Page 74 - Vitamin D and Cancer

3 Anti-inflammatory Activity of Calcitriol in Cancer 61

was seen only in primary cells derived from normal prostatic epithelium and
primary, localized adenocarcinoma but not in the established PCa cell lines derived
from PCa metastasis such as LNCaP, PC-3, or DU145. MKP5 is a member of the
dual specificity MKP family of enzymes that dephosphorylate, and thereby inac-
tivate, mitogen activated protein kinases (MAPKs). MKP5 specifically dephos-
phorylates p38 MAPK and the stress-activated protein kinase Jun-N-terminal
kinase (JNK), leading to their inactivation. Calcitriol inhibited the phosphoryla-
tion and activation of p38 in normal primary prostate cells in a MKP-5-dependent
manner as MKP5 siRNA completely abolished p38 inactivation by calcitriol [40].
A consequence of p38 stress-induced kinase activation is an increase in the pro-
duction of pro-inflammatory cytokines that sustain and amplify the inflammatory
response [95]. As interleukin-6 (IL-6) is a p38-regulated pro-inflammatory
cytokine implicated in PCa progression [96], we investigated the effect of calcit-
riol on IL-6 production. Stimulation of primary prostate cells with the pro-inflam-
matory factor, tumor necrosis factor a (TNFa), increased IL-6 mRNA stability
and concentrations of IL-6 in the conditioned media. Pretreatment of the cells with
calcitriol significantly attenuated the increase in IL-6 production following TNFa
treatment [40].
IL-6 is a major pro-inflammatory cytokine that participates in inflammation-
associated carcinogenesis [97] and has been implicated in the pathogenesis of
several cancers [96, 98, 99]. Serum IL-6 levels were significantly elevated and posi-
tively correlated to tumor burden in colon cancer patients [100]. Serum IL-6 levels
were also significantly elevated in BCa patients [101, 102] and in PCa patients,
where in addition a positive correlation between IL-6 levels and the number of bone
metastases was also seen [102]. IL-6 is known to be associated with PCa progres-
sion [96]. Our data demonstrate the ability of calcitriol to reduce the production of
pro-inflammatory cytokines such as IL-6 by inhibiting p38 activation via MKP5
upregulation as well as to interfere with the signaling of pleitropic inflammatory
cytokines such as TNFa [40]. These observations provide evidence of significant
anti-inflammatory effects of calcitriol in cancer cells. Interestingly, established
metastasis-derived PCa cell lines exhibited low levels of MKP5 and were unable to
induce MKP5 in response to calcitriol. We therefore speculate that a loss of MKP5
might occur during PCa progression, as a result of a selective pressure to eliminate
the tumor suppressor activity of MKP5 and/or calcitriol.

3.3.3 Inhibition of NFkB Activation and Signaling

NFkB comprises a family of inducible transcription factors ubiquitously present in
all cells. NFkB transcription factors are important regulators of innate immune
responses and inflammation [103]. In the basal state, most NFkB dimers are bound
to specific inhibitory proteins called IkB and pro-inflammatory signals activate
NFkB mainly through IkB kinase (IKK)-dependent phoshorylation and degrada-
tion of the inhibitory IkB proteins [42]. Free NFkB then translocates to the nucleus