Expounding Maspin and IRF6: identification and characterization of a novel serpin partnership

Maspin (Mammary Serine Protease Inhibitor) was first reported in 1994 as a serpin with tumor suppressive properties. Maspin was initially isolated through subtractive hybridization and differential display analysis as a 42kDa protein that is expressed in normal mammary epithelial cells but reduced or absent in breast carcinomas. Further research led to maspin’s characterization as a class II tumor suppressor based on its ability to inhibit cell invasion, promote apoptosis and inhibit angiogenesis. Since then, efforts have been made to characterize maspin’s tumor suppressive mechanisms. In particular, researchers have studied maspin localization, the regulation of maspin expression, and more recently, maspin protein interactions. We employed a maspinbaited yeast two-hybrid system and subsequently identified Interferon Regulatory Factor 6 (IRF6) as a maspin-binding protein. Whereas many of the IRF family members have been well characterized, IRF6 remains poorly understood. In this dissertation, we elucidate some of the complex mechanisms involved in maspin activity, specifically relating to IRF6 regulation and function. We have examined the expression of IRF6 in breast cancer cells and we show that, similar to maspin, IRF6 is reduced or absent in breast carcinomas. We further show that the re-expression of IRF6 in breast cancer cells results in genotypic and phenotypic changes which can be abrogated in the presence of maspin. We identify ERK1/2 as a kinase involved in IRF6 phosphorylation, and we demonstrate a possible role for toll-like receptor signaling in the activation of IRF6. We also evaluate the differential expression of maspin and IRF6 during murine mammary gland development and we show that both maspin and IRF6 are maximally expressed during lactation. These studies have increased our understanding of the complex, pleiotropic nature of maspin and provide an avenue to develop maspin’s potential as a diagnostic marker for cancer progression and as a potentially powerful therapeutic agent in the fight against breast cancer.