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Nuclear export of Smad2 and Smad3 by RanBP3 facilitates termination of TGF-beta signaling.

ABSTRACT: Smad2 and Smad3 (Smad2/3) are key intracellular signal transducers for TGF-beta signaling, and their transcriptional activities are controlled through reversible phosphorylation and nucleocytoplasmic shuttling. However, the precise mechanism underlying nuclear export of Smad2/3 remains elusive. Here we report the essential function of RanBP3 in selective nuclear export of Smad2/3 in the TGF-beta pathway. RanBP3 directly recognizes dephosphorylated Smad2/3, which results from the activity of nuclear Smad phosphatases, and mediates nuclear export of Smad2/3 in a Ran-dependent manner. As a result, increased expression of RanBP3 inhibits TGF-beta signaling in mammalian cells and Xenopus embryos. Conversely, depletion of RanBP3 expression or dominant-negative inhibition of RanBP3 enhances TGFbeta-induced antiproliferative and transcriptional responses. In conclusion, our study supports a definitive role for RanBP3 in mediating Smad2/3 nuclear export and terminating TGF-beta signaling.

SUBMITTER: Dai F

PROVIDER: S-EPMC2676691 | biostudies-literature | 2009 Mar

REPOSITORIES: biostudies-literature

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Nuclear export of Smad2 and Smad3 by RanBP3 facilitates termination of TGF-beta signaling.

Dai Fangyan F Lin Xia X Chang Chenbei C Feng Xin-Hua XH

Developmental cell 20090301 3

Smad2 and Smad3 (Smad2/3) are key intracellular signal transducers for TGF-beta signaling, and their transcriptional activities are controlled through reversible phosphorylation and nucleocytoplasmic shuttling. However, the precise mechanism underlying nuclear export of Smad2/3 remains elusive. Here we report the essential function of RanBP3 in selective nuclear export of Smad2/3 in the TGF-beta pathway. RanBP3 directly recognizes dephosphorylated Smad2/3, which results from the activity of nucl ...[more]

PMID: 19289081

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Project description:Prevailing insulin resistance and the resultant hyperglycemia elicits a compensatory response from pancreatic islet beta cells (β-cells) that involves increases in β-cell function and β-cell mass. However, the sustained metabolic stress eventually leads to β-cell failure characterized by severe β-cell dysfunction and progressive loss of β-cell mass. Whereas, β-cell dysfunction is relatively well understood at the mechanistic level, the avenues leading to loss of β-cell mass are less clear with reduced proliferation, dedifferentiation, and apoptosis all potential mechanisms. Butler and colleagues documented increased β-cell apoptosis in pancreas from lean and obese human Type 2 diabetes (T2D) subjects, with no changes in rates of β-cell replication or neogenesis, strongly suggesting a role for apoptosis in β-cell failure. Here, we describe a permissive role for TGF-β/Smad3 in β-cell apoptosis. Human islets undergoing β-cell apoptosis release increased levels of TGF-β1 ligand and phosphorylation levels of TGF-β's chief transcription factor, Smad3, are increased in human T2D islets suggestive of an autocrine role for TGF-β/Smad3 signaling in β-cell apoptosis. Smad3 phosphorylation is similarly increased in diabetic mouse islets undergoing β-cell apoptosis. In mice, β-cell-specific activation of Smad3 promotes apoptosis and loss of β-cell mass in association with β-cell dysfunction, glucose intolerance, and diabetes. In contrast, inactive Smad3 protects from apoptosis and preserves β-cell mass while improving β-cell function and glucose tolerance. At the molecular level, Smad3 associates with Foxo1 to propagate TGF-β-dependent β-cell apoptosis. Indeed, genetic or pharmacologic inhibition of TGF-β/Smad3 signals or knocking down Foxo1 protects from β-cell apoptosis. These findings reveal the importance of TGF-β/Smad3 in promoting β-cell apoptosis and demonstrate the therapeutic potential of TGF-β/Smad3 antagonism to restore β-cell mass lost in diabetes.

Dataset Information

Nuclear export of Smad2 and Smad3 by RanBP3 facilitates termination of TGF-beta signaling.

Publications

Nuclear export of Smad2 and Smad3 by RanBP3 facilitates termination of TGF-beta signaling.

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