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André S D Molgat, AnneMarie Gagnon, Charlie Foster and Alexander Sorisky

Adipose tissue contains macrophages whose state of activation is regulated as obesity develops. Macrophage-secreted factors influence critical processes involved in adipose tissue homeostasis, including preadipocyte proliferation and differentiation into adipocytes. Macrophage-conditioned medium (MacCM) from J774A.1 macrophages protects 3T3-L1 preadipocytes from apoptosis through platelet-derived growth factor (PDGF) signaling. Here, we investigated the effect of macrophage activation on MacCM-dependent preadipocyte survival. MacCM was prepared following activation of either J774A.1 macrophages with lipopolysaccharide (LPS) or human primary monocyte-derived macrophages (MD-macrophages) with LPS or interleukin 4 (IL4). 3T3-L1 and human primary preadipocytes were induced to undergo apoptosis in MacCM, and apoptosis was quantified by cell enumeration or Hoechst nuclear staining. Preadipocyte PDGF signaling was assessed by immunoblot analysis of phosphorylated PDGF receptor, Akt, and ERK1/2. Pro-inflammatory activation of J774A.1 macrophages with LPS inhibited the pro-survival activity of MacCM on 3T3-L1 preadipocytes, despite intact PDGF signaling. Upregulation of macrophage tumor necrosis factor a (TNFα) expression occurred in response to LPS, and TNFα was demonstrated to be responsible for the inability of LPS-J774A.1-MacCM to inhibit preadipocyte apoptosis. Furthermore, MacCM from human MD-macrophages (MD-MacCM) inhibited apoptosis of primary human preadipocytes. MD-MacCM from LPS-treated macrophages, but not IL4-treated anti-inflammatory macrophages, was unable to protect human preadipocytes from cell death. In both murine cell lines and human primary cells, pro-inflammatory activation of macrophages inhibits their pro-survival activity, favoring preadipocyte death. These findings may be relevant to preadipocyte fate and adipose tissue remodeling in obesity.

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AnneMarie Gagnon, Charlie Foster, Anne Landry and Alexander Sorisky

When adipose tissue accumulates in obesity, the ability of preadipocytes to differentiate permits a hyperplastic expansion of functional adipocytes that preserves insulin sensitivity. Adipose infiltration by macrophages is associated with an adipogenic deficit and the appearance of inflamed, insulin-resistant hypertrophied adipocytes. Interleukin 1β (IL1β) has been reported to account for the anti-adipogenic action of macrophages in a mouse model. Using the THP-1 human macrophage cell line and human primary preadipocytes, our objective was to determine whether IL1β was necessary for the ability of conditioned medium from THP-1 macrophages (THP-1-MacCM) to: i) stimulate human preadipocyte inhibitor of κB kinase β (IKKβ) and ii) inhibit human adipocyte differentiation. IL1β is present in THP-1-MacCM, and THP-1-MacCM or IL1β (500 pg/ml; its concentration in THP-1-MacCM) acutely stimulated IKKβ phosphorylation and inhibitor of κB (IκB) degradation in preadipocytes. IL1β was sufficient to inhibit adipogenesis on its own, and this was blocked by SC-514, an IKKβ inhibitor, as has been reported for THP-1-MacCM. IκB degradation by IL1β-immunodepleted THP-1-MacCM was attenuated, whereas IKKβ phosphorylation and the inhibition of adipocyte differentiation were unchanged. Therefore, in contrast to what has been suggested for mouse cell models, IL1β is not required for the ability of MacCM to inhibit adipogenesis in human cell models.