We have demonstrated that hemoglobin is expressed by human and rodent pulmonary epithelial cells, including alveolar type II (ATII) cells and a bronchiolar Clara-like cell line. This novel finding may have enormous implications in the physiology and pathology of the lung due to the many defined roles of hemoglobin, including gas exchange, nitric oxide (NO) metabolism, blood pressure regulation, and protection against oxidative and nitrosative stress. Also, because it is accepted dogma that globin genes are normally expressed solely by erythroid tissue in most vertebrates, these results raise many important questions in hematology, genetics, and developmental biology. In addition to their central role in the development and regeneration of the alveolar epithelium, ATII cell function in surfactant production and innate immunity of the lung, processes known to be affected by NO, and consequently, could also be affected by hemoglobin. Furthermore, the proteolytic turnover of hemoglobin yields several bioactive peptides, collectively termed hemorphins, that could potentially affect the activity of angiotensin-converting enzyme (ACE), which plays an important role in blood pressure regulation and is expressed by ATII cells. We hypothesize that hemoglobin is expressed in ATII cells, that this expression may involve some of the same regulatory mechanisms as those found in erythroid cells, that some of the functions of hemoglobin in the lung are related to NO and pulmonary surfactant production, and that peptides derived from hemoglobin may affect ACE activity in ATII cells.
The specific aims that will test this hypothesis are 1) to further characterize the expression of globin genes by ATII cells, 2) to define the relationships between globin gene expression, nitric oxide physiology, and surfactant production in ATII cells, and 3) to determine effects of globin protein turnover and hemorphins in ATII cells. Project Narrative: The proposed experiments will define the functions of hemoglobin in airway cells. Results of this research may lead to the identification of targets for therapeutic agents to treat airway diseases such as persistant pulmonary hypertension (elevated blood pressure) in premature babies, pulmonary fibrosis where cells are damaged from oxidative stress, and acute respiratory distress.
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