Human macrophage differentiation induces OCTN2–mediated L‐carnitine transport through stimulation of mTOR–STAT3 axis

• Published in: Journal of leukocyte biology Vol. 101; no. 3; pp. 665 - 674
• Main Authors: Ingoglia, Filippo, Visigalli, Rossana, Rotoli, Bianca Maria, Barilli, Amelia, Riccardi, Benedetta, Puccini, Paola, Milioli, Marco, Di Lascia, Maria, Bernuzzi, Gino, Dall’Asta, Valeria
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 01.03.2017
• Subjects: Activation; Affinity; Amino acids; Biological Transport; Carnitine; Carnitine - metabolism; Cell Differentiation; Differentiation; Fatty acids; GM‐CSF; Granulocyte-macrophage colony-stimulating factor; Granulocyte-Macrophage Colony-Stimulating Factor - metabolism; Humans; Immune response; Kinases; Kinetics; L-Carnitine; Macrophages; Macrophages - cytology; Macrophages - metabolism; Models, Biological; Monocytes; Monocytes - cytology; Organic Cation Transport Proteins - genetics; Organic Cation Transport Proteins - metabolism; Oxidation; Phosphorylation; rapamycin; RNA, Messenger - genetics; RNA, Messenger - metabolism; Signal Transduction; Signaling; Solute Carrier Family 22 Member 5; Stat3 protein; STAT3 Transcription Factor - metabolism; Time Factors; TOR protein; TOR Serine-Threonine Kinases - metabolism; Transcription; Transport; monocytes; GM-CSF; rapamycin
• ISSN: 0741-5400; 1938-3673
• DOI: 10.1189/jlb.1A0616-254R

Monocyte‐to‐macrophages differentiation associates with upregulation of carnitine transport due to mTOR‐dependent activation of STAT3. l‐Carnitine, in addition to playing a fundamental role in the β‐oxidation of fatty acids, has been recently identified as a modulator of immune function, although the mechanisms that underlie this role remain to be clarified. In this study, we addressed the modulation of l‐carnitine transport and expression of related transporters during differentiation of human monocytes to macrophages. Whereas monocytes display a modest uptake of l‐carnitine, GM‐CSF–induced differentiation massively increased the saturable Na+‐dependent uptake of l‐carnitine. Kinetic and inhibition analyses demonstrate that in macrophage l‐carnitine transport is mediated by a high‐affinity component (Km ∼4 µM) that is identifiable with the operation of OCTN2 transporter and a low‐affinity component (Km > 10 mM) that is identifiable with system A for neutral amino acids. Consistently, both SLC22A5/OCTN2 and SLC38A2/SNAT2 are induced during the differentiation of monocytes to macrophages at gene and protein levels. Elucidation of GM‐CSF signaling demonstrates that the cytokine causes the activation of mTOR kinase, leading to the phosphorylation and activation of STAT3, which, in turn, is responsible for OCTN2 transcription. SLC22A5/OCTN2 therefore emerges as a novel member of the set of genes markers of macrophage differentiation.