Inosine Production and Cytoprotective Activity in a Gradient Model of Breast Cancer Hypoxia Nutrient Exclusion (CHyNE)

Inosine has long been regarded as a biologically inert purine compound, signifying initiation of the purine waste pathway. Although extracellular purines such as ATP and adenosine have recently been established as potent immuno-modulators within the tumor microenvironment via the CD73 ectoenzymatic cascade, the production and functionality of inosine via ADA has remained relatively unexplored in breast cancer to date. As extracellular adenosine is constitutively generated from extracellular ATP under hypoxic conditions, and because inosine is the primary byproduct of adenosine degradation by adenosine deaminase (ADA), the focus of this project was to determine the production and cytoprotective activity of inosine during cancer hypoxia. In addition, the in-vitro¬ means of generating hypoxia were re-examined. As a result, the Cancer Hypoxia Nutrient Exclusion (CHyNE) method was formulated, accounting for the means by which hypoxia is generated in the tumor microenvironment in vivo. Inosine was found to exhibit a bioactivity during breast cancer hypoxia. Most importantly, the CHyNE method has revealed that much of our understanding of the CD73 ectoenzymatic cascade must be revised to include ADA, as the present data indicates that inosine, and not adenosine, is the primary cytoprotective compound during breast cancer hypoxia.