Target validation uncouples TSPO from 19-Atriol-mediated inhibition of steroidogenesis and reveals true enzymatic targets
The mitochondrial translocator protein (TSPO) was once proposed to mediate mitochondrial cholesterol import for steroid hormone biosynthesis, but genetic deletion studies in multiple models have refuted this role. Nevertheless, the idea that pharmacological ligands of TSPO can modulate steroid output continues to be invoked. One such compound, 19-Atriol (androst-5-ene-3{beta},17{beta},19-triol), was reported to inhibit progesterone synthesis via TSPO binding in MA-10 Leydig cells. To evaluate this proposed mechanism, we used CRISPR/Cas9-generated Tspo-deleted MA-10 cells to study 19-Atriol activity. We found that 19-Atriol inhibited Bt2-cAMP-stimulated steroid output independent of TSPO expression; it acted as a competitive inhibitor of 3{beta}-hydroxysteroid dehydrogenase (3{beta}-HSD), blocking the conversion of pregnenolone to progesterone. Mass spectrometry revealed that 19-Atriol is also a substrate for 3{beta}-HSD, yielding 19-hydroxytestosterone (19-OHT), which itself inhibits 3{beta}-HSD activity. In addition to this effect, both 19-Atriol and 19-OHT decreased cholesterol-to-pregnenolone conversion during stimulation. Partial inhibition of 22R-hydroxycholesterol metabolism by CYP11A1 was observed with 19-Atriol, but not 19-OHT, suggesting direct or indirect effects on this upstream step, potentially involving the steroidogenic acute regulatory protein (STAR). These findings decisively exclude TSPO as a functional mediator of 19-Atriol activity and instead identify direct enzymatic targets within the de novo steroidogenic pathway. By resolving a key mechanistic misattribution, this study underscores the importance of rigorous target validation, particularly for compounds previously assumed to act via TSPO.