Abstract PO3-26-10: Small molecule inhibitor of PELP1 exhibit cytotoxic effects on breast cancer cells by blocking ribosome biogenesis

Abstract Introduction: Breast cancer (BC) is the second-leading cause of cancer-related deaths in women. Despite responsive to endocrine therapy, a sizeable portion of estrogen receptor positive (ER+) BC goes on to develop therapy-resistant breast cancer (TR-BC). Furthermore, triple-negative breast cancer (TNBC) has a more aggressive clinical course with fewer targeted therapeutic options. The development of more effective therapies for TR-BC or TNBC represent an unmet need. Excessive ribosome biogenesis has recently been linked to BC progression, and resistance to therapy. Oncogene PELP1, which is required for ribosome biogenesis, is frequently dysregulated in BC and serves as a prognostic factor for poor BC survival and therapy resistance. We recently identified SMIP34 as a PELP1 small molecule inhibitor of its kind and it is effective against both TR-BC and TNBC. The objective of this study is to determine the mechanisms by which SMIP34 exhibit cytotoxic effects on BC cells. Methods: We have used multiple ER+, therapy resistant and TNBC models in cell viability, and apoptosis assays. Modelling studies were done based on Cryo-EM structure of the WDR18/PELP1 Rix1 complex (PDB code 7UWF). Biochemical assays include immune precipitation, reporter assays, GST pulldown, and Western Blotting. SMIP34 effect on protein synthesis was measured by using Western blot of puromycin treated total lysates for puromycinylated protein content. SMIP34 effect on protein synthesis assessed using Cayman’s Protein Synthesis Assay Kit. Ribosome biogenesis in SMIP34 treated cells was monitored using confocal microscopy using rpS6 antibody. Results: In in vitro cell based assays, SMIP34 was highly effective in reducing the cell viability, reducing colony formation and inducing apoptosis of ER+, endocrine therapy resistant, and TNBC model cells. PELP1 homodimer has recently been predicated to act as the core of the WDR18/PELP1 assembly. Our biochemical assay results suggest that SMIP34 binds to the same region involved in the PELP1 homodimerization and disrupts the formation of PELP1 homodimer and subsequently blocks the formation of WDR18/PELP1 Rix1 complex. Modeling studies predicted that altering the conformation of PELP1 aa 696-720 loop by SMIP34 binding may influence the conformation of the following C-terminal region, which may disturb the interaction of other PELP1 partners such as SENP3 and TEX10. Accordingly, the results from immunoprecipitation experiments demonstrated decreased association of Rix1 complex proteins WDR18, TEX10, and SENP3 with PELP1 under conditions of SMIP34 treatment. Further, Western blotting analysis confirmed that SMIP34 treatment decreased levels of PELP1 and its complex proteins, such as WDR18, TEX10, LAS1L, and SENP3. SMIP34 significantly reduced ribosomal biogenesis in a dose dependent manner. Additionally, puromycin labelling experiments confirmed a dose dependent decrease in new protein synthesis upon SMIP34 administration. Data from the DepMap database analysis revealed that PELP1 is required for cancer cell survival. Additionally, analysis of TCGA breast cancer datasets revealed that PELP1 expression is positively correlated with WDR18, TEX10, LAS1L, and SENP3. In in vivo xenograft assays and ex vivo explant assays using tumor tissue, SMIP34 strongly inhibited the growth of ER+, endocrine treatment resistant, and TNBC model cells. Conclusion: These findings provide solid evidence that the Rix1 complex, which is required for ribosomes production, is disturbed by SMIP34 binding to PELP1. SMIP34 represents a new therapeutic for the treatment of both ER+ and TNBC breast cancer. Supported by VA grant 1 I01 BX004545-01A1. Citation Format: Xue Yang, Khaled Mohamed Nassar, Uday Pratap, Rahul Gopalam, Behnam Ebrahimi, Durga Meenakshi Panneerdoss, Xiaonan Li, Yaxia Yuan, Daohong Zhou, Gangadhara R. Sareddy, Rajeshwar R. Tekmal, Manjeet Rao, Suryavathi Viswanadhapalli, Ratna Vadlamudi. Small molecule inhibitor of PELP1 exhibit cytotoxic effects on breast cancer cells by blocking ribosome biogenesis [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 PO3-26-10.