Association of Age with the Expression of Hypoxia-Inducible Factors HIF-1α, HIF-2α, HIF-3α and VEGF in Lung and Heart of Tibetan Sheep

Simple Summary The heart and lung play an essential role in physiological homeostasis, especially in a hypoxic environment. The effect of aging on HIF-1 alpha, HIF-2 alpha, HIF-3 alpha and VEGF expression in the heart and lung of Tibetan sheep that were adapted to hypoxia was evaluated in this study. We conclude that HIF-3a and VEGF are important in how the heart responds to hypoxia and that HIF-1a and HIF-2a may help mediate the adaptation by the sheep to hypoxia. The results suggested that the altered expression of these proteins due to hypoxia is regulated at the protein as well as gene levels. The expression of these proteins in alveolar macrophages suggests these cells play an important role in adaption to hypoxia. The research could provide insight into the role of inflammation in response to reduced alveolar PO2, and is useful in understanding how age influences the hypoxia adaption mechanisms of the heart and lung. This may allow a better understanding of chronic mountain sickness that is commonly observed in Tibetan people living at high altitude on the Qinghai-Tibetan plateau. Hypoxia-inducible factors (HIFs) play an important role in mediating the physiological response to low oxygen environments. However, whether the expression of HIFs changes with age is unknown. In the present study, the effect of aging on HIF-1 alpha, HIF-2 alpha, HIF-3 alpha and VEGF expression in the heart and lung of 30 Tibetan sheep that were adapted to hypoxia was evaluated. The 30 sheep were subdivided into groups of 10 animals that were 1, 2 or 6 years of age. Immunohistochemistry for HIF-1 alpha, HIF-2 alpha, HIF-3 alpha and VEGF revealed that the immunostaining intensity of VEGF protein in the heart and lung was significantly higher than the intensity of immunostaining against the HIFs (p < 0.05). HIF-1 alpha and HIF-2 alpha protein translocated into the nucleus of cardiac muscle cells. However, immunostaining for HIF-3 alpha was restricted to the cytoplasm of the myocardial cells. Immunostaining for HIF-1 alpha, HIF-2 alpha, HIF-3 alpha and VEGF was detected within alveolar macrophages. The concentration of HIF-1 alpha and HIF-2 alpha was higher in the lung of 1-year-old than 6-year-old sheep (p < 0.05). In contrast, HIF-3 alpha and VEGF immunostaining was most prominent in the hearts of the oldest sheep. However, when RT-PCR was used to evaluate RNA within the tissues, the expression of all four studied genes was higher in the lung than in the heart in the 1-year-old sheep (p < 0.05). Furthermore, VEGF and HIF-3 alpha gene expression was higher in the heart from 1-year old than 6-year old sheep (p < 0.05). However, in the lung, HIF-1 alpha and HIF-2 alpha gene expression was lower in 1-year old than 6-year old sheep (p < 0.05). We conclude that HIF-3 alpha and VEGF may play be important in how the heart responds to hypoxia. Additionally, HIF-1 alpha and HIF-2 alpha may have a role in the adaptation of the lung to hypoxia. The expression of these proteins in alveolar macrophages suggests a potential role of these cells in the physiological response to hypoxia. These results are useful in understanding how age influences the hypoxia adaption mechanisms of the heart and lung and may help to better understand chronic mountain sickness that is commonly observed in Tibetan people living on the Qinghai-Tibetan plateau.