106 Randel Lecture: The history of the discovery of the Steroidogenic Acute Regulatory (StAR) Protein

This two-part presentation regarding acute regulation of steroid biosynthesis documents discovery of the StAR protein and resolves controversy regarding mitochondrial cholesterol transport. The acute regulation of steroid biosynthesis was known to require de novo synthesis of a regulator protein to mediate the transfer of cholesterol, the substrate for steroids, from the outer to the inner mitochondrial membrane where it was converted to pregnenolone by the cytochrome P450 side chain cleavage enzyme. We discovered a novel protein that was tightly correlated with steroid biosynthesis and had the requisite characteristics for the putative acute regulator of cholesterol transfer for steroid synthesis. Further studies confirmed that StAR protein is an indispensable component in the process of mitochondrial uptake of the cholesterol substrate for steroidogenesis. The translocator protein (TSPO) is a mitochondrial outer membrane protein suggested to import cholesterol to the inner mitochondrial membrane. However, it was demonstrated in vivo in Leydig cell specific TSPO conditional knockout mice that TSPO was not required for testosterone production or fertility. Similarly, global TSPO knockout (TSPO/-) mice were viable and fertile with fecundity equivalent to TSPO floxed (TSPOfl/fl) controls. Adrenal and gonadal steroidogenesis did not differ between TSPOfl/ fl and TSPO-/- mice. In vitro use of different steroidogenic cell line models (MA-10, MLTC, Y-1, H295R and R2C) demonstrated that siRNA-knockdown of TSPO did not affect steroidogenesis. Also, CRISPR/ Cas9-mediated TSPO deletion did not affect MA-10 cell steroidogenesis. These results directly 1) refute the suggestion that TSPO is indispensable for viability and steroid hormone biosynthesis; and, 2) substantiate the primal role of the StAR protein as the rate limiting factor in steroid hormone biosynthesis.