Abstract

The main purpose of this study is to review the 6-gingerol effect in antidiabetic in mammalian system. Among them 6-gingerol have the potential to prevent the development of diabetes complications. This study was reviewed in mammalian systems by giving 6-gingerol orally with a specific defined dose to diabetes rats. 6-gingerol generally activates the Adenosine Monophosphate-Activated Protein Kinase (AMPK) path, which is the main target for the development of anti-diabetes drugs. These results indicate a significant antidiabetic potential of 6-gingerol. Treatment of doses depends on 6-gingerol at concentration varies indicating potential mechanisms in improving the effects of diabetes, both in in vitro and in vivo models with AMPK activation. Data relevant to the 6-gingerol role in diabetes is limited. Although some clinical trials have proven that 6-gingerol have efficiency in diabetes complications, the exact mechanism of AMPK activation by 6-gingerol must be further investigated. In this review, most of the focused on 6-gingerol are known to activate AMPK by increasing intracellular calcium levels, further studies are needed to determine the exact role of gingerol in the activation of the Ca²⁺/calmodulin-dependent protein kinase kinase (CAMKK)-AMPK pathway. Although ginger is considered safe in some cases, it might cause nausea, vomiting, gastric irritability, excessive bleeding in pregnant women and death in some rats. Therefore, clinical trials must be carried out further to estimate acute and chronic toxicity to assess the safety of ginger and related bioactive compounds.

1. Introduction

Ginger is a popular spice extensively used all over the World including Asian Countries. Ginger is known to contain more than 400 chemical compounds. Principally ginger rhizomes are known to contain 50-70% Carbohydrates, 3-8% Lipids and Terpenes and Phenolic compounds. Moreover the Terpenes known to possess different components such as zingiberene, β-bisabolene, α-farmesene, β-sesquiphellandrene and α-curcumene, and phenolic compounds represented by gingerol (23-25%), paradols and shogaols (18-25%) ¹.

In addition to the above, Terpenes and Phenolic compounds, amino acids, raw fibers, ash, protein, vitamins including Nicotonic acid and Vitamin A, Phytosterols, Minerals were also present. Zingiberene and bisabolene belongs to the class of aromatic constituents, and gingerols and shogaols belongs to the class of spicy constituents were present in Ginger. Compounds like 6-paradol, 1-dehydrogingerdione, 6-gingerdione and 10-gingerdione, 4-gingerdiol, 6-gingerdiol, 8-gingerdiol, 10-gingerdiol and diarylheptonoids and also present in ginger. The assumption of odor and flavor of ginger may be attributed due to the presence of volatile oils including shogaols and gingerols ².

Diabetes Mellitus (DM), a serious metabolic and most common disease observed in human beings and known to cause morbidity and mortality relatively at higher rates. Due to diabetes, generally the failure of increase glucose absorption into the peripheral tissues due to insulin availability, which subsequently causes an increase in glucose levels in circulation. The current focus of anti-diabetes medical research is identification of suitable compounds for stimulation of absorption of cell glucose. Ginger is identified as widely used species in functional foods and the nature of the drug associated with it includes potential anti-oxidant, anti-arthritic, anti-inflammatory, analgesic, anti-migraine, anti-thrombotic, anti-cancer, hypolipidemic, hypocholesterolaemic properties. Treatment with ginger juice resulted a significant increase in insulin levels and also significantly decreased the fasting glucose levels in STZ-induced diabetes rats ³.

DM is a common endocrine disorder that generally affects around 100 million people all over the World and is one of the heterogenous group of diseases lead to an increased concentrations of glucose i.e. hyperglycemia and is characterized by a condition such as polyuria and polyphagia. DM represents a chronic disorder of metabolism of endocrine system characterized by disturbances in glucose, lipid, protein disorders associated with insulin secretion rates. Diabetes is considered to be a major health problem all over the World and it is expected to increase significantly from 382-471 million individuals by 2035 ⁴. All the three Types of diabetes, Type-1- diabetes (T-1-D) associated with auto-immune disorder leading to the destruction of pancreatic β-cells, whereas Type-2-diabetes (T-2-D), a more common and basically caused by impairment in glucose regulation due to combinational dysfunction of pancreatic β-cells, insulin resistance and Gestational Diabetes Mellitus (GDM).

