A New Signal that Shrinks Fat

Abstract

Healthy white adipose tissue is dependent on proliferation of endothelial cells to maintain homeostasis or undergo expansion. A new study shows that endothelial cells communicate with adipocytes via polyamines to promote vascularization of adipose tissue, thereby reversing the metabolic effects of obesity.

The blood vessels in adipose tissue are lined by a single monolayer of quiescent endothelial cells (ECs), which can rapidly switch and proliferate to form new blood vessels. The change from a quiescent to proliferative state is mediated by a balance of pro-angiogenic factors and inhibitors. Governing this process are reciprocal interactions between ECs and adipocytes that regulate angiogenesis and that are fundamental to maintain white adipose tissue homeostasis. In this issue of Nature Metabolism, Monelli et al. demonstrate that the activation of the phosphoinositide 3-kinase (PI3K) pathway in ECs increases the proliferation of blood vessels selectively in WAT, reduces fat mass, and improves systemic glucose tolerance (1). In addition, the authors determine that the key mediators of this EC to AT crosstalk are polyamines released by ECs. These EC-derived polyamines stimulate adipocyte lipolysis. The resulting FFAs are then taken up by ECs and oxidized for energy production to support proliferation. These remarkable observations of Monelli and colleagues advance our understanding of EC biology in WAT and shed light on how specific properties of ECs can be dictated by their tissue microenvironment.

Adipose tissue is one of the tissues in the body with highest plasticity, as it has the ability to expand and shrink during development and adulthood (2). Healthy WAT readily expands and stores excess lipids safely in lipid droplets, preventing toxic lipid deposition in other organs. This WAT expansion is critically dependent on angiogenesis, a process during which ECs proliferate and remodel to form functional new blood vessels (3). Obesity and insulin resistance are frequently associated with a reduced vascular density of adipose tissue, resulting in a relative depletion of oxygen and nutrients. As a result, dysfunctional under-vascularized adipose tissue is characterized by unresolved inflammation and fibrosis. It is therefore critical to understand how angiogenesis is regulated in WAT to determine potential pathways that can improve obesity-induced dysfunction during the expansion process.

ECs respond to diverse signals, but EC proliferation is particularly sensitive to fluctuations in PI3K signaling, downstream of insulin action. Activation of PI3K catalyzes the formation phosphatidylinositol (3,4,5)-trisphosphate (PIP3), a lipid that activates protein kinase B (PKB/AKT), leading to an increase in glucose uptake (4). During obesity and type 2 diabetes, PI3K signaling is defective in endothelial cells, and impairs insulin-induced capillary recruitment (5). Monelli et al., studied this relationship between PI3K activity in EC and vascularization of WAT. To do this, the authors generated mice with an EC-specific phosphatase and tensin homolog (Pten) deletion (Pten^iΔEC), inducible by tamoxifen administration in postnatal mice. Pten is a lipid phosphatase that opposes the action of PI3K. Effectively, the loss of Pten supports sustained PI3K activity in the ECs of these mice.

Pten^iΔEC mice were fed a normal chow diet or challenged with a high fat/high sucrose (HFHS) diet. Loss of Pten on either diet reduced body weight and fat mass, and resulted in smaller average adipocyte size, without significant changes in other tissues. Pten^iΔEC mice have an over two-fold increase in vascular density in both the visceral and subcutaneous WAT depots, and increases to a slightly lesser extent in brown adipose tissue. It is remarkable, but not surprising, that EC proliferation specifically in WAT alters whole body metabolism. This is consistent with observations that reported adipocyte-specific VEGF-A expression yielding enhanced beiging of adipose tissue with an improved systemic phenotype (6).

The authors observed ECs isolated from Pten^iΔEC WAT displayed increased fatty acid oxidation (FAO), and increased mitochondrial respiration that was further enhanced when lipids were supplemented in the media. Furthermore, the loss of Pten in ECs increased gene expression related to lipid transport, such as CD36. This observation is unexpected, since ECs are typically relying preferentially on glycolysis. However, the data presented here suggests that FAO is necessary to sustain EC proliferation in adipose tissue from Pten^iΔEC mice. The obvious source of FFA for ECs would originate from WAT lipolysis, the process by which triacylglycerols break down into glycerol and free fatty acids (FFAs). Indeed, the authors demonstrate that Pten deletion in EC stimulates a lipolytic response in adipocytes.

