Abstract 4116: A novel phytoalexin, glyceollins, trigger anti-proliferative and apoptotic effects in aromatase inhibitor resistant breast cancer cells

Abstract

Aromatase inhibitors (AIs) are the standard endocrine treatment for postmenopausal women with estrogen-dependent metastatic breast cancer. While effective, some patients develop resistance leading to tumor relapse, metastasis, and more aggressive phenotypes. Previous research suggested that AI resistance arises as a result of increased growth factor pathways (i.e., HER2 and EGFR) and is associated with increased cellular motility. Our lab previously demonstrated that a novel phytoalexin, glyceollins, inhibits proliferation, survival, and migration of hormone independent letrozole-resistant breast cancer cells. However, many postmenopausal women with AI-resistant tumors remain hormone dependent. Therefore, there is a need to understand distinctions between estrogen receptor positive (ER+) and ER negative (ER-) AI resistant tumors and their response to therapy. We hypothesize that treating ER+ letrozole-resistant breast cancer with a combination of glyceollin and lapatinib (a dual EGFR/HER2 inhibitor) will reduce growth factor signaling, inhibit proliferation, and induce apoptosis. T47Darom cells (T47D cells which stably express the aromatase gene) and T47DaromLR cells (letrozole-resistant T47Darom cells) were characterized by measuring protein expression. As cells transition from letrozole-sensitive to letrozole-resistance ERα and aromatase expression were slightly decreased while HER2 was increased. A receptor tyrosine kinase phospho-antibody array was performed to evaluate changes in protein expression between the two cell lines. While many proteins were altered, two cell cycle regulators (p38 and p53) were significantly downregulated by in the T47DaromLR cells suggesting that T47DaromLR cells have acquired the ability to progress uncontrollably through the cell cycle, thereby causing enhanced proliferation and cell survival. To interrogate the hypothesis, both cell lines were treated with lapatinib, glyceollin, or the combination and cell viability was measured. Results demonstrated that drug treatment had no effect on the proliferation of T47Darom cells, however, glyceollin alone and glyceollin + lapatinib inhibited T47DaromLR cell viability by 46% and 59%, respectively. Glyceollin alone or the combination caused a 33% and 60% reduction in T47Darom colony formation, respectively. Interestingly, glyceollin alone or the combination of glyceollin + lapatinib significantly inhibited T47DaromLR colony formation by greater than 90%. Apoptosis studies were conducted to measure caspase 3/7 activity in both cell lines. Lapatinib alone had no effect on apoptosis, however glyceollin and the combination significantly induced apoptosis in both cell lines; the most dramatic effect was observed in T47Darom cells, suggesting that additional mechanisms may contribute to the anti-cancer effect observed in T47DaromLR cells. Taken together, these results suggest that dual inhibition using glyceollins and lapatinib may have potential as a novel combination therapy approach for postmenopausal patients with hormone-dependent, letrozole-resistant breast cancer. This work was funded by an NIH grant awarded to SL Tilghman (1SC1GM125617). Citation Format: Rashidra R. Walker, Jankiben Patel, A. Michael Davidson, Syreeta L. Tilghman. A novel phytoalexin, glyceollins, trigger anti-proliferative and apoptotic effects in aromatase inhibitor resistant breast cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4116.

Acquisition of Letrozole Resistance Through Activation of the p38/MAPK Signaling Cascade

“…Although these cells are ER+, they are cross resistant to the selective estrogen receptor modulator tamoxifen, (3) further demonstrating their aggressive nature and limited therapeutic options. As the AI-sensitive cells transitioned to AIresistance, both HER2 and MAPK expression were induced (6). This was a similar phenomenon previously observed in the hormone-independent AI-resistant LTLT-Ca cells (2).…”

“…To study multiple proteins within this pathway and other receptor tyrosine kinase signaling cascades, the MAPK and RTK phosphoantibody arrays were utilized to measure the activation of key signaling components between the T47Darom and the T47DaromLR cells. We chose both arrays because a previous finding from our group demonstrated that HER2 expression was increased in the T47DaromLR cells (6). Although total HER2 expression was previously shown to be increased, activated ErbB2 (HER2) levels were decreased by 1.40-fold (p<0.05), suggesting this may not be a critical driver associated with ER+ AI resistance (Figure 2).…”