Rhizoma coptidis as a Potential Treatment Agent for Type 2 Diabetes Mellitus and the Underlying Mechanisms: A Review

Diabetes mellitus, especially type 2 diabetes mellitus (T2DM), has become a significant public health burden. Rhizoma coptidis (RC), known as Huang Lian, is widely used for treating diabetes in China. The bioactive compounds of RC, especially alkaloids, have the potential to suppress T2DM-induced lesions, including diabetic vascular dysfunction, diabetic heart disease, diabetic hyperlipidemia, diabetic nephropathy, diabetic encephalopathy, diabetic osteopathy, diabetic enteropathy, and diabetic retinopathy. This review summarizes the effects of RC and its bioactive compounds on T2DM and T2DM complications. Less research has been conducted on non-alkaloid fractions of RC, which may exert synergistic action with alkaloids. Moreover, we summarized the pharmacokinetic properties and structure-activity relationships of RC on T2DM with reference to extant literature and showed clearly that RC has potential therapeutic effect on T2DM.


INTRODUCTION
Diabetes is an incurable metabolic disorder which affects more than 135 million people worldwide (Zhu and Zhang, 2016). It has been estimated that nearly 95% of diabetics worldwide are type 2 diabetes mellitus (T2DM) (Jhong et al., 2015;Thomas and Philipson, 2015). The characteristics of T2DM are mainly insulin resistance and insufficient insulin secretion, which result in high levels of blood glucose (DeFronzo, 2004). Long-term hyperglycemia results in inflammation, oxidative stress, and other pathological changes, which eventually lead to multiple organ injury, including diabetic nephropathy, diabetic neuropathy, and diabetic cardiomyopathy (Forbes and Cooper, 2013;Konig et al., 2013;Rios et al., 2015;Labazi and Trask, 2017). At present, the therapeutic strategies used for diabetes are mainly insulin injection and administration of chemical hypoglycemic drugs. However, these treatments are associated with adverse side effects, such as gastrointestinal problems and hypoglycemia (Teng and Chen, 2017). Many traditional Chinese medicines have been used in treating T2DM. It is of interest that these traditional medicines have acceptable preventive and therapeutic effects on diabetic complications through their multiple constituents and multiple routes of action (Chen et al., 2018).
Rhizoma coptidis (RC) is the dried rhizome of medicinal plants from the family Ranunculaceae: Coptis chinensis Franch., Coptis deltoidea C. Y. Cheng et Hsiao, and Coptis teeta Wall (Figure 1) (Tan et al., 2016). RC and its processed products Jiuhuanglian have been used against diabetes in China for more than 1,400 years (Zhou et al., 2008;Wang et al., 2014). Many famous formulas containing RC, such as San-Huang-Xie-Xin-Tang (Wu et al., 2016), and Jiao-Tai-Wan (Chen et al., 2013a), have also been shown to exert therapeutic effects on T2DM. More than 100 chemical constituents have been identified in RC. These include alkaloids, simple phenylpropanoids, flavonoids, and other compounds (Wang et al., 2019). Researches of RC in T2DM have mainly focused on berberine, coptisine, palmatine, epiberberine, jatrorrhizine, and magnoflorine. Other alkaloids, including berberrubine, columbamine, and berbamine, also have been shown to possess anti-diabetic potential. Non-alkaloidal constituents of RC, such as polysaccharides and polyphenols, also have potential therapeutic effects on T2DM and may promote the effects of alkaloids (Chen et al., 2012;Jiang et al., 2013).
Given the increasing populations of T2DM patients, the potential benefits in exploration of new drugs for T2DM have attracted many researchers to carry out in-depth studies on the effect of RC on  Table 1, RC regulates various signaling pathways and regulates the expressions of proteins and genes in many tissues and organs. Thus, it has good preventive and therapeutic effects on the complications of T2DM. However, the underlying mechanisms are still largely unknown. More molecular studies in the future are expected to elucidate the definite mechanism of its actions, especially the synergism between non-alkaloids and alkaloids. Large-scale clinical researches are necessary for the evaluation of its safe clinical dosage, interaction with other drugs, and specific indications in T2DM.

Adjustment of Glycometabolism
Inhibition of Digestion and Absorption of Sugar in the Digestive Tract α-Glucosidase and disaccharidases are necessary for conversion of carbohydrates into simple sugars and their absorption in the intestine (Liu et al., 2010a;Hui et al., 2017). RC, coptisine, epiberberine, jatrorrhizine, berberine, and palmatine possess strong α-glucosidase inhibitory effects (Li et al., 2012b;Ge et al., 2014;Zhou et al., 2014;Chen et al., 2017a). It has been reported that berberine significantly inhibited the activity of disaccharidases in protein kinase A-dependent pathway and reduced the mRNA expressions of sucraseisomaltase complex in diabetic rats and normal rats (Liu et al., 2010a). Berberrubine dose-dependently decreased the activity of intestinal α-glucosidase, and interestingly, berberrubine (50 mg/kg) showed stronger hypoglycemic effect than berberine (120 mg/kg) (Yang et al., 2017b). Therefore, by inhibition of carbohydrate digestion, RC directly inhibits increases in postprandial blood glucose levels.

