Mumbaistatin

A better understanding of the metabolic adaptations of the vascular endothelial cells (EC) that mediate tumor vascularization would help the development of new drugs and therapies. Novel roles in cell survival and metabolic adaptation to hypoxia have been ascribed to the microsomal glucose-6-phosphate translocase (G6PT). While antitumorigenic properties of G6PT inhibitors such as chlorogenic acid (CHL) have been documented, those of the G6PT inhibitor and semi-synthetic analog AD4-015 of the polyketide mumbaistatin are not understood. In the present study, we evaluated the in vitro antiangiogenic impact of AD4-015 on human brain microvascular endothelial cells (HBMEC), which play an essential role as structural and functional components in tumor angiogenesis. We found that in vitro HBMEC migration and tubulogenesis were reduced by AD4-015 but not by CHL. The mumbaistatin analog significantly inhibited the phorbol 12-myristate 13-acetate (PMA)-induced matrix-metalloproteinase (MMP)-9 secretion and gene expression as assessed by zymography and RT-PCR. PMA-mediated cell signaling leading to cyclooxygenase (COX)-2 expression and IkappaB downregulation was also inhibited, further confirming AD4-015 as a cell signaling inhibitor in tumor promoting conditions. G6PT functions may therefore account for the metabolic flexibility that enables EC-mediated neovascularization. This process could be specifically targeted within the vasculature of developing brain tumors by G6PT inhibitors.

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% O(2)) exposure on G6PT gene expression. We found that MSCs significantly expressed G6PT and the glucose-6-phosphatase catalytic subunit beta, whereas expression of the glucose-6-phosphatase catalytic subunit alpha 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)-1alpha 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-1alpha. Gene silencing of HIF-1alpha prevented G6PT and VEGF induction in hypoxic MSCs whereas generation of cells stably expressing HIF-1alpha resulted in increased endogenous G6PT gene expression. A semisynthetic analog of the polyketide mumbaistatin, a potent G6PT inhibitor, specifically reduced MSC-HIF-1alpha 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.

The structurally unique polyketide mumbaistatin is the strongest naturally occurring inhibitor of glucose-6-phosphate translocase-1 (G6P-T1), which is a promising target for drugs against type-2 diabetes mellitus and angiogenic processes associated with brain tumor development. Despite its high relevance, mumbaistatin has so far withstood all attempts towards its total synthesis. In the present study an efficient total synthesis of a deoxy-mumbaistatin analogue containing the complete carbon skeleton and a spirolactone motif closely resembling the natural product in its cyclized form was elaborated. Key steps of the synthesis are a Diels-Alder cycloaddition for the construction of the fully functionalized anthraquinone moiety and an anionic homo-Fries rearrangement to build up the tetra-ortho-substituted benzophenone core motif, from which a spiroketal lactone forms in a spontaneous process. The elaborated strategy opens an entry to a variety of new analogs of mumbaistatin and cyclo-mumbaistatin and may be exploited for the total synthesis of the natural product itself in the future.