Abstract P2-11-14: Endocrine agents promote adverse responses in ER+ breast cancer cells through a mechanism involving PELP1
Authors: Rees Michael , Smith Chris , Barrett-Lee Peter , Hiscox Steve 
Published: March 2020

Abstract
Introduction: Endocrine therapy is the mainstay of treatment for patients with ER+ breast cancer. However, not all patients respond equally leading and a significant proportion will develop resistance with associated disease progression and poor prognosis. Intriguingly, we have shown that endocrine agents themselves may induce an invasive phenotype in ER+ breast cancers that have low/absent expression of the cell adhesion protein, E-cadherin. Here we investigate this phenomenon further and provide data supporting a role for the ER co-receptor PELP-1, in mediating an adverse response to endocrine agents. Methods: The effects of 4-hydroxy tamoxifen (‘tam’), fulvestrant (‘fas’) and oestrogen withdrawal (‘-E2’; used to model aromatase inhibitor therapy) on the invasive and migratory nature of endocrine-sensitive MCF-7 and T47D cells, in the presence or absence of functional E-cadherin and/or PELP-1 (using siRNA knockdown), was assessed via Matrigel invasion and Boyden chamber migration assays. The effects of these agents ± E-cadherin/PELP-1 modulation on cell proliferation were determined using cell counting and Ki67 proliferation assays. Western blotting using phospho-specific antibodies was performed to investigate signalling pathway changes associated with endocrine-induced changes in invasion and migration. Results: Both tam and fas induced invasive and migratory behaviour in both ER+ breast cancer cell lines; these cells also displayed a high basal expression of PELP-1 (mean fold increase in invasion with endocrine treatment was 3.1±1.2; p=0.002 [MCF7+tam] and 2.7±1.4; p=0.002 [MCF7+fas]. Mean fold increase in invasion with endocrine treatment + E-cadherin siRNA was 22.4±5., p&lt;0.001 [MCF7+tam] and 18.6±2.2 [MCF7+fas]).PELP-1 expression was further enhanced following the suppression of E-cadherin and E-cadherin deficient cells exhibited a further augmentation of invasion and migration following endocrine treatment. Endocrine-induced invasion/migration occurred in a Src-dependent manner with Src inhibition reversing this behaviour (mean fold increase in invasion with endocrine treatment + E-cadherin siRNA in the absence of Src activity was 1.3±0.5, p&lt;0.001 [MCF7+tam] and 0.8±0.5; p&lt;0.001 [MCF7+fas]). siRNA-mediated suppression of PELP1 also reduced endocrine treatment-induced invasion and migration to a similar extent as Src suppression. In contrast to tam and fas, -E2 did not induce an invasive or migratory phenotype irrespective of E-cadherin or PELP1 expression level. Conclusion: Our data points to the ability of anti-oestrogens to promote an aggressive phenotype in ER-positive breast cancer models expressing PELP-1, which is further augmented in the absence of homotypic cell-cell contact. Identification of biological markers that predict response to treatment are valuable when deciding on an optimum choice of endocrine therapy. Our data suggests that E-cadherin and PELP-1 may help guide such decisions in the future, where high PELP-1 / low E-cadherin expression may benefit from oestrogen withdrawal therapy via aromatase inhibition, instead of direct ER modulation/antagonism.
Citation Format: Michael Rees, Chris Smith, Peter Barrett-Lee, Steve Hiscox. Endocrine agents promote adverse responses in ER+ breast cancer cells through a mechanism involving PELP1 [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-11-14.