Letrozole increases growth of rat theca-interstitial cells and CYP17A1 gene expression in the rat ovary

Abstract

Letrozole (LET) is a non-steroidal aromatase inhibitor, blocking the conversion of androgens to estrogens in granulosa cells. However, little is known regarding the effect of LET on theca-interstitial cell function. This study evaluated the effect of LET in vivo and in cultures of theca-interstitial cells. In-vitro and in-vivo studies. Effects of LET were studied in intact rats receiving either LET (90-day-continuous-release sc pellets, 400 μg/day) or placebo pellets (control group). After 10 weeks, the animals were sacrificed, ovaries were isolated and blood samples collected. In vitro experiments evaluated theca-interstitial cells isolated from 30 day-old female rats and cultured for up to 48 hrs in the absence (control) or in the presence of LET (0.1-1μM). Cell proliferation was evaluated by quantification of DNA synthesis as determined by radiolabelled thymidine incorporation assay. CYP17A1 gene expression was evaluated by qrt-PCR using HPRT as a reference gene. Androstenedione levels in plasma were determined by liquid chromatography-mass spectrometry. Comparisons between groups were performed by ANOVA followed by post-hoc pairwise testing. LET, in vivo, induced an increase in ovarian size compared to the control group: 179.1 ± 9.46 mg vs. 115 ± 5.19 mg (P<0.05). LET also induced a 57-fold (P<0.001) increase in CYP17A1 gene expression and a 52-fold (P<0.001) increase in plasma androstenedione level. In vitro, LET stimulated theca-interstitial cell proliferation in a concentration-dependent fashion by up to 65% (P<0.01) at the highest concentration (1 μM). LET stimulates growth of the theca-interstitial cell compartment and increases expression of the key enzyme (CYP17A1) regulating the androgen biosynthesis pathway in the rat. The present findings reveal novel mechanisms of action of LET in the rat ovary.