Page 212 - Vitamin D and Cancer

9 Molecular Biology of Vitamin D Metabolism and Skin Cancer 199

action of this molecule. The stimulation of phospholipase C (PLC) b and PLCg by
G proteins and protein-tyrosine kinase receptors respectively, leads to the hydrolysis
of phosphatidylinositol 4,5-bisphosphate (PIP ) in the inner layer of the plasma
2
membrane to form the second messengers diacylglycerol (DAG) and inositol 1,4,5-
triphosphate (IP ). DAG remains at the plasma membrane and activates kinases in
3
the protein kinase C (PKC) family. On the other hand, IP is release to the cytoplasm
3
2+
to stimulate the release of Ca from intracellular stores to increase cytosolic calcium
2+
(Ca ) levels. The Ca released can either act on protein kinases (some members of
2+
the PKC need both DAG and Ca to be activated) or cause the opening of calcium
2+
2+
channels in the plasma membrane to allow the influx of extracellular Ca for a more
sustained response. PIP can also initiate another second messenger signaling path-
2
way when it is phosphorylated by phosphatidylinositide 3-kinase (PI3K) to produce
PIP . PIP acts to recruit the protein kinases Akt and PDK1 to the plasma membrane.
3
3
Akt is subsequently phosphorylated and activated to phosphorylate downstream
targets such as regulators proteins for cell survival, transcription factors and other
protein kinases. Additionally, activation of the G protein can also stimulate adenylyl
cyclase (AC) activity. AC synthesizes cyclic AMP (cAMP) from ATP. cAMP then
binds to the regulatory subunits of protein kinase A (PKA) to release the catalytic
subunits which are now able to phosphorylate their target proteins [30].
Activation of the PKC and PKA in the non-genomic pathway can phosphorylate
the VDR involved in the genomic pathway to modulate its activity (Fig. 9.2) [38].
This suggests that kinase activation on the non-genomic pathway may have a role
in determining the functional outcome of the VDR in the genomic pathway.
In addition to the VDR, target proteins of PKC, PI3K and PKA pathways also
include proteins involved in the Raf-MEK-MAPK-ERK pathway (Fig. 9.2). This is
initiated by the activation of Ras which in turn activates the Raf protein serine/
threonine kinase and subsequently the MEK-MAPK-ERK cascade. This ultimately
allows ERK to phosphorylate a range of targets such as other protein kinases and
transcription factors. Thus, the PKA, PKC and ERK signaling pathway intersects
with the classical genomic pathway to provide “cross-talk” between the non-classical
membrane receptor pathway and the classical genomic pathway (Fig. 9.2). This
allows a complex fine tune regulatory mechanism to action of 1,25(OH) D in regu-
2
3
lating mineral and bone homeostasis, cellular proliferation and differentiation, that
are important in healthy and diseased states.

9.3.4 Classical Roles of 1,25-Dihydroxyvitamin D
3

The most well known and classical role of 1,25(OH) D is its function in calcium
2
3
and phosphate homeostasis and bone mineral metabolism [67]. The vitamin D
endocrine system maintains mineral homeostasis and bone metabolism by the
appropriate transcriptional activation of genes or repression of target genes in cells
that are involved in these processes [98]. The importance of this role is shown in
studies using 1a-hydroxylase, vitamin D receptor and a combination of