Zingiber officinale, which belongs to the Zingiberaceae family and the Zingiber genus, has been commonly consumed as a spice and an herbal medicine for a long time. Plants naturally known to contain several drug compounds which specifically control this metabolic syndrome i.e. diabetes. Ginger, is widely used in some food products including soft drinks and also in many Types of pharmaceutical formulations. 6-gingerol, a principle component of ginger known to possess anti-oxidant, anti-inflammatory and anti-cancer activities. Though much information is available on 6-gingerol with reference to the above activities, anti-hyperglycemic effects and its implications are not being explored completely. In this study, we rviewed the effects of 6-gingerol through the study of the molecular mechanism using the cultural rat muscle cells (L6 myocytes) and also examined whether or not 6-gingerol would protect pancreatic b-cells from reactive oxygen species (ROS) -induced stress employing cultured pancreatic b-cells (RIN-5F). To confirm antihyperglycemic effects 6-gingerol in vivo, the data was proofed to know its effect on fasting blood glucose level, glucose intolerance and gene expression of hepatic enzymes related to glucose metabolism in Type-2 diabetic model db/db mice ⁵.

Research has suggested that ginger may increase blood sugar in the long-term for Type-2-diabetes. In addition, ginger is rich in gingerol, which is the main active compound of ginger that can increase glucose absorption into muscle cells, thus imitating insulin actions, in maintaining blood sugar levels. Indeed, the researchers also reported that active compounds extracted from ginger can interact with serotonin receptors and reverse their influence on insulin secretion. Furthermore, the ginger aqueous extract was studied for hypoglycemic capabilities in diabetes-induced rats, which showed that the ginger aqueous extract may have reduced blood glucose levels up to 35% and about 10% increase insulin levels found in plasma. Other research also studied the effect of ginger powder on insulin resistance and glycemic index in patients with Type-2-diabetes mellitus, which indicates that the use of 3 grams of ginger per day for 8 weeks significantly reduces the glycemic index. However, consuming 2 grams of ginger every day for 8 weeks is not significant in reducing fasting blood glucose. Interestingly, small doses of ginger can also delay the onset and the development of cataracts; which is one of the complications of long-term diabetes related views, in diabetes induced mice ⁶.

Despite the hypoglycemic effect of Zingiber officinale extract has been reported, the exact mechanism of this effect has not been explored. To get an understanding of the anti-diabetes properties of Zingiber officinale, the present review on the biological activity of Zingiber officinale about glucose transportation in L6 myotubes and its downstream regulation mechanism through the insulin signal glucose transportation pathway.

2. Methodology

A bibliographic investigation was carried out by analysing classical text and reference books, articles, and peer-reviewed papers, as well as a thorough consultation of worldwide accepted scientific databases. We performed CENTRAL, EMBASE, and PubMed searches using terms such as “anti-diabetic” activity of plants. The final data collected through the authors’ discussions were then compiled, evaluated, compared and conclusion are drawn accordingly.

We used summary statistics to the effect of 6-gingerol in antidiabetic. The 6-gingerol activates AMPK pathway, which is the major target for anti-diabetic activity. All data are given as mean ± standard deviation (SD). Statistical analysis was performed with past (Version 3) several sample tests (ANOVA, Kru-wal) followed by Tukey’s pairwise test for multiple comparisons. Values of p<0.05 were considered significant.

3. Ginger

Several cultivars of ginger are grown in different growing regions in India and are generally named for the region in which they are mostly cultivated as in Table 1 & Table 2. Other studies suggest that the response to the ginger component depends on the concentration of the dose. Some experimental studies published anti-diabetes, hypolipidemic and anti-oxidative properties of controversial ginger and more investigations can clarify their potential in the protection and treatment of metabolic disorders ⁷.

The LD50 of methanol and water extracts of ginger was 10.25 g/kg and 11.75 g/kg bw by oral- administration in mice. Administration of ginger powder up to 2 g/kg body weight/day does not cause mortality or abnormal changes in hematological parameters in rats. Thus, if ginger has proven effective in reducing the hyperglycemia it can be used safely for its treatment ⁸. Therefore, ginger is considered as herbal medicine with strong health benefits. Though ginger promotes many beneficial effects in several diseases, it might cause some of the side effects when used beyond the required quantity. Consumption of ginger in higher doses showed mild symptoms like nausea, diarrhea, hypothermia, anorexia, drowsiness, hypotension, bradycardia and abdominal discomfort (Figure 1). Several cultivars of ginger are grown in different growing regions in India and are generally named for the region in which they are mostly cultivated as shown in Table 1 & Table 2.

