Role of ROS/Kv/HIF Axis in the Development of Hypoxia-Induced Pulmonary Hypertension

Wen Wu, Yan Li, Dunquan Xu＊

1Department of Ultrasound, 2Medical Examination Center, General Hospital of the Army, Beijing 100007, China 3Clinical Laboratory, the Xigaze Branch of Xinqiao Hospital, the Army Medical University,Xigaze, Tibet 857000, China

Role of ROS/Kv/HIF Axis in the Development of Hypoxia-Induced Pulmonary Hypertension

Wen Wu1, Yan Li2, Dunquan Xu3＊

1Department of Ultrasound,2Medical Examination Center, General Hospital of the Army, Beijing 100007, China3Clinical Laboratory, the Xigaze Branch of Xinqiao Hospital, the Army Medical University,Xigaze, Tibet 857000, China

hypoxia-induced pulmonary hypertension; reactive oxygen species; hypoxia inducible factors; potassium channels; vasoconstriction; arterial remodeling

Hypoxic pulmonary hypertension (HPH) is a common complication in patients with chronic obstructive pulmonary disease (COPD), sleep-disordered breathing, or dwellers in high altitude. The exact mechanisms underlying the development of HPH still remain unclear. Reactive oxygen species (ROS),hypoxia inducible factors (HIF), and potassium channels (KV) are believed as the main factors during the development of HPH. We propose that the “ROS/Kv/HIF axis” may play an important initiating role in the development of HPH. Being formed under a hypoxic condition, ROS affects the expression and function of HIFs or KV, and consequently triggers multiple downstream signaling pathways and genes expression that participate in promoting pulmonary vasoconstriction and arterial remodeling. Thus, further study determining the initiating role of “ROS/Kv/HIF axis” in the development of HPH could provide theoretic evidences to better understand the underlying mechanisms of HPH, and help identify new potential targets in the treatment of HPH.

PULMONARY hypertension, a condition associated with increased mortality, is a severe comorbidity in many diseases. Clinically it is characterized by progressive increase in pulmonary arterial pressure and pulmonary vascular resistance.1According to the latest clinical classification of the guidelines for the diagnosis and treatment of pulmonary hypertension,2pulmonary hyper-tension is classified into 5 groups, where hypoxic pulmonary hypertension (HPH) is a subtype of pulmonary hypertension due to lung diseases and/or hypoxia. Totally, there are 7 etiologies for HPH: chronic obstructive pulmonary disease, interstitial lung disease, other pulmonary diseases with mixed restrictive and obstructive pattern, sleepdisordered breathing, alveolar hypoven-tilation disorders,chronic exposure to high altitude, and developmental abnormalities.

Hypoxic vasoconstriction, one of the important physio-logical responses to hypoxia, known as the von Euler-Liljestrand mechanism, is meaningful for maintaining ventilation/perfusion ratio.3However, generalized pulmonary arteriole constriction results in elevated pulmonary arterial pressure, and long-term elevation of pulmonary arterial pressure promotes thickening of pulmonary arterial wall.Thus pulmonary vascular remodeling together with enhanced pulmonary vascular resistance eventually lead to pulmonary hypertension.

PATHOGENESIS FOR HYPOXIC PULMONARY HYPERTENSION

The mechanisms underlying the pulmonary vascular remodeling and vasoconstriction during hypoxic pulmonary hypertension remain to be fully explored. To date, there exist several cellular and molecular mechanisms explaining pulmonary vascular remodeling and vasoconstriction during HPH. Stenmark et al summarized that the cellular and molecular mechanisms are varied, and depend on the cellular composition of vessels at particular sites along the longitudinal axis of the pulmonary vasculature, as well as on local environmental factors.4They also elucidated that the resident vascular cell types play specific roles in the overall remodeling response, which undergo site and time-dependent changes in proliferation, matrix protein production, expression of growth factors, etc.4In another review, Stenmark and colleagues proposed that the adventitia played critical roles in hypoxia-induced pulmonary vascular remodeling, during which adventitial fibroblasts were activated and underwent phenotypic changes including proliferation, differentiation, recruitment of inflammatory and progenitor cells to the vessel wall, etc.5

