Adenosine is an important signaling molecule that regulates multiple physiologic processes and exerts major anti-inflammatory actions.
Tumors have high concentrations of
adenosine, which could inhibit the function of
tumor-infiltrating lymphoid cells. We investigated the ability of
adenosine and its stable analogue
2-chloroadenosine (CADO) to inhibit
cytokine production and cytotoxic activity of lymphokine-activated killer (LAK) cells and determined whether both these effects are initiated via a common pathway. CADO strongly inhibited cytotoxic activity of LAK cells and attenuated the production of IFN-gamma,
granulocyte macrophage colony-stimulating factor,
tumor necrosis factor alpha, and macrophage inflammatory protein-1alpha by LAK cells stimulated by cross-linking of the Ly49D receptor. These inhibitory effects were associated with the ability of CADO to stimulate
cyclic AMP (cAMP) production and activate
protein kinase A (PKA). Using cAMP analogues with different affinities for the A and B sites of the regulatory subunits of PKA types I and II, we found that activation of PKA I, but not PKA II, mimicked the inhibitory effects of CADO on LAK cell cytotoxic activity and
cytokine production. Inhibitors of the PKA catalytic subunits (
H89 and PKI(14-22)
peptide) failed to abrogate the inhibitory effects of CADO whereas
Rp-8-Br-cAMPS, an antagonist of the RI subunit, blocked the inhibitory effects of CADO. We conclude that the inhibitory effects of
adenosine are probably mediated via cAMP-dependent activation of the RI subunits of PKA I but are independent of the catalytic activity of PKA.
Tumor-produced
adenosine could be a potent
tumor microenvironmental factor inhibiting the functional activity of
tumor-infiltrating immune cells.