Novel Regulation of Ski Protein Stability and Endosomal Sorting by Actin Cytoskeleton Dynamics in Hepatocytes

Genaro Vázquez-Victorio,Cassandre Caligaris,Eugenio Del Valle-Espinosa,Marcela Sosa-Garrocho,Nelly R González-Arenas,Guadalupe Reyes-Cruz,Marco A Briones-Orta,Marina Macías-Silva

doi:10.1074/jbc.m114.579532

Abstract

TGF-β-induced antimitotic signals are highly regulated during cell proliferation under normal and pathological conditions, such as liver regeneration and cancer. Up-regulation of the transcriptional cofactors Ski and SnoN during liver regeneration may favor hepatocyte proliferation by inhibiting TGF-β signals. In this study, we found a novel mechanism that regulates Ski protein stability through TGF-β and G protein-coupled receptor (GPCR) signaling. Ski protein is distributed between the nucleus and cytoplasm of normal hepatocytes, and the molecular mechanisms controlling Ski protein stability involve the participation of actin cytoskeleton dynamics. Cytoplasmic Ski is partially associated with actin and localized in cholesterol-rich vesicles. Ski protein stability is decreased by TGF-β/Smads, GPCR/Rho signals, and actin polymerization, whereas GPCR/cAMP signals and actin depolymerization promote Ski protein stability. In conclusion, TGF-β and GPCR signals differentially regulate Ski protein stability and sorting in hepatocytes, and this cross-talk may occur during liver regeneration.

Highlights

The Ski oncoprotein and tumor suppressor is a negative regulator of the antimitotic TGF-␤/Smad pathway
Ski protein is distributed between the nucleus and cytoplasm of normal hepatocytes, and the molecular mechanisms controlling Ski protein stability involve the participation of actin cytoskeleton dynamics
We demonstrate that Ski protein stability is regulated differentially by TGF-␤ and G protein-coupled receptor (GPCR) signals in hepatocytes and that the molecular mechanisms involved are influenced by the dynamics of the actin cytoskeleton

Summary

Background

The Ski oncoprotein and tumor suppressor is a negative regulator of the antimitotic TGF-␤/Smad pathway. Conclusion: TGF-␤/Smads and GPCR/actin cytoskeleton-dynamic signals regulate Ski protein stability via the proteasome in hepatocytes. TGF-␤ and GPCR signals differentially regulate Ski protein stability and sorting in hepatocytes, and this cross-talk may occur during liver regeneration. A major cross-talk among diverse signaling pathways occurs during liver regeneration, where the antiproliferative actions of TGF-␤ may be under tight control exerted by different pathways, mainly those implicated in cell cycle promotion, such as the pathways downstream of growth factors receptors and GPCRs, among others [2]. We demonstrate that Ski protein stability is regulated differentially by TGF-␤ and GPCR signals in hepatocytes and that the molecular mechanisms involved are influenced by the dynamics of the actin cytoskeleton. We show that Ski protein stability is increased during liver regeneration, where it may facilitate hepatocyte proliferation by controlling TGF-␤ signaling

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION