Regulation of Smad signaling by PARG and PARP-1.

<p>(<b>a, b</b>) Real-time RT-PCR analysis of endogenous <i>fibronectin (FN1)</i> (a) and <i>PAI-1</i> (b) mRNAs in HaCaT cells transiently transfected with the indicated siRNAs (bottom of panel b) prior to stimulation (or not) with 5 ng/ml TGFβ1 for 24 h. The data are graphed as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103651#pone-0103651-g007" target="_blank">Fig. 7g, h</a>. Stars (panels a, b) indicate statistical significance, <i>p</i><0.05. The figure shows representative experiments from four or more repeats. (<b>c</b>) A model depicting TGFβ dimeric ligand that activates its cell surface type II (RII) and type I (RI) receptors, which phosphorylates Smad2 and Smad3, leading to oligomerization of Smad2, Smad3 and Smad4 into trimeric complexes. Smad oligomers enter the nucleus via nuclear pores and associate with chromatin in order to regulate transcription of target genes such as <i>Smad7</i>, <i>fibronectin (FN1)</i> and <i>PAI-1</i>. Nuclear PARP-1 and PARP-2 in complex associate with the Smad oligomer. For simplicity distinct complexes between Smads and PARP-1 and Smads and PARP-2 are not shown but their presence is supported by the experimental evidence. PARP-1/PARP-2 use NAD and oligo(ADP-ribosyl)ate Smad3 and Smad4 (ADP-ribose chains in red) and assist dissociation of Smads from DNA (as demonstrated in ref. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103651#pone.0103651-Lnn2" target="_blank">[9]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103651#pone.0103651-Zhang1" target="_blank">[10]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103651#pone.0103651-Huang1" target="_blank">[11]</a>). PARG associates with ADP-ribose chains on the Smad complex and removes ADP-ribose units (ADPr) possibly generating mono(ADP-ribosyl)ated Smads (not shown). PARG therefore promotes Smad association with DNA and is required for optimal gene expression in response to TGFβ.