Adipose tissue (AT) is composed not only of adipocytes but also of macrophages, endothelial cells and preadipocytes [12]. Mesenchymal stem cells (MSCs) are the major source of adipocytes, and they initially differentiate into preadipocytes that are committed to the adipogenic lineage. Consequently, preadipocytes give rise to enlarged mature adipocytes that can synthesize lipid droplets, secrete specific adipocyte factors, and regulate energy metabolism. The differentiation of adipocytes from MSCs is believed to play a vital role in maintaining the adipose tissue homeostasis. The endothelial cells, which line blood vessels, are stimulated by adipokines and can cause unhealthy fat tissue expansion by crosstalking with adipocytes [13]. Macrophages resident in the adipose tissue are distinguished by two subtypes, the M1-like and M2-like macrophages. The M1 macrophages produce pro-inflammatory cytokines, such as Tumor Necrosis Factor (TNF)-α, interleukin IL-6 and MCP-1, which contribute to the development of insulin resistance. On the other hand, M2 macrophages are anti-inflammatory, are involved in the maintenance of tissue homeostasis, and are typically present in the adipose tissue of non-obese individuals. The imbalance between M1 and M2 macrophages has been found to be responsible for chronic inflammatory milieu in adipose tissue and insulin sensitivity. In lean individuals, macrophages dispersed throughout adipose tissues are predominantly polarized toward the M2 phenotype, while in obese individuals, an elevated number of infiltrating M1 subtype macrophages are present [14]. Different mechanisms control adipocyte differentiation and macrophage differentiation and infiltration. AMPK and PPAR pathways and the decrease of inflammatory response are the main targets of the anti-obesity strategies (Figure 1). 2.1. The Activation of AMPK PathwayThe AMP-activated protein kinase (AMPK) pathway is a crucial energy sensor that regulates energy metabolism in multiple tissues. AMPK is a conserved, ubiquitously expressed, heterotrimeric serine/threonine kinase whose short-term activation has multiple beneficial metabolic effects. Different subunits take part in the complex, which plays critical roles in regulating growth and reprogramming the metabolism [15].The alpha subunit can phosphorylate itself in the Thr172 position or self-inhibit. The beta subunit has a glycogen-binding domain (GBD) and a domain that is able to modify its conformation by favoring the binding of the alpha subunit to the gamma. In the gamma subunit there are four AMP/ATP allosteric binding sites that regulate the activity of the complex itself. When the intracellular energy decreases, the AMP level is very high compared to ATP and ADP levels. In this condition, the AMP binds a cystathionine-beta-synthase (CBS) domain present in the gamma subunit, inducing a change in its conformation which in turn promotes the self-phosphorylation of the alpha subunits [16]. Active AMPK binds other kinases, such as calcium/calmodulin-dependent protein kinases (CaMKs) and Transformation Growth Factor (TGF)-β activated kinase-1 (TAK1).Furthermore, AMPK is involved in the cellular processes including autophagy and cell polarization [15]. It is activated when modest decreases in ATP production result in relative increases in AMP or ADP concentration. In response, AMPK promotes catabolic pathways to generate more ATP to inhibit anabolic pathways. In brown adipocytes the activation of AMPK signaling stimulates thermogenesis. This mechanism corresponds to fat burning, which produces an increase in temperature, instead of ATP, through thermogenin or an uncoupling protein1 (UCP1) and, in turn, induces the increase in energy expenditure, leading to weight loss. However, it should be taken into consideration that the energy consumption, due to thermogenesis, represents only 10% of total energy expenditure. Furthermore, the subunit AMPKα appears to regulate the browning of white adipocytes, increasing the energy expenditure [17,18]. Activation of AMPK has been proposed as an attractive strategy for the treatment of obesity and its complications. However, some careful consideration must be taken since, in an animal model, the chronic and non-specific tissue activation of the AMPKγ2 subunit resulted in inducing hyperautophagy and dysglycemia [19]. Some nutraceuticals present in herbal extracts resulted in an ability to activate the AMPK pathway. The characterization of active substances in extracts from anti-obesity plants revealed that they reduce