3-hydroxy-3-methyl glutaryl coenzyme A reductase: An essential actor in the biosynthesis of cantharidin in the blister beetle Epicauta chinensis Laporte

Cantharidin (C₁₀H₁₂O₄) is a monoterpene defensive toxin in insects involved in chemical defence as well as in courtship and mating behaviours. It is relatively well known in the medical literature because of its high anticancer activity and as an effective therapy for molluscum contagiosum. However, little is known about its biosynthesis pathway in vivo, and no enzyme involved in cantharidin biosynthesis has been identified. The purpose of this study was to identify the crucial enzyme that is involved in the biosynthesis of cantharidin. Using the homology cloning method, a 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMGR) gene, the rate-limiting enzyme in the mevalonate pathway, was cloned from the blister beetle Epicauta chinensis. Quantitative reverse transcription PCR and gas chromatography methods revealed that the HMGR transcripts had a positive correlation with cantharidin production in the beetles (R = 0.891). RNA interference (RNAi) knockdown of HMGR mRNA expression was achieved by microinjection of a specific double-stranded RNA with more than 90% RNAi efficiency, and an apparent decrease of cantharidin production was observed. Furthermore, the HMGR mRNA was greatly upregulated by exogenous juvenile hormone III (JH III), and cantharidin production was also raised in males; however, when injecting the JH III with RNAi of HMGR mRNA at the same time, cantharidin production did not rise. These results demonstrate that HMGR is an essential enzyme in cantharidin biosynthesis in the blister beetle E. chinensis, which further verifies previous research results demonstrating that cantharidin is synthesized de novo by the mevalonate pathway in blister beetles.