Abstract P5-04-04: Cutting the ties that bind: Targeting chaperonin-containing T-complex (CCT) for therapeutic intervention in the treatment of advanced stage breast cancer

Abstract

Metastatic disease is a principal cause of death from breast cancer. This is due in part to the development of resistance to current therapeutics and the often debilitating side effects that impair quality of life. The challenge is to therapeutically target an essential physiological function of cancer cells not found in normal, non-transformed cells. To this end, we discovered CT20p, a therapeutic peptide that causes cancer-specific death in human breast tumor cells and tumor regression in xenograft models of breast cancer. Using a proteomics approach, we found that CT20p directly binds to multiple subunits of a type II chaperonin called chaperonin containing T-complex or CCT. CCT forms a large macromolecular complex composed of 8 subunits (CCTα, CCTβ, CCTγ, CCTδ, CCTϵ, CCTζ, CCTη, CCTω). Each subunit contains an ATP binding site as well as substrate binding sites. Inhibition of CCT by CT20p depletes the pool of native actin and tubulin (obligate clients of CCT), impairing the polymerization of cytoskeletal elements needed to support cell adhesion and motility. As a result cancer cells lose the ability to migrate and die from loss of substrate survival signals. We found that expression levels of CCT varied among different triple negative breast cancer (TNBC) cell lines, with the highest expression occurring in those of the mesenchymal stem-like (MSL) subtype with metastatic potential. Lowest levels of CCT were found in normal breast epithelial cells. Sensitivity to killing by CT20p thus correlated with levels of CCT, with cancer cells expressing high amounts of CCT being the most susceptible. Using tissue microarrays (TMAs) of breast cancer progression, we developed an immunohistochemistry staining procedure for CCTβ. Results were interpreted on a scale of 1 to 4 (with 4 being the strongest staining). CCTβ expression was statistically higher in invasive ductal carcinoma (IDC) as compared to normal and cancer adjacent tissue (CAT) (p<0.0001). Within the types of IDC, CCTβ was highest in tumors that exceeded 5 cm across (T3), grew in chest wall or skin and in inflammatory breast cancer (T4) (p<0.05). Examining CCTβ levels in different molecular types showed little correlation with estrogen receptor (ER) positivity but strong correlation with ER and progesterone receptor (PR) positivity or PR alone (p>0.001). Statistical correlations were also observed with Her2 positivity (p>0.05). However, no statistically significant correlations were observed with TNBC tissues, with CCTβ staining ranging from the strongest staining (4) to lowest (1). These results are similar to that observed with the TNBC cell lines and indicate that CCT expression may reflect the heterogeneity of TNBC. These results suggest that CCT is a promising target for therapeutic intervention due to its increased expression in advanced stage breast cancer, independence of molecular identity and dependence by cancer cells to support essential cytoskeletal changes associated with the metastatic stem-like phenotype. Citation Format: Khaled AS, Vishnubhotla P, Khaled AR, Bassiouni R. Cutting the ties that bind: Targeting chaperonin-containing T-complex (CCT) for therapeutic intervention in the treatment of advanced stage breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-04-04.