Purpose: IFN-α is administered to melanoma patients and its endogenous production is essential for immune-mediated tumor recognition. We hypothesized that a reduced capacity for signal transducer and activator of transcription (STAT) 1 activation allows melanoma cells to evade the direct actions of IFN-α.

Experimental Design: Tyr⁷⁰¹-phosphorylated STAT1 (P-STAT1) was measured by flow cytometry in IFN-α–stimulated human melanoma cell lines, melanoma cells derived from patient tumors, and peripheral blood mononuclear cells (PBMC). Expression of other Janus-activated kinase (Jak)-STAT intermediates (STAT1, STAT2, Jak1, tyrosine kinase 2, IFN-α receptor, STAT3, and STAT5) was evaluated by flow cytometry, immunoblot, or immunohistochemistry.

Results: Significant variability in P-STAT1 was observed in human melanoma cell lines following IFN-α treatment (P < 0.05) and IFN-α–induced P-STAT1 correlated with the antiproliferative effects of IFN-α (P = 0.042). Reduced formation of P-STAT1 was not explained by loss of Jak-STAT proteins or enhanced STAT5 signaling as reported previously. Basal levels of P-STAT3 were inversely correlated with IFN-α–induced P-STAT1 in cell lines (P = 0.013). IFN-α–induced formation of P-STAT1 was also variable in melanoma cells derived from patient tumors; however, no relationship between P-STAT3 and IFN-α–induced P-STAT1 was evident. Because IFN-α acts on both tumor and immune cells, we examined the ability of IFN-α to induce P-STAT1 in patient-derived melanoma cells and PBMCs. IFN-α induced significantly lower levels of P-STAT1 in melanoma cells compared with matched PBMCs (P = 0.046). Melanoma cells and human melanocytes required 10-fold higher IFN-α doses to exert P-STAT1 levels comparable with PBMCs.

Conclusions: Melanoma cells are variable in their IFN-α responsiveness, and cells of the melanocytic lineage exhibit a lower capacity for IFN-α–induced Jak-STAT signaling compared with immune cells.