Abstract
The A(1), A(2A), A(2B) and A(3) G-protein-coupled cell surface adenosine
receptors (ARs) are found to be upregulated in various tumor cells.
Activation of the receptors by specific ligands, agonists or antagonists,
modulates tumor growth via a range of signaling pathways. The A(1)AR
was found to play a role in preventing the development of glioblastomas.
This antitumor effect of the A(1)AR is mediated via tumor-associated
microglial cells. Activation of the A(2A)AR results in inhibition
of the immune response to tumors via suppression of T regulatory
cell function and inhibition of natural killer cell cytotoxicity
and tumor-specific CD4+/CD8+ activity. Therefore, it is suggested
that pharmacological inhibition of A(2A)AR activation by specific
antagonists may enhance immunotherapeutics in cancer therapy. Activation
of the A(2B)AR plays a role in the development of tumors via upregulation
of the expression levels of angiogenic factors in microvascular endothelial
cells. In contrast, it was evident that activation of A(2B)AR results
in inhibition of ERK1/2 phosphorylation and MAP kinase activity,
which are involved in tumor cell growth signals. Finally, A(3)AR
was found to be highly expressed in tumor cells and tissues while
low expression levels were noted in normal cells or adjacent tissue.
Receptor expression in the tumor tissues was directly correlated
to disease severity. The high receptor expression in the tumors was
attributed to overexpression of NF-kappaB, known to act as an A(3)AR
transcription factor. Interestingly, high A(3)AR expression levels
were found in peripheral blood mononuclear cells (PBMCs) derived
from tumor-bearing animals and cancer patients, reflecting receptor
status in the tumors. A(3)AR agonists were found to induce tumor
growth inhibition, both in vitro and in vivo, via modulation of the
Wnt and the NF-kappaB signaling pathways. Taken together, A(3)ARs
that are abundantly expressed in tumor cells may be targeted by specific
A(3)AR agonists, leading to tumor growth inhibition. The unique characteristics
of these A(3)AR agonists make them attractive as drug candidates.
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