Vol. 285, Issue 3, 1296-1302, June 1998

Both the Immunosuppressant SR31747 and the Antiestrogen Tamoxifen Bind to an Emopamil-Insensitive Site of Mammalian 8-7 Sterol Isomerase

Raymond Paul¹ , Sandra Silve¹ , Nathalie De Nys, Pascal-Henry Dupuy, Christine Labit-Le Bouteiller, Jorge Rosenfeld, Pascual Ferrara, Gérard Le Fur, Pierre Casellas and Gérard Loison

Sanofi Recherche, F-34184 Montpellier Cédex 04, France (R.P., N.De-N., P.C.); Sanofi Recherche, F-31676 Labège Cédex, France (S.S., P.-H.D., C.L.-LeB., J.R., P.F., G.L.); and Sanofi Recherche, 75374 Paris Cédex, France (G.LeF.)

SR31747 is a novel agent that elicits immunosuppressive and anti-inflammatory effects. This drug was shown to inhibit 8-7 sterol isomerase in yeast. To test whether this enzyme could also be an SR31747 target in mammals, the binding, antiproliferative and sterol biosynthesis inhibitory properties of various drugs were studied in recombinant sterol isomerase-producing yeast cells. Our results clearly show that SR31747 is a high affinity ligand of recombinant mammalian sterol isomerase (K_d = 1 nM). Tridemorph, a sterol biosynthesis inhibitor that is widely used in agriculture as an antifungal agent, is also a powerful inhibitor of murine and human sterol isomerases (IC₅₀ value in the nanomolar range). Some drugs, like cis-flupentixol, trifluoperazine, 7-ketocholestanol and tamoxifen, inhibit SR31747 binding only with the mammalian enzymes, whereas other drugs, like haloperidol and fenpropimorph, are much more effective with the yeast enzyme than with the mammalian ones. Emopamil, a high affinity ligand of human sterol isomerase, is inefficient in inhibiting SR31747 binding to its mammalian target, suggesting that the SR31747 and emopamil binding sites on mammalian sterol isomerase do not overlap. In contrast, SR31747 binding inhibition by tamoxifen is very efficient and competitive (IC₅₀ value in the nanomolar range), indicating that mammalian sterol isomerase contains a so-called antiestrogen binding site. Tamoxifen is found to selectively inhibit sterol biosynthesis at the sterol isomerase step in the cells that are producing the mammalian enzyme in place of their own sterol isomerase. Finally, we also show that tridemorph, a sterol biosynthesis inhibitor widely used in agriculture as an antifungal agent, is not selective of yeast 8-7 sterol isomerase but is also highly efficient against murine 8-7 sterol isomerase or human 8-7 sterol isomerase. This observation contrasts with our already published results showing that fenpropimorph, another sterol isomerase inhibitor used in agriculture, is only poorly efficient against the mammalian enzymes.