UV INDUCTION OF TRANSFORMING GROWTH FACTOR-ALPHA IN MELANOMA CELL-LINES IS A POSTTRANSLATIONAL EVENT

Low, mitogenic fluences of UVC (3.7-5.6 Jm-2) have previously been shown to cause increases of radioimmunoassayable transforming growth factor alpha (TGF-alpha) in the medium and cells of cultures of melanocytes, melanoma lines, and HeLa cells (Ellem, K.A.O., Cullinan, M., Baumann, K.C., Dunstan, A.: Carcinogenesis 9:797-801, 1988). Here the cellular mechanism of this increase is explored by Northern blotting to detect any changes in TGF-alpha mRNA levels, and the use of inhibitors of macromolecular synthesis to attempt to block the increase in TGF-alpha protein. We were unable to detect any increase in TGF-alpha mRNA levels attributable to UVC between 2 and 24 hours after irradiation. Inhibition of DNA synthesis (arabinosylcytosine, 10-mu-M), RNA synthesis (actinomycin D, 3-mu-g/ml; DRB 93-mu-M), or protein synthesis (cycloheximide, 10-mu-g/ml) failed to prevent the UVC induced increase in TGF-alpha. We conclude that the UVC induction of TGF-alpha is by a posttranslational mechanism. There was considerable discordance between the amount of TGF-alpha protein and its mRNA in cultures of 15 different melanoma cell lines, which again emphasized that posttranscriptional mechanisms modulate the release of immunodetectable TGF-alpha. We also found that the inhibitors themselves were capable of inducing an increase in TGF-alpha in MM229 cultures. This suggests that the inhibitors and UV may effect the increase by a common mechanism, perhaps the activation of cell surface proteases as suggested for other stimuli (e.g., Pandiella, A., and Massague, J.: Proc. Natl. Acad. Sci., USA 88:1726-1730, 1991) and that the response may be part of a global response to perturbation of DNA synthesis.