Taming the Flames: Targeting White Adipose Tissue Browning in Hypermetabolic Conditions

Abstract

In this era of increased obesity and diabetes prevalence, the browning of white adipose tissue (WAT) has emerged as a promising therapeutic target to induce weight loss and improve insulin sensitivity in this population. The browning process entails a shift in the WAT from primarily storing excess energy to the dissipation of energy as heat. However, this idealistic view of WAT browning being the savior of the metabolic syndrome has been criticized by studies in burn and cancer patients that have shown browning to be detrimental rather than beneficial. In fact, in the context of hypermetabolic states, the browning of WAT has presented with substantial clinical adverse outcomes related to cachexia, hepatic steatosis, and muscle catabolism. Therefore, the previous thought construct of understanding browning as an all-beneficial physiologic event has now been met with skepticism. In this review, we focus on current knowledge of browning of WAT and its adverse metabolic alterations during hypermetabolic states. We also discuss the regulators and signaling pathways involved in the browning process and their potential for being targeted by new or existing drugs to inhibit or alleviate browning, potentially leading to decreased hypermetabolism and improved clinical outcomes. Lastly, the imminent clinical applications of pharmacological agents are explored in the perspective of attenuating WAT browning and its associated adverse side effects reported in burn patients.

In conditions of hypermetabolism browning of adipose leads to cachexia and organ steatosis. Thus, drugs to inhibit browning in these patients can potentially attenuate some of these adverse effects.

Essential Points

• The browning of white adipose tissue (WAT) has emerged as a promising therapeutic target to induce weight loss and improve insulin sensitivity in obesity and diabetes

• However, in burns and cancer, browning has been shown to be detrimental rather than being beneficial, as it facilitates cachexia and organ steatosis in these patients

• As a consequence of the unbalanced focus on browning from a purely beneficial standpoint, there has been a limited understanding as to what regulates and initiates browning in disease states

• Given the wide-ranging damaging impacts of browning from cachexia to organ steatosis, we believe it is paramount to initiate a discussion on the agents and pathways that drive this browning process in catabolic conditions, as they can potentially provide therapeutic opportunities by the unusual approach of inhibiting browning of the WAT

Hypermetabolism, a complex pathophysiologic condition that commonly affects burn and cancer patients, is widely defined as an elevation of resting energy expenditure with or without accompanying hyperglycemia, lipolysis, and organ and muscle catabolism. Despite these varying definitions of hypermetabolism, one concept becomes very clear: it is a multifactorial response involving adverse changes in several metabolic pathways (gluconeogenesis, lipolysis, and proteolysis) in many organs (liver, adipose, and muscle) (1, 2). Initially beneficial to meet the heightened energy demands of the condition (burns, cancer), chronic hypermetabolism results in significant organ, muscle, protein and lipid wasting, hepatic steatosis, and immunosuppression (2–4). Although, the mediators underlying hypermetabolism in both cancer and burns are myriad and complex, the presence of an elevated adrenergic state [catecholamine (CA) surge] and systemic inflammation (proinflammatory cytokines) has been implicated (5, 6). Over the past several decades, researchers have focused primarily on the hypermetabolic alterations of the liver and skeletal muscle and how these alterations affect patient outcome and prognosis (3, 7). Advances in these areas have certainly improved outcomes in these patients, but persistent hypermetabolism continues to be associated with significant morbidity and mortality in both burns and cancer, indicating the culprit is not limited to the liver and muscle.

In recent years, interest in the adipose tissue has seen a renaissance, particularly in the context of hypermetabolism. What accounts for this newfound respect for the adipose? It is the discovery of browning, a process by which white adipose tissue (WAT) converts into a fat-burning engine (8). In fact, several recent studies in both cancer and burns have highlighted the important role of WAT browning in the development and progression of a persistent hypermetabolic and catabolic state (9–11). Despite the sentinel findings that browning can be detrimental, these studies were controversial, as they have challenged the widely held paradigm that all browning is beneficial (12). In fact, prior to these studies, most research into the browning of WAT has concentrated on obesity and diabetes (12). As a consequence of this unbalanced focus on browning from a purely beneficial standpoint, there has been a limited understanding as to what regulates and initiates browning in disease states. Additionally, this bias in support of the activation of browning has also spilled over into the drug therapy market, where there has been a race to discover therapeutic compounds to activate browning in obese conditions, a model that does not reflect the microenvironment of burns and cancer. Given the wide-ranging damaging impacts of browning from cachexia to atherosclerosis, we believe it is paramount to initiate a discussion on the agents and pathways that drive this browning process in catabolic conditions, as they can potentially provide therapeutic opportunities by the unusual approach of inhibiting browning of the WAT.

In this review, we provide an overview and highlight some of the crucial distinctions of browning in obesity and hypermetabolic conditions (cancer, burns). We then discuss agents that have recently been shown to have an important role in mediating browning in burns and cancer. Lastly and most importantly, we focus on how the factors and signaling pathways that drive browning during these conditions (cancer, burns) can be exploited therapeutically to mitigate the damaging effects of hypermetabolism, with special emphasis on Food and Drug Administration–approved pharmacological agents.