the absorption of glucose and, at the same time, increase its energy consumption by acting on the AMPK pathway. The Gelidium species, a red alga, has been found to act on the AMPK-PR domain-containing the 16-uncoupling protein-1 pathway (PRDM16-UCP1), with a healthy effect in obese people. Moreover, in a mice model of obesity, an extract of the red alga was found to enhance the effect of Orlistat, a known lipase inhibitor [20,21]. An extract from Paullinia cupana seeds has been found to reduce obesity by inducing the overexpression of the UCP-1 via activation of AMPK, too [18,22]. Thermogenesis can also be induced when white adipocytes differentiate in beige via a browning mechanism. Polyphenols from Zingiber officinale have been found to be able to promote adipocyte browning in an animal model [23]. WAT browning is regulated by several genes, such as UCP1, PR domain containing 16 (PRDM16), cell death activator CIDE-A and PGC-1a, whose expression is under the control of the AMPK and sirtuin 1 (SIRT1) protein [24].Panax ginseng has been shown to contain the Ginsenoside Rg1, which is able to promote browning, by inducing UCP1 expression and by modulating the transcription factor PRDM16 and PGC1α [25,26]. However, this modulation depends on Irisin, an adipomyokine, which promotes WAT browning, acting both in adipose and muscle tissue [27,28]. Other plants have been found to induce heat dissipation, and among them the root extract from Pueraria montana var. lobata has been shown to stimulate thermogenesis in BrAs; however, other species of Puerariae have also been described as having a similar activity [29,30].Many natural compounds, such as Capsaicin, Curcumin, polyphenols found in cranberry and flavonoids in rose, have been proposed as adipocyte browning agents [31]. Mostly used are nutraceutical compositions of extracts, such as a composition of radix from Angelicae sinensis and rhizome from Zingiber officinale. Z. officinale has also been widely reported to activate the AMPK signalling pathway [32,33]. A similar effect was observed using an ethanolic extract of Scutellaria baicalensis; in this case, however, the biological effect relates to the wingless integrated protein (WNT)/β-catenin pathway. Baicalin, and Baicalein, have been found to be active against metabolic syndrome by activation and upregulation of AMPK and PPARγ [34,35]. The AMPK pathway is also stimulated by Iris rossii, which inhibits lipid accumulation; its extract, containing mangiferin, was also found to inhibit preadipocyte differentiation [36,37]. Salvia hispanica, also known as chia, extracts act on glucose metabolism by increasing AMPK mRNA [38,39].However, the AMPK subunits are differentially and tissue-specifically expressed, and AMPK activators (agonists) fail to function over a long time due to their non-specific activity [40] (Table 1).Moreover, the AMPK pathway is related to the fat mass and obesity-associated (FTO) gene, which regulates the lipid accumulation in skeletal muscle via a N6-Methyladenine demethylation mechanism [41]. This type of demethylation is dependent on epigenetic changes [42]. However, the FTO gene has been also associated with genetic polymorphisms that affect the individual response to anti-obesity drugs [43].An extract from Angelica sinensis and two extracts from Solanum melogena and S. aethipicum, respectively, have been shown to contribute to reducing obesity via modulation of the FTO gene [44,45,46,47] (Table 2). 2.2. The Inhibition of the PPAR PathwayThe peroxisome proliferator-activated receptors (PPARs) are transcription factors, grouped within the ligand-inducible nuclear hormone receptor superfamily, that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Three major isoforms, PPARα (or NR1C1), PPARβ/δ (or NR1C2), and PPARγ (or NR1C3), are implicated in the control of lipid metabolism, and they are also considered insulin sensors [48]. The different PPAR subunits are expressed in many tissues, including adipose, liver, muscle, and endothelial tissues. PPARα mainly influences fatty acid metabolism, and its activation lowers lipid levels, while PPARγ is mostly involved in the regulation of adipogenesis, the energy balance, and lipid biosynthesis, and its activation affects insulin resistance [49]. PPARβ/δ participates in fatty acid oxidation, mainly in skeletal and cardiac muscles, and regulates blood glucose and cholesterol levels. PPARs can also form heterodimers with retinoid X receptor (RXR) modulating the expression of genes involved in lipid metabolism, adipogenesis, maintenance