Promotion of Glucose Uptake and Catabolism
Insulin (INS) induces the activation of insulin receptors, which leads to the recruitment of insulin receptor substrate (IRS) protein and activation of PI3K/Akt/AMPK, as well as glucose transporters (GLUTs) which promote glucose uptake (Wilcox, 2005;Hu et al., 2017). Studies have shown that RC polysaccharides decreased fasting blood glucose (FBG) by regulating JNK/IRS1/PI3K pathway (Jiang et al., 2015a), improved the phosphorylation of AMPK in muscle and liver, and mediated glucose uptake in 3T3-L1 adipocytes by regulating PI3K/AMPK pathway (Cui et al., 2016). Berberine increased mRNA and protein expressions of insulin receptor in various human cell lines (Zhang et al., 2010). It promoted the expression of GLUT-4 mRNA in skeletal muscle (Liu et al., 2015) and improved the expression of insulin receptor substrate (InsR) 1, GLUT-1, and GLUT-3 in brain (Sandeep and Nandini, 2017). Palmatine also promoted glucose uptake of L6 cells by promoting the expression of GLUT-4 (Sangeetha et al., 2013). It has been demonstrated that the natural mass ratio of berberine and ferulic acid was approximately 20:1, and their synergistic effect indicated that RC has better clinical application value than berberine (Chen et al., 2012). Dietary polyphenols are effective supplements for the treatment of T2DM (Cao et al., 2018a). Octadecyl caffeate, a dietary polyphenol of RC, stimulated glucose uptake at a dose of 50 μg/ ml in skeletal muscles (Yang et al., 2014a). Coptisonine from RC also stimulated glucose uptake at a dose of 25 μg/ml in skeletal muscles (Yang et al., 2014a).
When cells are in a state of low energy charge, AMPK is activated to promote glucose uptake and utilization (Merrill et al., 1997). In HepG2 hepatocytes and C2C12 myotubes, berberine bi-directionally regulated the expression of AMPK and promoted glucose metabolism by stimulating glycolysis, an effect which may not be related to AMPK activity (Xiao et al., 2018). Similarly, a study has also indicated that AMPK plays an important but not dependent role in the ability of berberine to promote glucose uptake .

Protection of Islet β Cells
Cytokines produced by the infiltration of immune cells in islets are important mediators of β cell destruction in T2DM. RC extract and berberine exerted protective effect on FIGURE 3 | Mechanisms of RC on diabetic vascular dysfunction and diabetic heart disease. -• with different colors indicate inhibition/reduction while → with different colors indicate increase/promotion. Arrows of different colors depict different references, and when the results of mechanism research overlap, just randomly select one of the colors. BBR represents berberine. July 2019 | Volume 10 | Article 805 Frontiers in Pharmacology | www.frontiersin.org FIGURE 4 | Mechanisms of RC on diabetic nephropathy. -• with different colors indicate inhibition/reduction while → with different colors indicate increase/ promotion. Arrows of different colors depict different references, and when the results of mechanism research overlap, just randomly select one of the colors. BBR, Cop, and Jat represent berberine, coptisine, and jatrorrhizine.
FIGURE 5 | Mechanisms of RC on diabetic encephalopathy. -• with different colors indicate inhibition/reduction while → with different colors indicate increase/ promotion. Arrows of different colors depict different references, and when the results of mechanism research overlap, just randomly select one of the colors. BBR and Pal represent berberine and palmatine.
cytokine-induced apoptosis of β cells (Kim et al., 2007;Wang, 2013). When Rin-5F cells were treated with RC and berberine, glucose-stimulated insulin release, cell cycle, lipotoxic islet cell proliferation, and protein expression of poly(ADP-ribose) polymerase 1 (PARP-1) were altered and improved, which suggest that RC may protect β cells by improving islet β cell proliferation and PARP-1 protein expression . RC extract inhibited cell apoptosis and necrosis of Rin-m5F cells induced by S-nitroso-N-acetylpenicillamine via elimination of disruption of mitochondrial membrane potential (Kwon et al., 2005). In addition, islet cells pretreated with RC extract retained insulin secretion even after treatment with interleukin (IL)-1β (Kwon et al., 2005). Toll-like receptor 4 (TLR4) inflammatory pathway mediates β cell injury in T2DM. Berberine attenuated lipopolysaccharide (LPS)-induced inflammatory and apoptotic responses in β cells via TLR4-independent JNK/NF-κB pathway (Wang, 2013). Berberine also inhibited apoptosis of β cells by down-regulation of Bax/Bcl-2 gene expressions (Chueh and Lin, 2012). RC polysaccharides also significantly inhibited the formation of protein glycation and advanced glycation end products (AGEs) in pancreas and suppressed hyperglycemiainduced damage to the pancreas (Yang et al., 2016b).
Oxidative stress is a pathological process of tissue and organ damage caused by imbalance between oxidants and antioxidants in vivo, which is a crucial risk factor for diabetes mellitus (Giacco and Brownlee, 2010;Rochette et al., 2014). Silent information regulator 1 (SIRT1) exerts both anti-oxidative and anti-inflammatory effects (Chen et al., 2013b). The SIRT1 is the target gene of miR-106b, and over-expression of miR-106b can reverse the anti-oxidative effect of berberine in NIT-1 cells cultured in a high-glucose medium (Chen and Yang, 2017). This suggests that berberine protects islet β cells partially via SIRT1/miR-106b pathway.

