Role of NOX family NADPH oxidases

NOX family NADPH oxidases are transmembrane proteins that transfer electrons across biological membranes. The biological function of NOX enzymes represents the first step that control the oxidative stress cascade and consists in the generation of reactive oxygen species (ROS) from oxygen. ROS avidly interact with a large number of molecules including other small inorganic molecules as well as proteins, lipids, carbohydrates and nucleic acids. Through such interactions, ROS may irreversibly destroy or alter the function of the target molecule. Consequently, ROS have been increasingly identified as major contributors to damage in biological organisms.



ROS generation as a byproduct occurs with mitochondria, peroxisomes, cytochromes P450, and other cellular elements. However, the NOX family NADPH oxidases was demonstrated to generate ROS not as a byproduct but rather as the primary function of the enzymes.

Biological functions of NOX family NADPH oxidases and focused diseases


The NADPH oxidase enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other down-stream reactive oxygen species (ROS). Six homologues of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1 and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91phox), the homologues are now referred to as the NOX family of NADPH oxidases.



Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes includes host defense, post-tranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX actvity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration.

- Idiopathic pulmonary fibrosis (IPF, orphan disease) - Idiopathic pulmonary fibrosis (IPF) represents a particularly aggressive and unrelenting form of progressive pulmonary fibrosis, the aetiology of which is unknown. IPF is best defined as a ‘clinical–radiological–pathological’ syndrome with the finding of usual interstitial pneumonia (UIP) on surgical lung biopsy. UIP/IPF is distinguished from other forms of idiopathic interstitial pneumonia (IIP) by its overall poor prognosis and unresponsiveness to therapy.  Nox4 regulates myofibroblast synthesis of fibronectin and contractile responses to the multifunctional cytokine, transforming growth factor-β1. Transforming growth factor-β1 (TGF-β1) induces Nox4 expression in lung mesenchymal cells via SMAD-3, a receptor-regulated protein that modulates gene transcription. Nox4–dependent generation of hydrogen peroxide (H2O2) is required for TGF-β1–induced myofibroblast differentiation, extracellular matrix (ECM) production and contractility. Nox4 is upregulated in lungs of mice subjected to noninfectious injury and in cases of human idiopathic pulmonary fibrosis (IPF). Genetic or pharmacologic targeting of Nox4 abrogates fibrogenesis in murine models of lung injury. There is compelling evidence that supports a function for Nox4 in tissue fibrogenesis and provide proof of concept for therapeutic targeting of Nox4 in recalcitrant fibrotic disorders.

- Metastatic Renal Cancer –
Inactivation of the von Hippel-Lindau tumor suppressor (VHL) is an early event in >60% of sporadic clear cell renal cell carcinoma (RCC). Loss of VHL E3 ubiquitin ligase function results in accumulation of the -subunit of the hypoxia inducible heterodimeric transcription factor (HIF-) and transcription of an array of genes including vascular endothelial growth factor, transforming growth factor-, and erythropoietin. Studies have shown that HIF- can be alternatively activated by reactive oxygen species. Nox4 is an NADP(H) oxidase that generates signaling levels of superoxide and is found in greatest abundance in the distal renal tubules. Specific catalytic NAD(P)H oxidases Nox1 and Nox4 and their regulatory subunit p22phox are up-regulated and activated by VHL deficiency, leading to increased HIF-2 expression and tumorigenesis in vitro and in vivo. Thus, pharmacological inhibition of NADPH oxidases represents an alternative approach to the down-regulation of HIF-2 expression and its target pathways involved in renal carcinogenesis.

- Metabolic diseases –
Diabetes mellitus is a major cause of end-stage renal disease (ESRD). Clinical hallmarks of diabetic nephropathy include progressive increase in urinary albumin excretion and decline in glomerular filtration rate (GFR), which occurs in association with an increase in blood pressure, leading to ESRD. Molecular mechanisms underlying nephropathy in diabetes involve hyperglycemia, impaired insulin signaling and activation of the renin-angiotensin system. Common to these processes is increased bioavailability of reactive oxygen species (ROS) (oxidative stress), leading to inflammation, fibrosis and endothelial dysfunction. Clinical and experimental studies show increased circulating levels of oxidative markers in diabetes. Of the many enzymatic systems implicated in ROS generation in the kidney, including mitochondrial respiration, uncoupled eNOS and nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase, NADPH oxidase appears to be particularly important. Nox4 (Renox) is the most abundantly expressed NADPH oxidase in the kidney. Since changes in cellular function resulting in oxidative stress play an important role in the development and progression of diabetic nephropathy and chronic kidney disease (CKD), interventions to reduce ROS production by targeting Nox4 appears be an attractive therapeutic strategy.

- Cardiovascular diseases –
Reactive oxygen species (ROS) appear to play a key role in the atherosclerotic process via a series of molecular changes that lead to macrophage infiltration in the endothelium and to plaque formation. In particular, ROS elicit the formation of oxidized LDL, activate matrix metalloproteinases (MMPs), increase smoth  muscle cell growth and induce production of inflammatory molecules, such as MMP-1, intercellular adhesion molecule 1 (ICAM-1) and vascular cellular  molecule (VCAM-1). ROS are implicated in arterial dysfunction via inactivation of nitric oxide (NO), a potent vasodilatator and antiaggregating molecule produced by endothelium and/or inhibition of endothelial NO synthase. Even if several enzymatic pathways, such as xanthine oxidase, myeloperoxidase or NO synthase, can generate ROS, it appears that NADPH oxidases (Nox) are the most important enzyme responsible for ROS formation in human vessels. There is experimental and clinical evidence that Nox enzymes are upregulated in human atherosclerosis, where it maximally contributes to the formation of ROS. Nowadays it appears that Nox enzymes may be causally implicated in the process of atherosclerosis. There is also evidence for the involvement of NOX enzymes in cardiac hypertrophy, progression towards heart failure, hypertension and to some extent myocardial infarction, ischemia/reperfusion injury and atrial fibrillation.

- Neurodegenerative diseases –
The presence of ROS in the central and peripheral nervous systems has received considerable attention over the past several decades. The nervous system accounts for over 20% of the oxygen consumed by the body, and as a result produces large quantities of ROS. Additionally, the nervous system is particularly sensitive to oxidative stress because of enrichment of polyunsaturated fatty acids in many of the membranes. ROS generation by NOX enzymes has been implicated in a variety of diseases of the CNS such as amyotrophic lateral sclerosis, Ischemic Stroke, Alzheimer's disease, Parkinson’s disease and HIV dementia.