Human Monocyte Recognition of Adenosine-Based Cyclic Dinucleotides Unveils the A2a G_αs Protein-Coupled Receptor Tonic Inhibition of Mitochondrially Induced Cell Death

Abstract

Cyclic dinucleotides are important messengers for bacteria and protozoa and are well-characterized immunity alarmins for infected mammalian cells through intracellular binding to STING receptors. We sought to investigate their unknown extracellular effects by adding cyclic dinucleotides to the culture medium of freshly isolated human blood cells in vitro. Here we report that adenosine-containing cyclic dinucleotides induce the selective apoptosis of monocytes through a novel apoptotic pathway. We demonstrate that these compounds are inverse agonist ligands of A2a, a G_αs-coupled adenosine receptor selectively expressed by monocytes. Inhibition of monocyte A2a by these ligands induces apoptosis through a mechanism independent of that of the STING receptors. The blockade of basal (adenosine-free) signaling from A2a inhibits protein kinase A (PKA) activity, thereby recruiting cytosolic p53, which opens the mitochondrial permeability transition pore and impairs mitochondrial respiration, resulting in apoptosis. A2a antagonists and inverse agonist ligands induce apoptosis of human monocytes, while A2a agonists are antiapoptotic. In vivo, we used a mock developing human hematopoietic system through NSG mice transplanted with human CD34⁺ cells. Treatment with cyclic di-AMP selectively depleted A2a-expressing monocytes and their precursors via apoptosis. Thus, monocyte recognition of cyclic dinucleotides unravels a novel proapoptotic pathway: the A2a G_αs protein-coupled receptor (GPCR)-driven tonic inhibitory signaling of mitochondrion-induced cell death.

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ACKNOWLEDGMENTS

We acknowledge the technical advice from members of the laboratory of J.-J.F. and Isabelle Loubinoux (INSERM U825, Toulouse, France) and Guillaume Lebon (IGF Montpellier) for Adora2a cDNA and CHO cells. We are grateful to Nancy Aubry for critical reading and Kelly Thornber for editing of the manuscript.

This study was funded by INSERM, Université de Toulouse III Paul Sabatier, CNRS, and by contracts from INCa (TUMOSTRESS), Institut Carnot Lymphome (CALYM), and Laboratoire d 'Excellence Toulouse Cancer (TOUCAN).