Promotion of Insulin Secretion
Oral administration of berbamine (50,100,200 mg/kg) for 56 days improved insulin secretion in a dose-dependent manner in T2DM rats (Sankaranarayanan et al., 2018). Oral administration of palmatine, jatrorrhizine, or magnoflorine (10, 20, and 40 mg/kg) raised the serum insulin level in glucose-fed rats (Patel and Mishra, 2011). RC polysaccharides also restored islet size and increased insulin secretion in a T2DM model (Cui et al., 2016).
Incretin is a gut-derived hormone, which responses to nutrient intake (mainly glucose and fat). It is considered to be the strongest insulin-secreting hormone, and it inhibits appetite and glucagon secretion and delays gastric emptying (Yabe and Seino, 2011). Glucagon-like peptide-1 (GLP-1) is an intestinal hormone with incretin-like function (Chu et al., 2007). It is rapidly inactivated by dipeptidyl peptidase 4 (DPP-4) enzyme (Simona and Itamar, 2011). Thus, inhibition of DPP-4 and increase in GLP-1 level in vivo can improve the responsiveness of β   cells to glucose and promote insulin secretion, while inhibiting glucagon secretion, thereby reducing FBG and postprandial blood glucose (Drucker and Nauck, 2006;Del Prato et al., 2011). Berberine is a DPP-4 inhibitor (Al-Badri et al., 2018). In a study, it was shown that berberine reduced plasma glucose level of streptozotocin (STZ)-induced diabetic rats by inhibiting DPP-4 and increasing GLP-1 level in intestine (Wang et al., 2016a). Berberine is a ligand for bitterness receptors. Studies have shown that berberine stimulates GLP-1 secretion by activating intestinal bitterness receptor TAS2R38 in a phospholipase C (PLC)-dependent manner . The L-cells are located in the colon and are responsible for secretion of GLP-1. Berberine also improved GLP-1 expression by up-regulating the expression of GPR43 (a receptor of SCFAs) and suppressed ATP overproduction-induced mitochondrial stress in L-cells (Sun et al.). After GLP-1 activation, adenylate cyclase is activated and cyclic adenosine monophosphate is produced, resulting in activation of the second messenger pathway and closure of the adenosine triphosphate-dependent potassium channel. Increased intracellular potassium induces depolarization and leads to calcium influx through voltage-dependent calcium channels. The increase in intracellular calcium stimulates the migration and exocytosis of insulin (Cicero and Tartagni, 2012). Uncoupling protein 2 (UCP2) regulates glucose-stimulated insulin secretion. A study has shown that berberine inhibited oxidative stress and restored insulin secretion in high-glucosetreated INS-IE cells and db/db mice islets via the AMPK/UCP2 cascade .

Mitigation of Insulin Resistance
The development of T2DM is usually associated with insulin resistance and impaired glucose uptake in peripheral tissues (DeFronzo, 2004). Peroxisome proliferator-activated receptor (PPAR) γ is a well-known anti-diabetic insulin sensitizer receptor related to fat production and glucose homeostasis (Sharma and Staels, 2007;Cheatham, 2010;Sangeetha et al., 2013). Five known alkaloids, berberine, epiberberine, coptisine, palmatine, and magnoflorine inhibited the downregulation of PPARγ in 3T3-L1 cells in a dose-dependent manner (Choi et al., 2014). In L6 cells, palmatine positively modulated the expressions of PPARα and PPARγ (Sangeetha et al., 2013). Berberine also reduced fasting serum insulin and enhanced glucose uptake and insulin sensitivity through several mechanisms including 1) up-regulation of the expression of insulin receptor in a protein kinase C (PKC)-dependent manner (Kong et al., 2009); 2) downregulation of the expressions of resistin and leptin in insulin resistant 3T3-L1 adipocytes (Tu et al., 2016); 3) regulation of bone morphogenetic protein (BMP) 4 transcriptional pathways and brown adipose tissue transcriptional pathways in white/ brown adipose tissues ; and 4) regulation of transcription of sterol regulatory element-binding transcription factors (SREBPs), liver X receptors (LXRs), and PPARs in liver and visceral white adipose tissue (Liu et al., 2010b;Li et al., 2011). In some insulin-resistant patients, protein tyrosine phosphatase 1B (PTP1B) interacted with insulin receptor and induced insulin receptor dephosphorylation, resulting in inactivation of insulin signaling pathway (Ahmad et al., 1997;Combs, 2010).
Alkaloids of RC, such as berberine, epiberberine, magnoflorine, and coptisine showed inhibitory effects on PTP1B due to their higher affinity and tighter binding capacity to the active site of PTP1B (Bustanji et al., 2006;Choi et al., 2015). Berberine and epiberberine showed mixed-type inhibition against PTP1B, while magnoflorine and coptisine noncompetitively inhibited PTP1B (Choi et al., 2015). Retinol-binding protein 4 (RBP4), a cytokine secreted by adipocytes, led to insulin resistance and negatively correlated with the expression of GLUT-4 in insulinresistant states (Yang et al., 2005;Graham et al., 2006). Berberine lowered serum RBP4 level and up-regulated the expression of GLUT-4, thereby promoting glucose uptake in insulin-resistant cells .

