Lychee Seed as a Potential Hypoglycemic Agent, and Exploration of its Underlying Mechanisms

Food is people’s primal want. A reasonable diet and healthy food not only provide nutrients for human growth but also contribute to disease prevention and treatment, while following an unhealthy diet can lead to an increased risk of many diseases, especially metabolic disorders, such as diabetes. Nature is enriched with different food sources, and it seems that purely natural products are more in line with the current concept of health, which enhance the formation of the notion that “Food/Diet Supplements from Natural Sources as a Medicine.” As a delicious fruit, the medicinal values such as anticancer, antibacterial, antioxidation, and antiglycating properties of lychee have been found. Lychee (Litchi in Chinese) is a subtropical fruit plant belonging to the family Sapindaceae. It has been widely cultivated in warm climates worldwide, particularly in China, for thousands of years. In recent years, various phytochemical components such as quercetin, procyanidin A2, and (2R)-naringenin-7-O-(3-O-αL-rhamnopyranosyl-β-D-glucopyranoside) have been identified in a lychee seed, which may lend a lychee seed as a relatively safe and inexpensive adjuvant treatment for diabetes and diabetic complications. In fact, accumulating evidence has shown that lychee seed, lychee seed extracts, and related compounds have promising antihyperglycemic activities, including improving insulin resistance, anti-inflammatory effect, lipid regulation, neuroprotection, antineurotoxic effect, and renoprotection effect. In this review, we summarized publications on antiglycemic effects and mechanisms of lychee seed, lychee seed extracts, and related compounds, which included their efficacies as a cure for diabetes and diabetic complications in cells, animals, and humans, attempting to obtain a robust evidence basis for the clinical application and value of lychee seed.

Frontiers in Pharmacology | www.frontiersin.org October 2021 | Volume 12 | Article 737803 problem. Individuals with diabetes are more prone to develop complications such as retinopathy, nephropathy, coronary artery disease, peripheral arterial disease, and stroke, contributing to higher mortality rates (Ford, 2011;Tandon et al., 2012). Thus, the prevention and treatment of diabetes hold considerable importance. Currently, the main treatments for diabetes include insulin injection, oral diabetes medications, and pancreatic islet transplantation (Ryan et al., 2005;Doyle-Delgado et al., 2020). However, the available treatments only delay the progress of the disease rather than curing it, leading to the lengthy and costly therapy, and comprise side effects, which impart a heavy economic and psychological burden on patients.
Food is the first necessity of people. Poor diet is associated with a higher risk of many diseases (GBD 2017 Diet Collaborators, 2019), especially diabetes (Srour et al., 2020), while some healthy food is reported to improve glycemic control (Reynolds et al., 2020). Studies supported a positive association between dietary intake of momordica charantia and blood sugar reduction (Kibiti and Afolayan, 2015). Buckwheat also had effects on reducing serum glucose concentrations in diabetic rats (Kawa et al., 2003). Consequently, the notion that "Food/Diet Supplements from Natural Sources as a Medicine" has become popular and appealing among diabetic patients. In China, herbs in nature with homology of medicine and food have been widely studied. Lychee, a fruit tree belonging to family Sapindaceae, originating from China, is widely cultivated in warm climates in many regions around the world and is botanically related to Litchi chinensis Sonn (Jiang, 2003;Yang et al., 2006;Huang et al., 2014). Due to the high nutrients and savory flavor as well as the attractive red, lychee is widely favored by humans. Lychee seed, the dried mature seed of lychee, an ancient traditional drug-food homologous herbal medicine, was used to smooth Qi, dispel cold, alleviate polydipsia, and relieve pain in China (Kilari and Putta, 2016). Accumulating research studies recently focused on the antidiabetic activity of lychee seed, although the underlying mechanisms of action have not been studied thoroughly. A summary of antidiabetic studies of lychee seed will be helpful for offering a reference basis for deeper investigations and clinical use of this natural herb medicine ( Table 1). We examined the electronic resources with the PubMed, EMBASE, Web of Science, and China National Knowledge Infrastructure based on the information limited to English and Chinese literatures up till Jun 2021.

