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

Abstract

Using Western blot as the major technique, we studied the effects of the three Food and Drug Administration (FDA)-approved aromatase inhibitors (AI) on aromatase protein stability in the aromatase-overexpressing breast cancer cell line MCF-7aro. We have found that exemestane treatment significantly reduces aromatase protein level. Exemestane induces aromatase degradation in a dose-responsive manner (25-200 nmol/L), and the effect can be seen in as early as 2 hours. Metabolic labeling with S 35 -methionine was used to determine the half-life (t 1/2 ) of aromatase protein. In the presence of 200 nmol/L exemestane, the t 1/2 of aromatase was reduced to 12.5 hours from 28.2 hours in the untreated cells. Furthermore, exemestane-induced aromatase degradation can be completely blocked by 10 Mmol/L MG132, indicating that the degradation is mediated by proteasome. We also examined the effect of exemestane on aromatase mRNA level using realtime reverse transcription-PCR. No significant changes in mRNA level were detected after 8 hours of treatment with exemestane (200 nmol/L). This is the first report on the evaluation of three FDA-approved AIs on the stability of the aromatase protein. We have found that exemestane, different from letrozole and anastrozole, can destabilize the aromatase protein. (Cancer Res 2006; 66(21): 10281-6)

The HDAC inhibitor LBH589 (panobinostat) is an inhibitory modulator of aromatase gene expression

“…5). The proliferation of hormone-dependent breast cancer also can be suppressed by sulphatase inhibitors (53) and aromatase degraders (8). We now report that LBH589 can suppress aromatase promoters I.3/II in a selective and potent manner.…”

“…Anastrozole and letrozole act as competitive inhibitors with respect to the androgen substrates. Exemestane is a mechanism-based inhibitor that is catalytically converted into a chemically reactive species, leading to irreversible inactivation of aromatase, as well as degradation of aromatase protein (8).…”

Genome-Wide Analysis of Aromatase Inhibitor-Resistant, Tamoxifen-Resistant, and Long-Term Estrogen-Deprived Cells Reveals a Role for Estrogen Receptor

“…Real-time PCR as well as Western analysis showed that no major changes had occurred in aromatase or ERa mRNA and protein levels in the LTEDaro or resistant cell lines, compared with MCF-7aro, except aromatase protein levels in T+EXE R (Supplementary Table S2). Wang and Chen (31) have reported that in addition to its action as a mechanism based inhibitor, EXE is also an aromatase destabilizer on the protein level. In addition, in-cell aromatase activity assays showed that aromatase was still enzymatically active and responded to AI treatment in the resistant cell lines, again, with the exception of T+EXE R lines due to mechanism based inhibition (Supplementary Table S2).…”

The Role of Amphiregulin in Exemestane-Resistant Breast Cancer Cells: Evidence of an Autocrine Loop

“…Exemestane binds to aromatase irreversibly and causes time-dependent aromatase inactivation (4). A recent study from our lab showed that exemestane could destabilize aromatase protein, in addition to inhibiting its activity (5).…”