Page 117 - Vitamin D and Cancer

104 Y. Ma et al.

and induces apoptosis in sprouting endothelial cells [76]. 1,25D prevents retinal
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endothelial cells from forming capillary networks in Matrigel, while cell prolifera-
tion or migration is not affected at similar concentrations of 1,25D [77]. 1,25D
3 3
and vitamin D analogs 7553, 6760, and EB1089 exert anti-proliferative effects on
tumor-derived endothelial cells (TDEC) [68]. The TDECs are isolated by enzyme
digestion from SCC VII/SF tumors in C3H/HeJ mice, and sorted by flow cytom-
etry using antibodies against endothelial cells markers [68]. 1,25D differentially
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regulates cell growth of Matrigel-derived endothelial cells (MDEC) and TDEC
isolated from SCC tumors [69]. VDR protein is expressed and its signaling axis
intact in both MDEC and TDEC. 1,25D induces G0/G1 cell cycle arrest and
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apoptosis in TDEC, which is accompanied by decreased p21 expression, increased
p27 expression, and reduced phosphorylation of Akt and ERK1/2. Increased
cleavage of pro-caspase 3 and poly (ADP-ribose) polymerase is observed in TDEC
following 1,25D treatment. In contrast, these effects are not observed in MDEC
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treated with 1,25D [69]. The difference in methylation status of the 24-hydroxylase
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(CYP24) promoter may be one of the mechanisms for these observations. 1,25D
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induces CYP24 mRNA and protein expression and enzymatic activity in MDEC
but not in TDEC [78]. VDR is recruited to the CYP24 promoter in MDEC but not
TDEC. Further studies show hypermethylation in two CpG islands located at the
5′ end in TDEC but not in MDEC, indicating methylation silencing of CYP24
[78]. Knocking down CYP24 by siRNA sensitizes MDEC to 1,25D -mediated
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growth inhibitory effect. On the other hand, when TDEC is treated with DNA
methyltransferase inhibitor 5-aza-2′-deoxycytidine, 1,25D induces CYP24
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expression and TDEC loses its sensitivity to 1,25D [78]. These results indicate
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that the methylation-mediated silencing of CYP24 in TDEC contributes to the
differential growth inhibitory effects of 1,25D on endothelial cells isolated from
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different microenvironments.

5.2.3 Effect on Angiogenesis Models

The effect of vitamin D on angiogenesis was first reported in 1990, when 1,25D
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and a synthetic analog 22-oxa-1,25D were found to inhibit embryonic angiogen-
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esis in chorioallantoic membranes in a dose-dependent manner [79].
Growing evidence supports an anti-angiogenic role of vitamin D in vivo in vari-
ous model systems. 1,25D inhibits the proliferation of TDEC from VDR wild-type
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mice but not from VDR knockout mice [80]. Tumors from VDR knockout mice
show enlarged blood vessels, increased vascular volume, less pericyte coverage on
vessels, and higher vascular leakage compared to those from wild-type mice. In
addition, HIF-1a, VEGF, Ang1, and PDGF-BB expressions are higher in tumors
from VDR knockout mice [80]. 1,25D reduces the mean vessel counts in retino-
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blastoma in a transgenic murine retinoblastoma model system [81]. In an MCF-7
tumor xenografts model which overexpress VEGF, treatment with 1,25D resulted
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in less vascularized tumors compared with vehicle-control-treated tumors [76].