The Impacts of Herbal Medicines and Natural Products on Regulating the Hepatic Lipid Metabolism

The dysregulation of hepatic lipid metabolism is one of the hallmarks in many liver diseases including alcoholic liver diseases (ALD) and non-alcoholic fatty liver diseases (NAFLD). Hepatic inflammation, lipoperoxidative stress as well as the imbalance between lipid availability and lipid disposal, are direct causes of liver steatosis. The application of herbal medicines with anti-oxidative stress and lipid-balancing properties has been extensively attempted as pharmaceutical intervention for liver disorders in experimental and clinical studies. Although the molecular mechanisms underlying their hepatoprotective effects warrant further exploration, increasing evidence demonstrated that many herbal medicines are involved in regulating lipid accumulation processes including hepatic lipolytic and lipogenic pathways, such as mitochondrial and peroxisomal β-oxidation, the secretion of very low density lipoprotein (VLDL), the non-esterified fatty acid (NEFA) uptake, and some vital hepatic lipogenic enzymes. Therefore, in this review, the pathways or crucial mediators participated in the dysregulation of hepatic lipid metabolism are systematically summarized, followed by the current evidences and advances in the positive impacts of herbal medicines and natural products on the lipid metabolism pathways are detailed. Furthermore, several herbal formulas, herbs or herbal derivatives, such as Erchen Dection, Danshen, resveratrol, and berberine, which have been extensively studied for their promising potential in mediating lipid metabolism, are particularly highlighted in this review.


INTRODUCTION
Generally, liver regulates lipid metabolism by three major processes: (1) uptake free fatty acids from circulation, and de novo fatty acid synthesis (FAS); (2) lipid storage, including converting fatty acids into triglyceride (TG) and other lipid droplets, which are subsequently exported to adipose tissue or stored in liver; and (3) lipid consumption, including lipolysis, b-oxidation, and the generation of lipoproteins (Reddy and Rao, 2006;Musso et al., 2009;Ponziani et al., 2015;Mato et al., 2019). These processes are presented in Figure 1. Correct control of lipid level is critical for cellular and organismal homeostasis, while interferences with the lipogenic pathways are accompanied with a variety of metabolic syndromes. The disorders of lipid metabolism, such as decreased b-oxidation, enhanced lipolysis, and secretion of very low-density lipoprotein (VLDL), as well as altered pathways involved in the FAS, drive the accumulation of lipid droplets into the hepatocytes, eventually leading to the development of hepatic steatosis, which is a common pathological feature in various liver diseases (Reddy and Rao, 2006;Nguyen et al., 2008;Tessari et al., 2009;Perla et al., 2017).
The most prevalent liver diseases resulting from lipid metabolism disorder are alcoholic and non-alcoholic fatty liver diseases. Except difference in alcohol consumption, alcoholic and non-alcoholic fatty liver diseases show similar pathological process, which is characterized by long-term excessive fat accumulation in the liver (Younossi, 2019). They represent a wide range of liver injury, from simple fatty liver through steatosis with necrosis and inflammation to fibrosis and cirrhosis (Lomonaco et al., 2013;Heeboll et al., 2018). In particular, non-alcoholic fatty liver diseases (NAFLD), as the metabolic diseases induced by obesity and type 2 diabetes mellitus, are the second leading causes of death globally, becoming a heavy economic burden in many countries due to the high prevalence (Albhaisi and Sanyal, 2018;Al-Dayyat et al., 2018). Since inordinate lipid metabolism is intensively involved in fatty liver diseases progression, reducing lipid accumulation is a major target of development of pharmaceutical agents for various liver diseases (Ipsen et al., 2018). Simvastatin has been used as lipidlowing drug in patients with hyperlipidemia (Aronow, 2006). However, it shows side effects, such as constipation headaches, nausea, myopathy, elevated blood sugar, and even liver damage. As a matter of fact, there is currently no satisfying therapeutic drug for fatty liver diseases (Issa et al., 2018;Moctezuma-Velazquez, 2018).
