HDL inhibits endoplasmic reticulum stress-induced apoptosis of pancreatic β-cells in vitro by activation of Smoothened.

Loss of pancreatic β-cell mass and function as a result of sustained endoplasmic reticulum (ER) stress is a core step in the pathogenesis of diabetes mellitus type 2. The complex control of β-cells and insulin production involves hedgehog signaling pathways as well as cholesterol-mediated effects. In fact, data from studies in humans and animal models suggest that HDL protects against the development of diabetes through inhibition of ER stress and β-cell apoptosis. We investigated the mechanism by which HDL inhibits ER stress and apoptosis induced by thapsigargin, a sarco/ER Ca2+-ATPase inhibitor, in β-cells of a rat insulinoma cell line, INS1e. We further explored effects on the hedgehog signaling receptor Smoothened (Smo) with pharmacologic agonists and inhibitors. Interference with sterol synthesis or efflux enhanced β-cell apoptosis and abrogated the anti-apoptotic activity of HDL. During ER stress, HDL facilitated the efflux of specific oxysterols, including 24-hydroxycholesterol (24-OHC). Supplementation of reconstituted HDL with 24-OHC enhanced and-in cells lacking ATP-binding cassette transporter ABCG1 or the 24-OHC synthesizing enzyme CYP46A1-restored the protective activity of HDL. Inhibition of Smo countered the beneficial effects of HDL but also LDL, and Smo agonists decreased β-cell apoptosis in the absence of ABCG1 or CYP46A1. The translocation of the Smo-activated transcription factor glioma-associated oncogene GLI-1 was inhibited by ER stress but restored by both HDL and 24-OHC. In conclusion, the protective effect of HDL to counter ER stress and β-cell death involves the transport, generation and mobilization of oxysterols for activation of the hedgehog signalling receptor Smo.