Cardiac atrial natriuretic peptide (ANP), via its guanylyl cyclase-A (GC-A) receptor, has important endocrine functions in the maintenance of blood pressure and volume homeostasis and local, paracrine functions to moderate pathological cardiac growth. In patients with hypertensive cardiac hypertrophy and heart failure the plasma levels of ANP are markedly increased, but the effects of the peptide are clearly diminished, indicating a receptor or postreceptor defect. The present study characterizes posttranslational modifications involved in the inactivation and homologous (ANP evoked) desensitization of GC-A, especially the role of phosphorylation / dephosphorylation.

METHODS and RESULTS: 

HEK293 cells stably expressing FLAG-tagged GC-A were incubated with vehicle or 0.1 mM ANP during 60 min. After cell lysis and obtention of plasma membranes, GC-A was enriched and partially purified by immunoprecipitation with anti-FLAG antibody. The enriched GC-A receptor was subjected to in-gel digestion with trypsin. The phosphorylated tryptic peptides were then enriched with TiO₂ affinity chromatography and analyzed by mass spectrometry. We identified seven phosphorylated residues, all located within the intracellular protein kinase homology domain of GC-A. Six of them have been already suggested by previous studies in metabolically labelled cells (Potter and Hunter, J Biol Chem 2001). Additionally one so far unknown phosphorylated residue was detected. For relative quantification Multiple Reaction Monitoring (MRM) was used, which allows the detection of fragment ions corresponding to a predefined m/z transition resulting in a highly specific and sensitive analysis. The changes in the phosphorylation pattern of GC-A after treatment of cells with a high ANP concentration suggests a diminished population of completely phosphorylated receptor.

CONCLUSION: 

In contrast to G-protein coupled receptors, which are inactivated by phosphorylation, the GC-A receptor seems to be inactivated by dephosphorylation. These modifications of GC-A could contribute to the aforementioned loss of function of the ANP/GC-A system in cardiovascular diseases and thereby contribute to the progression of hypertension and cardiac hypertrophy.

Supported by Sonderforschungsbereich SFB 487.