This review focuses on the differentially expressed proteins during hypoxia, based on the gene regulation associated with high altitude hypoxia. Genomic screening has revealed a number of candidate genes having role in high altitude adaptation. Genomic findings have provided information for the identification of HAPS (hypoxia associated proteins). However, only a few proteins have been identified and categorised on the basis of their role in hypoxia. Though genomics and proteomics are interrelated, it has been found that up regulation of a transcription do not necessarily leads to up regulation of corresponding protein to the same extent. Recent advances in techniques for proteomic analysis have opened new vistas to understand the functional aspect of differential gene expression and provide a correlation between the gene regulation and differential expression of proteins during high altitude hypoxia. Finding newer proteins and their functional analysis provides better idea of understanding the physiological mechanism underlying adaptation to hypoxia. New information from the analysis of some of the novel proteins offers opportunities to further analyse the mechanisms of cellular responses to hypobaric hypoxia.
Key words: Hypobaric hypoxia; gene regulation; proteomics; hypoxia associated proteins; two-dimensional electrophoresis; mass spectrometry.
Introduction
The human habitation includes diverse range of environments; some of the most extreme of these environments are found at high-altitudes. The three major high altitude (HA) regions with long-term resident people are: the Himalayan region the Tibetan Plateau (Tibetans, Ladakhi, and Sherpas), Northern Africa (Ethiopians) and the Andes (Quechua and Ayamara). Residents of these altitudes descend from a long line of highland ancestors who have lived there for generations despite the physiological challenges associated with chronic oxygen deprivation. The study of indigenous HA residents therefore, provide the opportunity to identify genes and ultimately the proteins that might have played a role in hypoxia adaptation. Hypoxic stress due to high altitude environment affects the well-characterized physiological pathways related to oxidative energy metabolism which has facilitated the identification of HA adaptation mechanism in nonhuman animals [1, 2]. Naturally, this has stimulated interest for understanding the basic human physiological response to hypoxia [3].
High altitude environment is characterized by hypobaric hypoxia (HH) and it provides a natural experimental condition to study the genetic and physiological effects of HH. The decreased pressure of oxygen at high altitude results in lower circulating oxygen levels in the body, [4, 5]. The cellular responses to hypoxia are rather complex and characterized by alterations in number of genes expression, which are genes related to stress and proteins that are necessary to maintain homeostasis in cellular system. Genomic changes during hypoxia have been extensively investigated and it was found that HIF1 (hypoxia inducible factor1), which is a transcriptional regulator, controls cellular oxygen homeostasis and plays an important role in energy metabolism [6]. Scaning of genome to identify candidate genes for HA adaptation revealed that noncoding variants in and around the genes EPAS1 (endothelial PAS domain protein 1), EGLN1 (Egl nine homolog 1; a regulator of HIF), and PPARA (peroxisome proliferator activated receptor-α; a transcriptional target of HIF) are strongly associated with reduced blood concentration of hemoglobin in Tibetan highlanders [7, 8].
Recently it has been found that the frontal lobe of brain is highly sensitive to acute HH and there is involvement of limbic and central structures to blood gas changes; thus emphasizing the involvement of these brain areas in acute hypoxia [9]. Repeated mild HH exposure results in modification in expression of mitochondrial thioredoxin-2 in hippocampus of rat [10]. Another finding has revealed that activity-dependent neuroprotective protein (ADNP)-derived peptide (NAP) is associated with improved antioxidant status of brain during HH [11]. Recently it has been reported that exposure to high altitude may lead to compromise in colour discrimination. Colour discrimination thresholds for tritan (blue), protan (red) and deutan (green) axes were measured extensively in two male participants during an expedition to Mt. Everest, using a quantitative, computer controlled psychophysical colour vision test and it was found that with increasing altitude, colour discrimination thresholds were found to rise, predominantly for the tritan axes, deutan thresholds were minimally elevated at high altitude, whereas protan was altered in one of the observers [12]. Further studies are needed to understand the overall mechanis underlying hypoxic adaptation. These interesting findindings in the field of gene have generated the idea of exploring the global proteome to relate with each other. Hypoxia-induced changes in the proteome of mammalian cells are still in their early phase of investigation. Although several studies have been reported on the influence of hypoxia on expression of gene and post-translational modification of proteins, only a few reports have really elaborated the proteome wide alterations during hypoxia. Based on these findings differentially expressed proteins as perspective biomarkers have been discussed further in the article.