ROS, HIFS, AND KV IN HYPOXIC PULMONARY HYPERTENSION

Reactive oxygen species (ROS)

Sommer et al exhibited that the mechanisms responsible for hypoxic pulmonary vasoconstriction include ion channels,reactive oxygen species (ROS), and redox couples.6According to recent research, hypoxia leads to exaggerated increase of ROS in the pulmonary arterial smooth muscle cells.7-9Accumulating data have shown that oxidative stress plays important roles in mediating pathological changes in the pulmonary arterioles and the right ventricle.10-12Additionally, ROS affects cells’ sensibility to oxidative stress, cell migration, proliferation, apoptosis, and matrix protein deposition, all of which are related with vasoconstriction and vascular remodeling.10,13Lately, another team reported that ROS mediated physio-logical angiogenesis due to aerobic endurance exercise, in which HIF may play as an exercise-sensitive oxygen sensor and a redox regulator.14Taken together, ROS is believed to be an initiating factor for vascular response under hypoxia exposure. Thus,oxidative stress is becoming a new target for treating pulmonary hypertension.15

Hypoxia inducible factors (HIFs)

Hypoxia inducible factors (HIFs) are demonstrated to be the main modulators in cellular response to hypoxia, and play key roles in the development of organs and the progression of diseases.16,17Recently, a controversy about whether ROS participates in HIF-1α modulating has arisen.18An opinion considers that hypoxia induces superoxide production in compound Ⅲ of the mitocho-ndrial electron transport chain, and the activity of prolyl hydroxylase (PHD)is inhibited by oxidation of the nonheme Fe Ⅱ, which stabilizes HIF-1α.19On the contrary, another opinion propose that the decreased activity of mitochondrial electron transport chain can result in elevation of oxygen concentration in cytoplasm.20Consequently, PHD can be reactivated, and then HIF-1α degradation occurs. HIF-2α is also believed to be regulated via this mechanism.21Calvani M et al reported that hypoxia induces the produc-tion of ROS in mitochondria, and inhibition of PHD stabi-lizes HIF-1α.22Consequently, the expression of vascular endothelial growth factor (VEGF) increases. When VEGF binds to VEGF receptor-2, nicoti-namide adenine dinucleo-tide phosphate(NADPH), an oxidase, is activated, resulting in a secondary increase of ROS. As HIF-1α is further stabilized, cells acclimatize to oxidative stress. Besides, Prabhakar and colleagues also demon-strated that, besides by hypoxia,HIF-1α could be activated by nitric oxide (NO) and ROS.23Furthermore, study showed that the mechanism for ROS increase under chronic intermittent hypoxia is related with the imbalance of the pro-oxidation and anti-oxidation genes encoded by HIF-1α and HIF-2α.24

Potassium channels (Kv)

Research showed that oxygen-sensitive potassium channels (Kv) are predominantly expressed in resistant pulmonary arterioles, and are responsible for hypoxia induced pulmonary vasoconstriction occurring mainly in this area.25A team found that the ROS-K+signaling pathway modulates NO, endothelin 1 (ET-1), and VEGF secretion in human pulmonary arterial endothelium under oxidative stress. They also demonstrated that Kv1.5 may play an important role in ROS-K+signaling pathway, and intracellular Ca2+may be related with the secretion regulation of endo-thelium by ROS-K+.26Yasui et al reported that H2O2directly inhibits the activity of KATPat the concentration of 10 μM.27

ROS/KV/HIF AXIS

Based on the research findings and questions mentioned above, we believe that HIFs and KVmay exert key roles during the development of HPH. Consequently, we propose the concept of “ROS/Kv/HIF axis”. Firstly, hypoxia causes exaggerated increase of ROS in pulmonary arterioles. The increased ROS then inhibits the activity of Kv located in pulmonary arterioles, meanwhile, stabilizes HIFs activity. The decreased Kv activity could explain the early pulmonary vascular constriction reaction under hypoxia exposure. On the other hand, the stabilized HIF axis could activate its downstream genes, and subsequently cause the expression of various hypoxia-related proteins that participate in pulmonary vascular remodeling during HPH. We hypothesize that during the development of HPH, the production of ROS inhibits Kv activity,stabilizes HIF axis, and further leads to imbalances of pulmonary vascular constriction/dilation and proliferation/apoptosis.