The Adipose Tissue (White, Beige, and Brown)

The adipose tissue is a key metabolic tissue that influences systemic metabolism via its role in glucose and lipid synthesis, as well as endocrine functions. It also displays a high degree of plasticity, releasing free fatty acids during times of energy scarcity and storing excess energy in the form of triglycerides during periods of positive energy balance to carefully manage energy homeostasis (8, 13). However, recent work has established that the adipose tissue is not homogenous in terms of metabolic and endocrine function, with distinct adipose depots playing specific and often times contrasting roles metabolically. Like in real estate, when it comes to the adipose tissue, it’s all about location, location, location.

Brown adipose tissue

In humans, the brown adipose tissue (BAT) is primarily found in the interscapular region of the body, whereas in rodents it is found in the interscapular, perirenal, and periaortic regions. Interestingly, whereas interscapular BAT depots in humans disappear with age, they remain throughout life in rodents (13, 14) (Fig. 1). Characteristically, brown adipocytes are multilocular in morphology and are packed with mitochondria for heat production via the expression of uncoupling protein 1 (UCP1) (Fig. 1) (13, 14). UCP1 expression in these brown adipocytes uncouples electron transport from ATP synthesis to a predominant production of heat (13). Distinctively, the BAT is also highly innervated by the sympathetic nervous system and a well-structured vascularization that enables supply of oxygen and transport of heat (13, 14). Thus, the aforementioned characteristics allow the BAT to have a significant impact on whole-body energy metabolism.

Figure 1.

Anatomical locations and properties of the different adipose tissue depots in humans and mice. Left panel, top: human BAT (acquired from the supraclavicular region of a burn patient) is characterized by a multiocular morphology and increased staining for the UCP1. Middle: browning of subcutaneous white adipose (induced by burn injury in the image illustrated) leads to the formation of multiocular and UCP1-expressing beige/brite adipocytes. Bottom: human WAT (acquired from subcutaneous abdominal depot) have uniocular morphology and do not stain positive for the UCP1. Right panel, top: mice BAT (acquired from interescapular region) showing multiocular morphology and increased staining for the UCP1 protein. Middle: browning of inguinal WAT (induced by mice subjected to a 30% total body surface area burn injury) results in the formation of a multiocular and UCP1-expressing beige/brite adipocytes. Bottom: mice WAT (acquired from inguinal depot) have uniocular morphology and do not stain positive for the UCP-1 protein.

White adipose tissue

WAT is found dispersed throughout the body in both humans and rodents. Generally, subcutaneous (inguinal in rodents) and visceral (epididymal) adipose tissue encompass the largest fat depots in the body (13). The white adipocytes residing within WAT are characterized by unilocular morphology, reduced mitochondrial content, and the lack of UCP1 expression (Fig. 1) (13). In contrast to brown fat, this lack of UCP1 and decreased mitochondrial density facilitates excess energy storage in the form of triglycerides in white adipocytes. Additionally, the WAT also serves as an endocrine organ through the secretion of a number of adipokines such as leptin, adiponectin, and interleukin (IL)-6 (13). Accordingly, the preponderance to store fat has largely vilified the WAT in the obesity epidemic.

Beige/Brite adipose tissue

Until recently, the prevailing view was that adipose tissue only existed in the aforementioned classifications. However, the discovery of multilocular, UCP1-positive adipocytes within certain WATs in mice and humans has reformed our understanding of the adipose into one that is not only heterogeneous, but also very fluid in nature (13). It is a major task to categorize beige/brite adipocytes within WAT, as herein lies important information regarding their putative origins (15). In fact, two distinct prevailing theories exist to explain the source and development of these beige adipocytes within WAT. On one side of the aisle, the de novo hypothesis postulates that these beige adipocytes are adipocytes that have differentiated from specific precursor cells (EBF2 + PDGFRa⁺), distinct in source from that of both white and brown adipocytes (15). In contrast, the transdifferentiation hypothesis suggests that beige adipocytes arise and differentiate from white adipocytes in response to β-adregenic stimulation (15). Regardless, both hypotheses agree that the resulting beige adipocytes express UCP1 and show elevations in mitochondrial content, facilitating the dissipation of excess energy in the form of heat production (8, 15). Although it may seem that the de novo and transdifferentiation hypotheses conflict with each other, evidence suggests that both hypotheses coexist and can occur in timely independent processes within the WAT (15). Thus, we pragmatically define the concept of browning of WAT as the presence of UCP1 expressing adipocytes within traditionally WAT depots.

