Aromatase inhibitors and enzyme stability.

Abstract

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 in vitro and in vivo. Immunocytochemistry revealed increased quantities of immunoreactive aromatase in human choriocarcinoma-derived JEG-3 cells in response to pretreatment with the non-steroidal inhibitors. To elucidate this effect in detail, aromatase protein in JEG-3 cells after treatment with various inhibitors was quantified using an enzyme-linked immunosorbent assay (ELISA). A time-dependent increase in aromatase protein in the cells was observed with all the aromatase inhibitors except 4-hydroxyandrostenedione, whereas aromatase mRNA levels in the cells remained unchanged during the inhibitor treatment. The three non-steroidal agents caused an approximately fourfold increase in aromatase protein in the cells 24 h after the treatment, as compared with untreated controls. The increase in aromatase protein in the cells was not blocked by treatment with cycloheximide, an inhibitor of protein synthesis. The inhibitors also appeared to block the rapid degradation observed in JEG-3 cells after induction by forskolin. In vivo, daily injection of the inhibitors into adult female mice caused increases in levels of both aromatase mRNA and protein in the ovary. The increase in aromatase mRNA in this in vivo study could be explained by an increase in gonadotropin concentrations in response to decreased plasma concentrations of estrogens. In conclusion, we suggest that aromatase inhibitors increase aromatase Endocrine-Related Cancer (1999) 6 211-218 protein through stabilization and reduced protein turnover.

Aromatase Destabilizer: Novel Action of Exemestane, a Food and Drug Administration–Approved Aromatase Inhibitor

“…They reported a time-dependent increase of aromatase protein with three nonsteroidal inhibitors ( fadrozole, vorozole, and pentrozole) and a steroidal inhibitor (atamestane), whereas aromatase mRNA levels were not affected. Harada et al (16) did not find any inhibitors that could induce aromatase degradation.…”

Mechanism of PTC124 activity in cell-based luciferase assays of nonsense codon suppression

“…Firefly luciferase contains two domains, linked by a hinge region that, upon substrate binding, reorientate to form a closed conformation (26,27). Ligand binding to firefly luciferase has been shown to protect the enzyme from proteolysis, and a similar stabilization mechanism has been found for other enzymes (6,28). We examined the ability of PTC124 to protect purified FLuc and RLuc from trypsin digestion (Fig.…”

Use of aromatase inhibitors in breast carcinoma.

“…Interestingly, products secreted by epithelial cells in the breast tumors appear to stimulate aromatase in the stroma and provide a means for autoregulation of tumor growth through estrogen production. A rather novel means of regulation of aromatase levels was also recently described -the stabilization of degradation of enzyme (Harada et al 1999). Aromatase inhibitors bind to the active site of the enzyme and, through mechanisms not completely understood, prevent proteolysis of the aromatase protein.…”