Introduction
Low pO2 conditions, also termed environmental hypoxia,
are expressly encountered as one ascends to altitude. In a previous
study, we delineated the effects of varying levels of
pO2 during acute exposure of 24 h on the lung and plasma
proteomes. We observed that the lung’s redox homeostasis, cytoskeletal
integrity and energy homeostasis was greatly perturbed at very high
altitude zone (simulated) when the ascent rate was very high. In
addition, we also observed global proteome responding to these
perturbations and affirming systemic redox and energy homeostasis (Paul,
Gangwar, Bhargava, & Ahmad, 2018). However, the effects of extended
exposure to low pO2 conditions at varying time points
remain unknown. Hence, we needed to investigate the effects of
increasing the duration of low pO2. We chose the lowest
survivable pO2 (8.19 kPa) corresponding to
PB 40 kPa which is the atmospheric pressure encountered
at 25,000 ft (7620 m; beginning of death zone altitude zone).
In this study, we focus on the effects of increasing durations of low
pO2 exposure (8.19 kPa) on the lung and plasma
proteomes. Previous studies regarding extended low pO2(environmental hypoxia) exposure have elucidated multiple facets of
exposure to chronic environmental hypoxia at high altitude. A pioneering
human study on arterial pH and hemoglobin affinity during extended
environmental hypoxia exposure was conducted by Chiodi in 1957 (Chiodi,
1957). Mazzeo and co-workers have reported a higher arterial
norepinephrine and epinephrine concentration after 21 days at Pikes Peak
(~PB 61 kPa; pO2 12.81
kPa) in human subjects (Mazzeo et al., 1991). Other studies on endocrine
system during extended low pO2 exposure report a
suppressive effect of endocrine mediators to facilitate maximal energy
efficiency even at the cost of better oxygen delivery (Barnholt et al.,
2006; Chaiban, Bitar, & Azar, 2008). Weight loss is a highly studied
phenomenon during chronic low pO2 exposure and is
attributed in part to hormones like leptin (Boyer & Blume, 1984;
Butterfield et al., 1992; Hamad & Travis, 2006; Surks, Chinn, &
Matoush, 1966; Tschöp & Morrison, 2001; Tschöp, Strasburger, Hartmann,
Biollaz, & Bärtsch, 1998). Physiological features like heartbeat
interval distributions were reported to be affected by chronic
environmental hypoxia (Meyer et al., 1998). Ostadal and Kolar have
extensively reviewed the cardiac adaptations during extended low
pO2 exposures, particularly the underlying molecular
mechanisms (Ostadal & Kolar, 2007). An exquisite review focused on
cardiac metabolic perturbations during extended exposure to low
pO2 was authored by Essop (Essop, 2007). Even in the
early 80s, researchers had implicated energy efficiency with survival
during chronic low pO2 exposures owing to increased
energy expenditure and reduced caloric intake (Durand, 1982). A greater
emphasis was laid on the muscle tissue due to their energy intensive
nature during extended exposure to low pO2. Cerretelli
and Hoppeler provide an early review of metabolic consequences of
extended environmental hypoxia exposure on the muscle (Cerretelli &
Hoppeler, 1996). Early reports stated changes in muscle capillary
geometry during chronic environmental hypoxia in rats (Poole &
Mathieu-Costello, 1989). The same authors also elucidate the effects of
chronic environmental hypoxia on deer mouse muscle fiber size
(Mathieu-Costello, Poole, & Logemann, 1989). Human muscle studies
revealed a lower mitochondrial density and fiber size with unchanged
capillary network during mountaineering expeditions which involved low
pO2 exposures for extended time periods (Hoppeler et
al., 1990). Similar studies revealed an overall shift to anaerobic
metabolism in muscles (Howald et al., 1990). Muscle metabolism
selectively utilizing free fatty acids instead of glycogen reserves
after extended exposure to low pO2 was reported by Young
and colleagues (Young, Evans, Cymerman, Pandolf, & Knapik, 1981). The
influence of exercise on muscle during high altitude ascent and extended
environmental hypoxia exposure has also been discussed by Kayser and
co-workers in context of muscle fatigue where they showed that the
nervous system had a bigger role to play rather than metabolic
intermediates like lactate in causing exhaustion (Kayser, Narici,
Binzoni, Grassi, & Cerretelli, 1994). A more recent study focused on
the muscle mitochondrial processes during low pO2exposure (pO2 9.87 kPa) of up to 66 days and stated that
muscle energy utilization and oxidative stress were modulated towards
efficient ATP generation (Levett et al., 2011).
