Atrial natriuretic peptide is phosphorylated by intact cells through cAMP‐dependent ecto‐protein kinase

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Abstract
Recently we demonstrated the presence of cell‐surface‐located cAMP‐dependent protein kinase (ecto‐PK A) activity in a number of different cell types [Kübler, D., Pyerin, W., Bill, O., Hotz, A., Sonka, J. and Kinzel, V. (1989) J. Biol. Chem. 264, 14549–14555]. The question of the physiological role of externally directed kinase activity prompted a search for potential natural substrates present in the intercellular fluid. In the present study we have investigated the phosphorylation by ecto‐PK A of the human atrial natriuretic peptide ANP99–126, a hormone released by cardiac cells. This 28‐amino‐acid peptide carries the phosphorylation consensus sequence Arg‐Arg‐Ser‐Ser for the PK A. Incubation of various cell lines (including epithelial, epidermal, myoblast and lymphoma cells) or freshly isolated blood cells (macrophages, erythrocytes and platelets) with ANP in the presence of low micromolar concentrations of ATP resulted in the phosphorylation of ANP at Ser residues. The ANP phosphorylation reaction proved strictly dependent on cAMP; cAMP could not be replaced by cGMP. The phosphorylation was inhibited by the PK A‐specific inhibitory peptide and increased linearily for up to 15 min and with a Km value of 3–5 μM for ANP. At higher ATP concentrations (> 100 μM) the incorporation rates amounted to about 0.3 mmol P (mol ANP)−1 min−1. The rise of intracellular cAMP in HEL30 (an epidermal cell line) after application of the β‐adrenergic receptor agonist isoproterenol led to an approximately three‐fold stimulation of ANP phosphorylation which appears to be brought about by an efflux of intracellular cAMP. Employing cell supernatant fluids and cell sonicates, it could be shown that the phosphorylation of ANP results from the ecto‐PK A. Comparison of ANP with ANP phosphorylated in vitro using purified catalytic subunit of PK A showed that phosphorylation is accompanied by certain changes in the average solution conformation of the peptide, consistent with the changes known to occur in its biological activity. Our results demonstrate cAMP‐dependent phosphorylation of the peptide hormone analogue ANP99–126 by intact cells through ecto‐PK A, an intriguing mechanism for post‐translational processing of ANP.

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