P4-01-01: Preclinical and Clinical Studies of Estrogen Deprivation Support the PDGF/Abl Pathway as a Novel Therapeutic Target for Overcoming Resistance.

Abstract

Abstract Aim: To determine the relevance of PDGF/Abl signaling pathway as a therapeutic target in endocrine resistance in vitro and in vivo. Rationale: Targeting estrogenic stimulation reduces mortality from ER-positive (ER+) breast cancer (BC) but resistance remains a major clinical problem. To identify the molecular mechanisms associated with resistance to estrogen-deprivation, we previously assessed the temporal changes in gene expression during adaptation to long-term culture of MCF7 human breast cancer cells in the absence of estradiol (E2) (LTED), modeling resistance to an aromatase inhibitor (AI). Platelet-derived growth factor (PDGF)/Abl signaling was the top adaptive pathway at the point of resistance (p=1.15 E-07), but there is limited evidence for it playing a role clinically. Methods: Gene expression data from postmenopausal women with ER+ BC at pre-treatment and at 2-week on-treatment with neoadjuvant anastrozole were interrogated. Immunohistochemical staining using antibodies against PDGFRα, β and Abl was assessed in paired clinical specimen of 45 patients who had received an AI and presented progressive disease. Proliferation assays, immunoblots and ChIP assays were carried out using either the PDGFR/Abl inhibitor nilotinib or siRNA knockdowns in LTED and wild-type MCF7 cells. Results: In vitro: PDGFRβ and Abl expression and phosphorylation were elevated in LTED cells. Cell proliferation was decreased in LTED cells by siRNA knock-down of either PDGFRβ (50% p<0.01), Abl (40% p<0.01) or the combination (70% p<0.01). Inhibition of Abl activity using nilotinib decreased ER protein levels and suppressed ER-mediated transcription by reducing recruitment of AIB1 and CBP to the promoter of E-responsive genes TFF1 and GREB1. Clinical: Data from 81 patients treated with an AI in the neoadjuvant setting, showed increases in PDGFRβ gene expression after two weeks (1.25 fold, p=0.003). Low PDGFRβ at baseline was associated with a better response. Of note, paired pre-AI-treatment and relapse clinical specimens from a cohort of patients treated in the primary or advanced setting revealed increases in tumor protein expression of PDGFRα (1.39 fold, p=0.0065), PDGFRβ (4.32 fold, p=0.006) and Abl (1.8 fold, p=0.001) at the point of relapse. Tumor (T) and stromal (S) expression of PDGFRα as well as PDGFRβ was significantly correlated in pre-treatment and relapse samples (Table 1). High post-treatment tumor and stromal PDGFRβ levels were associated with a short time to treatment failure (TTF) (T: Rs −0.284, p=0.066; S: Rs −0.307, p=0.046). Expression of PDGFRα in relapsing tumor specimens was correlated with Abl expression (Rs 0.342, p=0.027) and Ki67 levels (Rs 0.39, p=0.01). Furthermore, changes in Abl correlated significantly with changes in ER expression (Rs 0.307, p=0.048). Discussion: These in vitro and clinical data support a biological interaction between PDGF/Abl and ER-signaling and suggest that the PDGF signaling pathway warrants clinical evaluation as a therapeutic target in endocrine resistant breast cancer. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-01-01.