The cytochrome P-450 monooxygenase system of the mammalian lung is known to be associated with the microsomal subcellular fraction and has been demonstrated in two pulmonary cell types rich in endoplasmic reticulum: Clara cells and type II pneumocytes. However, analysis of ultracellular fractions, isolated cell preparations, or light microscopic immunohistochemical studies of tissue sections has permitted only limited resolution of the distribution of this enzyme system within the 40 or more cell types of the lung. Therefore, we have used the greater resolving power of transmission electron microscopy and immunogold labeling to characterize the cellular and subcellular distribution of the cytochrome P-450 system in the lung. In Lowicryl-embedded sections of lung from adult rabbits, antisera (1:10,000) against the constitutive pulmonary microsomal cytochrome P-450 monooxygenase isozymes 2 and 5 and NADPH-cytochrome P-450 reductase (anti-2, anti-5 and anti-R) bound specifically to regions known to be rich in agranular endoplasmic reticulum (AER) in the cytoplasm of Clara cells. The plasma membranes of bronchiolar Clara cells, the tips of microvillae of ciliated cells, secretory granules of goblet cells, and the cell membrane and pinocytotic vesicles of endothelial cells were all intensely labeled with anti-2 and anti-5 but not with anti-R, even at a 10-fold higher concentration. The intensity of labeling of AER in Clara cells with anti-R and anti-2, but not anti-5, appeared to correlate positively with the cellular content of secretory granules. The golgi membranes of ciliated cells were labeled intensely with anti-5 only. The plasma membrane of type II pneumocytes was not labeled by any of the antisera, but with anti-2 and anti-5 there was labeling of AER-associated vacuoles, the membranous residue of lamellar bodies, and, to some extent, mitochondria; at 1:5,000 but not 1:10,000 dilution, staining with anti-R was qualitatively similar. Type I pneumocytes, ciliated cell cytoplasm, and nuclei were essentially unlabeled. Immunoblots (Western) of tracheal homogenates yielded no evidence for epitopes other than those in microsomal fractions from whole lung. Contact blots of fresh whole trachea, before but not after lavage, bound anti-2 and anti-R. Thus, we have demonstrated for the first time that components of the pulmonary cytochrome P-450 monooxygenase, although localized in the AER-rich regions of the Clara cells and type II pneumocytes, are not restricted to these cell types or to the endoplasmic reticulum. Labeling of the endothelium, plasma membranes, and the tracheal extracellular matrix indicates the possibility of cytochrome P-450-mediated metabolism at the luminal interface in the airways and the vasculature.
|Original language||English (US)|
|Number of pages||11|
|State||Published - Jan 1 1988|
ASJC Scopus subject areas
- Molecular Medicine