Regulation of Gut Microbiota
T2DM is associated with imbalance in gut microbiota (Cani et al., 2008). Regulation of gut microbiota improved glucose intolerance and changed inflammation and metabolic status (Han et al., 2011). After administration of RC extract, the diversity of gut microbiota in T2DM rats was changed, suggesting that RC ameliorated the disordered gut microbiota in T2DM (Ning-Ning et al., 2017). Due to the poor absorption of berberine, the regulation of intestinal microorganisms has been assumed to be one of the mechanisms involved in its anti-diabetic effects (Han et al., 2011). The balance between beneficial gut bacteria (e.g., SCFAs-producing bacteria) and opportunistic pathogens (e.g., endotoxin-producing bacteria and sulfate-reducing bacteria) is important for intestinal homeostasis (Qin et al., 2012). Berberine inhibited high-fat diet (HFD)-induced reduction of intestinal butyric acid bacteria, i.e., Blautia, Bacteriodes, Butyricoccus, Allobaculumand, and Phascolarctobacterium, resulting in antiinflammatory properties and improved insulin resistance (Jakobsdottir et al., 2013;Zhang et al., 2015;Wang et al., 2017). It has been reported that dihydroberberine has better hypoglycemic effect than berberine because it is more easily absorbed by intestinal epithelial cells (Turner et al., 2008). Further studies have shown that dihydroberberine is quickly converted to berberine after absorption, suggesting that berberine is the real hypoglycemic agent. Moreover, gut microbiota can convert berberine into dihydroberberine, thereby enhancing the absorption of berberine (Feng et al., 2015).

Diabetic Vascular Dysfunction
Endothelial dysfunction, as characterized by impaired endothelium-dependent vasodilation and reduced bioavailability of nitric oxide (NO), is closely related to T2DM (Huang, 2003;Félétou and Vanhoutte, 2006;Forstermann and Munzel, 2006). Studies have shown that berberine promoted insulininduced vasodilatation in rats and ameliorated cell viability and autophagy in human artery endothelial cells by activation of the InsR/AMPK/Akt/eNOS pathway (Geng et al., 2016). In aortic endothelial cells, berberine prevented hyperglycemia-induced endothelial injury and enhanced vasodilatation via downregulation of endothelial NO and reactive oxygen species (ROS) synthase through AMPK-dependent approaches (Wang et al., 2009b). In palmitic acid-treated human umbilical vein endothelial cells, berberine also significantly increased the bioavailability of NO and reduced the production of ROS with the up-regulation of p-AMPK protein levels . Altogether, these findings suggest that the activation of AMPK is necessary for berberine to regulate endothelium-dependent vasodilation. In rat brain, berberine protected microvascular endothelial cells from hypoxia/high-glucose-induced impairment through the SIRT1/HIF-1α/VEGF pathway (Mi et al., 2019).
The tolerance of diabetic patients to myocardial ischemia is significantly reduced by microvascular damage, leading to increase in myocardial infarction area and poor recovery of cardiac function after reperfusion (Iwakura et al., 2003;Kurisu et al., 2003). Berberine exerted protective effects against ischemic arrhythmias in T2DM rats possibly by regulation of ion channels and recovery of the balance of electric parameters (Wang et al., 2011;Wang et al., 2012). It also inhibited apoptosis and improved cardiac function after ischemia/reperfusion by activating AMPK and PI3K/Akt/GSK3β/eNOS signaling pathway in diabetic rats Chang et al., 2016).

