Inhibitory Cross-talk between Estrogen Receptor (ER) and Constitutively Activated Androstane Receptor (CAR)

Abstract

Estrogen receptor (ER) activity can be modulated by the action of other nuclear receptors. To study whether ER activity is altered by orphan nuclear receptors that mediate the cellular response to xenobiotics, cross-talk between ER and constitutive androstane receptor (CAR), steroid and xenobiotic receptor, or peroxisome proliferator-activated receptor ␥ was examined in HepG2 cells. Of these receptors, CAR substantially inhibited ER-mediated transcriptional activity of the vitellogenin B1 promoter as well as a synthetic estrogen responsive element (ERE)-containing promoter. Treatment with an agonist of CAR, 1,4-bis-(2-(3,5-dichloropyridoxyl))benzene, potentiated CAR-mediated transcriptional repression. In contrast, an antagonist of CAR, androstenol, alleviated the repression effect. Although CAR interacted with the ER in solution, CAR did not interact with the ER bound to the ERE. CAR/retinoid X receptor bound to the ERE but with much lower affinity than ER. Incremental amounts of CAR elicited a progressive reduction of the ER activity induced by the p160 coactivator glucocorticoid receptor interacting protein 1 (GRIP-1). In turn, increasing amounts of GRIP-1 progressively reversed the depression of ER activity by CAR. An agonist or antagonist of CAR potentiated or alleviated, respectively, the CAR-mediated repression of the GRIP-1-enhanced ER activity, which is consistent with the ability of theses ligands to increase or decrease, respectively, the interaction of CAR with GRIP-1. A CAR mutant that did not interact with GRIP-1 did not inhibit ER-mediated transactivation. Our data demonstrate that xenobiotic nuclear receptor CAR antagonizes ER-mediated transcriptional activity by squelching limiting amounts of p160 coactivator and imply that xenobiotics may influence ER function of female reproductive physiology, cell differentiation, tumorigenesis, and lipid metabolism.

Structural Determinants of Constitutive Androstane Receptor Required for Its Glucocorticoid Receptor Interacting Protein-1-mediated Nuclear Accumulation

“…Plasmids-The expression vectors pGEX2TKCAR, pGEX2TKCAR⌬8 (CAR (1-350)), and pEGFPC1CAR have been described (9,17). The mutations in the mCAR mutants, C21A/C24A, L326A, and L322A/ L326A/L329A, were introduced by the QuikChange site-directed mutagenesis system in the pEGFPC1CAR or pGEX2TKCAR vectors as described by the manufacturer (Stratagene, La Jolla, CA).…”

Endocrine disruptive chemicals: mechanisms of action and involvement in metabolic disorders

“…Although ARNT is not considered to be a classical transcriptional co-activator, it shares many properties with the SRC family of co-activators and has been shown to act as co-activator of the ERs (Brunnberg et al 2003), in particular of ERb (Rü egg et al 2007). Competition for common co-activators occurs also within the NR family; it has been shown that activation of CAR, a NR involved in metabolism of xenobiotic substances, inhibits ER activity by reducing the available levels of the p160 co-activator GRIP-1 (Min et al 2002).…”

Estrogen modulates transactivations of SXR-mediated liver X receptor response element and CAR-mediated phenobarbital response element in HepG2 cells

“…Recent studies (Min et al, 2002b) demonstrated that the action of ER in transcriptional activity can be modulated by functional cross-talk between ER and other nuclear receptors. Specifically, the xenobiotic nuclear receptors, constitutive androstane receptor (CAR) and steroid and xenobiotic receptor (SXR), inhibit ER-mediated transactivation.…”

“…The liver is the major organ that metabolizes steroids and xenobiotic chemical compounds, and one of the target organs for estrogen action in the body (Min et al, 2002b). ER and the orphan nuclear receptors, SXR and CAR, and their heterodimeric partner RXR are all expressed in the liver (Kliewer et al, 1999).…”

“…The orphan nuclear receptor CAR mediates induction of cytochrome P450 (CYP)2B genes, which are steroid hydroxylases, by the classical inducer of drug metabolism, phenobarbital (PB), in the liver (Wei et al, 2000;Min et al, 2002aMin et al, , 2002b. CAR is sequestered in the cytoplasm and upon exposure to agonists, such as PB and 1 , 4 -b is -( 2 -( 3 ,5 -d ic h lo r o p y r id o x y l) ) b e n z e n e (TCPOBOP) (Tzameli et al, 2000), translocated into the nucleus (Kawamoto et al, 1999;Min et al, 2002a), where it binds to its cognate recognition sites called nuclear receptor (NR)-1 and NR-2 as a CAR/retinoid X receptor (RXR) heterodimer in the phenobarbital-responsive enhancer (PBRU) of the CYP2B genes (Trottier et al, 1995;Honkakoski et al, 1998;Kawamoto et al, 1999;Kim et al, 2001;Min et al, 2002a).…”