The role of classical transient receptor potential channels in the regulation of hypoxic pulmonary vasoconstriction

Transcriptomic analysis of the hypoxia-inducible factor 1α impact on the gene expression profile of chicken fibroblasts under hypoxia

Hypoxia-inducible factor 1 (HIF-1) is a transcriptional regulator that mediates cellular adaptive responses to hypoxia. Hypoxia-inducible factor 1α (HIF-1α) is involved in the development of ascites syndrome (AS) in broiler chickens. Therefore, studying the effect of HIF-1α on the cellular transcriptome under hypoxic conditions will help to better understand the mechanism of HIF-1α in the development of AS in broilers. In this study, we analyzed the gene expression profile of the chicken fibroblast cell line (DF-1) under hypoxic conditions by RNA-seq. Additionally, we constructed the HIF-1α knockdown DF-1 cell line by using the RNAi method and analyzed the gene expression profile under hypoxic conditions. The results showed that exposure to hypoxia for 48 h had a significant impact on the expression of genes in the DF-1 cell line, which related to cell proliferation, stress response, and apoptosis. In addition, after HIF-1α knockdown more differential expression genes appeared than in wild-type cells, and the expression of most hypoxia-related genes was either down-regulated or remained unchanged. Pathway analysis results showed that differentially expressed genes were mainly enriched in pathways related to cell proliferation, apoptosis, and oxidative phosphorylation. Our study obtained transcriptomic data from chicken fibroblasts at different hypoxic times and identified the potential regulatory network associated with HIF-1α. This data provides valuable support for understanding the transcriptional regulatory mechanism of HIF-1α in the development of AS in broilers.

Crucial Role of Stromal Interaction Molecule-Activated TRPC-ORAI Channels in Vascular Remodeling and Pulmonary Hypertension Induced by Intermittent Hypoxia

Obstructive sleep apnea (OSA), a sleep breathing disorder featured by chronic intermittent hypoxia (CIH), is associate with pulmonary hypertension. Rats exposed to CIH develop lung vascular remodeling and pulmonary hypertension, which paralleled the upregulation of stromal interaction molecule (STIM)-activated TRPC-ORAI Ca2+ channels (STOC) in the lung, suggesting that STOC participate in the pulmonary vascular alterations. Accordingly, to evaluate the role played by STOC in pulmonary hypertension we studied whether the STOC blocker 2-aminoethoxydiphenyl borate (2-APB) may prevent the vascular remodeling and the pulmonary hypertension induced by CIH in a rat model of OSA. We assessed the effects of 2-APB on right ventricular systolic pressure (RVSP), pulmonary vascular remodeling, α-actin and proliferation marker Ki-67 levels in pulmonary arterial smooth muscle cells (PASMC), mRNA levels of STOC subunits, and systemic and pulmonary oxidative stress (TBARS) in male Sprague-Dawley (200 g) rats exposed to CIH (5% O2, 12 times/h for 8h) for 28 days. At 14 days of CIH, osmotic pumps containing 2-APB (10 mg/kg/day) or its vehicle were implanted and rats were kept for 2 more weeks in CIH. Exposure to CIH for 28 days raised RVSP > 35 mm Hg, increased the medial layer thickness and the levels of α-actin and Ki-67 in PASMC, and increased the gene expression of TRPC1, TRPC4, TRPC6 and ORAI1 subunits. Treatment with 2-APB prevented the raise in RVSP and the increment of the medial layer thickness, as well as the increased levels of α-actin and Ki-67 in PASMC, and the increased gene expression of STOC subunits. In addition, 2-APB did not reduced the lung and systemic oxidative stress, suggesting that the effects of 2-APB on vascular remodeling and pulmonary hypertension are independent on the reduction of the oxidative stress. Thus, our results supported that STIM-activated TRPC-ORAI Ca2+ channels contributes to the lung vascular remodeling and pulmonary hypertension induced by CIH.