Browning of WAT in Health and Disease

Browning in obesity

The concept that certain white adipose depots could be induced to develop brownish characteristics (UCP1, mitochondrial biogenesis), thereby transforming them into a fat-burning engine, has recently attracted much attention as a new therapeutic target for the metabolic syndrome (8, 12). For instance, this has been considered with regard to obesity, a condition in which accumulation of excess fat results from a prolonged imbalance between energy intake and energy expenditure (16). In this context, it is hypothesized that the activation of browning can increase energy expenditure, thereby attaining a sustained negative energy balance and the oxidation of excess nutrients (14). In fact, a number of studies in mice have shown that the activation of browning in WAT facilitates weight loss, ameliorates insulin resistance, and corrects hyperlipidemia in obese states (17–19). The current belief is that the increase in mitochondrial biogenesis that accompanies browning enhances oxidative metabolism within the adipose tissue by activating a futile cycle; this cycle begins with the breakdown of the adipose stores, allowing free fatty acid efflux to fuel diminished mitochondrial ATP production (ironically, this was disrupted by the browning process itself) (8). Additional metabolic benefits of browning in the context of obesity are further illustrated via the endocrine factor, fibroblast growth factor 21, which is secreted from the newly formed beige adipocytes (20). Beige fat-derived fibroblast growth factor 21 acts as an autocrine factor, where it enhances glucose uptake in the liver and preserves β-cell function in the pancreas, suggesting an antidiabetic function of browning (20, 21). Unfortunately, many of the studies in browning and obesity have largely been conducted in mice, leaving the potential of browning in obese patients largely unexplored. However, the beneficial findings of browning in obese mice and the rare clinical studies done in human adults that have shown activating BAT improves oxidative metabolism, glucose uptake, and decreases body fat mass are promising (14, 17) (Fig. 2).

Figure 2.

Context-dependent metabolic effects of browning. Left panel: activation of browning results in beneficial metabolic effects (green) in obesity, mediated by both fat catabolism and paracrine factors, released postbrowning like fibroblast growth factor 21 (FGF-21). Right panel: conversely, activation of browning in hypermetabolic conditions (burns, cancer) leads to adverse side effects (red), namely cachexia, hepatic steatosis, lean muscle loss, and, ultimately, impairments in wound healing.

“It is becoming more apparent now that browning indeed has many faces.”

Browning in hypermetabolic states (burns, cancer)

The seemingly beneficial attributes of browning in obesity are detrimental in burns and cancer. Indeed, browning in cancer has been shown to contribute to the progression of cancer-associated cachexia, a condition characterized by severe weight loss and muscle catabolism (12). Wagner et al. (9) have reported that WAT browning facilitates cachexia in both cancer patients and mouse models of cancer. Similarly, Spiegelman et al. (22) have also implicated WAT browning in the progression of cachexia in lung cancer patients and mouse models of lung cancer. Interestingly, these detrimental features of WAT browning have also been extended to other hypermetabolic conditions such as burns (10, 11). In fact, it has been reported in both pediatric and adult burn patients that browning drives severe hypermetabolism and metabolic dysfunction (10, 11). Although the metabolic effects of browning are identical in both obesity and hypermetabolic conditions (burns, cancer), the systemic results are opposite (Fig. 2). In fact, browning-mediated lipid mobilization and energy expenditure result in weight loss and improved insulin sensitivity in obesity, but cachexia and hepatic steatosis in hypermetabolic conditions (12). How do we explain this discrepancy? The mechanisms accounting for this differential finding lie in the contexts/patients in which the browning process is activated. In obesity, patients have excess adipose tissue reserves to handle the heightened resting energy expenditure state stimulated by browning, conferring protection against muscle wasting and cachexia. Analogous to the accumulation of excessive fat stores in obese patients, burn and cancer patients are in a heightened negative energy state with little to normal adipose stores, thereby becoming prone to excessive muscle and fat wasting with the activation of browning (12). Additionally, it has been speculated that the well-documented ectopic fat accumulation in the liver postburn injury may be linked to WAT browning-induced lipolysis (23). Next to playing a critical role in glycogen storage and drug detoxification, the liver is a central tissue involved in lipid metabolism (24). Considering this role in lipid trafficking, hepatic failure seen in burn patients is postulated to arise from browning-induced lipid overload (12, 23, 25). The hepatic steatosis detected in burn patients is similar to nonalcoholic fatty liver disease, and is mediated by the heightened lipolysis and mitochondrial dysfunction milieu seen postburn injury. It is also likely that dysfunctional mitochondria and associated reductions in β oxidation favor increased hepatic triglyceride accumulation (3, 26, 27). Mitochondria and β oxidation are not as severely affected in other contexts, such as obesity, thereby reducing some of the risks associated with browning-induced lipotoxicity. Adding insult to injury, it has been suggested that specific lipid metabolites like palmitic and oleic free fatty acids released postbrowning may be toxic to hepatocytes and cardiomyocytes by inducing endoplasmic reticulum stress-mediated apoptosis (28, 29). How browning-induced chronic lipotoxicity impacts hepatic mitochondrial oxidative metabolism and leads to hepatic dysfunction is beyond the scope of this work, but has been extensively reviewed elsewhere (12). Instead, our aim in this study was to briefly highlight the pathology associated with browning in the context of hypermetabolism.