Hypoxia is a pathophysiological condition in which the body as a whole which is generalized hypoxia or a particular region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Generalized hypoxia may occurs in any healthy people when they ascend to HA, where it cause altitude sicknessand may leads to potentially fatal complications like high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE). Hypoxia leads to a number of physiological consequences as well as alternate responses which lead to lowering its ill effect (Fig 1)
Fig1 Multiple physiological consequences and responses of hypoxia
There are different phases of HH depending upon the altitude and corresponding reduced air pressure. Four different stages of hypoxia have been mentioned which occurs as one ascends to high altitude from sea level (Table1).
Table1 Stages of hypoxia in aviation depending on altitude
Stages
Altitude
PaO2(mm Hg)
% saturation
of haemoglobin
Indifferent
0-10,000ft
100-60mmHg
95-90%
Compensatory
10,000-15,000ft
60-56mmHg
90-80%
Disturbance stage
15,000-20,000ft
45-35mmHg
80-70%
Critical Stage
20,000-23,000ft
35-30mmHg
70-60%
Hypoxia related gene-regulation
Hypoxic regulation of genes has been studied extensively but entire mechanism is yet to be understood. Information has been gathered reflecting some of the organs which have major correlation with regulatory mechanism during hypoxia so as to correlate it with upcoming proteomic findings. Physiological pathways existing in the body to maintain homeostasis moves around different organs and cannot be isolated separately. Lung, liver, kidney, heart and brain are the major organs that are affected by hypoxic exposure and thus have well evolved regulatory mechanisms both at genomic as well as proteomic level. Genes or proteins which are differentially expressed in these organs during hypoxic conditions are interrelated during homeostasis. Significant work has been done to understand the regulation of gene expression especially at gene level, in these organs during hypoxia.
The study of temporal changes in gene expression in murine kidney in response to acute hypobaric hypoxia (AHH) revealed altered expression of genes involved in metabolism, transcription, translation, signal transduction, apoptosis and protein folding [13]. Differential expression of a number of UPR (unfolded protein response) related genes such as Grp78, Grp94, Canx, Calr, DnaJ (Hsp40) homolog subfamily B, member 11 (DnaJb11) and activating transcription factor 4 (Atf 4) was found in this study, emphasing that HH has notable effect on kidney homeostasis. Recently it has been found that NFAT5 (Nuclear factor of activated T-cells) a member of the Rel family of transcriptional activators, which includes nuclear factor kB (NFkB) is activated by hypoxia [14]. This factor is necessary for survival of renal cells in challenging conditions and thus its activation confers of protection against stress and its up-regulation during hypoxia is the part of the adaptative process.
Studies on differential gene expression analysis of murine liver exposed to AHH showed that hypoxia has major impact on liver [15]. In this study a number of genes involved in various biological processes such as metabolism, signal transduction, transcription, apoptosis, cell cycle, and ubiquitination pathways were found to be differentially expressed. Out of the 15,000 genes analyzed, 512 genes (3.5%) were either induced or repressed suggesting that liver homeostasis has a strong correlation with hypoxia. In case of heart, gene expression pattern has been studied in mouse model in response to chronic constant or intermittent hypoxia (CCH/CIH) [16], which showed that some of the related genes responded qualitatively in a similar fashion during both CCH as well as CIH in the heart. These included stress-responding genes such as heat shock and redox genes, genes involved in vascular dilation, angiogenesis, and heme biosynthesis genes. The gene which encodes a thioredoxin-interacting protein that inhibits the function of thioredoxin was found to be down-regulated in both CCH and CIH [16]. Angiogenesis is a critical component of mammalian brain adaptation to prolonged hypoxia. Hypoxia-induced angiogenesis is mediated by hypoxia inducible factor-1 (HIF-1) dependent transcriptional activation of growth factors, such as vascular endothelial growth factor [17]. Acute mountain sickness (AMS) develops within a few hours after arrival at HA and High-altitude cerebral edema (HACE) is considered to be the end stage of severe AMS. Recently in one of the study it has been shown that in acute hypoxic condition there is disruptrion of blood brain brarrier (BBB) which may be attributed to AMS and HACE [18].