There are also many other phenolic compounds in ginger, such as quercetin, zingerone, gingerenone-A, and 6-dehydrogingerdione. In addition, there are several components of terpene in ginger, such as β-bisabolene, α-curcumene, zingiberene, α-farnesene, and β-sesquiphellandrene, which is considered the main constituent of ginger essential oils. In addition, polysaccharides, lipids, organic acids, and raw fibers are also present in ginger ⁹.

In addition to general and Segal, ginger has a number of phytochemicals such as zingerone, paradols, β- phellandrene, curcumene, cineole, geranyl acetate, terphineol, terpenes, borneol, geraniol, limonene, β- elemene, zingiberol, linalool, α-zingiberene, β-sesquiphellandrene, β-bisabolene, zingiberenol and α-farmesene, and galanols (A and B) with a number of medicinal and therapeutic values ¹⁰ (Table 3).

For centuries, it is an important ingredient in herbal medicine in the treatment of rheumatism, gingivitis, toothache, asthma, stroke, vomiting and diabetes ¹¹. Gingerol belongs to the class of organic compounds known as gingerols. Gingerols are compounds containing a gingerol moiety, which is structurally characterized by a 4-hydroxy-3-methoxyphenyl group substituted at the C6 carbon atom by a 5-hydroxy-alkane-3-one. Gingerol is a pungent tasting compound. Gingerol is found, on average, in the highest concentration within gingers, Zingiber officinale (Figure 2).

Because of the widespread uses of ginger as a spice, dietary supplements, tea, cream, household remedy, as well as an ingredient of various natural health products, it is essential to standardize ginger formulations. Many analytical methods have been reported in the analysis of 6-gingerol in its extract, commercial formulations and biological fluids. Most of these methods are high performance liquid chromatography (HPLC) and used for analysis of 6-gingerol either in biological fluids or in its extract. Only three high performance thin layer chromatography (HPTLC) densitometric methods are available for analysis of 6-gingerol in the extract of Zingiber officinale and its commercial or Ayurvedic formulations. ¹².

6-gingerol is soluble in ethanol, methanol and other organic solvents and is unsTable at room temperature in the presence of oxygen and light and sTable over an extended period at -20°C. The 6-gingerol content may also decrease during postharvest storage, maturation, and thermal processing ¹³ due to gingerol become dehydrated in order to produce shogaol ¹⁴. The data for the bioavailability and concentration of 6-gingerol of ginger are limited (Table 4).

4. Role of 6-gingerol in Diabetes

Ginger has efficacy in ameliorating the hyperglycemic effects and suggested that gingerols are the most prominent components that promote glucose uptake and exhibits promising therapy for anti-diabetic effects with positive effects resulting from its primary bioactive ingredients such as gingerols, shogaols, zingerone, and paradols ³⁷. Among all the gingerols, 6-gingerol showed higher hypoglycemic activity ³⁸ and also promotes increase in insulin release and insulin sensitivity. 6-gingerol has the potentiality to exert hypoglycemic effects through AMPK activation. These effects were promoted by 6-gingerol upon elevating the intracellular calcium levels on AMPK phosphorylation. Insulin sensitivity is increased upon AMPK activation in the presence of 6-gingerol. In arsenic intoxicated mice, 6-gingerol showed hypoglycemic property and improved the impaired insulin signaling and restored the plasma insulin levels and showed a protective effect on pancreatic β-cells. 6-gingerol may reduce the accumulation of lipids and delay insulin resistance in diabetes ³⁹.

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) - AMPK pathway is also involved in stimulating glucose uptake by 6-gingerol. In 3T3-L1 adipocytes and L6 myotube cells, AMPK is associated with 6-gingerol stimulated glucose uptake and GLUT4 translocation in vitro. Increased calcium from exercise might activate the CaMKK which increase the activity of markers that enhance glucose uptake ⁴⁰. 6-gingerol supplementation for 3 weeks reduced fasting blood glucose levels and increased GLUT4 translocation, thereby activated AMPK in vivo. The Rab-GTPase activating protein [Rab-GAP], AS160 phosphorylation is increased in the presence of 6- gingerol that promotes glucose uptake. In diabetic mice, 6-gingerol suppressed the fasting blood glucose levels and improved glucose intolerance. 6-gingerol may inhibit absorption of intestinal glucose and may promote hypoglycemia in the glucose-fed rats. In addition to ameliorating hyperglycemia, 6-gingerol improved the cardiac dysfunction induced by diabetes and ⁴¹. Activation of AMPK mediated pathway showed much beneficial effects in presence of 6-gingerol. Hence, activation of AMPK by 6-gingerol showed much prominence in treating diabetes. The role of AMPK against diabetic effects is presented in Table 5.