The aforementioned findings suggest that these hypermetabolic patients are at a significantly higher risk of death from browning-induced cachexia and hepatic steatosis (3, 9, 25). There is a growing consensus about developing more aggressive screening and intervention strategies for those at the greatest risk for developing such complications (i.e., burn and cancer patients). However, any therapeutic intervention will only be as effective as its ability to target key downstream and upstream regulators that lead to the activation of browning in these patients. The key factors regulating browning in the context of hypermetabolic states are described in the next chapters.

Upstream Regulators of Browning

A collection of agents has been implicated in the activation of WAT browning encompassing a diverse array of drugs, nutritional components, exercise, and specific genes, with this list growing ever more by the day. Because several exceptional reviews have recently been published just on the inducers of browning, we will not attempt to discuss these agents further, as it is beyond the scope of this review (30). Rather, for purposes of simplicity, we will focus on those agents that have been shown to induce browning in the context of burns and in other similar hypermetabolic disorders like cancer.

Catecholamines

CA are hormones produced primarily in the adrenal medulla and released into the circulation in response to stress or injury (30). Upon release, they bind to their respective receptors, activating a wide variety of physiologic processes ranging from nerve transmission to energy metabolism and cardiac function (30). Besides the various metabolic and cardiac activities that CAs have been linked to, they have also been shown to play a key role in the browning of adipose tissue (11, 15, 30). In fact, the release of CAs at sympathetic nerve terminals that supply BAT and WAT has been show to stimulate WAT browning and induction of UCP1. CAs released activate the β-adrenegic receptors, particularly β-3-adrenergic receptors expressed in adipocytes, to activate downstream targets that stimulate lipolysis, UCP1 gene expression, and metabolic activity (11, 15, 30). From a burn injury perspective, CAs were recently identified as the major drivers of browning in both patients and mice (10, 11). Typically, these burn patients have chronic levels of CA production, which persists months after the initial insult, and is believed to fuel persistent hypermetabolism via browning (5).

The above discussions would imply that browning of WAT would always be paralleled by (or preceded by) the release of CAs at sympathetic nerve terminals and that the adrenal gland is mandatory for this browning process. However, this adrenal medulla–centered paradigm has shifted with the implication of macrophages in the regulation of WAT browning (31). Because tissue-resident and recruited macrophages serve pivotal roles in responses to infection and injury, they have been labeled as troublemakers in the context of obesity and diabetes in adipose tissue (32). However, a groundbreaking study by Chawla et al. (33) has recently debunked the old view of adipose–macrophage crosstalk, by discovering that polarized macrophages promote the browning of WAT during cold exposure. Other studies in cancer- and fasting-induced browning have also supported a critical role of polarized macrophages in orchestrating the development of beige fat (9, 34). Mechanistically, it has been postulated that these polarized macrophages recruited to the adipose tissue produce cateochalmines to initiate the browning process (33). Thus, it is likely that CA-triggered browning is regulated by both the neuronal circuits emanating from the adrenal medulla, along with the orchestration of polarized macrophages.

Cytokines

The development of beige adipose tissue (browning) and the immunometabolism crosstalk discussed above does not only require innate immune cells like macrophages as a prerequisite, but also necessitates the release of anti-inflammatory cytokines. Specifically, recent studies have found that eosinophils promote type 2 macrophage polarization by secreting type 2 cytokines such as IL-4 and IL-13, consequently leading to the browning of WAT in cold-induced models of thermogenesis (33, 35). Interestingly, in hypermetabolic and systemically inflamed contexts, type 2 cytokines have not been suggested/reported to be involved. Instead, the proinflammatory cytokine IL-6 is thought to be the main driver of browning (10, 36). Indeed, in both cancer and burns, WAT browning was significantly impaired when the IL-6 cytokine was genetically silenced, indicating that IL-6 acts as the major mediator of browning during hypermetabolic states (9, 36). Similar to the notion that WAT browning is a double-edged sword, the role of IL-6 in metabolism is also perplexing (37). This paradox stems from studies done in obesity, cancer, and burns that have all shown that IL-6 has contradictory metabolic functions (9, 36, 38, 39). For instance, prolonged activation of IL-6 signaling induces WAT browning and increases energy expenditure in the context of cancer and burns (40). In contrast, in obesity, IL-6 has been shown to be the main mediator of the advantageous metabolic effects of browning (40). This relationship was demonstrated in mice, where it was shown that transplantation of BAT from healthy wild-type mice improved body weight and glucose homeostasis in obese recipient mice (40). Additionally, these beneficial metabolic effects were lost when these same obese mice received BAT transplantation from IL-6 knockout donor mice (40). Therefore, this suggests that IL-6 not only regulates the browning of WAT, but also functions as a BATokine (40). Although it is difficult to reconcile the paradoxical effects of IL-6 in browning, it does appear that these metabolic effects depend on the location of IL-6 secretion. In the framework of burns and cancer, bone marrow–derived IL-6 and tumor-derived IL-6 in the latter have been shown to be critical to the browning process observed in these conditions.

