Effects of adenylyl cyclase and protein kinase A inhibition on signaling enzymes in natural killer cells: comparison to tributyltin

The role of Gαs in activation of NK92-MI cells by neuropeptide substance P

Substance P (SP) is well known for its immunoregulatory influence on NK cells. The biological actions of SP are mediated primarily through the high-affinity neurokinin-1 receptor (NK-1R), a G protein-coupled receptor (GPCR). Receptor binding triggers a cAMP signaling pathway and intracellular levels of cAMP are regulated via Gαs and Gαi. In this study NF449, a Gαs-selective G protein antagonist, was used to study the role of Gαs in the activation of NK92-MI cells by SP. Results show that 10(-12)M SP enhances the expression of Gαs and Gαi3 in NK92-MI cells promoting a cytotoxic phenotype characterized by expression of perforin and granzyme B. Development of a cytotoxic phenotype in NK92-MI cells stimulated with SP is blunted by inhibition of Gαs by NF449. In summary, SP signaling through NK-1R promotes a cytotoxic phenotype in NK92-MI cells characterized by upregulation of both Gαs and Gαi3. NF449 inhibits Gαs, blunts SP-induced expression of perforin and granzyme B, and represents a potential therapeutic avenue for reducing NK-cell mediated cytotoxicity.

Production of nitric oxide by carp (Cyprinus carpio L.) kidney leukocytes is regulated by cyclic 3',5'-adenosine monophosphate

The inducible nitric oxide synthase (iNOS) plays a central role in the inflammatory reactions that follow infection or tissue damage. Induction of nitric oxide (NO) synthesis by bacterial lipopolysaccharide (LPS) depends on activation of G protein-coupled receptors in mammals. Thus, it was our intention to evaluate whether similar mechanisms are involved in iNOS activation in fish leukocytes. Therefore, the participation of membrane-bound receptors which activate effectors via G proteins has been confirmed using the G protein inhibitor suramin. Furthermore, the NO produced by iNOS performs both beneficial and detrimental actions. It is thus conceivable that regulatory mechanisms exist which control the timing and intensity of NO production by iNOS in order to outweigh protective effects against detrimental ones. The second messenger cAMP produced by adenylyl cyclases (ACs) plays a key role in the regulation of many cellular functions. Since cAMP signaling inhibits numerous immunological reactions, studies have been carried out to determine whether cAMP-dependent pathways could inhibit NO production by carp leukocytes as well. To measure cellular responses such as NO production by carp leukocytes derived from head and trunk kidneys treatments were performed with the cAMP elevating agents forskolin and dibutyryl-cAMP (db-cAMP) prior to stimulation with Aeromonas hydrophila. Pharmacological studies in stimulated kidney leukocytes showed that increased intracellular cAMP levels lead to reduced NO formation. This reduction of NO production was not due to decreased cell numbers, since a tetrazolium dye-based assay revealed no reduction of cell viability by cyclic nucleotide elevating agents. Thus, our data provide evidence that the AC/cAMP signaling pathway is well established in carp leukocytes. Cyclic AMP leads to type II immune response. We provide evidence that the predominant AC in fish leukocytes is a particulate enzyme due to its sensitivity to forskolin. Treatment of leukocytes with agents increasing intracellular cAMP gave clear evidence for participation of this cyclic nucleotide in immune signaling.