The Molecular Brakes of Adipose Tissue Lipolysis

Introduction

Maintaining whole-body energy homeostasis during times of dramatic fluctuations in energy supply and demand is a fundamental property of animal life. Coping with these energy fluctuations has entrained animals with the ability to orchestrate energy metabolism for optimal substrate storage and use during states of either food surplus or famine, and periods of either rest or increased energy demand (Smith et al., 2018). Deficiency in this ability is implicated in the pathogenesis of metabolic diseases such as insulin resistance (IR), type 2 diabetes (T2D) and cancer cachexia. Adipose tissue is a central metabolic organ that responds to fluctuations in nutrient availability to maintain systemic metabolic homeostasis. The ability of adipocyte to efficiently store energy after meals and release it when needed to other cells in the body is critical for integrating metabolism (Frühbeck et al., 2001). Tightly regulated fat accumulation and mobilization are a central characteristic of energy homeostasis.

In mammals, adipose tissue can be divided into white adipose tissue (WAT), brown adipose tissue (BAT) and beige/brite (brown in white) adipose tissue (Li et al., 2014b; Peirce et al., 2014). While white adipocytes, characterized by a single, large intra-cellular lipid droplet, harbored eccentric nucleus and few mitochondria, store energy in the form of triglycerides (TGs), brown and brite adipocytes, which express unique thermogenic uncoupling protein 1 (UCP1), contain multiple small lipid droplets of varied sizes as well as an almost centrally localized nucleus, and most importantly, equipped with an abundant number of mitochondria, specialize in energy dissipation. Under conditions of positive energy balance, expansion of adipose depots can be driven either by the increase in adipocyte size (hypertrophy) or by the formation of new adipocytes from tissue-resident progenitor differentiation in the process of adipogenesis (hyperplasia), thus protecting peripheral tissues from ectopic lipid deposition (Ghaben and Scherer, 2019). Conversely, during energy-demanding conditions such as exercise, fasting or cold stimulus, these stored fats are mobilized to supply the body with free fatty acids (FFAs) as energy substrates. In thermogenic brown and brite adipose tissues, FFAs activate and fuel the UCP1-dependent non-shivering thermogenesis (Cannon and Nedergaard, 2004; Li et al., 2014a). The thermogenic, energy dissipating phenotype of brown and brite adipocytes gains particular therapeutic interest considering the continuously increasing world-wide burden of obesity.

Mobilization of FFAs from stored TGs in adipocytes requires ester hydrolysis by a sequential enzymatic process called lipolysis, which is a pivotal metabolic reaction for energy mobilization. Once released, FFAs have a variety of fates including oxidation for energy in the form of ATP (adenosine triphosphate), re-esterification back into TGs, and also function as signaling molecules (Papackova and Cahova, 2015; Li et al., 2017). Despite their essential roles, high concentrations of FFAs are toxic because of their limited solubility, amphipathic nature and their active transformation into highly bioactive and cytotoxic lipid species. These destructive effects of FFAs — collectively referred to as “lipotoxicity” — can lead to cellular dysfunction and cell death (Unger, 2002). As such, elevated circulating FA (fatty acid) levels contribute to insulin resistance in both animals and humans (Savage et al., 2007). The findings of increased fat mobilization as well as brown fat thermogenesis during cancer cachexia indicate that dysfunction of adipose tissue contributes to the imbalance of energy homeostasis involved in catabolic wasting (Tsoli et al., 2012). Together with anorexia, the aberrant fat mobilization to generate heat represents a maladaptive response experienced by cancer patients (Tsoli et al., 2016). Therefore, intracellular lipolysis is a critical metabolic process of virtually all eukaryotic cells, and the tight control is important to metabolic health. Lipolysis is thus under tight positive and negative physiological regulation. The disruption of this subtle balance between energy storage and mobilization will result into IR, T2D and cancer cachexia (Kahn et al., 2006; Yang and Mottillo, 2020).

