The Molecular Mechanisms of Thymidine Phosphorylase in Survival of Monocytic Leukemia Cell

Thymidine phosphorylase (TP; EC 2.4.2.4), catalyzing the reversible reaction of thymidine to thymine and 2-deoxy-α-D-ribose-1-phosphate, plays roles not only in dTTP metabolism but also angiogenesis and anti-apoptosis. Previously, our laboratory found that depletion of TP in monocytic leukemia cell line U937 and THP-1 cell lines by lentiviral-based small hairpin RNA (shRNA) leads to apoptosis. Interestingly, expression of catalytic dead TP was able to rescue these cells, but not addition of TP catabolites to the culture medium. These results indicate that TP provides a survival signal independent of the catalytic function. In this study, I further dissect the mechanism of TP-mediated cell survival. Several signal cascades involved in cell survival, such as ERK1/2 and Akt, were found to be reduced by TP depletion. In addition, perturbation of mitochondrial membrane potential is accompanied by a decrease in Bcl-2 expression in cells depleted of TP. Contrarily, STAT3, one of the signals in JAK/STAT cascade involved in cell survival, stayed more active in these cells with the increased levels of COX2, SOCS3 and GM-CSF transcription. In order to understand the mechanism, I performed yeast two-hybrid screening to search for TP interacting proteins. Despite several proteins were identified capable of interacting with TP in yeast, co-expression of each of these proteins with TP did not form complex in human. Therefore, it remains to learn how TP exerts its biological function in influencing cell survival signal in monocytic leukemia cells