Over the past decades, due to the positive efficacy and minimal side effects, herbal medicines, and natural products have obtained increasing attention as alternative therapeutic agents for liver disorders and dyslipidemia (Xiao et al., 2013;Yao et al., 2016;Liu Q. et al., 2017). Growing evidence from preclinical studies suggests that many herbs and isolated compounds could inhibit the progression of hepatic steatosis (Dong et al., 2012;Liu Z. L. et al., 2013). A variety of mechanisms have been demonstrated to be implicated in preventing hepatic steatosis, including reducing lipogenesis, enhancing b-oxidation, increasing insulin sensitivity, suppressing oxidative stress, and inhibiting activation of inflammatory pathways (Dong et al., 2012;Yao et al., 2016). In recent studies, sterol regulatory elementbinding protein 1c (SREBP-1c), peroxisome proliferator activated receptor a (PPARa), AMPactivated protein kinase (AMPK) and sirtuin 1 (SIRT1) signaling pathways have been highlighted as crucial molecular targets of action mechanisms by which herbal medicines regulate hepatic lipid metabolism (Liu Z. L. et al., 2013). In this review, herbal medicines involved in regulating hepatic lipolytic and lipogenic pathways, such as mitochondrial and peroxisomal b-oxidation, the secretion of very low-density lipoprotein (VLDL), the nonesterified fatty acid (NEFA) uptake, and some vital hepatic lipogenic enzymes are summarized. Current clinical evidences and meta-analysis in the positive impacts of herbal medicines on the hepatic lipid metabolism pathways have also been reviewed. Furthermore, several herbal formulae, herbs or herbal derivatives, such as Erchen Dection, Danshen, resveratrol, and berberine which have been extensively studied for their promising potential in mediating lipid metabolism, are particularly highlighted in this review. This review aims to update and summarize current evidence from laboratory and clinic studies to provide alternative and complementary medical therapies with the regulatory property of hepatic lipid metabolism to current pharmaceuticals for the treatment of liver diseases.

HERBAL MEDICINES AND NATURAL PRODUCTS REGULATE ON THE HEPATIC LIPID METABOLISM PATHWAYS
Increasing evidence indicated that many herbs, natural products, and their derived compounds could inhibit the progression of hepatic steatosis. A variety of mechanisms have been demonstrated to be implicated in preventing hepatic steatosis and modulating lipid metabolism by herbs, including anti-oxidative stress, anti-inflammation, reducing hepatocyte fatty acid uptake and trafficking, reducing hepatic de novo lipogenesis, increasing lipolysis, induction of lipophagy, enhancing fatty acid b-oxidation. In particular, SREBP-1c, PPARa, AMPK, and SIRT1 signaling pathways have been highlighted as crucial molecular targets of action mechanisms by which herbal medicines regulate hepatic lipid metabolism. In Table 1, we reviewed the effects and mechanisms of herbs and some natural products on fatty liver diseases from recent studies. In the following section, we will discuss herbs that attenuate hepatic steatosis via reducing hepatocyte fatty acid uptake and trafficking, reducing hepatic de novo lipogenesis, increasing lipolysis, induction of lipophagy, and enhancing fatty acid b-oxidation in detail.

Reducing Hepatocyte Fatty Acid Uptake and Trafficking
Nonesterified fatty acids (NEFAs) and glycerol are generated and released from adipose tissue via lipolysis (Kawano and Cohen, 2013). Then NEFAs enter into hepatocytes principally through CD36, and fatty acid transports (FATPs) (Kawano and Cohen, 2013). Several mediators have been demonstrated to play a role in regulating CD36 and FATPs, such as pregnane X receptor (PXR), which impact the hepatocyte fatty acid uptake. Increasing evidence has shown that a variety of herbs and natural compounds attenuate hepatic steatosis via modulating genes for fatty acid uptake.
Scutellarin, one of the Traditional Chinese Medicines (TCM) used for liver diseases and diabetes, was found to reduce insulindependent lipid accumulation and the mRNA expression of CD36 in HepG2 cells-treated with palmitic acid (Luan et al., 2019). Several other TCM and isolated compounds, babaodan, licorice extract, polyphenol-enriched fraction from Herba Erigerontis, and magnesium lithospermate B, reduced hepatic CD36 expression in mice fed with High Fat Diet (HFD) (Wu and Wang, 2012;Wang et al., 2016;Sheng et al., 2019). Dansameum reduced the expression level of CD36 in liver of apolipoprotein E-Knockout mice with NAFLD (Ahn et al., 2019). In another mice model of NAFLD induced by high-fat and high-cholesterol diet, gypenosides which are a type of TCM extracted from plants downregulated CD36 level in the liver, alleviating the progression of hepatic steatosis . Berberine attenuated fat accumulation in the liver partially via suppressing the expression of FATP gene in HFD-fed mice .