Stim-activated TRPC-ORAI channels in pulmonary hypertension induced by chronic intermittent hypoxia

Obstructive sleep apnea (OSA), a breathing disorder featured by chronic intermittent hypoxia (CIH) is associated with pulmonary hypertension (PH). Rodents exposed to CIH develop pulmonary vascular remodeling and PH, but the pathogenic mechanisms are not well known. Overexpression of Stim-activated Transient Receptor Potential Channels (TRPC) and Calcium Release-Activated Calcium Channel Protein (ORAI) TRPC-ORAI Ca²⁺ channels (STOC) has been involved in pulmonary vascular remodeling and PH in sustained hypoxia. However, it is not known if CIH may change STOC levels. Accordingly, we studied the effects of CIH on the expression of STOC subunits in the lung and if these changes paralleled the progression of the vascular pulmonary remodeling and PH in a preclinical model of OSA. Male Sprague-Dawley rats (∼200 g) were exposed to CIH (5%O₂, 12 times/h for 8 h) for 14, 21, and 28 days. We measured right ventricular systolic pressure (RVSP), cardiac morphometry with MRI, pulmonary vascular remodeling, and wire-myographic arterial responses to KCl and endothelin-1 (ET-1). Pulmonary RNA and protein STOC levels of TRPC1, TRPC4, TRPC6, ORAI 1, ORAI 2, and STIM1 subunits were measured by qPCR and western blot, and results were compared with age-matched controls. CIH elicited a progressive increase of RVSP and vascular contractile responses to KCl and ET-1, leading to vascular remodeling and augmented right ventricular ejection fraction, which was significant at 28 days of CIH. The levels of TRPC1, TRPC4, TRPC 6, ORAI 1, and STIM 1 channels increased following CIH, and some of them paralleled morphologic and functional changes. Our findings show that CIH increased pulmonary STOC expression, paralleling vascular remodeling and PH.

Canonical Transient Receptor Potential Channels and Their Link with Cardio/Cerebro-Vascular Diseases

The canonical transient receptor potential channels (TRPCs) constitute a series of nonselective cation channels with variable degrees of Ca²⁺ selectivity. TRPCs consist of seven mammalian members, TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7, which are further divided into four subtypes, TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7. These channels take charge of various essential cell functions such as contraction, relaxation, proliferation, and dysfunction. This review, organized into seven main sections, will provide an overview of current knowledge about the underlying pathogenesis of TRPCs in cardio/cerebrovascular diseases, including hypertension, pulmonary arterial hypertension, cardiac hypertrophy, atherosclerosis, arrhythmia, and cerebrovascular ischemia reperfusion injury. Collectively, TRPCs could become a group of drug targets with important physiological functions for the therapy of human cardio/cerebro-vascular diseases.

Transcription factors, transcriptional coregulators, and epigenetic modulation in the control of pulmonary vascular cell phenotype: therapeutic implications for pulmonary hypertension (2015 Grover Conference series)

Pulmonary hypertension (PH) is a complex and multifactorial disease involving genetic, epigenetic, and environmental factors. Numerous stimuli and pathological conditions facilitate severe vascular remodeling in PH by activation of a complex cascade of signaling pathways involving vascular cell proliferation, differentiation, and inflammation. Multiple signaling cascades modulate the activity of certain sequence-specific DNA-binding transcription factors (TFs) and coregulators that are critical for the transcriptional regulation of gene expression that facilitates PH-associated vascular cell phenotypes, as demonstrated by several studies summarized in this review. Past studies have largely focused on the role of the genetic component in the development of PH, while the presence of epigenetic alterations such as microRNAs, DNA methylation, histone levels, and histone deacetylases in PH is now also receiving increasing attention. Epigenetic regulation of chromatin structure is also recognized to influence gene expression in development or disease states. Therefore, a complete understanding of the mechanisms involved in altered gene expression in diseased cells is vital for the design of novel therapeutic strategies. Recent technological advances in DNA sequencing will provide a comprehensive improvement in our understanding of mechanisms involved in the development of PH. This review summarizes current concepts in TF and epigenetic control of cell phenotype in pulmonary vascular disease and discusses the current issues and possibilities in employing potential epigenetic or TF-based therapies for achieving complete reversal of PH.

Role of Hypoxia-Induced Brain Derived Neurotrophic Factor in Human Pulmonary Artery Smooth Muscle

Background

Hypoxia effects on pulmonary artery structure and function are key to diseases such as pulmonary hypertension. Recent studies suggest that growth factors called neurotrophins, particularly brain-derived neurotrophic factor (BDNF), can influence lung structure and function, and their role in the pulmonary artery warrants further investigation. In this study, we examined the effect of hypoxia on BDNF in humans, and the influence of hypoxia-enhanced BDNF expression and signaling in human pulmonary artery smooth muscle cells (PASMCs).

