Inhibitors of aromatase prevent degradation of the enzyme in cultured human tumour cells

The effects of two steroidal (4-hydroxyandrostenedione and atamestane) and three non-steroidal (fadrozole, vorozole and pentrozole) aromatase inhibitors on the levels of aromatase mRNA and protein were examined using cultured JEG-3 and HepG2 cells. Immunocytochemical studies demonstrated increased quantities of immunoreactive aromatase in both cell types as a result of these treatments. To clarify this effect in detail, quantitation of aromatase protein in JEG-3 cells was performed after various treatments using an enzyme-linked immunosorbent assay. Time-dependent increase was observed with all the aromatase inhibitors except 4-hydroxyandrostenedione. The three non-steroidal agents caused an approximately fourfold elevation in the cells 24 h after the treatment compared with untreated controls. The inhibitors also appeared to block the rapid degradation observed in JEG-3 cells after induction with forskolin. However, aromatase mRNA levels in JEG-3 cells remained unchanged, Furthermore, the increase in aromatase protein in JEG-3 cells due to the inhibitor action was not blocked by treatment with cycloheximide, an inhibitor of protein synthesis. These results thus suggest that aromatase inhibitors increase aromatase protein through stabilization and reduced protein turnover as a side-effect of their binding.