As aberrant adipocyte lipolysis is involved in various metabolic diseases and releasing the brakes on lipolysis in thermogenic adipocytes may activate thermogenesis, targeting adipocyte lipolysis is thus of therapeutic interest. Despite a very detailed knowledge of the biochemical and physiological control of lipolysis activation, the molecular brakes are poorly understood. This review focuses on the molecular brakes of lipolysis and discusses some of the divergent fine-tuning strategies in the negative feedback regulation of lipolysis, including delicate negative feedback loops, intermediary lipid metabolites-mediated allosteric regulation and dynamic protein–protein interactions.

The Canonical Pathway of Adipocyte Lipolysis

In adipocytes, three major neutral lipases participate in the canonical pathway of lipolysis (Figure 1): adipose triglyceride lipase (ATGL)—also termed patatin-like phospholipase domain-containing protein 2 (PNPLA2) or desnutrin, hormone-sensitive lipase (HSL), and monoglyceride lipase (MGL) (Haemmerle et al., 2002; Zimmermann et al., 2004; Taschler et al., 2011). These enzymes operate in a sequential and controlled manner in conjunction with several cofactors to break down TGs stored in lipid droplets. ATGL, considered the rate-limiting enzyme, catalyzes the first step, hydrolyzing TGs to diglycerides (DGs), which in turn are degraded by HSL into monoglycerides, with the release of a FFA in each step (Haemmerle et al., 2002; Zimmermann et al., 2004). Finally, MGL completes the process through the hydrolyzation of the last ester bond in monoglycerides, generating one last FFA and a glycerol backbone (Taschler et al., 2011).

Adipose tissue lipolysis is tightly regulated by endocrine, paracrine, autocrine and autonomic nervous systems, which modulate the activity of lipases by determining their intracellular locations and interactions with regulatory factors. Under non-stimulated conditions (basal), such as feeding, ATGL can be found on the surface of lipid droplets as well as in the cytoplasm, while HSL predominantly remains in the latter (Bezaire et al., 2009). Meanwhile, the lipid droplet-associated protein Perilipin 1 (PLIN1) limits ATGL function by sequestering its co-activator, comparative gene identification 58 (CGI58), also known as ABHD5 (α-β Hydrolase domain-containing protein 5) (Girousse and Langin, 2012). Other accessory proteins can further inhibit ATGL activity through their direct protein-protein interaction with ATGL, including G0/S2 switch protein 2 (G0S2), hypoxia-induced lipid droplet-associated protein (HILPDA), and fat specific protein 27 (FSP27) (Yang X. et al., 2010; Grahn et al., 2014). As a result, the rate of TG hydrolysis within the basal state is low (Figure 1).

In contrast, during periods of high energy demand, such as fasting or cold exposure, lipolysis is activated mainly via the action of catecholamines (Heeren and Scheja, 2018). Upon stimulation, sympathetic nerve fibers innervating the adipose tissue release norepinephrine, which binds to β-adrenergic receptors (β-ARs) located on the plasma membrane of adipocytes. Hormonal binding initiates a stimulatory G protein (Gs)-mediated cascade, elevating intracellular levels of the second messenger cyclic AMP (cAMP, cyclic adenosine monophosphate), through the activity of adenylyl cyclase (AC). An accumulation of cAMP in the cytoplasm promotes protein kinase A (PKA) activation, which in turn modulates lipolysis via phosphorylation of key effector factors (Langin, 2006). For instance, the phosphorylation of PLIN1 leads to the release of CGI58, enabling it to interact with ATGL, therefore, fully activating its TG hydrolase activity (Lass et al., 2006). In addition, PKA, together with phosphorylated PLIN1, promote HSL translocation from the cytosol to the lipid droplet surface, inducing an acute activation of TG hydrolysis in conjunction with ATGL and MGL (Strålfors et al., 1984; Arner and Langin, 2014).

