Evidence for Transcriptional Regulation of the Glucose-6-Phosphate Transporter by HIF-1 α : Targeting G6PT with Mumbaistatin Analogs in Hypoxic Mesenchymal Stromal Cells

Abstract

Mesenchymal stromal cell (MSC) markers are expressed on brain tumor-initiating cells involved in the development of hypoxic glioblastoma. Given that MSCs can survive hypoxia and that the glucose-6-phosphate transporter (G6PT) provides metabolic control that contributes to MSC mobilization and survival, we investigated the effects of low oxygen (1.2% O2) exposure on G6PT gene expression. We found that MSCs significantly expressed G6PT and the glucose-6-phosphatase catalytic subunit β, whereas expression of the glucose-6-phosphatase catalytic subunit α and the islet-specific glucose-6-phosphatase catalytic subunit-related protein was low to undetectable. Analysis of the G6PT promoter sequence revealed potential binding sites for hypoxia inducible factor (HIF)-1α and for the aryl hydrocarbon receptor (AhR) and its dimerization partner, the AhR nuclear translocator (ARNT), AhR:ARNT. In agreement with this, hypoxia and the hypoxia mimetic cobalt chloride induced the expression of G6PT, vascular endothelial growth factor (VEGF), and HIF-1α. Gene silencing of HIF-1α prevented G6PT and VEGF induction in hypoxic MSCs whereas generation of cells stably expressing HIF-1α resulted in increased endogenous G6PT gene expression. A semisynthetic analog of the polyketide mumbaistatin, a potent G6PT inhibitor, specifically reduced MSC-HIF-1α cell survival. Collectively, our data suggest that G6PT may account for the metabolic flexibility that enables MSCs to survive under conditions characterized by hypoxia and could be specifically targeted within developing tumors.