Critical factors regulating site-specific brain metastasis of murine melanomas

The intracarotid injection of B16 melanoma cells syngeneic to C57BL/6 mice and K-1735 melanoma cells syngeneic to C3H/HeN mice results in site-specific brain metastasis in C57BL/6 x C3H/HeN F1 mice. The K-1735 cells produce lesions only in the brain parenchyma, whereas the B16 cells produce lesions only in the meninges and ventricles. To determine the mechanisms that regulate this site-specific brain metastasis, we transfected the melanoma cells with DNA from plasmids pSV2neo or pSV2hvgro, which confer resistance to the drugs neomycin and hygromycin, respectively. Hybrids between the B16 and K-1735 cells were obtained by fusion. Cells of the K-1735 x K-1735 hybrid produced lesions only in the brain parenchyma of C57BL/6 x C3H/HeN F1 mice, whereas all B16 x K-1735 hybrids produced lesions only in the meninges and the ventricles. Initial cell arrest in the meninges or the brain parenchyma, production of collagenolytic activity, motility, and expression of CD44 did not predict or correlate with site-specific brain metastasis. The response of the different melanomas and hybrid cells to transforming growth factor-beta (TGF-beta) correlated with growth in the brain parenchyma. B16 cells and B16 x K-1735 hybrids bound more TGF-beta than K-1735 cells. The in vitro growth of B16 cells and all B16 x K-1735 hybrid cells was significantly inhibited by TGF-beta1 and TGF-beta2, whereas the growth of K-1735 cells and K-1735 x K-1735 hybrids was enhanced. Since TGF-beta is abundant in brain tissue, the results suggest that the ability of melanoma cells to proliferate in the brain parenchyma determines the production of site-specific brain metastasis.