of metabolic homeostasis, and inflammation. Activated cytoplasmic PPARs can translocate into the nucleus, where they bind DNA responsive elements, either as a homodimer or heterodimer complexed with retinoid acid receptor (RXR), and each transcriptional complex needs coactivators [50,51]. Moreover, the PPAR transcriptional activity can be modulated through a nongenomic cross talk with phosphatases and kinases, including ERK1/2, p38-MAPK, PKA, PKC, AMPK, and GSK3 [48]. Moreover, the members of the PPAR family also modulate basic processes, such as proliferation, differentiation, and postnatal development [52].Activation of PPARγ by thiazolidinediones can reduce insulin resistance and hyperglycemia in type 2 diabetes, but these drugs can also cause weight gain. Apparently, a moderate reduction of PPARγ activity, observed in heterozygous PPAR γ-deficient mice or in the Pro12Ala polymorphism in human PPARγ protein, has been shown to prevent insulin resistance and obesity induced by a high-fat (HF) diet. Furthermore, it has been observed that a moderate reduction of PPARγ with an RXR antagonist or a PPARγ antagonist decreases triglyceride (TG) content in white adipose tissue, skeletal muscle and liver. However, severe reduction of PPARγ by treatment of heterozygous PPARγ-deficient mice with a RXR antagonist or a PPAR antagonist depletes white adipose tissue and markedly decreases leptin and adiponectin levels and energy dissipation, which increases TG content in skeletal muscle and liver, thereby leading to the re-emergence of insulin resistance [53].PPARγ is considered the master activator of adipogenesis, and its inhibition is a preferred strategy to screen anti-obesity agents. PPARγ inhibitors were shown to be useful in treating obesity, through the inhibition of expression of the adipose tissue genes, but the drugs can have undesirable effects in other tissues. PPARγ activators are used as insulin sensitizers in order to improve insulin resistance in diabetic patients. PPARγ comprises seven gene isoforms, among them PPARγ1 and PPARγ2, which are prevalently expressed in adipocytes where they are involved in lipid metabolism and cell differentiation, and in macrophages where they trigger the initial inflammatory response. PPARγ2, which is highly expressed in adipocytes, is the key fat-selective PPAR subtype, when it is phosphorylated at Ser273 promotes the lipogenesis; thus, it is a good target of the anti-obesity agents. Many studies showed that both isoforms, γ1 and γ2, are increased in obese people, whereas other studies have stated that only the γ2 isoform is increased. These inconsistent results could depend on the different gene expression regulation in different tissues and on post-translational modification of PPARγ isoforms. Ligands blocking the Ser273 phosphorylation in γ2 improve insulin resistance, whereas they induce weight gain; thus, they have been thoroughly studied [54]. Moreover, the block of Ser273 phosphorylation positively correlates with increased expression of the growth differentiation factor-3 (GDF-3), which is pro-inflammatory and promotes fat accumulation [55,56]. Several herbs have been described to act on PPAR pathway, among them the Spermacoce hispida, the seeds of which contain deacetylasperulosidic acid, an iridoid glycoside, acting on the PPARα and exhibiting an antihyperlipidemic activity [57]. The seeds of S. hispida significantly reduce the proliferation of mature adipocytes and decrease fat accumulation. A similar effect was shown by Rosa rugosa extract, rich in flavonoids, which reduces hypertriglyceridemia by acting as PPARα agonist [58,59]. The polymethoxyflavones (Sinensetin, nobiletin, 3,5,6,7,8,3′,4′-Heptamethoxyflavone, and Tangerine) from Citrus reticulata, ‘Chachi’ peels, exhibit biological effects similar to those observed for S. hispida and R. Rugosa; however, their mechanism of action is still to be elucidated [60,61]. Apparently, the Nigella sativa oil, containing a Thymoquinone, a PPARγ agonist, decreases the expression of TNF-α and adipokines in obese and overweight people [62,63]. Astragali membranaceus var. mongholicus increase thermogenesis via PPARγ activation [64,65]. Moreover, the tissue-specific expression of PPAR subunits account for healthy effects on short-time and failure on long-time and chronic use (Table 3).Some nutraceuticals have been shown to affect the PPAR signaling via different patterns; some herbal extracts modulate the CCAAT-enhancer-binding (C/EBP) alpha and beta subunits [66]. Ziziphus jujuba