Peroxisomes and sterol carrier protein-2 in luteal cell steroidogenesis: a possible role in cholesterol transport from lipid droplets to mitochondria

In the present investigation, we have studied peroxisomes and sterol carrier protein-2 (SCP₂) in control and luteinizing hormone stimulated rat luteal cells. Superovulated immature rats in mid-luteal phase (8 days after ovulation) were divided into two groups (n=4/group) and treated with vehicle (0.2 ml saline), or luteinizing hormone (LH, 20 μg/rat). In this animal model, LH acutely stimulates steroidogenesis. Thirty minutes later, corpora lutea were fixed by whole body perfusion and processed for (1) electron microscopic immunocytochemistry to localize SCP₂ via the protein A gold immunolabeling technique, and for (2) electron microscopic histochemistry to stain peroxisomal catalase via the alkaline 3,3′-diaminobenzidine tetrahydrochloride method. In the steroidogenic, mid-phase luteal cells of vehicle treated rats (controls), SCP₂ was highly concentrated in peroxisomes and sparsely scattered on mitochondria, but no labeling was observed in lipid droplets. In the luteal cells of rats acutely stimulated with LH, peroxisomes immunolabeled for SCP₂ were observed within the luteal cell lipid droplets and mitochondria, and in union with lipid droplets and mitochondria. Moreover, in contrast to control luteal cells, significant immunolabeling for SCP₂ was detected within the lipid droplets and mitochondria in luteal cells of LH-treated rats. As SCP₂ binds cholesterol to 1:1 molar ratio and is known to be involved in the intracellular movement of cholesterol, these findings suggest that peroxisomes and SCP₂ may possibly be involved in delivering cholesterol from lipid droplets to the mitochondria when luteal cell steroidogenesis is acutely stimulated by LH.