BOTANICAL DESCRIPTIONS OF LYCHEE
Lychee (Litchi chinensis Sonn.) is a subtropical medium evergreen dome-shaped tree with a glossy grayish stem belonging to the family Sapindaceae. It generally grows to less than 10 m in height, rarely up to 15 m or more. The pinnate leaves which consist of 4-8 pairs of elliptic or lanceolate, long acuminate, glabrous leaflets are leathery, 5-7 cm long, and 2-4 cm broad. The yellowish-white flower is small in size with a tetramerous calyx. Terminal inflorescence is about 5-30 cm long with multibranched panicles and slender pedicels. The fruit is ellipsoidal or nearly round in shape, estimated at 2.5 cm in diameter and clothed with a coarse thicker ring or pericarp with strawberry to red in color at maturity. Inside the pericarp is lychee aril that is milky-white and semitransparent with a sweet, juicy, and delicious taste. A seed with a smooth and glossy surface is brown or reddish brown in color and elliptic to ovate in shape covered by a fleshy aril. The size of the seed varies greatly between 1 and 2 cm in length, as shown in Figure 1.

THE HISTORY OF LYCHEE SEED
Lychee seed, the dry mature seed of Litchi chinensis Sonn, is known to have a remarkable medicinal value in ancient China. The oldest available Chinese written source which described the application of lychee seed is Ben Cao Yan Yi traced back to the Song dynasty (AD 1116). In ancient Chinese medical practices, lychee seed was always used for hernia, orchitis, ulcers, and intestinal troubles. Diabetes-related symptoms were known as "Xiaoke" (emaciation and thirst) in the ancient Chinese medical literature (Tong et al., 2012). In Compendium of Materia Medica (Ben Cao Gang Mu) written by Li Shizhen (from 1518 to 1593 AD) during the Ming dynasty, the lychee seed was warm in nature, sweet in taste, and could act as a beneficial agent in thirstquenching. Numerous Chinese patent medicines such as Jinlida granule (Tian et al., 2018) and Jiangtangtongmai tablets (Su et al., 2017) approved by the Chinese Food and Drug Administration, containing lychee seed, were clinically used for the treatment of diabetes.