extract, containing apigenin, betulinic acid, and maslinic acid, was linked with a decrease of protein levels of PPARγ and C/EBPα. Ziziphus jujuba fructus has been shown to stimulate adipocyte browning with the effect of stimulating thermogenesis [67].The theobromine from Theobroma cacao beans reduces the expression of C/EBPα and in turn PPARγ, and the total effect is to stimulate thermogenesis via activating the β3-Adenergic receptor [68,69]. The isoimperatorin from Angelica dahuricae significantly increased mRNA expression, and protein production, of the C/EBPα and PPARγ [70,71]. A fraction of Bidens pilosa extract containing cytopiloyne has been found to reduce adipogenesis by inhibiting expression of PPARγ and C/EBP [72]. An extract, rich in alkaloids from Coptis chinensis, showed an anti-adipogenic activity linked to its effect to downregulate C/EBP-α and PPAR-γ factors [73]. An extract, rich in terpenoids from the Isodon adenantha, has been shown to inhibit adipogenesis in 3T3-L1 and mouse embryonic fibroblast models. Adenanthin, an ent-Kaurane diterpenoid, decreases inflammation and ROS by inhibiting the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signalling [74]. NF-κB can interact with the PPARγ subunit, promoting cell inflammation [75] (Table 4).Other herbal extracts indirectly act on the PPAR pathway through very different biological targets. An extract from Barringtonia acutangula inhibits the 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1); however, this enzyme has been shown to decrease the expression of PPARγ and increase that of PPARα [76].The leaf extract from Melissa officinalis reduces the mRNA levels of genes involved in lipogenesis, such as the fatty acid synthase (FAS), stearoyl-CoA desaturase 1 (SCD1), and sterol regulatory element-binding protein 1c (SREBP-1c) [77]. The extract of M. officinalis acts via the C/EBPα and the PPAR pathway [78]. The extract ALS-L1023 confirmed that the effects of nutraceuticals present in this herb reduce weight gain. This extract, which increases the level of AMPKα2, alleviates abdominal obesity and insulin resistance in C57BL/6J mice by increasing the mRNA levels of carnitine palmitoyltransferase 1 (CPT-1), medium-chain acyl-coenzyme A dehydrogenase (MCAD) [79]. Thus, the effects of the herbs depend on the different nutraceuticals obtained according to solvents used to prepare the extracts.An extract from Momordica charantia seed exhibits lipid-lowering activity by reducing the insulin-stimulated IRS-1 tyrosine phosphorylation; however, IRS-1 is linked to PPARγ subunit [80]. In particular the IRS-1 variant (C189T > G) was associated with post-weaning body weight and body weight gains [81] (Table 5).Overall, these results confirm that many mechanisms regulate, directly or indirectly, the cellular fat mass. Moreover, these mechanisms interfere each other, confirming the difficulties in treating obesity over a long term due to a complex pattern of inter and intra-cellular signals (Figure 2). 2.3. Reducing InflammationChronic inflammation impacts the fat storage in adipose tissue, resulting in free fatty acid and triglyceride excesses in bloodstream and induction of insulin resistance in muscle and liver. Adipocytes crosstalk with macrophages, which are able to infiltrate fat masses, leading to obesity, inducing ectopic fat deposition in these tissues. [82]. Enlarged adipocytes, together with the infiltrated macrophages, act in a synergistic manner to cause aberrant production of pro-inflammatory molecules including inducible nitric oxide, cytokines such as TNF-α, interleukine-6 (IL-6) and the chemokine monocyte chemoattractant protein-1 (MCP-1). Both TNF-α and IL-6 can lead to insulin resistance by triggering different key steps in the insulin signalling pathway. However, several different factors can induce inflammation, especially chronic low-grade inflammation, and cause obesity. Proinflammatory pathways, free radical increase, and epigenetic modulation of some genes, such as methylation of the IGF-1 encoding gene, can affect the growth of fat masses. These pathways involve different protein complexes and metabolites, for example by activation of inflammasome, kinases, and related coactivators. 