Biochemical Characterizations of the Steroidogenic Acute Regulatory (StAR) Protein

While the first step in the production of steroid hormones in the body is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage system (CSCC), the rate-limiting step is the delivery of substrate cholesterol from the outer to the inner mitochondrial membrane and the CSCC. Although it has been known for over three decades that this requires the de novo synthesis of a protein(s) in response to trophic hormone stimulation, the identity of this factor has remained unknown. Our laboratory has recently cloned and characterized a candidate mitochondrial protein that can induce steroid production in the absence of stimulation in transfected MA-10 mouse Leydig tumor cells. As a result, it was named the Steroidogenic Acute Regulatory (StAR) protein.

In this work, we showed that all previously described forms of StAR can be generated from a single mRNA, the 37 kDa form of StAR is the precursor form, and that upon stimulation, transcription of StAR is upregulated. StAR was demonstrated to be sufficient to induce steroidogenesis in isolated MA-10 cell mitochondria, in a specific and time- and dose-dependent manner. Together with other lines of evidence, we conclude that StAR is the long-sought acute regulator of steroidogenesis.

We have further investigated requirements for StAR function. StAR-induced cholesterol transfer was determined to require ATP hydrolysis and a mitochondrial electrochemical gradient in vitro. Consistent with the former requirement, phosphorylation of StAR at serine 194 in mouse and 195 in human was found to be critical for maximal activity. This phosphorylation appears to occur as the precursor form of StAR is processed by the mitochondria to die 32 kDa intermediate.

Disruption of either the electrochemical gradient or the potential across the inner mitochondrial membrane in whole cells resulted in inhibition of steroidogenesis induced by StAR as well as mitochondrial import. Therefore, either import of StAR or another factor. such as mitochondrial calcium, is requisite for stimulation of intermembrane cholesterol delivery to the CSCC. Since we find that StAR can induce cholesterol transfer in nonsteroidogenic cells, the mechanism by which StAR acts is probably conserved through different cell types.