Nitric oxide prevents inducible cyclooxygenase expression by inhibiting nuclear factor-kappa B and nuclear factor-interleukin-6 activation

Sodium nitroprusside restored lipopolysaccharide-induced learning and memory impairment in male rats via attenuating inflammation and oxidative stress

Inflammation and oxidative stress upset memory. We explored influence of sodium nitroprusside (SNP) on memory deficits resulted from lipopolysaccharide (LPS).Groups include control, LPS, LPS + SNP 1 mg/kg, LPS + SNP 2 mg/kg, and LPS + SNP 3 mg/kg. Morris water maze and passive avoidance tests and biochemical measurements were carried out.In Morris water maze, LPS prolonged time and distance for finding the platform. In probe trial, it diminished time spent and traveled distance in the target zone. Injection of 2 and 3 mg/kg of SNP overturned the effect of LPS. In passive avoidance task, LPS postponed entrance into darkroom and reduced time spent in light room and incremented time spent in darkroom in 3, 24, and 72 h after electrical shock. All three doses of SNP restored the effects of LPS. Biochemical experiments confirmed that LPS elevated interleukin-6 and malondialdehyde concentration and declined total thiol content and superoxide dismutase and catalase activity in the hippocampus and cortex tissues. SNP particularly at a 3 mg/kg dose ameliorated LPS effects on these parameters.SNP attenuated memory disabilities resulting from LPS through modifying inflammation and boosting antioxidant defense.

Nitric oxide induces SOCS-1 expression in human monocytes in a TNF-α-dependent manner

In contrast to the thoroughly characterized mechanisms of positive regulation within cytokine signaling pathways, our knowledge of negative feedback loops is comparatively sparse. We and others have previously reported that IRAK-M down-regulates inflammatory responses to multiple stimuli. In particular, we could show that the nitric oxide (NO) donor, GSNO, induces IRAK-M overexpression in human monocytes. Here we study the expression of another important negative regulator of cytokine signaling, SOCS-1, in human monocytes exposed to GSNO. The NO donor induced significant levels of SOCS-1 mRNA and protein, 6 h and 16 h after stimulation, respectively. Monocytes stimulated with GSNO for longer periods (24 h and 48 h) failed to express IL-6 and IP-10 upon LPS challenge. In addition, and in line with previous reports of NO-mediated induction of TNF-α, we have found that exposure to this cytokine induces SOCS-1 mRNA in human monocytes. A blocking antibody against TNF-α impaired SOCS-1 expression upon GSNO treatment and re-instated IL-6 and IP-10 mRNA levels after LPS challenge in cultures pretreated with the NO donor. We conclude that NO stimulates SOCS-1 overexpression in a pathway at least partially regulated by TNF-α.

Nitric oxide donors as neuroprotective agents after an ischemic stroke-related inflammatory reaction

Cerebral ischemia initiates a cascade of detrimental events including glutamate-associated excitotoxicity, intracellular calcium accumulation, formation of Reactive oxygen species (ROS), membrane lipid degradation, and DNA damage, which lead to the disruption of cellular homeostasis and structural damage of ischemic brain tissue. Cerebral ischemia also triggers acute inflammation, which exacerbates primary brain damage. Therefore, reducing oxidative stress (OS) and downregulating the inflammatory response are options that merit consideration as potential therapeutic targets for ischemic stroke. Consequently, agents capable of modulating both elements will constitute promising therapeutic solutions because clinically effective neuroprotectants have not yet been discovered and no specific therapy for stroke is available to date. Because of their ability to modulate both oxidative stress and the inflammatory response, much attention has been focused on the role of nitric oxide donors (NOD) as neuroprotective agents in the pathophysiology of cerebral ischemia-reperfusion injury. Given their short therapeutic window, NOD appears to be appropriate for use during neurosurgical procedures involving transient arterial occlusions, or in very early treatment of acute ischemic stroke, and also possibly as complementary treatment for neurodegenerative diseases such as Parkinson or Alzheimer, where oxidative stress is an important promoter of damage. In the present paper, we focus on the role of NOD as possible neuroprotective therapeutic agents for ischemia/reperfusion treatment.

