Page 273 - Vitamin D and Cancer

260 R. Okamoto et al.

levels of Bcl-2, but this may not be the case with myeloid cells. A down-regulation
of Bcl-2 was observed both at the mRNA and protein levels after HL-60 cells were
exposed to 1,25(OH) D [62].
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Activation of the proto-oncogene c-myc is a typical feature of human leukemias.
The HL-60 leukemia cell line is characterized by high levels of expression of c-myc
due to gene amplification [64, 65]. Treatment of this cell line with 1,25(OH) D
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results in a down-regulation of expression of this oncogene associated with cell
differentiation [66]. Suppression of c-myc by 1,25(OH) D and its non-calcemic
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analogs occurs at the transcriptional level in HL-60 cells [67, 68]. Exposure of
HL-60 cells to 1,25(OH) D induces the expression of the proto-oncogene c-fms,
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which encodes the receptor for M-CSF. It occurs in parallel with the induction of
CD14 expression and a block of the cell cycle in the G /G phase [69].
0 1
1,25(OH) D up-regulates the protein coding for the homeobox gene, HOXB4,
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that binds to the first exon/intron border of MYC to prevent transcriptional elonga-
tion, a process dependent on activation of PKC-b [70, 71]. Another homeobox
gene, HOXA10, was found by differential display to be a gene transcriptionally
induced by 1,25(OH) D through binding to the VDRE in the promoter during dif-
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ferentiation of U937 cells [72, 73].
Besides MYC and HOX genes, 1,25(OH) D can induce other transcription fac-
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tors and coactivators to regulate gene expression. For example, exposure of U937
cells to 1,25(OH) D induced the expression of PU.l, IRF8 and C/EBPb [74]. In
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contrast, exposure of U937 cells to 1,25(OH) D (10 M) down-regulated the
–8
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expression of TEL2, which is a member of the ETS family [75]. Interestingly,
forced overexpression of TEL2 inhibited 1,25(OH) D -induced differentiation.
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The ligand-activated VDR can bind to the AP-l complex. Exposure of the
chronic myelogenous leukemia (CML) cell line RWLeu-4 to 1,25(OH) D inhibited
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their proliferation and enhanced the binding activity of the proto-oncogene junD to
VDRE. This binding activity decreased in a 1,25(OH) D -resistant variant JMRD
2 3 3
cells. Although these cells exhibit no detectable differences in the VDR, alterations
in the interaction with the VDRE were important [76]. Exposure of HL-60 cells to
1,25(OH) D , up-regulated expression of genes that code for the AP-l complex
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–7
including c-jun, ATF-2, jun-B and fos-B [15, 77]. Moreover, 1,25(OH) D (l0 M)
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was also able to induce expression of the subunits of the transcriptional coactivator,
Thyroid hormone Receptor-Associated Polypeptide (TRAP, also called DRIP) as
early as 6 h in the HL-60 cells [78]. The TRAP complex plays a role in direct
communication between the nuclear receptors and the general transcriptional
machinery through direct interaction with RNA polymerase II [79]. The murine
Trap220(-/-) yolk sac hematopoietic progenitor cells, as well as, TRAP knockdown
HL-60 cells are resistant to induction of differentiation by 1,25(OH) D .
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Fusion proteins involving the retinoic acid receptor alpha (RARa) with either the
PML or PLZF nuclear proteins are the genetic markers of acute promyelocytic leuke-
mias (APLs). APL cells expressing PML-RARa are sensitive to retinoid induced dif-
ferentiation to granulocytes in the presence of retinoic acid. In contrast, forced
expression of either PML-RARa or PLZF-RARa in either U937 or HL-60 cells blocks
their terminal differentiation after exposure to 1,25(OH) D [80]. Both PML-RARa
2 3