Expression of SAA1, SAA2 and SAA4 genes in human primary monocytes and monocyte-derived macrophages.

Claire Jumeau,Dominique Valeyre,Irina Giurgea,Sonia-Athina Karabina,Gilles Grateau,Serge Amselem,Jean-François Bernaudin,Eman Assrawi,Philippe Duquesnoy,Laetitia Cobret,Fawaz Awad

doi:10.1371/journal.pone.0217005

Abstract

Circulating serum amyloid A (SAA) is increased in various inflammatory conditions. The human SAA protein family comprises the acute phase SAA1/SAA2, known to activate a large set of innate and adaptive immune cells, and the constitutive SAA4. The liver synthesis of SAA1/SAA2 is well-established but there is still an open debate on extrahepatic SAA expression especially in macrophages. We aimed to investigate the ability of human primary monocytes and monocyte-derived macrophages to express SAA1, SAA2 and SAA4 at both the transcriptional and protein levels, as previous studies almost exclusively dealt with monocytic cell lines. Monocytes and derived macrophages from healthy donors were stimulated under various conditions. In parallel with SAA, pro-inflammatory IL1A, IL1B and IL6 cytokine expression was assessed. While LPS alone was non-effective, a combined LPS/dexamethasone treatment induced SAA1 and to a lesser extent SAA2 transcription in human monocytes and macrophages. In contrast, as expected, pro-inflammatory cytokine expression was strongly induced following stimulation with LPS, an effect which was dampened in the presence of dexamethasone. Furthermore, in monocytes polarized towards a pro-inflammatory M1 phenotype, SAA expression in response to LPS/dexamethasone was potentiated; a result mainly seen for SAA1. However, a major discrepancy was observed between SAA mRNA and intracellular protein levels under the experimental conditions used. Our results demonstrate that human monocytes and macrophages can express SAA genes, mainly SAA1 in response to an inflammatory environment. While SAA is considered as a member of a large cytokine network, its expression in the monocytes-macrophages in response to LPS-dexamethasone is strikingly different from that observed for classic pro-inflammatory cytokines. As monocytes-macrophages are major players in chronic inflammatory diseases, it may be hypothesized that SAA production from macrophages may contribute to the local inflammatory microenvironment, especially when macrophages are compactly organized in granulomas as in sarcoidosis.

Highlights

In humans, serum amyloid A (SAA) is encoded by four SAA genes, mapping to a 150-kb region of chromosome 11p15.1 [1,2,3]
We used human monocytes from peripheral blood, selected by adherence or by CD14-positive selection, and macrophages that were differentiated from monocytes after 7, 14 or 21 days in culture; during this period, cells acquire the morphological and functional characteristics of tissue macrophages, which after 14 days in culture, present an epithelioid cell pattern similar to that of macrophages observed in granulomas [51,52]
We clearly show an induction of SAA1 and SAA2 gene transcription in monocytes and monocyte-derived macrophages in response to a combined stimulation with LPS and glucocorticoids

Summary

Introduction

Serum amyloid A (SAA) is encoded by four SAA genes, mapping to a 150-kb region of chromosome 11p15.1 [1,2,3]. Recombinant human SAA (rhSAA) has been shown to induce chemotactic activity in neutrophils, dendritic cells, monocytes and T lymphocytes [8,11,19,20,21] and secretion of cytokines [22,23]. These pleiotropic functions of SAA are thought to be mediated through signalling pathways following engagement of cell-surface receptors like the Toll-like and scavenger receptors [24,25,26]. SAA is an acute inflammation response mediator, and plays a significant role in the pathogenesis of various chronic diseases at the crossroad of autoimmunity and autoinflammation such as Behcet’s disease [27,28] or sarcoidosis [29,30]

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