The Critical Role of the Adipocytokine NOV in Obstructive Sleep Apnea Induced Cardiometabolic Dysfunction: A Review

Abstract

Obstructive sleep apnea (OSA) is highly prevalent and associated with oxidative stress, chronic inflammation, and adverse cardiovascular consequences. The comorbid condition of obesity remains epidemic. Both obesity and OSA are highly comorbid in patients with cardiovascular disease including atrial fibrillation, resistant hypertension, congestive heart failure, and coronary artery disease. Patients with these preexisting cardiovascular conditions should be screened for OSA with a low threshold to treat, even if OSA severity is mild. Nephroblastoma overexpressed (NOV/CCN3) protein has been identified in multiple chronic inflammatory states, most notably in obesity and more recently in OSA, even in the absence of obesity. As such, NOV may represent an important biomarker for oxidative stress in OSA and may lead to a deeper understanding of the relationship between OSA and its clinical sequelae.

Obstructive sleep apnea (OSA) is a highly prevalent condition characterized by repetitive episodes of upper airway closure due to the collapse of the upper airway lumen in response to an active inspiratory effort that results in a reduction (ie, hypopnea event) or complete (ie, apnea event) cessation of airflow during sleep.¹ The diagnosis and severity of OSA are determined via polysomnography and are quantified using the apnea-hypopnea index (AHI) and oxyhemoglobin saturation indices.² The prevalence of at least moderate OSA in adults ages 30–70 years old in women and men is 6% and 13%, respectively, with increased risk of developing OSA by 2% annually.³ Risk factors for OSA include elevated body mass index (BMI), gender (men more than women), age, ethnicity (African American, Asian, or Hispanic), menstrual status, and upper airway anatomic narrowing or insufficiency.⁴

The airflow reduction in apnea and hypopnea events in OSA leads to reduced alveolar ventilation, intermittent hypoxia, and/or arousals, which causes systemic redox imbalance that favors the oxidative state with an increase in inflammatory cytokines, and upregulation of lipid peroxidation.^5,6 Indeed, the population at increased risk for OSA already has equivalent risk factors for other cardiometabolic disorders, and OSA adds additional risk in developing comorbid cardiovascular and metabolic disease, neurocognitive defects, daytime sleepiness and motor vehicle accidents, depression and mood disorders, and reduced quality of life.^7,8 The common driving pathophysiology behind these entities is likely explained by oxidative stress with increased production of reactive oxygen species (ROS) from adipocyte mitochondria, and inflammatory cytokines, which further contribute to insulin resistance, decreased adiponectin, and heme oxygenase.^6,9 Some novel biomarkers of inflammation such as nephroblastoma overexpressed (NOV) and endothelial cells that have been sloughed into the circulation due to endothelial damage and/or inflammation, called circulating endothelial cells (CEC), have been undergoing investigation in various inflammatory disorders including OSA. These studies showed a correlation to the severity of OSA as ROS-like peroxynitrite, hydrogen peroxide, and superoxides cause the sloughing of endothelial cells that present as CEC, which causes the accelerated production of dysfunctional endothelial progenitor cells from the bone marrow to replace the sloughed endothelial cells.^10–13 As a result of OSA’s downstream inflammatory cascade due to oxidative stress and endothelial dysfunction, untreated OSA places individuals at elevated risk for cardiovascular disease.

NOV, OXIDATIVE STRESS AND CARDIOMETABOLIC DYSFUNCTION

NOV is a novel extracellular matrix protein of the CCN gene family, a new family of regulatory proteins, found to have multiple functions through various extracellular receptors in cellular regulation, wound healing and angiogenesis, bone differentiation, and immunomodulation regulated by various chemo-/cyto-kines; as well as being potent regulators to produce inflammatory molecules.¹⁴ NOV plays a role in chronic inflammatory diseases leading to uncontrolled or prolonged inflammation that occurs in fibrosis, vascular disease, chronic inflammatory or autoimmune diseases, and progression of cancer, making the CCN gene family an attractive area of study for potential therapeutic targets.¹⁴ There have been multiple studies demonstrating the correlation of inflammatory diseases with established inflammatory cytokines and their correlation with other co-morbid inflammatory disorders such as obesity and metabolic syndrome.^14–16 Only few studies have observed OSA in relation to established inflammatory cytokines involved in cell signaling (such as Interleukin 6 (IL-6) and Tumor necrosis factor (TNF)-α) and novel biomarkers of inflammation.^10,11,17

