The complex role of Wnt ligands in type 2 diabetes mellitus and related complications

Abstract Type 2 diabetes mellitus (T2DM) is one of the major chronic diseases, whose prevalence is increasing dramatically worldwide and can lead to a range of serious complications. Wnt ligands (Wnts) and their activating Wnt signalling pathways are closely involved in the regulation of various processes that are important for the occurrence and progression of T2DM and related complications. However, our understanding of their roles in these diseases is quite rudimentary due to the numerous family members of Wnts and conflicting effects via activating the canonical and/or non‐canonical Wnt signalling pathways. In this review, we summarize the current findings on the expression pattern and exact role of each human Wnt in T2DM and related complications, including Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11 and Wnt16. Moreover, the role of main antagonists (sFRPs and WIF‐1) and coreceptor (LRP6) of Wnts in T2DM and related complications and main challenges in designing Wnt‐based therapeutic approaches for these diseases are discussed. We hope a deep understanding of the mechanistic links between Wnt signalling pathways and diabetic‐related diseases will ultimately result in a better management of these diseases.


| INTRODUC TI ON
Diabetes, caused by the deficiency of insulin secretion and/or insulin resistance and characterized by chronic hyperglycaemia, is currently one of the most important metabolic diseases worldwide. Diabetic patients have a higher risk of developing a series of acute metabolic complications, such as diabetic ketoacidosis, and chronic vascular complications (angiopathy) including microvascular diseases such as diabetic retinopathy (DR), diabetic peripheral neuropathy (DPN), diabetic nephropathy (DN) and diabetic foot, and macrovascular diseases including cardiovascular disease manifesting as myocardial infarction and cerebrovascular disease resulting in strokes. 1,2 The prevalence and incidence rate are increasing rapidly in most countries, according to the latest data from International Diabetes Federation, more than 463 million adults are suffering from diabetes and the number is expected to rise to 700 million by 2045. Diabetes caused 4.2 million deaths and 10% of total health expenditure on adults in 2019. More severely, about 50% people with diabetes have not been diagnosed and about 79% of adults with diabetes are living in developing countries. Diabetes is mainly divided into type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM) and gestational diabetes mellitus (GDM); among them T2DM is the most common type and accounts for above 90% of all diabetes cases. 3 It is commonly believed that insulin resistance is the initial factor for the occurrence of T2DM, whereas dysfunction of pancreatic β-cells is the determinant factor. 4,5 There is no cure for T2DM currently, the cornerstone of the treatment is reducing insulin resistance and stimulating pancreas to secret more insulin. Therefore, it is urgent to reveal the underlying pathogenic mechanisms of T2DM for a better therapeutic management.
The aetiology of T2DM has not been fully elucidated, and it is considered to be a complex polygenetic disease attributed to the interaction between hereditary predisposition and multiple acquired disposition; the latter of which includes the risk factors such as overweight, unhealthy diet, physical inactivity, increasing age and hypertension. 6,7 The abnormalities in many important signalling transduction pathways are critically involved in the occurrence and progression of T2DM and related complications. [8][9][10][11] Among them Wnt signalling pathways attract more attention due to the essential role in the embryogenesis and tissue homeostasis, and notorious role in the pathogenesis of multiple human diseases, especially in cancers. [12][13][14] The relationship between Wnt signalling pathways and T2DM was firstly documented by Grant et al in 2006; they found genetic polymorphism of TCF7L2 gene, which encodes an important transcription factor TCF4 in Wnt signalling pathways, contributed to the risk of T2DM through regulation of the expression of proglucagon gene. 15 Subsequently, emerging studies proved that dysregulation of Wnt signalling pathways participate in the occurrence and progression of T2DM through directly influencing the differentiation and proliferation of pancreatic β-cells and the secretion and action of insulin. [16][17][18] However, due to the numerous components and resulting intricate networks, the role of Wnt pathways in the pathogenesis of T2DM and related complications seems to be contradictory, sometimes they function as protectors, while their activation is simultaneously required for the development of these disorders, and our understanding on their relationship is still quite rudimentary. Therefore, a more comprehensive understanding of their relationship will be helpful for a better therapeutic effect.