Improving Insulin Resistance
Insulin has a pivotal function in ensuring the homeostasis of energy metabolism through a coordination of the storage and Frontiers in Pharmacology | www.frontiersin.org October 2021 | Volume 12 | Article 737803 utilization of fuel molecules in insulin-targeted organs (Castan-Laurell et al., 2012). Insulin resistance (IR) is a pathological condition defined by the inability of insulin to stimulate glucose disposal and is considered as a key player in the development of type 2 diabetes mellitus (Brown and Walker, 2016). Although the precise pathophysiology of IR in diabetes has not yet been delineated, inflammatory response, oxidative stress, insulin receptor mutations, endoplasmic reticulum stress, and mitochondrial dysfunction are currently regarded as the possible underlying mechanisms (Yaribeygi et al., 2019). Consequently, numerous genes such as INS, AKT1, IL-6, TP53, TNF, VEGFA, MAPK3, EGFR, EGF, and SRC have been revealed to be associated with the development of IR (Gao et al., 2020). The relatively prominent signaling pathways involved in the formation of IR are the pathways of insulin resistance, adipocytokine, insulin, PI3K-Akt, ERK, AMPK, and HIF-1 (Ozaki et al., 2016;Huang et al., 2018;Gao et al., 2020). In the glucose tolerance test, intragastric administration of a lychee seed water extractant remarkably decreased hyperinsulinemia and potentiated insulin sensitivity (Guo et al., 2003a). Another study indicated that lychee seed extracts could increase the quality of life of streptozotocin (STZ) combined with a high-fat diet-induced type 2 diabetes rats. Compared to the control group, the insulin resistance index in the lychee seed extract group was dramatically reduced, which in turn increased the insulin sensitivity index progressively (Man et al., 2016). The PI3K/AKT/mTOR signaling pathway makes essential contribution to the occurrence of IR. Activation of the PI3K/ AKT/mTOR signaling pathway could improve insulin-induced glucose Han et al., 2020). Lychee seed extracts significantly improved IR in a type 2 diabetes mouse model by elevating the expression levels of PI3K, AKT, and mTOR to trigger the PI3K/AKT/mTOR signaling pathway (Man et al., 2017). Recently, growing evidence has shown that microRNAs as crucial regulators of gene expression perform a critical role in the development of IR (Honardoost et al., 2014;Wen et al., 2014;Xihua et al., 2019). One study showed that the microRNA expression changed significantly in db/db mouse administered extract of lychee seed (0.015 g/d, i.g.) (Zhang et al., 2013). In addition, abundant studies have demonstrated that endoplasmic reticulum stress-induced pancreatic β-cell destruction is one of the vital mechanisms of progression for both insulin-dependent diabetes and non-insulin-dependent diabetes (Cnop et al., 2017). Endoplasmic reticulum stress can not only directly damage the insulin signaling pathway but also further promote IR in a variety of ways (Ozcan and Tabas, 2012;Dong et al., 2017). Experiments in vitro have confirmed that lychee semen effective constituents can significantly reduce the mRNA expression of glucose regulatory protein 78 (Grp78) (Liao et al., 2014;Li et al., 2015) which contributes to endoplasmic reticulum stress and the activation of unfolded protein response (UPR). Elevated proinflammatory cytokine tumor necrosis factor-α (TNF-α) and leptin levels have been demonstrated to be closely associated with IR (Ayina et al., 2016;Alzamil, 2020). In addition, plasma FIGURE 1 | Lychee fruit, lychee aril, and lychee seed (color version of these figures is available at https://699pic.com/tupian-500450779.html?bindPhone 1, https://item.m.jd.com/product/53077629044.html?). free fatty acid (FFA) is viewed as a potential factor to IR and disrupts insulin secretion (Bergman and Ader, 2000). Lychee seed extracts could improve insulin sensitivity by reducing the levels of TNF-α, hyper-leptinemia, and FFA in diabetic rats (Guo et al., 2004).

Antioxidant Effect
Oxidative stress is induced by an imbalance between the production of free radicals and the antioxidant mechanisms, which is a well-known contributor to the pathogenesis and progression of diabetes via several molecular mechanisms, such as β-cell dysfunction and defects of the normal insulin signaling pathways (Yaribeygi et al., 2020). In addition, the excessive production of reactive oxygen species (ROS) inside the cell occupies a pivotal role in the onset of oxidative stress . The body produces excess ROS, which is known to enhance nuclear factor (NF)-κB activity (Zheng et al., 2015), β-cell maturation, and apoptosis increase. In a 1,1diphenyl-2-picrylhydrazyl (DPPH) radical scavenging assay, the total flavonoids of lychee seed showed a potent antioxidant activity (Ren et al., 2017). Moreover, lychee seed extracts could significantly accelerate the clearance of O 2 − in the cerebrum of mice with diabetes induced by alloxan . Malondialdehyde (MDA), an oxidative stress marker, is produced when ROS within cells oxidize unsaturated fatty acids (Kwiecien et al., 2014). Several animal studies revealed that lychee seed extracts remarkably improved the activity of superoxide dismutases (SODs) which was the central antioxidant defense system against O 2 − (Fukai and Ushio-Fukai, 2011) and decreased MDA in animal models of diabetic rats (Pan et al., 1999;Guo et al., 2003b;Guo et al., 2004).