2.3.1. Activation of InflammosomeThe inflammasome complex is a multimer consisting of pattern-recognition receptors (PRR); each one is a monomer sensor that responds to inflammatory stimuli; after stimulation, they oligomerize and form a pro-caspase-1 activation platform. Several members of PRRs have been confirmed to form inflammasomes; among these are the NLRP3, a member of the leucine-rich repeat (LRR)-containing proteins (NLRP) [83]. The NLRP3 inflammasome is activated by diverse stimuli and multiple molecular and cellular events, including ionic flux, mitochondrial dysfunction, such as the production of reactive oxygen species (ROS), and lysosomal damage. In obese individuals, stressed adipocyte and macrophage cells respond by activating the NLRP3 inflammasome, which modulates the MAPK signalling or decreases the expression of proinflammatory cytokines such as TNF-α and IL-6, and could improve inflammation-related diseases. However, aberrant activation of NLRP3 inflammasome has been related to obesity. An extract from Coptidis rhizoma has been shown to have successful effects in obese rats affected by glomerulopathy [84]. Subsequent studies on Coptidis rhizome, used in oriental medicine, showed that its anti-inflammatory activity is linked to berberine derivatives [85]. The analyses of its biological effects revealed that it inhibits dysregulated NLRP3 inflammasome complex. Morus alba is traditionally used as an antioxidant and anti-inflammatory agent, and it shows an anti-obesity effect [86,87]. Some research describes the effects of these herbs on TNF-α and IL-6. Recently, administration to HFD-induced obese C57BL/6 mice of an extract of leaves from M. alba, fermented with fungus Cordyceps militaris, has been shown to prevent fat accumulation through the inhibition of lipogenesis and stimulation of lipolysis [88]. However, the fermented powder can contain by-products from fungal fermentation, involved in the mechanism of action, whose secondary effects are still unknown (Table 6). 2.3.2. TNF-α Pathway

The activation of immune cells, such as macrophages, is stimulated by secretion of TNF-α which, in paracrine and autocrine manner, regulates a number of critical cell functions including cell proliferation, survival, differentiation, and apoptosis. Macrophages are the major producers of TNF-α and interestingly are also highly responsive to TNF-α itself. Aberrant TNF-α production and TNF receptor signaling have been associated with the pathogenesis of several diseases, including rheumatoid arthritis, Crohn’s disease, atherosclerosis, psoriasis, sepsis, diabetes, and obesity.

Curcuma longa rhizome contains the curcuminoids, which reduce inflammation and improve obesity. Curcuminoids have also been proposed as browning promoting agents, leading to weight loss by suppressing chronic inflammation in white adipocytes through the modulation of TNF-α [89]. Unfortunately, curcuminoids are effective at high dosages which induce undesirable effects, and therefore some Food Safety Agencies have decreased the amounts allowed in the formulation of food supplements.Nelumbo nucifera contains quercetin-3-O-ß-glucuronide (Q3GA) and quercetin; experiments conducted in vitro confirmed that quercetin and Q3GA affect lipid metabolism by promoting triglyceride degradation through inhibition of the cAMP pathway which in turn blocks the TNF-α pathway [90].Garcinia mangostana contains xanthones, which act on inflammatory and anti-inflammatory pathways [91].The total fraction of soluble compounds present in the aqueous extract of the Cichorium intybus was added to the diet of obese diabetic mice, reducing obesity [92] (Table 7). 2.3.3. MAPK SignallingMAPK pathways are signaling modules that transduce extracellular and intracellular signals to regulatory networks within the cell via the phosphorylation of key protein targets. Each cascade is activated by extracellular signals which lead to the activation of the MAPK, which in turn become substrates of a second specific kinase (MAPKKK). A subsequent third kinase (MAPKK) completes the signal transduction. However, three different families of kinases, ERKs, JNKs, and p38/SAPKs, respond to different extracellular signals of stress [93]. Although they show some differences in the modulation of different genes, all these pathways can induce obesity. We report that some herbal extracts, having the MAPK pathways as target, are able to reduce obesity.A Cordyceps militaris extract, containing cordycepin, has been shown to prevent overweight, fat accumulation, liver hypertrophy, and lowered triglyceride levels by modulating gene expression. The targeted genes were related to the insulin signalling pathway, insulin resistance, the MARK signalling pathway and the PI3K–Akt signalling pathway [94].Sorghum bicolor has been found to contain a fraction extract rich in 3-Deoxyanthocyanines; the extract reduces inflammation and the ROS cascade and promotes the gut Bacteroidetes which are highly present in lean individuals [95]. However, S. bicolor has been shown to activate the MAPK cascade, suggesting an intracellular mechanism to reduce lipid accumulation [96] (Table 8). 