5. AMPK Pathway – Role in Diabetes

AMPK [Adenosine monophosphate–activated protein kinase] is a serine/threonine protein kinase complex consisting of a catalytic α-subunit and a regulatory β-subunit and γ-subunit. AMPK is present in different organs and a number of signalling pathways are initiated in liver, heart, brain, lung, kidney and skeletal muscle and plays a prominent role in metabolism of lipids, glucose and proteins. Activation of AMPK is regulated by upstream kinases, LKB1 (Liver-kinase-B1), CaMKKβ (Ca²⁺/calmodulin-dependent protein kinase β), and TAK-1 [transforming growth factor β-activated kinase 1] phosphorylate Thr-172 of the AMPKα subunit that is essential for full activation of AMPK ⁵⁶.

The major AMPK activity in liver corresponds to upstream kinase LKB1 that plays a key role in hepatic AMPK activation. Binding of AMP or ADP promotes phosphorylation of Thr-172 by enhancing LKB1 at γ-subunit in vivo. Phosphorylation of Thr-172 can increase the AMPK activity up to 100-fold. Hence 6-gingerol uptake might activate AMPK pathway and lead to activation of hepatic genes in the liver involved in synthesis of bio-molecules promoting anti-diabetic effects. Deletion of LKB1 in the liver results in a decreased AMPK phosphorylation at Thr-172 rendering AMPK insensitive to stimuli which normally activate it. If LKB1 is lacking in mice, it failed to reduce fasting blood glucose levels on treatment with metformin, furthermore, it abolished AMPK activity completely and rendered it inactive.

The second AMPKK phosphorylated the Thr-172 by CaMKKβ, activates AMPK more than CaMKKα in response to increase in intracellular Ca²⁺ without changing AMP or ADP levels ⁵⁷. Hence AMPK activity is stimulated independently of the cellular AMP/ATP ratios if intracellular calcium levels are elevated. Rise in intracellular Ca²⁺ levels, promotes binding of Ca²⁺ to calmodulin, and form Ca²⁺/calmodulin complex. Thus formed complex binds with CaMKK and increase CaMKKα/β activity.

The third AMP-Activated Protein Kinase Kinase (AMPKK), Transforming growth factor-β-activated kinase 1 (TAK-1) promotes phosphorylation at Thr-172 and activates AMPK, but the physiological significance and intrinsic mechanism of TAK-1 remains yet to be established. TAK-1, is also activated by various cytokine receptors, thus connecting AMPK activation to extracellular inflammatory, cell growth, and apoptotic signals ⁵⁸.

AMPK may be a potential and a pharmacological target for many metabolic diseases, including treatment of Type-2-diabetes. AMPK activation is mediated by increase in AMP. AMP binding to γ-subunit activates the enzymes by 3 complementary mechanisms:

• By promoting Thr-172 phosphorylation of α-subunit of AMPK by AMPK regulators LKB1 and CaMKKβ and is inactivated through dephosphorylation of this site by protein phosphatases in response to changes of cellular Adenosine monophosphate (AMP): Adenosine Diphosphate (ADP): Adenosine Triphosphate (ATP) ratio ⁵⁹.

• Binding of AMP and ADP to γ-subunit extends the functional “half-life” of AMPK by inhibiting dephosphorylation of Thr-172 by protein phosphatases 2A/2C. In the absence of such protection, pThr-172 is rapidly dephosphorylated by phosphatases, including protein phosphatase 2A [PP2A] and 2C [PP2C] ⁵⁸.

• Enhancement of AMPK activity by allosteric activation of phosphorylated kinase is by binding of AMP to AMPK upto 10-fold ⁶⁰.

Binding of AMP/ADP promotes allosteric activation of AMPK at Thr-172 of α-subunit and activates AMPK but AMP binding to the AMPKγ subunit serves as an important regulatory feature that acts as a conformational switch to activate AMPK complex. AMP and ADP activate AMPK by promoting phosphorylation. Increased cytosolic Ca²⁺ concentration enhances the levels of phosphorylated AMPKα via modulation by Ca²⁺/Calmodulin-dependent protein kinase kinase (CaMKK) and activate AMPK. Some of the AMPK activators increase the AMP concentration, increased AMP binds to the γ-subunit of AMPK and serves as an important regulatory feature of the conformational switch that activates the AMPK complex ⁶¹. AMPK is inhibited at high concentrations of ATP, by binding of ATP to γ-subunit. Glycogen binds to the glycogen-binding domain of β-subunit allosterically and inhibits AMPK phosphorylation. The phosphorylated AMPK is active but under basal conditions the rate of dephosphorylation is high and the phosphate is immediately removed by protein phosphatase 2A/2C and may inactivate AMPK. Hence Protein phosphatases 2A and 2C dephosphorylate and inactivate AMPK in vitro.

