The Mitochondria-Associated ER-Membrane (MAM) is a small section of the outer mitochondrial membrane (OMM) tethered to the endoplasmic reticulum (ER) by lipid and protein filaments. One such MAM protein is the sigma-1 receptor, which contributes to multiple signaling pathways. We found that siRNA-mediated knockdown of sigma-1 reduced pregnenolone synthesis by 95% without affecting expression of the inner mitochondrial membrane (IMM) resident enzyme, 3-beta hydroxysteroid dehydrogenase2 (3βHSD2). To explore the underlying mechanism of this effect, we generated a series of sigma receptor ligands: KSCM-1, KSCM-5 and KSCM-11 specifically bound to sigma-1 in the nanomolar range while KSCM-5 and KSCM-11 also bound to sigma-2. Treatment of cells with the KSCM ligands led to decreased cell viability, with KSCM-5 having the most potent effect followed by KSCM-11. KSCM-1 increased sigma-1 expression by four-fold and progesterone synthesis, whereas the other compounds decreased progesterone synthesis. These differences likely are due to ligand molecular structure: For example, KSCM-1 has two methoxy substituents at C-5 and C-6 of the benzofuran ring while KSCM-11 has one at C-6. KSCM ligands or sigma-1 knockdown did not alter expression of ER resident enzymes that synthesize steroids. However, co-immunoprecipitation of the sigma-1 receptor pulled down voltage-dependent anion channel 2 (VDAC2), whose expression was enhanced by KSCM-1. This VDAC2 plays a key role in cholesterol transport into the mitochondria, suggesting that the sigma-1 receptor at the MAM coordinates with steroidogenic acute regulatory protein (StAR) for cholesterol trafficking into the mitochondria for metabolic regulation.
- Received July 6, 2012.
- Revision received August 20, 2012.
- Accepted August 23, 2012.
- The American Society for Pharmacology and Experimental Therapeutics