Combination studies of
celecoxib and chemotherapeutic agents suggest that combining
cyclooxygenase-2 inhibitors with other agents may have supra-additive or synergistic effects on
tumor growth inhibition.
Carboxyamido-triazole (CAI), a voltage-independent
calcium channel inhibitor, has been shown to induce growth inhibition and apoptosis in
cancer cells. We found that continuous exposure to
cytostatic doses of CAI and
LM-1685, a
celecoxib analogue, reduced the proliferation and survival of seven human
cancer cell lines by at least one log (P < or = 0.001) over either agent alone. To explore the mechanism of action of this combination, we further studied the effects of LM-1685/CAI on CCL-250
colorectal carcinoma cells. We found that the supra-additive antiproliferative effects occurred throughout a range of
LM-1685 doses (5-25 micromol/L) and paralleled a decrease in COX-2 activity as measured by
prostaglandin E2 production. In these cells, treatment with LM-1685/CAI suppressed the
extracellular signal-regulated kinase pathway within the first hour but ultimately results in high, sustained activation of ERK over a 9-day period (P = 0.0005). Suppression of
cyclin D1 and phospho-AKT, and cleavage of
caspase-3 and PARP were concomitant with persistent ERK activation. Addition of
PD98059, a MEK-1 inhibitor, suppressed ERK activation and significantly but incompletely reversed these signaling events and apoptosis. Flow cytometry experiments revealed that the CAI/LM-1685 combination induced a 3-fold increase in apoptosis over control (P = 0.005) in 3 days. We show that the combination of CAI and
LM-1685 produces a cytotoxic effect by suppressing proliferation and triggering apoptosis.