Background

Diabetes mellitus (DM) is an endocrine disease that is prevalent worldwide, and its incidence has been rapidly increasing in recent years. According to the World Health Organization, DM is expected to affect over 693 million people by 2045 [1]. DM is a metabolic disorder that is marked by either absolute or relative insulin deficiency or insulin resistance, resulting in a clinical syndrome that manifests as a range of symptoms [2]. In addition, DM is a systemic vascular disorder that occurs when metabolic irregularities in the body cause glucose buildup, leading to a range of diabetic vascular complications, including chronic microvascular disease and macrovascular atherosclerosis [3, 4]. The disease can affect multiple organs and systems in the body and it is a leading cause of premature death [5]. The occurrence of DM is caused by the combined effects of genetic and environmental factors [6, 7]. DM is a complex condition that can be broadly categorized into four types based on its etiology: type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM), gestational diabetes, and other types of diabetes [8]. Currently, the treatment of DM primarily relies on administration of exogenous insulin or oral intake of hypoglycemic drugs. However, these methods can only alleviate the symptoms of patients and cannot cure DM completely [9]. Therefore, once diagnosed with DM, patients need to continuously take medication or use insulin to control blood sugar, alleviate symptoms, and prevent disease progression [10]. This has a multifaceted impact on the daily life of patients, not only requiring them to constantly monitor the side effects and risks of hypoglycemic drugs but also inflicting a heavy economic burden to their families and society [11]. Therefore, exploring alternative treatments for DM and maintaining their therapeutic effects is necessary.

Various diseases can be treated with stem cells because of their ability to self-renew, differentiate into other cell types, and regulate immunity. There is an ongoing ethical debate surrounding the procurement and utilization of embryonic stem cells [12]. Compared with other stem cells, the adipose-derived stem cells (ADSCs) have wider and more convenient sources, such as adipose tissue in the abdomen, limbs, and face areas and obtaining ADSCs only causes minor damage [13, 14]. Animal models of DM have been treated with ADSCs, and some human clinical trials (phase I/II) have also utilized these cells [15], with a few even progressing to phase III trials [16]. This article focuses on the research progress of ADSCs in the treatment of DM and its complications and explores its underlying mechanisms of action.

ADSCs

In recent years, there have been experiments using mesenchymal stem cells (MSCs) from different sources to treat diabetes. The early focus was on bone marrow-derived MSCs (BM-MSCs), which contain various types of stem cells including hematopoietic stem cells, mesenchymal stem cells, and endothelial progenitor cells. After the application of BM-MSCs, the patient’s insulin requirement decreased, insulin sensitivity increased, and β-cell function improved [17,18,19]. However, BM-MSCs are obtained invasively through the femur or iliac bone, which is not only painful but also yields a small quantity of cells and carries the risk of infection after extraction [20]. Umbilical cord-derived MSCs have a higher similarity to embryonic stem cells and possess greater differentiation potential compared to other common types of MSCs. After the infusion of Umbilical cord-derived MSCs, there was an increasing trend in the number of regulatory T (Treg) cells and a slight decrease in insulin requirements [21]. Jiang et al. [22] studied the use of placenta-derived MSCs in treating type 2 diabetes mellitus (T2DM), which resulted in a decrease of ≥ 50% in insulin requirements and improvement in kidney and heart function to some extent. However, MSCs derived from fetal appendages are obtained after birth, which poses potential risks of allogeneic stem cells and corresponding ethical issues [23]. Following BM-MSCs and fetal appendage-derived MSCs, ADSCs have become an alternative choice for clinical cell therapy due to their easy accessibility, abundant source, subcutaneous location, and longer culture time [24, 25]. Compared with other types of MSCs, such as BM-MSCs, ADSCs have similar proliferation and differentiation abilities and can be obtained with less pain [26, 27]. The cell surface molecules of ADSCs have been found to include differentiation clusters such as CD9, CD29, CD36, CD44, CD49d, CD49e, CD51, CD55, CD73, CD82, CD105, CD106, CD271, and von Willebrand factor (vWF), among which CD36 and CD49d are unique to ADSCs, whereas CD3, CD11a, CD11c, CD31, CD33, CD45, CD133, c-Kit, Lin, major histocompatibility complex II, and human leukocyte antigen (HLA)-DR surface proteins are lacking [28].

