SCATTER FACTOR STIMULATES TUMOR-GROWTH AND TUMOR ANGIOGENESIS IN HUMAN BREAST CANCERS IN THE MAMMARY FAT PADS OF NUDE-MICE

Scatter factor (SF) (also known as hepatocyte growth factor) is a plas minogen-related growth factor that induces tumor cell motility, invasi on, and angiogenesis. Its receptor is a tyrosine kinase encoded by c-m et, a protooncogene. Human breast cancer cells express SF and c-met in vivo; but human breast cancer cell lines do not produce SF in vitro. To determine whether SF can modulate the in vivo growth of human breas t cancers within a natural mammary environment, we studied the orthoto pic growth of SF-transfected (SF+) versus control (SF-) clones of MDAM B231 human mammary carcinoma cells in the mammary fat pads of athymic nude mice. SF+ clones expressed SF mRNA and produced very high titers of SF protein, whereas SF- clones did not express SF mRNA or produce d etectable SF protein. Two SF+ clones (21 and 29) showed significantly increased tumor growth rates, reaching 3- to 4-fold larger primary tum or volumes and weights by time of killing (p < 0.001), as well as high er rates of axillary lymph node metastasis (p < 0.02), as compared wit h two SF- clones (32 and 34). In contrast, in vitro proliferation rate s, two-dimensional colony formation, and soft agar colony formation we re no greater in SF+ than in SF-clones. We performed further studies t o investigate the discrepancy between the in vivo and in vitro growth results. Tumor extracts from SF+ clone (21 + 29) tumors had 50-fold hi gher SF content than did SF- clone (32 + 34) tumors, confirming high-l evel SF expression in vivo in SF+ tumors. Immunostaining of tumor sect ions for proliferating cell nuclear antigen revealed only a modest inc rease in the proportion of cycling cells in SF+ versus SF- tumors (70% versus 60%, respectively). The terminal deoxytransferase-labeling ind ex was equally low (approximately 1%) in SF+ and SF- tumors, suggestin g that apoptosis was not responsible for the slower growth of SF- tumo rs. However, SF+ tumors had significantly higher tumor microvessel den sities than SF- tumors (p < 0.001). Moreover, there were much higher t iters of chemotactic activity for microvascular endothelial cells in c ell-conditioned media and primary tumor extracts from SF+ clones as co mpared with SF- clones. As demonstrated using the rat cornea assay, th ere was more angiogenic activity in SF+ tumor extracts than in SF- ext racts. The increased chemotactic and angiogenic activities in SF+ tumo r extracts were not explained by secondary alterations in the content of the angiogenic mediator, vascular endothelial growth factor, or the antiangiogenic glycoprotein, thrombospondin-1; and those activities w ere neutralized using an anti-SF monoclonal antibody. These findings s uggest that SF promotes the orthotopic growth of human breast cancers, at least in part, by stimulating tumor angiogenesis.