Abstract LB185: BBX and PLK1 feedback mechanisms regulate mitotic entry & exit

Abstract

Listen

Translate article

Abstract Bobby Sox Homolog (BBX) is a highly conserved member of the high mobility group (HMG) box family of transcription factors, but its function remains largely unknown. We find human papillomavirus positive oropharyngeal squamous cell carcinomas (HPV-OPSCC) are enriched for BBX amplification and single nucleotide variants clustered within the BBX lysine rich domain. We therefore used the immortalized keratinocyte cell line HaCaT to elucidate BBX function. BBX is localized primarily in the nucleus. Cell synchronization at G1-S or G2-M shows BBX mRNA transcription and protein expression are highest at G1-S and rapidly decline at G2-M. Immunofluorescence (IF) shows high nuclear BBX expression during interphase and rapid reduction at mitosis (M). Induced overexpression of BBX decreases cellular proliferation, prolongs M phase, induces senescence, increases frequency of binucleated cells, and increases DNA damage as measured by γH2AX. Knockdown of BBX expression decreases cell proliferation, shortens M phase, and results in several mitotic defects, including lagging chromosomes, micronuclei, multi-polar spindles, and binucleated cells. Because effects of BBX dysregulation mirror those of Polo-like kinase-1 (PLK1), we investigated possible BBX-PLK1 interactions. BBX induction increases, whereas knockdown decreases, PLK1 mRNA and protein expression. BBX strongly activated transcription of the PLK1 promoter in a luciferase reporter assay. In contrast, PLK1 dramatically decreased BBX protein, but not mRNA, expression, and this effect was attenuated by a PLK1 T210 inactivating mutation and by a PLK1 small molecule inhibitor. We found a strong positive correlation between BBX and PLK1 mRNA expression in HNSCC tumors, but not in normal tissues. We conclude that BBX and PLK1 interact in a feedback mechanism to precisely regulate the G2-M transition. Future work will investigate interactions between BBX, HPV oncoproteins, and regulation of the cell cycle. Citation Format: Gaiyun Li, Maura L. Gillison. BBX and PLK1 feedback mechanisms regulate mitotic entry & exit [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB185.