Our data indicate that in select settings activation of acid sphingomyelinase (ASMase)/ceramide signaling in tumor endothelial cells by radiation and certain chemotherapies synergizes with direct tumor cell damage to impact outcome. ASMase is a lysosomal hydrolase preferentially expressed in endothelial cells up to 20-fold compared with other mammalian cells. Mechanistically, endothelial ASMase activation leads within min to formation of plasma membrane ceramide-rich platforms (CRPs), macrodomains that organize apoptotic signaling programs. Support for our concept derives from studies showing xenografts of all histologies, when implanted in asmase-/- host mice become resistant to gemcitabine, paclitaxel, etoposide, and high single dose radiotherapy, effects reversed by exclusive adenoviral asmase gene delivery to tumor microvasculature. We recently discovered VEGF is the principal inhibitor of endothelial ASMase, and that anti-angiogenic drugs de-repress ASMase, amplifying tumor responses to anti-cancer therapies, but only under specific conditions. We found irrespective of t1/2 or anti-angiogenic class, these drugs enhance endothelial apoptosis and tumor response only if scheduled at 1-2h preceding chemotherapy, as ASMase can be de-repressed for only 1-2h. Based on these data, the MSK Sarcoma Service performed a Phase II trial that showed sphingolipid-based timing of bevacizumab vs. conventional synchronous timing improved metastatic sarcoma response to the cytidine analogue gemcitabine from 38 to 93% (p=0.0024; Tap and Kolesnick, unpublished). The current application will help establish the mechanism by which temporal delivery of a short-acting anti-angiogenic drug simultaneously enhances gemcitabine-induced endothelial and tumor cell apoptosis. The overarching hypothesis of this application is that the principal nucleoside transporter in mammalian cells, ENT1, required for gemcitabine uptake, is not constitutively ?on? as generally accepted but must insert into CRPs on endothelial and tumor cells for functionalization. This new membrane-based mechanism of gemcitabine action will be explored in 3 aims designed to examine mechanism of ENT1 functionalization via CRPs in both endothelial and sarcoma cells, VEGF inhibition of ASMase-generated CRPs, and pharmacologic strategies to enhance endothelial ASMase- ceramide signaling in vivo to improve ENT1-mediated gemcitabine uptake and cell death. A major concept to be explored is that gemcitabine-induced ASMase secreted by endothelium triggers ?bystander? gemcitabine uptake via ENT1 in tumor cells. As such, these investigations potentially define failure to stimulate ASMase/ceramide signaling as mediating a new form of chemoresistance. It is anticipated that information derived from studies proposed here will inform a planned follow up clinical trial for advanced sarcoma to be performed by the Sarcoma Service at MSK.
Transport of nucleoside chemotherapeutic drugs is primarily accomplished in mammalian cells through Equilibrative Nucleoside Transporter 1 (ENT1), considered constitutively on, redistributing drugs across a bilayer via facilitated diffusion. Preliminary evidence, however, indicates that ENT1 transport of the chemotherapeutic gemcitabine in endothelial cells requires gemcitabine-induced acid sphingomyelinase (ASMase) activation, formation of ceramide-rich platforms (CRPs) on the external plasma membrane, and ENT1 insertion into CRPs for optimized transport function. We propose the existence of a previously- unrecognized form of chemoresistance involving suppression of ENT1 function, reversible by pharmacologic activation of ASMase/Ceramide Signaling.
