Thioredoxin, thioredoxin reductase and tumour necrosis factor-α 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)-1 alpha, IL1 beta, IL2, IL4, IL6, IL8, IL10, tumour necrosis factor-alpha (TNF alpha) and interferon-gamma (IFN gamma) 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 TNF alpha. Thirdly, we explored the possible correlation between Trx/TrxR expression and resistance to exogenous TNF alpha. 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 TNF alpha, IL1 alpha and IL1 beta. In spite of its presence in the Golgi apparatus, none of the cell lines secreted TNF alpha constitutively, and only one melanoma, FM3, released detectable amounts after stimulation. In contrast, U-937 monocyte control cells released high amounts of TNF alpha on identical stimulation. All the melanoma cell lines were relatively resistant against exogenous TNF alpha, and there was a significant correlation (P < 0.01) between intracellular Trx/TrxR expression and TNF alpha resistance (IC50) In conclusion, Trx and TrxR, as well as TNF alpha, IL1 alpha and IL1 beta, were highly expressed in cultured normal skin melanocytes and malignant melanoma cell lines. In contrast to U-937 monocytic cells, TNF alpha showed a secretory block in these cells, suggesting a cytoprotective and possible autocrine role for TNF alpha. The intracellular expression of Trx and TrxR together with endogenous TNF alpha was correlated with the resistance to TNF alpha-induced cytotoxicity. (C) 2000 Lippincott Williams & Wilkins.