Desensitization of melanoma cells to autocrine TGF-beta isoforms

Previous studies have suggested that transforming growth factor-beta 1 (TGF -beta 1) acts as an autocrine growth inhibitor on normal human melanocytes, while melanoma cells may not respond to this stimulus. The role of other T GF-P isoforms such as TGF-beta 2 and TGF-beta 3 remained less well characte rized. In the present study, the mRNA and protein levels of all three isofo rms of TGF-beta were analyzed in a panel of human melanoma cell lines and i n cultures of normal human melanocytes in vitro. Northern analysis showed t hat the degree of TGF-beta 1 -beta 2, -beta 3 mRNA expression varied consid erably in melanoma cells, whereas TGF-beta expression was very low in melan ocytes. In melanoma cells, secreted amounts of TGF-beta 1 and TGF-beta 3 we re found increased in comparison to normal melanocytes: 615 pg/ml vs. 118 p g/ml and 193 pg/ml vs. 30 pg/ml (mean values). In addition, low levels of T GF-beta 2 were detected (mean value: 28 pg/ml). Although TGF-beta secretion increased, the proliferation of melanoma cells was found to be only modera tely inhibited by TGF-beta isoforms, in contrast to its strong antiprolifer ative effect on normal human melanocytes: -15%, -11%, and -18% vs. -52%, -4 6%, and -50% average inhibition at 0.5 ng/ml TGF-beta 1, -beta 2, and -beta 3, respectively. The different efficacy of TGF-beta on melanocyte and mela noma cells was highly significant (P < 0.0001); in addition, TGF-beta-depen dent growth inhibition of melanoma cells from primary tumors vs. cells from metastases showed a trend for further decreased response for the metastati c populations (P less than or equal to 0.075). Measurements of DNA synthesi s revealed even more pronounced differences between melanocytes (-86%, -78% , and -80% inhibition, respectively, for TGF-beta 1, -beta 2, and -beta 3) and melanoma cells (no inhibition). Our data show loss of responsiveness of melanoma cells to the growth-inhibitory function of TGF-P isoforms bur not of melanocytes. Although melanoma cells are not growth-inhibited by all th ree TGF-beta isoforms, they secrete significantly higher levels of TGF-beta , as compared to melanocytes. The reduced response indicates their escape f rom TGF-P surveillance with ongoing tumor progression. J Cell Physiol 178: 179-187, 1999. (C) 1999 Wiley-Liss, Inc.