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.