D. M. Stocco

The acute response of steroidogenic cells to trophic hormone stimulation is the mobilization of cholesterol from cellular stores to the mitochondrial outer membrane and the transfer of this cholesterol to the mitochondrial inner membrane where the first enzymatic step in steroidogenesis occurs. The transfer of cholesterol across the mitochondrial membranes is dependent upon de novo protein synthesis, and this is the regulated step in the process. Although the newly synthesized regulatory protein(s) have yet to be identified, we previously have proposed a candidate protein which we identified in MA-10 cells that is synthesized in response to luteinizing hormone stimulation and that is localized to the mitochondria. In the present study, we report the isolation of a cDNA that encodes this luteinizing hormone-induced protein. Analysis of the cDNA and protein sequences reveals this is a novel protein. Importantly, we demonstrate for the first time that expression of the protein in MA-10 cells in the absence of hormone stimulation is sufficient to induce steroid production. We conclude that this protein is required in the acute regulation of steroidogenesis and propose to call this protein the Steroidogenic Acute Regulatory protein (StAR).

Steroid hormones are synthesized in the adrenal gland, gonads, placenta and brain and are critical for normal reproductive function and bodily homeostasis. The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step in steroid biosynthesis, i.e. the delivery of cholesterol from the outer to the inner mitochondrial membrane. The expression of the StAR protein is predominantly regulated by cAMP-dependent mechanisms in the adrenal and gonads. Whereas StAR plays an indispensable role in the regulation of steroid biosynthesis, a complete understanding of the regulation of its expression and function in steroidogenesis is not available. It has become clear that the regulation of StAR gene expression is a complex process that involves the interaction of a diversity of hormones and multiple signaling pathways that coordinate the cooperation and interaction of transcriptional machinery, as well as a number of post-transcriptional mechanisms that govern mRNA and protein expression. However, information is lacking on how the StAR gene is regulated in vivo such that it is expressed at appropriate times during development and is confined to the steroidogenic cells. Thus, it is not surprising that the precise mechanism involved in the regulation of StAR gene has not yet been established, which is the key to understanding the regulation of steroidogenesis in the context of both male and female development and function.

The steroidogenic acute regulatory or StAR protein has been shown to be instrumental in the acute regulation of steroid hormone biosynthesis through its action in mediating cholesterol transfer to the inner mitochondrial membrane and the cholesterol side chain cleavage enzyme system. Since the time of its cloning in 1994, a number of studies have been performed which underscore the important role that this protein plays in steroidogenesis. While it is now quite apparent that StAR fulfills the criteria for the acute regulator as proposed by early studies, several crucial areas remain poorly understood. This list is topped by the so far intractable nature of the mechanism of action of StAR in transferring cholesterol to the P450scc enzyme. A second area which should prove to be of great interest is that of further understanding the regulation of the StAR gene which, like many genes, is quite complex. Lastly, with the recent demonstration of StAR being present in the brain, determining if StAR has a role in the synthesis of neurosteroids should prove to be of great importance.

A crucial event in the acute regulation of steroidogenesis by trophic hormones is the delivery of cholesterol into the mitochondria where it is converted to pregnenolone by the cholesterol side chain cleavage enzyme. Although considerable controversy exists regarding the exact mechanisms that underlie this acute response to hormone stimulation, recent studies suggest that the Steroidogenic Acute Regulatory (StAR) protein, a hormone-induced 30-kilodalton mitochondrial protein, plays an essential role. We now extend these studies by establishing in MA-10 mouse Leydig tumor cells a temporal relationship between levels of StAR expression and steroidogenesis in response to hormone stimulation. These data indicate that trophic hormones regulate StAR mRNA and protein within a time frame concomitant with the acute production of steroid hormones and provide the first evidence implicating changes in StAR transcription and/or mRNA stability in the functional response of steroidogenic cells to hormone action. In addition, in situ hybridization analyses of StAR expression in embryonic and adult mice demonstrated a precise spatial and temporal relationship in vivo between StAR expression and the capacity to produce steroid hormones. These experiments strengthen considerably the evidence that StAR is the key mediator of the acute induction of steroidogenesis and provide new insights into the mechanisms by which trophic hormones activate steroidogenesis in steroidogenic cells.