PII-045 - ACETYLATION-DEPENDENT REGULATION OF NUCLEAR PD-L1 IS CRITICAL TO SISTER CHROMATID COHESION IN TRIPLE NEGATIVE BREAST CANCER

M. Wang¹, H. Gao¹, D. Liu¹, x. wang¹, R. Weinshilboum¹, L. Wang¹; ¹Mayo Clinic.

Background: Triple negative breast cancer (TNBC) is a subtype of breast cancers with poor outcome that lacks the expression of estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2, receptors which are targets for pharmacotherapy. Immune checkpoint inhibitors such as programmed death ligand-1 (PD-L1) targeting therapy is being approved to treat metastatic TNBC. PD-L1 is expressed in a subgroup of TNBC and is implicated in protecting tumor cells from immune surveillance. However, accumulating evidence revealed a novel role of nuclear PD-L1 in promoting TNBC cell survival and proliferation by regulating chromosome segregation. Recent studies have reported that PD-L1 is acetylated in multiple sites. In this work, we investigated how acetylation influences the intra-nuclear function of PD-L1 in chromosome segregation by regulating cohesin complex.

Methods: In this study, we investigated how PD-L1 acetylation might regulate cohesin complex. Acetylation-incapable PD-L1 plasmids were constructed using site-directed mutagenesis. PD-L1 acetylation level and its interaction with cohesin complex were detected by Immunoprecipitation. Stability of sister chromatid cohesion was assessed by chromosome spreading assay and fluorescence in situ hybridization. Cell proliferation was evaluated through Cyquant and colony formation assays.

Results: We found that PD-L1 can be acetylated at Lys178. The mutation of this site (PD-L1 K178R) reduced the acetylation level of PD-L1 and partially disrupted its binding to the cohesin complex in TNBC cells, results suggesting that PD-L1 acetylation at Lys178 is critical for its interaction with the cohesin complex. Furthermore, we demonstrated that K178R mutation leads to significant loss of sister chromatid cohesion and chromosomal stability, rendering the disruption of chromatids formation during pre-mitosis phase, and result in G2M cell cycle arrest. Finally, this K178R mutation repressed the TNBC cell proliferation and colony formation, suggesting that inhibiting PD-L1 Lys178 acetylation and preventing it from binding to the cohesin complex, would repress TNBC cell growth.

Conclusion: Our study provided insights into the acetylation regulatory mechanism of PD-L1 in TNBC and suggested a potential therapeutic target for this aggressive breast cancer subtype.