Combined treatment with specific ligands for PPAR gamma : RXR nuclear receptor system markedly inhibits the expression of cytochrome P450arom in human granulosa cancer cells

Our previous study demonstrated that PPAR gamma specific ligand troglitazon e (TGZ) or RXR specific ligand LG100268 (LG) alone decreased the aromatase activity in cultured human ovarian granulosa cells from pre-ovulatory folli cles. and combined treatment caused an even greater reduction in this activ ity. Since similar manners of effects of TGZ or/and LG on the aromatase act ivity in human ovarian granulosa cancer cell line were observed, we perform ed the detailed analysis of the mechanisms of these effects using this cell line. The changes in the aromatase activity were associated with comparabl e changes in the P450arom mRNA levels based on a RNase protection assay, A nuclear run-on assay indicated the P450arom transcript to decrease by 40 an d 66% at 24 and 48 h, respectively, after TGZ plus LG treatment. An RNA sta bility analysis showed the half-life of P450arom mRNA to decrease from 13 t o 9 h after the TGZ plus LG treatment. The inhibitory effect of TGZ plus LG on the aromatase activity and P450arom mRNA may not be mediated by the cAM P-PKA pathway that is usually implicated in the regulation of aromatase act ivity in granulosa cells: because (1) the aromatase activity stimulated by forskolin was not inhibited by TGZ plus LG; (2) the specific PKA inhibitor H89 could not block the inhibitory effect of TGZ plus LG on the aromatase a ctivity; and (3) the luciferase activity of P450arom promoter II did not de crease by the addition of TGZ and LG in transfected cells either at a basic state or in the states stimulated by forskolin or PGE2, respectively. Take n together, these results indicate that TGZ plus LG inhibited the aromatase activity and also decreased the P450arom mRNA level in granulosa cancer ce lls, and the loss of P450arom mRNA expression was considered to be due to b oth the decreased transcription and rapid degradation of its RNA. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.