Reducing Hepatic De Novo Lipogenesis
De novo lipogenesis in the liver is tightly controlled by metabolic hormones such as insulin, and glucose level . In the normal physiological status, high level of glucose promotes the secretion of insulin, activates carbohydrate-responsive elementbinding protein (ChREBP), and meanwhile, provides substrate to facilitate lipogenesis in the liver . In terms of insulin, it activates sterol regulatory element-binding protein 1c (SREBP-1c) to up-regulate lipogenic enzymes, and then promotes de novo lipogenesis (Eissing et al., 2013;Chao et al., 2019). Figure 2 shows the overview of lipogenesis in hepatocytes. Herbs and isolated natural compounds have been demonstrated by animal studies and in vitro studies to alleviate hepatic steatosis by ChREBP pathway and insulin-SREBP-1c pathway, as well as other factors, such as AMPK, PPARg, SIRT1, inflammatory cytokines, immuno-modulation, and microRNAs. We summarized medicinal herbs and isolated natural compounds from recent literatures with the effects of reducing hepatic lipogenesis in Table 2 and discussed some representative studies in detail as following.

Rat model of HFD-induced NAFLD
Attenuated phenotypic characteristics of NAFLD By affecting insulin resistance and lipid metabolism related pathways (e.g., PI3K-Akt, AMPK); activating cholesterol secretio; increasing serum thyroid hormone levels, improving beta-oxidation (via modulation of TR beta 1 and CPT1A expression), metabolism and transport (through modulation of SREBP-1c, ACSL and ApoB100 expression) of fatty acid. Yang et al., 2017;Zhu et al., 2017) Chinese herb extract, QSHX (Bupleurum falcatum, Salvia miltiorrhiza, rhubarb, lotus leaf, capillary Artemisia, rhizome polygoni cuspidate and gynostemma pentaphyllum) High-fat and high-sugar dietinduced NAFLD in rat ↓Body weight, liver index, and serum levels of AST, ALT and TG; and increased the serum level of adiponectin Promoting the expression of HMW APN and DsbA-L, which may have been induced by inhibiting the activation and expression of FOXO1 in adipocytes  Qushi Huayu Decoction (Herba Artemisiae capillaris, Polygonum cuspidatum, Hypericum japonicum Thunb, Gardenia, and Rhizoma Curcumae Longae) NAFLD rats Attenuated phenotypic characteristics of NAFLD ↑Hepatic anti-oxidative mechanism, ↓hepatic lipid synthesis, and promoted the regulatory T cell inducing microbiota in the gut.  lipogenesis via FXR-dependent pathway. It decreased hepatic levels of SREBP-1c, FAS, ACC1 and SCD1, and promoted lipid metabolism via inducing PPAR a, CPT1 a, ACADS, and LPL (Meng et al., 2017). In an apolipoprotein E-knockout mice model, Dansameum (Salvia miltiorrhiza root), a kind of Korean polyherbal medicine, reduced hepatic lipogenesis, and inflammation via regulating PPAR-g, SREBP-1c, FAS, ACC1, and CD36 (Ahn et al., 2019). Dangguiliuhuang Decoction, a TCM formula composed of radix rehmanniae (root of Rehmannia Glutinosa), angelica (Angelica acutiloba Siebold et Zucc.), Coptis chinensis Franch., Radix Rehmanniae Praeparata (Rehmannia root), Astragalus propinquus (the root of astragalus membranaceus), Chinese skullcap (Scutellaria baicalensis) and Phellodendron amurense (Phellodendron chinense Schneid.), is used for the treatment of autoimmune diseases and diabetes (Cao et al., 2017;Cao et al., 2018). In a study of ob/ob mice model, it normalized glucose and insulin level, diminished fat accumulation and lipogenesis, increased the expression of adiponectin, and promoted glucose uptake (Cao et al., 2017). It showed modulation abilities on inflammation and immune response. Dangguiliuhuang Decoction (composition as listed above) promoted the shift of pro-inflammatory to anti-inflammatory cytokines. Furthermore, it decreased T cells proliferation while increased regulatory T cells (Tregs) differentiation, reduced dendritic cells (DCs) maturation and secretion of IL-12p70 cytokine, decreased DCs-stimulated T cells proliferation, and promoted, the interaction of DCs with Tregs. In adipocytes and hepatocytes as well as DCs and T cells, Dangguiliuhuang Decoction treatment altered PI3K/Akt signaling pathway and increased PPAR-g expression, indicating the ameliorated glucose and lipid metabolism (Cao et al., 2017).
