Abstract PO5-23-08: Adenylosuccinate lyase is essential for proliferation and mitochondrial function of endocrine therapy resistant breast cancer cells

Abstract Eighty percent of breast cancers express estrogen receptor alpha (ERα, ESR1) at the time of diagnosis. Endocrine therapy (ET) is the standard treatment that either block estrogen mediated ER activation (such as tamoxifen, fulvestrant) or suppress estrogen synthesis (such as exemestane, letrozole, or anastrozole). While ET is initially effective, emergence of ET-resistance is common and possess a major clinical challenge. Hence, there is an ongoing need to develop new treatments for ET-resistant-ERα positive breast cancer. In this study we investigated the role of adenylosuccinate lyase (ADSL), an enzyme of the de-novo purine biosynthesis pathway as a potential target of ET-resistant breast cancer. The protein level of ADSL in two independent ET-resistant cell line model LCC9 and T47D-4HT cells were significantly higher in comparison to its parental ET-sensitive cell lines, MCF7 and T47D cells, respectively. Transient knockdown of ADSL using two independent siRNA in both LCC9 and T47D-4HT cells results in a significant decrease of cell growth, colony, and spheroid formation. Notably, reduced ADSL levels in LCC9 cells prevented the cells from progressing from G1 to S phase of cell cycle with concurrent elevation of cyclin D1/D3, CDK2/4, and cyclin E. On the other hand, in T47D-4HT cells, ADSL reduction caused cells to accumulate in S-phase of cell cycle and showed lower levels of total cyclin D1 protein. Mechanistically, in LCC9 cells (but not in T47D-4HT cells), ADSL depletion induced DNA replication stress which activates (phosphorylate) ataxia-telangiectasia-mutated-and-Rad3-related kinase (ATR) and its major downstream effector checkpoint kinase 1 (Chk1) that in turn failed to de-phosphorylate the inhibitory-phospho-groups of cyclin-dependent kinase 2 (CDK2). In addition, ADSL depletion perturbed the mitochondrial function in both LCC9 and T47D-4HT cells. Oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) was lower in both the cells along with the mitochondrial membrane potential. Pertinently, lower concentration of AICAR (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside), a product of ADSL enzyme function, partially rescued the cell growth and restored mitochondrial membrane potential in ADSL-depleted LCC9 cells. To further understand the global effects of ADSL depletion we performed RNA-sequencing of LCC9 and T47D-4HT cells after ADSL depletion and compared with respective control siRNA transfected cells followed by gene set enrichment analysis. Intriguingly, diverse effect was observed in LCC9 and T47D-4HT cells as very few genes were found to be commonly regulated between these two cell lines. Analysis of clinical data sets revealed high level of ADSL transcript was associated with adverse progression free survival and overall survival in breast cancer patients treated with endocrine therapies. Overall, our findings demonstrate that ADSL-mediated de novo purine synthesis is critical for cellular growth, proliferation, and mitochondrial function of endocrine therapy-resistant breast cancer cells. Therefore, targeting ADSL is a novel potential therapeutic approach for ERα positive ET- resistant breast cancer. Citation Format: Anil Yadav, Lu Jin, Robert Clarke, Surojeet Sengupta. Adenylosuccinate lyase is essential for proliferation and mitochondrial function of endocrine therapy resistant breast cancer cells [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-23-08.