Methods and Results

48h of 1% hypoxia enhanced BDNF and TrkB expression, as well as release of BDNF. In arteries of patients with pulmonary hypertension, BDNF expression and release was higher at baseline. In isolated PASMCs, hypoxia-induced BDNF increased intracellular Ca²⁺ responses to serotonin: an effect altered by HIF1α inhibition or by neutralization of extracellular BDNF via chimeric TrkB-Fc. Enhanced BDNF/TrkB signaling increased PASMC survival and proliferation, and decreased apoptosis following hypoxia.

Conclusions

Enhanced expression and signaling of the BDNF-TrkB system in PASMCs is a potential mechanism by which hypoxia can promote changes in pulmonary artery structure and function. Accordingly, the BDNF-TrkB system could be a key player in the pathogenesis of hypoxia-induced pulmonary vascular diseases, and thus a potential target for therapy.

Hypoxia-dependent reactive oxygen species signaling in the pulmonary circulation: focus on ion channels

SIGNIFICANCE

An acute lack of oxygen in the lung causes hypoxic pulmonary vasoconstriction, which optimizes gas exchange. In contrast, chronic hypoxia triggers a pathological vascular remodeling causing pulmonary hypertension, and ischemia can cause vascular damage culminating in lung edema.

RECENT ADVANCES

Regulation of ion channel expression and gating by cellular redox state is a widely accepted mechanism; however, it remains a matter of debate whether an increase or a decrease in reactive oxygen species (ROS) occurs under hypoxic conditions. Ion channel redox regulation has been described in detail for some ion channels, such as Kv channels or TRPC6. However, in general, information on ion channel redox regulation remains scant.

CRITICAL ISSUES AND FUTURE DIRECTIONS

In addition to the debate of increased versus decreased ROS production during hypoxia, we aim here at describing and deciphering why different oxidants, under different conditions, can cause both activation and inhibition of channel activity. While the upstream pathways affecting channel gating are often well described, we need a better understanding of redox protein modifications to be able to determine the complexity of ion channel redox regulation. Against this background, we summarize the current knowledge on hypoxia-induced ROS-mediated ion channel signaling in the pulmonary circulation.

TRP Channels in Respiratory Pathophysiology: the Role of Oxidative, Chemical Irritant and Temperature Stimuli

There is rapidly growing evidence indicating multiple and important roles of Ca(2+)- permeable cation TRP channels in the airways, both under normal and disease conditions. The aim of this review was to summarize the current knowledge of TRP channels in sensing oxidative, chemical irritant and temperature stimuli by discussing expression and function of several TRP channels in relevant cell types within the respiratory tract, ranging from sensory neurons to airway smooth muscle and epithelial cells. Several of these channels, such as TRPM2, TRPM8, TRPA1 and TRPV1, are discussed in much detail to show that they perform diverse, and often overlapping or contributory, roles in airway hyperreactivity, inflammation, asthma, chronic obstructive pulmonary disease and other respiratory disorders. These include TRPM2 involvement in the disruption of the bronchial epithelial tight junctions during oxidative stress, important roles of TRPA1 and TRPV1 channels in airway inflammation, hyperresponsiveness, chronic cough, and hyperplasia of airway smooth muscles, as well as TRPM8 role in COPD and mucus hypersecretion. Thus, there is increasing evidence that TRP channels not only function as an integral part of the important endogenous protective mechanisms of the respiratory tract capable of detecting and ensuring proper physiological responses to various oxidative, chemical irritant and temperature stimuli, but that altered expression, activation and regulation of these channels may also contribute to the pathogenesis of respiratory diseases.