Radak et al have provided a
detailed review of oxidative stress during environmental hypoxia (Dosek,
Ohno, Acs, Taylor, & Radak, 2007) and linked DNA repair with fatty acid
metabolism in human skeletal muscle exposed to environmental hypoxia
(Acs et al., 2014). Another study during Caudwell Xtreme Everest
expedition concluded that chronic environmental hypoxia exposure caused
significant muscle atrophy, however, muscle function remained constant
(Edwards et al., 2010).
Lung (Pulmonary) system has also been studied during extended exposures
to low pO2. Few of the earliest publications detail
carotid body insensitivity during extended low pO2exposure in man (Severinghaus, Bainton, & Carcelen, 1966; Weil,
Byrne-Quinn, Sodal, Filley, & Grover, 1971). In 1975, Tucker and
co-workers implicated lung vascular smooth muscle as determinant of
pulmonary hypertension, common during extended low pO2exposures (Tucker et al., 1975). Later on, mechanical properties of lung
as an organ were elucidated (Weil, 1986). Decreased pressor response and
increased mast cell density have been reported in rats exposed to longer
durations of low pO2 (McMurtry, Petrun, & Reeves, 1978;
Tucker, McMurtry, Alexander, Reeves, & Grover, 1977). Pulmonary
hypertension during low pO2 exposure at high altitude
has been reported in numerous studies and said to be a causative factor
of HAPE and CMS (Aldashev et al., 2002; Antezana et al., 1998; Ge &
Helun, 2001; Maggiorini & Leon-Velarde, 2003; Naeije, De Backer,
Vachiery, & De Vuyst, 1996; Rabinovitch, Gamble, Miettinen, & Reid,
1981; Scherrer et al., 1999; Vender, 1994; Yagi, Yamada, Kobayashi, &
Sekiguchi, 1990). Molecular aspects of the hypoxic lung have also been
investigated. HO-1, VEGF and Rho kinase based signaling events have been
implicated in chronic hypoxic lung (Gao et al., 2007; Tuder, Flook, &
Voelkel, 1995). However, rat lung proteome analysis focused on extended
exposure to low pO2 are rare (Ohata et al., 2013). Only
recently, studies on the effects of acute low pO2exposure on rat lung proteome have been reported (Ahmad et al., 2015).
Thus, this study focusing on extended low pO2 exposure
induced lung proteome alterations in rat was explorative in nature.
Furthermore, we also investigated changes in the plasma proteome of same
animals so as to find the interlinks between the two proteomes. This was
done to further facilitate their translational application during later
investigations. This study elucidated many crucial aspects of lung and
plasma proteome during extended exposure to low pO2. One
of the key findings was that unlike redox homeostasis being a central
process during acute exposure (Paul et al., 2018), extended exposure
caused cytoskeletal re-arrangements to dominate in lungs while plasma
proteome was dominated by proteins involved in metabolic and immune
processes. An overall view that emerges from this study is the
cytoskeletal remodeling that occurs in the lung signals the systemic
response manifested in energy metabolism and inflammation signaling
processes. A common low pO2 mediator between lung and
plasma is STAT3 (Signal transducer and activator of transcription 3)
(Paul et al., 2018). In an attempt to gather qualitative insights in
humans undergoing extended exposure to low pO2conditions, we performed ELISA based validations of Alpha-1-antitrypsin,
cofilin-1 and S100A8 in humans exposed to 14,000 ft
(~pO2 12.81 kPa) for 7, 30 and 120 days.
The selection of these proteins was from the protein datasets of rats,
with the focus being that each of the proteins represent the processes
found perturbed in rat, i.e. lung cytoskeletal re-modeling, and its
derived innate immune/inflammatory signaling.
Overall, we observed that extended exposure to low pO2perturbed lipid metabolism due to redox stress in pursuit of redox
homeostasis. The disturbed lipid metabolism led to inflammatory
processes being activated. STAT3 played an important upstream regulatory
role which may be specifically evaluated in future investigations.