Purpose: Extensive studies in the last decade have shown that ER stress and cardiovascular pathophysiology are intimately intertwined. Recent evidence sheds light on the role of protein homoeostasis in the high altitude related disorders including pulmonary hypertension. Although individuals vary in their ability to cope with the harsh environmental conditions, oxidative modifications to the myocardial proteins, may contribute significantly to the hypoxic vulnerability. Assessing the hitherto unexplored significance of proteostasis in the differential response to hypoxic stress forms the crux of the present study. The study evaluates the potential of ER stress modulators as therapeutic agents in enhancing hypoxic tolerance.
Methods: Sprague Dawley rats were exposed to simulated hypoxia equivalent to 223 mm Hg pressure, screened on the basis of gasping time and categorized as susceptible (<10 min), normal (10-25 min) or tolerant (>25 min). Animals were administered either ER stress inducer tunicamycin (0.3mg/kg BW) or chemical chaperone, 4-phenylbutyric acid (200mg/kg BW) to assess the role of protein folding homeostasis. Biochemical assays were performed to identify protein modifications, proteolytic and apoptotic activities in the myocardial tissue extracts. Immunoblotting, immunohistochemistry and ELISA based expression analysis was done to elucidate expression of ER stress markers in the heart tissue.
Results: Animals administered Tm showed a significant decrease in gasping time, along with greater protein oxidation and expression of UPR markers, including GRP78 and PERK highlighting the role of ER stress activation in enhancing hypoxic susceptibility. Conversely, 4-PBA significantly elevated the hypoxic tolerance as evident by the increase in gasping time and reduced levels of oxidative protein modifications. There was an appreciable increase in heat shock protein expression, as well as enhanced nitric oxide synthesis. This may be mediated in part by the induction of pro-survival signalling cascade including PI3K/Akt/mTOR/S6Kinase pathway and inhibition of apoptosis.
Conclusion: 4-PBA thus increases stress tolerance through the attenuation of hypoxia induced protein misfolding. With recent research suggesting the occurrence of ER stress in high altitude related disorders like pulmonary hypertension, administration of chemical chaperones may form a novel therapeutic agent in increasing tolerance to environmental stress.