Anti-Inflammatory Effect
The relationship between inflammation and diabetes has received extensive attention. It is believed that diabetes is a chronic inflammatory state (Wellen and Hotamisligil, 2005). Indeed, recent studies have emphasized and found substantial evidence that many inflammatory cytokines such as transforming growth factor beta 1 (TGF-β1) (Herder et al., 1984(Herder et al., -2002Olivieri et al., 2010;Shi et al., 2018), monocyte chemotactic peptide 1 (MCP-1) (Reddy et al., 2017), and macrophage migration-inhibitory factor (MIF) (Sánchez-Zamora and Rodriguez-Sosa, 2014; Abu El-Asrar et al., 2019) are reported to be responsible for the pathogenesis of the development of diabetes or diabetes complications. The currently available medical therapy mainly targets the underlying etiology. Hence, inhibition of excessive inflammatory responses might provide a potentially promising candidate for future therapeutics of diabetes. Lychee seed extracts could alleviate the inflammation reaction in rats with impaired glucose tolerance, which was associated with the downregulation expression of TGF-β1, MCP-1, and MIF (Qi, 2017). NF-κB is central to inflammatory responses and is tightly linked to various inflammatory diseases. Lychee seed extracts directly affected the mRNA levels of NF-κB, which prevented diabetes (Man et al., 2016). An experiment in diabetic nephropathy mice models has revealed that the saponin of lychee seed could delay the diabetic kidney inflammation development through inhibiting the expression of MCP-1 and intercellular cell adhesion molecule-1 (ICAM-1) protein in the kidney tissue, reducing the content of pro-inflammatory cytokines including interleukin-1β (IL-1β) and interleukin-6 (IL-6) in the serum (Qin, 2017).

Lipid Regulation
Glucose and lipid metabolism are intrinsically related to one another in many aspects. Diabetic dyslipidemia is common in individuals with diabetes (Athyros et al., 2018). The pathophysiological mechanism of diabetic dyslipidemia is highly complex and multifactorial, yet accepted as a preponderant contributor in the occurrence of diabetic dyslipidemia is IR with an attendant increase in free fatty acid flux into the liver (Mooradian, 2009). The most predominant clinical presentation of the interaction is marked by elevated triglycerides (TGs), decreased high-density lipoprotein cholesterol (HDL-C), and predominance of small-dense lowdensity lipoprotein (LDL) (Parhofer, 2015). The saponin of lychee seed affected the lipid metabolism in dexamethasone (DX)-induced insulin-resistant rats by lowering the content of total cholesterol (TC), TG, and low-density lipoprotein cholesterol (LDL-C) (Guo et al., 2003b). Simultaneously, the potential lipid-modifying effect of lychee seed extracts was also demonstrated by the other two animal studies (Pan et al., 1999;Li et al., 2015).

Kidney Protection Effect
Diabetic kidney disease (DKD), a severe microvascular complication of diabetes, is the primary cause of end-stage renal failure and the single strongest predictor of mortality in diabetic patients (Reidy et al., 2014;Thomas et al., 2016). Strict glycemic management dramatically reduces DKD morbidity, which suggests that metabolic disorders resulting from hyperglycemia, including changes in energy utilization and mitochondrial damage, exert a critical role in the disease progression (Reidy et al., 2014). Presently, multifactorial management of DKD primarily includes diet therapy, glucoselowering therapy, lipid control, and preserving renal function (Selby and Taal, 2020). Despite various therapeutic strategies, the morbidity and mortality of DKD remain high throughout the world. Traditional Chinese herbal medicine can possess antidiabetic effects and improve renal function on DKD obviously. Research showed that saponin of lychee seed could reduce the blood glucose and ameliorate pathological damage and kidney lesions of diabetic nephropathy model rats through repressing the expression of inflammatory factors and attenuating inflammatory responses in kidney tissue (Qin, 2017). Out of many cytokines implicated in fibrosis, transforming growth factor-β1 (TGF-β1), fibronectin (FN), and collagen IV (Col IV) promoting extracellular matrix (ECM) accumulation are the most notorious (Downer et al., 1988;Chen et al., 2017). In the rat mesangial cells induced by high glucose and tumor necrosis factor-α (TNF-α), the total flavonoids of lychee seed distinctly decreased the protein expression of TGF-β1, FN, and Col IV, which indicated the total flavone might improve the diabetic nephropathy fibrosis process (Liu, 2016;Liu et al., 2016). In addition, the saponin of the lychee seed was Frontiers in Pharmacology | www.frontiersin.org October 2021 | Volume 12 | Article 737803 confirmed to obviously reduce the content of IL-6 and IL-1β secreted by human glomerular mesangial cells (HMC) and decrease the secretion of ECM to slow down the sclerosis process of glomerulus (Zhang, 2016).

