Hypoxia induced ATF3 escalates breast cancer invasion by increasing collagen deposition
Activating transcription factors (ATFs), members of the adaptive-response gene family, participate in cellular processes to aid adaptations in response to extra- and/or intracellular changes. In this study, we observed that one of the ATFs Activating transcription factor 3 (ATF3) is upregulated under hypoxia via alterations in the epigenetic landscape of its promoter, followed by transcriptional upregulation. Under hypoxic conditions, Hypoxia-inducible factor 1-alpha (HIF1?) alleviates methylation at the ATF3 promoter by recruiting TET1 and  induces ATF3 transcription. Additionally, our RNA-seq analysis showed that ATF3 globally affects transcription under hypoxia and controls the processes of EMT and cancer invasion by stimulating the transcription of Prolyl 4-Hydroxylase Subunit Alpha 1 (P4HA1), an enzyme which enhances invasion conducive extracellular matrix (ECM) under hypoxic condition. Prolyl hydroxylases play a critical role in the hydroxylation and deposition of collagen in the extracellular matrix (ECM) during the evolution of cancer, which is necessary for metastasis. Importantly, P4HA1 undergoes alternative splicing under hypoxia, where the inclusion of exon 9a is increased. It is interesting to note that ATF3's involvement in P4HA1 splicing was also evident, as binding of ATF3 at intron 9a led to demethylation of this DNA region via recruitment of TET1. Further, we also show that the demethylated DNA region of intron 9a then becomes accessible to CTCF. Thus, a cascade of demethylation via ATF3 recruited TET1, followed by increased RNA PolII pause via CTCF at the intron 9a leads to inclusion of exon 9a. The P4HA1 9a isoform leads to enhanced invasion under hypoxic conditions by increasing deposition of collagen in the ECM. These results provide a novel  hypoxia-induced HIF1?-ATF3-P4HA1 axis which can be potentially exploited as a therapeutic target to impede EMT and ultimately breast cancer invasion. Overall design: shControl and shATF3 MCF7 cells were treated under hypoxic conditions for 24 hours