Localisation and function of mammary serine protease inhibitor (maspin)

Maspin (SERPINB5), a member of the clade B subgroup of the serpin superfamily, is reported to function as a tumour suppressor, but its mechanism remains to be elucidated, as both extracellular and intracellular functions have been proposed. Members of the clade B serpin lack a classical secretory peptide and thus, are predominantly intracellular and nucleocytoplasmic. Despite this, numerous studies still suggest an extracellular mechanism of function for maspin without providing evidence of its release from cells. In addition, proposed mechanisms of function for maspin are based on single cell clones of cancer cells engineered to overexpress maspin, which leaves open the possibility that these observations may be due to clonal artefacts. Taken together with the contrasting results from epidemiological reports in various cancers regarding the prognostic utility of maspin, these observations suggest that maspin may not function as a tumour suppressor. This thesis shows, using immortalised, non-transformed human mammary epithelial cells naturally expressing maspin, that maspin is an obligate intracellular serpin and cannot be secreted even when provided with a conventional signal sequence. Maspin has a nucleocytoplasmic localisation, but redistributes to the cytosol following differentiation of the epithelial cells. Furthermore, based on studies using whole populations of transduced, but uncloned, breast cancer cells, this study also shows that maspin re-expression does not inhibit cancer cell invasion, migration and clonogenicity in vitro, or primary tumour growth and metastasis in vivo. Hence, maspin is unlikely to be a tumour suppressor.