Although the
sphingolipid ceramide exhibits potent
tumor suppressor effects, efforts to harness this have been hampered by poor solubility, uptake, bioavailability, and metabolic conversion. Therefore, identification of avenues to improve efficacy is necessary for development of
ceramide-based
therapies. In this study, we used mutant p53,
triple-negative breast cancer (TNBC) cells, a type of
breast cancer highly refractory to treatment, and cell-permeable nanoliposomal
C6-ceramide in conjunction with the
antiestrogen tamoxifen, which has been shown to be an effective modulator of
ceramide metabolism. We show for the first time that nanoliposomal
tamoxifen enhances nanoliposomal
C6-ceramide cytotoxicity in cultured TNBC cells, a response that was accompanied by induction of cell-cycle arrest at G(1) and G(2),
caspase-dependent induction of DNA fragmentation, and enhanced mitochondrial and lysosomal membrane permeability at 18 and 2 hours, respectively.
Tamoxifen metabolites were also effective. Only
tamoxifen promoted lysosomal membrane permeability. In addition, we show for the first time that
tamoxifen inhibits
acid ceramidase, as measured in intact cell assays; this effect was irreversible. Together, our findings show that
tamoxifen magnifies the antiproliferative effects of
C6-ceramide via combined targeting of cell-cycle traverse and lysosomal and mitochondrial integrity. We adduce that C6-ceramide-induced apoptosis is amplified by
tamoxifen's impact on lysosomes and perhaps accompanying inhibition of
acid ceramidase, which could result in decreased levels of
sphingosine 1-phosphate. This drug regimen could serve as a promising
therapy for chemoresistant and triple-negative types of
breast cancer, and thus represents an indication for
tamoxifen, irrespective of
estrogen receptor status.