Lung is one of the vital organs that plays major role in maintaining homeostasis during hypoxic exposure and alveolar epithelial cells have the adaptability to cope up with oxygen deprivation and allow them to maintain ATP content near to those of normoxic cells. Hypoxic exposure down-regulates tropoelastin gene in rat lung fibroblast [19]. Alveolar hypoxia up-regulates the expression of extracellular matrix proteins and platelet-derived growth factor-B in lung parenchyma suggesting that alveolar hypoxia causes vascular remodelling in lung parenchyma [20]. In another study genes for VEGF (vascular endothelial growth factor), and adhesion molecules (intracellular adhesion molecule-1 [ICAM -1] and vascular cell adhesion molecule-1 [VCAM-1] were found to be up-regulated in lung epithelial cells during hypoxia [21]. However the functional relevance of these observations is not fully understood.
This information is useful in understanding the molecular basis of body's adaptation to oxygen in cellular environment by regulating the expression of related genes. The mRNA level by transcriptional control in turn, regulates the proteins that are affected during lower oxygen condition in the cells. Some of the proteins that have major role in regulation of physiological mechanism of the body during hypoxic condition have been identified
Proteomic analysis
Proteomics has rapidly progressed in the post genomic era and is generally accepted as a complementary method to genetic profiling. The advantage of a proteomic rather than a transcriptomic approach is that protein expression levels are measured directly, rather than being inferred from abundance of the corresponding mRNAs, which are imperfectly correlated to protein concentration [22] due to variable rates of post-transcription modification, differences in message stability and translation efficiency [23]. Proteomic analysis provides information pertaining to compensatory changes taking place at the level of protein expression in response to environmental perturbations. The techniques commonly used for the global analysis of protein expression include two-dimensional Electrophorsis (2DE) in association with mass spectrometry (MS) [24, 25], multidimensional chromatography coupled with tandem MS [26, 27], and chip technologies coupled with either antigens [28, 29] or antibodies [30, 31].
The idea that multiple proteins can be analyzed in parallel grew from two-dimensional gel maps [32, 33]. Two-dimensional gels have provided much valuable information and these will continue to be an integral part of proteomics research for the foreseeable future [34, 35]. Two-dimensional electrophoresis coupled with matrix-assisted laser desorption/ionization (MALDI) is the most extensively used approach for identifying and quantifying the changes in protein expression [36, 25]. On the other hand, the multidimensional chromatography, in conjuction with tandem MS strategy and isotope-labeled affinity tags, may ensure more sensitive and better separation and quantification of proteins than that achieved by 2DE [37].
Hypoxia and proteomics
Comparative analysis of certain hypoxia related proteins suggests that hypoxia up-regulates or down-regulates these proteins in a cell-type or tissue dependent manner. Major consequences and responses of hypoxia depicted in figure 1 are result of the alteration in expression of these proteins. The hypoxia-evoked proteins can be classified into several groups depending on their activities (Table 2).
