SKI is critical for repressing the growth inhibitory function of TGF-beta in human melanoma

The propensity of invasive circulating tumor cells (iCTCs) in metastatic progression and therapeutic responsiveness

Circulating tumor cells (CTCs) are important clinical indicators of metastatic progression and treatment efficacy. However, because of their low number and heterogeneity, reliable patient-derived CTC models are not readily available. We report here the isolation and characterization of the invasive population of CTCs, iCTCs, from blood of 10 patients with epithelial ovarian cancer (EOC) and one pancreatic cancer patient based on the avidity of tumor cells toward an artificial collagen-based adhesion matrix (CAM), in comparison with tumor progenitor (TP) cells isolated from tumor cell lines, tumors and ascites from EOC patients. CAM-avid cells identified to be iCTCs were indistinguishable with TP cells using either functional CAM uptake or surface markers (seprase and CD44). In addition, iCTCs were characterized using peritoneal and spontaneous metastasis models in vivo to evaluate their metastatic propensity and therapeutic response. TP cells and iCTCs had a doubling time of about 34-42 hours. TP cells were rare (<3.5%) in most patient-derived specimens, however, iCTCs emigrated into blood, at a high frequency, 64.2% (n = 49). Approximately 500 patient-derived iCTCs recapitulated formation of iCTCs in mouse blood and formed micrometastases in the liver and/or lung, a degree of metastatic spread equivalent to the inoculation of 5 × 105 bulk tumor cells isolated from ascites and tumors. iCTCs were shown to be novel therapeutic targets for blocking metastasis using the reduced formation of iCTCs and micrometastases by RNAi, peptides, and monoclonal antibodies against seprase.

The Ski Protein is Involved in the Transformation Pathway of Aurora Kinase A

Oncogenic kinase Aurora A (AURKA) has been found to be overexpresed in several tumors including colorectal, breast, and hematological cancers. Overexpression of AURKA induces centrosome amplification and aneuploidy and it is related with cancer progression and poor prognosis. Here we show that AURKA phosphorylates in vitro the transcripcional co-repressor Ski on aminoacids Ser326 and Ser383. Phosphorylations on these aminoacids decreased Ski protein half-life. Reduced levels of Ski resulted in centrosomes amplification and multipolar spindles formation, same as AURKA overexpressing cells. Importantly, overexpression of Ski wild type, but not S326D and S383D mutants inhibited centrosome amplification and cellular transformation induced by AURKA. Altogether, these results suggest that the Ski protein is a target in the transformation pathway mediated by the AURKA oncogene.

Ski modulate the characteristics of pancreatic cancer stem cells via regulating sonic hedgehog signaling pathway

Evidence from in vitro and in vivo studies shows that Ski may act as both a tumor proliferation-promoting factor and a metastatic suppressor in human pancreatic cancer and also may be a therapeutic target of integrative therapies. At present, pancreatic cancer stem cells (CSCs) are responsible for tumor recurrence accompanied by resistance to conventional therapies. Sonic hedgehog (Shh) signaling pathway is found to be aberrantly activated in CSCs. The objectives of this study were to investigate the role of Ski in modulating pancreatic CSCs and to examine the molecular mechanisms involved in pancreatic cancer treatment both in vivo and in vitro. In in vitro study, the results showed that enhanced Ski expression could increase the expression of pluripotency maintaining markers, such as CD24, CD44, Sox-2, and Oct-4, and also components of Shh signaling pathway, such as Shh, Ptch-1, Smo, Gli-1, and Gli-2, whereas depletion of Ski to the contrary. Then, we investigated the underlying mechanism and found that inhibiting Gli-2 expression by short interfering RNA (siRNA) can decrease the effects of Ski on the maintenance of pancreatic CSCs, indicating that Ski mediates the pluripotency of pancreatic CSCs mainly through Shh pathway. The conclusion is that Ski may be an important factor in maintaining the stemness of pancreatic CSCs through modulating Shh pathway.

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

Background

SKI 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.

Results

In 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.

Conclusions

Despite 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.

Resistance to transforming growth factor β-mediated tumor suppression in melanoma: are multiple mechanisms in place?

Resistance to transforming growth factor (TGF) β-mediated tumor suppression in melanoma appears to be a crucial step in tumor aggressiveness since it is usually coupled with the ability of TGFβ to drive the oncogenic process via autocrine and paracrine effects. In this review, we will focus mainly on the mechanisms of escape from TGFβ-induced cell cycle arrest because the mechanisms of resistance to TGFβ-mediated apoptosis are still essentially speculative. As expected, some of these mechanisms can directly affect the function of the main downstream effectors of TGFβ, Smad2 and Smad3, resulting in compromised Smad-mediated antiproliferative activity. Other mechanisms can counteract or overcome TGFβ-mediated cell cycle arrest independently of the Smads. In melanoma, some models of resistance to TGFβ have been suggested and will be described. In addition, we propose additional models of resistance taking into consideration the information available on the dysregulation of fundamental cellular effectors and signaling pathways in melanoma.