It should be noted that products and derivatives liberated through lipolysis do not only serve as substrates for metabolic fuel, as it was first thought, but also influence key signaling pathways involved in energy homeostasis. For instance, it has been demonstrated that lipolysis-derived fatty acids are shuttled by fatty acid-binding proteins (FABPs) to the nucleus, where they are necessary for proper peroxisome proliferator-activated receptor (PPAR) signaling (Mottillo et al., 2012). These lipids stimulate expression of target genes through either direct (as ligands of PPAR) or indirect (as precursors for other PPAR ligands) interaction with PPARs. Accordingly, ATGL-mediated lipolysis is essential for PPARα signaling in cardiac muscle and its ablation in mice leads to impaired thermogenesis due to reduced PPARα binding to BAT-specific genes such as UCP1 (Ahmadian et al., 2011; Haemmerle et al., 2011). Furthermore, the HSL-mediated reaction provides ligands to activate central adipogenesis transcription factors, including PPARγ and/or retinoid X receptor alpha (RXRα) (Ström et al., 2009; Shen et al., 2011). Therefore, lipolysis per se is multifunctional (Figure 1).

The Alternative, Non-Canonical Pathway of Lipolysis: Lipophagy

In addition to the above-mentioned canonical lipase-mediated lipolysis, autophagy-based lipophagy offers an alternative mechanism for the control of fat mobilization and breakdown in various cell types, including hepatocytes (Singh et al., 2009; Zechner and Madeo, 2009), enterocytes (Khaldoun et al., 2014), macrophages (Ouimet et al., 2011), brown adipocytes (Zhang et al., 2020) and neurons (Martinez-Lopez et al., 2016).

Under conditions of nutrient deprivation, induced by the major metabolic kinases mTORC1 (Farah et al., 2016; Martinez-Lopez et al., 2016) and AMPK (Garcia and Shaw, 2017; Seo et al., 2017), lipid droplets (LDs) and autophagic machinery (phagophore) associate to form autophagic vacuoles (autolipophagosome) and subsequently autophagy promotes lipid breakdown by releasing engulfed LDs to lysosomes for degradation (Singh et al., 2009; Zechner et al., 2017). Lysosomal acid lipases are ultimately responsible for the acid hydrolysis of the LD-stored neutral lipids and subsequent release of free fatty acids (Dubland and Francis, 2015). Ever since its discovery in 2009 in hepatocytes, lipophagy has emerged as a significant component of lipid metabolism and serves a critical role in maintaining overall lipid homeostasis in physiological status (Singh et al., 2009; Zechner and Madeo, 2009). There are many excellent reviews of this topic (Schulze et al., 2017; Zechner et al., 2017; Zhang et al., 2018; Filali-Mouncef et al., 2021).

As autophagic engulfment of cytosolic cargo is size-restricted, only tiny or smaller lipid droplets can be directly targeted by lipophagy, whereas larger lipid droplets seem to be preferentially broken down into smaller ones via lipolysis (Schott et al., 2019). Therefore, lipolysis and lipophagy are sequential pathways, these two paralleled and distinct pathways function in a coordinated manner contributing to the regulation of the lipid turnover and the lipid metabolism homeostasis (Zechner et al., 2017; Filali-Mouncef et al., 2021).

Defective lipophagy has been linked to numerous metabolic disorders such as fatty liver (Carotti et al., 2020), obesity (Yang L. et al., 2010) and atherosclerosis (Ouimet et al., 2011), and the age-dependent decrease in autophagy (Cuervo, 2008) could underline the basis for the metabolic syndrome of aging (Singh and Cuervo, 2012). Therefore, beyond lipid mobilization, lipophagy can be considered as a defensive mechanism against ectopic lipid accumulation (Zechner and Madeo, 2009). Activation of lipophagy is a viable therapeutic target to prevent or treat such diseases where canonical lipolysis stimulation is not achievable. Generally, nutrient abundance inhibits lipophagy, while energy deprivation promotes it. Consistently, HFD feeding suppresses autophagy in mouse liver (Yang L. et al., 2010). Fatty acids are also known to regulate lipophagy. Acute stimulation with oleic acid activates lipophagy in hepatocytes (Singh et al., 2009). This activation is considered as a mechanism to buffer excess lipid influx into the cell. Nevertheless, despite considerable progress has been made in understanding lipophagy, specific molecular processes of lipophagy remain largely unknown. In addition, understanding the molecular brakes of lipophagy may provide a new approach to increase autophagic function to prevent metabolic disorders.