Decline in AMPK activity corresponds to the increased risk of Type-2-diabetes and obesity. Hence AMPK is one of the potential therapeutic target in treating Type-2-diabetes and obesity. Activated AMPK promotes glucose, lipid and energy metabolism and suppresses the effects of Type-2-diabetes mellitus. Depletion in cellular energy levels, nutrient deprivation by glucose, heat shock, exercise, oxidative stress, neurohumoral factors like leptin, adiponectin, and adrenoreceptor agonists, anoxia, hypoxia, anti-diabetic drugs and ischemia results in increased AMP/ATP ratio and activates AMPK in response to metabolic stress ⁶².

Several anti-diabetic drugs like Thiazolidinediones, biguanides and Metformin are known to increase the AMP/ATP ratio and the increased AMP binds to γ-subunit by displacing ATP and activates AMPK. But Synthetic drugs might promote Oxidative stress in animals by producing reactive oxygen species (ROS) hence oxidative stress is dependent on AMP. In a similar manner inhibition of mitochondrial ATP synthesis may occur due to oxidative stress by rising AMP and ADP levels. Hence, increased AMP/ATP ratio activates AMPK. Fall in cellular energy occurs when utilization of ATP exceeds the rate of ATP generation. It promotes an increase in ATP breakdown and may increase the AMP ratio by deaminating part of adenine nucleotide to inosine monophosphate (IMP) and ammonia (NH₃). The IMP can either be reaminated back to AMP or its metabolites. Hence AMPK activation promotes an increase in AMP: ATP ratio.

Activation of AMPK is dependent on changes in the cellular adenosine nucleotides (AMP, ADP or ATP) and regulated by sensing changes in AMP: ATP and ADP: ATP ratios. ATP depletion causes increase in AMP and ADP, and AMPK gets activated by inhibiting ATP utilizing pathways (anabolic pathways) and activating ATP generating pathways (Catabolic pathways). AMPK activation alters various metabolic processes in the liver, skeletal muscle, heart, adipose tissue and hypothalamus. Depletion of energy leads to activation of AMPK and promotes ATP generating pathways such as glucose uptake, glycolysis, fatty acid oxidation, apoptosis, glucose transporter protein type-4 (GLUT-4) expression, mitochondrial biogenesis and autophagy in different tissues. Hence it generates and maintains ATP levels under metabolic stress situations by activating ATP [adenosine triphosphate] generating pathways and promotes its breakdown and inhibits their synthesis and storage ⁶³. On the other hand it inhibits ATP utilizing pathways like gluconeogenesis, glycogen synthesis, fatty acid synthesis, triglyceride synthesis, lipolysis, lipogenesis, isoprenoid synthesis, protein synthesis and DNA replication. Hence it influences almost all physiological processes in the cells, and acts as a master switch regulating glucose, lipid and protein metabolism.

AMPK activation increases the supply of ATP by promoting glucose uptake, fatty acid oxidation, insulin sensitivity, apoptosis and reduces further ATP expenditure by inhibiting fatty acid synthesis, lipogenesis, TG synthesis, lipolysis, Tumour Necrosis Factor-α (TNF-α) and Interleukin-6 (IL-6) secretion in adipose tissue. Lipopolysaccharide can induce inflammation in adipocytes via enhancing TNF-α levels to induce insulin resistance ⁶⁴. TNF-α and IL-6 acts as a link between obesity and insulin resistance in diabetics, because of over expression of this cytokines in obese humans. Overproduction of TNF-α and IL-6 induced lipolysis and apoptosis in adipocytes due to oxidative stress. IL-6 is a proinflammatory cytokine elevated during obesity is inhibited in adipose tissue on AMPK activation. Diminished AMPK activity is seen in skeletal muscle or adipose tissue of humans with T2D.

The liver is the major organ involved in the metabolism of carbohydrates and lipids and regulates energy metabolism and blood glucose levels in the body. Activated AMPK would be able to reduce lipid stores in liver and hence improve insulin sensitivity. AMPK activation is associated with the stimulation of glucose uptake, mitochondrial biogenesis and fatty acid oxidation in liver and inhibits gluconeogenesis, cholesterol and fatty acid biosynthesis, lipogenesis, glycogen synthesis and protein synthesis ⁶⁵. Hyperglycaemia is ameliorated by inhibiting gluconeogenesis in the liver.

