Thioredoxin, thioredoxin reductase and tumour necrosis factor-alpha expression in melanoma cells: correlation to resistance against cytotoxic attack

Although malignant melanomas are often associated with cytotoxic lymphocyte infiltration, these cells are largely ineffective in inducing tumour cell kill, indicating that the melanoma cells have protective mechanisms. These mechanisms are not fully understood, but cytokines and redox-active antioxidant proteins such as catalase, superoxide dismutase, thioredoxin (Trx) and Trx reductase (TrxR) present in the tumour cells constitute part of this protection. In this study firstly we investigated the constitutive intracellular expression of Trx, TrxR, the cytokines interleukin (IL)-1alpha, IL1beta, IL2, IL4, IL6, IL8, IL10, tumour necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma) in normal melanocytes and ten primary and metastatic malignant melanoma cell lines. Secondly, we analysed whether redox stimulation by Trx alone or in combination with the phorbol ester PMA affected the expression and release of TNFalpha. Thirdly, we explored the possible correlation between Trx/TrxR expression and resistance to exogenous TNFalpha. All the cultured cells showed intracellular overexpression of Trx and TrxR, which was not always the case for melanoma cells in vivo (tissue sections). The predominant intracellular cytokines found were TNFalpha, IL1alpha and IL1beta. In spite of its presence in the Golgi apparatus, none of the cell lines secreted TNFalpha constitutively, and only one melanoma, FM3, released detectable amounts after stimulation. In contrast, U-937 monocyte control cells released high amounts of TNFalpha on identical stimulation. All the melanoma cell lines were relatively resistant against exogenous TNFalpha, and there was a significant correlation (P < 0.01) between intracellular Trx/TrxR expression and TNFalpha resistance (IC50). In conclusion, Trx and TrxR, as well as TNFalpha, IL1alpha and IL1beta, were highly expressed in cultured normal skin melanocytes and malignant melanoma cell lines. In contrast to U-937 monocytic cells, TNFalpha showed a secretory block in these cells, suggesting a cytoprotective and possible autocrine role for TNFalpha. The intracellular expression of Trx and TrxR together with endogenous TNFalpha was correlated with the resistance to TNFalpha-induced cytotoxicity.