As a significant player in atherosclerosis initiation and progression, studies have suggested that the inflammatory response in endothelial dysfunction can be driven by IKKβ/NF-κB signaling (14, 21, 26). Gareus et al. previously demonstrated that inhibition of NF-κB activity through the deletion of IKKγ, also known as NF-κB essential modulator (NEMO), or expression of a dominant-negative IκBα decreases atherosclerosis in atherogenic prone mice (14). They also found that inhibition of NF-κB in endothelial cells reduced the expression of proinflammatory cytokines, chemokines, and adhesion molecules, leading to decreased monocyte recruitment into the plaque (14). Consistently, inhibition of IKKβ in human umbilical vein endothelial cells has been shown to block NF-κB activation, leading to decreased adhesion molecule gene expression including E-selectin, ICAM-1, and VCAM-1 (27). These adhesion molecules are essential for the attachment and infiltration of the recruited monocytes into the intimal layer (16–18). By contrast, constitutive activation of endothelial IKKβ in mice increased monocyte infiltration into the subintimal space, which contributed to exacerbating early and late-stage atherosclerosis (28). Indeed, the rise of age-associated endothelial dysfunction is correlated with increased IKK activation in arteries while pharmacological inhibition of IKK by salicylate has been shown to improve age-related endothelial dysfunction (29). Thus, targeting endothelial cell IKKβ may have beneficial effects against atherosclerosis development.

The M1, or proinflammatory, macrophage plays a key role in atherosclerosis development, while M2, or anti-inflammatory, macrophages enhance plaque regression and stability (30). The link between macrophage polarization and IKKβ remains elusive, though evidence suggests that IKKβ/NF-κB pathway activation polarizes macrophages to the M2, anti-inflammatory phenotype through negative crosstalk with STAT1 (31, 32). To study the role of macrophage IKKβ in atherosclerosis, Kanters et al. transplanted IKKβ-deficient bone marrow-derived macrophages into atherogenic prone low-density lipoprotein receptor-deficient (LDLR^−/−) mice. They found that the mice receiving IKKβ-deficient macrophages exhibited enhanced atherosclerotic lesion development and increased necrosis, which suggest a protective role of bone marrow-derived macrophage IKKβ against atherosclerosis development (33). However, the same group used a similar method to delete IκBα in myeloid cells, aimed to activate NF-κB signaling. Interestingly, those mice displayed increased atherosclerosis lesion size and leukocyte adhesion without significantly increasing NF-κB targeted genes (34), indicating pro-atherogenic effects of canonical NF-κB activation. Several other studies have also found that macrophage IKKβ/NF-κB pathway has pro-atherogenic effects (35, 36). For example, inhibition of NF-κB in macrophages through the overexpression of a trans-dominant and non-degradable form of IκBα can reduce macrophage foam cell formation in vitro (35). Further, myeloid-specific IKKβ deficiency decreased diet-induced atherosclerosis in LDLR^−/− mice by diminishing macrophage inflammatory responses such as adhesion, migration and lipid uptake in macrophages (36). Collectively, these results indicate the role of macrophage IKKβ/NF-κB in atherogenesis is complex and more studies are needed to completely understand how IKKβ functions in myeloid cells to regulate atherosclerosis development.

In addition to endothelial and immune cells, vascular smooth muscle cells (VSMCs) also play an important role in atherogenesis. In the early stages of atherosclerosis, VSMCs undergo a phenotypic switch from contractile to synthetic where they gain the ability to proliferate and migrate into the intimal layer. This provides a beneficial effect as these VSMCs proliferate and migrate to the cap of the plaque and reinforces its stability, lowering the risk for plaque rupture (37). An earlier study demonstrated that IKKα and IKKβ was activated in IL-1β-induced proliferative response of human saphenous vein smooth muscle cells (38). Notably, the proliferative ability of human VSMCs were diminished in IKKα and IKKβ mutant transfected cells (38). The role of VSMC IKKβ in atherosclerosis was also investigated in LDLR^−/− mice (39). Deficiency of IKKβ in VSMCs driven by a SM22Cre-IKKβ-flox system protected LDLR^−/− mice from diet-induced vascular inflammation and atherosclerosis development (39). Since inhibition of NF-κB activity in endothelia cells also decreased vascular inflammation and atherosclerosis in ApoE^−/− mice (14), these studies suggest that inhibiting IKKβ/NF-κB signaling in the vasculature has anti-atherogenic effects.

