Abstract P6-07-04: Targeting ErbB2 with human PEPD
Authors: Yang Lu , Li Yun , Bhattacharya Arup , Zhang Yuesheng 
Published: May 2015

Abstract
ErbB2, also known as Her2 or Neu, belongs to the ErbB family of plasma membrane-bound receptor tyrosine kinases, which also include ErbB1, ErbB3 and ErbB4. ErbB2 is best known for its involvement in human breast cancer. ErbB2 gene amplification occurs in ∼20% of breast cancer, and ErbB2 amplification or overexpression is a strong predictor of poor disease prognosis. ErbB2-targeted therapies, particularly humanized monoclonal antibody trastuzumab (Ttzm) in combination with chemotherapy, have shown considerable clinical efficacy. However, primary and secondary resistance remains a clinical challenge, and Ttzm, produced in mammalian cells, is very expensive.
We have found that human prolidase, also known as peptidase D (PEPD) among several other names, binds to ErbB2 with high affinity (Kd = ∼7 nM) and binds as a homodimer (493 amino acids per subunit) to subdomain 3 in the extracellular domain of ErbB2. Each monomer of PEPD binds to one copy of ErbB2. However, PEPD is a weak ErbB1 binder (Kd = ∼5 μM) and does not bind to ErbB3 or ErbB4. PEPD is the first-ever natural ligand of ErbB2, and unlike the other ligands of ErbB receptors, it is devoid of an EGF motif. PEPD has been long known to hydrolyze dipeptides with proline or hydroxylproline at the carboxy terminus, but the dipeptidase activity of PEPD is not involved in ErbB2/ErbB1 modulation. In cells overexpressing ErbB2, where both activated dimers and inactive monomers of ErbB2 exist, as ErbB2 overexpression causes spontaneous dimerization, auto-tyrosine phosphorylation and recruitment and activation of downstream signals, PEPD rapidly binds to ErbB2 homodimers (&lt;∼10 min) and silences the existing ErbB2-SRC signaling, a key oncogenic pathway of ErbB2, by disrupting SRC association with ErbB2. In contrast, PEPD binds to ErbB2 monomers relatively slowly (&gt;∼30 min), but this binding causes ErbB2 dimerization, ErbB2 phosphorylation and downstream signaling. PEPD binding to ErbB2 subsequently causes pronounced ErbB2 depletion, resulting from its internalization and degradation. PEPD also strongly inhibits the DNA synthesis, anchorage-independent growth and invasion of cells that overexpress ErbB2, but has no effect on cells without overexpression of ErbB2. In fact, cells become sensitized to inhibition by PEPD upon achieving stable ErbB2 overexpression. Thus, the overall impact of PEPD on ErbB2 is inhibitory, and PEPD targets cells addicted to ErbB2. In ErbB2-overexpressing cells, at equimolar concentrations, PEPD was more effective than Ttzm in driving ErbB2 depletion, but is weaker than Ttzm in stimulating ErbB2 phosphorylation. In mouse tumor models, PEPD administered by intraperitoneal injection (Monday, Wednesday, Friday) at 0.2-2 mg/kg body weight strongly inhibited the growth of ErbB2-overexpressing tumors, but had no impact on tumors without ErbB2 overexpression, and the PEPD-treated mice showed no adverse effects.
Given that the findings described above were made using human PEPD generated in bacteria, there is a distinct possibility that recombinant human PEPD may be a low cost alternative to Ttzm. Further investigation of the antitumor activity of PEPD and its modulation of ErbB2 signaling is warranted.
Citation Format: Lu Yang, Yun Li, Arup Bhattacharya, Yuesheng Zhang. Targeting ErbB2 with human PEPD [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-07-04.