody production. Th2 responses are generally considered anti-inflammatory but can contribute to chronic inflammation in periodontal tissues. Th17 cells produce IL-17 and IL-22, which recruit neutrophils and other immune cells to the site of infection. Th17 cells are implicated in the chronic inflammation and bone resorption seen in periodontitis. These cells can directly kill infected cells and release cytokines that exacerbate inflammation. Their role in periodontal disease involves the destruction of periodontal tissues, contributing to disease progression [53]–[55]. Tregs produce anti-inflammatory cytokines like IL-10 and transforming growth factor-beta (TGF-β), which help to regulate the immune response and limit tissue damage. However, in periodontal disease, their regulatory function may be impaired, leading to unchecked inflammation. B cells secrete various cytokines that can influence the immune response. For example, B cells can produce IL-10, which has anti-inflammatory effects, but they can also secrete pro-inflammatory cytokines that contribute to periodontal tissue destruction [56]. The cytokines produced by T and B cells, such as RANKL (receptor activator of nuclear factor kappa-Β ligand), stimulate osteoclast activity, leading to increased bone resorption [57]. This is a key factor in the alveolar bone loss observed in periodontitis.

In terms of innate immune cells, neutrophils are the first ones to arrive at the site of infection in periodontal diseases. They play a crucial role in controlling bacterial infections through phagocytosis, degranulation, and the release of reactive oxygen species (ROS). However, excessive neutrophil activity can contribute to tissue damage and the progression of periodontal diseases [58]. Additionally, macrophages play varied roles in inflammation and immunological balance and can adopt different activation states, including the pro-inflammatory M1 and anti-inflammatory M2 phenotypes. In periodontal diseases, M1 macrophages produce high levels of pro-inflammatory cytokines, ROS, and nitric oxide (NO), contributing to tissue destruction. M2 macrophages, on the other hand, promote tissue repair and resolution of inflammation. Thus, the balance between M1 and M2 macrophages influences the progression and resolution of periodontal inflammation [59]. The non-classical monocytes are referred to as inflammatory and it constitutes 10% of the pool of monocytes in normal individual .We observed doubling of this percentage of non-classical monocytes in T2DM with periodontitis [60].

The PKR-DM-Apoptosis-Inflammation Axis in Periodontal Diseases

PKR plays a critical role in the development of insulin resistance, a hallmark of Type 2 diabetes. Activation of PKR in response to metabolic stress and inflammatory signals leads to the phosphorylation of insulin receptor substrates (IRS), impairing insulin signaling. This results in reduced glucose uptake by cells and increased blood glucose levels. The inhibition of PKR has been shown to improve insulin sensitivity and glucose metabolism in animal models of diabetes. In diabetes, PKR activation contributes to the dysfunction and apoptosis of pancreatic β-cells, which are responsible for insulin production. Chronic hyperglycemia and inflammatory cytokines induce PKR activation in β-cells, leading to ER stress and apoptosis. The loss of β-cells exacerbates insulin deficiency and hyperglycemia, further contributing to the progression of diabetes. PKR interacts with various inflammatory pathways in diabetes, including the NF-κB and JNK pathways. We have demonstrated presence of glucose transporter GLUT-4 in healthy gingival cells [61], [62].

Activation of these pathways promotes the production of pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6, which further impair insulin signaling and β-cell function. The chronic inflammatory state in diabetes perpetuates a cycle of metabolic and cellular stress, contributing to the development and progression of diabetic complications. Periodontal pathogens, such as Porphyromonas gingivalis, can activate PKR in periodontal tissues. These bacteria produce virulence factors, such as lipopolysaccharides (LPS) and gingipains, which stimulate PKR activation in host cells. PKR activation in response to periodontal pathogens enhances the production of pro-inflammatory cytokines and promotes apoptosis in periodontal cells, contributing to tissue destruction [63], [64].

PKR-mediated apoptosis plays a significant role in the breakdown of periodontal tissues. Activation of PKR in response to bacterial infection and inflammatory signals induces apoptosis in gingival fibroblasts, periodontal ligament cells, and osteoblasts. This apoptosis contributes to the loss of connective tissue and alveolar bone, leading to the progression of periodontal diseases. PKR modulates inflammatory responses in periodontal diseases through its interaction with NF-κB and JNK pathways. Activation of PKR enhances NF-κB signaling, leading to increased production of pro-inflammatory cytokines and chemokines. This amplifies the inflammatory response and perpetuates tissue destruction in the periodontium. Additionally, PKR can interact with other inflammatory pathways, such as the p38 MAPK pathway, to promote inflammation and apoptosis [65] (Fig. 1).

Fig. 1. Multiple functions of PKR in cell functioning including insulin resistance.

Therapeutic Implications and Future Directions

Given the central role of PKR in the PKR-DM-apoptosis-inflammation axis, targeting PKR represents a promising therapeutic strategy for managing periodontal diseases in diabetic patients. Small molecule inhibitors of PKR, such as C16 and imoxin [66], have shown potential in preclinical studies for reducing inflammation and improving glucose metabolism. These inhibitors can mitigate PKR-mediated apoptosis and inflammatory responses, preserving tissue integrity and promoting healing. In periodontal therapy, PKR inhibitors could be used to reduce inflammation and apoptosis in periodontal tissues. Combination therapies that target PKR along with conventional periodontal treatments, such as scaling and root planing, may enhance clinical outcomes in diabetic patients with periodontal diseases. Additionally, the use of PKR inhibitors could be explored in the context of regenerative therapies, such as guided tissue regeneration and bone grafting, to improve the healing and regeneration of periodontal tissues. Future research should focus on elucidating the detailed molecular mechanisms underlying PKR activation and its effects on apoptosis and inflammation in periodontal diseases. Investigating the role of PKR in different cell types within the periodontium, including gingival fibroblasts, periodontal ligament cells, and osteoblasts, will provide a comprehensive understanding of its impact on periodontal health. Additionally, studies should explore the interactions between PKR and other signaling pathways involved in periodontal inflammation and tissue destruction. Clinical studies are needed to evaluate the efficacy and safety of PKR inhibitors in patients with periodontal diseases and diabetes. Long-term studies should assess the potential benefits of PKR-targeted therapies in reducing periodontal inflammation, preventing tissue destruction, and improving glycemic control. Additionally, clinical trials should investigate the optimal dosing, delivery methods, and potential side effects of PKR inhibitors in the context of periodontal therapy.

Conclusion

The PKR-DM-apoptosis-inflammation axis plays a critical role in the pathogenesis of periodontal diseases in diabetic patients. PKR serves as a key mediator linking metabolic stress, apoptosis, and inflammation, contributing to the severity of periodontal diseases. Understanding the complex interactions within this axis is essential for developing effective therapeutic strategies to manage periodontal diseases in the context of diabetes. Targeting PKR offers a promising approach to reducing inflammation, preventing tissue destruction, and improving periodontal health in diabetic patients. Future research should focus on elucidating the molecular mechanisms of PKR activation, investigating the potential of PKR inhibitors in clinical settings, and exploring novel therapeutic approaches to address the challenges of periodontal diseases in diabetic populations.