Under pathological conditions, adipose tissue is at a chronic low level of inflammation (3). The circulating inflammatory mediators secreted by adipocytes participate in vascular dysfunction, which can lead to atherosclerosis (40). However, the role of adipocyte IKKβ signaling in atherogenesis is poorly understood. A recent study found that adipocyte-specific deletion of IKKβ did not affect obesity and atherosclerosis in lean LDLR^−/− mice when fed a low-fat diet (41). When fed a high-fat diet, however, IKKβ-deficient LDLR^−/− mice had defective adipose remodeling, leading to increased adipose tissue and systemic inflammation (41). Deficiency of adipocyte IKKβ did not affect atherosclerotic lesion size but resulted in enhanced lesional inflammation and increased plaque vulnerability in obese IKKβ-deficient LDLR^−/− mice (41). In addition to regular fat depots, adipocytes can also be found adjacent to the vascular wall called perivascular adipose tissue (PVAT). Under homeostatic conditions, PVAT holds a protective role on vascular homeostasis by secreting bioactive molecules like adiponectin, nitric oxide (NO), and IL-10 (42, 43). However, under pathological conditions, PVAT switches to a proinflammatory phenotype by secreting adipokines, cytokines, and chemokines (43). The role of PVAT in atherosclerosis and vascular injury has not been extensively investigated. However, studies have found that PVAT may contribute to endothelial dysfunction (42), macrophage migration, and VSMC proliferation and migration (44). The role of PVAT IKKβ in vascular function and atherosclerosis remains elusive. Future studies should be considered to investigate the role of PVAT IKKβ/NF-κB signaling on vascular function and atherosclerotic development under normal or pathological conditions (e.g., obesity).

While deletion of IKKβ in VSMCs decreased atherosclerosis development in LDLR^−/− mice (39), those mice were also protected from diet-induced obesity and insulin resistance. Interestingly, many adipocyte precursor cells express SMC markers and ablation of IKKβ blocked adipocyte differentiation in vitro and in vivo, suggesting that IKKβ functions in adipocyte precursor cells to regulate adiposity (39). Indeed, selective deletion of IKKβ in the white adipose lineage further elucidated the role of adipose progenitor cell IKKβ signaling in regulating adiposity and metabolic function (39, 54). Deficiency of adipose progenitor IKKβ decreased high-fat feeding-induced adipogenesis and systemic inflammation, resulting in decreased adiposity and insulin resistance in those mice (39, 54). The function of IKKβ in the regulation of adipogenesis was further confirmed in mesenchymal stem cells (MSCs) (55). Mechanistic studies then revealed an important crosstalk between IKKβ and Wnt/β-catenin signaling (Figure 1) (55). Interestingly, IKKβ is a β-catenin kinase that can directly phosphorylate the conserved degron motif of β-catenin to prime it for β-TrCP-mediated ubiquitination and degradation (10, 55). Wnt/β-catenin signaling has been well studied to inhibit adipocyte differentiation (56, 57) and the impact of IKKβ signaling on adipogenesis was abolished in β-catenin-deficient MSCs (10, 55). Thus, IKKβ-mediated β-catenin phosphorylation may play a critical role in regulating adipocyte differentiation and adiposity in obesity (Figure 1).

While studies have suggested a pro-obesogenic role of progenitor IKKβ, the function of IKKβ in mature adipocytes is apparently more complicated. Constitutive activation of IKKβ in adipocytes has been demonstrated to increased energy expenditure in mice, leading to protective effects against diet-induced obesity and insulin resistance (58). However, targeted deletion of IKKβ in adipocytes did not affect obesity but resulted in increased tissue inflammation, impaired adipose remodeling, and exacerbated metabolic disorders (59, 60). In addition to mediating inflammation, IKKβ can also promote cell survival by upregulating NF-κB-mediated anti-apoptotic gene expression (61–63) and by direct phosphorylation of pro-apoptotic protein, BAD (64). Previous reports have linked adipocyte death with obesity, adipocyte macrophage infiltration, and systemic insulin resistance (65). IKKβ has been shown to be a key adipocyte survival factor in obesity, and deficiency of IKKβ in adipocytes can lead to high fat feeding-elicited cell death, impaired adipose tissue remodeling and partial lipodystrophy in visceral adipose tissue (59, 60). Further studies are required to completely understand the role of adipocyte IKKβ in regulating energy expenditure, homeostasis, and adiposity.

