Defective activations of STAT3 Ser727 and PKC isoforms lead to oncostatin M resistance in metastatic melanoma cells.

Stage III melanoma is refractory to common therapies and shows resistance to the anti-proliferative activity of cytokines in vitro. We previously demonstrated that, for 30% of the metastatic melanoma cell lines, oncostatin M (OSM) resistance is due to the epigenetic silencing of its receptor OSMRbeta. Here we analyse, on a larger panel of short-term cultures derived from melanoma-invaded lymph nodes, other mechanisms potentially implicated in OSM resistance. For 18% of the cell lines, OSM resistance is associated with a phosphorylation defect of signal transducer and activator of transcription (STAT)3 on serine (Ser)727, in concordance with defects in the activation of various protein kinase C (PKC) isoforms, especially PKCdelta. For 21% of the cell lines, OSM resistance is associated with a defect in the activation of Akt on Ser473. By the use of inhibitors, dominant negatives and small interfering (si)RNA, we show that the PKC-STAT3 Ser727, but not the Akt, pathway appears necessary for OSM anti-proliferative activity. Moreover, we bring evidence that OSM or interleukin (IL)-6, produced in lymph nodes and/or melanoma cells, could be involved in the establishment of OSM resistance during melanoma progression. These findings could be relevant for the prognosis and the treatment of stage III melanoma patients.