Page 164 - Vitamin D and Cancer

7 Induction of Differentiation in Cancer Cells by Vitamin D 151

role of differentiation in clinical trials [72]. A study of the role of 1,25D in the
differentiation of the normal rat prostate gland was based on morphological
characteristics, which included an increased abundance of cytoplasmic secretory
vesicles [73]. This characteristic has been used as a differentiation marker, along
with the expression of keratins 8, 17, and 18 in human prostate cancer PC-3 cells
[74]. In other studies [75, 76], the increased expression of E-cadherin was used as
a maker of differentiation. However, although many reports on the effects of 1,25D
on prostate cancer cells include the word “differentiation,” the documentation most
often focuses on the anti-proliferative effects of 1,25D exposure, which may, or
may not be associated with phenotypic differentiation.
In a recent microarray analysis of 1,25D regulation of gene expression in
LNCaP cells, Krishman et al. [77] reported several findings that appear relevant to
1,25D-induced differentiation. In addition to the major upregulation of the expres-
sion of the insulin-like growth factor binding protein-3 (IGFBP-3), which functions
to inhibit cell proliferation by upregulating p21/Cip1 [78], it was noted that among
about a dozen genes upregulated by 1,25D was the “prostate differentiation factor,”
a member of the bone morphogenetic protein (BMP) family, which is generally
involved in growth and differentiation of both embryonic and adult tissues [79].
Also interesting was the finding that in these cells 1,25D regulates those genes
which are androgen-responsive, and the genes which encode the enzymes involved
in androgen catabolism. Further, Feldman and colleagues showed that 1,25D up-
regulates the expression and activity of the androgen receptor (AR) [80, 81], raising
the possibility that the differentiation effects of 1,25D on prostate cells are not
direct, but are due to modifications of the level or the activity of AR. Interestingly,
it has also been suggested that androgens upregulate the expression of VDR [82];
thus, a positive feedback loop that includes 1,25D activation of VDR could be a
factor in inducing differentiation of cancer cells derived from the hormonally regu-
lated tissues (Fig. 7.2), while in normal cells the sex hormone (androgen or estro-
gen) is sufficient to promote differentiation. Since 1,25D has an established
anticancer activity in prostate cells, it can be assumed that in this scenario VDR
selectively enhances the AR-mediated androgenic pro-differentiation, but not the
proliferation-enhancing activity (Fig. 7.2). In addition, it is likely that nuclear
receptors for retinoids, glucocorticoids, and PPAR affect the signaling pathways,
directly or indirectly. Whether the demonstrated 1,25D-induced decrease in the
expression of COX-2 and an increase in 15-PGDH in prostate cancer cells [77, 83]
have any relationship to cell differentiation, remains to be established.
Prostate cancer cells are also known to undergo “trans-differentiation” to a
neuroendocrine phenotype, and when this phenotype is found in human tumors it
may indicate an aggressive form of the disease [84]. Although currently 1,25D has
no known role in this form of differentiation, this may be a promising area of future
research, since recent studies point to a key role of NFkB, as well as IL-6 in this
process [85, 86]. This suggestion is based on the finding that in some cells 1,25D
upregulates the expression of C/EBP b [87], which cooperates with NFkB in
regulation of the secretion of the cytokine IL-6 in neuroendocrine human prostate
cancer cells [85].