Glucose uptake, glycolysis and fatty acid oxidation are promoted in cardiac muscle by switching on ATP-generating systems and protein synthesis are inhibited in cardiac muscle by switching off ATP-utilizing systems.

Contraction of skeletal muscle by exercise promotes AMPK activation, aiming at conserving ATP and it stimulates glucose uptake through translocation of GLUT4, fatty acid oxidation, expression of glucose transporter Type-4 (GLUT-4) in mitochondria, mitochondrial biogenesis and inhibits protein and glycogen synthesis in skeletal muscle ⁶⁰. In skeletal muscle physical exercise activated AMPK, and increased glucose uptake. In similar manner insulin also promotes glucose uptake in muscle and glucose perform glycogen synthesis, and provides energy. Hence insulin sensitivity is increased in muscle.

In thyroid it promotes Glucose uptake and inhibits iodide uptake. In the pancreas it inhibits or modulates insulin secretion by pancreatic β-cells and in hypothalamus it activates and increase food intake. Hence, activation of AMPK promotes glucose uptake, fatty acid oxidation and improves insulin sensitivity in almost all tissues like liver, heart, skeletal muscle and adipose tissue. Improved AMPK activity increased Mitochondrial biogenesis in liver, muscle and may regulate lipid metabolism. AMPK reduces lipogenic enzymes activation and promotes oxidation of fatty acids in the liver, muscle and heart. All these attributes of AMPK provides opportunity to develop a selective AMPK activator to treat lipid and glucose abnormalities ⁶⁶.

Insulin resistance promoted hyperglycaemic condition and impaired glucose uptake. Such a insulin resistance is seen in different organs that include liver, skeletal muscle, and adipose tissue. More over AMPK activation promoted glucose uptake and develops insulin sensitivity. AMPK activation increased Akt substrate of 160 kDa (AS160) and promoted glucose uptake. Hence glycolysis is promoted by phosphofructokinase-2 (PFK-2) activity by production of fructose 2,6-bisphosphate that stimulates glycolysis. Activation of AMPK alters glycogen synthesis by inhibiting glycogen synthase activity by phosphorylation. Glycogen synthase (GS) activity is inhibited both in liver by glutamine synthetase (GS2) and in skeletal muscle by glutamine synthetase (GS1) isoforms that prevents glucose storage by lowering phosphorylation of glucose-6-P Hence AMPK suppresses lipid and sterol synthesis by inhibiting acetyl-CoA carboxylase, activity leading to reduced glycogen storage by glycogen synthases ⁶⁷.

An increase in the rate of gluconeogenesis is associated with an increase in hepatic glucose output. Increased hepatic glucose output promotes excess glucose release into the blood and abnormal regulation of gluconeogenic enzymes is associated with the development of T-2-D. Therefore, inhibition of gluconeogenesis by AMPK activation is essential for future development of antidiabetic drugs. The gluconeogenic enzymes-glucose-6-phosphatase (G-6-P) and phosphoenolpyruvate carboxykinase [PEPCK] activity was decresed in liver upon AMPK activation and decreased gluconeogenesis. Decrease in gluconeogenesis is dependent on G-6-P and PEPCK expression that regulates glucose metabolism and improves hyperglycemia. In addition to PEPCK and G-6-P, cyclic-AMP-regulated transcriptional co-activator (CRTC2), CREB-regulated transcription Co-activator-2 (CREB-2) and class II HDACs (histone deacetylases) down regulates the transcription of gluconeogenic genes. Class IIA histone deacetylases (HDAC4/HDAC5/HDAC7) expression was down regulated and inhibited gluconeogenic enzymes ⁶⁸, hence inactivation of class IIa HDACs reduced hyperglycemic effets. Diabetes altered the regulation of gluconeogenesis, glycogenolysis and glycogenesis (Figure 3).

These enzyme expressions suppressed the presence of 6-gingerol whereas, AMPK activation also reduces their expression. Insulin also inhibits gluconeogenesis. Liver glycogen phosphorylase (LGP), the enzyme involved in glycogenolysis is suppressed and the glycogen synthase (GS) the enzyme involved in glycogenesis is promoted in the presence of 6-gingerol. Hence glucose metabolism is regulated in the presence of 6-gingerol by inhibiting gluconeogenesis and glycogenolysis, thus promoting glycogenesis ⁵. Hence it regulated hepatic glucose production. Activation of 6-gingerol also involved in the inhibition of the enzymes, α-amylase and α-glucosidase involved in carbohydrate digestion and potentially inhibited hyperglycemia ³⁸.

