Co-expression of vascular endothelial growth factor (VEGF) and its receptors (flk-1 and flt-1) in hormone-induced mammary cancer in the Noble rat

Vascular endothelial growth factor (VEGF) is recognized to play a predomina nt role in breast cancer prognosis. The action of VEGF is mediated by two h igh-affinity receptors with ligand-stimulated tyrosine kinase activity: VEG FR-1/flt-1 and VEGFR-2/flk-1, which are expressed mainly in vascular endoth elial cells. To the best of our knowledge, no previous studies on the expre ssion of these receptors in breast cancer cells has been made. We have esta blished a new animal model for breast cancer, using a combination of 17 bet a-oestradiol and testosterone as 'carcinogens'. Taking advantage of the ani mal model, we have demonstrated that mammary cancer cells expressed not onl y high levels of VEGF but also, surprisingly, its receptors (flt-1 and flk- 1) in mammary cancer cells, Intense reactivities to VEGF, flt-1 and flk-1 w ere observed in mammary cancer cells, especially in invasive mammary carcin oma. Western blot analysis confirmed the increase in flk-1 and flt-1 protei ns in induced mammary cancers. Based on these observations, we hypothesize that in mammary cancer, VEGF regulates, in addition to endothelial prolifer ation and angiogenesis, also growth of cancer cells by an autocrine mechani sm mediated through its receptors. To further verify this hypothesis, we in vestigated the correlation between cellular proliferation and the expressio n of VEGF, flt-1 and flk-1. Using double-labelling immunocytochemistry, we have shown a correlation between high VEGF activity and Ki-67 expression. T he Ki-67 indices in the areas of strong and weak VEGF reactivities were 58. 3% and 3.7% respectively. Similarly, there was also a correlation of strong flk-1 and Ki-67 reactivity, The Ki-67 indices for areas of strong and weak flk-1 reactivities were 53.9% and 3.1% respectively. On the other hand, th ere was a reverse correlation between flt-1 and Ki-67 activities. These res ults indicate that overexpression of VEGF and flk-1 is correlated with high Ki-67 index, The data, therefore, suggest that VEGF may act as an autocrin e growth factor for mammary cancer cells in vivo and this autocrine regulat ory role may be mediated through flk-1. The present study is the first repo rt showing that VEGF may act as a growth stimulator for mammary cancer cell s. (C) 1999 Cancer Research Campaign.