S-Nitrosoglutathione administration ameliorates cauda equina compression injury in rats

Lumbar spinal stenosis (LSS) causes ischemia, inflammation, demyelination and results in dysfunction of the cauda equina (CE), leading to pain and locomotor functional deficits. We investigated whether exogenous administration of S-nitrosoglutathione (GSNO), an endogenous redox modulating anti-neuroinflammatory agent, hastens functional recovery in a CE compression (CEC) rat model. CEC was induced in adult female rats by the surgical implantation of two silicone blocks within the epidural spaces of L4-L6 vertebrae. GSNO (50 μg/kg body weight) was administered by gavage 1 h after the injury, and the treatment was continued daily thereafter. GSNO induced change in the pain threshold was evaluated for four days after the compression. Tissue analyses and locomotor function evaluation were carried out at two weeks and four weeks after the CEC respectively. GSNO significantly improved motor function in CEC rats as evidenced by an increased latency on rotarod compared with vehicle-treated CEC rats. CEC induced hyperalgesia was decreased by GSNO. GSNO also increased the expression of VEGF, reduced cellular infiltration (H&E staining) and apoptotic cell death (TUNEL assay), and hampered demyelination (LFB staining and g-ratio). These data demonstrate that administration of GSNO after CEC decreased inflammation, hyperalgesia and cell death leading to improved locomotor function of CEC rats. The therapeutic potential of GSNO observed in the present study with CEC rats suggests that GSNO is a candidate drug to test in LSS patients.

S-nitrosoglutathione reduces oxidative injury and promotes mechanisms of neurorepair following traumatic brain injury in rats

BACKGROUND

Traumatic brain injury (TBI) induces primary and secondary damage in both the endothelium and the brain parenchyma, collectively termed the neurovascular unit. While neurons die quickly by necrosis, a vicious cycle of secondary injury in endothelial cells exacerbates the initial injury in the neurovascular unit following TBI. In activated endothelial cells, excessive superoxide reacts with nitric oxide (NO) to form peroxynitrite. Peroxynitrite has been implicated in blood brain barrier (BBB) leakage, altered metabolic function, and neurobehavioral impairment. S-nitrosoglutathione (GSNO), a nitrosylation-based signaling molecule, was reported not only to reduce brain levels of peroxynitrite and oxidative metabolites but also to improve neurological function in TBI, stroke, and spinal cord injury. Therefore, we investigated whether GSNO promotes the neurorepair process by reducing the levels of peroxynitrite and the degree of oxidative injury.

METHODS

TBI was induced by controlled cortical impact (CCI) in adult male rats. GSNO or 3-Morpholino-sydnonimine (SIN-1) (50 μg/kg body weight) was administered orally two hours following CCI. The same dose was repeated daily until endpoints. GSNO-treated (GSNO group) or SIN-1-treated (SIN-1 group) injured animals were compared with vehicle-treated injured animals (TBI group) and vehicle-treated sham-operated animals (Sham group) in terms of peroxynitrite, NO, glutathione (GSH), lipid peroxidation, blood brain barrier (BBB) leakage, edema, inflammation, tissue structure, axon/myelin integrity, and neurotrophic factors.

RESULTS

SIN-1 treatment of TBI increased whereas GSNO treatment decreased peroxynitrite, lipid peroxides/aldehydes, BBB leakage, inflammation and edema in a short-term treatment (4-48 hours). GSNO also reduced brain infarctions and enhanced the levels of NO and GSH. In a long-term treatment (14 days), GSNO protected axonal integrity, maintained myelin levels, promoted synaptic plasticity, and enhanced the expression of neurotrophic factors.

CONCLUSION

Our findings indicate the participation of peroxynitrite in the pathobiology of TBI. GSNO treatment of TBI not only reduces peroxynitrite but also protects the integrity of the neurovascular unit, indicating that GSNO blunts the deleterious effects of peroxynitrite. A long-term treatment of TBI with the same low dose of GSNO promotes synaptic plasticity and enhances the expression of neurotrophic factors. These results support that GSNO reduces the levels of oxidative metabolites, protects the neurovascular unit, and promotes neurorepair mechanisms in TBI.

N-acetylcysteine is able to reduce the oxidation status and the endothelial activation after a high-glucose content meal in patients with Type 2 diabetes mellitus

Post-prandial hyperglycemia seems to play a pivotal role in the pathogenesis of the cardiovascular complications of diabetes mellitus, as it leads to an oxidative stress which in turn causes a reduced NO bioavailability. These conditions produce an endothelial activation.

