Vanadium Induces VL30 Retrotransposition at an Unusually High Level: A Possible Carcinogenesis Mechanism

Carcinogenesis by vanadium is thought to occur through induction of DNA-double-strand breaks (DSBs) but its mechanism is not fully understood. We investigated the effect of vanadium on induction of viral-like 30 element (VL30) retrotransposition using a NIH3T3 cell-retrotransposition assay based on a recombinant VL30/EGFP element. Incubation of assay cells with vanadyl sulphate (VOSO4) induced retrotransposition frequency in a dose and time-dependent manner, measured by fluorescence-activated cell scanning (FACS) and retrotransposition events were confirmed by UV microscopy and PCR analysis. Among vanadium salts with different valence tested, vanadyl (4+) ions were the most potent retrotransposition inducers. VOSO4, at 50 μM induced retrotranspositions at an unusually high frequency of up to 0.185 events per cell per generation. VOSO4, acting at the transcription level, strongly induced VL30 and endogenous reverse transcriptase (enRT) transcripts with maxima at 50 μM and 100 μM of 22 and 18-fold, respectively. VOSO4-induced retrotransposition frequency was inhibited by 42% with efavirenz, an inhibitor of enRTs, while paraquat, a DNA-DSBs inducer, had no effect. Furthermore, it was completely abolished with deferoxamine, a metal chelator, while reduced by 75% with N-acetyl-cysteine, a general antioxidant. Remarkably, H2O2 reproduced inducible retrotransposition linking for the first time oxidative stress to induction of retrotransposition. We propose that VOSO4-induced VL30 retrotransposition through H2O2 generation may be an alternative mutagenic, DNA-DSBs independent, mechanism leading to carcinogenesis.