Desensitization of melanoma cells to autocrine TGF-? isoforms

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.

Transcriptional Regulation of Seprase in Invasive Melanoma Cells by Transforming Growth Factor-β Signaling

“…In addition, human metastatic melanoma cell lines were found to be more resistant to TGF-␤-dependent growth inhibition as compared with cells from primary tumors (60). Despite resistance to TGF-␤-induced growth arrest, TGF-␤-resistant melanoma cell lines still exhibit intact Smad-dependent regulation of gene expression in response to TGF-␤ that occurs in an autocrine manner (63).…”

Stable Overexpression of Smad7 in Human Melanoma Cells Impairs Bone Metastasis

“…TGF-b2 was expressed at very low levels (100-to 1,000-fold less than TGF-h1). These results are consistent with previously published observations (35). Incubation of some of these cell lines, selected based on their robust Smad-specific transcriptional response to TGF-h (see Fig.…”

Expression of the embryonic morphogen Nodal in cutaneous melanocytic lesions

“…8,9 TGF-b has potent antiproliferative effects in normal melanocytes, and this antiproliferative activity is diminished with melanoma progression. [10][11][12] It has also been found that TGF-b signaling promotes the anchorage-independent growth and metastatic capability of melanoma cells. 13,14 Consistent with a similar role for Nodal in melanoma aggressiveness, blockade of Nodal signaling in cultured melanoma cells resulted in decreased invasiveness, anchorage-independent growth, and xenograft tumor formation.…”