TGF-β signaling: A new piece of the puzzle

TGF-β signaling promotes the nuclear accumulation of CLIC4, which mediates the transcriptional response to TGF-β by protecting phospho-Smad2 and 3 from dephosphorylation.

Chloride intracellular channel 4 (CLIC4) has an important role in a number of processes, including cell-cycle control and apoptosis. Stressors such as DNA damage induce CLIC4 nuclear translocation; furthermore, CLIC4 is excluded from the nucleus in some cancers. The nuclear function of CLIC4, and the mechanisms regulating its translocation, have not yet been clearly defined. In Nature Cell Biology, Stuart Yuspa and colleagues now report that TGF-β signaling promotes the accumulation of CLIC4 in the nucleus, where it mediates the transcriptional response to TGF-β by preventing dephosphorylation of phospho-Smad2 and 3.

Previous studies had uncovered a link between TGF-β signaling and CLIC4 expression in fibroblasts. In a screen for novel CLIC4 interacting proteins, Yuspa and colleagues identified the TGF-β pathway protein Schnurri-2. TGF-β1 treatment increased the expression of and interaction between CLIC4 and Schnurri-2. Exogenous Schnurri-2 promoted CLIC4 nuclear translocation and upregulated TGF-β-dependent transcriptional activity and growth inhibition, whereas depletion of Schnurri-2 blocked these effects. However, nuclear targeting of CLIC4 circumvented the requirement for Schnurri-2 in stimulating TGF-β transcriptional activity, indicating that Schnurri-2 regulates CLIC4 nuclear translocation but not its activity.

Intriguingly, nuclear targeting of CLIC4 inhibited DNA synthesis and caused a marked increase in TGF-β transcriptional responses, such as downregulation of c-Myc and upregulation of p21, even in the absence of TGF-β1. Indeed, nuclear CLIC4 enhanced the nuclear accumulation of phosphorylated Smad2 and 3 — the transcription factors that are required for the transcriptional response to TGF-β signaling — and small hairpin (sh)RNA against CLIC4 decreased phospho-Smad2/3 levels independently of TGF-β1. CLIC4 was found to stabilize phospho-Smad levels by directly interacting with phospho-Smad2/3 and preventing their association with the Smad phosphatase PPM1a.

These data suggest that TGF-β1 stimulates the interaction between CLIC4 and Schnurri-2, which increases CLIC4 nuclear translocation. Nuclear CLIC4 then binds phospho-Smad2/3, preventing their dephosphorylation and stimulating the TGF-β transcriptional program. Thus, CLIC4 represents a new essential component of TGF-β signaling, which is itself induced by TGF-β signaling. It will be important to explore the regulation of this pathway and the mechanism by which CLIC4 is retained in the cytoplasm in cancer cells in future studies.

Emily J. Chenette
Signaling Gateway

Reference:
Shukla, A. et al.
TGF-β signalling is regulated by Schnurri-2-dependent nuclear translocation of CLIC4 and consequent stabilization of phospho-Smad2 and 3
Nature Cell Biology 11, 777-784 (2009)
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