But what is the angiogenic factor originating from ECs responsible for stimulating lipolysis in WAT? The authors obtained conditioned media from Pten^iΔEC EC cultured cells and filtered it to exclude proteins and extracellular vesicles. Targeted metabolomics revealed elevated polyamine levels. This auspicious finding was of great interest, since polyamines are secreted and can act in a paracrine manner. Polyamines, such as spermine, spermidine, and putrescine, are small positively charged polycations that can regulate many physiological functions, including cell proliferation and differentiation (7). Polyamine rich foods have been associated with a high antioxidant activity, especially spermine (8). These molecules readily bind to negatively charged molecules, such as DNA, RNA, ATP, proteins, and phospholipids (7). Although it is established that the biosynthesis of polyamines is highly regulated in cells and tissues, the biological function of polyamines is only partly understood. An EC-based communication axis with AT via polyamines has not yet been previously reported to date.

The authors excluded proteins and extracellular vesicles (EVs) from the metabolomic analysis of the Pten^iΔEC conditioned media. EVs contain an array of proteins and metabolites that modulate signaling between endothelial cells and adipocytes. Interestingly, a proteomic analysis of small EVs secreted from WAT revealed an enrichment of proteins involved in polyamine metabolism (9). Therefore, it will be interesting to explore whether ECs can deliver polyamines via multiple pathways when they communicate with WAT, and whether additional cell types in the stromal vascular fraction of adipose tissue can also be a source of polyamines.

The polyamine spermidine plays a crucial role in adipogenesis by regulating genes required for differentiation of preadipocytes (10). Thus, one could speculate that polyamines secreted from ECs would also have an impact on neighboring preadipocytes, or any of the diverse cell types found in WAT. This draws attention to a drawback to the approach used by the authors by using tamoxifen as an inducer of the elimination of Pten in the ECs. Tamoxifen treatment alters patterns of adipogenesis, is a lipodystrophic agent, and remains bioavailable for weeks post administration (11). Therefore, the off-target effects of tamoxifen on adipogenesis and lipid and glucose homeostasis can make the physiological interpretation of these models challenging.

Monelli et al. emphasize that polyamine-mediated stimulation of lipolysis in WAT was inhibited by propranolol, suggesting polyamine action in adipocytes is partially dependent on the b adrenergic receptor (ßAR). Typically, stimulation of ßAR signaling increases uncoupling protein 1 (UCP1) gene expression and protein, resulting in adipocytes having a brown-fat-like appearance and thermogenic gene expression profile (12). The authors confirmed that spermidine increased the production of the ßAR second messenger cAMP in WAT, but paradoxically Pten^iΔEC mice do not display any UCP1-dependent or independent WAT beiging. The results raise the intriguing question as to how polyamines interact with the ßAR or possibly that ßAR activity is upstream of the effects polyamine metabolism (13) (Figure 1).

Figure 1. Endothelial cells exhibit self-directed regulation of angiogenesis in adipose tissue.

A) Obesity is characterized by a rapid expansion of WAT associated with impaired angiogenesis, the process by which ECs proliferate and form new blood vessels. The lack of sufficient angiogenesis leads to decreased oxygen supply and triggers inflammation and dysfunctional WAT. B) Targeted deletion of Pten in ECs increases vascularization specifically in WAT. As a result, adipose tissue function is restored, and the reduction of body weight and fat mass ultimately rescues the pathophysiological effects of HFD. C) Pten deletion promotes PI3K signaling in ECs, and stimulates the synthesis of polyamines, namely spermine, small polycation metabolites capable of paracrine signaling. ECs and adipocytes are in direct contact, engaging in cellular crosstalk. Spermine enhances ßAR receptor signaling in adipocytes and promotes lipolysis and expression of genes that mediate FFA transport. Consequently, adipocytes release FFAs for adjacent ECs to use as fuel for fatty acid oxidation that provides the energy needed to promote proliferation. Therefore, ECs undergo cell-autonomous angiogenic signaling in WAT, via the secretion of polyamines that stimulate lipolysis in adipocytes that provide FFAs for the growth of ECs.

Overall, the findings of Monelli et al. are consistent with previous studies that show supplementation of spermine improves glucose metabolism, reduces fat mass, and ameliorates the pathophysiological response to HFD (14). It is interesting to note that polyamines trigger lipolysis and FAO in WAT simultaneously. Several studies show polyamine flux and catabolism deplete acetyl CoA and cellular energy that causes a futile cycle that consumes WAT, and improves glucose and lipid metabolism (14, 15). This possibility was not explored in the current study, but may be also a contributor to the improved metabolism in Pten^iΔEC mice. Nevertheless, the authors demonstrate ECs are a novel regulator of extracellular polyamines in WAT.

Currently, pharmacological treatments for obesity remain limited and ineffective. Understanding the interplay of EC angiogenesis in AT can offer a therapeutic approach for obesity and its comorbidities. It is tempting to speculate that many clinical complications related to obesity could potentially be treated with angiogenic factors. Humans may benefit from supplementation with polyamines, as Monelli et al. report that obese humans and mice display low WAT polyamine levels. However, the potential drawback of such a therapeutic approach is the inherent danger that increased angiogenesis also offers a fertile environment for tumor growth.