| THE C ANONI C AL AND NON -C ANONIC AL WNT S IG NALLING PATHWAYS
The Wnt signalling pathway is roughly divided into β-catenindependent (canonical) and β-catenin-independent (non-canonical) signalling pathways, which activates distinct intracellular signalling pathways ( Figure 1). Among them the canonical Wnt signalling gets more attention and is well understood. 19 The most crucial event in this signalling is the regulation in the turnover of β-catenin, a pivotal component that acts as a transcriptional co-activator in this cascade.
In resting cells, the production of Wnts is suppressed and the protein level of cytoplasmic β-catenin is low due to the activity of destruction complex composed of Axin, adenomatosis polyposis coli (APC), casein kinase 1 (CK1α) and glycogen synthase kinase (GSK-3β). βcatenin is captured and phosphorylated by the destruction complex, 20 followed by ubiquitinated by β-transducin repeat-containing protein (β-TRCP) and dispatched to the proteasome for complete degradation, 21 without enough β-catenin in nucleus, the bidirectional T cell factor/lymphoid enhancer-binding factors (TCF/LEF) begin to recruit transducin-like enhancer protein (Groucho/TLE) and histone deacetylases (HDACs) to form a repressive complex, thus to inhibit the transcription of Wnt target genes. Conversely, the activation of canonical Wnt signalling is initiated by the formation of complex among Wnts, their cognate receptor Frizzled (Fzd) and coreceptor low-density lipoprotein receptor-related protein 5/6 (LRP5/6) on the cell membrane. Consequently, the effector protein dishevelled (DVL) is recruited and polymerized to inactivate the destruction complex, which leads to the stabilization and accumulation of β-catenin in the cytoplasm and the subsequent translocation into the nucleus to form an active complex with TCF/LEF by removing TLE/Groucho complexes and recruiting transcriptional co-activators such as B cell CLL/lymphoma 9 (BCL9), Brahma-related gene 1 (BRG1), CBP/p300 and Pygo. Finally, the transcription of Wnt target genes is driven and results in the changes of series of cellular processes. Collectively, the activation of canonical Wnt signalling mainly includes the following biological processes: the production and secretion of Wnts, the recognition of Wnts by their receptors, the inactivation of destruction complex, the accumulation of β-catenin and translocation into nucleus and the activation of transcriptional complex of target genes.
The non-canonical Wnt signalling is mainly subdivided into Wnt/ Ca 2+ and Wnt/planar cell polarity (PCP) signalling pathways, which are activated by some Wnts proteins, such as Wnt5a, Wnt5b and Wnt11, and eventually regulate the cellular polarity and migrationrelated signalling pathways that have important roles in cell orientation during development and cell migration in metastasis formation. 22 The Wnt/Ca 2+ signalling is activated by the complex formation among Wnt, Fzd, DVL and G proteins and the resulting activation of phospholipase C (PLC) activity and subsequent calcium influx, increased intracellular Ca 2+ concentration activates various signalling pathways, including protein kinase C (PKC), Ca 2+ /calmodulindependent protein kinase II (CAMKII) and Ca 2+ /calcineurin, leading to the phosphorylation of retinoic acid-related orphan nuclear receptor α (RORα) and/or the translocation of transcription factors, such as nuclear factor activated in T cells (NFAT) and Nemo-like kinase (NLK). 23 Intriguingly, RORα and NLK function as inhibitors of canonical Wnt signalling via reducing the binding of β-catenin to TCF/LEF transcription factors. 24,25 In the Wnt/PCP signalling, Wnts bind to the receptor like tyrosine kinase (RYK) to active SRC, or to the receptor tyrosine kinase-like orphan receptor 1/2 (ROR1/2)-Fzd complex to activate DVL and further activate small Rho GTPases, including Rac family small GTPase 1 (RAC1), RhoA and cell division cycle 42 (CDC42), in a DVL-dependent way. RhoA triggers ROCK and c-Jun N-terminal kinase (JNK) but RAC1 only activates JNK, thereby regulating rearrangements of cytoskeleton and/or activating related transcription factors, such as activator protein 1 (AP-1) and NFAT.

| Wnt1, Wnt2 and Wnt2b
At present, no direct evidence is available for the link between Wnt1 and T2DM; few studies on its role in different diabetic complications is also controversial. In T1DM-induced rats, the inhibition of Wnt1/

| Wnt3 and Wnt3a
Wnt3 and Wnt3a share 85% amino acid sequence identity. At present, studies on Wnt3 are mainly focused on its role in malignancies rather than in diabetes; we have revealed the carcinogenic role of in a latest study on human-induced pluripotent stem cell (hiPSC)derived S7 cells, activating the canonical Wnt3a/β-catenin and noncanonical Wnt4/5a/5b signalling pathways simultaneously did not alter or improve glucose-simulated insulin secretion (GSIS); instead, inhibition of these endogenous Wnts even modestly promoted the maturation of β-cells. 48 In another study, activation of Wnt3a/βcatenin pathway was found to improve the impaired osteointegration under diabetic condition. 49 Wnt3a/β-catenin signalling is also overactivated in the kidneys of diabetic patients and animal models, hence inhibiting this signalling by PPARα displays a protective effect on DN. 50

| Wnt4
Wnt4 is the most abundantly expressed Wnt protein in β-cells, whereas its role in diabetes is also contradicting because it func-

| Wnt5a and Wnt5b
Wnt5a, another initiator that activates the canonical and noncanonical Wnt pathways, is closely related to a variety of metabolic disorders such as obesity and T2DM. Compared with that in healthy subject, plasma Wnt5a level is significantly decreased in patients with the onset T2DM, and a negative correlation is found between the Wnt5a level and fast blood glucose (FBG)/HbA1c levels.
However, Wnt5a level is gradually increased in patients with longterm T2DM or after 3 months of treatment, 70

| Wnt6
Wnt6 is highly homologous to Wnt1 but these two Wnts only share

| Wnt7a and Wnt7b
Wnt7a and Wnt7b are also secreted proteins with 78% amino acid sequence identity, and both can activate the canonical and non-

| Wnt10a and Wnt10b
Wnt10a is another biphasic Wnt ligand that shares 62% amino acid sequence identity with Wnt10b. Although the exact role of

| Wnt11
Wnt11 shows no homology to other Wnts and only shares 41% amino acid sequence identity with Wnt4. Wnt11 is expressed at low level throughout the development; it is mainly expressed in mesenchymal

| Wnt16
Wnt16 is another Wnt ligand having two distinct mRNA isoforms, Wnt16a and Wnt16b, which only differ in the composition of

| ROLE S OF MAIN ANTAG ONIS TS AND CORECEP TOR OF WNTS IN T2DM AND REL ATED COMPLIC ATIONS
The signalling transduction of Wnt pathways will be blocked if their receptors are bound by competitive antagonists. Secreted frizzledrelated proteins (sFRPs) and Wnt inhibitory factor-1 (WIF-1) are classical Wnt antagonists that block all Wnt signalling pathways.

| CON CLUS IONS
The

CO N FLI C T O F I NTE R E S T
The authors have no conflicts of interest to disclose in relation to this review.