Endogenous arginase 2 as a biomarker for PEGylated arginase 1 treatment in squamous cell lung carcinoma xenograft models

Abstract

Background: Arginine depletion induced by PEGylated arginase 1 (BCT-100) has shown encouraging anticancer effects among arginine auxotrophic cancers which lack argininosuccinate synthetase (ASS1) and ornithine transcarbamylase (OTC). High endogenous arginase 2 (ARG2) was previously reported in human lung cancers. Although high ARG2 level does not cause immunosuppression nor affect disease progression, it may affect the efficacy of PEGylated arginase 1 therapy. ARG2 was highly expressed in H520 lung squamous cell carcinoma (SCC) xenograft while undetectable in SK-MES-1 lung SCC xenograft. We proposed that high endogenous ARG2 expression might impede anti-tumour effect of PEGylated arginase 1 in lung SCC. Methods: The in vivo effect of PEGylated arginase 1 was investigated using two lung SCC xenograft models (SKMES-1 and H520). Protein expression, apoptosis, and arginine concentration were investigated by western blot, terminal deoxynucleotidyl transferase dUTP nick end labelling assay, and enzyme-linked immunosorbent assay, respectively. Results: PEGylated arginase 1 (60 mg/kg) decreased rate of tumour growth in SK-MES-1 but not in H520 xenograft. ASS1 was highly expressed in SK-MES-1 xenograft while OTC expression remained low in both xenografts. Serum arginine level was declined significantly by PEGylated arginase 1 in both xenograft models. Intratumoural arginine level was decreased by PEGylated arginase 1 in SK-MES-1 xenograft only. In H520 xenograft, intratumoural arginine level in control group was already very low that could not be further reduced in PEGylated arginase 1 treatment arms. G1 arrest was indirectly demonstrated by suppression of cyclin A2, B1, D3, E1, and CDK4 with PEGylated arginase 1 in SK-MES-1 xenograft only. Moreover, downregulation of proliferation factor Ki67 and activation of apoptosis were observed in SK-MES-1 xenograft by PEGylated arginase 1 treatment only. Conclusion: PEGylated arginase 1 treatment (BCT-100) was effective in lung SCC xenograft with low endogenous ARG2 level. High endogenous ARG2 expression may induce low intratumoural arginine level in lung SCC xenograft. ARG2 may serve as the third predictive biomarker, other than ASS1 and OTC, in PEGylated arginase 1 treatment in lung SCC. Acknowledgement This research was supported by Hong Kong Anti-Cancer Society, Hong Kong SAR.