Calcitriol as a Chemopreventive and Therapeutic Agent in Prostate Cancer: Role of Anti‐Inflammatory Activity

Calcitriol, the hormonally active form of vitamin D, inhibits the growth and development of several cancers. Inflammation has been implicated in the development and progression of many cancers, including prostate cancer (PCa). Recent research from our laboratory suggests that calcitriol exhibits anti‐inflammatory actions that may contribute to its inhibitory effects in PCa. We found that calcitriol inhibits the synthesis and actions of pro‐inflammatory prostaglandins (PGs) by three mechanisms: (1) inhibition of the expression of cyclooxygenase‐2 (COX‐2), the enzyme that synthesizes PGs, (2) induction of the expression of 15‐prostaglandin dehydrogenase (15‐PGDH), the enzyme that inactivates PGs, and (3) decreasing the expression of prostaglandin E and prostaglandin F PG receptors, which are the mediators of PG signaling. The combination of calcitriol and nonsteroidal anti‐inflammatory drugs (NSAIDs) result in a synergistic inhibition of PCa cell growth and offers a potential therapeutic strategy. Acting on a separate anti‐inflammatory pathway, calcitriol induces the expression of mitogen‐activated protein kinase phosphatase 5 (MKP5), a member of a family of phosphatases that are negative regulators of MAP kinases, causing the selective dephosphorylation and inactivation of the stress‐activated protein kinase p38. Because p38 activation may be both procarcinogenic and promote inflammation, this calcitriol action, especially coupled with the inhibition of the PG pathway, may contribute to the chemopreventive activity of calcitriol. We conclude that calcitriol exerts several anti‐inflammatory actions in prostate cells, which contribute to its potential as a chemopreventive and therapeutic agent in PCa.