Table 2 Categorization of hypoxia evoked differentially expressed proteins
Classification
Proteins
Physiological function
References
Proteins of energy metabolism including
Glycolytic enzymes
Often referred as hypoxia-associated proteins (HAPs), these proteins include the glycolytic enzyme GAPDH and non-neuronal enolase unique to endothelial cells. Expression of these proteins helps in up regulated tolerance and adaptation to lack of oxygen. This may be one of the mechanisms to overcome the ischemic injury during hypoxia
[38]
Stress related proteins
Heat shock Proteins (HSPs)
The enhanced synthesis and accumulation of several distinct families of HSPs, such as HSP70, HSP90, HSP60, and HSP27 is one of the cellular mechanisms for protection against the stress induced by environmental cues. Inflammatory responses during hypoxia are due to over expression of these proteins.
[39]
Membrane-bound proteins that include transporters and receptors
Acid-base transporters, glucose transporter
The effect of intermittent hypoxia in the mouse central nervous system especially in the cerebellum leads to decreased expression of sodium/hydrogen exchanger (NHE) isoform 1 and sodium-bicarbonate co-transporter. Alteration in these proteins leads Edema of CNS during hypoxia.
[40]
Cytosolic proteins
Antioxidants, signalling cascade proteins
The expression of antioxidants and the proteins associated with signalling cascade is altered in hypoxia. The expression of nitric oxide synthase (NOS) endothelial (eNOS) and inducible (iNOS) forms is increased during hypoxic condition. Overexpression of NOS leads to vasodilation to minimise the effect of hypoxia.
[41]
While many of these proteins were identified using proteomic analysis, relatively fewer studies have been carried out using proteomic based approaches for identification of novel protein(s) for high altitude adaptation/ HH. A study on comparative profile of skeletal muscle proteome from human volunteer natives of high altitude (Tibetans) and low altitude reported seven differentially regulated proteins [42]. It was found that glutathione-S-transferase P1-1, Δ2-enoyl-CoA-hydratase, phosphoglycerate mutase, and myoglobin overexpressed while NADH-ubiquinone oxidorductase was only slightly overexpressed in highlanders. On the other hand, glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase were slightly down-regulated in high landers. In this above study it became clear that hypoxia affects mostly energy metabolism pathways. Recently it has been found that chronic hypobaric hypoxia (CHH) increases isolated rat fast-twitch and slow-twitch limb muscle force and fatigue [43].
Differential expression of lung proteome has been studied during pulmonary arterial hypertension (PAH) in male Sprague-Dawley rats which is a common manifestation of HA exposure. Differential expression of HSP27, septin 2, tropomyosin β-chain, annexin 3, HSP70, F-actin capping protein, biliverdin reductase and ERp29 (endoplasmic reticulum protein) in PAH as compared to control (without PAH) was reported [44]. In another proteomic study conducted on patients with idiopathic PAH it was found that out of 300 proteins detected in lung protein homogenate, 25 were differentially expressed [45]. Few of them are chloride intracellular protein1, annexin A3, phosphoglucomutase-like protein 5, and serum deprivation-response protein. These are probably the proteins that could be associated with cell growth, proliferation, intracellular trafficking, and signaling. Some of these proteins have been validated by Western blotting. These include periostin, haptoglobin, chloride intracellular channel protein 1 (CLIC1) and chloride intracellular channel protein 4 (CLIC4) [45]. Acute hypoxia also induces apoptosis of pancreatic β-cells by activation of the unfolded protein response and upregulation of CHOP (C/EBP homologous protein) which is a pro-apoptotic transcription factor [46] indicating insulin metabolism impairment and ultimately energy metabolism. In another report it was found that chronic intermittent hypoxia (CIH) and chronic sustained hypoxia (CSH) differentially regulate left ventricular inflammatory and extracellular matrix responses. Matrixmetalloprotein MMP-9 protein and fibronectin protein levels were found to be decreased in CIH showing decreased inflamatory status whereas in case of CSH extracellular matrix and adhesion molecule were found to be increased indicating that inflammation increases in response to CSH and decreases in response to CIH [47]. All these changes show that the effect of hypoxia is not limited organ specific changes, rather the machinery of the whole body is affected and by its effect.