A mutation in TRPC6 channels abolishes their activation by hypoosmotic stretch but does not affect activation by diacylglycerol or G protein signaling cascades

Canonical transient receptor potential-6 (TRPC6) channels have been implicated in the pathogenesis of kidney disease and in the regulation of vascular smooth muscle tone, podocyte function, and a variety of processes in other cell types. The question of whether their gating is intrinsically mechanosensitive has been controversial. In this study we have examined activation of two alleles of TRPC6 transiently expressed in CHO-K1 cells: the wild-type human TRPC6 channel, and TRPC6-N143S, an allele originally identified in a family with autosomal dominant familial focal and segmental glomerulosclerosis (FSGS). We observed that both channel variants carried robust cationic currents that could be evoked by application of membrane-permeable analogs of diacylglycerol (DAG) or by the P2Y receptor agonist ATP. The amplitudes and characteristics of currents evoked by the DAG analog or ATP were indistinguishable in cells expressing the two TRPC6 alleles. By contrast, hypoosmotic stretch evoked robust currents in wild-type TRPC6 channels but had no discernible effect on currents in cells expressing TRPC6-N143S, indicating that the mutant form lacks mechanosensitivity. Coexpression of TRPC6-N143S with wild-type TRPC6 or TRPC3 channels did not alter stretch-evoked responses compared with when TRPC3 channels were expressed by themselves, indicating that TRPC6-N143S does not function as a dominant-negative. These data indicate that mechanical activation and activation evoked by DAG or ATP occur through fundamentally distinct biophysical mechanisms, and they provide support for the hypothesis that protein complexes containing wild-type TRPC6 subunits can be intrinsically mechanosensitive.

Sildenafil inhibits hypoxia-induced transient receptor potential canonical protein expression in pulmonary arterial smooth muscle via cGMP-PKG-PPARγ axis

Transient receptor potential canonical (TRPC) proteins play important roles in chronically hypoxic pulmonary hypertension (CHPH). Previous results indicated that sildenafil inhibited TRPC1 and TRPC6 expression in rat distal pulmonary arteries (PAs). However, the underlying mechanisms remain unknown. We undertook this study to investigate the downstream signaling of sildenafil's regulation on TRPC1 and TRPC6 expression in pulmonary arterial smooth muscle cells (PASMCs). Hypoxia-exposed rats (10% O2 for 21 d) and rat distal PASMCs (4% O2 for 60 h) were taken as models to mimic CHPH. Real-time PCR, Western blotting, and Fura-2-based fluorescent microscopy were performed for mRNA, protein, and Ca(2+) measurements, respectively. The cellular cyclic guanosine monophosphate (cGMP) analogue 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate sodium salt (CPT-cGMP) (100 μM) inhibited TRPC1 and TRPC6 expression, store-operated Ca(2+) entry (SOCE), and the proliferation and migration of PASMCs exposed to prolonged hypoxia. The inhibition of CPT-cGMP on TRPC1 and TRPC6 expression in PASMCs was relieved by either the inhibition or knockdown of cGMP-dependent protein kinase (PKG) and peroxisome proliferator-activated receptor γ (PPARγ) expression. Under hypoxic conditions, CPT-cGMP increased PPARγ expression. This increase was abolished by the PKG antagonists Rp8 or KT5823. PPARγ agonist GW1929 significantly decreased TRPC1 and TRPC6 expression in PASMCs. Moreover, hypoxia exposure decreased, whereas sildenafil treatment increased, PKG and PPARγ expression in PASMCs ex vivo, and in rat distal PAs in vivo. The suppressive effects of sildenafil on TRPC1 and TRPC6 in rat distal PAs and on the hemodynamic parameters of CHPH were inhibited by treatment with the PPARγ antagonist T0070907. We conclude that sildenafil inhibits TRPC1 and TRPC6 expression in PASMCs via cGMP-PKG-PPARγ-dependent signaling during CHPH.

Functional expression of the transient receptor potential channel TRPA1, a sensor for toxic lung inhalants, in pulmonary epithelial cells

