Hypoxia-Driven Ribosome Biogenesis: Unveiling a Novel Regulatory Role of HIF1a in Breast Cancer Progression and Metastasis [SUM1315_Hypoxia_RNA-seq]

Solid tumors such as breast cancer have intratumoral regions of low oxygen tension, which promotes cancer cell invasion and metastasis. Hypoxia-Inducible Factor 1-alpha (HIF1a) represents the principal transcription factor orchestrating cellular responses to hypoxic conditions, mediating the regulation of genes implicated in oxygen homeostasis. Here we describe our findings that demonstrate upregulation of RNA Polymerase I activity in cells exposed to hypoxic conditions. RNA Polymerase I is responsible for transcription of rDNA. Using confocal microscopy, we discovered that in hypoxia, HIF1? localizes to the nucleolus, dependent on a previously unknown nucleolar localization signal in HIF1?. Further detailed analysis by chromatin immunoprecipitation revealed that HIF1? binds rDNA promoter at -230 and -350 bp. Ribosome biogenesis is important in supporting cellular metabolic processes and invasion and metastasis. Our analysis of publicly available data sets reveals that HIF1? and rRNA biogenesis are upregulated in breast cancer patients and are associated with poor prognosis. Our studies show that inhibition of RNA Pol I diminished hypoxia-triggered invasion of breast cancer cells. Cumulatively our work unravels a novel role of HIF1? in regulating rRNA biogenesis in breast cancer. Overall design: We investigated the impact of hypoxia on ribosome and rRNA biogenesis in human breast cancer SUM1315 cells. The cells were cultured for 24 hours under hypoxic (1% O2) and normoxic (21% O2) conditions to study the molecular mechanisms involved. We performed RNA-sequencing on total RNA from hypoxic and normoxic SUM1315 cells. This high-throughput sequencing approach provided a global view of the transcriptomic changes due to hypoxia, offering insights into cancer cell adaptation to low oxygen levels.