Wnt and beta-catenin signaling target the expression of ecto-5'-nucleotidase and increase extracellular adenosine generation

Solid tumors, which routinely experience necrosis and ischemia, release and degrade adenine nucleotides. This process may lead, depending on the expression of enzymes that regulate adenosine, to the generation of extracellular adenosine. Since genes encoding ecto-5'-nucleotidase (eN) and adenosine deaminase (ADA) contain TCF/LEF consensus binding sites, we asked whether Wnt/beta-catenin signaling, a pathway that is deregulated in several human tumors, targets the expression of these genes and thus influence extracellular adenosine generation. Our results show that beta-catenin strongly increased the activity of the 969-bp promoter of eN and this increase depended on the presence of TCF-1 transcription factor. Reciprocally, the eN promoter activity was decreased by co-transfection of APC, a beta-catenin antagonist. The expression of endogenous eN mRNA was increased either in Cos-7 cells transfected with a mutated beta-catenin and TCF-1 or in Rat-1 cells transformed by the Wnt-1 oncogene. In Rat-1 cells, expression of Wnt-1 correlated with increased eN protein levels and enzymatic activity and a concomitant decrease of adenosine deaminase mRNA and enzymatic activity. This expression profile is accompanied by a threefold increase in the generation of extracellular adenosine. Our study demonstrates a link between the Wnt signaling and the regulation of two enzymes that control the metabolism of adenosine.