Regulatory mechanism of some of the important proteins during hypobaric hypoxia
Activation of transcription factors is one of the most rapid cellular events that occur in response to cellular stress caused by hypoxia. Heat shock factor-1 (HSF-1) and hypoxia-inducible factor-1 (HIF-1) represent two separate classes of transcription factors that are specifically and rapidly activated in response to cellular stress [48, 49].
HIF-1 is central to key molecules involved in hypoxia. It is a heterodimeric complex composed of HIF-1α and HIF-1β subunits. The dimer binds to specific DNA enhancer sequences and regulates the expression of target genes. Both HIF-1α and HIF-1β are constitutively expressed under normal oxygen conditions (normoxia). However, the HIF-1α protein is quickly degraded before its dimerization with HIF-1β [50]. Normoxic HIF-1α degradation is mediated by a series of hydroxylations and ubiquitinations that tag HIF-1α for disposal through the proteosomes [51]. Following a shift to low oxygen environment, the α-subunit gets stabilized and translocates to nucleus. HIF-1α protein have an oxygen-dependent domain (ODD) that is important for regulation of its subunit stability. Under normal oxygen tension, the ODD is identified by the product of Von Hippel Lindau (pVHL) suppressor gene, a component of a complex of multisubunit ubiquitin-protein that binds the subunit with polyubiquitin to promote HIF-degradation. When the cells are in hypoxic state, pVHL is not able to recognize the HIF-1α, allowing its accumulation and transportation to nucleus. Once in the nucleus, HIF-1α dimerizes with HIF-1β and control the expression of hypoxia-inducible genes [52] (Fig 2a and 2b).
Fig 2a Hypoxia-inducible gene regulation by HIF-α In normoxia, HIF-α is transcriptionally inactive and is rapidly degraded by the ubiquitin (Ub) proteasome pathway. In hypoxia, HIF-α undergoes protein stabilization and translocation from the cytoplasm to the nucleus, where it dimerizes with HIF-β to induce the transcription of related genes
Fig 2b Stablization of HIF-α by pVHL protein. (+) shows recognition of ODD by pVHL and (-)shows failure of recognition of ODD by pVHL
Heat shock factor-1 (HSF-1) is important for the induction of heat shock proteins (HSPs) such as HSP27, HSP70 which in turn act as chaperones preventing the aggregation and inactivation of essential cellular proteins [48]. HSPs are vital in maintaining cellular homeostasis and are up-regulated during hypoxic contition [44]. In unstressed cells, HSF-1 is bound to HSP-90 and remains in inactive form. During cellular stress, HSF-1 rapidly dissociates from HSP-90, gets trimerized and is translocated to the nucleus, where it activates the transcription of genes that contain the characteristic heat shock response element such as HSP27 and HSP70 thereby regulating hypoxia [53-55].
Erythropoietin (EPO) is required for the proliferation and differentiation of erythroid progenitor cells to yield red blood cells and elicits a response in different tissues, depending upon the level of expression of its receptor (EPOR). During low oxygen condition HIF-1 binds to hypoxia response elements and activates the transcription of EPO (and other hypoxia-responsive genes). The EPOR expression and the biological response to EPO have been observed in cardiac, muscle, endothelial, neural, and certain other cell types [56, 57]. In neuronal cells, EPORs are induced by hypoxia resulting in increased EPO response due to its binding to EPORs [58]. EPO regulates several activities in cardiovascular system; EPO mediated cascade in hypoxic conditions leads to induction of NO production and provides one of the explanations for the proposed protection by EPO in myocardial infarction as it may rapidly increase vasodilatation and facilitate effective collateral circulation [59]. The up-regulation of EPOR in human endothelial cells in vitro with their consequent increased sensitivity to EPO at low oxygen supports that EPOR stimulation of endothelial cells in vivo may require an elevated level of EPO (by hypoxic induction or other means) [60].
