Neutral endopeptidase (EC 3.4.24.11) is highly expressed on osteoblastic cells and other marrow stromal cell types.

A series of cultured mouse marrow stromal cell lines of different phenotype characteristics were examined for their neutral endopeptidase (NEP) activity using the highly selective chromophoric substrate of enzyme, 3-carboxypropanoyl-alanyl-alanyl-leucine 4-nitroanilide. All the cell lines tested contained appreciable amounts of NEP activity, but the cells expressing an osteoblastic phenotype, MBA-15, showed the high levels. Other non-stromal osteoblastic cell lines, MC-3T3-E1, ROS 17/2.8 cells derived from mouse long bone explants, were also rich in NEP. A four-fold stimulation of NEP activity was observed when certain MBA-15 clones were cultured in the presence of 1 alpha,25-dihydroxyvitamin D3. Since NEP is an effective cell-surface endopeptidase, it may play a role in the dynamics of bone formation, via interaction with biologically active polypeptides.