“It drives a lipotoxic environment in burns and cancer, yet promotes insulin sensitivity in obesity.”

In summary, given the wealth of cytokines, adipokines (fibroblast growth factor 21, adiponectin), and substrates (meteorin-like, parathyroid hormone–related protein) found to regulate adipose browning, it seems likely that the already long list of browning agents will continue to grow in the near future (20, 22, 41).

Signaling Pathways That Regulate WAT Browning

Elucidating whether the browning agents described above are truly browning agents necessitates demonstrating that they also activate the signaling pathways that regulate the expression of thermogenic genes, most importantly UCP1. A spectrum of these pathways has been reported to play a role in regulating browning (Table 1). It should be pointed out, however, that far more is understood about the agents that govern WAT browning than about the signaling pathways that regulate such a transition. Even more problematic has been that most of the signaling pathways studied have been conducted in physiological (cold exposure) models of browning, and thus it is unknown whether the ‘‘white to beige fat switch’’ seen in conditions of hypermetabolism also uses a similar signaling cascade. Nevertheless, we briefly review the major signaling pathways involved in adipose tissue browning, and those newly discovered pathways that may also regulate the development and activation of beige adipocytes for their future therapeutic exploitation.

Table 1.

Upstream Regulators and Signaling Pathways Implicated in the Regulation of Browning

Regulator	Target Adipose Tissue	Effect on Thermogenesis
Upstream regulators
Catecholamines	White, beige, and brown	Induces thermogenesis via UCP1 expression (30, 33)
Cytokines (IL-4, IL-6)	White, beige, and brown (IL-6 only)	Induces thermogenesis via macrophage polarization (30, 33, 35–36)
Downstream regulators
MAPK signaling pathway	White, beige, and brown	Induces thermogenesis via UCP1 expression (30, 42)
The mTOR signaling pathway	White, beige, and brown	Contrasting effects on thermogenesis:
		mTORC1 is required for thermogenesis via effects on UCP1 expression in WAT and BAT mitochondrial biogenesis (47)
		Hyperactivation of mTORC1 signaling inhibits the expression of several thermogenic genes, including UCP1 and PGC-1α in BAT (48)
		mTORC2 is required for BAT glucose uptake, growth, differentiation, and thermogenesis (46, 49)
		Rapamycin inhibits thermogenesis by decreasing UCP1 expression; decreases BAT activity by suppressing UCP1 expression and respiration (75)
AMPK signaling pathway	White, beige, and brown	Contrasting effects on thermogenesis:
		AICAR use in rats increased energy expenditure and mitochondrial content, but failed to induce UCP1 expression (52)
		AICAR use in mice induced browning via UCP1 expression and migration of brown adipocytes to WAT (52)

Aminoimidazole-4-carboxamide ribonucleotide (AICAR) is an AMPK agonist.

The mitogen-activated protein kinase signaling pathway

To harness the metabolic power of brown and beige adipocytes requires the expression of UCP1—and thus signaling pathways that stimulate UCP1 expression are, therefore, thought to control both browning and the metabolic functions of these cells. One such signaling pathway that has been implicated in UCP1 induction, as well as mediating the browning effects of catechoalmines, is the mitogen-activated protein kinase (MAPK) pathway (8, 42). CAs upon release bind to the stimulatory G protein–coupled β3-adrenoreceptor to activate adenylyl cyclase, an enzyme that converts ATP to 3′-5′-cyclic adenosine monophosphate in cytoplasm (8). The 3′-5′-cyclic adenosine monophosphate subsequently binds to the regulatory subunit of the enzyme protein kinase A, thereby activating it. Upon activation, protein kinase A phosphorylates p38 MAPK, which in turn initiates the activation of key transcription factors that control thermogenic potential. The best studied of these is peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), which induces mitochondrial biogenesis and oxidative metabolism, important prerequisites for WAT browning (8, 30). Additionally, PGC-1α binds to and complexes with the transcription factor peroxisome proliferator-activated receptor γ, which both activate UCP1 expression (8, 30). It is important to highlight that the MAPK signaling pathway represents the better characterized signaling pathways that regulate WAT browning.

The mammalian target of rapamycin signaling pathway

The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase belonging to the phosphatidylinositol 3-kinase family that regulates cell growth, proliferation, survival, mitochondrial function, and biogenesis (43). mTOR is the catalytic subunit for two macromolecular complexes: mTORC1 (nutritional sensor and control of protein synthesis; mitochondrial biosynthesis and energy balance) and mTORC2 (cell survival and division) (43). These two mTOR subunits differ primarily in their components, namely, mTORC1 comprising the regulatory-associated protein of mTOR (Raptor), whereas mTORC2 contains the Raptor-independent companion of mTOR. Recent studies in rodents have shown that both mTOR complexes have important regulatory and functional roles in white and BAT thermogenesis (44–47). For instance, hyperactivation of mTORC1 signaling by genetic knockout of the tuberous sclerosis 1 gene inhibits the expression of several thermogenic genes, including UCP1 and PGC-1α in BAT (48). Similarly, WAT tissue-specific Raptor knockout mice show elevated expression of thermogenic genes such as UCP1, cidea, and PGC-1α (44). However, it is important to note that the inhibitory effects of the mTORC1 pathway on thermogenesis are context dependent, as it has been shown that mTORC1 is required for adipose browning mediated by CAs and β-adrenergic receptor activation (47). Collectively, these studies demonstrate that mTORC1 signaling regulates thermogenesis in both BAT and WAT in a context-dependent process.