Inhibition of mammalian target of rapamycin complex 1 (mTOR), eukaryotic elongation factor 2 kinase (eEF2K) and transcription initiation factor IA (TIF-1A) caused AMPK to suppress protein synthesis by blocking ribosomal RNA synthesis through phosphorylation. Hence AMPK inhibits protein synthesis by the inhibition of the mechanistic target of rapamycin complex 1 (mTORC1), which plays a central role in protein translation and cell growth ⁶⁹. AMPK regulates TSC [tuberous sclerosis complex] activation and inhibits the mTORC1 that leads to inhibition of protein synthesis by blocking ribosomal RNA synthesis through phosphorylation. It may also activate protein translation, and stimulates cell growth by TSC1/TSC2/mTOR. Inhibition of AMPK occurs by activation of TSC2. mTORC1 activation phosphorylates the eEF2K expression and inhibited protein elongation thus decreasing protein synthesis ⁷⁰. Decreased protein synthesis by eEF2K phosphorylation decreased stress induced by hypoxia and might promote beneficial effects.

Lipid metabolism involves a balance between synthesis of fatty acids and fatty acid oxidation. Fatty acids stored in the form of triglycerides in cells are an important source of energy. But the excessive accumulation of triglycerides may increase the risk of metabolic disorders. Induction of diabetes in rats by STZ showed an increase in the levels of total cholesterol (TC), triglyceride (TG), low density lipoprotein-cholesterol (LDLC), very low density lipoprotein-cholesterol [VLDL-C] and decreased high density lipoprotein-cholesterol (HDL-C) levels ⁷¹. In obese and diabetics there is an elevation in the circulating free fatty acid and triglycerides, thus lowering the capacity of insulin to suppress hepatic glucose production by the activation of gluconeogenesis and inhibition of glycolysis ⁷² (Figure 4).

6-gingerol up-regulated and maintained the cholesterol metabolism in the liver through activation of Sterol regulator element binding proteins (SREBP). SREBP’s are involved in the maintenance of cholesterol homeostasis in liver that govern the transcription of 3-Hydroxy-3-Methylglutaryl-CoA Reductase (HMGCR) and low-density lipoprotein receptor (LDLR). Inhibition of HMG-CoA reductase is involved in lowering cholesterol levels. Plasma triglyceride, total cholesterol, free fatty acid, low-density lipoprotein, plasma insulin levels, fasting blood glucose were lowered and glucose tolerance was improved significantly in diabetic mice upon supplementation of 100 mg/kg bw of 6-gingerol. The key transcription factors involved in this process are Acetyl CoA carboxylase (ACC), fatty acid synthase (FAS), carnitine palmitoyltransferase (CPT), sterol regulatory element binding proteins (SREBPs), Malonyl-CoA decarboxylase, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoA), Glycerol phosphate acyl transferase (GPAT), Hepatocyte nuclear factor (HNF)-4α and Carbohydrate-responsive element-binding protein (CREBP).

AMPK inactivates ACC isoforms ACC1 (predominately expressed in lipogenic tissues such as liver and adipose tissue), and ACC2 (which is more common in heart and skeletal muscle) causing inhibition of fatty acid synthesis thereby promoting fatty acid oxidation ⁷³. Fatty acid oxidation is promoted by inhibition of ACC2 and Malonyl Co-A in many tissues upon AMPK activation and by inhibition of fatty acid synthesis by suppression of ACC1. Inactivation of ACC leads to the decreased conversion of acetyl-CoA to malonyl CoA. Malonyl–coenzyme A, is a key energy regulator in the cell synthesized by ACC1 and is used in fatty acid synthesis, and the malonyl-CoA generated by ACC2 is involved in the control of fatty acid oxidation. Malonyl-CoA decarboxylase (MCD) is an enzyme that degrades malonyl-CoA and promotes Fatty acid oxidation in the liver by inhibiting fatty acid synthesis on AMPK activation ⁷⁴. Hence, by inhibiting ACC and activating MCD, AMPK promotes fatty acid oxidation.