Aberrant Lipolysis in Health and Disease

Adipocytes are the cell type specialized in the regulation of lipid metabolism and energy storage for the body in the form of lipids, while cells in other tissues like skeletal muscle and liver are not suited for the safe long-term storage of lipids (Pond, 1992; Li et al., 2015). The phenomenon of chronic and excess lipid accumulation in non-adipose tissues that eventually leads to IR and inflammation in the targeted tissue is termed lipotoxicity (Engin, 2017). Lipotoxicity has emerged as a key feature underlying the development of the metabolic syndrome, which itself is a major risk factor for the development of T2D and cardiovascular disease (Ballantyne et al., 2008; Unger and Scherer, 2010). The root cause for lipid accumulation in non-adipose tissues is a prolonged metabolic imbalance between lipid release and uptake as seen in obesity. An overload of energy is usually safely stored in WAT in the form of lipids, but only until the fat storage capacity of the adipocytes is reached (Gray and Vidal-Puig, 2007). Adipocytes that reached their fat storage capacity can no longer incorporate lipids from the circulation and turn into a state of lipid spillover which leads to a release of FFAs into the circulation, increasing the already high plasma lipid levels caused by chronic energy overload (Katsanos, 2018). High plasma levels of FFA lead to systemic IR via at least partially negating the anti-lipolytic effect of insulin on adipose tissue (Carlson et al., 2007). Consequentially, an ever-increasing level of FFA in the plasma, supported by the now uncontrolled lipolysis, eventually leads to ectopic lipid buildup and its tissue specific pathologic consequences (Schaffer, 2003).

The pathologic consequences of ectopic lipids are not primarily based on the buildup of intracellular TGs or FFAs and their hydrophobic effects on the cellular organelles. So called bioactive lipids, namely DGs and ceramides that derive from the intracellular FFA and TAG pool have been causally linked to the development of lipid-induced IR. DGs are lipid species that occur as intermediates during the esterification of FAs to TGs. Hepatic accumulation of DGs, especially sn-1,2 DG, activates PKC (protein kinase C) isoform ε, which can phosphorylate and inhibit the activity of the insulin receptor, reducing the responsiveness of the liver to insulin (Lyu et al., 2020). A similar mechanism was also proposed in DG-induced IR in skeletal muscle (Timmers et al., 2008).

Ceramides, key molecules for the biosynthesis of sphingolipids, are mostly synthesized from the saturated fatty acid thioester palmitoyl CoA and are heavily implicated in the development of IR (Holland et al., 2007; Young et al., 2012). Ceramides have emerged as potent signaling factors that are able to stimulate a number of kinases and phosphatases, most importantly PKC isoform ζ and PP2A (protein phosphatase 2A), which lead to an impairment of the insulin-dependent membrane recruitment of protein kinase B (PKB, also named as AKT) (Ruvolo, 2003; Stratford et al., 2004).

An uncontrolled basal lipolytic rate in adipocytes appears to be a key influencing factor in the development of IR and lipotoxicity. Unrestrained lipolysis in humans with mutations in PLIN1 leads to severe IR (Gandotra et al., 2011). The severity of IR varies depending on which fat depot becomes exceedingly enlarged upon prolonged metabolic imbalance. Especially, the enlargement of visceral WAT is closely correlated with IR since it exhibits a high basal lipolytic rate and secreted FFAs directly go to the liver via portal circulation, interfering with the liver’s regulatory role in glucose metabolism (Koska et al., 2008). In contrast, enlargement of subcutaneous fat seems to be rather metabolically protective (Suárez-Cuenca et al., 2021).