Nevertheless, does the “ROS/KV/HIF axis” really exist in pulmonary circulation during the development of HPH?What is the exact nature of the “ROS/KV/HIF axis”, and what is the underlying molecular mechanism? These questions need further investigation.

The characteristics of HPH are remarkable pulmonary arterial remodeling and elevated vascular resistance. The specific pathological changes in resistant arterioles during HPH are exhibited as: thickening of pulmonary arteriole wall with proliferated and hypertrophic medial smooth muscle cells, inflammatory cell infiltration, and enlarged adventitia along with extracellular matrix accumulation.The pulmonary vasoconstriction response to hypoxia is initially a physiological adaptation to maintain ventilation/perfusion ratio, which eventually leads to elevation of vascular resistance. Chronic hypoxia exposure induces durable vasoconstriction and vascular resistance, which synergistically promote pulmonary arteriole remodeling. As a consequence,pulmonary arteriole remodeling inversely promotes vasoconstriction and vascular resistance. The positive feedback path during chronic hypoxia exposure further exacerbates HPH. However, the exact mechanisms underlying HPH still remain further exploration.

Studies showed that intracellular redox reaction could affect cellular signaling transduction and genes expression,which exerts important function during cell proliferation,growth inhibition, and apoptotic pathophysiological processes. ROS are produced during intracellular redox reaction. Mitochondrial electron transport chain and NADPH oxygenase (Nox) are the sources of ROS in pulmonary arterial smooth muscle cells (PASMCs) under the hypoxic condition. Wang and colleagues showed that hypoxia leads to increase of ROS in PASMCs, along with significant elevation of intracellular [Ca2+]i, followed by the constriction of pulmonary arterial smooth muscle cells.28Sommer et al demonstrated that endothelial cells regulated the vascular tone partially through regulating the ROS production in cells.6In the early phase of hypoxia, decreasing of hydrogen carriers accompanied with reducing of oxygen radical causes elevation of GSH/GSSG and NADPH/ NADP. Next, the intracellular deoxidation tendency increases, and the calciumdependent potassium channels are inhibited. Transmembrane influx of Ca2+increases afterwards, followed by pulmonary vascular constriction.6,28Additionally, Rathore and colleagues showed that ROS production through the ROS-Protein Kinase C Epsilon (PKCe)-Nox axis played a pivotal role in transmembrane Ca2+influx and the constriction of PASMCs.29N-acetyl-cysteine, an antioxidant could effectively inhibit peroxides from increasing,and prevent vascular pathological changes during the development of HPH.30However, another team showed that as a key endothelium-derived hyperpolarizing factor,H2O2mediates flow-induced dilation of human coronary arterioles through activating BK(Ca) channels in smooth muscle cells.31,32To summarize those aforesaid studies,different concentra-tions of ROS affect the pathophysiological processes of vascular constriction/dilation and cell proliferation/apoptosis.