The cation channel TRPA1 functions as a chemosensory protein and is directly activated by a number of noxious inhalants. A pulmonary expression of TRPA1 has been described in sensory nerve endings and its stimulation leads to the acceleration of inflammatory responses in the lung. Whereas the function of TRPA1 in neuronal cells is well defined, only few reports exist suggesting a role in epithelial cells. The aim of the present study was therefore (1) to evaluate the expression of TRPA1 in pulmonary epithelial cell lines, (2) to characterize TRPA1-promoted signaling in these cells, and (3) to study the extra-neuronal expression of this channel in lung tissue sections. Our results revealed that the widely used alveolar type II cell line A549 expresses TRPA1 at the mRNA and protein level. Furthermore, stimulating A549 cells with known TRPA1 activators (i.e., allyl isothiocyanate) led to an increase in intracellular calcium levels, which was sensitive to the TRPA1 blocker ruthenium red. Investigating TRPA1 coupled downstream signaling cascades it was found that TRPA1 activation elicited a stimulation of ERK1/2 whereas other MAP kinases were not affected. Finally, using epithelial as well as neuronal markers in immunohistochemical approaches, a non-neuronal TRPA1 protein expression was detected in distal parts of the porcine lung epithelium, which was also found examining human lung sections. TRPA1-positive staining co-localized with both epithelial and neuronal markers underlining the observed epithelial expression pattern. Our findings of a functional expression of TRPA1 in pulmonary epithelial cells provide causal evidence for a non-neuronal TRPA1-mediated control of inflammatory responses elicited upon TRPA1-mediated registration of toxic inhalants in vivo.

Hypoxia-induced ventilatory responses in conscious mice: gender differences in ventilatory roll-off and facilitation

The aim of this study was to compare the ventilatory responses of C57BL6 female and male mice during a 15 min exposure to a hypoxic-hypercapnic (H-H) or a hypoxic (10% O(2), 90% N(2)) challenge and subsequent return to room air. The ventilatory responses to H-H were similar in males and females whereas there were pronounced gender differences in the ventilatory responses during and following hypoxic challenge. In males, the hypoxic response included initial increases in minute volume via increases in tidal volume and frequency of breathing. These responses declined substantially (roll-off) during hypoxic exposure. Upon return to room-air, relatively sustained increases in these ventilatory parameters (short-term potentiation) were observed. In females, the initial responses to hypoxia were similar to those in males whereas roll-off was greater and post-hypoxia facilitation was smaller than in males. The marked differences in ventilatory roll-off and post-hypoxia facilitation between female and male C57BL6 mice provide evidence that gender is of vital importance to ventilatory control.

Enhanced Ca(2+)-sensing receptor function in idiopathic pulmonary arterial hypertension

RATIONALE

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary arterial smooth muscle cells (PASMC) is an important stimulus for pulmonary vasoconstriction and vascular remodeling. Increased resting [Ca(2+)](cyt) and enhanced Ca(2+) influx have been implicated in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH).

OBJECTIVE

We examined whether the extracellular Ca(2+)-sensing receptor (CaSR) is involved in the enhanced Ca(2+) influx and proliferation in IPAH-PASMC and whether blockade of CaSR inhibits experimental pulmonary hypertension.

METHODS AND RESULTS

In normal PASMC superfused with Ca(2+)-free solution, addition of 2.2 mmol/L Ca(2+) to the perfusate had little effect on [Ca(2+)](cyt). In IPAH-PASMC, however, restoration of extracellular Ca(2+) induced a significant increase in [Ca(2+)](cyt). Extracellular application of spermine also markedly raised [Ca(2+)](cyt) in IPAH-PASMC but not in normal PASMC. The calcimimetic R568 enhanced, whereas the calcilytic NPS 2143 attenuated, the extracellular Ca(2+)-induced [Ca(2+)](cyt) rise in IPAH-PASMC. Furthermore, the protein expression level of CaSR in IPAH-PASMC was greater than in normal PASMC; knockdown of CaSR in IPAH-PASMC with siRNA attenuated the extracellular Ca(2+)-mediated [Ca(2+)](cyt) increase and inhibited IPAH-PASMC proliferation. Using animal models of pulmonary hypertension, our data showed that CaSR expression and function were both enhanced in PASMC, whereas intraperitoneal injection of the calcilytic NPS 2143 prevented the development of pulmonary hypertension and right ventricular hypertrophy in rats injected with monocrotaline and mice exposed to hypoxia.

CONCLUSIONS

The extracellular Ca(2+)-induced increase in [Ca(2+)](cyt) due to upregulated CaSR is a novel pathogenic mechanism contributing to the augmented Ca(2+) influx and excessive PASMC proliferation in patients and animals with pulmonary arterial hypertension.