Although much work has been done in uncovering the function of mTORC1 signaling in the regulation of thermogenesis, less is known about the role that mTORC2 signaling plays in this process. However, some progress has been made recently with regard to the function of mTORC2 in thermogenesis. In fact, it was recently shown in mice that myogeneic factory 5 lineage-specific deficiency of Raptor-independent companion of mTOR causes increased diet-induced thermogenesis, via the upregulation of genes involved in thermogenesis like UCP1, PGC-1α, and TFAM in BAT (49). Additionally, mTORC2 signaling has been implicated in stimulating glucose uptake in brown adipocytes, thus regulating fuel supply for BAT thermogenesis (46). Based on these two opposing studies, it is too early to conclude as to whether mTORC2 is a positive or negative regulator of adipose tissue thermogenesis; instead, further investigations are warranted. It is also important to note that most of these studies have focused on studying BAT, leaving the role and the importance of mTORC2 in WAT function relatively unexplored.

The adenosine monophosphate kinase signaling pathway

The adenosine monophosphate kinase (AMPK) is a critical enzyme involved in the regulation of energy homeostasis. During periods of energy stress in which ATP levels are depleted, AMPK is activated to re-establish physiological homeostasis (50). It does this by mobilizing energy-producing pathways like β-oxidation, as well as increasing the capacity to produce energy via increases in glucose transport and mitochondrial biogenesis (50). Because two of the main hallmarks of WAT browning are increased, fat oxidation and mitochondrial biogenesis, and because AMPK is implicated in these two processes, it has been suggested to influence adipose tissue browning. Two studies have directly examined this and found conflicting results. In the first study by Gaidhu et al. (51), using the AMPK activator 5-aminoimidazole-4-carboxamide ribonucleotide in rats increased energy expenditure and mitochondrial content, but failed to promote UCP1 expression in WAT. This is in contrast to the study by Vila-Bedmar et al. (52), showing that AMPK activation via 5-aminoimidazole-4-carboxamide ribonucleotide administration in mice promotes WAT browning, via increased UCP1 expression and brown adipocyte migration into WAT. Other studies have indirectly linked AMPK signaling in the browning of WAT via mediators like myostatin and desnutrin (53, 54). In summary, the preponderance of evidence suggests that AMPK most likely has a role in WAT browning, but further studies are required to fully unravel its involvement.

“It enhances fat loss yet has been suggested to exacerbate cachexia.”

Pharmacologically Blocking WAT Browning

Although WAT browning has beneficial metabolic effects in the context of obesity, it is accompanied by a myriad of unwanted side effects ranging from cachexia to heart disease in hypermetabolic conditions. The majority of these adverse side effects to browning have largely been ignored, most likely because of the overwhelming benefits seen in obese conditions. Nevertheless, we consider it prudent that we at least start a discussion of how we can preserve the strong metabolic effects of browning, while also eliminating many of the unwanted side effects. As we have previously discussed, attenuating the hypermetabolic response to burn injury regardless of WAT browning can be achieved in a variety of ways, but may ultimately include a combination of approaches, including pharmacotherapy and nutritional support (55). Due to the scope of this review, however, we limit our discussion to pharmacological strategies to inhibit WAT browning that might be exploited for clinical benefit (Table 2). It is also critical we convey that this discussion of targeting WAT browning is largely focused in the context of burns and cancer, two major hypermetabolic conditions in which browning has been shown to be detrimental for patient outcome.

Table 2.

Antibrowning Effects of Selected Drugs for Potential Therapeutic Exploitation in Hypermetabolic Conditions