Diabetic Hyperlipidemia
Abnormalities in lipid metabolism, including disturbance of fatty acids and lipoproteins, are among the major factors contributing to an increased cardiovascular risk in T2DM (Verges, 2015;Soran et al., 2016). Diabetic hyperlipidemia is usually reflected in increase in free fatty acids (FFA), triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), apolipoprotein B (ApoB), and decreased levels of high-density lipoprotein cholesterol (HDL-C) (Betteridge, 1999;Goldberg, 2001). In a T2DM model, RC reduced blood lipid level and down-regulated expressions of sterol regulatory element-binding protein 1c (SREBP-1c) and SREBP cleavage-activating protein (SCAP) in the liver . Berberine mitigated disorder in lipid metabolism partly by decreasing expressions of TNF-α and FFA, while increasing the activity of lipoprotein lipase (Zhou and Zhou, 2010). Jatrorrhizine also down-regulated SREBP-1c and fatty acid synthase (FAS) mRNA levels in liver, while up-regulating the expressions of PPAR-α and carnitine pecalmitoyltransferase 1A (CPT1A), thereby lowering serum TG, TC, and LDL-C levels and increasing serum HDL-C level (Yang et al., 2016a). By up-regulation of HNF-4α and fetoprotein transcription factor (FTF), columbamine promoted cholesterol 7α-hydroxylase (CYP7A1) promoter activity and enhanced the conversion of cholesterol to bile acid and its excretion in the feces (Wang et al., 2016b). Berberrubine exerted lipid-lowering effects through regulation of low-density lipoprotein receptor (LDLR) and pro-protein convertase subtilisin/kexin type 9 (PCSK9) in the extracellular signal-regulated kinase 1/2(ERK1/2) signaling pathway (Cao et al., 2018b). RC polysaccharides also reduced the levels of TG and TC in diabetic mice (Jiang et al., 2013).

Diabetic Nephropathy
Hyperglycemia leads to abnormal glycosylation of proteins (Lassila et al., 2004), disorders in lipid metabolism (Sieber et al., 2010), abnormal changes in glomerular hemodynamics (Reudelhuber, 2010), and activation of protein kinase pathways (Soetikno et al., 2011). These changes eventually lead to local renal microcirculation disorders, thickening of glomerular basement membrane (GBM), deposition of extracellular matrix (ECM), glomerulosclerosis, and interstitial fibrosis. Berberine inhibits or delays the occurrence and development of diabetic nephropathy through a variety of mechanisms, as shown in Table 1 and Figure 4.
Berberine significantly reduced the kidney weight, urinary protein content, and blood urea nitrogen (BUN) levels in T2DM (Liu et al., 2008a). The TGF-β1 plays an extremely important role in the therapeutic effect of berberine. Through regulation of upstream targets such as nuclear factor-erythroid 2-related factor 2 (Nrf2) , AGEs , and activator protein-1 (AP-1) (Lan et al., 2014), berberine inhibited the expression of TGF-β1, thereby influencing downstream signals via 1) activation of PI3K/Akt pathway (Wang et al., 2018c), 2) inhibition of the expressions of Smad2 and Smad3, and 3) suppression of the matrix metalloproteinases (MMPs)/ tissue inhibitor of matrix metalloproteinases (TIMPs) system (Ni et al., 2015). These changes ultimately lead suppression of the thickening of GBM, deposition of ECM, glomerulosclerosis, and interstitial fibrosis.
The suppression of p38MAPK up-regulates the expressions of cAMP-response element binding protein (CREB) and fibronectin (FN), suggesting that p38MAPK participates in the accumulation of ECM. Berberine partly reduced highglucose-induced FN expression and collagen synthesis in mesangial cells by suppression of p38MAPK (Liu et al., 2009). It inhibited glomerular mesangial cell proliferation through regulation of PGE2-EPs-G protein-adenylyl cyclase (AC)-cAMP-Ca 2+ signaling pathway Wang et al., 2013a;Yang et al., 2014b;Ni et al., 2016) and inhibited Snail expression through regulation of Notch signaling pathway, leading to suppression of the high-glucose-induced epithelialto-mesenchymal transition (Yang et al., 2017a). It has been reported that the anti-inflammatory and anti-oxidative activities of berberine partly contributed to its therapeutic effects on diabetic nephropathy, including inhibition of the expressions of Nrf2/HO-1, NF-κB, and the production of ROS Zhu et al., 2018).
Cellular polyol pathway is activated by persistent high-glucose stimulation that leads to cellular dysfunction accompanied by oxidative stress injury (Sytze et al., 2013). The alkaloids berberine, palmatine, coptisine, and jatrorrhizine, which contain isoquinoline/ bis(isoquinoline) and related ring structures, have exhibited strong aldose reductase (AR) inhibitory activities and can be used to control diabetic complications including nephropathy (Liu et al., 2008a;Liu et al., 2008b;Gupta et al., 2014).