MicroRNA (miR), a small non-coding RNA molecule, has been recently demonstrated to play a role in mediating the antihepatic steatosis effects of natural compounds derived from herbs. Berberine reduced steatosis in MIHA and HepG2 cells by mechanism associating with up-regulation of miR-373, which decreased its mRNA level target gene AKT serine/threonine kinase 1 (AKT1), resulting in the suppression of AKT-mTOR-S6K signaling pathway in hepatocytes . Genipin reduced HFD-induced hyperlipidemia and hepatic lipid accumulation in mice via increasing the expression levels of miR-142a-5p, which bound to 3'untranslated region of SREBP-1c, thus leading to the inhibition of lipogenesis (Zhong et al., 2018).

Increasing Lipolysis
Lipolysis is the catabolic process of hydrolytic cleavage of ester bonds in TG, leading to the production of fatty acids and glycerol, which could be further utilized for b-oxidation and subsequent ATP generation (Lass et al., 2011). It predominantly occurs in adipose tissues, but also in the liver, with different physiological functions. Dietary fat is digested into the gut lymphatic system as chylomicrons, which arrives at the liver through the circulation and release NEFAs through lipolysis which mediated mainly by lipoprotein lipase (LPL) (Rui, 2014). Other lipolytic enzymes contributing to hepatic TG metabolism include adiponutrin/  (Sheng et al., 2011) Osthol Alcohol-induced fatty liver in mice Inhibit alcohol-induced fatty liver Anti-oxidation and suppression of TNF-a production (Sun et al., 2009) ↑ means increase and up-regulate and ↓ means decrease and down-regulate.
patatin-like phospholipase domain containing 3 (PNPLA3) (Kumashiro et al., 2013), lysosomal acid lipase (LAL) (Quiroga and Lehner, 2018), arylacetamide deacetylase (Lo et al., 2010), hepatic lipase (HL) (Chatterjee and Sparks, 2011) and some members of the carboxylesterase family. In adipose tissue, inhibition of lipolysis improves glucose metabolism and insulin sensitivity, whereas in liver tissue, increasing lipolysis facilitates the attenuation of hepatic steatosis. As far from now, limited herbs were found to show regulatory effect on hepatic lipolysis. Lavatera critica (Cornish mallow), a green leafy vegetable, attenuated hepatic lipid accumulation induced by HFD via reversing lipolysis genes acetyl-CoA carboxylase (Veeramani et al., 2017). Nitraria retusa (Forssk.) Asch. ethanolic extract modulated the lipolysis-lipogenesis balance in the liver of db/db mice (Veeramani et al., 2017). Caffeic acid upregulated the phosphorylation of AMPK and its primary downstream targeting enzyme, acetyl-CoA carboxylase, to promote the lipolysis in HepG2 cells with oleic acid administration (Liao et al., 2014). Polygonatum stenophyllum (PS) Maxim. rhizome showed efficacy on menopausal obesity by activating lipolysisrelated genes including hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) (Lee J. E. et al., 2016). Mulberry (Fructus Mori) water extracts promoted hepatic lipolysis and protected liver from steatosis in obesity (Peng et al., 2011). More herbs or natural compounds exerted effects on lipolysis in adipose tissues and attenuated hepatic steatosis via liver-adipose tissue crosstalk, which are not going to be discussed in detail here.

Induction of Lipophagy
In addition to lipolysis, lipid breakdown can also be accessed via lipophagy, a special kind of autophagy to degrade lipid droplets (Singh and Cuervo, 2012;Kounakis et al., 2019). It is a process that the double membrane wraps lipid droplets and sends them to lysosomes to form autolysosomes for degradation of excessive lipid droplets deposited in cells (Liu and Czaja, 2013;Ward et al., 2016). It plays a vital role in maintaining the cellular steady state.
