Lipoproteins Regulate Expression of the Steroidogenic Acute Regulatory Protein (StAR) in Mouse Adrenocortical Cells

Abstract

The production of hormones in steroidogenic tissues is dependent upon the availability of free cholesterol at the inner mitochondrial membrane, the site of the cholesterol side chain cleavage. In adrenocortical cells, adrenocorticotropin (ACTH) 1 stimulates hydrolysis of cholesteryl esters in lipid droplets, and transport of free cholesterol to mitochondria for use in steroid synthesis (1, 2). How free cholesterol in the outer mitochondrial membrane is transferred to the P450-cholesterol side chain cleavage (P450-SCC) complex on the inner mitochondrial membrane is unclear, and the existence of a hormonally regulated, labile protein which facilitates this transfer had been predicted for many years (1, 3). The recently discovered steroidogenic acute regulatory protein (StAR) is a 30-kDa mitochondrial protein which has many of the characteristics of this regulator (3,4). StAR has been demonstrated to promote movement of cholesterol from the outer to the inner mitochondrial membrane where P450-SCC catalyzes the first, and rate-limiting, enzymatic step in steroid synthesis (5, 6). In addition, expression of StAR promotes steroidogenesis, and directly facilitates the transfer of cholesterol and other sterols in reconstituted mitochondria (7-9). Perhaps the most convincing evidence that StAR is critical for this process in vivo is the observation by Miller and others (10, 11) that mutations in the StAR gene cause congenital lipoid adrenal hypoplasia. In this autosomal recessive disorder, gonadal and adrenal steroid production are blocked at the level of cholesterol side chain cleavage, resulting in a massive accumulation of cholesterol in the adrenal glands (10). Likewise, mice in which the StAR gene is disrupted have a severe reduction in corticosterone synthesis and accumulate massive lipid deposits in their adrenal cortex (12).Expression of StAR is acutely regulated in response to hormonal stimulation by a process which does not require de novo protein synthesis (5). Phosphorylation of StAR has also been demonstrated to regulate its steroidogenic activity (13). cAMPdependent protein kinase (PKA), as well as the transcription factors steroidogenic factor-1 (SF-1) and C/EBP␤, are required for basal transcription of StAR (14 -19). Other reports demonstrate that StAR transcription can be regulated by oxysterols (20) and agents known to increase cellular calcium can also regulate StAR transcription (21). Since the net result of ACTH stimulation is an increase in free cholesterol at the outer mitochondrial membrane, the current studies were undertaken to ask if the expression of StAR is regulated directly by lipoproteins, the major source of cholesterol for steroid synthesis in adrenocortical cells. Our studies demonstrate that lipoproteins regulate StAR expression by a mechanism that is dependent on cAMP-dependent protein kinase, and the transcription factor SF-1. The regulation of StAR expression by cholesterol may represent a positive feedback circuit designed to maintain maximal output of steroid hormone. * This work ...

Oxysterols regulate expression of the steroidogenic acute regulatory protein

“…This experiment was performed twice with duplicate samples. conversion of cholesterol to hydroxycholesterol or steroid, affects LDL-induced increases in StAR mRNA levels (Reyland et al 2000). Taken together, these data suggest that increased substrate cholesterol availability may promote StAR protein synthesis.…”

“…In contrast to oxysterols, LDL increases StAR expression through upregulation of StAR promoter activity using an SF-1 site at 135 in the murine StAR promoter, suggesting involvement of SF-1 (Reyland et al 2000). The effect of LDL would probably not be enhanced by oxysterols since neither the P450 scc inhibitor aminoglutethimide, nor ketoconazole, which inhibits synthesis and Figure 7 Acute regulation of StAR protein by oxysterol is inhibited by cycloheximide (cyc).…”

“…StAR protein expression was detected by immunoblot analysis as previously described using 50 µg whole Y1 cell protein and 25 or 50 µg mitochondrial protein from MA-10 cells (Wang et al 1998, Reyland et al 2000. Two different antisera generated against recombinant N62-truncated human StAR were generously provided by Drs Jerome F Strauss III (University of Pennsylvania Medical Center, Philadelphia, PA, USA) and Walter L Miller (University of California at San Francisco, CA, USA) and used to detect StAR protein in Y1 and MA-10 protein blots respectively (Pollack et al 1997, Bose et al 1999.…”

“…Interestingly, elevations in cholesterol supplies derived from lipoprotein increase StAR mRNA and protein levels (Reyland et al 2000), suggesting that substrate availability and steroid synthesis are linked in part through regulation of StAR activity. Other evidence indicates that StAR protein levels are also positively regulated by oxysterols.…”

Liver X receptors regulate adrenal cholesterol balance

“…44). Oxysterols and lipoproteins have also been identified as chronic regulators of StAR gene and protein expression (45)(46)(47). Using an in vitro reporter assay, King et al (46) concluded that oxysterols regulate StAR in a process thought to be independent of new gene transcription.…”