Abstract
BACKGROUND
A distinguishing characteristic of all cancers is uncontrolled cell division, and they require additional nucleotide bases such as purines, the building blocks of DNA and RNA, to sustain their uncontrolled growth. Purines can be synthesized from scratch by de novo pathway or salvaged by recycling surrounding nucleotides that are released by hydrolytic degradation. Even though the central nervous system (CNS), as well as CNS associated malignancies like glioblastoma (GBM), rely more heavily on the salvage pathway due to its energy efficiency, its precious role in promoting chemoresistance and GBM recurrence is yet to be elucidated.
MATERIAL AND METHODS
We have examined the role of purine biosynthesis in GBM by using stable isotope tracing analysis as well as utilizing a knockdown (KD) system to investigate its effect on i) DNA damage response during temozolomide (TMZ) therapy, ii) tumor engraftment and iii) therapeutic responses in vivo.
RESULTS
Through gene expression and protein-protein interaction analysis, we have identified ARL13B, member of ADP-ribosylation factor-like protein family, as a novel regulator of the purine biosynthesis pathway in GBM. ARL13B can physically interacting with the inosine monophosphate dehydrogenase 2 (IMPDH2), a key rate-limiting enzyme for purine biogenesis. Isotope tracer analysis under normal physiological conditions revealed that during TMZ treatment, salvage recycling activity was decreased by 50% while de novo pathway activity remains unchanged. In contrast, TMZ treatment of ARL13B knock-out cells results in a ~50% decrease in de novo pathway activity (p-value=0.004), whereas purine salvage pathway activity is upregulated ~6-fold (p-value <0.0001). ARL13B knockdown cells treated with TMZ show a robust increase in DNA double-strand breaks compared to control cells exposed to TMZ, as demonstrated by gH2X staining. Mice orthotopically engrafted with KD cells experience prolonged survival relative to mice engrafted with unmodified cells.
CONCLUSION
We propose that ARL13B-IMPDH2 interaction has two consequences: i) augmentation of de novo purine biosynthesis activity, and ii) inhibition of nucleotide recycling. The increasing de novo purine biosynthesis during TMZ therapy helps GBM cells reduce the recycling of nucleotides via the salvage pathway that have been modified as a result of TMZ alkylation. This, in turn, protects the cells from deleterious effects of incorporating modified nucleotides into newly-synthesized DNA while maintaining a supply of purine building blocks to support uncontrolled proliferation. Our results indicate that the interaction of ARL13B-IMPDH2 functions as a purine biosynthesis regulator that could be targeted for increasing efficacy of TMZ treatment of GBM.
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