AMPK activation promotes increased MCD activity that down regulates Malonyl CoA and inhibits CPT1 ⁷⁵. AMPK activators might increase fatty acid oxidation in the liver and skeletal muscle through a pathway requiring ACC phosphorylation. Hence inhibition of ACC1 reduces lipogenic genes, such as fatty acid synthase and malonyl-CoA decarboxylase (MCD) and ACC2 inhibition may convert acetyl-CoA into malonyl-CoA and inhibits CPT1 activity thus promotes fatty acid oxidation. Hence Fatty acid synthase also plays an important role in lipogenesis. Inhibition of these enzymes regulates insulin signaling and manages hepatic metabolic disorders ⁶⁶.

AMPK controls lipid metabolism and inhibits two key enzymes acetyl-CoA carboxylase-1 (ACC1) and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoA) responsible for fatty acid and cholesterol synthesis respectively. Inhibition of HMGCoA effects ACC1 and ACC2 by inhibiting lipid and sterol synthesis in the cell ⁷⁶. AMPK phosphorylation suppresses the glycerol-3-phosphate acyltransferase (GPAT) enzyme activity, that is involved in triglyceride and phospholipid synthesis ⁶⁶. Among the isoforms of GPAT, glycerol-3-phosphate acyltransferase (GPAT1) located on the outer mitochondrial membrane has a prominent role in saturation of fatty acids, where its over expression is involved in insulin resistance.

In addition to ACC1, HNF-4α also reduced fatty acid synthesis. Hepatocyte nuclear factor (HNF)-4α is a transcription factor involved in glucose, cholesterol and fatty acid metabolism. Decreased HNF-4α activity by AMPK activation prevented lipid export by the liver, and uses energy for ATP production. Increased expression of SREBP and HMG-CoA reductase is associated with high levels of cholesterol and LDL in diabetics. SREBP-1c is a key lipogenic transcription factor, that decreases lipogenesis by controlling synthesis and uptake of fatty acids, cholesterol, and triglyceride Decreased expression of SREBP1 (sterol regulatory element binding protein 1) controls lipid metabolism, but increased expression is associated with dyslipidemia in Type-2-diabetes. In addition to this AMPK inhibits Carbohydrate-responsive element-binding protein (CREBP) and decreases transcription of lipogenic genes ⁶⁰ (Figure 5).

6. Conclusions

In conclusion, This review aims to summarize, we have analysed the content of 6-gingerol along with its beneficial effects in treating diabetic complications. Activation of AMPK pathway by 6-gingerol helps in restoring the β-cell functioning capacity due to its hypoglycemic effects. 6-gingerol altered the Metabolic pathways, including Carbohydrates, Lipids and Proteins and paved a way to decline the diabetic effects in presence of AMPK pathway. Though ginger has many beneficial effects uptake beyond the limits might promote harmful effects. Hence, further research is essential to carry out on 6-gingerol to check that at which dose of ginger it promotes beneficial effects.

Abbreviations

6-G: 6-Gingerol; G: Ginger; AD: Anti-diabetic; AMPK: Adenosine Monophosphate-Activated Protein Kinase; TX: Toxicity; BC: Bio-active Compounds; HPLC: High Performance Liquid Chromatography; AMP: Adenosine monophosphate; ATP: Adenosine Triphosphate; ADP: Adenosine Diphosphate; GPAT: Glycerol Phosphate Acyl Transferase; HNF: Hepatocyte Nuclear Factor; ACC: Acetyl CoA Carboxylase; FAS: Fatty Acid Synthase; CPT: Carnitine Palmitoyltransferase; SREBPs: Sterol Regulatory Element Binding Proteins; MCD: Malonyl-CoA Decarboxylase; HMGCoA: 3-Hydroxy-3-Methylglutaryl-CoA Reductase; GPAT: Glycerol Phosphate Acyl Transferase; HNF-4α: Hepatocyte Nuclear Factor; CREBP: Carbohydrate-responsive Element-binding Protein.

Acknowledgements

The authors would like to acknowledge the Department of Zoology of the Sri Venkateswara University for allowing us to use their laboratory fecilities for our research work.

Declaration of Competing Interest

The authors have declared that they have no conflicts of interest in this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Financial Support

This research acquired no particular provide from any funding agency in the public, commercial or non-income sectors.

Authorship Contribution Statement

P.V.R: Reviewed the manuscript. Y.A: Was a major contributor in editing and revising. M.S.R: conceptualized and reviewed the draft, Supervised data collection, Data curation, Formal analysis, Investigation, Methodology, Resources, Software, Supervision, Validation, Visualization, Writing-original draft, Writing-review and editing. Both authors critically edited the text and reviewed the final version.

Ethics Approval

Not applicable.

Consent of Publication

Not applicable.

Availability of Data and Materials

The references were used to gather all the information in the manuscript.

References

Graphical Abstract