AIM OF THE STUDY

The aim of this study was to assure that the administration of N-acetylcysteine (NAC), thiolic antioxidant, is able to decrease the oxidation status and endothelial activation after a high-glucose content meal.

SUBJECTS AND METHODS

Ten patients with Type 2 diabetes mellitus (DMT2) (Group 1) and 10 normal subjects (Group 2) were studied. They assumed a high-glucose content meal without (phase A) or after (phase B) the administration of NAC. Glycemia, insulinemia, intercellular adhesion molecule 1, vascular adhesion molecule 1 (VCAM-1), E-selectin, malonaldehyde (MDA), and 4-hydroxynonenal (HNE) were assessed at -30, 0, +30, +60, +90, +120, and +180 min with respect to the meal consumption.

RESULTS

During the phase A in Group 1, only HNE and MDA levels increased after the meal assumption; all parameters remained unchanged in Group 2. During the phase B, in Group 1, HNE, MDA, VCAM-1, and E-selectin levels after the meal were lower than those in phase A, while no change for all variables were observed in Group 2.

CONCLUSIONS

A high-glucose meal produces an increase in oxidation parameters in patients with DMT2. The administration of NAC reduces the oxidative stress and, by doing so, reduces the endothelial activation. In conclusion, NAC could be efficacious in the slackening of the progression of vascular damage in DMT2.

Long-term N-acetylcysteine and L-arginine administration reduces endothelial activation and systolic blood pressure in hypertensive patients with type 2 diabetes

OBJECTIVE

Reactive oxygen and nitric oxide (NO) have recently been considered to be involved in the cardiovascular complications of patients with type 2 diabetes, as NO is thought to lose its beneficial physiological effects in the presence of oxygen radicals. For this reason, we tested the effects of l-arginine (ARG) and N-acetylcysteine (NAC) administration in increasing NO bioavailability by reducing free radical formation.

RESEARCH DESIGN AND METHODS

A double-blind study was performed on 24 male patients with type 2 diabetes and hypertension divided into two groups of 12 patients that randomly received either an oral supplementation of placebo or NAC + ARG for 6 months.

RESULTS

The NAC + ARG treatment caused a reduction of both systolic (P < 0.05) and diastolic (P < 0.05) mean arterial blood pressure, total cholesterol (P < 0.01), LDL cholesterol (P < 0.005), oxidized LDL (P < 0.05), high-sensitive C-reactive protein (P < 0.05), intracellular adhesion molecule (P < 0.05), vascular cell adhesion molecule (P < 0.01), nitrotyrosine (P < 0.01), fibrinogen (P < 0.01), and plasminogen activator inhibitor-1 (P < 0.05), and an improvement of the intima-media thickness during endothelial postischemic vasodilation (P < 0.02). HDL cholesterol increased (P < 0.05). No changes in other parameters studied were observed.

CONCLUSIONS

NAC + ARG administration seems to be a potential well-tolerated antiatherogenic therapy because it improves endothelial function in hypertensive patients with type 2 diabetes by improving NO bioavailability via reduction of oxidative stress and increase of NO production. Our study's results give prominence to its potential use in primary and secondary cardiovascular prevention in these patients.

Preoptic nitric oxide attenuates endotoxic fever in guinea pigs by inhibiting the POA release of norepinephrine

