Alzheimers disease (AD) is associated with abnormal cholesterol metabolism.Recent studies have suggested that AD is a neuroendocrine disease in which substantial abnormalities arise due to impairments of insulin and insulin-like growth factor (IGF) signaling.We speculated that the reduced cholesterol level in neurons might contribute to the impairment of insulin/IGF-l signaling.3-Beta-hydroxysterol delta-24-reductase (DHCR24) catalyzes the conversion of desmosterol to cholesterol,and is a multi-function enzyme that exhibits anti-apoptotic activity.Our previous work demonstrated that DHCR24 could play an important role in insulin-Akt cell survival signaling by maintaining cholesterol biosynthesis and the normal structure and function of caveolae in mouse embryonic fibroblasts.In the present study,we targeted DHCR24 by inhibiting its enzyme activity with U18666A,a chemical inhibitor of DHCR24,and RNA interference to determine the role of DHCR24 and cholesterol biosynthesis in insulin/IGF-1 signaling in rat adrenal pheochromocytoma(PC12) cells.Treatment with U18666Ain serum-free medium blocked the neuron-protective function of IGF-1,as demonstrated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and in situ apoptosis assays.siRNA targeting DHCR24 also showed similar results.These results suggest that the reduced intracellular cholesterol level might contribute to the impairment of IGF-1 function.Double immunocytochemical fluorescent probing revealed that the IGF-1 receptor is localized in caveolae,cholesterol-rich microdomains on the plasma membrane of PC12 cells.However,treatment with U18666A removed the localization of IGF-1 receptor in caveolae.Consistent with this,western blotting results demonstrated that the phosphorylation of insulin receptor,insulin receptor substrate,Akt,and Bad induced by IGF-1 exposure were significantly inhibited in U18666A-treated cells compared to controls.Taken together,these results demonstrate that a normal intracellular cholesterol level and normal expression and function of DHCR24 are very important for maintaining the function of the insulin/IGF-1-Akt survival cascade in neuronal cells.