Page 71 - Vitamin D and Cancer

58 A.V. Krishnan and D. Feldman

mediate a wave of COX-2 expression at the tumor sites not only in the cancer cells
themselves but also in the surrounding stromal cells and infiltrating macrophages
as well as endothelial cells, thereby promoting tumor progression.

3.3.1.4 Calcitriol Effects on the PG Pathway in Prostate Cells

Our studies demonstrate that calcitriol regulates the expression of PG pathway
genes in multiple PCa cell lines as well as primary prostatic epithelial cells estab-
lished from surgically removed prostate tissue from PCa patients [35]. We found
measurable amounts of COX-2 mRNA and protein in various PCa cell lines as well
as primary prostatic epithelial cells derived from normal and cancerous prostate
tissue, which were significantly decreased by calcitriol treatment. We also found
that calcitriol significantly increased the expression of 15-PGDH mRNA and pro-
tein in various PCa cells. We further showed that by inhibiting COX-2 and stimulat-
ing 15-PGDH expression, calcitriol decreased the levels of biologically active PGs
in PCa cells, thereby reducing the growth stimulation due to PGs. Our data also
revealed that calcitriol decreased the expression of EP and FP PG receptors. The
calcitriol-induced decrease in PG receptor levels resulted in the attenuation of
PG-mediated functional responses even when exogenous PGs were added to the
cell cultures. Calcitriol suppressed the induction of the immediate-early gene c-fos
and the growth stimulation seen following the addition of exogenous PGs or the PG
precursor arachidonic acid to PCa cell cultures [35]. We postulate that the down-
regulation of PG receptors by calcitriol would inhibit the positive feedback exerted
by PGs on COX-2, thereby limiting the wave of COX-2 expression at the tumor
sites and slowing down tumor progression. Thus, calcitriol inhibits the PG pathway
in PCa cells by three separate mechanisms: decreasing COX-2 expression, increas-
ing 15-PGDH expression, and reducing PG receptor levels. We believe that these
actions contribute to the suppression of the proliferative and angiogeneic stimuli
provided by PGs in PCa cells. The regulation of PG metabolism and biological
actions constitutes an important novel pathway of calcitriol action mediating its
anti-inflammatory effects.

3.3.1.5 Combination of Calcitriol and NSAIDs as a Therapeutic
Approach in PCa

NSAIDs are a class of drugs that decrease PG synthesis by inhibiting COX-1 and
COX-2 enzymatic activities. Several NSAIDs nonselectively inhibit both the con-
stitutively expressed COX-1 and the inducible COX-2, while others have been
shown to be more selective in preferentially inhibiting COX-2 enzymatic activity.
We tested the effect of combinations of calcitriol and various NSAIDs on PCa cell
proliferation [35]. These studies were based on our hypothesis that the action of
calcitriol at the genomic level to reduce COX-2 expression, leading to decreased
COX-2 protein levels, will allow the use of lower concentrations of NSAIDs to