During the early stage of NAFLD, lipophagy is activated in response to acute increase in lipid availability, thus reduce lipid deposition (Czaja, 2016;Ipsen et al., 2018). However, in the condition of such as long-lasting high fat dieting, hepatic lipophagy is impaired when lipids are sustained overwhelmed (Kwanten et al., 2014;Czaja, 2016;Ipsen et al., 2018). Growing evidence raised from recent studies indicate that lipophagy is partially suppressed in patients and animal models of NAFLD and restoring lipophagy may slow the progression of hepatic steatosis. Lipophagy could be activated by various approaches, such as mTOR and AMPK-targeting agents. Glycycoumarin, a representative of coumarin compounds isolated from licorice, mitigated hepatic steatosis partially through AMPK-mediated lipophagy in a murine model of NAFLD induced by MCD diet . Dioscin is a saponin extracted and isolated from Polygonatum zanlanscianense Pamp. It has been proposed as a healthcare product against hepatic fibrosis with remarkable ability to inhibit the expression of p-mTOR/mTOR level and sequentially promote autophagy (Xu et al., 2017). In another study, Bergamot polyphenol fraction prevents NAFLD via stimulation of lipophagy in cafeteria dietinduced rat model of metabolic syndrome. The increased levels of LC3 and Beclin 1, and concomitant reduction of SQSTM1/p62 proved the promoted lipophagy with the treatment of Bergamot polyphenol fraction (Parafati et al., 2015). Increasing number of herbs or natural products have been demonstrated to exert significant effects on regulating lipophagy in the liver. Current understanding of mechanisms associated with autophagy/ lipophagy of herbal medicines and natural products in preventing and treating NAFLD has been well reviewed in Zhang et al. (2018), which could be referred for further reading.
In alcoholic liver diseases (ALD), upon acute consumption of alcohol, lipophagy is activated in hepatocytes, serving as a defensive mechanism against injury to steatosis . However, it is impaired by chronic alcohol exposure, which is likely due to the activation of mTOR signaling and decreased lysosomal biogenesis in hepatocytes (Kounakis et al., 2019;. There are  growing number of herbs and natural products have been found to protect liver from injury induced by alcohol by mechanism of lipophagy stimulation. Corosolic acid, a compound derived from the leaves of Langertroemia speciosa L Pers., protected the liver from alcoholic-induced liver injury partially via restoring hepatic lipophagy due to mTORC1 suppression after AMPK activation (Guo et al., 2016). Another natural compound, quercitin, which is extensively found in many fruits and herbal plants, remarkably reversed the alcohol-induced blockade of TFEB nuclear localization, via restoring lysosome function and autophagic flux in livers of ethanol-fed C57BL6 mice . Salvianolic acid A, a phenolic carboxylic acid extracted from Salvia miltiorrhiza Bunge, reduced hepatic steatosis induced by alcohol administration in rats. The action mechanism is attributed to enhanced autophagosome-lysosome fusion after restoring lysosomal cathepsin activities . As a matter of fact, the field of lipophagy in liver diseases has yet to be fully developed. Its pathological role in different stages and circumstances of various liver disorders still needs to be revealed. Nevertheless, current studies concerning lipophagy have already provided new insights on lipid metabolism and energy homeostasis in the liver. It represents a promising path forward to the therapeutic of hepatic steatosis. Pharmaceutic agents including herbs, natural products or compounds targeting lipophagy in the liver deserve to be further investigated in future basic and clinic researches.

Enhancing Fatty Acid b-Oxidation
Fatty acid could be oxidized by b-oxidation, a-oxidation, omegaoxidation, and peroxisomal oxidation, among which b-oxidation is the major type occurring in the mitochondria matrix (Wanders et al., 2015). In b-oxidation, two carbon subunits from fatty acids are removed repeatedly until the fatty acid carbon chain is fully degraded to form acetyl-CoA, which is further oxidized to carbon dioxide and H 2 O in the tricarboxylic acid cycle (TCA) (Canbay et al., 2007). boxidation plays a vital role in hepatic lipid consumption. A variety of proteins and enzymes are involved in the process of mitochondrial fatty acid b-oxidation, such as plasma membrane fatty acid binding protein (FABPpm) (Furuhashi and Hotamisligil, 2008), fatty acid transport protein (FATP) (Ouali et al., 2000), carnitine acylcarnitine translocase (CACT) (Pierre et al., 2007), carnitine palmitoyltransferases 1 and 2 (CPT1/2), etc. (Bonnefont et al., 2004;Houten and Wanders, 2010). More importantly, mitochondrial fatty acid b-oxidation is regulated by both transcriptional and posttranscriptional mechanisms. Peroxisome proliferator-activated receptors (PPARs) are activated by fatty acids, having specific roles in physiology of different tissues (Yu et al., 2003;Lamichane et al., 2018). In liver, PPARa controls many genes involved in mitochondrial fatty acid b-oxidation (Lamichane et al., 2018). In terms of posttranscriptional mechanism, the inhibition of CPT1 by malonyl-CoA is a vital regulatory step. The levels of malonyl-CoA in hepatocytes are regulated via degradation induced by malonyl-CoA decarboxylase and via production by acetyl-CoA carboxylase (ACC) (Park et al., 2002). PPARs-mediated activation persuades transcription of malonyl-CoA decarboxylase, and phosphorylated AMPK inactivated ACC (Saha and Ruderman, 2003). They stimulate mitochondrial fatty acid b-oxidation by reducing malonyl-CoA levels. Additionally, peroxisome proliferator activated receptor gamma coactivator 1-alpha (PGC-1a) has also been regarded as a factor of posttranscriptional regulation of b-oxidation (Fernandez-Marcos and Auwerx, 2011). The activation of PGC-1a is mediated by AMPK via SIRT1mediated deacetylation (Canto and Auwerx, 2009).
