Page 166 - Vitamin D and Cancer

7 Induction of Differentiation in Cancer Cells by Vitamin D 153

recruitment of phosphatidylinositol 3-kinase (PI3K) to a complex at the cell plasma
membrane consisting of E-cadherin, b-catenin, and p120-catenin. This complex is
postulated to activate PI3K leading to the accumulation of phosphatidylinositol
3,4,5-triphosphate (PIP ), which binds to and activates phospholipase C gamma-1
3
(PLC-g1) [93, 94]. The activated phospholipase generates inositol triphosphate
(IP ) which stimulates the release of calcium from the intracellular stores in the
3
endoplasmic reticulum, and diacylglycerol, which together with increased intracel-
lular calcium activates PKC. PKC, and perhaps calcium activation of other
enzymes, then initiate signaling cascades that impinge on nuclear transcription
factors such as AP-1, which lead to differentiation [95].
How much of this description applies to the 1,25D-induced differentiation is
less clear, but Bikle et al. [91] presented a plausible model in which 1,25D inter-
acts with calcium to induce keratinocyte differentiation. This model also includes
a G-protein-coupled calcium-sensing surface receptor (CaR), which when acti-
vated by 1,25D leads to the activation of PKC, with consequences described
above. The associated influx of calcium, which occurs in human keratinocytes
after exposure to 1,25D has been recently shown to be mediated, at least in part,
by the calcium-selective channel TRPV6 upregulated at the mRNA and protein
levels by 1,25D [96]. A cohesive picture of 1,25D-induced keratinocyte differen-
tiation is quite well, but perhaps not completely developed. For instance, regula-
tion of AP-1 activity in cultured human keratinocytes by 1,25D was reported to
be independent of PKC [97], in contrast to the model presented by Bikle et al.
[91]. Takahashi et al. [98] reported that treatment of normal human keratinocytes
with 1,25D increases the expression of cystatin A, a cysteine protease inhibitor
which is a component of the cornified envelope, and that it is the suppression of
the Raf-1/MEK-1/ERK signaling pathway which is responsible for this effect.
However, cystatin A expression is stimulated by the Ras/MEKK-/MKK7/JNK
pathway [99], consistent with the schematic model of Bikle et al. [91], and
explaining why PKC activation may not be essential for AP-1 activation in this
cell system. An enigmatic role of caspase-14 in keratinocyte differentiation
induced by 1,25D has been reported [100], and it was suggested that the absence
of caspase-14 contributes to the psoriatic phenotype. Since caspase-14 is a non-
apoptotic protein, it is unclear if this is related to the report that 1,25D protects
keratinocytes from apoptosis [101]. On the other hand, the identification of Kruppel-
like factor 4 (KLF-4) and c-fos as 1,25D-responsive genes in gene expression
profiling of 1,25D-treated keratinocytes [102] fits in well with the existing
knowledge of differentiation signaling, as c-fos is a component of the AP-1 tran-
scription factor, and KLF-4 is a transcription factor with a major role in cell fate
decisions [103–105]. Recently, it was reported that yet another transcription factor,
PPAR gamma, also has a major role in 1,25D-induced differentiation of keratino-
cytes [106]. In these studies, dominant negative (dn) PPAR gamma inhibited the
expression of involucrin (a differentiation marker), suppressed AP-1 binding to
DNA, and prevented the 1,25D-induced phosphorylation of p38. Thus, the kera-
tinocyte system provided a wealth of interesting information on 1,25D as a
differentiation-promoting and survival-regulating agent.