Delineating the functional role of the tumor suppressor protein PAR-4 in breast cancer

• Applicant: Dr. Jörg Hartkamp
• Subject Area: Cell Biology ; Biochemistry
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 288871592

Most deaths from breast cancer result from disease relapse following treatment of the primary tumor. Because breast cancer recurrence is typically an incurable disease, understanding the mechanisms by which residual cancer cells survive treatment is required to improve therapies for breast cancer patients following disease relapse. Recent evidence implicates down-regulation of the tumor suppressor protein prostate apoptosis response-4 (PAR-4) as a critical step for breast tumor recurrence. Down-regulation of PAR-4 allows tumor cells to survive tumor regression following targeted therapy and chemotherapy and is both a necessary and sufficient step for tumor recurrence. Therefore, strategies that restore PAR-4 expression could pay therapeutic dividends but the mechanisms or signaling pathways that regulate PAR-4 function or expression are ill characterized. Recent advances in analyzing apoptotic signaling networks have uncovered an important role for activation of caspase-8 following DNA damage caused by genotoxic drugs. Our own published and unpublished data demonstrate that PAR-4 functions downstream of caspase-8 by cleavage-induced nuclear translocation of the C-terminal part. Therefore, we aim to analyze the mechanism leading to caspase-8 activation following DNA damage in breast cancer and subsequent PAR-4 activation. Moreover, loss of PAR-4 in a defined subset of breast cancer cells mediates resistance to DNA damage-induced apoptosis. In this proposal we aim to identify the downstream PAR-4 target(s), which mediate and are required for PAR-4s pro-apoptotic function in breast cancer using mass spectrometry approaches. Low PAR-4 expression promotes tumor cell survival following therapy but the pathways controlling PAR-4 expression are not identified. Using an unbiased mass spectrometry approach we were able to identify nine physical PAR-4 interaction partners out of which two belong to the family of E3 ubiquitin-protein ligases (MYCBP2 and UBR5), and one to the family of deubiquitylases (USP7/HAUSP). Our first experiments suggest that USP7/HAUSP deubiquitylates PAR-4 and thereby stabilizes the protein, indicating that PAR-4 protein stability might be regulated through ubiquitylation. We speculate that the down-regulation of PAR-4 in chemo-resistant breast cancer might be directly linked to aberrant ubiquitylation signaling and we will investigate the capacity of our identified candidates to regulate PAR-4 expression. Therapies that restore PAR-4 expression may hold significant clinical promise for relapsing breast cancer patients and our research aims to identify the biochemical mechanisms that control PAR-4, which could be exploited for drug targeting.

• DFG Programme: Research Grants