Diabetic Encephalopathy
Chronic and persistent hyperglycemia may impair central nervous system function, affect cognitive function, and lead to dementia (Luchsinger et al., 2001). As shown in Table 1 and Figure 5, RC exerts therapeutic effect on diabetic encephalopathy through different channels. Total alkaloids of RC alleviated cognitive impairment in type 2 diabetic rats by reducing Aβ deposition and enhancing insulin signaling (Li et al., 2018c). Palmatine inhibited the expression of P2X receptor in hippocampus, resulting in attenuation of diabetic neuropathy, which may be related to the inhibition of ERK1/2 phosphorylation and the release of tumor necrosis factor (TNF)-α and IL-1β in hippocampus (Shen et al., 2018). Berberine easily penetrates the blood-brain barrier, and its elimination is slow in brain (Wang et al., 2005). It affects diabetic encephalopathy through the following ways: 1) regulation of SIRT1/endoplasmic reticulum (ER) stress pathway and Nrf2/HO-1 pathway (Hsu et al., 2013), leading to enhancement of the antioxidant capacity of nerve cells (Li et al., 2018b); 2) restoration of hippocampal synaptic plasticity (Kalalian- Moghaddam et al., 2013) and activation of PI3K/Akt/GSK-3β/tau pathway to prevent axonopathy ; 3) reduction of inflammation by inhibition of NF-κB/MAPK pathway Chen et al., 2017b); 4) inhibition of the expression of beta-site amyloid precursor protein-cleaving enzyme 1 (BACE-1) leading to reduction of Aβ deposition ; 5) inhibition of neuronal apoptosis by regulation of mitochondrial pathway (Friedemann et al., 2014); 6) prevention of pain through inhibition of the PKC/TRPV1 pathway; and 7) promotion of the expression of GLUT3 to enhance glucose absorption and utilization in the brain.
Diabetic vascular dysfunction is usually accompanied by impairment of intracellular Ca 2+ handling in vascular smooth muscle cells (VSMCs) (Fernández-Velasco et al., 2014). Hyperglycemia induces increases in intracellular Ca 2+ by activating long-lasting voltage-dependent Ca 2+ channels, thereby promoting Ca 2+ influx and altering Ca 2+ release from the ryanodine receptors (RyRs) in VSMCs (Navedo et al., 2010;Krishnamoorthynatarajan and Koide, 2016). In cerebral VSMCs, berberine regulated intracellular Ca 2+ handling by inhibition of current density and α1C subunit expression in long-lasting voltage-dependent Ca 2+ channels, reducing the release of RyRs and the concentration of intracellular Ca 2+ , leading to reduction in blood pressure and reversal of impaired vascular relaxation in STZ-induced T2DM model (Ma et al., 2016b;Ma et al., 2017).

Diabetic Osteoporosis
Diabetic osteoporosis is a serious complication of diabetes mellitus in the skeletal system, which is characterized by destruction of bone tissue structure, reduction in bone mass, increase in bone fragility, and susceptibility to fractures (Kanazawa, 2017;Rathinavelu et al., 2018;Zhang et al., 2019a). Hyperglycemia also stimulates oxidative stress in bone tissue and promotes bone reabsorption of osteoclasts (Cunha et al., 2014). In STZ and HFD-induced diabetic osteoporosis, berberine significantly up-regulated the activity of serum antioxidants, thereby lowering oxidative stress-induced damage to DNA . Hyperosmotic state induced by hyperglycemia affects osteoblast formation by promoting urinary calcium excretion and reducing blood calcium concentration and bone calcium deposition (Moerman et al., 2004). Insulin promotes the metabolism of osteoblasts and increases bone formation, whereas the reduction of insulin secretion may result in decreased metabolism and bone formation by osteoblasts (Hamann et al., 2012). Bone marrow mesenchymal stem cell differentiation into adipocytes is promoted by PPAR-γ, which also inhibits osteoblast differentiation, thereby reducing bone mineral density and increasing fracture risk (Kahn et al., 2006;Smith et al., 2015). Berberine treatment alone, or in combination with pioglitazone, remarkably ameliorated expressions of AMPK and PPAR-γ mRNA and reversed abnormal urinary calcium and serum insulin levels in diabetic rats (Adil et al., 2017). In some studies on STZinduced diabetic rats, bone resorption markers such as urinary calcium and serum tartrate-resistance acid phosphatase (TRAP) were activated but were down-regulated by berberine (Adil et al., 2017;Xie et al., 2018). Receptor activator for nuclear factor-κB ligand (RANKL) is a binding partner for osteoprotegerin (OPG). Berberine improved osteoclast and osteoblast differentiation by regulating RANKL and OPG expressions in bone marrow stromal cells (Xue et al., 2012). Runt-related transcription factor 2 (Runx2) regulates the expressions of various osteoblastrelated ECM proteins in osteoblasts, including osteocalcin (a bone formation marker) to promote bone formation (Haxaire et al., 2016;Liu et al., 2016). Berberine treatment alone or in combination with pioglitazone remarkably ameliorated RANKL, OPG, Runx2, and osteocalcin in diabetic rats (Adil et al., 2017).

Diabetic Enteropathy
Compared with non-diabetic patients, diabetic patients are more likely to have gastrointestinal symptoms, including abdominal pain, abdominal discomfort, heartburn, and diarrhea (Bytzer et al., 2001;Bytzer et al., 2002). Diabetes mellitus is accompanied by inflammation and oxidative stress, which lead to impaired intestinal permeability, and transfer of enteric LPS into the blood, eventually leading to endotoxemia (Cani et al., 2009;Yamamoto and Yamamoto, 2013). In STZ plus HFD-induced diabetic model, berberine regulated TLR4/MyD88/NF-κB signaling pathway in intestinal tissues (Gong et al., 2017) and promoted the expressions of glutamine-induced glucagon-like peptide-2 (GLP2) and tight junction protein zonula occludens 1 (ZO1), resulting in significant recovery of intestinal villus/mucosa structure and less infiltration of inflammatory cells, amelioration of intestinal barrier abnormalities, and endotoxemia (Shan et al., 2013). The regulation of intestinal flora disorders has already been described in 1.7.