On the other hand, HIFs play a key role in response to hypoxia, and modulate genes transcription during various pathophysiological processes. The battery of genes regulated by HIF-1 and HIF-2 are overlapping but distinct, and are dependent on cell types. For example,HIF-2α is present but does not activate the transcription in mouse embryonic stem cells.33HIF-1α modulates various genes expression such as endothelin-1, platelet derived growth factor, heme oxygenase-1, and inducible nitric oxide synthase-1, all of which play important roles during hypoxia-induced vasoconstriction and arterial remodeling.34,35Moreover, HIF-2α plays a significant role in tumorigenesis.36,37

HIF-1 is a heterodimer composed with HIF-1α and HIF-1β, which is regulated by Fe and oxygen dependent PHD.The main viewpoint on the increase of HIF-1α under hypoxia is that this is due to the decrease of oxygendependent degradation, and the increase of oxygenindependent protein synthesis.38,39A recent study exhibited that, besides hypoxia, NO and ROS can also activate HIF-1α.23Chronic intermittent hypoxia leads to increased synthesis and stability of HIF-1α, and the calpaindependent degradation of HIF-2α. Thus, the loss of redox homeostasis due to the imbalance of HIF-1α-dependent and pro-oxidant/HIF-2α-dependent anti-oxidant activity is considered as the main mechanism underlying the pathogenesis of autonomic morbidities associated with chronic intermittent hypoxia.24Mechanisms responsible for increased ROS generation may involve transcriptional dysregulation of genes encoding pro- and anti-oxidant enzymes by HIF-1 and HIF-2, respectively.40Therefore, we consider that the interaction of ROS and HIF mutually promotes each other, forming a positive feedback loop, which disturbs the constriction/dilation of pulmonary vessels and cell proliferation/apoptosis during the pathophysiological progression of HPH.

Hypoxia induced pulmonary vasoconstriction mainly occurs in resistant arterioles (the forth division, diameter＜200 μm), not in conduit arteries.41The anatomical(proximal-distal) and functional (conduit-resistance) differences also exhibit at the cellular and molecular level, such as the difference of K+channel distribution in different arterial segments.42,43In conduit PASMCs, the whole cell potassium currents (IK) exhibit contributions from both voltagegated (KV)and large-conductance, calcium-sensitive(BKCa) channels. However, IKin resistance PASMCs mainly manifests Kv current.44The reason for these differences may lie in the different origins of conduit and resistance arteries in the embryological vascular beds: the conduit arteries originate from the 6th aortic arch, whereas the resistance arterioles originate from the mesenchymal lung bud by capillary plexus expansion.45Hypoxia-induced pulmonary vasoconstriction is dependent on the inhibition of KV, for 4-AP pretreatment could completely inhibit the activity of KV. Along with the longitude of pulmonary arteries, the mRNA levels of KV1.2, KV1.5, KV3.1, KV4.3,and KV9.3 all increase. However, only the protein level of KV1.5 increases in resistance arterioles.25Study showed that K+is transported to mitochondria more quickly in hypoxia-resistant rats than in hypoxia-sensitive rats;meanwhile, the concentration of K+in hypoxia-resistant rats is higher than that in hypoxia-sensitive rats.46The study also showed that adaptation to hypoxia manifests not only as a faster transportation of K+, but also as the exchange of K+/H+. When KATPchannels are blocked, the production of H2O2in mitochondria of hypoxia-resistant rats is faster than that of hypoxia-sensitive rats. Extralife,a flavonoid extract, was shown to increase the transportation of ATP-dependent K+, and elevate the tolerance ability by 5 times in hypoxia-sensitive rats.46Brown et al found that hypoxia that lasts several seconds to minutes inhibits KVthrough decreasing ROS production in mitochondria and directly stimulating catecholamine secretion in adrenal medulla chromatocytes; meanwhile, chronic hypoxia leads to the production of HIF-2α by a ROS-independent pathway in fetus-derived chromatocytes, and intermittent chronic hypoxia results in anti-oxidative response modulated by Nrf-2 that is associated with increases of ROS and HIF-1α.47Dong et al found that, with 18-hour hypoxia exposure,HIF-1αincreased hypoxia-induced expression of Kv channels in cultured PASMCs through erythropoietin (EPO) enhancer effects.48According to the research above, HIF-1α may have protective effect in vitro during early hypoxic phase.Thus, the role of HIF-1α during hypoxia process should be fully explored. Besides, Shin and colleagues found that HIF-1α upregulated a two-pore domain K+channel in mouse B cells, which may be due to humoral immune responses and B cell differentiation.49Taken together,there are different types of potassium channels existing invascular endothelial cells and smooth muscle cells,whose activation or inhibition play important roles in regulating vascular constriction/dilation. The ROS production,diversity of K+channels expression, and HIF axis activation may be closely related with hypoxia-induced vasoconstriction and vascular remodeling during chronic hypoxia.

