Steroid ligand-independent activation

Ligand-Independent Activation of Steroid Hormone Receptors and Non-genomic Effects of Steroids... 

Power, R.F., Maui, S.K., Codina, J., Conneely, O.M.and O Malley, B.W. Dopaminergic and Ligand-independent activation of steroid hormone receptors (1991) Science 254,1636-1639... 

The ligand-independent activation of AF-1 is presumed to be closely related to phosphorylation of steroid receptors by cellular protein kinases (Weigel, 1996). 

Our observations suggest that Ser-106 and Ser-124 are both required in vivo to fully recruit SRC-1. In addition, when cells were treated with factors known to activate Ras, such as EGF or IGF-1 (data not shown), the in vivo interaction between SRC-1 and ER(3 was also enhanced, thus mimicking the results obtained in the presence of activated Ras. This study demonstrates for the first time that phosphorylation of the AF-1 domain of a member of the nuclear receptor superfamily enhances the recruitment of a steroid receptor coactivator (SRC-1) and provides a molecular basis for ligand-independent activation of ER(3 via the MAPK cascade. 

Weigel NL, Zhang Y. 1998. Ligand-independent activation of steroid hormone receptors. J. Molec. Med. 76 469-79... 

Peroxisome proliferator activated receptors (PPARs) are members of the nuclear hormone receptors superfamily of ligand-activated transcription factors that are related to retinoid, steroid and thyroid receptors. All members of this superfamily have a similar structure the amino-terminal region allows ligand-independent activation, confers con-... 

The phosphorylation of nuclear receptors on Ser/Thr residues is a key mechanism for the coupling of nuclear receptor signaling to other signaling pathways of the cell. Many nuclear receptors are isolated as phosphoproteins, and their phosphorylation provides a means for ligand-independent activation and regulation. The steroid... 

Furthermore, post-translational modifications activate steroid hormone receptors in a ligand-independent fashion (Fig. 5), as shown for the ERa which is phosphorylated on serine residue 118 in the AF-1 domain by the Erkl/2 kinase [71]. In vitro, the serine-118 phosphorylated ERa is transcriptionally active in a ligand-independent fashion. 

The two ERs share many functional characteristics based on their well conserved modular structure. As summarized above, AF-2 is responsible for estrogen-dependent activation through recruitment of coactivator proteins including members of the steroid receptor coactivator (SRC) family (Anzick et al., 1997 Chen et al., 1997 Hong et al., 1996 Kamei et al., 1996 Li et al., 1997 Onate et al., 1995 Torchia et al., 1997 Voegel et al., 1996). On the other hand, AF-1 activity is constitutive and ligand-independent (Berry et al., 1990 Kumar et al., 1987 Metzger et al., 1995). 

O Malley BW, Schrader WT, Mani S, Smith C, Weigel NL, et al. 1995. An alternative ligand-independent pathway for activation of steroid receptors. Recent Prog. Horm. Res. 50 333 17... 

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