In some experiments, ADSCs showed genetic and epigenetic stability [29] and did not show significant immune response [30] and tumorigenicity [31]. Additionally, there have been no significant safety issues observed when ADSCs are transplanted into animal models [16, 29] or when used in human clinical trials [32]. According to existing research, ADSCs have not shown apparent safety issues and have a low potential for stimulating anti-HLA immune responses. There have been few reports of adverse reactions to ADSC-based treatments, but the long-term immunogenicity effects still need to be considered [33]. At the same time, in some studies, researchers have observed pulmonary embolism and infarction after injecting ADSCs into mice or patients [34, 35]. Other literatures have also reported an increase in levels of thrombin-antithrombin and D-dimer after intravenous infusion of allogeneic ADSCs, both of which are markers of coagulation activation [36].

Several experiments have explored the functions of ADSCs in different cell lineages [37, 38]. The advantages of ADSCs in cell replacement therapy and cell repair functions have been validated using animal models [39]. Some studies have demonstrated that direct intravenous or in situ injection of ADSCs restores the vitality of transplanted cells, which then differentiate and integrate ADSC functions in the body [40]. Transplanted ADSCs secrete several repair molecules such as neurotrophic factors [41], chemokines [42], immune regulatory factors [43], and inflammation regulatory factors [44]. To date, the therapeutic ability of ADSCs for diseases has been verified in several clinical trials [16, 38, 45,46,47]. These findings provide evidence of the safety of ADSCs and their potential in regenerative medicine, suggesting that ADSCs hold promise for use in human clinical trials.

Application of ADSCs in type 1 diabetes mellitus

In T1DM, due to autoimmune reactions, T helper 1 (Th1) cells attack pancreatic β cells, leading to the loss of insulin-producing cells (IPCs) [48]. The presence of macrophages, dendritic cells, natural killer cells, and lymphocytes also accelerates the progression of T1DM [49]. CD4+ T cells and inflammatory factors, including interferon-gamma (IFN-γ), interleukin-2 (IL-2), and tumor necrosis factor-alpha (TNF-α), play important roles in the process of β cell damage [50]. As pancreatic β cells are the sole producers of insulin in the body, their death leads to a complete absence of insulin secretion, ultimately resulting in the development of diabetes [51].

Long-term complications, such as vascular degeneration, renal failure, and blindness, cannot be prevented with the current interventions. The methods that have been applied in clinical practice to replace β cells mainly include whole pancreas and islet cell transplantation [52]. However, there are still many obstacles to the development of these methods, such as a lack of suitable islet donors, the need for lifelong immunosuppressive therapy after transplantation, and the exhaustion of transplanted organs and cells in diabetic patients, all of which restrict the development of this technology [53]. In recent years, ADSCs have gradually gained the attention of researchers owing to their ability to self-renew, differentiate into other cell lineages, and regulate the immune system. It is hoped that the characteristics of ADSCs can be utilized to achieve the goal of curing T1DM.

Animal models and human clinical trials

In the application of ADSCs to animal models, undifferentiated ADSCs or differentiated IPCs from ADSCs can be transplanted via intravenous, intraperitoneal, or renal capsule injection [44, 50, 54,55,56,57,58,59,60,61], as shown in Table 1. According to research, the mortality rate of mice within 24 h is close to 85% when a large number of ADSCs are administered through the tail vein. Reducing the injection quantity can avoid similar occurrences [35]. Some studies use intraperitoneal or renal capsule injection to attempt to avoid this problem [50, 55, 57, 58]. Currently, the use of ADSCs for treating T1DM patients is still in the preliminary research stage. There are not many studies related to this direction, and there are differences in administration routes. Currently, the administration routes that have been used include differentiating ADSCs into IPCs and then injecting them into the portal vein, thymus, or subcutaneous tissue [62]. Alternatively, ADSCs can be induced to become insulin-secreting ADSCs and co-transplanted with unfractionated cultured bone marrow cells into the portal vein of diabetic patients [15, 63]. It is administered through portal vein infusion because it allows the cells to stay in the liver microcirculation, and the liver, being a tolerant organ, would not reject cell engraftment. However, there is currently no consensus on the specific administration method that can achieve better therapeutic effects. In addition, regarding the number of transplanted ADSCs, there are also variations in current studies. Some studies used a level of 106 cells, while others used a level of 108 cells, as shown in Table 2 [15, 62, 63]. However, the specific number of cells required to achieve therapeutic effects while minimizing the potential risks of ADSCs still needs further investigation.

