Aberrant redox regulation in human metastatic melanoma cells compared to normal melanocytes.
Melanocytes and melanoma cells contain melanin, a complex polymer that modulates redox changes in these cells. Relative intracellular hydrogen peroxide levels measured by dichlorodihydrofluorescein are similar in the two cell types, but the levels of superoxide anion measured by dihydroethidium were markedly increased in melanoma cells. Chelator-induced oxidative stress is efficiently suppressed by melanocytes without substantial recruitment of the transcription factors NF-kappaB and AP-1 as measured by electrophoretic mobility shift assay and quantitated by densitometry or by a change in frequency of apoptosis as determined by annexin V binding. In contrast, NF-kappaB in melanoma cells is strongly recruited by changes in redox status and exhibits a correlative relationship to intracellular hydrogen peroxide (but not superoxide anion). However, the response of the NF-kappaB pathway to intracellular hydrogen peroxide is anomalous, including downregulation of p65 and IkappaBalpha RNA expression (Northern blot). Additionally, recruitment of AP-1 binding in melanoma cells was directly correlated with intracellular levels of superoxide anion (but not hydrogen peroxide). Neither the degree of NF-kappaB nor AP-1 binding in melanoma cells was related to the frequency of apoptosis. The responsiveness of NF-kappaB and AP-1 recruitment to intracellular levels of hydrogen peroxide and superoxide anion without concomitant control of apoptosis provides a general mechanism by which these cells can escape noxious injury (e.g., chemotherapy). The marked enhancement of apoptosis in melanoma cells by chelators indicates, however, that this alteration can be circumvented and offers a unique therapeutic window to explore.