Page 271 - Vitamin D and Cancer

258 R. Okamoto et al.

–7
–10
HL-60 myeloblastic cell line cultured with 1,25(OH) D (l0 –10 M, for
2 3
7 days) morphologically and functionally differentiate toward macrophages,
becoming adherent to charged surfaces, developing pseudopodia, staining posi-
tively for nonspecific esterase (NSE), reducing nitroblue tetrazolium (NBT), and
acquiring the ability to phagocytose yeast [36, 51, 52]. In addition, these cells have
the ability to degrade bone marrow matrix in vitro, raising the possibility that the
cells may have acquired some osteoclast-like characteristics. Leukemic cells from
AML patients respond to vitamin D compounds when cultured in vitro; however,
they are often less sensitive to this seco-steroid than are the HL-60 cell lines. They
frequently undergo partial monocytic differentiation as assessed by NBT reduction,
morphology, and phagocytic ability. Furthermore, their clonal growth is often
inhibited [36, 53].

11.4.2 Molecular Mechanisms of Action of Vitamin D
Compounds Against Leukemic Cells

Vitamin D compounds can exert their biological effects by genomic (Sect. 11.4.2.1)
and/or nongenomic (Sect. 11.4.2.2) pathways. Both pathways require ligand bind-
ing to the VDR. The former pathway relies on a 1,25(OH) D activated VDR/RXR
2 3
complex binding to VDREs in order to modulate the transcription of various target
2+
genes. The latter increases rapid intracellular Ca influxes resulting in activation of
kinases within seconds to minutes [54]. It is still unknown whether the nongenomic
actions are mediated through the classical nuclear VDR, a membrane-associated
VDR or other proteins. Exposure of hematopoietic cells to 1,25(OH) D controls
2 3
myriad of genes, including those responsible for the regulation of cellular prolifera-
tion, differentiation, apoptosis and angiogenesis. Modulation of these genes by
1,25(OH) D may not always be a direct effect on transcription of target genes, but
2 3
can reflect the entire process of differentiation associated with a series of interact-
ing transcription factors. Nonetheless, 1,25(OH) D -activated intracellular signal-
2 3
ing pathways require the presence of VDR to stimulate monocyte/macrophage
differentiation, as demonstrated by studies on bone marrow cells from VDR KO
mice [30] and cells from patients with vitamin D-dependent rickets type II [55, 56].
The rapid, nongenomic activities of vitamin D are described in detail in Chapter xx.
The molecular targets of vitamin D compounds in leukemic cells are summarized
in Table 11.2.

11.4.2.1 Molecular Mechanisms of Genomic Action of 1,25(OH) D
2
3
in Leukemic Cells
Myeloid leukemic cell lines cultured with 1,25(OH) D undergo an initial prolifera-
2 3
tive burst, which is followed by growth inhibition, terminal differentiation and
subsequent apoptosis [57, 58]. Levels of cyclin A, D1 and E increase in the U937