Due to recent use of short-chain
ceramides in preclinical studies, we characterized
C6-ceramide metabolism in
cancer cell lines and assessed metabolic junctures for enhancing efficacy. MDA-MB-231
breast cancer cells decreased the amount of
C6-ceramide metabolized to C6-sphingomyelin (C6-SM) and increased the amount metabolized to C6-glucosylceramide (C6-GC) in response to increasing concentrations. A similar trend was seen in DU-145 (
prostate cancer), PANC-1 (
pancreatic cancer), and LoVo (
colorectal cancer) cells. KG-1
leukemia cells favored C6-SM synthesis at low (0.6muM) and high-dose (12muM)
C6-ceramide. Partnering
C6-ceramide with
tamoxifen, a
P-glycoprotein antagonist that impedes
ceramide glycosylation, was an effective regimen for enhancing cytotoxicity in cells. Experiments to assess the mechanism of cell death using KG-1 cells showed that
tamoxifen inhibited synthesis of C6-GC and C6-SM from
C6-ceramide by 80% and 50%, respectively, which was accompanied by enhanced apoptosis. Radiolabeling of KG-1 cells with [(3)H]
palmitic acid produced a 2-fold increase in (3)H-long-chain
ceramides when unlabeled
C6-ceramide was added and a 9-fold increase when
C6-ceramide and
tamoxifen were added. The increase in (3)H-palmitate radiolabeling of long-chain
ceramides was blocked by inclusion of a
ceramide synthase inhibitor; however, inhibiting synthesis of long-chain
ceramide did not rescue cells. These studies show that
tamoxifen enhances the apoptotic effects of
C6-ceramide. The proposed mechanism involves blocking short-chain
ceramide anabolism to favor hydrolysis and generation of
sphingosine. We propose that use of
tamoxifen and other
P-glycoprotein antagonists can be an effective means for enhancing cytotoxic potential of short-chain
ceramides in the treatment of
cancer.