The role of protein kinase C in the synergistic interaction of safingol and irinotecan in colon cancer cells

Colon cancer represents one of the most common solid tumors in adults. Although 5-fluorouracil (5-FU) and irinotecan have been frequently administered in colon cancer patients, low response rates to these single drug therapies were reported. It is therefore imperative to search for new targeted combination therapies that are effective. In this study, we investigated the anti-cancer effect of safingol as a single agent or in combination with irinotecan using HT-29 and LS-174T colon cancer cells as our in vitro models. As a single agent, safingol was more potent than irinotecan and 5-FU, with IC50 values of 2.5±1.1 µM and 3.4±1.0 µM achieved in HT-29 and LS-174T cells, respectively. However, protein kinase C (PKC) was not inhibited with concentrations of safingol which could induce substantial cell kill. The combination of safingol/irinotecan at 1:1 molar ratio was found to be additive in HT-29 cells (CI=0.94) and synergistic in LS-174T cells (CI=0.68), and resulted in concentration- and time-dependent down-regulation of p-PKC and p-MARCKS. The drug effect of the safingol/irinotecan combination was further modulated in the presence of a PKC stimulator (phorbol 12-myristate 13-acetate) or a PKC inhibitor (staurosporine). Furthermore, the 1:1 safingol/irinotecan combination inhibited the adhesion of colon cancer cells to the extracellular matrix 4-h post-treatment. Taken together, modulation of the PKC pathway could be a possible molecular basis for the observed synergism of the safingol/irinotecan combination, and these results demonstrate the therapeutic potential of this drug combination in colon cancer treatment.