Many herbs and active compounds protect liver from steatosis via regulation of fatty acid b-oxidation. Herbacetin is a dietary flavonoid with plenty of pharmacological activities. Its antihyperglycemic and anti-hyperlipidemic properties was associated with up-regulation of CPT to enhanced b-oxidation and hepatic lipid metabolism (Veeramani et al., 2018). Acteoside, a major compound isolated from leaves of Aloysia citriodora Palau (syn. Lippia triphylla), promoted lipolysis and fatty acid oxidation by enhancing mRNA expression level of adipose triglyceride lipase (ATGL) and CPT-1, and thus improved hepatic lipid metabolism . Cordycepin enhanced b-oxidation and suppressed lipid accumulation via regulating AMPK pathway and mitochondrial fusion in hepatocytes (Uen et al., 2018).

HepG2
Attenuated high glucose-induced hepatic lipogenesis Prevented lipid accumulation by blocking the expression of SREBP-1c and FAS through LKB1/SIRT1 and AMPK activation (Pil Hwang et al., 2013) Fructus Xanthii (Xanthium strumarium) HFD-fed rats Attenuated hepatic steatosis ↓The expression of lipogenic genes  ↑ means increase and up-regulate and ↓ means decrease and down-regulate. found to activate hepatic expression of PPAR a and its target genes associated with fatty acid b-oxidation (Ren et al., 2019). Babaodan, a TCM, up-regulated the expression of CPT-1 and PPAR a in liver of HFD-fed mice with NAFLD, leading to the enhanced b-oxidation (Sheng et al., 2019). Rosa rugosa Thunb., another TCM, is used for treatment of cardiovascular diseases and diabetes, hypertension, hyperlipidemia, and inflammation. R. rugosa flavonoids, the major components in R. rugosa Thunb., were observed to up-regulate the mRNA expression of PPAR a and its downstream gene of acylcoenzyme A oxidas X (ACOX) in a mouse model of hypertriglyceridemia (Baiyisaiti et al., 2019). Thereby, R. rugosa flavonoids could reduce TG in hepatocytes via rising b-oxidation. Gynura procumbens Merr., one of precious medicinal herbs of Asterceaes, up-regulated the mRNA expression of genes involved in b-oxidation, including PPAR a, CPT1 a, ACOX, fatty acid-binding proteins 5 (FABP5), stearoyl-coenzyme A desaturase-1 (SCD-1), glycerol-3-phosphate acyltransferase (mGPAT), microsomal triglyceride transfer protein (MTTP), to increase b-oxidation and efflux of fatty acids in liver of mice fed with MCD diet, and consequently decreased hepatic lipid accumulation ). An herbal formula Gyeongshingangjeehwan 18 (GGEx18), composed of Laminaria japonica Aresch (Laminariaceae), Rheum palmatum L. (Polygonaceae) and Ephedra sinica Stapf (Ephedraceae), has traditionally been described to against obesity and related metabolic disease such as dyslipidemia. In HFD-fed mice receiving GGEx18, genes related to hepatic fatty acid b-oxidation was higher compared to mice fed with only HFD . Evidence from recent studies has also indicated that some natural compounds promoted fatty acid oxidation by regulating the AMPK/PGC-1a signaling pathway. Yellow pigments, monascin, and ankaflavin, as secondary metabolites derived from monascus-fermented products, could reduce fatty acid accumulation partly mediated by the AMPK signaling activation and enhancement of b-oxidation by PGC-1a (Hsu et al., 2014). Myricetin, a natural flavonol with many biological activities, decreased PGC-1a acetylation through SIRT1 activation, and thus enhanced mitochondrial activity, suggesting its potential role in regulating hepatic lipid metabolism (Jung et al., 2017).

