Maspin (SERPINB5) is an obligate intracellular serpin.

Maspin (SERPINB5) is a tumor suppressor lost in breast and prostate cancer whose molecular function is unknown. It is a non-inhibitory member of the clade B serpins suggested to play a role in a plethora of intracellular and extracellular settings, yet its normal cellular distribution has never been clarified. Here we investigate the distribution of maspin in non-transformed human epithelial cells. By indirect immunofluorescence, maspin has a nucleocytoplasmic distribution in breast (MCF10A) and prostate (RWPE-1) cells and, by immunoblotting and pulse-chase analyses, is neither glycosylated nor secreted. Cell surface biotinylation studies also show that maspin is not present at the cell surface. Differentiation of MCF10A cells into three-dimensional acini results in the redistribution of maspin from the nucleus to the cytoplasm but does not result in secretion. Addition of an efficient conventional signal peptide to maspin directs it into the secretory pathway and results in glycosylation but not secretion. We further show that maspin in the cytoplasm of MCF10A cells is a soluble monomeric protein that is not detectably associated with the cytoskeleton or other extractable components. Taken together, these results suggest that maspin is restricted to an intracellular, possibly nuclear, role in which it influences cell-matrix interactions indirectly. It is probably released only as a consequence of cell damage or necrosis.

Maspin is not required for embryonic development or tumour suppression.

Maspin (SERPINB5) is accepted as an important tumour suppressor lost in many cancers. Consistent with a critical role in development or differentiation maspin knockout mice die during early embryogenesis, yet clinical data conflict on the prognostic utility of maspin expression. Here to reconcile these findings we made conditional knockout mice. Surprisingly, maspin knockout embryos develop into overtly normal animals. Contrary to original reports, maspin re-expression does not inhibit tumour growth or metastasis in vivo, or influence cell migration, invasion or survival in vitro. Bioinformatic analyses reveal that maspin is not commonly under-expressed in cancer, and that perturbation of genes near maspin may in fact explain poor survival in certain patient cohorts with low maspin expression.

A versatile monoclonal antibody specific to human SERPINB5.

Maspin (SERPINB5) is a member of the Clade B subgroup of the large superfamily of serine protease inhibitors. It is proposed that maspin is a tumor suppressor; however, its molecular role remains to be elucidated. Here we report the characterization of a mouse monoclonal antibody directed against human maspin. This antibody, 16F7, recognizes maspin in both its native and denatured form, unlike several other commercial antibodies tested in this study. It will be a useful and versatile tool for future analyses of the biological function of maspin.