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Desensitization of melanoma cells to autocrine TGF-beta isoforms
K Krasagakis1, S Krüger-Krasagakes, S Fimmel
1Department of Dermatology, University Medical Center Benjamin Franklin, The Free University of Berlin, Germany.
Abstract:
Previous studies have suggested that transforming growth factor-beta 1 (TGF-beta1) acts as an autocrine growth inhibitor on normal human melanocytes, while melanoma cells may not respond to this stimulus. The role of other TGF-beta isoforms such as TGF-beta2 and TGF-beta3 remained less well characterized. In the present study, the mRNA and protein levels of all three isoforms of TGF-beta were analyzed in a panel of human melanoma cell lines and in cultures of normal human melanocytes in vitro. Northern analysis showed that the degree of TGF-beta1, -beta2, -beta3 mRNA expression varied considerably in melanoma cells, whereas TGF-beta expression was very low in melanocytes. In melanoma cells, secreted amounts of TGF-beta1 and TGF-beta3 were found increased in comparison to normal melanocytes: 615 pg/ml vs. 118 pg/ml and 193 pg/ml vs. 30 pg/ml (mean values). In addition, low levels of TGF-beta2 were detected (mean value: 28 pg/ml). Although TGF-beta secretion increased, the proliferation of melanoma cells was found to be only moderately inhibited by TGF-beta isoforms, in contrast to its strong antiproliferative effect on normal human melanocytes: - 15%, -11%, and -18% vs. -52%, -46%, and -50% average inhibition at 0.5 ng/ml TGF-beta1, -beta2, and -beta3, respectively. The different efficacy of TGF-beta on melanocyte and melanoma cells was highly significant (P<0.0001); in addition, TGF-beta-dependent growth inhibition of melanoma cells from primary tumors vs. cells from metastases showed a trend for further decreased response for the metastatic populations (P< or = 0.075). Measurements of DNA synthesis revealed even more pronounced differences between melanocytes (-86%, -78%, and -80% inhibition, respectively, for TGF-beta1, -beta2, and -beta3) and melanoma cells (no inhibition). Our data show loss of responsiveness of melanoma cells to the growth-inhibitory function of TGF-beta isoforms but not of melanocytes. Although melanoma cells are not growth-inhibited by all three TGF-beta isoforms, they secrete significantly higher levels of TGF-beta, as compared to melanocytes. The reduced response indicates their escape from TGF-beta surveillance with ongoing tumor progression.
Insights
Melanoma cells lose sensitivity to the growth-inhibiting effects of transforming growth factor-beta (TGF-beta) isoforms, unlike normal melanocytes. This reduced response, despite increased TGF-beta secretion by melanoma, suggests an escape from cellular surveillance during tumor progression.
Area of Science:
- Cell Biology
- Cancer Research
- Molecular Biology
Background:
- Transforming growth factor-beta 1 (TGF-beta1) is a known autocrine inhibitor of normal human melanocyte growth.
- The roles of other TGF-beta isoforms (TGF-beta2, TGF-beta3) and their differential effects on melanoma cells are less understood.
- Melanoma cells may exhibit altered responses to TGF-beta compared to their normal counterparts.
Purpose of the Study:
- To investigate the expression and function of all three TGF-beta isoforms in human melanoma cell lines and normal melanocytes.
- To compare the antiproliferative effects of TGF-beta isoforms on melanoma cells versus normal melanocytes.
- To determine if melanoma cells secrete altered levels of TGF-beta isoforms.
Main Methods:
- Analysis of TGF-beta1, TGF-beta2, and TGF-beta3 mRNA and protein expression in melanoma cell lines and normal melanocytes.
- Quantification of secreted TGF-beta isoforms using protein assays.
- Assessment of cell proliferation and DNA synthesis inhibition in response to TGF-beta isoforms.
Main Results:
- Melanoma cells exhibited variable mRNA expression for all TGF-beta isoforms, while normal melanocytes showed low expression.
- Melanoma cells secreted significantly higher levels of TGF-beta1 and TGF-beta3 compared to normal melanocytes.
- Melanoma cells showed significantly reduced inhibition of proliferation and DNA synthesis by TGF-beta isoforms compared to normal melanocytes, with a trend towards decreased responsiveness in metastatic cells.
Conclusions:
- Melanoma cells demonstrate a loss of responsiveness to the growth-inhibitory functions of TGF-beta isoforms.
- Despite increased secretion, melanoma cells escape TGF-beta-mediated growth control, indicating a mechanism for tumor progression.
- These findings highlight the critical role of TGF-beta signaling in melanoma development and progression.