Abstract
Abscisic acid (ABA) is a plant hormone involved in the response to environmental stress. Recently, ABA has been shown to be present and active also in mammals, where it stimulates the functional activity of innate immune cells, of mesenchymal and hemopoietic stem cells, and insulin-releasing pancreatic β-cells. LANCL2, the ABA receptor in mammalian cells, is a peripheral membrane protein that localizes at the intracellular side of the plasma membrane. Here we investigated the mechanism enabling ABA transport across the plasmamembrane of human red blood cells (RBC). Both influx and efflux of [(3)H]ABA occur across intact RBC, as detected by radiometric and chromatographic methods. ABA binds specifically to Band 3 (the RBC anion transporter), as determined by labeling of RBC membranes with biotinylated ABA. Proteoliposomes reconstituted with human purified Band 3 transport [(3)H]ABA and [(35)S]sulfate, and ABA transport is sensitive to the specific Band 3 inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Once inside RBC, ABA stimulates ATP release through the LANCL2-mediated activation of adenylate cyclase. As ATP released from RBC is known to exert a vasodilator response, these results suggest a role for plasma ABA in the regulation of vascular tone.
Highlights
The plant stress hormone abscisic acid (ABA) is present and active in mammalian cells
We further provide direct evidence, by different methodological approaches, that the transmembrane anion exchange protein Band 3 mediates Abscisic acid (ABA) influx into erythrocytes and that extracellular ABA stimulates ATP release from intact red blood cells (RBC) via lanthionine synthetase C-like protein 2 (LANCL2)-mediated adenylate cyclase activation
ABA Binding and Influx into Erythrocytes—Previous experiments of [3H]ABA binding to intact human granulocytes [12] had revealed two processes characterized by different affinity constants: (i) a high affinity binding (Kd ϭ 11 nM), which was interpreted as ABA binding to receptor and (ii) a low affinity binding (Kd ϭ 500 M), which was interpreted as ABA interacting with intracellular proteins, after influx into the cells
Summary
The plant stress hormone abscisic acid (ABA) is present and active in mammalian cells. Results: Band 3 protein is required for ABA influx into red blood cells (RBC); intracellular ABA activates adenylate cyclase resulting in [cAMP]i increase and subsequent ATP release. LANCL2, the ABA receptor in mammalian cells, is a peripheral membrane protein that localizes at the intracellular side of the plasma membrane. Once inside RBC, ABA stimulates ATP release through the LANCL2-mediated activation of adenylate cyclase. Because PYR/ PYL/RCAR are intracellular soluble receptors and ABA may be produced by cell types different from those that are functionally responsive to the hormone, ABA transport across the cell membranes is necessary to the action of the hormone. We further provide direct evidence, by different methodological approaches, that the transmembrane anion exchange protein Band 3 mediates ABA influx into erythrocytes and that extracellular ABA stimulates ATP release from intact RBC via LANCL2-mediated adenylate cyclase activation
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