Drug	General Effects	Target Tissues	Preclinical or Clinical Studies	Key Findings
Glucocorticoids
Prednisolone	Immune metabolic	Adipose liver immune	Human: lean healthy men	Human: ↑ BAT respiration and UCP1 expression (59)
			Animal: male 129/Ola mice	Mice: ↓ Beige respiration and UCP1 expression (59)
Dexamethasone	Immune metabolic	Adipose liver immune	Animal: C57BL/6J male mice	Mice: ↓ Browning of WAT by suppressing UCP1 expression and reduced mitochondrial respiration (59)
			Obese C57BL/6J male mice	Obese mice: ↓ BAT respiration, UCP1 expression, and energy expenditure (59)
Corticosterone	Immune metabolic	Adipose liver immune	Animal: C57BL/6J male mice	Mice: ↓ Browning of WAT by suppressing UCP1 expression
				↓ BAT activity by suppressing UCP1 expression (57)
β-3 blockers
Propranolol	Immune metabolic	Adipose liver heart immune	Human: burn patients	Human: ↓ Browning of WAT by suppressing UCP1 expression, resting energy expenditure, and IL-6 levels (10)
			Animal: C57BL/6J male mice
SR 59230A	Immune metabolic	Adipose liver heart immune	Animal: Kras-lung cancer male mice	Mice: ↓ Browning of WAT by suppressing UCP1 expression
			Sprague–Dawley rats	Rats: ↓ BAT activity and thermogenesis (63)
mTOR inhibitors
Rapamycin	Immune metabolic	Adipose liver immune	Animal: C57BL/6N male mice	Mice: ↓ Browning of WAT by suppressing UCP1 expression and mitochondrial biogenesis (77)
			Sprague–Dawley rats	Rats: ↓ BAT activity by suppressing UCP1 expression and respiration (78)
IL-6 receptor inhibitors
Mouse IL-6 monoclonal antibody (clone: MP5-20F3)	Immune metabolic	Adipose liver immune	Animal: Kras-lung cancer male mice	Mice: ↓ Browning of WAT by suppressing UCP1 expression (9)

Sirolimus is the equivalent name of rapamycin and is widely used clinically. Mouse IL-6 monoclonal antibody is equivalent to the human anti–IL-6 receptor blocker Tocilizumab widely used clinically.

Glucocorticoids

Glucocorticoids (GC) are steroid hormones that exert a broad range of effects throughout the body to modulate metabolic and immune responses. Although they act upon multiple target systems to increase energy substrate availability, they primarily affect the activity of adipose and immune cells (56). Additionally, the actions of GCs on target cells are thought to be mediated by the type 2 GC receptor (56). In the perspective of WAT browning, GCs have been postulated to inhibit browning via two distinct processes. In the first aspect, it is believed that GCs directly inhibit browning via the suppression of UCP1 transcriptional expression (57–59). In fact, it has been shown that GC administration to rodents decreases thermogenic activity and UCP1 expression (56, 58). Interestingly, it was also recently reported that GC administration stimulated UCP1 expression and BAT activity in lean healthy men, whereas GC administration inhibited browning of WAT by suppressing UCP1 expression, suggesting adipose tissue–specific effects of GCs (60). Conversely, GCs are also thought to inhibit WAT browning directly via their immunomodulatory actions. Categorized as anti-inflammatory, GCs act to suppress the production of proinflammatory cytokines, including tumor necrosis factor-α, IL-6, and IL-1β, inhibiting inflammatory responses (61). This inhibitory effect on IL-6 production is significant in the context of burns and cancer, as chronic activation of IL-6 signaling promotes WAT browning and increases energy expenditure in these conditions (9, 10). Additionally, GCs have been shown to inhibit the recruitment of inflammatory cells to sites of inflammation, primarily by suppressing the release of chemokines (61). This is noteworthy in the context of WAT browning, as infiltration of macrophages to the adipose tissue has been shown to regulate cold-induced thermogenesis. Thus, it is apparent that GCs impair browning and facilitate a brown to white phenotypic conversion.

Propranolol

Propranolol is a nonselective β-adrenergic receptor antagonist that has been shown to attenuate hypermetabolic and hyperdynamic states following injury (55, 62). Because of its inhibitory effects on β-adrenergic receptors, which facilitate CA-induced WAT browning, they have been proposed to mitigate browning. Animal studies have shown that inhibiting β-3 adrenergic receptor via the use of propranolol or other β-3 antagonists like SR59230A inhibited both BAT thermogenesis and WAT browning (9, 63). Prior to the discovery of WAT browning in burns, propranolol has been widely used clinically in the management of the hypermetabolic response to burn injury (62, 64). In fact, a number of randomized control trials performed in severe burn patients have found that propranolol decreases lean muscle mass loss and lipid catabolism safely with no adverse effect on mean arterial blood pressure (64–66). At the time when these clinical trials were being conducted, WAT browning was not yet discovered in these patients; thereby, little mechanistic details were known on how propranolol was meditating its beneficial effects on adipose tissue metabolism and energy expenditure. However, the link between WAT browning and propranolol was recently made by Patsouris et al. (10), in which they showed that adult severe burn patients treated with propranolol had reduced UCP1 expression and energy expenditure. These recent findings provide reason to believe that the wide use of propranolol in the care burn patients as well as in the care of other hypermetabolic conditions will be strengthened.

