Modification of sphingolipid metabolism by tamoxifen and N-desmethyltamoxifen in acute myelogenous leukemia-Impact on enzyme activity and response to cytotoxics

The triphenylethylene antiestrogen, tamoxifen, can be an effective inhibitor of sphingolipid metabolism. This offtarget activity makes tamoxifen an interesting ancillary for boosting the apoptosis-inducing properties of ceramide, a sphingolipid with valuable tumor censoring activity. Here we show for the first time that tamoxifen and metabolite, N-desmethyltamoxifen (DMT), block ceramide glycosylation and inhibit ceramide hydrolysis (by acid ceramidase, AC) in human acute myelogenous leukemia (AML) cell lines and in AML cells derived from patients. Tamoxifen (1-10 mu M) inhibition of AC in AML cells was accompanied by decreases in AC protein expression. Tamoxifen also depressed expression and activity of sphingosine kinase 1 (SphK1), the enzymecatalyzing production of mitogenic sphingosine 1-phosphate (S1-P). Results from mass spectroscopy showed that tamoxifen and DMT (i) increased the levels of endogenous C16:0 and C24:1 ceramide molecular species, (ii) nearly totally halted production of respective glucosylceramide (GC) molecular species, (iii) drastically reduced levels of sphingosine (to 9% of control), and (iv) reduced levels of S1-P by 85%, in vincristine-resistant HL-60/VCR cells. The co-administration of tamoxifen with either N-(4-hydroxyphenyl)retinamide (4-HPR), a ceramide-generating retinoid, or a cell-deliverable form of ceramide, C6-ceramide, resulted in marked decreases in HL-60/VCR cell viability that far exceeded single agent potency. Combination treatments resulted in synergistic apoptotic cell death as gauged by increased Annexin V binding and DNA fragmentation and activation of caspase-3. These results show the versatility of adjuvant triphenylethylene with ceramide-centric therapies for magnifying therapeutic potential in AML Such drug regimens could serve as effective strategies, even in the multidrugresistant setting. (C) 2015 Elsevier B.V. All rights reserved.