Abstract 1303: In vivo analysis of EGFR family signalling as a bypass mechanism in prostate cancer
Authors: Jathal Maitreyee K. , Steele Thomas M. , Siddiqui Salma , Mooso Benjamin A. , D’Abronzo Leandro S. , Drake Christiana M. , Ghosh Paramita M. 
Published: July 2016

Background: Prostate cancers (PCa) rely on androgenic ligands and the androgen receptor (AR) for their growth and survival, making AR inhibition a predominant therapeutic strategy for these tumors. Some prostate tumors however fail this therapy due to ‘bypass’ mechanisms that emerge as a result of prolonged AR targeting. This in vivo study attempted to assess the expression and activation of the epidermal growth factor receptor (EGFR) family (whose role is well-documented in PCa) in response to androgen deprivation therapy (ADT).

Methods: Nude mice were implanted (s.c.) with CWR22 tumors (human-patient-derived, androgen-dependent ‘AD’) and its castration-resistant (‘CR’) subline CWR22-Rv1 (relapsed CWR22). Androgen deprivation (i.e. AR inhibition) was achieved by surgical or ‘sham’ castration of mice. Tumors were analyzed (immunohistochemistry/immunoblot) for EGFR/ErbB2/ErbB3/AR proteins and proliferative/apoptotic markers.

Results: Castration caused significant tumor regression in AD but not CR tumors. in vitro viability assays demonstrated that castration (mimicked by using charcoal-stripped serum, ‘css’) did not slow down CR cells to the same degree as it did AD cells. At baseline, intratumoral EGFR protein was unchanged in R22 tumors, ErbB2 levels decreased and ErbB3 protein increased in Rv1 tumors. Castration increased ErbB3 but not EGFR or ErbB2 proteins in CWR22 tumors. Phosphorylated forms of these receptors were generally difficult to detect but there was more phosphorylated ErbB3 protein in Rv1 tumors. Downstream of the EGFR family, there was less phosphorylated Erk but not Akt protein in CWR22-Rv1 tumors. Castration decreased Erk protein in AD tumors but increased it in CR tumors. Immunohistochemical quantification revealed that cytoplasmic EGFR and ErbB3 proteins were elevated in CR tumors but reduced in AD tumors. Castration greatly decreased Ki-67 staining in AD but not in CR tumors while the number of TUNEL-positive nuclei and intensity of PARP staining decreased in castrated CR but not in AD tumors. ErbB3 and AR proteins were significantly correlated with DNA damage and proliferation in CWR22 tumors but only nuclear AR levels and proliferation were significantly correlated in CR tumors.

Conclusions: We conclude that androgen deprivation therapy may alter EGFR and ErbB3 protein levels and localization in androgen-dependent and castration-resistant tumors. The EGFR family is typically activated at the cell surface hence their presence and activity there, in response to castration, may initiate signalling pathways encouraging tumor cell proliferation and survival.

Citation Format: Maitreyee K. Jathal, Thomas M. Steele, Salma Siddiqui, Benjamin A. Mooso, Leandro S. D’Abronzo, Christiana M. Drake, Paramita M. Ghosh. In vivo analysis of EGFR family signalling as a bypass mechanism in prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1303.