Vascular endothelial growth factor (VEGF) is an important growth and permeability factor for endothelial cells and is expressed in lungs in high amounts. Hypoxia is the best-characterized potent inducer of VEGF mRNA expression. This regulation occurs at the transcriptional level by activating HIF-1α and/HIF-2α, which bind to response element in VEGF gene. In alveolar epithelial cells, hypoxia induces an up-regulation of VEGF mRNA transcripts due to transcriptional activation of VEGF gene without change in mRNA stability [61]. There is strong evidence that HIF plays a major role in hypoxia-induced VEGF regulation. VEGF modulates epithelial cell proliferation and surfactant protein expression [62]. The role of increased VEGF secretion in overall adaptation process of alveolar epithelial cells in response to hypoxia remains to be elucidated.
Prospective biomarkers for high altitude adaptation
Angiogenesis is one of the important mechanisms that have been studied during hypoxic condition. Oxygen carrying capacity is directly related to number of blood vessels and in turn increased blood flow. This compensates for hypoxic condition by increasing the availability of oxygen content to the cells. Although there are a number of other physiological mechanisms operating in body to maintain the homeostasis during low oxygen condition, angiogenesis plays a very crucial and important role. Molecular characterization of angiogenic pathways and identification of hypoxia-inducible factor (HIF) as a key transcriptional regulator of molecules related to hypoxic response has great promise in understanding the regulatory mechanisms during hypoxic exposure
Pur-α: Pur-α is a transcriptional activator protein that has been implicated as a novel hypoxia response factor. It is responsible for coordinated induction of β-2 integrin family [63]. Increased abundance of Pur-α has also been found in lung tissue of the patient with idiopatic PAH [45]. Pur-α expression leads to increased angiogenesis.
Chloride intracellular channel protein-4: CLCI-4 has been reported to have role in angiogenesis by supporting acidification of vacuoles along the intracellular tubulogenic pathway [64]. Angiogenesis is one of the common adaptative response while one ascents to high altitude.
Periostin: Periostin (encoded by Postn) is a TGF-β inducible, 90-kDa protein which advances the atherosclerotic and rheumatic cardiac valve degeneration by inducing angiogenesis. Up-regulation of this protein has also been reported in patients with idiopatic PAH [45].
Macrophage migration inhibitory factor (MIF): It has been reported that hypoxia accelerates the expression of macrophage migration inhibitory factor in human vascular smooth muscle cells via HIF-α dependent pathway [65]. In this study the expression of MIF mRNA and protein both were found to be up-regulated as early as 2 hours after the cultured human VSMCs were exposed to moderate hypoxia condition (3-10% O2).
HSP70: Prior induction of HSP70 leads to hypoxic tolerance and facilitates acclimatization to acute HH in mouse brain [66]. Up-regulation of HSP70 has also been reported in lung tissues of male Sprague-Dawley rats after induction of PAH [44].
RhoA: It has been reported that prolonged hypoxia increases RhoA and ROS signaling and activation in pulmonary artery smooth muscles and endothelial cells [67]. Hypoxia activates RhoA and subsequently RhoA kinase activity [68]. However the mechanism by which RhoA is activated during hypoxia not well established.
Dicer: To date little is known about the link between the microRNA pathway and hypoxia signaling. However, it has been reported that hypoxia impairs with Dicer expression and activity (Dicer is an endoribonuclease that cleaves doble stranded-RNA). Dicer is found to have role in regulation of gene expression and functions in human endothelial cells during angiogenesis [69]. It has also been reported that Dicer is down-regulated in chronic hypoxia and this maintains the induction of hypoxia-inducible-factor-α subunits and hypoxia-responsive genes and ultimately maintains the homeostasis [70].
