Some relationship between abnormal
cholesterol content and impairment of
insulin/
insulin-like growth factor I (IGF-1) signaling has been reported in the pathogenesis of
Alzheimer's disease (AD). However, the underlying mechanism of this correlation remains unclear. It is known that 3-β hydroxycholesterol Δ 24
reductase (DHCR24) catalyzes the last step of
cholesterol biosynthesis. To explore the function of
cholesterol in the pathogenesis of AD, we depleted cellular
cholesterol by targeting DHCR24 with
siRNA (siDHCR24) or
U18666A, an inhibitor of DHCR24, and studied the effect of the loss of
cholesterol on the IGF-1-Akt signaling pathway in vitro and in vivo. Treatment with
U18666A reduced the cellular
cholesterol level and blocked the anti-apoptotic function of
IGF-1 by impairing the formation of caveolae and the localization of
IGF-1 receptor in caveolae of the PC12 cells. Downregulation of the DHCR24 expression induced by
siRNA against DHCR24 also yielded similar results. Furthermore, the phosphorylation levels of
IGF-1 receptor,
insulin receptor substrate (IRS), Akt, and Bad in response to
IGF-1 were all found to decrease in the U18666A-treated cells. Rats treated with
U18666A via intracerebral injection also exhibited a significant decrease in the
cholesterol level and impaired activities of IGF-1-related signaling
proteins in the hippocampus region. A significant accumulation of
amyloid β and a decrease in the expression of
neuron-specific enolase (NSE) was also observed in rats with
U18666A. Finally, the Morris water maze experiment revealed that U18666A-treated rats showed a significant
cognitive impairment. Our findings provide new evidence strongly supporting that a reduction in
cholesterol level can result in neural apoptosis via the impairment of the IGF-1-Akt survival signaling in the brain.