Immunohistochemical localization of napsin and its potential role in protein catabolism in renal proximal tubules.

In a previous in situ hybridization study, we demonstrated the mRNA expression of napsin, an aspartic protease of the pepsin family, in the kidney, lung, and lymphoid organs of mice. However, findings on the cellular localization of napsin at the protein level are controversial, and no information on the subcellular localization is available. The present immunohistochemical study revealed the cellular and subcellular localization of napsin in mice and rats, and also analyzed the influences of chemical-induced proteinuria on the renal expression of this enzyme in rats. Immunohistochemistry using a polyclonal antibody against mouse napsin showed that napsin immunoreactivity was noticeable in lysosomes of renal proximal tubule cells and in lamellar bodies of pulmonary type II alveolar cells. In the lung, immunoreactivity was also found in lysosomes of alveolar macrophages and on the surface of type I alveolar cells; the immunoreactivities in these cells may be due to the uptake and adhesion of napsin secreted from type II alveolar cells, since they did not express napsin mRNA. Conversely, immunoreactivity for napsin was undetectable in B lymphocytes with intense mRNA expression. In puromycin- or doxorubicin-induced proteinuria, napsin mRNA expression was markedly elevated in renal proximal tubules, showing characteristic distribution patterns. Immunostaining of kidneys with proteinuria showed intense immunoreactivity for napsin in congested and enlarged lysosomes, called protein absorption droplets. These results indicate that napsin functions as a lysosomal protease and is involved in protein catabolism in renal proximal tubules.