Page 22 - Vitamin D and Cancer

1 Vitamin D: Synthesis and Catabolism 9

established breast cancer cell lines showed a wide range of vitamin D hydroxylase
expression. In general, however, CYP27B1 mRNA expression is relatively low and
that of CYP24A1 is rather high. For example, hydroxylation of 25-(OH)D in
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MCF-7 cells occurred primarily on the C-24 pathway [38], though we were able to
demonstrate that 17b-estradiol elevates CYP27B1 mRNA expression and activity
in these cells as well [70]. Kemmis and Welsh [36] recently showed that oncogenic
transformation of human mammary epithelial cells was associated with reduced
1,25-(OH) D synthesis and decreased sensitivity to its antimitotic action. This sug-
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gests enhanced expression of the catabolic CYP24A1 during progression.
Growth and function of the prostate is dependent on androgens. Initial endocrine
therapy in prostate cancer aims to eliminate androgenic activity from cells.
However, cells invariably become refractory to this therapy and grow androgen-
independently. During this progression, estrogen influence appears to increase and
oxidative and reductive 17b-hydroxysteroid dehydrogenase activities are modified
[75]. In another report, 17b-hydroxysteroid dehydrogenase subtypes 2, 4, and 5
were up-regulated in prostatic cell lines treated with 1,25-(OH) D [76]. Interestingly,
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aromatase enzymatic activity was enhanced by 1,25-(OH) D in prostate cancer cell
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lines suggesting synthesis of estradiol from testosterone, whereas 5a-reductase was
not modified [77]. On the other hand, 1,25-(OH) D apparently inhibited androgen
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glucuronidation and thus androgen inactivation [78], while it stimulated androgen
receptor expression [79]. Quantification of CYP27B1 mRNA [80] and of enzy-
matic activity in prostate cancer compared with normal cells suggested deficiency
during progression [35], which would result in reduced dependence on 25-(OH)D
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for growth control.

1.2.4 Regulation of CYP27B1 and of CYP24A1 Expression
by Splicing Mechanisms and Polymorphisms

Alternative gene splicing affects up to 70% of human genes and enhances genetic
diversity by generating proteins with distinct new functions. In line with many
cytochrome P450s, CYP27B1 is known to exhibit alternative splicing and, in kid-
ney cells, this led to modified 1,25-(OH) D synthesis [81]. There have been several
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reports on differential expression of splice variants for CYP27B1 also in cancerous
cells derived from diverse tissues suggesting a role for gene splicing in tissue-
specific regulation of 1,25-(OH) D production [82–85]. In MCF-7 mammary cells,
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and several subclones of this cell line, at least six splice variants of CYP27B1 were
detected resulting in at least six protein variants present in Western blots at varying
band intensity [85]. It is yet unknown whether some of these splice variants present
during breast tumor progression lack 1a-hydroxylation activity.
Splice variants of CYP24A1 could lead to abnormal vitamin D catabolism
respectively reduced or enhanced 1,25-(OH) D accumulation (see, e.g., [86, 128]).
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In prostate tumor-derived cell lines, constitutive CYP24A1 was expressed as a
splice variant in some cells, whereas others had CYP24A1 splice variants after