The anti-adipogenic effect of macrophage-conditioned medium requires the IKKβ/NF-κB pathway

Immune Cell Regulation of White Adipose Progenitor Cell Fate

Adipose tissue is essential for energy storage and endocrine regulation of metabolism. Imbalance in energy intake and expenditure result in obesity causing adipose tissue dysfunction. This alters cellular composition of the stromal cell populations and their function. Moreover, the individual cellular composition of each adipose tissue depot, regulated by environmental factors and genetics, determines the ability of the depots to expand and maintain its endocrine and storage function. Thus, stromal cells modulate adipocyte function and vice versa. In this mini-review we discuss heterogeneity in terms of composition and fate of adipose progenitor subtypes and their interactions with and regulation by different immune cell populations. Immune cells are the most diverse cell populations in adipose tissue and play essential roles in regulating adipose tissue function via interaction with adipocytes but also with adipocyte progenitors. We specifically discuss the role of macrophages, mast cells, innate lymphoid cells and T cells in the regulation of adipocyte progenitor proliferation, differentiation and lineage commitment. Understanding the factors and cellular interactions regulating preadipocyte expansion and fate decision will allow the identification of novel mechanisms and therapeutic strategies to promote healthy adipose tissue expansion without systemic metabolic impairment.

Cecal Ligation and Puncture-Induced Sepsis Promotes Brown Adipose Tissue Inflammation Without Any Impact on Expression of Thermogenic-Related Genes

Background and Aims: The negative effects of chronic low-level inflammation on adipose tissue physiology have been extensively demonstrated, whereas the effects of acute inflammation are less studied. Here, we aimed to investigate the effects of sepsis-induced acute inflammation on gene expression markers of brown and white adipose tissue functionality.

Methods: Brown adipose tissue (BAT) and perirenal white adipose tissue (prWAT) gene expression markers were analyzed in cecal ligation and puncture (CLP)-induced sepsis mice model.

Results: CLP-induced sepsis attenuated expression of adipogenesis-related genes, in parallel to increased Tnf, Il6, and Ltf gene expression in prWAT. In contrast, CLP-induced sepsis resulted in increased expression of pro-inflammatory genes (Il6, Ltf, and Lbp) in BAT, without affecting expression of genes encoding for thermogenic activity.

Conclusion: Sepsis promotes both prWAT and BAT inflammation, associated with reduced adipogenesis-related gene expression in prWAT, without significant effects on BAT thermogenic genes.

Hydrogen sulfide impacts on inflammation-induced adipocyte dysfunction

A dual role of hydrogen sulfide (H₂S) in inflammation is well-reported and recent studies demonstrated adipogenic effects of H₂S in 3T3-L1 cells. Here, we aimed to investigate the effects of H₂S on adipocyte differentiation and inflammation. H₂S concentration in 3T3-L1 culture media was increased during adipocyte differentiation in parallel to adipogenic and Cth gene expression, and its inhibition using DL-Propargyl Glycine (PPG) impaired 3T3-L1 differentiation. GYY4137 and Na₂S administration only in the first or in the last stage of adipocyte differentiation resulted in a significant increased expression of adipogenic genes. However, when GYY4137 or Na₂S were administrated during all process no significant effects on adipogenic gene expression were found, suggesting that excessive H₂S administration might exert negative effects on adipogenesis. In fact, continuous addition of Na₂S, which resulted in Na₂S excess, inhibited adipogenesis, whereas time-expired Na₂S had no effect. In inflammatory conditions, GYY4137, but not Na₂S, administration attenuated the negative effects of inflammation on adipogenesis and insulin signaling-related gene expression during adipocyte differentiation. In inflamed adipocytes, Na₂S administration enhanced the negative effects of inflammatory process. Altogether these data showed that slow-releasing H₂S improved adipocyte differentiation in inflammatory conditions, and that H₂S proadipogenic effects depend on dose, donor and exposure time.

