Binding features of steroidal and nonsteroidal inhibitors

Aromatase is the rate-limiting enzyme in estrogen biosynthesis. As a cytochrome P450, it utilizes electrons from NADPH–cytochrome P450 reductase (CPR) to produce estrogen from androgen. Estrogen is a key factor in the promotion of hormone-dependent breast cancer growth. Aromatase inhibitors (AIs) are drugs that block estrogen synthesis, and are widely used to treat estrogen-dependent breast cancer. Structure–function experiments have been performed to study how CPR and AIs interact with aromatase to further the understanding of how these drugs elicit their effects. Our studies have revealed a strong interaction between aromatase and CPR, and that the residue K108 is situated in a region important to the interaction of aromatase with CPR. The published X-ray structure of aromatase indicates that the F221, W224 and M374 residues are located in the active site. Our site-directed mutagenesis experiments confirm their importance in the binding of the androgen substrate as well as AIs, but these residues interact differently with steroidal inhibitors (exemestane) and non-steroidal inhibitors (letrozole and anastrozole). Furthermore, our results predict that the residue W224 also participates in the mechanism-based inhibition of exemestane, as time-dependent inhibition is eliminated with mutation on this residue. Together with previous research from our laboratory, this study confirms that W224, E302, D309 and S478 are important active site residues involved in the suicide mechanism of exemestane against aromatase.

► Aromatase forms an electron transfer complex with the CPR protein during estrogen biosynthesis.
► We revealed the importance of the K108 residue of aromatase in its association with CPR.
► Our data showed that aromatase residues F221, W224 and M374 are involved in androgen binding.
► These residues interact differently with steroidal and non-steroidal inhibitors of aromatase.
► Of these residues, only W224 participates in the mechanism-based inhibition of exemestane.