Abstract P4-08-03: Operation of eIF4A1-PD-L1 axis in therapy-naïve and drug-resistant TNBC
Authors: Boring Andrew , Dulal Dharmindra , Raman Dayanidhi 
Published: March 2023

Abstract
Operation of eIF4A1-PD-L1 axis in therapy-naïve and drug-resistant TNBC Background and Premise: Triple-negative breast cancer (TNBC) is an aggressive, metastatic disease with high mortality. The standard-of-care neoadjuvant chemotherapy (NACT) in TNBC shows an initial response but followed by an increase in tumor relapse and distant metastases. Furthermore, metastatic TNBC (mTNBC) patients often develop resistance to NACT and targeted therapies. Immunotherapy with therapeutic antibodies such as anti-programmed death ligand-1 (PD-L1) has shown efficacy in treating a subset of TNBC patients, but currently only 20% of patients qualify for that treatment. Hence, there is an unmet need for developing a precision approach against novel actionable targets along with immunotherapy to treat mTNBC. Eukaryotic initiation factor 4A1 (eIF4A1) is an integral part of the translational machinery for many oncogenic mRNAs including survivin, c-MYC, Rho kinase1, and Cyclins D1 and D3, which all require the helicase activity of eIF4A1 for their translation. Our lab previously established that the eIF4A1 is a vulnerable target in breast cancer stem-like cells (BCSCs) and bulk tumor cells. We further demonstrated a key role for eIF4A1 in cancer stemness and drug resistance. Interestingly, we also observed a reduction in levels of PD-L1 when eIF4A1 is pharmacologically inhibited or genetically ablated. Many downstream effectors of eIF4A1 such as c-MYC, STAT1 and ARF6 (involved in recycling of PD-L1 back to the plasma membrane) are indeed upstream regulators of PD-L1 gene expression and cell surface expression of PD-L1. In this study, we examine human TNBC biospecimens for total eIF4A1 level, levels of STAT1, c-MYC, ARF6 and PD-L1. This will help establish the differential operation of the eIF4A1-PD-L1 axis in therapy-naïve and drug-resistant primary and mTNBC. Approach: Human biospecimens were obtained from CHTN NIH repositories and now we have a collection of more than 100 TNBC tumor samples as well as some lymph node, lung and brain metastases and one sample of male breast primary tumor. These specimens are analyzed for role of eIF4A1 in translation of oncogenic mRNAs that contribute to the gene expression of immune checkpoint PD-L1 and other emerging immune checkpoint markers such as VISTA, TIGIT, and LAG3. To mimic the inclusion criteria for current PD-L1 monoclonal antibody treatment, TNBC tumors are examined for PD-L1 expression via immunohistochemistry after subjecting the samples to deglycosylation for effective detection and also by immunoblotting. Results: The levels of total eIF4A1 from 16 matched, therapy naive TNBC tissue were compared to adjacent normal breast tissue from the same patient via immunoblotting. Total eIF4A1 levels were found to be significantly elevated in TNBC compared to adjacent normal tissue. The downstream effectors of eIF4A1 such as STAT1 and ARF6 were also significantly elevated indicating a robust helicase activity of eIF4A1 in tumor samples. Importantly, PD-L1 levels was found to be elevated. Additionally, the total eIF4A1 level from tumor lysates from TNBC patients that had received a variety of NACT (i.e., patients with demonstrated drug resistance) were compared against normal untreated breast tissue from volunteers who underwent reduction mammoplasty by immunoblotting. The total eIF4A1 was found to be elevated in drug-resistant TNBC specimens compared to control, therapy-naïve TNBC samples. Conclusion: The eIF4A1-STAT1-PD-L1 axis is highly active in TNBC especially in drug-resistant situations. Currently, we are evaluating emerging immune checkpoints such as VISTA, TIGIT, and LAG3.
Citation Format: Andrew Boring, Dharmindra Dulal, Dayanidhi Raman. Operation of eIF4A1-PD-L1 axis in therapy-naïve and drug-resistant TNBC [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-08-03.