Cholesterol, LDL, and 25-hydroxycholesterol regulate expression of the steroidogenic acute regulatory protein in microvascular endothelial cell line (bEnd.3).

The steroidogenic acute regulatory (StAR) protein promotes intramitochondrial delivery of cholesterol to the cholesterol side-chain cleavage system. In this experiment, we first demonstrated that StAR expressed in endothelial cells as well. Immunochemistry showed positive staining of StAR in endothelial cells. To investigate whether steroids and oxysterols regulate StAR expression in endothelial cells, mouse brain microvascular endothelial cell line (bEnd.3) was treated with various steroids and oxysterols, including free cholesterol (CHO), low density lipoprotein (LDL), and 25-hydroxycholesterol (25-OH). All these three compounds increased StAR mRNA and protein expression in a time- and dose-dependent manner. When treated with CHO and LDL, the StAR mRNA change was prior to the protein change, suggesting that transcription may be one of the mechanisms of CHO and LDL regulation. In contrast to CHO and LDL, 25-OH increased StAR protein levels independently of mRNA amount. It suggested that 25-OH might regulate StAR activity at post-transcriptional level.