Y-box binding protein 1 (YB-1) is an oncogenic transcription and translation factor and is overexpressed in several types of cancer. Our previous data showed that YB-1 is upregulated and translocated to the nucleus during melanoma progression and that YB-1 is an important transcription factor regulating proliferation, survival, migration, invasion and chemosensitivity of melanoma cells. It has been suggested that YB-1 is activated and translocated to the nucleus after S102-phosphorylation in the DNA binding domain. In this study, we show that activation of YB-1 by S102-phosphorylation and nuclear translocation is increased during melanoma progression using a human tissue microarray with 100 melanocytic lesions. Furthermore, we analysed the mechanisms governing the expression and activity of YB-1 in melanoma cells. We show that the PI3K/AKT and p53 signalling, growth factors and chemotherapeutic agents increase YB-1 promoter activity. This, however, resulted in no or only modest increase in YB-1 protein expression. We show that the MAPK and PI3K/AKT signalling pathways, both activated in melanoma cells, as well as p53 overexpression increase YB-1 S102-phosphorylation, whereas NFκB signalling inhibits phosphorylation. Overexpression of YB-1 in melanoma cells inhibits translation efficiency and by this proliferation and survival of melanoma cells indicating that there is an autoregulatory loop restricting YB-1 protein expression. These data suggest that there is a tightly regulated feedback mechanism regulating YB-1 expression and activation, necessary for proper cell cycle progression of melanoma cells.

Recent studies have shown that a loss of methylthioadenosine phosphorylase (MTAP) gene expression exerts a tumor-promoting effect, including induction of invasiveness, enhanced cell proliferation, and resistance against cytokines. To date, the molecular mechanisms underlying these effects remain unknown. Since the loss of MTAP expression resulted in induced secretion of 5'-deoxy-5'-(methylthio)adenosine (MTA), we hypothesized that MTA might modulate the observed effects. We first determined MTA levels produced by tumor cells in vitro and in situ by means of stable isotope dilution liquid chromatography tandem mass spectrometry. Subsequently, we revealed induction of matrix metalloproteinase (MMP) and growth factor gene expression in melanoma cells accompanied by enhanced invasion and vasculogenic mimicry. In addition, MTA induced the secretion of basis fibroblast growth factor (bFGF) and MMP3 from fibroblasts and the upregulation of activator protein-1 (AP-1) activity in melanoma cells and fibroblasts. In summary, we demonstrated a tumor-supporting role of MTA.

Loss of E-cadherin-mediated cell-cell contacts can elicit a signaling pathway that leads to acquisition of an invasive phenotype. Here, we show that at the receiving end of this pathway is the proto-oncogene c-Jun, a member of the activator protein-1 family of transcription factors that play a key role in stimulation of cell proliferation and tumor promotion. Cell separation or abrogation of E-cadherin-mediated cell-cell contacts both cause a dramatic increase in accumulation of the c-Jun protein.

A central event in the development of malignant melanoma is the loss of the tumor-suppressor protein E-cadherin. Here, we report that this loss is linked to the activation of the proto-oncogene c-Jun, a key player in tumorigenesis. In vivo, malignant melanomas show strong expression of the c-Jun protein in contrast to melanocytes. Interestingly, c-Jun mRNA levels did not differ in the melanoma cell lines when compared to melanocytes, suggesting that c-Jun could be regulated at the post-transcriptional level. To uncover the link between E-cadherin and c-Jun, we re-expressed E-cadherin in melanoma cells and detected decreased protein expression and activity of c-Jun. Furthermore, c-Jun accumulation is dependent on active E-cadherin-mediated cell-cell adhesion and regulated via the cytoskeleton. Additionally, we determined that, with respect to c-Jun regulation, there are two melanoma subgroups. One subgroup regulates c-Jun expression via the newly discovered E-cadherin-dependent signaling pathway, whereas the other subgroup uses the MAPKinases to regulate its expression. In summary, our data provide novel insights into the tumor-suppressor function of E-cadherin, which contributes to the suppression of c-Jun protein translation and transcriptional activity independent of MAPKinases.

Recently, we discovered that the loss of E‐cadherin induces c‐Jun protein expression, which is a member of the AP‐1 transcription factor family and a key player in the processes of cell proliferation and tumor development and also found in elevated levels in melanomas. Notably, the mRNA level of c‐Jun was not affected, suggesting that c‐Jun is regulated at post‐transcriptional level. Here, we present data that suggest that the dynamic cytoskeletal network, linked to E‐cadherin, is involved in the regulation of the c‐Jun protein and transcriptional activity. In a signaling cascade, the loss of E‐cadherin activates the transcriptional regulator ETS‐1 and consequently leads to the induction of RhoC expression that stabilizes c‐Jun in melanoma. The link between RhoC and c‐Jun seems to be indirect via the cytoskeleton. We conclude that the loss of E‐cadherin mediated cell‐adhesion induces c‐Jun protein expression in a multistep process, offering several possibilities for therapeutic intervention.