AIM: The effects of vitamin D3 have been investigated on various tumors, including
colorectal cancer (CRC). 25-hydroxyvitamin-D3-24-hydroxylase (CYP24A1), the enzyme
that inactivates the active vitamin D3 metabolite 1,25-dihydroxyvitamin D3 (1,25-D3),
is considered to be the main enzyme determining the biological halflife of 1,25-D3.
During colorectal carcinogenesis, the expression and concentration of CYP24A1 increases
significantly, suggesting that this phenomenon could be responsible for the proposed
efficacy of 1,25-D3 in the treatment of CRC. The aim of this study was to investigate
the anti-tumor effects of vitamin D3 on the human CRC cell line Caco-2 after inhibition
of the cytochrome P450 component of CYP24A1 activity.
METHODS: We examined the expression of CYP24A1 mRNA and the effects of 1,25-D3 on
the cell line Caco-2 after inhibition of CYP24A1. Cell viability and proliferation
were determined by means of sulforhodamine-B staining and bromodeoxyuridine incorporation,
respectively, while cytotoxicity was estimated via the lactate dehydrogenase content
of the cell culture supernatant. CYP24A1 expression was measured by realtime reverse
transcription polymerase chain reaction. A number of tetralone compounds were synthesized
to investigate their CP24A1 inhibitory activity.
RESULTS: In response to 1,25-D3, CYP24A1 mRNA expression was enhanced significantly,
in a time- and dose-dependent manner. Caco-2 cell viability and proliferation were
not influenced by the administration of 1,25-D3 alone, but were markedly reduced by
coadministration of 1,25-D3 and KD-35, a CYP24A1-inhibiting tetralone. Our data suggest
that the mechanism of action of co-administered KD-35 and 1,25-D3 does not involve
a direct cytotoxic effect, but rather the inhibition of cell proliferation.
CONCLUSION: These findings demonstrate that the selective inhibition of CYP24A1 by
compounds such as KD-35 may be a new approach for enhancement of the anti-tumor effect
of 1,25-D3 on CRC.
