Abstract
Observations from nematodes to mammals indicate that insulin/insulin-like growth factor signaling (IIS) regulates lifespan. As in other organisms, IIS is conserved in mosquitoes and signaling occurs in multiple tissues. During bloodfeeding, mosquitoes ingest human insulin. This simple observation suggested that exogenous insulin could mimic the endogenous hormonal control of aging in mosquitoes, providing a new model to examine this phenomenon at the organismal and cellular levels. To this end, female Anopheles stephensi mosquitoes were maintained on diets containing human insulin provided daily in sucrose or three times weekly by artificial bloodmeal. Regardless of delivery route, mosquitoes provided with insulin at 1.7 x 10(-4) and 1.7 x 10(-3) micromol l(-1), doses 0.3-fold and 3.0-fold higher than non-fasting blood levels, died at a faster rate than controls. In mammals, IIS induces the synthesis of reactive oxygen species and downregulates antioxidants, events that increase oxidative stress and that have been associated with reduced lifespan. Insulin treatment of mosquito cells in vitro induced hydrogen peroxide synthesis while dietary supplementation reduced total superoxide dismutase (SOD) activity and manganese SOD activity relative to controls. The effects of insulin on mortality were reversed when diets were supplemented with manganese (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP), a cell-permeable SOD mimetic agent, suggesting that insulin-induced mortality was due to oxidative stress. In addition, dietary insulin activated Akt/protein kinase B and extracellular signal-regulated kinase (ERK) in the mosquito midgut, suggesting that, as observed in Caenorhabditis elegans, the midgut may act as a 'signaling center' for mosquito aging.