CLINICAL TRIALS
Given to the encouraging effects of herbal medicines on liver diseases, plenty of clinical trials have been extensively performed. The potential therapeutic benefits of herbal medicines in patients with NAFLD have been reviewed in several papers in recent years (Xiao et al., 2013;Bedi et al., 2016;Perumpail et al., 2018). In present review, we focused on the efficacy of herbal medicines to mediate lipid metabolism and attenuate hepatic steatosis.
Dava Al-Balgham, as one of the traditional medicine products composed of Nigella sativa L., Pistacia lentiscus L., Zataria multiflora Boiss. (ZM), and Trachyspermum ammi, was tested for its effect on NAFLD by a randomized, double-blinded, placebocontrolled trial with 76 NAFLD patients. Placebo or Dava Al-Balgham were consumed with each meal for three months. The results showed that Dava Al-Balgham could cause weight loss and have anti-hypolipidemic effect (Hormati et al., 2019).
The effect of Z. multiflora supplementation on NAFLD was studied by a randomized double-blind placebo-controlled clinical trial. Total 85 patients with NAFLD were treated with ZM powder (700 mg) or placebo twice daily for 3 months. However, no significant difference between two ZM-treated groups and placebo groups regarding ALT, TNF-a, grade of fatty liver in ultrasonography, lipid profiles, and high sensitive C-reactive protein (hs-CRP), while it could improve insulin resistance in patients with NAFLD. Further studies with larger sample size and longer duration are recommended (Zamani et al., 2018).
A 12-weeks randomized, controlled, double-blind trial included with was 44 NAFLD patients, was performed to evaluate the efficacy of Capparis spinosa L. on disease regression of NAFLD. Patients are randomly divided into control (n=22) or caper (n=22) group. The caper group was treated with 40-50 g caper fruit pickles with meals every day. Results obtained after treatment of 12 weeks indicated that the grade of fatty liver and serum lipoproteins were improved by C. spinosa administration (Khavasi et al., 2017).
We further checked the registered clinical trials about testing effects of the herbs and natural products on fatty liver via the website of www.clinialtrial.gov. The intensively studied herbs and derived compounds are resveratrol, ginseng, and ginger, which were discussed in detail in following. Other herbs and some natural products that are undergoing or were performed clinical trials on fatty liver diseases are listed in Table 3.
Resveratrol is a stilbenoid and a phytoalexin generated by several plants, such as red grapes in response to stimuli (Hasan and Bae, 2017). It is an activator of AMPK and SIRT1, and thus has a critical role in promoting fat breakdown and removal from the liver, preventing liver damage and inhibiting the progression of NAFLD (Shang et al., 2008;Charytoniuk et al., 2017;Theodotou et al., 2019). Resveratrol has been involved in three trials (NCT01446276; NCT01464801; NCT02030977) included patients of fatty liver, NAFLD, and obesity.
Another herb, ginseng, has been traditionally used for more than 2,000 years with various biological effects. A great deal of preclinical studies have demonstrated the protective effects of ginseng on liver diseases, including ALD and NAFLD. Korean Red Ginseng (Panax ginseng) (Park et al., 2017) enhanced the decreased phosphorylation of AMPK induced by ethanol consumption. Notably, it reduced the accumulation of fat in hepatocytes caused by ethanol via regulation of SREBP-1, SIRT-1 and PPAR-a (Huu Tung et al., 2012;Park et al., 2017). Clinical trial (NCT0394512) has been performed to study the effect of red ginseng on liver dysfunction. Fermented ginseng powder has also been tested to study its efficacy on NAFLD (NCT03260543).
Ginger is the root of Zingiber officinale Roscoe and is one of the most used spices in many countries (Huu Tung et al., 2012). It contains active compounds, such as shogaol, gingerol, zingerone, and b-bisabolene. It has been shown that ginger can reduce insulin resistance and serum TG level in patients with Type II diabetes and hyperlipidemia (Arablou et al., 2014). In a randomized, double-blind, placebo-controlled clinical trial with 44 patients of NAFLD, ginger supplementation significantly   reduced the levels of ALT, inflammatory cytokines, g-glutamyl transferase, as well as hepatic steatosis grade and the insulin resistance index in comparison to the control group. Another clinical trial of ginger supplement on fatty liver or Type 2 Diabetes Mellitus is still undergoing (NCT02289235).

