Determining the role of NAD biosynthetic pathways in chemosensitivity in glioblastoma

Abstract

Glioblastoma (GBM) is an aggressive form of brain cancer with a median survival time of only one year. Since the turn of the 21st century, the only major advancement in the treatment of GBM has been the discovery of the DNA damaging agent temozolomide (TMZ), but this only offers a small increase in survival time. Attempts to target dysregulated signalling pathways in GBM have offered very little clinical bene t. Cancers, including GBM, adapt their metabolism by increasing their consumption of glucose and glutamine, and activating metabolic pathways to promote cell growth, including nucleotide, amino acid and fatty acid biosynthesis. Cancer cells have also been shown to require rapid synthesis of nicotinamide adenine dinucleotide (NAD) to support metabolic reactions, regulation of the cell cycle by sirtuins (SIRTs), and repair of DNA damage by poly (ADP-ribose) polymerase (PARP). This allows them to maintain high rates of proliferation. Inhibition of a key enzyme in NAD biosynthsis, nicotinamide phosphoribosyltransferase (NAMPT), has demonstrated signi cant anti-neoplastic activity in vitro. However, translation to the clinic has been poor. This is largely due to the lack of patient strati cation and understanding of resistance mechanisms. I hypothesised that there is a subset of GBM tumours that lack alternative NAD biosynthetic pathways, rendering them sensitive to NAMPT inhibitors and unable to survive even in the presence of NAD precursors. I show that a proportion of GBM tumours lack key enzymes involved in salvage and de novo NAD biosynthetic pathways, including nicotinate phosphoribosyltransferase (NAPRT), ecto-5'-nucleotidase (NT5E) and quinolinate phosphoribosyltransferase (QPRT). This renders them sensitive to the NAMPT inhibitor FK866, and prevents rescue from FK866-induced growth arrest by different NAD precursors. Inhibition of NAMPT disrupts different aspects of cell cycle regulation in FK866-sensitive and FK866-resistant GBM cells lines. Finally, the combination of FK866 and TMZ limits the proliferation of GBM cells both in vitro, and in a murine orthotopic xenograft model, by initiating a G2/M cell cycle arrest. Combining NAMPT inhibitors with standard therapy may be an effective treatment for patients with GBM.