Cytochrome P450-derived epoxyeicosatrienoic acids and pulmonary hypertension: central role of transient receptor potential C6 channels

Hypoxia induces the constriction of pulmonary resistance arteries, which results in the redistribution of blood from poor to better ventilated areas, thus optimizing its oxygenation. Many different oxygen-sensing mechanisms have been proposed to regulate this process, including cytochrome P450 enzymes. These enzymes, which convert substrates such as arachidonic acid into bioactive epoxides (the epoxyeicosatrienoic acids [EETs]), are highly expressed in the lung as is the soluble epoxide hydrolase which metabolizes the epoxides to their less active diols. The EETs play a well-documented role as endothelium-derived vasodilators in the systemic vasculature, but in the pulmonary circulation, they are generated in vascular smooth muscle cells and potentiate vasoconstriction. Preventing the breakdown of 11,12-EET by the inhibition or genetic deletion of the soluble epoxide hydrolase strongly augments the response to hypoxia. Mechanistically, 11,12-EET potentiates the contractile response by recruiting transient receptor potential C6 channels to caveolae. Indeed, neither 11,12-EET nor hypoxia is able to elicit pulmonary vasoconstriction in TRPC6 knockout mice. The cytochrome and soluble epoxide hydrolase enzymes are also implicated in the vascular remodeling associated with chronic hypoxia and pulmonary hypertension. Thus, targeting this pathway may be in an attractive new therapeutic approach to treat this incapacitating disease.

Activity of Ca -activated Cl channels contributes to regulating receptor- and store-operated Ca entry in human pulmonary artery smooth muscle cells

Intracellular Ca²⁺ plays a fundamental role in regulating cell functions in pulmonary arterial smooth muscle cells (PASMCs). A rise in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) triggers pulmonary vasoconstriction and stimulates PASMC proliferation. [Ca²⁺]_cyt is increased mainly by Ca²⁺ release from intracellular stores and Ca²⁺ influx through plasmalemmal Ca²⁺-permeable channels. Given the high concentration of intracellular Cl^- in PASMCs, Ca²⁺-activated Cl^-(Cl_Ca) channels play an important role in regulating membrane potential and cell excitability of PASMCs. In this study, we examined whether activity of Cl_Ca channels was involved in regulating [Ca²⁺]_cyt in human PASMCs via regulating receptor- (ROCE) and store- (SOCE) operated Ca²⁺ entry. The data demonstrated that an angiotensin II (100 nM)-mediated increase in [Ca²⁺]_cyt via ROCE was markedly attenuated by the Cl_Ca channel inhibitors, niflumic acid (100 μM), flufenamic acid (100 μM), and 4,4’-diisothiocyanatostilbene-2,2’-disulfonic acid (100 μM). The inhibition of Cl_Ca channels by niflumic acid and flufenamic acid significantly reduced both transient and plateau phases of SOCE that was induced by passive depletion of Ca²⁺ from the sarcoplasmic reticulum by 10 μM cyclopiazonic acid. In addition, ROCE and SOCE were abolished by SKF-96365 (50 μM) and 2-aminoethyl diphenylborinate (100 μM), and were slightly decreased in the presence of diltiazem (10 μM). The electrophysiological and immunocytochemical data indicate that Cl_Ca currents were present and TMEM16A was functionally expressed in human PASMCs. The results from this study suggest that the function of Cl_Ca channels, potentially formed by TMEM16A proteins, contributes to regulating [Ca²⁺]_cyt by affecting ROCE and SOCE in human PASMCs.

Redox signaling and reactive oxygen species in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an essential physiological mechanism of the lung that matches blood perfusion with alveolar ventilation to optimize gas exchange. Perturbations of HPV, as may occur in pneumonia or adult respiratory distress syndrome, can cause life-threatening hypoxemia. Despite intensive research for decades, the molecular mechanisms of HPV have not been fully elucidated. Reactive oxygen species (ROS) and changes in the cellular redox state are proposed to link O2 sensing and pulmonary arterial smooth muscle cell contraction underlying HPV. In this regard, mitochondria and NAD(P)H oxidases are discussed as sources of ROS. However, there is controversy whether ROS levels decrease or increase during hypoxia. With this background we summarize the current knowledge on the role of ROS and redox state in HPV.