META-ANALYSIS STUDIES
HuoXueHuaYu (HXHY), a TCM formula, has been widely used in clinic for patients with NAFLD. Cai et al. performed a meta-analysis of randomized controlled trial of HXHY in NAFLD. There are 13 studies involving 1429 patients which 654 patients receiving conventional treatment group and 775 patients belonged to HXHY group. HXHY showed better ability on lowing TC and TG levels than that of conversational treatment. HXHY might be an effective and safe therapy for NAFLD, and trials with rigorous design, multicenter, large-scale, and high-quality worldwide are still expected (Cai et al., 2019). Erchen Decoction (ECD), a TCM formula, is often used in the therapy of various diseases. A meta-analysis of the efficacy of ECD for the treatment of NAFLD by PRISMA systematic review standard has been performed. Seven randomized controlled trial with a total of 1951 participants were included in this study. The analysis results showed that patients with ECD treatment showed an improved status compared to the conventional treatment. Longer follow-up periods and larger-scale randomized controlled trial are still required to evaluate the efficacy of ECD in NAFLD .
The efficiency and safety of a famous TCM Danshen in the treatment of NAFLD has also been analyzed by a meta-analysis study. Eight randomized controlled trials with 800 patients of NAFLD were identified. The results indicated that Danshen had improved total effectiveness rate, lower level of TC, TG, LDL, ALT, and AST, suggesting that Danshen may have potential effects on NAFLD, while multicenter large-sample randomized clinical trials are still expected to confirm the efficacy and safety of Danshen .
Another study performed by Narjes et al. on 2017 has evaluated the efficiency of all kinds of TCM on the treatment of NAFLD. Literature were searched on China National Knowledge and PubMed from 1995 to 2010. Total 5904 patients from 62 randomized controlled trials were included for meta-analysis. Results showed that TCM had a better effect on the normalization of ALT level and disappearance of radiological steatosis for the patients of NAFLD. Finally, authors concluded that TCM is of modest benefit to the therapy of NAFLD (Shi et al., 2012).

CONCLUSIONS AND PERSPECTIVES
Due to the positive efficacy and minimal side effects, herbal medicines have obtained increasing attention as alternative therapeutic agents for liver disorders and dyslipidemia. Increasing evidence from laboratory studies suggests that many herbs, natural products, and derived compounds could inhibit the progression of hepatic steatosis. A variety of mechanisms have been demonstrated to be implicated in preventing hepatic steatosis and modulating lipid metabolism by herbs, including reducing hepatocyte fatty acid uptake and trafficking, reducing hepatic de novo lipogenesis, increasing lipolysis, inducing lipophagy, enhancing fatty acid boxidation. In particular, SREBP-1c, PPARa, AMPK, and SIRT1 signaling pathways have been highlighted as crucial molecular targets of action mechanisms by which herbal medicines regulate hepatic lipid metabolism. Current clinical evidences and metaanalysis showing the positive impacts of herbal medicines on the hepatic lipid metabolism pathways are still not strong enough. Further multicenter large-sample randomized clinical trials are still required to confirm the efficacy and safety of herbal medicines on hepatic lipid metabolism. Herbs mix and single medical plants as well as their components have been widely applied in the treatment of NAFLD. We consider the main actor should be the active components. For both herbs mix and single medical plants, they are containing many compounds, which may act synergistically in ways to enhance the therapeutic effects. Identifying the active components in herbs is a crucial and significant subject for the development of TCM. Currently, network pharmacology-based strategy has been extensively used for the prediction of the active components from herbs. Network pharmacology is an approach based on systems biology, poly-pharmacology, and molecular networks, to analyze relationships between drugs and diseases in recent decade, which has attracted considerable attention among Chinese medicine researchers for its ability in predicting and illustrating interactive relationships between numerous components and targets of herbal medicines. Network-based pharmacological analysis is a desirable approach as well as a good tool of in silico prediction for investigating the mechanisms of action for herbs and formulae and their potential bioactive components at molecular and systematic levels, which renders more effective subsequent exploration with experimental approaches. With the promising and effective prediction, subsequently validation experiments in laboratory and bench would be performed to confirm their pivotal role. In conclusion, herbal medicines have the potency to be alternative and complementary medical therapies to current pharmaceuticals for the treatment of liver diseases with lipid metabolism disorder.

AUTHOR CONTRIBUTIONS
YF designed and conceived the study. SL and YF retrieved and analyzed the data, and drafted the manuscript. SL, YX, WG, FC, CZ, HT, and NW discussed and revised the manuscript. All authors confirmed final version of the manuscript.