Comparative study of the molecular mechanism of action of the synthetic progestins, Medroxyprogesterone acetate and Norethisterone acetate

___________________________________________________ Medroxyprogesterone acetate (MPA) and norethisterone (NET) and its derivatives (norethisterone enanthate (NET-EN); norethisterone acetate (NETA)), are used by millions of women as contraceptives and in hormone replacement therapy (HRT). Although both progestins are widely used, very little is known about their mechanism of action at the molecular level. In this thesis, the differential regulation of gene expression and molecular mechanism of action via different steroid receptors by these synthetic progestons, as compared to progesterone (Prog) was investigated in human cell lines. In the first part of the study, the effect of Prog, MPA and NET-A on the expression of endogenous cytokine genes was investigated in two epithelial cell lines of the human female genital tract, Ect1/E6E7 (an ectocervical cell line) and Vk2/E6E7 (a vaginal cell line). Quantitative realtime RT-PCR (QPCR) showed ligand-specific and cell-specific regulation of the interleukin (IL)-6, IL-8 and RANTES (Regulated-upon-Activation, Normal T cell Expressed and Secreted) genes with Prog, MPA and NET-A. Moreover, the repression of the TNFα-induced RANTES gene by MPA in the Ect1/E6E7 cell line was found to be mediated by the androgen receptor (AR). The second part of the study focused on elucidating the androgenic activities of these two progestins, in comparison to Prog. Competitive binding in whole cells revealed that Prog, MPA and NET-A have a similar binding affinity for the hAR as the natural androgen dihydrotestosterone (DHT). Both transactivation and transrepression transcriptional assays demonstrate that, unlike Prog, MPA and NET-A are efficacious AR agonists, with activities comparable to DHT. Using a mammalian two-hydrid assay, it was shown that MPA and NET-A exert their androgenic actions by different mechanisms. NET-A, like DHT and other well-characterised androgens, induces the ligand-dependent interaction between the NH2and COOH-terminal domains (N/C-interaction) of the AR independent of promoter-context, while MPA does this in a promoterdependent manner. In the third part of this study, competitive binding revealed that MPA and NET-A have a similar binding affinity to each other, but about a 100-fold lower affinity than Prog for the human mineralocorticoid receptor (hMR), while RU486 has an even lower affinity for the hMR. Promoter-reporter assays showed that MPA, NET-A and RU486 are all antagonists of the hMR, but unlike Prog, they have weak antagonistic activity. However, on the endogenous MR-regulated Orm-1 (α-glycolytic protein or orosomucoid-1) gene expressed in a rat cardiomyocyte cell line, NET-A and RU486, but not MPA, has similar antagonistic activity as Prog. This study is the first to show that, NET-A and RU486, but not MPA, can dissociate between transrepression and transactivation via the hMR. Taken together, these results show that natural Prog and the synthetic progestins, MPA and NET-A display differential promoter-, celland receptor-specific effects on gene expression. Furthermore they may have important implications for cervicovaginal immune function, cardiovascular and other physiological functions.