NOV plays a role in inflammation through multiple different extracellular and receptor interactions, mainly via integrins to produce a biochemical response.¹⁴ It can be produced by Treg cells but is mainly found in endotheliocytes, smooth muscle cells, chondrocytes, and fibroblasts.¹⁴ NOV plays a role in the inflammation observed in OSA and its associated co-morbid diseases such as obesity, cardiovascular disease (CVD), and/or metabolic syndrome.^10,14,15,18 In obesity, inflammatory adipocytokines are elevated due to a higher white-to-brown fat ratio and leptin/insulin resistance resulting in a baseline chronic inflammatory state.^15,19–21 Chronically elevated IL-1, IL-6, TNF-α, leptin, and insulin have all been independently shown to be correlated with prolonged inflammation in obesity, and NOV contributes by inducing cytokine formation increasing adipogenesis of white adipose tissue that has decreased numbers of mitochondria, and therefore, has higher proinflammatory properties.^22–24 In fact, knockout mice for NOV have shown changes in macrophage profile and reduced expression of adipocytokines and enhanced insulin signaling.²⁵ The NOV-driven alterations in macrophage profile from M1-like to M2-like leads to increased levels of ox-low density lipoprotein and subsequent atherosclerosis and inflammation.²⁶

OSA AS A MANIFESTATION OF METABOLIC SYNDROME

Metabolic syndrome is defined by a cluster of individual medical entities including obesity, hypertension, dyslipidemia, and impaired glucose tolerance all of which increase the risk of CVD due to chronic inflammation.²⁷ In fact, risk factors for CVD that are not components of metabolic syndrome, such as inflammatory and prothrombotic biomarkers, may have an equal or greater bearing on risk.²⁸ OSA is another co-morbid condition that is not part of metabolic syndrome but is a frequent comorbidity of metabolic syndrome due to its similar risk factors and clinical sequelae of glucose intolerance, dyslipidemia, inflammation, and vasculopathy independent of obesity.^29,30 As a result, OSA further increases cardiometabolic risk in obese and metabolic syndrome patients.^6,29,30 OSA has a prevalence of anywhere from 40% to 80% in a CVD practice; resistant hypertension and patients with atrial fibrillation must be carefully studied for the possibility of this bi-directional relationship between OSA and CVD.²⁸ Clinical tools such as the STOP-BANG questionnaire can help physicians screen high-risk patients for OSA but has decreasing sensitivity as the severity of OSA decreases, which places patients with preexisting cardiometabolic disease (eg, CVD, Congestive heart failure, diabetes, and metabolic syndrome) who need to be treated for even mild OSA at risk of underdiagnosis.

NOV AS POTENTIAL BIOMARKER FOR THE SEVERITY OF OSA

NOV was shown to be independently associated with OSA after adjusting for obesity in recent clinical studies.^10,11 NOV was correlated with the oxygen desaturation index, a measure of the frequency of oxygen desaturation that frequently aligns with the AHI, but not AHI itself, suggesting that elevations in NOV are more directly associated with intermittent hypoxia rather than the conventional AHI, which may or may not include oxygen desaturation indices as criteria for a positive event.^10,11 This relationship suggests that intermittent hypoxia leading to oxidative stress is a greater driving force for NOV elevation, and therefore, is more specific to the inflammation created by OSA. Studies have shown that initiation of Continuous positive airway pressure correlated with lower markers of oxidative stress and endothelium damage.³¹ To date, contemporary literature has not found specific biomarkers that better correlate with the severity of disease but is currently undergoing investigation.^32,33 This finding of NOV as a potential biomarker in OSA independent of obesity was underscored by 3 patients in the study that had normal BMI but significantly elevated NOV.^10,11 The other conventional biomarkers including leptin and IL-6 appeared to be modified by BMI.¹⁰ Due to the downstream effects of OSA pathophysiology and intermittent hypoxia leading to increased oxidative stress and inflammation, sympathetic arousal surges, and platelet activation, individuals with OSA regardless of obesity or metabolic syndrome have a higher risk for CVD making NOV a potential biomarker for OSA severity.^10,34,35