Lipopolysaccharide (LPS) administration induces hypothalamic nitric oxide (NO); NO is antipyretic in the preoptic area (POA), but its mechanism of action is uncertain. LPS also stimulates the release of preoptic norepinephrine (NE), which mediates fever onset. Because NE upregulates NO synthases and NO induces cyclooxygenase (COX)-2-dependent PGE2, we investigated whether NO mediates the production of this central fever mediator. Conscious guinea pigs with intra-POA microdialysis probes received LPS intravenously (2 μg/kg) and, thereafter, an NO donor (SIN-1) or scavenger (carboxy-PTIO) intra-POA (20 μg/μl each, 2 μl/min, 6 h). Core temperature (Tc) was monitored constantly; dialysate NE and PGE2 were analyzed in 30-min collections. To verify the reported involvement of α2-adrenoceptors (AR) in PGE2 production, clonidine (α2-AR agonist, 2 μg/μl) was microdialyzed with and without SIN-1 or carboxy-PTIO. To assess the possible involvement of oxidative NE and/or NO products in the demonstrated initially COX-2-independent POA PGE2 increase, (+)-catechin (an antioxidant, 3 μg/μl) was microdialyzed, and POA PGE2, and Tc were determined. SIN-1 and carboxy-PTIO reduced and enhanced, respectively, the rises in NE, PGE2, and Tc produced by intravenous LPS. Similarly, they prevented and increased, respectively, the delayed elevations of PGE2 and Tc induced by intra-POA clonidine. (+)-Catechin prevented the LPS-induced elevation of PGE2, but not of Tc. We conclude that the antipyretic activity of NO derives from its inhibitory modulation of the LPS-induced release of POA NE. These data also implicate free radicals in POA PGE2 production and raise questions about its role as a central LPS fever mediator.

Suppression of endotoxin-induced fever in near-term pregnant rats is mediated by brain nitric oxide

Over the last three decades, experiments in several mammalian species have shown that the febrile response to bacterial endotoxin is attenuated late in pregnancy. More recent evidence has established that the expression of nitric oxide synthase (NOS) enzymes is increased in the brain late in pregnancy. The current study investigated the possible role of brain nitric oxide in mediating the phenomenon of fever suppression. Core body temperature (Tb) of near-term pregnant rats ( day 19 and 20) was measured following inhibition of brain NOS and intraperitoneal injection of LPS (50 μg/kg); they were compared with both day 15 pregnant and virgin animals. Intracerebroventricular injection with an inhibitor of NOS, NG-monomethyl-l-arginine citrate (l-NMMA; 280 μg), in near-term pregnant rats restored the febrile response to LPS. As expected, near-term dams that received intracerebroventricular vehicle + IP LPS did not increase Tb, in contrast to the 1.0 ± 0.2°C rise in Tb in dams treated with ICV l-NMMA + IP LPS ( P < 0.01). In virgin females and day 15 pregnant controls receiving this treatment, the increases in Tb were 1.5 ± 0.3°C and 1.6 ± 0.4°C, respectively. Thus, blockade of brain NOS restored the febrile response to LPS in near-term dams; at 5 h postinjection, Tb was 60–70% of that observed in virgins and day 15 pregnant animals. Intracerebroventricular l-NMMA alone did not induce a significant change in Tb in any group. These results suggest that the mechanism underlying the suppression of the febrile response in near-term pregnancy is mediated by nitric oxide signaling in the brain.

Distinct patterns of intracerebral hemorrhage-induced alterations in NF-κB subunit, iNOS, and COX-2 expression

Transcription factor nuclear factor-kappaB (NF-kappaB), plays a key role in regulating inflammation in brain pathologies. The goal of this study was to characterize temporal changes in NF-kappaB activation, NF-kappaB subunit expression, and expression of selected NF-kappaB-regulated gene products [inducible form of nitric oxide synthase (iNOS) and cyclooxygenase-2], at the transcriptional and translational level in the brain after intracerebral hemorrhage (ICH). Employing the intrastriatal injection of autologous blood in rats to model ICH, we demonstrated, using NF-kappaB-DNA binding assay, a robust and prolonged NF-kappaB activation, starting as early as 15 min after the onset of ICH. Consequently, we demonstrated that the mRNA and protein for p50, p52, p65, c-Rel, and RelB NF-kappaB subunits, as well as IkappaBalpha were all up-regulated, with a time course ranging from minutes to days following ICH, depending on the subunit. Using reverse transcription-polymerase chain reaction to analyze mRNA and immunoblotting to analyze protein in ICH-affected tissue, we found robust induction of iNOS at both mRNA and protein levels that followed a time-course similar to changes in p65, p52, and RelB mRNA. Oddly, in contrast to iNOS, cyclooxygenase-2 mRNA and protein following an early transient increase demonstrated significant reduction in response to ICH. In summary, NF-kappaB activation occurs within minutes and persists for at least a week in response to ICH. This reaction utilizes different NF-kappaB regulatory subunits and is associated with the expression of selected target genes.

