PKM2-mediated epigenetic reprogramming regulates hypoxic expression of PFKFB3 to promote breast cancer progression

Abstract

The hypoxic milieu is a critical modulator of aerobic glycolysis, yet the regulatory mechanisms existing between the key glycolytic enzymes in hypoxic cancer cells are largely unexplored. In particular, M2 isoform of pyruvate kinase (PKM2) - the ratelimiting enzyme of glycolysis, is well-known to confer adaptive advantages under hypoxia. Herein, we report a non-canonical role of PKM2 in functioning as a co-activator of HIF-1 to govern the transcription of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3). Nuclear PKM2 enhances HIF-1 and p300 occupancy at PFKFB3 hypoxia responsive elements (HREs) resulting in its upregulation. Consequently, absence of nuclear PKM2 fails to recruit HIF-1 which activates an opportunistic occupancy of HIF-2 at PFKFB3 HREs. Enhanced binding of HDAC3 also occurs in the absence of PKM2 which prevents HIF-2 from efficiently inducing PFKFB3 to hamper proliferation of hypoxic breast cancer cells. In addition, clinical relevance of the study has been investigated by demonstrating that Shikonin blocks nuclear translocation of PKM2 to suppress PFKFB3 expression. Furthermore, MCF7 cells-derived xenograft tumors in mice exhibited substantial tumor growth inhibition when treated with shikonin, highlighting the vitality of targeting PKM2. Taken in concert, this work provides novel insights into contributions of PKM2 in modulating hypoxic transcriptome and a previously unreported molecular axis exhibited by the hypoxic breast cancer cells for ensuring the maintenance of PFKFB3 expression essential for achieving high glycolytic flux. Statement of significanceNuclear PKM2 orchestrates the binding of histone modifiers to epigenetically alter PFKFB3 promoter and affects the binding of HIF-2. Notably, targeting this axis attenuates proliferation of hypoxic breast cancer cells.