In situ localization and regulation of thromboxane A2 synthase in normal and LPS-primed lungs

Abstract

Thromboxane (Tx) A2 synthase catalyzes the conversion of prostaglandin H2 to the unstable metabolite TxA2, which is a potent mediator of vasoconstriction and bronchoconstriction. The cellular localization of TxA2 synthase was examined by immunohistochemistry and in situ hybridization in human and rat lung tissues. Bronchial epithelial cells, bronchial smooth muscle cells, peribronchial nerve fibers, single cells of bronchus-associated lymphoid tissue, single cells located in the alveolar septum, and alveolar macrophages exhibited positive immunostaining for TxA2 synthase protein in lung tissue of both species. In addition, vascular smooth muscle cells of muscular and partially muscular vessels displayed strong (rat) and moderate (human) immunostaining for TxA2 synthase. In situ hybridization performed in the rat lungs demonstrated TxA2synthase mRNA localization in accordance with the immunostaining pattern. Perfusing isolated rat lungs with endotoxin for 1 and 2 h resulted in a marked increase in TxA2 synthase protein staining intensity in most cell types as measured by quantitative image analysis, whereas the in situ hybridization signal was unchanged. We conclude that the pulmonary distribution of TxA2 synthase displays close similarity between rat and human lung tissues and matches well with the previously described immunolocalization of cyclooxygenase-1 and cyclooxygenase-2 in this tissue. Endotoxin challenge is suggested to cause a rapid upregulation of TxA2 synthase at the posttranscriptional level. These data provide a morphological basis for the understanding of the role of TxA2 in the regulation of lung bronchial and vascular tone and in immunologic events.