Transforming growth factor-beta1 inhibits tumor growth in a mouse melanoma model by down-regulating the plasminogen activation system.

The degradation of basement membranes by tumor cells involves secretion and activation of proteinases, such as matrix metalloproteinases (MMPs) and the plasminogen activation system (uPA, tPA, PAI-1), and results from an imbalance between their inhibitors and activators, controlled by various growth factors or cytokines. Among them, the TGF-beta family is one of the most intriguing because it has been reported either to decrease or promote cancer progression. In the present paper, we studied the effect of TGF-beta1 in a mouse melanoma model. In vivo, TGF-beta1 inhibited tumor growth after subcutaneous injection of B16F1 cells in syngenic mice. In vitro, TGF-beta1 did not alter B16F1 cell proliferation, but strongly decreased their migration through Matrigel-coated membranes. The protease production was analyzed by zymography, Western blot, or RT-PCR. MMP-2 and TIMP-2 expression were not altered by TGF-beta1. In contrast, TGF-beta1 triggered a large decrease of uPA and tPA, as well as a decrease of uPA and uPAR mRNAs. By Western blot and RT-PCR analyses, TGF-beta1 was shown to induce a strong increase of PAI-1 synthesis. Collectively, these results suggest that TGF-beta1 may inhibit melanoma tumor growth by specifically decreasing plasmin activity of tumor cells and play a protective role during the earliest stages of tumor progression.