1α,25-Dihydroxyvitamin D
3 [1α,25(OH)
2D
3], the active form of vitamin D
3, is known to exhibit an anti-tumor properties and markedly suppresses the growth of various human cancer cells. We synthesized novel vitamin D analogs, 1α,25-dihydroxyvitamin D
3-26,23-lactam (DLAMs), having a lactam moiety in the side chain, and examined the effects on cell growth of human prostate cancer cells LNCaP. 1α,25(OH)
2D
3 significantly suppressed both the number of cells and cell viability. The mRNA expression of p21, well-known as a tumor suppressive gene, was clearly induced by treatment with 1α,25(OH)
2D
3 in LNCaP cells. The effects of 1α,25(OH)
2D
3 on the growth suppression of LNCaP cells was attenuated by the simultaneous addition of (23
S,25
S)-DLAM-1P. In a computer docking simulation, (23
S,25
S)-DLAM-1P bound to vitamin D receptor (VDR), and its lactam moiety may interfere VDR helix-12 folding. Its stereoisomer (23
R,25
R)-DLAM-1P did not influence cell growth regulated by 1α,25(OH)
2D
3. The expression of p21 mRNA induced by 1α,25(OH)
2D
3 was suppressed by (23
S,25
S)-DLAM-1P but not by (23
R,25
R)-DLAM-1P in LNCaP cells. In the absence of 1α,25(OH)
2D
3, neither (23
S,25
S)-DLAM-1P nor (23
R,25
R)-DLAM-1P regulated the cell growth of LNCaP cells. Thus, (23
S,25
S)-DLAM-1P interferes with the VDR signal and acts as a vitamin D
3 antagonist in cancer cells.
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