The importance of aberrant adipocyte lipolysis is highlighted by the observation that partial inhibition of adipocyte lipolysis can improve insulin sensitivity and glucose metabolism, as seen with genetic and pharmacological inhibition of HSL or in adipocyte-specific ATGL-KO mice (Girousse et al., 2013; Schoiswohl et al., 2015). Additionally, having a higher number of smaller adipocytes appears to be protective against the development of IR and the metabolic syndrome (Roberts et al., 2009). The less adipocytes one has, the more of these adipocytes are of a very large and hypertrophic phenotype that already reached their capacity for lipid storage and exhibit elevated rates of basal lipolysis (Wueest et al., 2009). Indeed, people with obesity who are still metabolically healthy have more numerous and smaller adipocytes compared to people with obesity with metabolic syndrome (Weyer et al., 2000; Cotillard et al., 2014). Of note, both decreased and increased adipocyte lipolysis have been demonstrated to improve metabolic health (Ahmadian et al., 2009; Jaworski et al., 2009). These contradictory results clearly depend on specific contexts. For patients with T2D, targeting the lipolysis to reduce the circulating FFA levels might be an effective way, while for the prevention of obesity development, boosting lipolysis might be a conceivable strategy.

Uncontrolled lipolysis is also an important factor in the cachexic process. Cancer-associated cachexia is a state of progressive metabolic disturbance that leads to muscle and adipose tissue wasting in affected patients (Tisdale, 2009). The main mechanism responsible for the marked loss of fat mass is related to strongly elevated lipolysis. Cachexic cancer patients exhibit a twofold increase in basal lipolytic activity as compared to weight-stable cancer patients and weight-loss patients due to reasons other than cachexic cancer. Mature adipocytes derived from these cachexic cancer patients expressed elevated mRNA and protein levels of HSL and showed a 2–3-fold stronger lipolytic response to catecholamines and natriuretic peptide as compared to adipocytes derived from weight stable and weight loss controls (Agustsson et al., 2007). In line with these findings, subcutaneous fat of cachexic cancer patients was additionally found to express elevated protein levels of the pro-lipolytic β-1 adrenergic receptor (Cao, 2010). These findings further highlight the importance of uncontrolled lipolysis as the driving force behind the undesired and detrimental loss of fat mass in cancer-associated cachexia.

Although inhibition of uncontrolled lipolysis seems to be promising to improve insulin sensitivity in obesity and prevent fat loss in cancer cachexia, functional basal lipolysis is essential for maintaining adipogenesis, adipocyte function as well as human health. In fact, as already mentioned, lipase-mediated lipolysis generates endogenous ligands for nuclear receptors such as PPARγ and RXRα, which are required for transcriptional regulation of adipogenesis and maintenance of adipocyte function (Itoh et al., 2008; Ström et al., 2009). As such, HSL deficiency in human patients results in impaired adipocyte development and high plasma lipid levels which eventually leads to ectopic lipid accumulation and IR (Albert et al., 2014). These metabolic phenotypes highlight the physiological significance of functional basal lipolysis in adipocyte function and systemic metabolic homeostasis and underscore the severe metabolic consequences of non-functional lipolysis. Therefore, a dynamic and delicate regulation of lipolysis is crucial for a healthy metabolism.

Extracellular Hormonal and Autocrine/Paracrine Brakes of Adipocyte Lipolysis

The subtle balance between fat storage and mobilization is important for the whole-body energy homeostasis; its disruption has been implicated as contributing to the development of IR, T2D and cancer cachexia. As a result, adipocyte lipolysis requires delicate regulations at the physiological level by hormones that have opposing effects as well as by autocrine/paracrine feedback mechanisms to provide a balance between energy storage and mobilization (Figure 2).

Intracellular Brakes on Adipocyte Lipolysis

In addition to the regulation by extracellular hormonal and autocrine/paracrine signals, lipolysis is also controlled by intracellular mechanisms that involve triglyceride-fatty acid (TG-FA) cycling, intracellular energy state sensing, intermediary lipid metabolites, as well as protein-protein interactions mediated modification of key components of the lipolytic signaling pathways (Figure 3).