Lipopolysaccharides reduce adipogenesis in 3T3-L1 adipocytes through activation of NF-κB pathway and downregulation of AMPK expression

Lipopolysaccharides (LPS) from the outer membrane of Gram-negative bacteria serve as endotoxin to exert potent immune responses. However, the effect of LPS on adipogenesis has not been elucidated. The present study was designed to examine the effect of LPS on adipogenesis in 3T3-L1 preadipocytes and possible mechanism(s) of action involved. Our results revealed that LPS challenge significantly suppressed adipogenesis in 3T3-L1 preadipocytes mainly through downregulated expression of the late adipogenic markers PPARγ and aP2 as well as AMP-activated protein kinase (AMPK) expression and activity. As an inflammatory factor, LPS was found to lead to an overt reduction in IκBα levels compared with the time-matched controls, consolidating its pro-inflammatory property in 3T3-L1 preadipocytes. Our data also revealed that LPS retarded adipogenesis, the effect of which was partially reversed by the selective inhibitor of IKKβ. IκBα was found to be involved in the anti-adipogenic effect of LPS. In conclusion, LPS is capable of inhibiting adipogenesis in 3T3-L1 adipocytes possibly through activation of NF-κB and inhibition of AMPK. With the activation of NF-κB pathway and inhibition of AMPK, LPS suppresses C/EBP α DNA-binding activity and the expression of late adipogenic markers PPARγ and aP2.

Characterization of mesenchymal stem cells under the stimulation of Toll-like receptor agonists

Infective factors cause the perpetuation of inflammation as a result of the permanent exposure of the immune system to exogenous or endogenous products of virus or bacteria. Mesenchymal stem cells (MSCs) can be exposed to this infective environment, which may change the characteristics and therapeutic potency of these MSCs. MSCs have the ability to repair damaged and inflamed tissues and regulate immune responses. In this study, we demonstrated that MSCs express functional Toll-like receptors (TLR) 3 and 4, the Toll-like receptor families that recognize the signals of viral and bacterial mimics, respectively. The specific stimulations did not affect the self-renewal and apoptosis capabilities of MSCs but instead promoted their differentiation into the adipocytes and osteoblasts with the TLR3 ligand. The reverse of these results were obtained with the TLR4 ligand. The migration of the MSCs to stimulate either of the two specific ligands was inhibited at different times, whereas the immunogenicity and immunosuppressive properties of the MSCs were not weakened unlike in the MSCs group. These results suggest that TLR3 and TLR4 stimulation affect the characterization of MSCs.

The role of interleukin 1β in the anti-adipogenic action of macrophages on human preadipocytes

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.

Macrophage-induced adipose tissue dysfunction and the preadipocyte: should I stay (and differentiate) or should I go?

Adipose tissue can be regarded as a multidepot organ responsible for metabolic homeostasis by managing sophisticated energy transactions as well as by producing bioactive molecules that regulate insulin sensitivity and immune and vascular responses. Chronic nutrient excess expands adipose tissue, and concomitant variations in its cellular and matrix remodeling can affect the extent of the metabolic dysfunction that is associated with obesity. Preadipocytes, also termed adipose progenitor cells, play a pivotal role in determining whether a dysfunctional hypertrophic state arises as opposed to a hyperplastic process in which mature adipocytes remain relatively responsive. Obesity is associated with infiltration of macrophages, and these immune cells have been shown to communicate with preadipocytes to influence how they differentiate, survive, and proliferate. Understanding macrophage-preadipocyte interactions and their effect on adipose remodeling mechanisms may identify potential therapeutic molecular targets to improve adipose tissue function, even in the face of obesity.

THP-1 Macrophages and SGBS Adipocytes - A New Human in vitro Model System of Inflamed Adipose Tissue

Obesity is associated with an accumulation of macrophages in adipose tissue. This inflammation of adipose tissue is a key event in the pathogenesis of several obesity-related disorders, particularly insulin resistance. Here, we summarized existing model systems that mimic the situation of inflamed adipose tissue in vitro, most of them being murine. Importantly, we introduce our newly established human model system which combines the THP-1 monocytic cell line and the preadipocyte cell strain Simpson-Golabi-Behmel syndrome (SGBS). THP-1 cells, which originate from an acute monocytic leukemia, differentiate easily into macrophages in vitro. The human preadipocyte cell strain SGBS was recently introduced as a unique tool to study human fat cell functions. SGBS cells are characterized by a high capacity for adipogenic differentiation. SGBS adipocytes are capable of fat cell-specific metabolic functions such as insulin-stimulated glucose uptake, insulin-stimulated de novo lipogenesis and β-adrenergic-stimulated lipolysis and they secrete typical adipokines including leptin, adiponectin, and RBP4. Applying either macrophage-conditioned medium or a direct co-culture of macrophages and fat cells, our model system can be used to distinguish between paracrine and cell-contact dependent effects. In conclusion, we propose this model as a useful tool to study adipose inflammation in vitro. It represents an inexpensive, highly reproducible human system. The methods described here can be easily extended for usage of primary human macrophages and fat cells.