To summarize, the “ROS/Kv/HIF axis” partakes and greatly contributes in the process of hypoxia-induced pulmonary vasoconstriction and vascular remodeling. The schematic diagram below elucidates the “ROS/Kv/HIF axis”during HPH (Fig. 1). Hypoxia leads to the production of ROS, and ROS affects the balance of K+, followed by stabilizing of HIF system. Those aforesaid factors further cause the imbalances of pulmonary vascular constriction/dilation and proliferation/apoptosis, which promote the development of HPH. Further investigation in the “ROS/Kv/HIF axis” may offer new theoretical bases and potential targets for treating HPH.

Figure 1. Schematic flowchart of the pathway of ROS/ Kv/HIF axis in hypoxia-induced HPH.

Conflict of interest statement

The authors have no conflict of interest to disclose.

Acknowledgement

We thank professor Yanxia Wang from the Fourth Military Medical University for editing and polishing the language of this manuscript.

1. Shah SJ. Pulmonary hypertension. JAMA 2012; 308:1366-74. doi: 10.1001/jama.2012.12347.

2. Galiè N, Hoeper MM, Humbert M, Torbicki A, Vachiery JL,Barbera JA, et al. Guidelines for the diagnosis and treatment of pulmonary hypertension: the Task Force for the Diagnosis and Treatment of Pulmonary Hypertension of the European Society of Cardiology (ESC) and the European Respiratory Society (ERS), endorsed by the International Society of Heart and Lung Transplantation (ISHLT). Eur Heart J 2009; 30: 2493-537. doi: 10.1093/ eurheartj/ehp297.

3. Sylvester JT, Shimoda LA, Aaronson PI, Ward JP. Hypoxic pulmonary vasoconstriction. Physiol Rev 2012; 92:367-520. doi: 10.1152/physrev.00041.2010.

4. Stenmark KR, Fagan KA, Frid MG. Hypoxia-induced pulmonary vascular remodeling: cellular and molecular mechanisms. Circ Res 2006; 99: 675-91. doi: 10.1161/01.RES.0000243584.45145.3f.

5. Stenmark KR, Davie N, Frid M, Gerasimovskaya E, Das M. Role of the adventitia in pulmonary vascular remodeling. Physiology (Bethesda) 2006; 21: 134-45. doi: 10.1152/physiol.00053.2005.

6. Sommer N, Dietrich A, Schermuly RT, Ghofrani HA,Gudermann T, Schulz R, et al. Regulation of hypoxic pulmonary vasoconstriction: basic mechanisms. Eur Respir J 2008; 32:1639-51. doi: 10.1183/09031936.00013908.

7. Liu JQ, Zelko IN, Erbynn EM, Sham JS, Folz RJ. Hypoxic pulmonary hypertension: role of superoxide and NADPH oxidase (gp91phox). Am J Physiol Lung Cell Mol Physiol 2006; 290: L2-10. doi: 10.1152/ajplung.00135.2005.

8. Jankov RP, Kantores C, Pan J, Belik J. Contribution of xanthine oxidase-derived superoxide to chronic hypoxic pulmonary hypertension in neonatal rats. Am J Physiol Lung Cell Mol Physiol 2008; 294: L233-45. doi: 10.1152/ajplung.00166.2007.

9. Mittal M, Gu XQ, Pak O, Pamenter ME, Haag D, Fuchs DB,et al. Hypoxia induces Kv channel current inhibition by increased NADPH oxidase-derived reactive oxygen species. Free Radic Biol Med 2012; 52: 1033-42. doi: 10.1016/j.freeradbiomed.2011.12.004.