The IKKβ/NF-κB pathway has been demonstrated to be active in both obesity-dependent and independent insulin resistance (47, 53). Inhibition of IKKβ with salicylate or other methods is associated with reduced insulin resistance and glucose intolerance (54, 73–75). Previous studies demonstrated that constitutively active hepatic IKKβ induced obesity-independent systemic insulin resistance, while inhibiting hepatic NF-κB reversed both local and systemic insulin resistance (51, 76). These findings indicate an important role of IKKβ in regulating hepatic and systemic insulin resistance. Another study utilizing hepatocyte-specific IKKβ deficient mice found improved hepatic insulin response while maintaining systemic insulin resistance during obesity (77). These results can be attributed to obesity-associated systemic inflammation that cannot be alleviated by IKKβ knockdown in the liver alone. More recently, it has been reported that hepatic IKKβ in the liver can improve glucose homeostasis by interacting with x-box binding protein 1 (XBP1) and enhancing its activity, stabilization, and nuclear translocation (Figure 1) (78). While it is generally recognized that hepatic inflammation drives the detrimental perspectives of obesity-induced insulin resistance (1, 73, 79), upregulation of certain inflammatory signaling could have positive or negative contributions to whole-body metabolism, depending on conditions of signaling activation and related physiological statuses. Therefore, the hepatic IKKβ function in insulin resistance is complex and future studies are required to define the detailed mechanisms through which hepatic IKKβ regulates insulin responsiveness under normal and pathophysiological conditions.

Inflammation is an important contributor of insulin resistance, and adipose tissue is one of the important tissues for this high-fat feeding-elicited inflammatory response (80). Adipose IKKβ signaling has been implicated in obesity-associated insulin resistance. For example, studies have found that IKKβ deficiency in adipocyte precursors or adipose lineage cells can protect mice from diet-induced obesity, systemic inflammation and insulin resistance (39, 54). Several studies demonstrated that IKKβ deficiency and XBP1 overexpression attenuates FFA-induced inflammation and impairment of insulin signaling in cultured adipocytes (81, 82). While hepatic IKKβ increases nuclear translocation of XBP1 (78), adipocyte IKKβ is inhibited by XBP1 (82), indicating a more complex role of IKKβ/XBP1 interaction in cardiometabolic disease. Overexpression of IKKβ in adipocytes also led to increased adipose tissue inflammation in mice (58). Paradoxically, those mice were resistant to diet-induced obesity and insulin resistance, likely due to increased energy expenditure (58). Deletion of adipocyte IKKβ did not affect obesity in mice but resulted in elevated adipose tissue inflammation, increased macrophage infiltration and exacerbate insulin resistance (59, 60).

Skeletal muscle is another insulin responsive tissue that is impaired in obesity and diabetes (67). Studies revealed elevated IKKβ activity in isolated skeletal muscle of obese patients with type 2 diabetes and obese mice (83, 84). By contrast, inhibition of IKKβ or NF-κB signaling can restore insulin signaling in vitro (85, 86) and systemic IKKβ inhibition can alleviate skeletal muscle and systemic insulin resistance all together (73, 74). However, under obese conditions, targeting skeletal muscle IKKβ can only alleviate local insulin resistance, but not systemic insulin responsiveness (87).

While tissue-specific inhibition of IKKβ (i.e., liver, adipose, skeletal muscle) may be able to abrogate local insulin resistance, it may not be sufficient for systemic inflammation-induced insulin resistance under obese conditions. For example, it is reported that myeloid-specific IKKβ deficiency can improve obese-dependent systemic insulin resistance (77, 87), indicating that myeloid cell IKKβ plays a role in systemic insulin resistance and inflammation in obesity. Furthermore, Cai et al. linked the IKKβ/NF-κB pathway with paracrine IL-6 signaling (51), which is associated with type 2 diabetes and insulin resistance (88, 89). IL-6 can induce the expression of suppressor of cytokine signaling 3 (SOCS-3), which inhibits autophosphorylation of IRS-1 and insulin receptor (90). The IKKβ/NF-κB/IL-6 axis was confirmed to be involved in insulin resistance when IL-6 neutralization improved insulin resistance (51).