Fig 3 The interrelation between the differently expressed proteins during hypoxia
BMP: Bone morphogenetic protein, TGF-β :Transforming growth factor-β, CLIC-4: Chloride intracellular channel protein-4, VEGF: Vascular endothelial growth factor, MIF: Macrophage migration inhibitory factor, HSP: Heat shock protein, HIF: Hypoxia inducible factor, Pur-α :Transcriptional activator protein Pur-alpha
Fig 3 shows a diagrammatic representation of the interrelation between different proteins that are found differentially expressed during hypoxia. These include several pathways interrelated to angiogenesis that are altered to cope with hypoxia. TGF-β and BMP signalling are interrelated pathways and TGF-β plays significant role in angiogenesis. Many of the proteins that are altered during hypoxic condition revolve around these pathways. Exact role of each protein and their direct relationship is not clear, but attempts have been made to relate these molecules with each other and with above mentioned pathways.
p53 and hypoxia
The tumor suppressor activity of p53 is well documented, it has also been implicated to play a role in hypoxia. The p53 can be induced and activated by hypoxia. The pattern of expression and activity of p53 under hypoxia is similar to its induction of tumors when get mutated, because in tumor tissues, the condition mimics hypoxia [71]. Earlier studies support an indirect interaction between p53 and HIF-1α, probably through MDM2 [72]. However, the subsequent studies presented evidence of a direct interaction between p53 and HIF-1α. The two p53-binding sites have been studied within the oxygen dependent degradation (ODD) domain of HIF-1α, suggesting that one molecule of HIF-1α interacts with one p53 which is dimeric [73]. Recently a mechanism has been identified which states that hypoxia-mediated p53 activation is through the ATR-Chk1- MDMX-14-3-3γ signaling pathway [74]. The mechanism suggests that hypoxia activates ATR and then ChK1, which in turn phosphorylates MDMX at S367 and induces MDMX-14-3-3 binding, suppressing MDMX activity and activating p53 [74]. p53 posses some interesting properties also. It has been found that Wt p53 helps to rescue Mt p53 and leads to repression of hypoxic tumor [75]. This novel role of Wt p53 as a molecular chaperone put up a query whether if p53 self-rescue may exist as a cellular event in order to reverse the oncogenic properties of Mt p53 in cancer cell. Another important finding showed that there is a novel chaperone-like activity resides in p53-N-ter region. This study might have significance in understanding the role of p53-NTD in self stabilization of p53, conformational activation and apoptosis under heat-stress conditions [76]. Although the chaperone and apoptotic potential of Wt p53 are comparable with that of NTD, the tumor regression potential of NTD is about one half of Wt p53 which suggests that additional in vivo p53-bound modulating factors may also be involved for tumor regression [75]. All these important properties of p53 and further studies on the understanding of the relationship between p53 and HIF will provide new strategies for both hypoxic adaptation and cancer therapy.
Conclusion and future prospects
Hypoxia affects a number of cellular processes and has been recognized as an important contributing factor in a variety of pathophysological processes. Single or whole proteome analysis of expression and posttranslational modification of proteins in response to hypoxia have revealed that complex interplay of molecules that are present in multiple cellular compartments to cope up with the low oxygen availability for maintaining cellular functions. A number of findings have been reported on differentially expressed genes in liver, kidney, heart etc. During hypoxia most of the differentially expressed genes are involved in various biological processes such as metabolism, signal transduction, transcription, apoptosis, cell cycle, and ubiquitination pathways. Genomic studies have given an insight into the proteomics which can be correlated to hypoxia. Several proteins have been identified till date and categorised on the basis of their role during hypoxia. However, potential role of most of these proteins is not fully understood. Proteomics based studies during hypobaric hypoxia provide a new platform for the identification of novel proteins that can be useful for the development of new strategies in the therapeutics related to high altitude disorders. Increased application of proteomics in future, for the analysis of hypoxia related proteins will provide a new direction for better understanding of the underlying cellular mechanisms associated with the response and adaptation to hypoxia.
Conflict of interest: The authors declare that they have no conflict of interest.
Acknowledgement: Financial assistance from ICMR is gratefully acknowledged.
