The Balance between Acetylation and Deacetylation Controls Smad7 Stability

Abstract

Transforming growth factor beta (TGF␤) regulates multiple cellular processes via activation of Smad signaling pathways. We have recently demonstrated that the inhibitory Smad7 interacts with the acetyl transferase p300 and that p300 acetylates Smad7 on two lysine residues. These lysine residues are critical for Smurfmediated ubiquitination of Smad7, and acetylation protects Smad7 from TGF␤-induced degradation. In this study we demonstrate that Smad7 interacts with specific histone deacetylases (HDACs) and that the same HDACs are able to deacetylate Smad7. The interaction with HDACs is dependent on the C-terminal MH2 domain of Smad7. In addition, HDAC1-mediated deacetylation of Smad7 decreases the stability of Smad7 by enhancing its ubiquitination. Thus, our results demonstrate that the degradation of Smad7 is regulated by the balance between acetylation, deacetylation and ubiquitination, indicating that this could be a general mechanism to regulate the stability of cellular proteins.Transforming growth factor beta (TGF␤) 1 is a member of the TGF␤ superfamily of cytokines that regulate multiple cellular processes including extracellular matrix production, cell growth, apoptosis, and differentiation. Dysfunction of TGF␤ signaling has been implicated in various human disorders ranging from vascular diseases to cancer progression (for a recent review see Ref. 1). TGF␤ exerts its cellular effects via formation of a heteromeric complex of type I and type II serine/ threonine kinase receptors. The type II receptor phosphorylates and activates the type I receptor, which in turn phosphorylates and activates the receptor-activated Smads (Smad2 and Smad3) in their C-terminal SSXS motif. The activated Smads then interact with Smad4 and translocate into the nucleus where they act as transcription factors together with co-activators and co-repressors (2).The subfamily of inhibitory Smads consists of Smad6 and Smad7, which are immediate early target genes of TGF␤ (3-6). Smad7 is a nuclear protein in resting cells (7); after TGF␤ stimulation, Smad7 translocates out of the nucleus to the plasma membrane in a manner that is dependent on the Smurf E3-ubiquitin ligases (8 -10). At the cell membrane, the Smad7-Smurf complex interacts with the activated receptors and inhibits TGF␤ signaling by blocking the interaction between the receptor-activated Smads and the activated receptors (3, 6). In addition, Smurf ubiquitinates both the receptors and Smad7, thereby inducing the degradation of both the receptors and Smad7 (8, 9).We have recently shown that Smad7 interacts with the transcriptional co-activator p300, a protein acetyl transferase (11). p300-mediated acetylation transfers the acetyl moiety from acetyl coenzyme A to the amino group of a lysine residue of the acceptor protein. Acetylation is a dynamic process and the balance between acetylated and non-acetylated histones has major effects on chromatin structure and transcription (for a recent review see Ref. 12). Histones H3 and H4 are acetylated on specific lysine residues in ...

Regulation of Smad activities

“…It was shown that overexpression of p300 stabilized Smad7 protein, and such effect depends on the acetyltransferease function of p300. In addition, the carboxy-terminal region of Smad7 binds to the histone deacetylase HDAC1, which can reverse the acetylation of Smad7 by p300 [107]. As expected, acetylation of Lys4 and Lys70 prevented ubiquitination of Smad7, which probably accounted for the observed increase in Smad7 stability [106].…”

Lithium inhibits Smad3/4 transactivation via increased CREB activity induced by enhanced PKA and AKT signaling

“…The latter case is demonstrated in an earlier study indicating that Smad7, a TGF-β response gene that negatively regulates TGF-β signaling, is acetylated by p300 and becomes stabilized and protected from proteasome (Smurf-1)-mediated degradation (Gronroos et al, 2002). Conversely, histone deacetylases (HDAC) interact with Smad7 and promotes the degradation of Smad7 protein (Simonsson et al, 2005). The reduction of Smad7 protein expression could further lead to the upregulation of Smad3/4-dependent TGF-β signaling and cause an increase in transcription of downstream genes, such as PAI-1 and p21.…”

To (TGF)β or not to (TGF)β: Fine-tuning of Smad signaling via post-translational modifications

“…This acetylation antagonizes Smurf-mediated ubiquitination of Smad7, resulting in a higher concentration of Smad7 available to bind receptors and block TβR-RSmad interactions, and possibly to increase receptor turnover as Smad7 acetylation did not alter its nuclear export and may thus not prevent Smad7/Smurf mediated degradation of the receptor. Conversely, the same group more recently found that HDAC1-mediated deacetylation of Smad7 decreases the stability of Smad7 by enhancing its ubiquitination [109]. Thus, Smad7 degradation, and TβR level and activity as a consequence, appear to be regulated by a balance between acetylation, deacetylation and ubiquitination.…”