10. Xu S, Touyz RM. Reactive oxygen species and vascular remodelling in hypertension: still alive. Can J Cardiol 2006; 22: 947-51. doi: 10.1016/s0828-282x(06)70314-2.

11. Wong CM, Bansal G, Pavlickova L, Marcocci L, Suzuki YJ.Reactive oxygen species and antioxidants in pulmonary hypertension. Antioxid Redox Signal 2013; 1: 1789-96.doi: 10.1089/ars.2012.4568.

12. Voelkel NF, Bogaard HJ, Al Husseini A, Farkas L, Gomez-Arroyo J, Natarajan R. Antioxidants for the treatment of patients with severe angioproliferative pulmonary hypertension? Antioxid Redox Signal 2013; 18: 1810-7. doi:10.1089/ars.2012.4828.

13. Waypa GB, Guzy R, Mungai PT, Mack MM, Marks JD, Roe MW, et al. Increases in mitochondrial reactive oxygen species trigger hypoxia-induced calcium responses in pulmonary artery smooth muscle cells. Circ Res 2006; 99:970-8. doi: 10.1161/01.RES.0000247068.75808.3f.

14. Radak Z, Zhao Z, Koltai E, Ohno H, Atalay M. Oxygen consumption and usage during physical exercise: the balance between oxidative stress and ROS-dependent adaptive signaling. Antioxid Redox Signal 2013; 18: 1208-46. doi: 10.1089/ars.2011.4498.

15. Freund-Michel V, Guibert C, Dubois M, Courtois A, Marthan R, Savineau JP, et al. Reactive oxygen species as therapeutic targets in pulmonary hypertension. Ther Adv Respir Dis 2013; 7: 175-200. doi: 10.1177/1753465812 472940.

16. Semenza GL. Hypoxia-inducible factor 1: control of oxygen homeostasis in health and disease. Pediatr Res 2001;49: 614-7. doi: 10.1203/00006450-200105000-00002.

17. Semenza GL. Oxygen sensing, homeostasis, and disease.N Engl J Med 2011; 365: 537-47. doi: 10.1056/NEJMra 1011165.

18. Hagen T. Oxygen versus Reactive Oxygen in the Regulation of HIF-1alpha: The Balance Tips. Biochem Res Int 2012; 436981. doi: 10.1155/2012/436981.

19. Masson N, Singleton RS, Sekirnik R, Trudgian DC, Ambrose LJ, Miranda MX, et al. The FIH hydroxylase is a cellular peroxide sensor that modulates HIF transcriptional activity. EMBO Rep 2012; 13: 251-7. doi: 10.1038/embor.2012.9.

20. Chua YL, Dufour E, Dassa EP, Rustin P, Jacobs HT, Taylor CT, et al. Stabilization of hypoxia-inducible factor-1alpha protein in hypoxia occurs independently of mitochondrial reactive oxygen species production. J Biol Chem 2010;285: 31277-84. doi: 10.1074/jbc.M110.158485.

21. Brown ST, Nurse CA. Induction of HIF-2alpha is dependent on mitochondrial O2consumption in an O2-sensitive adrenomedullary chromaffin cell line. Am J Physiol Cell Physiol 2008; 294: C1305-12. doi: 10.1152/ajpcell.00007.2008.

22. Calvani M, Comito G, Giannoni E, Chiarugi P. Time-dependent stabilization of hypoxia inducible factor-1alpha by different intracellular sources of reactive oxygen species. Plos One 2012; 7: e38388. doi: 10.1371/journal.pone.0038388.

23. Prabhakar NR, Semenza GL. Adaptive and maladaptive cardiorespiratory responses to continuous and intermittent hypoxia mediated by hypoxia-inducible factors 1 and 2. Physiol Rev 2012; 92: 967-1003. doi: 10.1152/physrev.00030.2011.

