Modulators of ceramide metabolism sensitize colorectal cancer cells to chemotherapy: a novel treatment strategy.

Irinotecan is a first-line chemotherapeutic agent for patients with metastatic colorectal cancer (CRC). Response rates of less than 40% underscore the problem of treating CRC with irinotecan. Our studies have shown that chemosensitization correlates with high levels of ceramide, whereas resistance correlates with high levels of glucosylceramide (GlcCer). The purpose of this study was to characterize the role of ceramide in irinotecan-mediated CRC cell death. We used four human CRC cell lines to assess ceramide metabolism, cell viability, and apoptosis after treatment with irinotecan. Fumonisin B(1) (FB(1)) and 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) were used to inhibit de novo ceramide synthesis and GlcCer production, respectively. L-threo-dihydrosphingosine (safingol) was used to inhibit secondary proliferative pathways mediated by an atypical protein kinase C that is activated by ceramide. Irinotecan elicited dose- and time-dependent increases in ceramide, which preceded apoptosis. When FB(1) was added to irinotecan, CRC cell death was significantly decreased. A significant increase in intracellular levels of GlcCer also was noted after treatment with irinotecan. When GlcCer production was blocked by treating cells with PPMP in addition to irinotecan, ceramide levels increased to 228% of control values and cell death increased by 88%, compared to irinotecan alone. When irinotecan was combined with both PPMP and safingol, cell death was increased by 225% to 325%, compared to irinotecan lone. CRC cell death due to irinotecan is mediated, at least in part, by the de novo synthesis of ceramide. Blocking further metabolism of ceramide can enhance this cytotoxicity. Targeting ceramide pathways is a novel strategy for the treatment of patients with CRC.