StAR is a cholesterol transport protein that transports cholesterol from the cytoplasm into the mitochondrial matrix for further metabolism by CYP enzymes. Depending on the CYP expressed, this results in production of corticosteroids, gonadal steroids, neurosteroids or oxysterols. These products generally exert positive effects on cellular health and function but can also be harmful. We previously showed that StAR is produced in β cells and is increased under conditions of amyloid formation in human islet amyloid polypeptide (hIAPP) transgenic mouse islets, a model of human islet amyloidosis in type 2 diabetes (T2D). Further, CYP11A1, CYP11B1, CYP7A1 and CYP17A1 are not expressed in mouse islets, and CYP27A1 is expressed but decreases with amyloid deposition. Therefore, we hypothesized that increased StAR in islets is deleterious to β-cell health as the downstream CYP enzymes are not present to metabolize mitochondrial cholesterol.
We first demonstrated (by immunoblot) that StAR is expressed in human islets, a requirement if it is to play a role in T2D. Also in human islets, only CYP27A1 is detectable, while CYP11A1 and CYP7A1 are not (n=4). We next used a lentivirus to increase StAR expression (or a control lentivirus) in INS-1 cells, which resulted in reduced mitochondrial membrane potential (100±2.0% vs. 78.1±1.2%, n=2, p<0.05) and cell viability (100±4.1% vs. 89.7±7.7%, n=3, p<0.05). Finally, we determined whether reducing StAR expression with an siRNA oligonucleotide in hIAPP transgenic islets had beneficial effects. A 40% knockdown in StAR expression did not alter amyloid deposition, but tended to increase islet viability (100±3.1% vs. 134.7±12.2%, n=2, p=0.1).
In summary, StAR is expressed in human islets and increasing its expression is deleterious due to changes in mitochondrial function and β-cell survival. Thus, reducing StAR expression and/or increasing expression of downstream CYP enzymes may be a novel strategy to protect β cells in T2D.
M.F. Hogan: None. N. Esser: None. A.T. Templin: None. S. Zraika: Research Support; Self; Novartis Pharmaceuticals Corporation. R.L. Hull: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc. S. El-Osta: None. S.E. Kahn: Advisory Panel; Self; Boehringer Ingelheim Pharmaceuticals, Inc., Eli Lilly and Company, Intarcia Therapeutics, Inc., Janssen Pharmaceuticals, Inc., Merck & Co., Inc., Novo Nordisk A/S. Consultant; Self; Neurimmune. Other Relationship; Self; Boehringer Ingelheim Pharmaceuticals, Inc., Merck & Co., Inc., Novo Nordisk A/S.
American Diabetes Association (1-18-PDF-174 to M.F.H.); National Institutes of Health (T32HL007028, F32DK109584); U.S. Department of Veterans Affairs (BX001060); University of Washington