Diabetic Retinopathy
Diabetic retinopathy is the most common microvascular complication in diabetic patients (Calderon et al., 2017). Hyperglycemia induces blood-retinal barrier injury. Lowdensity lipoprotein (LDL) leaks through blood-retinal barriers and is subsequently modified by oxidation and glycosylation to products which induced death in several types of human retinal cells Fu et al., 2014). Müller cells are the major glia of the retina that provide support to retinal neurons. They control blood flow, modulate neuronal activity and glucose metabolism, and maintain the blood-retinal barriers (Canning et al., 2007;Ahmad et al., 2011). Berberine inhibited highly oxidized glycated-LDL-induced damage to Müller cells by activating the AMPK pathway (Fu et al., 2016). Studies have shown that leukocytes from diabetic mice caused direct injury and death of retinal endothelial cells (Li et al., 2012a;Talahalli et al., 2013). The secretion of inflammatory mediators leads to the recruitment and exudation of leukocytes, thereby enhancing the inflammatory processes (Chen et al., 2018). Berberine significantly inhibited leukocyte adhesion to human retinal endothelial cells as well as leukocyte-mediated killing of endothelial cells by inhibiting the downregulation of intercellular cell adhesion molecule-1 (ICAM-1) (on endothelial cells) and integrin ligand integrin beta-2 (CD18) (on leukocytes), suppressing the activation of NF-κB and improving antioxidant enzymes (SOD, CAT, and GSH-Px) in the retina (Tian et al., 2013). In IL-1β or TNF-α evoked human retinal pigment epithelial cell line (ARPE-19), berberrubine dose-dependently inhibited the expression and protein secretion of IL-8 and monocyte chemotactic protein-1 (MCP-1) through regulation of NF-κB pathway (Cui et al., 2006). AR is an important enzyme in the polyol pathway, which plays a key role in the occurrence of diabetic complications (Jesús Angel et al., 2003). Studies have shown that RC extract and its alkaloids (epiberberine, coptisine, groenlandicine, jatrorrhizine, palmatine, and magnoflorine) exhibited moderate inhibitory activities on rat lens aldose reductase (RLAR), which partly contributed to the anti-diabetic retinopathy effect of RC (Jung et al., 2008;Patel and Mishra, 2012).

PHARMACOKINETIC PROPERTIES OF RHIZOME COPTIDIS IN T2DM
It is generally known that patients are the consumers of drugs. Therefore, studies on the pharmacokinetic properties of RC in pathological conditions of T2DM are quite necessary. Pathological conditions of T2DM, including hyperglycemia, hyperlipidemia, and autonomic neuropathy, affect hemorheological parameters and change the absorption and distribution of drugs (Stacher et al., 2003). It has been reported that T2DM delayed gastric emptying, resulting in prolonged transit time of drugs in gastrointestinal tract, thereby promoting their absorption (Chiu et al., 2014). Moreover, it has been found that pharmacokinetic properties of berberine in normal and T2DM rats were quite different, and that the bioavailability of berberine was improved in T2DM rats (Jia et al., 2017).
Relative to normal rats, T2DM rats exhibited increased pharmacokinetic parameters for berberine (20 mg/kg). Cmax, t 1/2 , and AUC (17.35 ± 3.24 vs 34.41 ± 4.25 μg/L, 3.95 ± 1.27 vs 9.29 ± 2.75 h, 151.21 ± 23.96 vs 283.81 ± 53.92 μg⋅h/L, respectively) and oral clearance rates (134.73 ± 32.15 vs 62.55 ± 16.34 L/h/kg) were decreased significantly. Intestinal ATP-dependent efflux pump P-glycoprotein (PGP) is distributed widely in the intestinal epithelium and hepatocytes, and it reduces drug efficiency of protoberberine alkaloid (Pan et al., 2010;Zhang et al., 2019b). A study revealed that intestinal P-GP was impaired in STZinduced T2DM model, which partly enhanced the absorption of the five protoberberine alkaloids (berberine, palmatine, coptisine, epiberberine, and jatrorrhizine) (Yu et al., 2010). Small structural differences (e.g., the location and number of hydroxyl groups) led to different binding affinities to P-GP (e.g., berberine and coptisine) (Cui et al., 2015;Zhang et al., 2019b). In normal rats, berberine was quickly distributed in organs, and the concentrations in organs were higher than that in blood . Similarly, in HFD and STZ-induced diabetic rats, berberine was quickly distributed in the liver, kidney, and lung, and the concentrations in organs were higher than that in blood (Cui et al., 2018).
Berberine is the main active component of RC. However, the bioavailability of berberine is very low. Nevertheless, clinical applications of RC showed that it has high hypoglycemic effects. RC is generally used in the form of formula in traditional Chinese medicine, such as Jiao-Tai-Wan and San-Huang-Xie-Xin-Tang. It has been revealed that the Cmax and AUC(0-∞) of Jiao-Tai-Wan were higher than those of RC in diabetic mice, and that cinnamon in Jiao-Tai-Wan promoted bioavailability of RC (Chen et al., 2013a). In STZ and HFD-induced T2DM model, the pharmacokinetic curves (especially AUC, Tmax, and Cmax) of coptisine, berberine, and palmatine of San-Huang-Xie-Xin-Tang in normal and T2DM rats were quite different .

