Targeting Asparagine Synthetase in Tumorgenicity Using Patient-Derived Tumor-Initiating Cells

Reprogramming of energy metabolism is regarded as one of the hallmarks of cancer; in particular, oncogenic RAS has been shown to be a critical regulator of cancer metabolism. Recently, asparagine metabolism has been heavily investigated as a novel target for cancer treatment. For example, Knott et al. showed that asparagine bioavailability governs metastasis in a breast cancer model. Gwinn et al. reported the therapeutic vulnerability of asparagine biosynthesis in KRAS-driven non-small cell lung cancer. We previously reported that KRAS-mutated CRC cells can adapt to glutamine depletion through upregulation of asparagine synthetase (ASNS), an enzyme that synthesizes asparagine from aspartate. In our previous study, we assessed the efficacy of asparagine depletion using human cancer cell lines. In the present study, we evaluated the clinical relevance of asparagine depletion using a novel patient-derived spheroid xenograft (PDSX) mouse model. First, we examined ASNS expression in 38 spheroid lines and found that 12 lines (12/37, 32.4%) displayed high ASNS expression, whereas 26 lines (25/37, 67.6%) showed no ASNS expression. Next, to determine the role of asparagine metabolism in tumor growth, we established ASNS-knockdown spheroid lines using lentiviral short hairpin RNA constructs targeting ASNS. An in vitro cell proliferation assay demonstrated a significant decrease in cell proliferation upon asparagine depletion in the ASNS-knockdown spheroid lines, and this was not observed in the control spheroids lines. In addition, we examined asparagine inhibition with the anti-leukemia drug L-asparaginase (L-Asp) and observed a considerable reduction in cell proliferation at a low concentration (0.1 U/mL) in the ASNS-knockdown spheroid lines, whereas it exhibited limited inhibition of control spheroid lines at the same concentration. Finally, we used the PDSX model to assess the effects of asparagine depletion on tumor growth in vivo. The nude mice injected with ASNS-knockdown or control spheroid lines were administered with L-Asp once a day for 28 days. Surprisingly, in mice injected with ASNS-knockdown spheroids, the administration of L-Asp dramatically inhibited tumor engraftment. On the other hands, in mice injected with control spheroids, the administration of L-Asp had no effect on tumor growth inhibition at all. These results suggest that ASNS inhibition could be critical in targeting asparagine metabolism in cancers.