The role of neo-angiogenesis in
prostate cancer (PCA) growth and
metastasis is well established, but the development of effective and non-toxic pharmacological inhibitors of angiogenesis remains an unaccomplished goal. In this regard, targeting aberrant angiogenesis through non-toxic
phytochemicals could be an attractive angiopreventive strategy against PCA. The rationale of the present study was to compare the anti-angiogenic potential of four pure diastereoisomeric
flavonolignans, namely
silybin A,
silybin B,
isosilybin A and
isosilybin B, which we established previously as biologically active constituents in
Milk Thistle extract. Results showed that oral feeding of these
flavonolignans (50 and 100 mg/kg
body weight) effectively inhibit the growth of advanced human PCA DU145 xenografts. Immunohistochemical analyses revealed that these
flavonolignans inhibit
tumor angiogenesis
biomarkers (CD31 and
nestin) and signaling molecules regulating angiogenesis (
VEGF, VEGFR1, VEGFR2, phospho-Akt and HIF-1α) without adversely affecting the vessel-count in normal tissues (liver, lung, and kidney) of
tumor bearing mice. These
flavonolignans also inhibited the microvessel sprouting from mouse dorsal aortas ex vivo, and the
VEGF-induced cell proliferation, capillary-like tube formation and invasiveness of human umbilical vein endothelial cells (HUVEC) in vitro. Further studies in HUVEC showed that these diastereoisomers target cell cycle, apoptosis and
VEGF-induced signaling cascade. Three dimensional growth assay as well as co-culture invasion and in vitro angiogenesis studies (with HUVEC and DU145 cells) suggested the differential effectiveness of the diastereoisomers toward PCA and endothelial cells. Overall, these studies elucidated the comparative anti-angiogenic efficacy of pure
flavonolignans from Milk Thistle and suggest their usefulness in PCA angioprevention.