Abstract 5551: Apelin is Necessary for the Maintenance of Insulin Sensitivity
BACKGROUND: The recently discovered cardioactive peptide apelin has been shown to be involved in the maintenance of insulin sensitivity. However, questions persist regarding its precise role and molecular mechanism of action.
METHODS: Insulin (0.75 U/kg) tolerance testing (ITT) was performed on mice with generalized deficiency of the apelin gene (APKO). Additionally, fasting glucose, insulin, free fatty acid (FFA), and adiponectin levels were determined. To assess the impact of exogenously delivered apelin on insulin sensitivity, osmotic pumps containing pyroglutamated apelin-13 or saline were implanted in APKO mice for 4 weeks. Following the infusion, ITTs were repeated and the animals sacrificed. In selected animals, insulin (0.75 U/kg) was delivered 30 minutes prior to sacrifice; soleus muscles were then harvested, homogenized in lysis buffer, and insulin-induced Akt phosphorylation determined by Western blotting. Apelin-13 infusion, ITTs, and serum insulin, glucose, FFA, and adiponectin measurements were also performed in obese diabetic db/db mice. To probe the underlying mechanism for apelin’s effects, apelin-13 (1 μM) was also delivered to cultured C2C12 myotubes. 2-[3H]-deoxyglucose uptake and Akt phosphorylation were assessed.
RESULTS: APKO mice had significantly decreased insulin sensitivity by ITT, increased insulin (0.70±0.033 vs. 0.55±0.098 pg/mL; p < 0.05) and FFA (231±11 vs. 172±14 μmol/L; p < 0.01) levels, and decreased adiponectin levels (64±3.3 vs. 80±8.0 μg/mL; p < 0.05). Soleus lysates had decreased insulin-induced Akt phosphorylation. Administration of apelin to both APKO and db/db mice resulted in improvements in insulin sensitivity. No changes in body weight were observed in either animal model with or without apelin. In C2C12 myotubes, apelin increased glucose uptake (10.6±0.70 vs. 6.01±0.18 pmol/mg protein/min; p < 0.01) and Akt phosphorylation. These events were fully abrogated by inhibitors of Gi (pertussis toxin) and AMPK (Compound C), but only partially by phosphoinositide 3-kinase inhibition (LY294002).
CONCLUSIONS: Apelin is necessary for the maintenance of insulin sensitivity in vivo. Apelin’s effects on glucose uptake and Akt phosphorylation may be mediated by a Gi and AMPK-dependent pathway.