Cyclic AMP-dependent protein kinase in Molt 4b lymphoblasts: identification by photoaffinity labeling and activation in intact cells by vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI).

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Abstract
Molt 4b lymphoblasts have previously been shown to possess a single class of pharmacologically specific, high affinity receptors for vasoactive intestinal polypeptide (VIP). This study further explores the molecular basis for modulation of human lymphocyte function by VIP. Dose-dependent stimulation of adenylate cyclase was observed in Molt lymphoblasts over the range of 0.1 nM to 1 microM VIP. VIP-mediated by guanine nucleotide. Accumulation of intracellular cAMP was observed in the presence of either VIP or the diterpene, forskolin. The effects of these two agonists were synergistic. Two neuropeptides that share sequence homology with VIP were also studied; both peptide histidine isoleucine (PHI) and human pancreatic growth hormone releasing factor (1-44 GHRF) competed for 125I-VIP binding to Molt cells. PHI stimulated intracellular cAMP accumulation and demonstrated synergism with forskolin, whereas GHRF had no effect on cAMP. Photoaffinity labeling of 100,000 X G soluble proteins with 8-N3-[32P]cAMP followed by SDS gel electrophoresis demonstrated the presence of cAMP-dependent protein kinases I and II. Cyclic AMP-dependent protein kinase II predominated in the soluble fraction and was the only isozyme observed in particulate fractions. Protein phosphorylation was studied in Molt 4b cells preincubated with [32P]PO43- followed by addition of media alone, 1 microM peptide, or 10 microM forskolin. Cells were lysed and subjected to two-dimensional electrophoresis. Increased phosphorylation of a specific 41,000 Mr protein was observed after addition of forskolin, VIP, or PHI. A much lower concentration of VIP (1 nM) also caused a significant net increase in phosphorylation, which was of a lower magnitude. In contrast, no net effect on protein phosphorylation was seen with GHRF. These data demonstrate the presence of a functional VIP receptor that is linked to the G protein-adenylate cyclase complex. The demonstration of cAMP-dependent protein kinase and of VIP- and PHI-mediated protein phosphorylation in Molt 4b lymphoblasts provides evidence on a molecular level for neuropeptide modulation of human lymphocyte function.