Cancer therapies based on the inhibition of angiogenesis by endostatin have 
recently been developed. We demonstrate that a mutated form of human endostatin 
(P125A) can inhibit the angiogenic switch in the C3(1)/Tag mammary cancer model. 
P125A has a stronger growth-inhibitory effect on endothelial cell proliferation 
than wild-type endostatin. We characterize the angiogenic switch, which occurs 
during the transition from preinvasive lesions to invasive carcinoma in this 
model, and which is accompanied by a significant increase in total protein 
levels of vascular endothelial growth factor (VEGF) and an invasion of blood 
vessels. Expression of the VEGF(188) mRNA isoform, however, is suppressed in 
invasive carcinomas. The VEGF receptors fetal liver kinase-1 (Flk-1) and 
Fms-like tyrosine kinase-1 (Flt-1) become highly expressed in epithelial tumor 
and endothelial cells in the mammary carcinomas, suggesting a potential 
autocrine effect for VEGF on tumor cell growth. Angiopoietin-2 mRNA levels are 
also increased during tumor progression. CD-31 (platelet-endothelial cell 
adhesion molecule [PECAM]) staining revealed that blood vessels developed in 
tumors larger than 1 mm The administration of P125A human endostatin in 
C3(1)/Tag females resulted in a significant delay in tumor onset, decreased 
tumor multiplicity and tumor burden and prolonged survival of the animals. 
Endostatin treatment did not reduce the number of preinvasive lesions, 
proliferation rates or apoptotic index, compared with controls. However, mRNA 
levels of a variety of proangiogenic factors (VEGF, VEGF receptors Flk-1 and 
Flt-1, angiopoietin-2, Tie-1, cadherin-5 and PECAM) were significantly decreased 
in the endostatin-treated group compared with controls. These results 
demonstrate that P125A endostatin inhibits the angiogenic switch during mammary 
gland adenocarcinoma tumor progression in the C3(1)/Tag transgenic model.