2.3.4. Production Control and Scavenging of Free RadicalsHigh free radical levels arise after eating and they increase the sense of appetite, so that antioxidants could be useful to reduce obesity. There is evidence that excessive ROS contribute to develop metabolic disorders leading to inflammation and obesity [97]. Antioxidant natural extracts could ameliorate inflammation, especially low-chronic grade one, and reduce appetite by controlling the level of ROS and their activity [98]. However, in spite of a large number of natural antioxidant extracts, only few of them seem to be effective to treat obesity. A natural ent-Kaurane diterpenoid, Eriocalyxin B, extracted from Isodon eriocalyx, showing anti-inflammatory activity, was found inhibits adipogenesis in 3T3-L1 adipocytes [99]. Moreover, the role played by nitric oxide (NO) reactive species in obesity, received similar attention. A raw non-polar extract from the leaves of Angelica keiskei improves obesity by reducing the amount of NO [100,101]. NO has proposed as gaseous signalling molecule linked to onset of obesity by inducing low-chronic grade inflammation [102,103]. However, an extract rich in extract chalcones also decreases the TNF-α levels and in turn reduces insulin resistance [104,105] (Table 9). 2.4. Regulation of Lipase ActivityFatty acids are hydrolysed at gastrointestinal level and transported to liver where they are re-esterified into triglycerides and transported to other tissue by lipoproteins. Pancreatic lipase and Lipoprotein lipase are responsible for the conversion of fats into monoglyceride and free fatty acids, which can across the enterocytes. The fat can reach the adipocytes, where it is accumulated causing the enlargement of these cells. Thus, the inhibition of lipases and related enzymes is another strategy to reduce adipose tissue hyperplasia [106]. Delaying fat adsorption or inhibiting fat digestion, by suppressing lipase activity, decrease obesity as shown by the Orlistat, but unfortunately this medicine has secondary undesired effects [107]. The research of lipase inhibitors has been investigated in 76 plant species resulting able to inhibit lipases. Among them we report Hieracium and Pilosella, which are two strictly related species diffused as popular remedies of obesity [108].Taraxacum officinale, traditionally known as Dandelion, contains several active compounds such as taraxasterol, chlorogenic acid, α-amyrin, which exhibit anti-inflammatory effects, inhibiting NO synthase and COX-2 protein expression. Often, some nutraceuticals depending on extract fraction, in a dose dependent manner, inhibit pancreatic lipase and in turn improve fat deposition [109].Recently, an extract from Ginkgo biloba leaf was described to contain a novel pancreatic lipase inhibitor, but this is the only report describing its anti-lipolytic activity [110].Moreover, ginkgolides are considered the most interesting compounds improving insulin resistance via Toll-like receptors signalling cascades [111].Prunus armeniaca leaves contain phenolic compounds that show several activities, all of them have positive effect on reducing obesity, in particular inhibiting the pancreatic lipase [112].Rhus verniciflua extracts reduce appetite. An extract from this plant was found to act on dopaminergic cells, to inhibit the activity of glycolytic enzymes, such as alpha-glucosidase, and consequently to suppress obesity in an animal model [113]. A leaf extract from Rhus verniciflua, rich in quercetin derivatives, was found lowering SREBP-1 and triglyceride levels. It has been observed that PPARs and SREBP-1 interaction reduces lipid deposition [114]. The extract suppresses obesity in high fat diet-induced obese mice [115]. Rhus verniciflua ethanol extract revealed a strong alpha-glucosidase inhibitory activity, which was able to reduce weight gain and improve insulin resistance [116]. Furthermore, a R. verniciflua Stockes extract, used as traditional medicine in Korea, exhibited lipid-lowering effects by lowering SREBP-1 and triglyceride levels, and promoting the activation of PPARs and AMPK in an in vitro model of non-alcoholic fatty liver disease [117].An extract from Morus nigra acts on hormone-sensitive lipase showing anti-obesity effects [87,118] (Table 10).