24. Semenza GL, Prabhakar NR. The role of hypoxia-inducible factors in oxygen sensing by the carotid body. Adv Exp Med Biol 2012; 758: 1-5. doi: 10.1007/978-94-007-4584-1_1.

25. Archer SL, Wu XC, Thébaud B, Nsair A, Bonnet S, Tyrrell B, et al. Preferential expression and function of voltagegated, O2-sensitive K+channels in resistance pulmonary arteries explains regional heterogeneity in hypoxic pulmonary vasoconstriction: ionic diversity in smooth muscle cells. Circ Res 2004; 95: 308-18. doi: 10.1161/01.RES.0000137173.42723.fb.

26. Ouyang JS, Li YP, Li CY, Cai C, Chen CS, Chen SX, et al.Mitochondrial ROS-K+channel signaling pathway regulated secretion of human pulmonary artery endothelial cells. Free Radic Res 2012; 46: 1437-45. doi: 10.3109/10715762.2012.724532.

27. Yasui S, Mawatari K, Morizumi R, Furukawa H, Shimohata T, Harada N，et al. Hydrogen peroxide inhibits insulininduced ATP-sensitive potassium channel activation independent of insulin signaling pathway in cultured vascular smooth muscle cells. J Med Invest 2012; 59: 36-44. doi: 10.2152/jmi.59.36.

28. Wang YX, Zheng YM. Role of ROS signaling in differential hypoxic Ca2+and contractile responses in pulmonary and systemic vascular smooth muscle cells. Respir Physiol Neurobiol 2010; 174: 192-200. doi: 10.1016/j.resp. 2010.08.008.

29. Rathore R, Zheng YM, Niu CF, Furukawa H, Shimohata T,Harada N，et al. Hypoxia activates NADPH oxidase to increase [ROS]i and [Ca2+]i through the mitochondrial ROS-PKCepsilon signaling axis in pulmonary artery smooth muscle cells. Free Radic Biol Med 2008; 45:1223-31. doi: 10.1016/j.freeradbiomed.2008.06.012.

30. .Lachmanová V, Hnilicková O, Povysilová V, Hampl V,Herget J. N-acetylcysteine inhibits hypoxic pulmonary hypertension most effectively in the initial phase of chronic hypoxia. Life Sci 2005; 77: 175-82. doi: 10.1016/j.lfs.2004.11.027.

31. Liu Y, Bubolz AH, Mendoza S, Zhang DX, Gutterman DD.H2O2 is the transferrable factor mediating flow-induced dilation in human coronary arterioles. Circ Res 2011; 108:566-73. doi: 10.1161/CIRCRESAHA.110.237636.

32. Zhang DX, Borbouse L, Gebremedhin D, Mendoza SA,Zinkevich NS, Li R, et al. H2O2-induced dilation in human coronary arterioles: role of protein kinase G dimerization and large-conductance Ca2+-activated K+channel activation. Circ Res 2012; 110: 471-80. doi: 10.1161/CIRCRESAHA.111.258871.

33. Hu CJ, Iyer S, Sataur A, Covello KL, Chodosh LA, Simon MC. Differential regulation of the transcriptional activities of hypoxia-inducible factor 1 alpha (HIF-1alpha) and HIF-2alpha in stem cells. Mol Cell Biol 2006; 26: 3514-26.doi: 10.1128/MCB.26.9.3514-3526.2006.

34. Rankin EB, Giaccia AJ. Hypoxic control of metastasis. Science 2016; 352(6282): 175-80. doi: 10.1126/science.aaf4405.

35. Smith KA, Yuan JX. Hypoxia-inducible factor-1alpha in pulmonary arterial smooth muscle cells and hypoxiainduced pulmonary hypertension. Am J Respir Crit Care Med 2014; 189: 245-6. doi: 10.1164/rccm.201312-2148ED.

36. Zhao J, Du F, Shen G, Zheng F, Xu B. The role of hypoxiainducible factor-2 in digestive system cancers. Cell Death Dis 2015; 6: e1600. doi: 10.1038/cddis.2014.565.