Roles of Fine-Tuned Lipolysis in Thermogenic Fat Biology

In thermogenic brown and brite adipocytes, lipolysis stimulation is a canonical way for the activation of non-shivering thermogenesis. FFAs released by lipolysis are direct activators of uncoupling protein 1-dependent thermogenesis. In other words, UCP1 activity depends on the cellular availability of FFAs. Thus, pro- and anti-lipolytic mediators are bona fide modulators of thermogenesis in brown and brite adipocytes (Braun et al., 2018).

Hitherto, thermogenesis had always been correlated with the presence of BAT local lipolysis. It has been reported, however, that lipolysis in white adipocytes permits maximal stimulation of energy expenditure, as β3-ARs must be present in both white and brown adipocytes in order for β3-AR agonist (CL-316,243) to maximally increase thermogenesis, while lack of β3-ARs in white adipocytes significantly limits the stimulatory effect (Grujic et al., 1997). The mechanism by which β3-ARs in white adipocytes permit maximal stimulation of brown fat thermogenesis is unknown but might be related to their roles in mobilizing FFAs. When the FFA concentration reaches a sufficient level in the blood, FFAs can be used as activators and/or fuel for energy expenditure in brown adipocytes (Grujic et al., 1997). Consistent with this scenario, more recently, when Schreiber et al. (2017) tried to knock out the ATGL and Shin et al. (2017) deleted an ATGL-activating protein CGI58 specifically in BAT, respectively, it is observed that lipolysis did not necessarily occur locally within the brown fat cells to ignite thermogenesis in both cases during cold. In line with these findings, a recent study reported that even lipid droplets in brown fat were dispensable for thermogenesis in response to cold when other fuel sources were present (Chitraju et al., 2020). By ablation of TG biosynthesis enzyme acyl CoA:DGAT specifically in brown fat, the authors found that in the absence of TG storage in brown adipocytes, other substrates, such as circulating glucose and FAs, as well as increased brown adipocyte glycogen, appear to be sufficient to compensate to fuel respiration and thermogenesis (Chitraju et al., 2020). Overall, these studies imply that with the deficiency of lipolytic machinery and even lack of energy storage in BAT, the breakdown of stored TGs in WAT is a key feature for maintaining thermogenesis. WAT lipolysis promotes FFA release, which thereafter can be deployed into BAT as activators and/or fuel for energy dissipation via UCP1 (Schreiber et al., 2017; Shin et al., 2017; Chitraju et al., 2020). Aside from WAT lipolysis, other sources of FFAs can be derived through lipoprotein lipase (LPL)-mediated triglyceride-rich lipoprotein (TRL) degradation. This pathway liberates FFAs into the vascular lumen. In addition, a recent study demonstrates that TRL remnants can be fully incorporated into endothelial cells, followed by lysosomal acid lipase (LAL)-induced FFA release in the cells. Released FFAs will be further uptaken by brown adipocytes to induce thermogenesis (Bartelt et al., 2017; Lynes et al., 2017; Singh et al., 2018; Fischer et al., 2021).

Inhibition of intracellular lipolysis and loss of TG synthesis enzymes inside brown adipocytes substantially promotes the import of extracellular FFAs upon thermogenesis demand. However, the mechanism through which circulating FFAs produced from WAT can be imported into BAT has not been addressed yet. The possible pathway of how cold exposure induces FFA uptake in brown fat might be mediated through increasing in LPL activity as well as various transporters such as fatty acid transport protein (FATP), FABP, and the scavenger receptor CD36 (Klingenspor et al., 1996; Wu et al., 2006; Anderson et al., 2015; Shu et al., 2017). The circulating lipid chaperone called A-FABP (or known as FABP4) has been reported to facilitate FFA import from WAT into BAT (Shu et al., 2017). FABP is a carrier protein for FFAs not only derived from extracellular lipolysis but also from intracellular lipolysis (Mita et al., 2015). It was reported that FABP-FFA complex could be internalized through endocytosis (Shrestha et al., 2018; Villeneuve et al., 2018). The loss of the integral transmembrane transporter FATP1 in murine brown fat also marks attenuation in dissipating metabolic energy as heat (Wu et al., 2006).

