Nuclear receptors phosphorylation

Chromatin remodeling, transcription factor modification by various enzyme activities, and the communication between the nuclear receptors and the basal transcription apparatus are accomplished by protein-protein interactions with one or more of a class of coregulator molecules. The number of these coregulator molecules now exceeds 100, not counting species variations and splice variants. The first of these to be described was the CREB-binding protein, CBP. CBP, through an amino terminal domain, binds to phosphorylated serine 137 of CREB and mediates transactivation in response to cAMP. It thus is described as a coactivator. CBP and... 

Eventually, nuclear receptors can act as substrata of phosphorylation-dephosphorylationin response to signals originating in other pathways. The state of phosphorylation of the nuclear receptors integrates them within the system of cell membrane signaling. 

Regulation of the proteic activity by means of phosphorylation and dephosphorylation is well known. In the case of nuclear receptors, it has been descr ib ed that the state of phosphorylation affects not only their affinity for the hormone, but also their transcription activity. The process of phosphorylation seems to occur after the receptor binds with the hormone and frees the hsp90, which is a phosphoprotein (Mester et al. 1995). 

These findings complete the panorama relative to the mechanisms of hormonal action mediated by nuclear receptors. Thus, gene activation mediated by nuclear receptors can respond to three clearly differentiated modalities (1) receptor bound to hormone and not phosphorylated, (2) receptor bound to hormone and phosphorylated, and (3) receptor not bound to hormone and phosphorylated (Filardo 2002 Lee et al. 2002 Powles 2002). 

The histone deacetylases are found in large protein complexes, often together with repressive transcription factors. By this token, interactions of the repressive heterodi-meric transcription factor Mad-Max and a complex with the histone deacetylase HDAC I and the mSinSA protein have been demonstrated. A complex of HDAC I and the nuclear receptor-corepressor (see chapter 4) binds to unliganded nuclear receptors and is believed to exercise a repressive effect. A further example is the tumor suppressor protein pRb (see chapters 13,14), which can occur as a transcription repressor in the hypo-phosphorylated form and transcriptionally activating in the hyperphosphorylated form. The repressive form of the pRb protein recruits the histone deacetylase HDAC 1 to the DNA and thereby initiates an active repression of the gene (see 13.3.2). 

Apart from direct activation via ligand binding, nuclear receptors are also subject to regulation by phosphorylation. Thus, transcriptional activity of PPARy can be regulated by growth factor stimulation via the mitogen-activated protein (MAP) kinase pathway (34). 

We also assessed the effect of phosphorylation of the ERp AF-1 in regulating interaction with SRC-1 in vivo using transfection experiments. Treatment of transfected cells with PD98059, a selective inhibitor of MAPK activation, completely abrogated the SRC-1-mediated activation of unliganded ER 3, whereas use of staurosporin, which inhibits protein kinase C, had no significant effect (Tremblay et al., 1999). The interaction between ER 3 and SRC-1 in the absence of hormone was also demonstrated in vivo and shown to be influenced by factors known to change the phosphorylation status of nuclear receptors. 

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. 

Desclozeaux M, et al. Phosphorylation and intramolecular stabilization of the ligand binding domain in the nuclear receptor steroidogenic factor 1. Mol. CeU Biol. 2002 22 7193-7203. Wansa KD, Harris JM, Muscat GE. The activation function-1 domain of Nur77/NR4A1 mediates trans-activation, cell specificity, and coactivator recruitment. J. Biol. Chem. 2002 277 33001-33011. 

Ntcp and Oatpl are regulated by the nuclear receptor HNF4a (nuclear receptor 2A1), which is a key element of hepatocyte differentiation [64]. A functional downregulation occurs by phosphorylation by extracellular ATP and activation by protein kinase C, but not by protein kinase A [65, 66]. 

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... 

In addition to the receptors themselves, the nuclear receptor coactivators have also been found to be targets of regulatory phosphorylation events. 

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