Background: Low plasma levels of HDL-cholesterol are associated with increased risk of diabetes. The anti-diabetic activities of HDL include the inhibition of endoplasmic reticulum (ER) stress and apoptosis of pancreatic beta cells. Neither the underlying mechanism nor the protective agonist are known. 
Methods: The rat insulinoma cell line INS1e was used as the beta cell model. ER stress and apoptosis were induced by Thapsigargin (TG. Apoptosis was measured by free nucleosomes or caspase 3 activity. ER stress was recorded by qRT-PCR of CHOP, ATF4, and spliced XBP1 or by western blotting of phosphorylated JNK and eIF isoforms.
Results: TG induced apoptosis and ER stress of INS1e cells were dose-dependently inhibited by HDL. Interference with JNK prevented the anti-apoptotic effect of HDL. Because proteomics identified Sonic (SHH) and Indian Hedgehog (IHH) in HDL, we investigated the involvement of hedgehog signaling in apoptosis inhibition by HDL. The nuclear translocation of the hedgehog signaling induced transcripton factor GLI1 was inhibited by TG but promoted by HDL. However, neither the reconstitution of HDL with IHH or SHH nor the knock-down of IHH or SHH or their receptor Patched interfered with the anti-apoptotic activity of HDL. Interestingly, however, both pharmacological inhibition and knock-down of Smoothened, the downstream target of Patched, prevented the suppression of TG-induced apoptosis and ER-stress by HDL. Moreover, direct agonists of Smoothened such as 22S-hydroxycholesterol mimicked the anti-apoptotic activities of HDL. 
Conclusion: HDL inhibits ER-stress and apoptosis of pancreatic beta cells by interaction with Smoothened, possibly by providing oxysterols as the agonists.

02 - HIGH DENSITY LIPOPROTEINS (HDL) INHIBIT ER STRESS-INDUCED APOPTOSIS OF PANCREATIC BETA CELLS BY INTERACTION WITH SMOOTHENED