Cyclooxygenase-independent induction of p21(WAF-1/cip1), apoptosis and differentiation by L-745,337, a selective PGH synthase-2 inhibitor, and salicylate in HT-29 cells

In order to dissect out cyclooxygenase-dependent from cyclooxygenase- independent mechanisms in the antiproliferative effects of selective prostaglandin H synthase (PGHS)-2 inhibitors, we compared the effects of L- 745,337 (a highly selective PGHS-2 inhibitor) with sodium salicylate (a weak PGHS inhibitor) on prostanoid production, induction of the cyclin-dependent kinase inhibitor p21(WAF-1/cip1), mutant p53 (m273-p53) levels, apoptosis and differentiation in human colon adenocarcinoma HT-29 cells. L-745,337 dose- dependently suppressed the cyclooxygenase activity of HT-29 cells (IC₅₀:0.24 μM). Four-day treatment with L-745,337 caused a concentration- dependent inhibition of cell growth (IC₅₀: 0.9 mM) associated with the induction of p21(WAF-1/cip1) and an increase in the proportion of apoptotic nuclei (EC₅₀: 0.1 and 0.34 mM, respectively) while reducing the levels of m273-p53 (IC₅₀: 0.2 mM). Sodium salicylate, at the concentration of 10 mM that did not affect prostanoid formation, caused a 60% reduction of cell growth associated with a 3-fold induction of p21(WAF-1/cip1) and a 60% increase in the proportion of apoptotic nuclei. Ultrastructural analysis showed that L-745,337 (0.5 mM) and sodium salicylate (10 mM) caused the induction of a differentiated phenotype. We conclude that high concentrations of L-745,337 and sodium salicylate inhibit colon cancer cell growth by a mechanism unrelated to cyclooxygenase inhibition that may involve p53- independent induction of the tumor suppressor p21(WAF-1/cip1).