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, PEG-BCT-100 or rhArg1peg5000) has shown promising anticancer effects among arginine auxotrophic cancers that are deficient in argininosuccinate synthetase (ASS1) and ornithine transcarbamylase (OTC). High endogenous arginase 2 (ARG2) was previously found in human lung cancers. Although high ARG2 does not induce immunosuppression nor affect disease progression, it may potentially affect the efficacy of BCT-100 treatment. ARG2 was highly expressed in H520 squamous cell lung carcinoma (lung SCC) xenograft while undetectable in SK-MES-1 and SW900 lung SCC xenografts. We postulated that high endogenous ARG2 expression might hamper anticancer effect of PEGylated arginase 1 in lung SCC. Methods The in vivo effect of PEGylated arginase 1 was studied using 3 lung SCC xenograft models (SK-MES-1, SW900 and H520). Protein expression, arginine concentration and apoptosis were investigated by Western blot, ELISA and TUNEL assay respectively. ARG2 in H520 xenograft was knocked down using shRNA technique. Results PEGylated arginase 1 (60 mg/kg) suppressed tumor growth in SK-MES-1 and SW900 but not in H520 xenograft. Serum arginine level was decreased significantly by BCT-100 in all xenografts. On the other hand, intratumoral arginine level was reduced by BCT-100 treatment in SK-MES-1 and SW900 xenografts. In H520 xenograft, intratumoral arginine level in control arm could not be further lowered in BCT-100 treatment arms. G1 arrest was indirectly evidenced by downregulation of cyclin A2, B1, D3, E1 and CDK4 with BCT-100 in SK-MES-1 xenograft only. Moreover, activation of apoptosis was induced by BCT-100 in SK-MES-1 and SW900 xenografts. Knockdown of ARG2 in H520 xenograft restored sensitivity to BCT-100 treatment partially induced by G1 arrest via arginine depletion. Conclusion PEGylated arginase 1 treatment was effective in lung SCC xenograft with low endogenous ARG2 expression. High endogenous ARG2 level may explain resistance of BCT-100 treatment in lung SCC xenograft. ARG2 may serve as an additional predictive biomarker, other than ASS1 and OTC, in BCT-100 treatment in lung SCC.