Hypoxic regulation of transcription in HUVEC is mediated by EPAS1

Cells adapt to environmental changes, including fluctuations in oxygen levels, through the induction of specific gene expression programs. To identify genes regulated by hypoxia at the transcriptional level, we pulse-labeled HUVEC cells with 4-thiouridine and sequenced nascent transcripts. Then, we searched genome-wide binding profiles from the ENCODE project for factors that correlated with changes in transcription and identified binding of several components of the Sin3A co-repressor complex, including SIN3A, SAP30 and HDAC1/2, proximal to genes repressed by hypoxia. SIN3A interference revealed that it participates in the downregulation of 75% of the hypoxia-repressed genes in endothelial cells. Unexpectedly, it also blunted the induction of 47% of the upregulated genes, suggesting a role for this corepressor in gene induction. In agreement, ChIP-seq experiments showed that SIN3A preferentially localizes to the promoter region of actively transcribed genes and that SIN3A signal was enriched in hypoxia-repressed genes, prior exposure to the stimulus. Importantly, SINA3 occupancy was not altered by hypoxia in spite of changes in H3K27ac signal. In summary, our results reveal a prominent role for SIN3A in the transcriptional response to hypoxia and suggest a model where modulation of the associated histone deacetylase activity, rather than its recruitment, determines the transcriptional output. Overall design: Exponentially growing non-synchronized HUVEC were transduced with lentiviral particles encoding for shRNA targeting EPAS1 or control shRNA. 72h after infection, cells were exposed to normoxia or hypoxia (21% or 1% oxygen respectively) for 8 hours and pulse-labelled with 4-thiouridine during the last two hours of treatment. RNA was extracted from samples in each condition (total RNA) and an aliquot subjected to affinity chromatography to purify the 4-thiouridine-labelled RNA fraction (newly transcribed RNA, Newly Tr). Both RNA fractions from each condition were analyzed by high-throughput sequencing. Data includes 8 samples from a single biological replicate.