Neuroprotection and Cognitive Function Improvement
Cognitive dysfunction is considered as a serious and common comorbidity or even a complication of diabetes (Biessels and Despa, 2018). They share common biological mechanisms including deficits in insulin signaling, neuroinflammatory pathways, mitochondrial (Mt) metabolism, the sirtuinperoxisome proliferator-activated receptor-gamma coactivator 1α (SIRT-PGC-1α) axis, and Tau signaling (Zilliox et al., 2016). An animal test showed that the lychee seed extract fluid could protect the nervous system by significantly improving the transmit function of the cholinergic nervous system in the cerebrum of mice with diabetes induced by alloxan and accelerating the clearance of O 2 − . In another research, compared with the model group, amyloid beta (Aβ) and Tau deposition of the experimental rats in the medium-and highdose lychee seed extract administration groups [1.39 and 2.78 g/ (kg.d)] were significantly decreased (Zeng et al., 2016). Similarly, investigators have found that lychee seed extracts consisting of numerous ingredients such as adenosine, 5hydroxymethyluridine, and 4-p-coumaroylquinic acid dramatically protected against neuronal damage and prevented the decline in the cognitive function through lowering serum glucose, ameliorating IR, and suppressing the aggregation of Aβ, Tau protein, and advanced glycation end products (AGEs) in the hippocampus of type 2 diabetes rats (Tang et al., 2018), while further study demonstrated that polyphenols derived from lychee seed inhibited hyperphosphorylated Tau through improving IR via upregulating IRS-1/PI3K/Akt and downregulating GSK-3β (Xiong et al., 2020). Based on studies on diabetes and diabetic complication intervention with lychee seed in vivo and in vitro, the underlying hypoglycemic mechanisms of lychee seed are summarized in Figure 2.
Frontiers in Pharmacology | www.frontiersin.org October 2021 | Volume 12 | Article 737803 diabetic complications. Its beneficial effects have also been validated by several clinical observations (Zhang and Teng, 1985;Shen, 1991). Through the literature review, the underlying mechanisms, improving insulin resistance, antioxidant effect, anti-inflammatory effects, lipid regulation, kidney protection effect, and neuroprotection and cognitive function improvement of lychee seed in treating diabetes are also worth investigating. For further research of lychee seed within this field, several issues should be considered. An in vitro research showed that saponin of lychee seed had no effect on glycometabolism in an insulin resistance model of pepatocellular carcinoma (HepG2) cells (Qin, 2017), which may be related to the site of drug action. The impact of saponin of lychee seed on improving IR may not be effected in hepatocytes but in other peripheral tissues such as muscle and fat. Thus, the corresponding site of action of lychee seed needs to be explicitly investigated. Most of the elucidation of the antidiabetic mechanisms scratches only at the surface, and researchers need to probe deeper into analyzing the detailed molecular mechanisms of the effects of lychee seed intervention. Consequently, comprehensive and much more robust evidence is desperately needed. As outlined in the above review, although some clinical studies show positive results in the treatment of diabetes, large, double-blind, randomized, placebo-controlled, multicenter clinical trials are needed.
In conclusion, lychee seed might be developed as a multi-target agent and prescribed as a useful adjuvant to the current treatment for diabetes and especially diabetic complications. Despite the enormous therapeutic potential, further comprehensive investigation from bench to clinical reasearch is warranted.

AUTHOR CONTRIBUTIONS
FL designed the study and is the corresponding author. YZ, DJ, and XA drafted the manuscript and figure. LD and YD drafted the table. All authors approved the final version of the manuscript.