A potent, reversible inhibitor of AMPK (Ki = 109 nM); dose-dependently inhibits the bone morphogenetic protein type 1 receptors ACTR-I (ALK2), BMPR-IA (ALK3), and BMPR-IB (ALK6); downregulates the Akt/mTOR pathway to induce autophagy in U251 human glioma cells