The crosstalk between WAT and BAT in terms of the compensatory role as well as synergistic interaction for allowing maximal stimulation of adaptive thermogenesis provides evidences for a fine-tuned lipolysis in the activation of thermogenesis (Figure 5). The flexibility of UCP-1 activation through catecholamines-induced WAT lipid breakdown indicates an alternative pathway to increase energy combustion. Taken together, the concept that white adipocytes lipolysis permit maximal stimulation of energy expenditure provides significant implications for boosting energy expenditure and the treatment of human obesity.

Conclusion and Perspective

Adipose lipolysis not only provides metabolic fuel for the whole body, but also produces products that act as signaling molecules and transcriptional modulators, which are essential for preserving adipocyte function. Aberrant lipolysis, both excessive and impaired lipolysis, is tightly linked to severe metabolic disorders such as obesity, T2D, non-alcoholic fatty liver disease and cancer cachexia. Thus, lipolysis is under tight control involving extracellular circulating hormones, local autocrine/paracrine factors, as well as delicate intracellular interplays. Intricate cooperation of these regulatory signals leads to a fine-tuning of lipolysis in adipocytes that are essential for energy homeostasis. The existence of these distinct pathways will undoubtedly influence the approach to develop therapeutic treatments that target specific components of the lipolytic signaling pathway. Our understanding of these pathways is still in its infancy and many questions remain. Generally, while stimulation of adipocyte Gs-coupled GPCRs results in cAMP-mediated activation of PKA, resulting in the phosphorylation and activation of HSL and enhanced lipolysis, activation of adipocyte Gi-coupled receptors exerts an antilipolytic effect via decrease in cAMP levels. This counter-regulation between G protein-coupled receptors suggests a system evolved for defending a minimal but sufficient cAMP signaling capacity while protecting against excess activation. Intracellularly, a variety of negative feedback loops targeting key components of the signaling pathways leads to further multi-layered fine-tuning of lipolysis, which exemplifies the complexity and importance of feedback controls. A common feature of these regulations is that they interfere with assembly/composition of signalosome complexes (critical nodes) via protein-protein interactions. However, we are just beginning to appreciate these fine-tuning. Additional complex feedback loops and the coordinated interplays with lipolytic signalosomes as well as lipolytic machinery require further investigation. A comprehensive understanding of lipolytic regulatory factors remains a challenge and will require unbiased extensive research, such as CRISPR-based genome-wide screen, to identify new components in the signaling networks and to map their connectivity.

Of note, both the stimulation and inhibition of adipocyte lipolysis have been demonstrated to improve metabolic health. As such, tailored therapeutic strategies depend on subject metabolic context. In thermogenetic brown and brite adipocytes, the canonical UCP1-mediated thermogenesis is activated and fueled by FFAs to produce heat for body temperature maintenance. Pro-lipolytic factors are potential activators of thermogenesis in brown and brite adipocytes. It must be mentioned that while the mobilization of fatty acid supply is required for thermogenesis, this actually tends to raise fatty acid metabolite levels and reduces insulin sensitivity. It is only when the increased fatty acid oxidation capacity or reesterification cycle exceeds the mobilization that consequentially fatty acid metabolite levels fall and insulin sensitivity improves. Moreover, the synergic effects of white adipocytes lipolysis with brown fat thermogenesis activation in permitting maximal stimulation of energy expenditure should be kept in mind when developing novel interventions to boost thermogenesis and treat obesity-related metabolic diseases.

Author Contributions

YL conceived, structured, drafted, and edited the review. ML drafted the lipolysis pathway part and created Figures 1, 2. AW drafted the lipolysis in the health and disease section. DN drafted the thermogenesis part and created Figures 3–5. YW contributed to the lipophagy section. YL drafted the rest parts, with input from ZL for the brake parts. All authors read, edited, and approved the manuscript.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

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Funding

This work was supported by grants to YL from the Deutsche Forschungsgemeinschaft (LI 3716/1-1 and 2-1) and Technical University Munich within the Open Access Publishing Funding Program.