Tocilizumab

IL-6, a pleiotropic cytokine, has been implicated in numerous diseases associated with inflammation, including rheumatoid arthritis, inflammatory bowel disease, and several types of cancer (67–69). Notably, as discussed earlier, IL-6 has also been implicated as the driver of adipose browning in cancer and burns (9, 10, 36). It is not surprising then that a number of therapeutic agents have been developed that target the various aspects of the IL-6 signaling pathway (70, 71). One such drug that is widely used clinically is tocilizumab, a humanized IL-6 receptor inhibiting monoclonal antibody. Indeed, several studies have reported beneficial effects of tocilizumab use in rheumatoid arthritis, sepsis, and cancer (9, 67, 72, 73). In the context of the adipose tissue, very little studies have assessed whether blocking IL-6 signaling blocks browning. In fact, there has only been one report in which blocking IL-6 not only attenuated browning, but also protected mice from cancer-induced cachexia (9). Although the concept of blocking IL-6 signaling has not been studied clinically in burns and cancer to inhibit browning, we can speculate whether tocilizumab use will be beneficial in this setting. Certainly, given that the IL-6 cytokine has undeniably been linked to being the master regulator of both burn- and cancer-induced browning, tocilizumab use and the subsequent inhibition of the IL-6 signaling may be beneficial in these patients (9, 10, 36). In agreement with this hypothesis, increase in body weight and a protection against cancer-induced cachexia have been reported in a lung cancer patient treated with tocilizumab (73). However, whether this weight gain and defense against cachexia were due to the inhibition of WAT browning in this patient on tocilizumab has not been clearly established, and research in this area is lacking. It is also anticipated that future research will need to consider whether selective blockade of IL-6 signaling (pro- vs. anti-inflammatory effects) offers a clinical advantage over a more global inhibition of IL-6 in these patients (74). In essence, targeting the proinflammatory aspect of IL-6 has the potential to inhibit the harmful component of IL-6 signaling but might leave certain IL-6–regulated beneficial metabolic and immune processes intact. Such strategy would be critical to mitigating the propensity to infections reported in patients on tocilizumab.

“Hence, targeting browning for therapeutic purposes while minimizing side effects represents a great challenge.”

mTORC1 inhibitors

As discussed earlier, activation of the mTORC1 signaling pathway by CAs in mice has been linked to WAT browning, and inhibiting mTORC1 activity was shown to block CA-induced browning (47). Additionally, chronic elevated CA surge in burns and hyperactivation of the mTORC1 signaling pathway in cancer have been implicated in development of insulin resistance, muscle wasting, and tumor growth in the latter. Therefore, targeting mTORC1 is a rational therapeutic approach in the context of these hypermetabolic conditions. Sirolimus, also known as rapamycin, was one of the first compounds able to inhibit mTOR and originally used as an antifungal and immunosuppressive agent (75). Sirolimus has been widely used clinically in cancer patients to inhibit tumor growth, but little has been shown clinically on the metabolic benefits (75, 76). This is particularly important given that WAT browning has been linked to cancer-induced cachexia and poor outcome (9, 22). It is, however, known that rapamycin treatment inhibits WAT browning in mice and rats (77, 78). The metabolic effects of rapamycin treatment are murky, with paradoxical results seen in a number of mice studies. For instance, rapamycin treatment improves insulin sensitivity by disrupting S6 kinase-mediated inhibition of the insulin receptor substrate and protects against diet-induced obesity. In contrast, chronic rapamycin administration has been shown to impair whole-body insulin sensitivity and facilitate a diabetic phenotype in mice (79). Thus, major unanswered questions remain with regard to benefits, safety, and dosage of rapamycin use clinically. Fully addressing these questions will lead to an improved understanding as to how they modulate the metabolic pathways, which should streamline their wide therapeutic use clinically in humans.

In summary, the authors freely admit that some of the therapeutic agents discussed in this section are controversial, nor would everyone agree with some of the points put forth regarding their potential use to inhibit WAT browning. Also, given the effects of seasonal and circadian mediators in the browning process, the discordant results seen clinically with the use of the aforementioned pharmacological agents might be attributed to this. These circadian and seasonal confounding variables can be resolved with better controls, including the time of day that experiments or drug treatments are administered. Nevertheless, the aim of this section was to stimulate a conversation on the potential therapeutic benefits of inhibiting browning in hypermetabolic patients.

Conclusion

The complexity of browning compounded with the metabolic etiology of cancer and burn patients presents numerous clinical challenges but also several opportunities for therapeutic intervention. We have learned an extraordinary amount in a short time, and it seems certain that we will discover much more about this highly complex and relatively damaging process in the very near future. Despite the prevailing dogma, it is becoming more apparent now that browning indeed has many faces. It drives a lipotoxic environment in burns and cancer, yet promotes insulin sensitivity in obesity. It enhances fat loss, yet has been suggested to exacerbate cachexia. Hence, targeting browning for therapeutic purposes while minimizing side effects represents a great challenge. Nevertheless, it is clear that selective inhibition of browning in this subset of patients (cancer, burns) represents a promising therapeutic strategy to curtail the deleterious hypermetabolic response. Key to the medicinal utilization of the pharmacological agents discussed in this work, as with any intervention, will be specificity and efficacy. Some of these agents, like propranolol, are already widely used in burn patients and have shown promise, although their effects on the adipose tissue remain to be explored further. Overall, the purpose of this review was to advance the idea that blocking the thermogenic potential of white adipose during conditions of hypermetabolism is just as equally important in unlocking its thermogenic potential in the fight against the obesity epidemic.