Page 275 - Vitamin D and Cancer

262 R. Okamoto et al.

1,25(OH) D probably activates the PI3-K/AKT pathway in both a nongenomic
2 3
and genomic fashions. Activation of PI3-K may be required for the 1,25(OH) D -
2 3
stimulated myeloid differentiation, as monitored by induction of CD14 expression
[98]. For example, PI3-K was activated by 1,25(OH) D in THP-1 cells within 20
2 3
min. Pre-treatment with the PI3-K inhibitors, LY 294004 or wortmanin, inhibited
monocytic differentiation in response to 1,25(OH) D in HL-60 and THP-1 cells, as
2 3
well as peripheral blood monocytes [98, 99]. Antisense oligonucleotides against
PI3-K blocked induction of CD14 expression in THP-1 and HL-60 cells. Expression
of the VDR was required for activation of PI3-K; and interestingly, VDR was found
to be associated with the active form of the kinase. PI3-K activates (phosphorylates)
AKT, as well as of its downstream targets, were activated within 6–48 h of exposure
to 1,25(OH) D in HL-60 cells [100]. PI3-K inhibitors synergized with 1,25(OH) D
2 3 2 3
to induce cell cycle arrest of HL-60 cells, associated with a synergistic up-regula-
tion of p27. On the other hand, treatment with 1,25(OH) D for 4 days induced the
2 3
expression of PTEN, which could block the PI3-K/AKT pathway, resulting in dif-
ferentiation, cell death or inhibition of growth of HL-60 cells [50].
The MAPK pathway can also be activated by 1,25(OH) D , and this also prob-
2 3
ably involves genomic and nongenomic mechanisms. Exposure of either HL-60 or
NB4 cells to differentiation-inducing concentrations of vitamin D compounds
cause activation and nuclear translocation of MAPK [101–103]. Rapid changes of
MAPK phosphorylation occurred within 30 s of exposure to 1,25(OH) D in NB4
2 3
cells [102]. In addition, the vitamin D analog EB1089 was demonstrated to
3
induced apoptosis of B-cell chronic lymphocytic leukemia cells from patients, an
event preceded by stimulation of p38 MAPK and suppression of ERK activity
[104]. 1,25(OH) D stimulated the transient (24–48 h) phosphorylation of ERK1/2
2 3
in HL-60 cells. After 24 h, the level of phosphorylated ERK decreased to basal
levels, while differentiation continued over an additional 48 h [105]. Furthermore,
PD98059, an ERK1/2 inhibitor, blocked the 1,25(OH) D -stimulated differentia-
2 3
tion of HL-60 cells [106]. Kinase Suppressor of Ras-1 and -2 (KSR-1, -2) which
phosphorylate Raf-l and act as scaffolds increases the efficiency of signaling by
Raf-l [107, 108]. These two genes have an upstream promoter containing a func-
tional VDRE motif. Knocked-down of KSR-2 blocked 1,25(OH) D induced
2 3
myeloid differentiation. Signaling by Raf-1 is required for the later stage of
1,25(OH) D -induced differentiation and requires p90 RSK which is either directly
2 3
or indirectly phosphorylated by Raf-1 [109].

11.4.3 Vitamin D Compounds in Combination with Other Agents

Because of the potential toxicity of 1,25(OH) D and its analogs at the
2
3
concentrations required in vivo to inhibit proliferation and/or induce differentia-
tion of leukemia cells. Various attempts have been made to use them with other
compounds that might act synergistically and that have an acceptable toxicity. The
mechanism of action and toxicity (hypercalcemia) of vitamin D compounds differ