Abstract
cAMP and cGMP are well established second messengers that are essential for numerous (patho)physiological processes. These purine cyclic nucleotides activate cAK and cGK, respectively. Recently, the existence of cCMP was described, and a possible function for this cyclic nucleotide was investigated. It was postulated that cCMP plays a role as a second messenger. However, the functions regulated by cCMP are mostly unknown. To elucidate probable functions, cCMP-binding and -activated proteins were identified using different methods. We investigated the effect of cCMP on purified cyclic nucleotide-dependent protein kinases and lung and jejunum tissues of wild type (WT), cGKI-knockout (cGKI KO) and cGKII-knockout (cGKII KO) mice. The catalytic activity of protein kinases was measured by a (γ-32P) ATP kinase assay. Cyclic nucleotide-dependent protein kinases (cAK, cGKI and cGKII) in WT tissue lysates were stimulated by cCMP. In contrast, there was no stimulation of phosphorylation in KO tissue lysates. Competitive binding assays identified cAK, cGKI, and cGKII as cCMP-binding proteins. An interaction between cCMP/MAPK and a protein-protein complex of MAPK/cGK were detected via cCMP affinity chromatography and co-immunoprecipitation, respectively. These complexes were abolished or reduced in jejunum tissues from cGKI KO or cGKII KO mice. In contrast, these complexes were observed in the lung tissues from WT, cGKI KO and cGKII KO mice. Moreover, cCMP was also able to stimulate the phosphorylation of MAPK. These results suggest that MAPK signaling is regulated by cGMP-dependent protein kinases upon activation by cCMP. Based on these results, we propose that additional cCMP-dependent protein kinases that are capable of modulating MAPK signaling could exist. Hence, cCMP could potentially act as a second messenger in the cAK/cGK and MAPK signaling pathways and play an important role in physiological processes of the jejunum and lung.
Highlights
The principle of cyclic nucleotides as intracellular signaling molecules is well established in regard to the purinergic cyclic nucleotides cyclic adenosine monophosphate and cyclic guanosine monophosphate
Activation of cyclic nucleotide dependent kinases by cyclic cytidine monophosphate (cCMP) in vitro and in tissues In previous studies, we showed that cCMP is able to stimulate purified cyclic adenosine monophosphate (cAMP)- and cGMPdependent kinases cAMPdependent protein kinase (cAK) and cGMP-dependent protein kinase I (cGKI) [7,8]
In addition to the recently established cCMPbinding proteins cGKI and cAKRI, we identified cGMP-dependent protein kinase II (cGKII) and MAPK as possible signaling proteins that interact with cCMP
Summary
The principle of cyclic nucleotides as intracellular signaling molecules is well established in regard to the purinergic cyclic nucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). The synthesis of pyrimidinic cyclic nucleotides has been detected in bacterial cells (e.g., Pseudomonas aeruginosa) and was shown to be stimulated by bacterial toxins (e.g., edema factor) [3,4,5] These observations raise the question of whether possible target proteins and/or effector systems of these cyclic nucleotides exist. To elucidate possible targets of cCMP in tissues, we performed cCMP kinase activity assays and cCMP-affinity chromatography of lung or intestinal murine tissues and analyzed the relevant proteins using mass spectrometry and immunoblotting analysis. These tissues were selected because cGKI and/or cGKII are present in high concentrations. We confirmed that cGKI and cAK are cCMP-interacting proteins, and identified cGKII and MAPK as possible cCMP signaling molecules
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