Among diseases unique to pregnancy,
intrahepatic cholestasis of pregnancy is the most prevalent disorder with elevated serum
bile acid levels. We have previously shown that
estrogen 17β-estradiol (E2) transrepresses
bile salt export pump (BSEP) through an interaction between
estrogen receptor (ER)-α and farnesoid X receptor (FXR) and transrepression of BSEP by E2/ERα is an etiological contributing factor to
intrahepatic cholestasis of pregnancy. Currently the mechanistic insights into such transrepression are not fully understood. In this study, the dynamics of coregulator recruitment to BSEP promoter after FXR activation and E2 treatment were established with quantitative
chromatin immunoprecipitation assays. Coactivator
peroxisome proliferator-activated receptor-γ coactivator-1 was predominantly recruited to the BSEP promoter upon FXR activation, and its recruitment was decreased by E2 treatment. Meanwhile, recruitment of
nuclear receptor corepressor was markedly increased upon E2 treatment. Functional evaluation of ERα and ERβ chimeras revealed that domains AC of ERα are the determinants for ERα-specific transrepression on BSEP. Further studies with various truncated ERα
proteins identified the domains in ERα responsible for
ligand-dependent and
ligand-independent transrepression. Truncated ERα-AD exhibited potent
ligand-independent transrepressive activity, whereas ERα-CF was fully capable of transrepressing BSEP
ligand dependently in vitro in Huh 7 cells and in vivo in mice. Both ERα-AD and ERα-CF
proteins were associated with FXR in the coimmunoprecipitation assays. In conclusion, E2 repressed BSEP expression through diminishing
peroxisome proliferator-activated receptor-γ coactivator-1 recruitment with a concurrent increase in
nuclear receptor corepressor recruitment to the BSEP promoter. Domains AD and CF in ERα mediated
ligand-independent and
ligand-dependent transrepression on BSEP, respectively, through interacting with FXR.