Specific Activation of A3, A2A and A1 Adenosine Receptors in CD73-Knockout Mice Affects B16F10 Melanoma Growth, Neovascularization, Angiogenesis and Macrophage Infiltration.

CD73 (ecto-5'-nucleotidase), a cell surface enzyme hydrolyzing AMP to adenosine, was lately demonstrated to play a direct role in tumor progression including regulation of tumor vascularization. It was also shown to stimulate tumor macrophage infiltration. Interstitial adenosine, accumulating in solid tumors due to CD73 enzymatic activity, is recognized as a main mediator regulating the production of pro- and anti-angiogenic factors, but the engagement of specific adenosine receptors in tumor progression in vivo is still poorly researched. We have analyzed the role of high affinity adenosine receptors A1, A2A, and A3 in B16F10 melanoma progression using specific agonists (CCPA, CGS-21680 and IB-MECA, respectively). We limited endogenous extracellular adenosine background using CD73 knockout mice treated with CD73 chemical inhibitor, AOPCP (adenosine α,ß-methylene 5'-diphosphate). Activation of any adenosine receptor significantly inhibited B16F10 melanoma growth but only at its early stage. At 14th day of growth, the decrease in tumor neovascularization and MAPK pathway activation induced by CD73 depletion was reversed by all agonists. Activation of A1AR primarily increased angiogenic activation measured by expression of VEGF-R2 on tumor blood vessels. However, mainly A3AR activation increased both the microvessel density and expression of pro-angiogenic factors. All agonists induced significant increase in macrophage tumor infiltration, with IB-MECA being most effective. This effect was accompanied by substantial changes in cytokines regulating macrophage polarization between pro-inflammatory and pro-angiogenic phenotype. Our results demonstrate an evidence that each of the analyzed receptors has a specific role in the stimulation of tumor angiogenesis and confirm significantly more multifaceted role of adenosine in its regulation than was already observed. They also reveal previously unexplored consequences to extracellular adenosine signaling depletion in recently proposed anti-CD73 cancer therapy.

CD73 on B16F10 melanoma cells in CD73-deficient mice promotes tumor growth, angiogenesis, neovascularization, macrophage infiltration and metastasis.

Ecto-5'-nucleotidase (CD73), an enzyme providing interstitial adenosine, mediates diverse physiological and pathological responses. In tumor progression, it has primarily an immunosuppressive role but is also thought to regulate neovascularization. However, the latter role is still in debate. When B16F10 melanoma was subcutaneously injected into CD73 knockout mice, changes in the tumor vasculature were not always observed. However, we demonstrated earlier that the growth and vascularization of B16F10 melanoma in CD73 knockout mice depend on the low presence of CD73 on tumor cells. To further analyze the role of CD73 on tumor growth and vascularization, we compared the changes in B16F10 melanoma subcutaneously injected into right flank of wild-type mice, CD73 knockout mice lacking host CD73 only, and CD73 knockout mice with tumor cell CD73 either inhibited with AOPCP (adenosine α,ß-methylene 5'-diphosphate) or permanently knocked down through genetic modification. We report here that both inhibition and knockdown of tumor CD73 further inhibited tumor growth compared to host CD73 knockout alone. MAP-kinase signaling pathway activation also decreased more strongly in the stable knockdown. There was a significant reduction in the angiogenic activation of blood microvessels as observed by decreased anti-VEGFR2 staining. Stable CD73 knockdown also reduced endothelial cell proliferation as measured by anti-CD105 staining. However, only chemical inhibition with AOPCP significantly augmented the reduction in intratumoral microvessel density induced by host CD73 knockout. Such reduction was not observed when tumor CD73 was knocked down due to the much slower tumor growth and decreased oxygen demand as indicated by the low expression of Bad, a hypoxia marker. Decreased CD73 activity also led to the decreased expression of angiogenic factors, including VEGF and bFGF that was only partially reversed by hypoxia in tumors treated with AOPCP. Both inhibition and knockdown of tumor CD73 significantly decreased tumor macrophage infiltration and induced microenvironment changes, thereby influencing MI or MII macrophage polarization. Additionally, tumor cell CD73 is important in metastasis formation through adenosine-independent attachment to endothelium. We conclude that even low tumor cell CD73 expression has an undeniable role in melanoma progression, including the regulation of many aspects of angiogenesis. CD73 is thus a viable target in anti-angiogenic melanoma therapy.

Inhibition of CD73 stimulates the migration and invasion of B16F10 melanoma cells in vitro, but results in impaired angiogenesis and reduced melanoma growth in vivo.

The role of ecto-5'-nucleotidase (CD73), an enzyme providing interstitial adenosine, was investigated in B16F10 melanoma progression. Chemical inhibition of CD73 decreased adherence of cells to extracellular matrix proteins in vitro and led to enhanced migration and invasion. Both processes were reversed by adenosine receptor agonists. In CD73­deficient mice, tumor growth was decreased in comparison with that of wild-type animals. Additionally, the vasculature of CD73-inhibited tumors was impaired and neoangiogenesis in Matrigel plugs was reduced. It is, therefore, proposed that although CD73 shows anti-invasive and antimigratory function in B16F10 melanoma cells, its proangiogenic action is prevalent in vivo and may contribute to increased tumor growth.