Activation of lymphocytes by brominated nucleoside and cyclic nucleotide analogues: implications for the "second messenger" function of cyclic GMP.

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Abstract
The purine nucleoside guanosine, when derivatized at the C-8 position to give 8-bromoguanosine (8-BrGuo), acquires the capacity to stimulate high-level mouse spleen lymphocyte proliferation in the presence or absence of serum. Direct comparisons were undertaken to determine whether this activity is exerted only by virtue of the structural resemblance of 8-BrGuo to 8-bromo cGMP (8-BrcGMP) (a known intracellular lymphocyte mitogen). Of the brominated guanosine derivatives studied, 8-BrGuo is the primary activator because it is a far more potent lymphocyte activator than 8-BrcGMP, the order of mitogenic potency being 8-BrGuo > 8-bromo GMP (8-BrGMP) > 8-BrcGMP. 8-BrGuo acts much more rapidly than 8-BrcGMP and it is not metabolized to 8-BrcGMP or cGMP nor does it elevate intracellular cGMP content. cGMP is not likely to be the 2nd messenger serving to activate B cells because it does not induce significant proliferation unless brominated at the C-8 position; the brominated form is much less efficient than 8-BrGuo or 8-BrGMP; and 8-BrGuo and many other mitogens do not increase intracellular cGMP. Also, many agents that increase cGMP fail to initiate lymphocyte activation. Certain agents that increase cGMP (i.e., 15-hydroperoxyarachidonic acid, azide) inhibit lymphocyte activation. Addition of unbrominated cGMP to cultures stimulated with 8-BrGuo actually diminished stimulation. 8-BrGuo apparently triggers high level lymphocyte activation by interaction with cellular components. The role of cGMP as an intracellular 2nd messenger in lymphocyte proliferation is doubtful.