Advanced experimental cystic echinococcosis: reprogramming of the intermediary carbon metabolism in the parasite under metformin treatment in vivo

Abstract

Cystic echinococcosis is a progressive and chronic disease caused by the larval stage of the species complex Echinococcus granulosus sensu latu. New treatment options are needed especially for advanced disease. Since in these cestodes glycogen is the main energy storage molecule and glucose the major fermentative substrate, our approach was to target the metabolic pathways of the parasite involved in energy production, focusing on the AMPK/TOR pathway. Here, the aim was to assess the in vivo efficacy of metformin as an indirect AMPK agonist, in an advanced disease model (1year post-infection in mice), employing the highest dose of assayed drug (250 mg kg−1 day−1). Metformin-treated mice exhibited a reduction of cellular integrity of the germinal layer of cysts, registering a drug concentration of 1.7 mM (which inhibits mitochondrial respiratory chain complex I), a reduction in intracystic glucose with an increase in lactate concentration, consistent with the rise in the glycogen breakdown and in the LDH activity. Interestingly, the fraction of reducing soluble sugars decreased by 3 times in the cystic fluid and germinal cells after of drug-treatment. However, non-reducing soluble sugars, such as sucrose and trehalose, were consumed in the cystic fluid but showed a significant increase at the intracellular level in presence of the drug. It is surprising that trehalose and sucrose biosynthesis was upregulated during starvation induced by metformin. That is, a futile cycle of non-reducing sugars synthesis and glycogen catabolism during starvation. Function of these disaccharides as stress protectants during starvation provides some resolution to this paradox, as it also occurs in others invertebrates and plants. In the same line evidence, fasting and starvation induce hepatic gluconeogenesis in mammalians. Thus, in this parasite metformin affects glucose-starvation-induced AMPK activation and restructures carbohydrate metabolism prior induction of cell death.