Abstract
Glioblastoma multiforme (GBM) is the most aggressive brain tumor in adults, with a median survival for patients of 12–15 months after diagnosis. Standard therapy includes radiation and chemotherapy, with GBM recurrences refractory to second line treatments. Given the lack of therapeutic opportunities in GBM, new treatment strategies are urgently needed. Glioma stem cells (GSCs) have been shown to support GBM maintenance and exhibit enhanced resistance to ionizing radiation, a cornerstone of GBM therapy. Monocarboxylate transporter-4 (MCT4, SLC16A3) is highly expressed in the vast majority of GBMs and predominantly in cells that congregate in “palisades” around centers of necrosis or in cells where hypoxia-inducible factor-1alpha (HIF-1α) is expressed either due to reduced oxygen levels or other microenvironmental stresses. MCT4 appears to regulate proliferation, survival, and xenograft implantation of GSC neurosphere lines. Importantly, we found these effects to be independent of lactate homeostasis. Gene set enrichment analysis (GSEA) of differentially expressed genes revealed an enrichment of DNA replication stress response pathways including cell cycle (G2/M checkpoint), DNA replication, DNA damage response (DDR), and cellular deoxyribonucleotide metabolism in MCT4-depleted GSCs. To validate these data, after lentiviral silencing of MCT4 expression in two GBM neurosphere lines, we examined DNA repair potential by assessing the resolution of yH2AX and by performing alkaline comet assays. In both assays, reduced MCT4 levels resulted in a dramatic increase in the number of nuclear foci (p < 0.0001) and in the percentage of cells with comet tails (p=0.0003), respectively, confirming that reduced MCT4 expression decreases cellular ability to repair DNA damage. Moreover, we show that conditional MCT4 depletion alone (p=0.0001) or in combination with ionizing radiation (p=0.0006) efficiently enhance survival of mice orthotopically implanted with GSC-derived xenografts. We determined that MCT4 inhibition enhances the radiosensitivity of GBM and should be explored as a treatment strategy to complement conventional treatment.