Although there have been strong links between IKKβ and metabolic diseases within the periphery, more recently, inflammatory activation has been seen within the central nervous system (CNS). Specifically, IKKβ in the hypothalamus can be activated in obesity and obesity-related metabolic dysregulation such as energy, body weight, and glucose dysregulation (95–98). A study found that FFAs induce TLR4-mediated hypothalamic cytokine production and anorexigenic signal resistance which may lead to obesity (99). Signaling between the gut and brain (gut-brain-axis) is a major influencer in developing obesity. Obese mice and mice stimulated with overnutrition display overall higher levels of IKKβ within the hypothalamic neurons, which is consistent with the systemic trend (95, 100). However, it was observed that overnutrition-mediated activation of IKKβ/NF-κB was activated intracellularly by ER stress and prompted both hypothalamic leptin and insulin resistance through the induction of suppressor of cytokine signaling 3 (SOCS3), an inhibitor of leptin and insulin signaling (95, 101). ER stress can also lead to impaired hepatic insulin signaling, which was improved upon ER stress inhibition (102). TLR-dependent IKKβ activation in the CNS was also involved in obesity and leptin resistance (96). Deficiency of IKKβ in hypothalamic AGRP neurons displayed anti-obese phenotype along with preserved leptin and insulin signaling and reduced SOCS3 gene expression, and overexpression of SOCS3 reversed the protective effects of IKKβ knockout in mice (95). By contrast, activation of IKKβ in AGRP neurons resulted in impaired glucose homeostasis, without affecting body weight and leptin signaling (103).

While it is critical to study the effects of hypothalamic inflammation on obesity and metabolic syndromes, it is also important to investigate the upstream targets mediating hypothalamic inflammation. For example, astrocytes play essential roles in neuronal development; regulation of blood flow; fluid, ion, pH, and transmitter homeostasis; the regulation of synaptic transmission; and regulate immune response (104). Under pathological conditions or external stressors, astrocytes and other glial cells undergo gliosis, or astrogliosis, which is characterized by proliferation and accumulation of astrocytes (104, 105). Zhang et al. demonstrated an important role of astrocyte IKKβ in stimulating glucose intolerance, hypertension, and weight gain (106). While overnutrition and IKKβ overexpression inhibited proper astrocytic plasticity, inhibition of IKKβ prevented overnutrition-induced metabolic diseases and impaired astrocytic plasticity (106). Mechanistically, IKKβ-induced shortening of astrocyte processes led to increased extracellular GABA, an inhibitory neurotransmitter, and lower brain derived neurotrophic factor (BDNF) levels through inhibition of BDNF secreting neurons in the hypothalamus (106). Low levels of BDNF have been associated with metabolic disorders such as obesity, energy metabolism, and hyperglycemia (107). The protective role of IKKβ deficiency in astrocytes were reversed by BDNF inhibition, suggesting that the GABA-BDNF axis is important in regulating energy homeostasis and metabolic syndromes (106). In addition to developed cells within the CNS, the hypothalamic neural stem cells are important mediators for metabolic syndrome. IKKβ/NF-κB activation in the mediobasal hypothalamus can lead to obesity and insulin resistance, along with loss of neuronal development including POMC neurons (108).

Hypertension, a chronic elevation in arterial blood pressure, is one of the major risk factors for developing CVD such as myocardial infarction, stroke, and heart failure. Although there are therapeutic interventions aimed to target and treat hypertension, it is still a prevalent contributor to cardiometabolic disease burden (109). IKKβ in the CNS, mainly in the hypothalamus, can regulate blood pressure. Overexpression of a constitutively active form of IKKβ in the mediobasal hypothalamus induces hypertension in mice, while NF-κB inhibition attenuated high-fat feeding induced hypertension in mice (110). Additionally, astrocyte-specific IKKβ overexpression in mice led to higher daytime blood pressure, while NF-κB inhibition reversed obesity-induced hypertension in mice (106). In line with the previous discussion linking ER stress to insulin resistance, thapsigargin-induced ER stress increased blood pressure and phosphorylated IκB, but inhibition of NF-κB alleviated these effects (102).