Nitric oxide activates the expression of IRAK-M via the release of TNF-alpha in human monocytes

The activation of interleukin receptor associated kinases (IRAK) is an important event in several inflammatory processes. However, exposing monocytes to a nitric oxide (NO) donor inhibits the activity of IRAK-1 and its molecular interaction with TNF receptor associated factor-6 (TRAF6). Despite the fact that NO is known to regulate many events in the immune and vascular system, the mechanism that underlies this inhibition remains unknown. We have recently demonstrated that IRAK-M inhibits the TLR/IRAK pathway during endotoxin tolerance and thus, we hypothesized that IRAK-M may be involved in the inhibition of IRAK-1 activity in the presence of NO. Hence, we have analyzed the expression of IRAK-M in human monocytes following exposure to a NO donor (GSNO) and we have observed that GSNO was capable of inducing IRAK-M mRNA and protein expression 8 and 20 h after stimulation, respectively. It is known that NO induces the expression of TNF-alpha in monocytes and we found that exposure to TNF-alpha induced IRAK-M mRNA expression in human monocytes within 2 h of stimulation. Furthermore, the expression of IRAK-M induced by GSNO was inhibited by the presence of a blocking antibody raised against TNF-alpha. Thus, our data indicate that stimulation of human monocytes with a NO donor results in a clear induction of IRAK-M and this is dependent on the release of TNF-alpha by this kind of cells.

Local and systemic expression of inducible nitric oxide synthase in comparison with that of cyclooxygenase-2 in rat carrageenin-induced pleurisy

Expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) is up-regulated in response to inflammatory stimuli. To evaluate the extent to which local pleural inflammation involves additional site in the pleural cavity and elsewhere, we investigated the time course of the levels of iNOS and its product in the inflammatory and other sites, and compared those with a level of COX-2 in rat carrageenin-induced pleurisy. The exudate and plasma NOx levels rose, reaching peaks at 9 and 14 h, respectively. Both COX-2 and iNOS became detectable in exudate leukocytes, their levels reaching peaks at 3 and 9 h after irritation, respectively. COX-2 was detectable mainly in neutrophils, but iNOS was detectable in both neutrophils and mononuclear leukocytes. Furthermore, iNOS became detectable in neutrophils and mononuclear leukocytes in enlarged parathymic lymph nodes from 3h in addition to those in peripheral blood and Kupffer cells from 3 to 14 h, respectively. The gene product is also detectable in thymic large dendritic cells of pleurisy-induced rats as well as normal control rats. COX-2 became detectable in stellar dendritic cells of the enlarged draining lymph nodes from 14 h. Thus, these gene products were induced in the immediate proximity of regional lymph nodes, and even at a considerable distance of liver by the local inflammatory stimulus. Although their expression pattern was quite different from each other, these gene products were detectable in phagocytic cells.

Nitrosative stress and transcription

Low NO concentrations synthesized by constitutively expressed NO synthases act on several signaling pathways activating transcription factors (TF), such as NF-kappaB or AP-1, and thereby influence gene expression. In contrast, during inflammatory reactions the inducible NO synthase produces NO for prolonged periods of time. The resulting nitrosative stress directly affects redox-sensitive TF like NF-kappaB, AP-1, Oct-1, c-Myb, or zinc finger-containing TF, but also additional mechanisms have been identified. Nitrosative stress in some cases induces expression of TF (AP-1, p53), indirectly modulates activity or stability of TF (HIF-1, p53) or their inhibitors (NF-kappaB), or modulates accessibility of promoters via increased DNA methylation or histone deacetylation. Depending on the promoter the result is induced, increased, decreased or even totally inhibited expression of various target genes. In unstimulated cells nitrosative stress increases NF-kappaB- or AP-1-dependent transcription, while in activated cells nitrosative stress rather abolishes NF-kappaB- or AP-1-dependent transcription. Sometimes the oxygen concentration also is of prime importance, since under normoxic conditions nitrosative stress activates HIF-1-dependent transcription, while under hypoxic conditions nitrosative stress leads to inhibition of HIF-1-dependent transcription. This review summarizes what is known about effects of physiological NO levels as well as of nitrosative stress on transcription.

