Leydig cells contain significant amounts of constitutively produced
steroidogenic acute regulatory protein (STAR; STARD1).
Hormone-induced STAR plays an essential role in inducing the transfer of
cholesterol into the mitochondria for
hormone-dependent steroidogenesis. STAR acts at the outer mitochondrial membrane, where it interacts with a
protein complex, which includes the translocator
protein (TSPO). Mutations in STAR cause
lipoid congenital adrenal hyperplasia (lipoid CAH), a disorder characterized by severe defects in adrenal and gonadal
steroid production; in Leydig cells, the defects are seen mainly after the onset of
hormone-dependent
androgen formation. The function of constitutive STAR in Leydig cells is unknown. We generated STAR knockout (KO) MA-10 mouse
tumor Leydig cells and showed that STAR KO cells failed to form
progesterone in response to dibutyryl-cAMP and to TSPO
drug ligands, but not to
22(R)-hydroxycholesterol, which is a membrane-permeable intermediate of the
CYP11A1 reaction. Electron microscopy of STAR KO cells revealed that the number and size of lipid droplets were similar to those in wild-type (WT) MA-10 cells. However, the density of lipid droplets in STAR KO cells was drastically different than that seen in WT cells. We isolated the lipid droplets and analyzed their content by liquid chromatography-mass spectrometry. There was a significant increase in
cholesteryl ester and
phosphatidylcholine content in STAR KO cell lipid droplets, but the most abundant increase was in the amount of
diacylglycerol (DAG); DAG 38:1 was the predominantly affected species. Lastly, we identified genes involved in DAG signaling and lipid metabolism which were differentially expressed between WT MA-10 and STAR KO cells. These results suggest that constitutive STAR in Leydig cells is involved in DAG accumulation in lipid droplets, in addition to
cholesterol transport. The former event may affect cell functions mediated by DAG signaling.