Targeting hypoxia-induced circSTT3A decreases breast cancer stem cell formation via degradation of PGK1 protein and serine synthesis

Abstract Background Hypoxia is a key feature of tumor microenvironment that can cause fundamental changes in cancer cells, and may also lead to the development of breast cancer stem cells (BCSCs) with self-renewal ability. However, the mechanism of hypoxia in inducing BCSCs is not fully understood. Methods Performing RNA sequence and bioinformatics analysis, a hypoxia specific circular RNA (circRNA), named circSTT3A, was identified in hypoxic breast cancer cells and tissues. The clinical significance of circSTT3A was investigated in breast cancer (BC) tissues and tissue array. The loss and acquisition of circSTT3A were carried out in vivo and in vitro to confirm its functional roles in BCSC stemness maintenance. HIF1α droved circSTT3A expression was evaluated by chromatin immunoprecipitation and dual luciferase reporter assays. RNA pull-down, RNA immunoprecipitation, silver staining detection, mass spectrographic analysis, co-immunoprecipitation assays and western blotting were exerted to determine circSTT3A directly binding with HSP70 and PGK1 proteins. CircSTT3A-mediated serine metabolism was determined by UHPLC-QTRAP-MS system and ELISA kit. BC mouse model was used to assess the effects of circSTT3A/Hsp70/PGK1 on tumorigenesis and chemotherapy resistance in vivo. Results A novel hypoxia specific circSTT3A was significantly upregulated in clinical breast cancer tissues, and was related to the clinical stage and poor prognosis of BC patients. The hypoxia inducible factor 1 alpha (HIF1α)-regulated circSTT3A has remarkable effect on mammosphere formation in breast cancer cells. Our work revealed that circSTT3A directly interacting with nucleotide-binding domain of heat shock protein 70 (HSP70) increases the ability of HSP70 to recruit phosphoglycerate kinase 1 (PGK1) via its substrate-binding domain, which reduces the ubiquitination of PGK1 and increases the stability of PGK1. The enhanced PGK1 catalyzes 1,3-diphosphoglycerate (1, 3-BPG) into 3-phosphoglycerate (3-PG) leading to 3-PG accumulation and increase of serine synthesis, thus to facilitate BCSC enrichment under hypoxic microenvironment. Loss of circSTT3A or PGK1 substantially induces suppression in tumor initiation and tumor growth, which dramatically increases tumor sensitivity to Doxorubicin treatment in mice. Conclusions Hypoxia induced circSTT3A/HSP70/PGK1 axis plays a critical role in maintaining BCSC properties and may be meaningful for treating patients with breast cancer.