| Napsin A is a lysosomal and lamellar body–associated aspartic protease of the pepsin family encoded by NAPSA and is enriched in type II pneumocytes and renal proximal tubular epithelium, where it links tissue‑specific protein processing to lung surfactant homeostasis and epithelial proteolysis. The protein is synthesized as a glycosylated zymogen bearing an N‑terminal signal peptide, a propeptide that maintains the inactive state, and a C‑terminal catalytic domain with the conserved Asp‑Thr/Ser‑Gly motifs characteristic of aspartic proteases, and proteolytic removal of the propeptide in acidic compartments yields the active enzyme. Localisation to lamellar bodies and lysosomes in type II pneumocytes positions Napsin A to cleave prosurfactant protein B and related precursors, contributing to generation of mature surfactant protein B that is essential for the structure and function of pulmonary surfactant and thereby for alveolar stability and efficient gas exchange. In renal tubules, Napsin A participates in lysosomal protein catabolism and endocytic cargo degradation, integrating it into epithelial proteostasis and tubular protein handling, although detailed substrates in the kidney remain less well defined. Expression surveys show strong cytoplasmic granular Napsin A staining in normal lung type II pneumocytes and renal tubular epithelium, with weaker or focal expression in alveolar macrophages and limited expression in spleen and other tissues, establishing a characteristic lineage‑restricted profile. In tumor pathology, Napsin A is detected in the majority of primary lung adenocarcinomas, particularly those with terminal respiratory unit differentiation, and is largely absent from lung squamous cell carcinoma and small‑cell lung carcinoma, making it a highly informative marker for confirming primary lung adenocarcinoma when interpreted alongside TTF‑1 and cytokeratin profiles. Napsin A also appears in a substantial fraction of renal papillary and clear cell carcinomas and in subsets of ovarian and endometrial clear cell carcinomas, with much lower frequencies in breast, gastrointestinal, hepatobiliary, and other adenocarcinomas, so its expression pattern supports differential diagnosis and tumor origin assignment in metastatic settings when integrated into an immunohistochemical panel. |
Lane 1: NCI-H441
