Page 301 - Vitamin D and Cancer

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12.6 Vitamin D Resistance Pathways

Some transformed breast cells display reduced sensitivity to 1,25D, suggesting that
the vitamin D pathway may become deregulated during cancer development.
Multiple mechanisms have been identified that contribute to 1,25D resistance,
including loss of VDR expression, alterations in transcriptional co-regulators and
overexpression of Cyp24, the enzyme that catabolizes 1,25D. Stable expression of
the antiapoptotic protein bcl-2 rendered cancer cells resistant to 1,25D mediated
apoptosis, and expression of certain oncogenes (including ras and SV40 large T
antigen) abrogated VDR signaling [32, 63, 64]. Amplification of the Cyp24 gene
was reported in human breast tumors, and higher Cyp24 expression was detected in
tumors compared to adjacent normal tissue [54, 65]. De-sensitization of breast
cancer cells to growth inhibition by VDR ligands has also been associated with
changes in nuclear receptor co-repressors via epigenetic mechanisms, which are
potentially reversible [66–68]. Sub-clones of the MCF-7 breast cancer cell line
selected for resistance to 1,25D in vitro have been independently developed and
characterized [69, 70]. These cell lines retain low level expression of transcription-
ally active VDR but exhibit changes in protein expression that alter redox status,
favor autonomous growth signaling, and down-regulate the apoptotic pathway
[24, 26, 71]. One of these 1,25D resistant MCF-7 cell lines was tested in a xeno-
graft model and retained resistance to the antitumor effects of the vitamin D analog
EB1089, providing an in vivo model for the study of vitamin D resistance [45].
Notably, despite deregulation of multiple signaling pathways, the MCF-7 cells
selected for 1,25D resistance are cross-resistant to structurally related vitamin D
analogs but retain sensitivity to other growth inhibitory agents, including retinoids
and anti-estrogens. Uncovering the molecular basis for selective vitamin D resis-
tance will be critical in design and implementation of new vitamin D analogs for
clinical use.
Kemmis and Welsh [72] used a series of isogenic, progressively transformed HME cell
lines [73] to study the effects of transformation per se on the vitamin D pathway. In this
model, HME cells transduced with SV40 large T antigen and oncogenic ras undergo the
epithelial-mesenchymal transition (loss of E-cadherin) and acquire tumorigenic potential.
VDR expression in HME cells expressing SV40 or ras was reduced more than 70% com-
pared to the nontransformed HME cells from which they were derived. Loss of VDR may
be associated with up-regulation of transformation-associated corepressor proteins such as
slug or snail, which have been shown to directly repress the human VDR promoter in
breast and colon cancer cells [74, 75]. In the HME series, oncogenic transformation was
also associated with reduced Cyp27B1 expression and activity (as measured by 125D
synthesis), but the underlying molecular mechanisms for this change remain unknown.
The reductions in VDR and Cyp27B1 in the oncogene-transformed HME cells were of
sufficient magnitude to reduce cellular sensitivity to growth inhibition by both 1,25D and
25D approximately 100-fold. These studies provide evidence that disruption of the vita-
min D signaling pathway may occur early in the cancer development process, and that
cancer cells employ multiple mechanisms to evade the negative growth regulatory effects
of the vitamin D signaling pathway.