Efficient TGF-β/SMAD signaling in human melanoma cells associated with high c-SKI/SnoN expression

Delphine Javelaud,Vasileia I Alexaki,Alain Mauviel,Erwan Le Scolan,Leon Van Kempen,Kunxin Luo

doi:10.1186/1476-4598-10-2

Abstract

BackgroundSKI and SnoN proteins have been shown to inhibit TGF-β signaling, acting both as transcriptional co-repressors in the cell nucleus, and as sequestrators of SMAD proteins in the cytoplasm. TGF-β, on the other hand, induces rapid, proteasome-mediated, degradation of both proteins. How elevated SKI and SnoN protein levels co-exist with active autocrine TGF-β signaling in cancer cells is yet to be understood.ResultsIn this study, we found elevated SKI and SnoN protein levels in a panel of melanoma cell lines, as compared to normal melanocytes. There was no correlation between SKI protein content and the capacity of melanoma cells to invade Matrigel™, to form subcutaneous tumors, or to metastasize to bone after intracardiac inoculation into nude mice. Nor did we find a correlation between SKI expression and histopathological staging of human melanoma. TGF-β induced a rapid and dose-dependent degradation of SKI protein, associated with SMAD3/4 specific transcriptional response and induction of pro-metastatic target genes, partially prevented by pharmacologic blockade of proteasome activity. SKI knockdown in 1205Lu melanoma cells did not alter their invasive capacity or transcriptional responses to TGF-β, and did not allow p21 expression in response to TGF-β or reveal any growth inhibitory activity of TGF-β.ConclusionsDespite high expression in melanoma cells, the role of SKI in melanoma remains elusive: SKI does not efficiently interfere with the pro-oncogenic activities of TGF-β, unless stabilized by proteasome blockade. Its highly labile nature makes it an unlikely target for therapeutic intervention.

Highlights

SKI and SnoN proteins have been shown to inhibit Transforming growth factor-b (TGF-b) signaling, acting both as transcriptional co-repressors in the cell nucleus, and as sequestrators of SMAD proteins in the cytoplasm
SKI may localize in the cytoplasm of tumor cells [17], where it may interfere with TGF-b signaling by sequestering SMAD proteins and preventing their nuclear accumulation in response to TGF-b, as we demonstrated in the case of SnoN [18]
We report that SKI levels do not correlate with the tumorigenic or metastatic potential of melanoma cells, the latter largely depending upon constitutive TGF-b signaling, and do not correlate with the clinical or pathological stage of human melanoma lesions

Summary

Introduction

SKI and SnoN proteins have been shown to inhibit TGF-b signaling, acting both as transcriptional co-repressors in the cell nucleus, and as sequestrators of SMAD proteins in the cytoplasm. Receptor activation leads to phosphorylation of cytoplasmic protein substrates of the SMAD family and subsequent accumulation in the nucleus where they act as transcription factors to regulate target gene expression [1,2,3]. Expression levels of SKI family members may be downregulated by TGF-b, as the latter rapidly induces SKI protein poly-ubiquitination and degradation in a SMAD- and proteasome-dependent manner, allowing TGF-b target gene transactivation [22,23,24,25,26,27,28,29]

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