Cellular and subcellular localization of a binding protein for polychlorinated biphenyls in rat lung

Enriched cell populations from rat lung were isolated by use of elutriation. An in vitro ligand binding assay as well as a Western immunoblot assay were used to determine the levels of a binding protein for certain polychlorinated biphenyls (PCBs) in cytosolic preparations from these cell populations. The cell population enriched in Clara cells (30% Clara cells) contained by far the largest amount of the PCB-binding protein, 759 +/- 81 pmol/mg of cytosolic protein as judged by an in vitro ligand binding assay. Western immunoblot analysis of cytosolic preparations from the enriched cell preparations, using antibodies to the PCB-binding protein, showed levels of immunoreactive material in these fractions that corresponded to the level of the PCB-binding protein as determined by the in vitro ligand binding assay. By use of the peroxidase-antiperoxidase method of immunoperoxidase staining, antibodies to the PCB-binding protein were found to stain the Clara cells in sections of paraffin-embedded rat lungs. Intense immunoperoxidase staining of the material lining the airway epithelium was also observed. The protein was predominantly localized to the secretory granules in the apical cytoplasm of the Clara cells as determined using antibodies to the protein, protein A-gold, and electron microscopy. Previous studies have shown a selective in vivo accumulation of methylsulfonyl-PCBs in Clara cells of rodent lung and the present investigation, demonstrating the presence in the Clara cells of a secretory Mr 13,000 protein that binds methylsulfonyl-PCBs with high affinity, gives further support to the contention that the protein is an important factor in determining the in vivo disposition of these compounds.