SILENCING NOV AND THE EFFECT ON OXIDATIVE STRESS INDUCED CARDIOMETABOLIC DYSFUNCTION

A recent study in NOV knockout mice showed reduced insulin resistance, improved mitochondrial biogenesis, and improved insulin signaling in both adipose and cardiac tissue.³⁶ Furthermore, the upregulation of protein PGC1α, which plays a crucial role in mitochondrial and vascular function, alleviated metabolic abnormalities in obesity by upregulating biochemical processes that increase the expression of brown fat with a reduction in adipocytokines, including NOV.³⁷ A recent human study has shown NOV to be highly correlated with BMI and decreases with weight reduction.¹⁸ Furthermore, several studies have shown that epoxyeicosatrienoic acid was able to reduce NOV by upregulating heme oxygenase 1 (HO-1) and Wnt signaling in pericardial fat, reversing obesity-induced cardiomyopathy.^22,23 HO-1 and epoxyeicosatrienoic acid upregulation reduce NOV, IL-6, and TNF-α, and improved insulin receptor phosphorylation leading to enhanced insulin sensitivity and reversal of cardiomyopathy in animal studies.^22,38 In addition, NOV has been shown to maintain cardiovascular hemostasis by using integrins to regulate endothelial and vascular smooth muscle cell function and inflammation.¹⁴ High levels of NOV are associated with atherosclerosis likely through the mechanism of high density lipoprotein and low density lipoprotein peroxidation.^15,39,40 This promotes ROS formation causing endothelial damage resulting in elevated Endothelial progenitor cells and CECs as new endothelium begins the reparation process.^{12,13,15,41,42}

MECHANISM OF OXIDATIVE STRESS IN OSA

Oxidative stress in OSA is caused by the intermittent hypoxia driven by a disturbed sleep pattern that is characteristic of the disease.¹⁰ This oxidative stress causes an increase in ROS and inflammatory adipocytokines such as NOV, IL-6, TNF-α, and Leptin, accelerated by comorbid conditions such as obesity.^10,15,22,34 NOV has been linked to chronic inflammation, obesity, insulin resistance, and cardiometabolic dysfunction.^15,35,36 NOV induces inflammation and fibrosis, decreases AKT (protein kinase B), HO-1, and 5' adenosine monophosphate-activated protein kinase leading to mitochondrial dysfunction: This was evidenced by decreased mitochondrial uncoupling protein, mitofusion proteins 1 and 2, while increasing mitochondrial fission 1 protein³⁴ (Fig. 1).

FIGURE 1.

(Scheme) Diagram shows a possible mechanism for how hypoxic episodes in OSA drive inflammation and the cascade of events that increase NOV, the dysfunction of mitochondria and decreased Cardiac bioenergetics. Insulin resistance increases leading to a reduction in AKT (Protein kinase B) and AMPK. The decrease in the antioxidant HO-1 decreases the activity of PGC1α, severely decreasing mitochondrial function. Decreased AKT also drives a reduction in autophagy and may decrease mitophagy as well. The mitochondrial dysfunction is represented by the reduction in UCP1, MFN1,2 and an increase in FIS1. FIS1, indicates fission 1 protein; HO-1, heme oxygenase 1; MFN1,2, mitofusion proteins 1 and 2; NOV, nephroblastoma overexpressed; OSA, obstructive sleep apnea UCP1, uncoupling protein 1.

We have previously shown that HO upregulation, acting through upregulation of its nuclear co-activator PGC1α reduces oxidative stress and reduces NOV levels, thereby decreasing the resultant cardiometabolic dysfunction.^15,36 HO-1 does this through the degradation products of bilirubin and carbon monoxide increasing mitochondrial fusion, adiponectin levels, and reducing body weight, and thermogenesis.³⁴ It improves mitochondrial function and increases adipocyte mesenchymal stem cell differentiation into “mitochondrial rich” beige and brown fat, which is the holy grail of weight loss.³⁵ NOV levels and mitochondrial function are inversely proportional. Elevated NOV is the consequence of oxidative stress, regardless of the cause. The therapy of OSA is aimed at reducing the hypoxic episodes that are contributing to oxidative stress.^10,11

SUMMARY

OSA is increasing in prevalence as the epidemic of obesity remains unabated. OSA carries a significant risk for CVD that includes hypertension, cardiac dysrhythmias such as atrial fibrillation, pulmonary hypertension, and coronary artery disease. NOV is an adipocytokine that has been linked to various chronic inflammatory conditions, most notably obesity. NOV has been identified as a potential biomarker in OSA. Because NOV may play a mechanistic role in the biochemical pathway of mitochondrial destruction and cardiometabolic dysfunction, further study is essential. Therapy must be aimed at reducing NOV and increasing antioxidant function such as HO upregulation.

ACKNOWLEDGMENTS

We thank Mr. Tomer Mouzica for his editorial assistance.