37. Yang SL, Liu LP, Niu L, Sun YF, Yang XR, Fan J, et al.Downregulation and pro-apoptotic effect of hypoxiainducible factor 2 alpha in hepatocellular carcinoma. Oncotarget 2016; 7: 34571-81. doi: 10.18632/oncotarget. 8952.

38. Huang LE, Gu J, Schau M, Bunn HF. Regulation of hypoxia-inducible factor 1alpha is mediated by an O2-dependent degradation domain via the ubiquitin-proteasome pathway. Proc Natl Acad Sci USA 1998; 95:7987-92. doi: 10.1073/pnas.95.14.7987.

39. Semenza GL. HIF-1: mediator of physiological and pathophysiological responses to hypoxia. J Appl Physiol 2000;88: 1474-80. doi: 10.1152/jappl.2000.88.4.1474.

40. Prabhakar NR, Kumar GK, Peng YJ. Sympatho-adrenal activation by chronic intermittent hypoxia. J Appl Physiol 2012; 113: 1304-10. doi: 10.1152/japplphysiol.00444. 2012.

41. Kato M, Staub NC. Response of small pulmonary arteries to unilobar hypoxia and hypercapnia. Circ Res 1966; 19:426-40. doi: 10.1161/01.res.19.2.426.

42. Madden JA, Vadula MS, Kurup VP. Effects of hypoxia and other vasoactive agents on pulmonary and cerebral artery smooth muscle cells. Am J Physiol 1992; 263: L384-93. doi: 10.1152/ajplung.1992.263.3.L384.

43. Vadula MS, Kleinman JG, Madden JA. Effect of hypoxia and norepinephrine on cytoplasmic free Ca2+in pulmonary and cerebral arterial myocytes. Am J Physiol 1993;265: L591-7. doi: 10.1152/ajplung.1993.265.6.L591.

44. Archer S, Michelakis E. The mechanism(s) of hypoxic pulmonary vasoconstriction: potassium channels, redox O(2)sensors, and controversies. News Physiol Sci 2002; 17:131-7. doi: 10.1152/nips.01388.2002.

45. Hall SM, Hislop AA, Pierce CM, Haworth SG. Prenatal origins of human intrapulmonary arteries: formation and smooth muscle maturation. Am J Respir Cell Mol Biol 2000; 23: 194-203. doi: 10.1165/ajrcmb.23.2.3975.

46. Mironova GD, Shigaeva MI, Gritsenko EN, Murzaeva SV,Gorbacheva OS, Germanova EL, et al. Functioning of the mitochondrial ATP-dependent potassium channel in rats varying in their resistance to hypoxia. Involvement of the channel in the process of animal's adaptation to hypoxia. J Bioenerg Biomembr 2010; 42: 473-81. doi:10.1007/s10863-010-9316-5.

47. Brown ST, Buttigieg J, Nurse CA. Divergent roles of reactive oxygen species in the responses of perinatal adrenal chromaffin cells to hypoxic challenges. Respir Physiol Neurobiol 2010; 174: 252-8. doi: 10.1016/j.resp.2010.08.020.

48. Dong Q, Zhao N, Xia CK, Du LL, Fu XX, Du YM. Hypoxia induces voltage-gated K+(Kv) channel expression in pulmonary arterial smooth muscle cells through hypoxia-inducible factor-1 (HIF-1). Bosn J Basic Med Sci 2012; 12:158-63. doi: 10.17305/bjbms.2012.2463.

49. Shin DH, Lin H, Zheng H, Kim KS, Kim JY, Chun YS, et al.HIF-1alpha-mediated upregulation of TASK-2 K+channels augments Ca2+signaling in mouse B cells under hypoxia. J Immunol 2014; 193: 4924-33. doi: 10.4049/jimmunol.1301829.

10.24920/J1001-9294.2017.037

October 24, 2016.

Tel: 86-17710560131, E-mail:149011273@qq.com