Metastatic melanoma is a lethal neoplasm with rapid progression and systemic dissemination. Melanoma has an increasing incidence rate and in Australia, it is the third most common cancer. The five-year survival rate of metastatic melanoma is poor with 70%-39% for patients with lymph node involvement and 18%-6% with distant metastases. This is due not only to the aggressive nature of the disease but also the current lack of effective therapies. The unfolded protein response (UPR), an endoplasmic reticulum stress response, is activated in various cancers. In melanoma activation of the UPR positively correlates with tumour progression, metastasis and poor outcome. To date, the full function of the UPR is yet to be determined and its complete role in cancer progression is undefined.
In the present study, proteomic analyses have been used to identify proteins involved in UPR. Subcellular proteomes of human melanoma cell lines (Mel-RM and WMM1175) treated with thapsigargin (TH), an inducer of ER stress, were isolated by differential centrifugation. Tryptic peptides were labeled with isobaric tags for relative and absolute quanitation and anaylsed using 2D liquid chromatography coupled to tandem mass spectrometry (2D LC-MS/MS). Differentially abundant proteins were validated using selected reaction monitoring (SRM) mass spectrometry and western blotting.
Following ER-stress and activation of the UPR 284 differentially abundant proteins were identified. These proteins are involved in stress response, cell migration and adhesion, metabolism, transcriptional and translational regulation, immune response and apoptosis/survival.
One of the pathways affected by the UPR is the granzyme A/B pathway, with decreased abundance of several proteins in the SET complex that induce apoptosis. Proteins involved in invasion and metastasis also changed in abundance, including proteins that modulate epithelial adherens junction signaling. Identification of these proteins and pathways contribute to our understanding of the metastatic mechanisms of melanoma and may provide new therapeutic targets.