Abstract P4-05-11: A proteasomal signature of breast cancer cells

Abstract

Abstract The multisubunit, multifunctional protease called proteasome as a part of the essential proteasome-ubiquitin pathway constitutes an attractive target for anti-cancer drugs. A proteasome inhibitor bortezomib is successful in blood cancers treatment, however trials with breast cancers yield disappointingly inconsistent results, likely stemming from the cancers’ diversity. Since bortezomib preferentially induces an apoptosis in cancerous cells, it is likely that differences in the proteasome pathway between the control and cancerous cells, and also between distinct cancers, contribute to this effect. Here we compared components of proteasome assemblies and their peptidase activities in seven breast cancer and two control cell lines in an attempt to find a common proteasomal signature of breast cancer. Defining a common proteolytic denominator for breast cancer cells should facilitate a rational drug design based on proteasomal inhibition. For this purpose, we analyzed cytosolic, nuclear and microsomal extracts from human cultured breast cells: control (MCF10A, Hs 578 Bst) and derived from distinct types of breast cancer: luminal A MCF7 and T47D, triple negative basal A MDA-MB-468, and triple negative basal B MDA-MB-231, Hs 578 T and BT-549. We found a high content of total proteasome in a microsomal fraction with a relatively prominent contribution from immuno proteasomes in all cancer cell lines. These proteasomes are believed to be localized on the outside of endoplasmic reticulum membrane and presumably take part in the endoplasmic reticulum - associated degradation system. Markedly, the pattern of peptidase activities of proteasome and content of subunits of its exchangeable sub-assemblies were specific for each fraction and discriminated between cancerous and control cells. Statistical analysis identified two clusters formed by the cell lines in the microsomal and nuclear fractions and a distinct clustering of the cytosols. Furthermore, three groups of variables, which tend to always cluster together were identified: the 19S regulatory particle subunits, immunoproteasome subunits and peptidolytic activities. Variables that the best quantitatively discriminate between the cell lines at a given level of total proteasome were immunoproteasome subunits and 19S cap subunit Rpt5. Additionally, the housekeeping subunit beta5 in the cytosol, the PA28 activator gamma subunit in the microsomal fraction and chymotrypsin like activity in the nuclear fraction qualitatively were the best markers aiding classification of the cell lines. We speculate that the distinct properties of proteasomes in the microsomal and nuclear fractions may contribute to specific response of the cancer cell lines to the treatment with proteasomal inhibitors. The study shows that a comprehensive analysis of the proteasome system in the established breast cancer cell lines may help to guide diagnostics and therapy design. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-05-11.