STRUCTURE-ACTIVITY RELATIONSHIPS OF PROTOBERBERINE ALKALOIDS IN T2DM
Substitutions at positions C-2, C-3, C-9, and C-10 play important roles in anti-diabetic effects of protoberberine alkaloids of RC. It has been shown that the binding affinity to β-cell sulfonylurea receptors disappeared when the methylenedioxy group in C-2 or C-3 was replaced by other groups (Bian et al., 2006). When C-9 and C-10 were both substituted by methoxyl group, the hypoglycemic effect was stronger. However, when C-9 or C-10 was substituted by only one methoxy or benzyloxy group, the hypoglycemic effect was reduced (Bian et al., 2006). Moreover, it has been demonstrated that berberine and coptisine had more significant hypoglycemic effects than palmatine, epiberberine, and jatrorrhizine (Ma et al., 2016a). Berberine had the highest hypoglycemic effect partly due to its methoxyl groups in C-9 and C-10 positions (Ma et al., 2016a). This is consistent with the results obtained in another study (Chen et al., 2012). Therefore, methylenedioxy groups in C-2, C-3 and two methoxy substituents in C-9, C-10 contribute greatly to the hypoglycemic effect of protoberberine alkaloids of RC. The methylenedioxy group in D-ring may contribute to the inhibitory activity of RLAR (Jung et al., 2008). When the quaternary salts were reduced to tertiary amines, the binding affinity of berberine to β-cell membranes disappeared, and the aromatic C ring became unnecessary for the hypoglycemic effects of berberine (Bian et al., 2006). The electrostatic effect of positively charged nitrogen atoms is important for anti-diabetic activity of alkaloids. A study has shown that the positively charged isoquinolinium nitrogen atom of berberine could bind to the anionic acid site ASP48 of PTP1B and competitively inhibit the recombination of PTP1B (Bustanji et al., 2006).

CONCLUSION
Chinese herbal medicines with multiple-components are often used to successfully manage miscellaneous diseases, particularly chronic diseases triggered by multiple factors ( Gao et al., 2017). RC has been shown to possess beneficial effects on various diseases by multi-targeted therapies (Wang et al., 2018a). In this review, the most relevant articles were evaluated to reveal how RC exerts curative effect on T2DM and its complications through multi-component and multi-target ways. In the present review, it has been shown that RC with multi-component and multi-target characteristics effectively treats T2DM and its complications by adjusting glycometabolism and inhibiting T2DM-induced damage to the kidneys, pancreatic islets, and other tissues and organs. However, the precise mechanism which underlies these effects has not yet been completely elucidated. The pharmacokinetic properties and structure-activity relationships of protoberberine alkaloids of RC in T2DM have been investigated. In protoberberine alkaloids, rather small structural changes can bring about significant differences in actions. Long-term clinical application has demonstrated that RC has good hypoglycemic effects, but bioavailabilities of protoberberine alkaloids (mainly active ingredients) are rather low, which cannot explain the clinical hypoglycemic effect. The problem may be partly explained in the following ways: 1) it is possible that the protoberberine alkaloids exert hypoglycemic effect by direct impact on the intestinal tract and intestinal flora, 2) the bioavailability of RC may be significantly improved in the pathological conditions of T2DM, 3) the synergistic effect between alkaloids, and between alkaloids and non-alkaloids (e.g., polysaccharides and ferulic acid), may have improved the hypoglycemic effect of RC. At present, the hypoglycemic effect of RC is mainly focused on the alkaloid fraction, while the effect of the non-alkaloid fraction is mostly neglected. It has been reported that after removing the six main protoberberine alkaloids, the rest still had hypoglycemic effects similar to that of berberine, especially at low concentrations (Chen et al., 2012).
With the increasing number of T2DM patients, it is of great significance to find safe and effective drugs for reducing blood glucose and preventing T2DM complications in natural medicines. Although RC has been applied in clinical treatment of diabetes for many years, there is still a big gap between theoretical research and clinical practice. More investigations, especially reliable clinical trials, including large-scale, rigorously controlled, and multicenter randomized controlled clinical trials, are needed to assess its long-term safety.

AUTHOR CONTRIBUTIONS
Q-WH is the corresponding author of the study. QR and JW are the first authors and responsible for collecting materials and writing the paper. LW, H-RZ and X-BY helped organizing the information and edited the article pictures. All authors read and approved the final manuscript.

ACKNOWLEDGMENTS
We are indebted to our alma mater, Chengdu University of Traditional Chinese Medicine, for providing convenience in the collection of documents. Thanks for all the help from everyone in our lab.