Proteolysis targeting chimeras (PROTACs) are bivalent molecules that bring a cellular protein to a ubiquitin ligase E3 for ubiquitylation and subsequent degradation.Although PROTAC has emerged as a promising therapeutic means for cancers as it rewires the ubiquitin pathway to destroy key cancer regulators, the degradation signals/pathwaysfor PROTACs remain underdeveloped.Here we append single amino acids, the simplest degradation signal, to a ligand specific for estrogen related receptor a (ERRa) and demonstrate their utility in ERRa knockdownviathe N-end rule pathway and also their efficiency inthe growth inhibition of breast cancer cells. The modular designdescribedoffers unique advantages including smaller molecular size with shortest degradation sequences and degradation speed modulation with different amino acids.Our study expands the repertoire of limited ubiquitin pathways currently available for PROTACs and could be easily adapted for broad use in targeted protein degradation.

Most patients with estrogen receptor alpha–positive (ER+) breast cancers initially respond to treatment but eventually develop therapy resistance with disease progression. Overexpression of oncogenic ER coregulators, including proline, glutamic acid, and leucine-rich protein 1 (PELP1), are implicated in breast cancer progression. The lack of small molecules that inhibits PELP1 represents a major knowledge gap. Here, using a yeast-two-hybrid screen, we identified novel peptide inhibitors of PELP1 (PIP). Biochemical assays demonstrated that one of these peptides, PIP1, directly interacted with PELP1 to block PELP1 oncogenic functions. Computational modeling of PIP1 revealed key residues contributing to its activity and facilitated the development of a small-molecule inhibitor of PELP1, SMIP34, and further analyses confirmed that SMIP34 directly bound to PELP1. In breast cancer cells, SMIP34 reduced cell growth in a dose-dependent manner. SMIP34 inhibited proliferation of not only wild-type (WT) but also mutant (MT) ER+ and therapy-resistant breast cancer cells, in part by inducing PELP1 degradation via the proteasome pathway. RNA sequencing analyses showed that SMIP34 treatment altered the expression of genes associated with estrogen response, cell cycle, and apoptosis pathways. In cell line–derived and patient-derived xenografts of both WT and MT ER+ breast cancer models, SMIP34 reduced proliferation and significantly suppressed tumor progression. Collectively, these results demonstrate SMIP34 as a first-in-class inhibitor of oncogenic PELP1 signaling in advanced breast cancer. Significance: Development of a novel inhibitor of oncogenic PELP1 provides potential therapeutic avenues for treating therapy-resistant, advanced ER+ breast cancer.