The effect of nitric oxide on cyclooxygenase-2 (COX-2) overexpression in head and neck cancer cell lines

The overexpression of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) has been previously reported in head and neck squamous cell carcinoma (HNSCC), as well as in many cancers. We hypothesized that endogenous nitric oxide (NO) might increase the expression of COX-2 in cancer cells. Therefore, we investigated the cross-talk between NO and the prostaglandin (PG) pathways in HNSCC cell lines. We found that COX-2 and iNOS expressions were elevated simultaneously. On adding the NO donor, SNAP, the PGE2 level was increased 2-20 times due to increased COX-2 expression. This increase of COX-2 expression by SNAP or PMA (potent inducer of both iNOS and COX-2) was blocked to various degrees by NO scavengers and NOS inhibitors (L-NAME and 1400W). Also, the expression of COX-2 in resting cells was inhibited by NOS inhibitors. Moreover, COX-2 expression, induced by SNAP, was inhibited by ODQ, a soluble guanylate cyclase (sGC) inhibitor. The effect of dibutyryl-cGMP on COX-2 expression was similar to that of SNAP. These results imply that endogenous or exogenous NO activates sGC and that the resulting increase of cGMP induces a signaling that upregulates the expression of COX-2 in HNSCC cell lines. We also observed that NO increased COX-2 expression in different cancer cell lines, including cervic and gastric cancer cell lines. These findings further support the notion that NO can be associated with carcinogenesis through the upregulation of COX-2, and that NOS inhibitor may be also useful for cancer prevention.

Suppression of NF-kappa B survival signaling by nitrosylcobalamin sensitizes neoplasms to the anti-tumor effects of Apo2L/TRAIL

We have previously demonstrated the anti-tumor activity of nitrosylcobalamin (NO-Cbl), an analog of vitamin B12 that delivers nitric oxide (NO) and increases the expression of tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) and its receptors in human tumors. The specific aim of this study was to examine whether NO-Cbl could sensitize drug-resistant melanomas to Apo2L/TRAIL. Antiproliferative effects of NO-Cbl and Apo2L/TRAIL were assessed in malignant melanomas and non-tumorigenic melanocyte and fibroblast cell lines. Athymic nude mice bearing human melanoma A375 xenografts were treated with NO-Cbl and Apo2L/TRAIL. Apoptosis was measured by TUNEL and confirmed by examining levels and activity of key mediators of apoptosis. The activation status of NF-kappa B was established by assaying DNA binding, luciferase reporter activity, the phosphorylation status of I kappa B alpha, and in vitro IKK activity. NO-Cbl sensitized Apo2L/TRAIL-resistant melanoma cell lines to growth inhibition by Apo2L/TRAIL but had minimal effect on normal cell lines. NO-Cbl and Apo2L/TRAIL exerted synergistic anti-tumor activity against A375 xenografts. Treatment with NO-Cbl followed by Apo2L/TRAIL induced apoptosis in Apo2L/TRAIL-resistant tumor cells, characterized by cleavage of caspase-3, caspase-8, and PARP. NO-Cbl inhibited IKK activation, characterized by decreased phosphorylation of I kappa B alpha and inhibition of NF-kappa B DNA binding activity. NO-Cbl suppressed Apo2L/TRAIL- and TNF-alpha-mediated activation of a transfected NF-kappa B-driven luciferase reporter. XIAP, an inhibitor of apoptosis, was inactivated by NO-Cbl. NO-Cbl treatment rendered Apo2L/TRAIL-resistant malignancies sensitive to the anti-tumor effects of Apo2L/TRAIL in vitro and in vivo. The use of NO-Cbl and Apo2L/TRAIL capitalizes on the tumor-specific properties of both agents and represents a promising anti-cancer combination.

Regulation of gene expression by oxygen: NF-kappaB and HIF-1, two extremes

Aerobic life is dependent on molecular oxygen for ATP regeneration, but only possible in a narrow range of oxygen concentrations. Increased oxygen tension is toxic through the generation of reactive oxygen species (ROS), while a decrease in oxygen concentration impairs energy availability and, hence, cell viability. Cells have developed strategies to respond to changes in oxygen tension: specific systems detect excessive ROS and hypoxia, leading to the activation of specific transcription factors and expression of appropriate target genes. The aim of this review is to describe how hypoxia-inducible factor-1 (HIF-1) and nuclear factor-kappaB (NF-kappaB) are regulated and what could be the sensors to the changes in oxygen levels. Some of the physiological responses initiated by these transcription factors are also mentioned.