Table 1 Animal experiments of application of ADSCs in T1DM
Full size table
Table 2 Clinical trials using ADSCs in T1DM
Full size table

Mechanism of ADSCs in T1DM

ADSCs differentiation to IPCs

According to current research, one of the mechanisms for using ADSCs to treat T1DM is to transplant differentiated IPCs or ADSCs into animals or humans and utilize their ability to secrete insulin. Timper et al. [68] conducted the first experiment to differentiate human ADSCs into IPCs. The glucagon-like peptide-1 (GLP-1) was used in another study to induce differentiation of human ADSCs into IPCs. Insulin and C-peptide were released by IPCs in a glucose concentration-dependent manner [61]. Additionally, the injection of differentiated IPCs or undifferentiated ADSCs into diabetic animals resulted in a rise in insulin levels in diabetic animal serum and a return to normal blood glucose levels, as shown in Table 1.

The differentiation of ADSCs into IPCs is influenced by many factors, including the Wnt signaling pathway. Pancreatic development, islet function, and insulin production and secretion depend on this pathway [69]. Some studies have found that activating the Wnt signal can induce ADSCs of rats to differentiate into IPCs, which can be identified by detecting the expression levels of genes such as INS (insulin), pancreatic and duodenal homeobox 1 (PDX1), and GLP-1, as well as the protein expression levels of PDX1, cytokeratin 19, nestin, insulin, and C-peptide [70]. Additionally, the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway is also crucial in the differentiation of IPCs. The PI3K/Akt signaling pathway is significantly activated during the differentiation process of ADSCs into IPCs, mediated by the stromal cell-derived factor 1α and basic fibroblast growth factor [71]. According to a recent study, the upregulation of miR-375 is a crucial aspect of ADSC differentiation into IPCs [72]. A further aspect of its expression is that it is related to the secretion of insulin as well as cell proliferation [73]. Finally, IPCs are also developed through the Sonic hedgehog signaling pathway. To promote the development of IPCs, it is necessary to remove the inhibition of the Sonic hedgehog signaling pathway on them [74].

Restore the function of residual pancreatic islets in the body

ADSCs can not only serve as a source of IPCs, but also support the function of residual pancreatic islets in patients with diabetes [75]. In these experiments, it was observed that the function of residual pancreatic was restored after injection of ADSCs or IPCs differentiated from ADSCs [50, 54, 59, 64, 66, 67, 76]. These transplanted ADSCs release a variety of cytokines, including interferon-induced protein 10, eosinophil chemotactic factor (eotaxin), vascular endothelial growth factor (VEGF), and tissue inhibitor of metalloproteinase 1 (TIMP-1), all of which can prevent apoptosis of β cells and promote β cell proliferation [54]. In addition, transplantation of ADSCs effectively improved the autoimmune mechanism of diabetes in non-obese diabetic mice by reducing the Th1 immune response and inducing the proliferation of Tregs to improve the high blood glucose levels of early-onset autoimmune diabetes [50]. Compared to the untreated diabetes group, the IPCs transplantation group showed increased pancreatic regeneration, as well as a significant increase in the number of islet cells, islet area and density, and C-peptide immunoreactive area. The percentage of collagen fiber area in the islets of the IPCs transplantation group also decreased [67].

Maintain the function of pancreatic islet grafts in vivo or in vitro

ADSCs can also be used for preconditioning of pancreatic islet grafts in vitro to enhance the viability of the transplanted islets. Existing studies have shown that co-culturing ADSCs with syngeneic islets in vitro can significantly increase the level of insulin release compared to islets cultured alone. These pre-cultured pancreatic islet transplants have a higher success rate during transplantation and significantly improve the hyperglycemic condition in diabetic mice [54, 55, 76,77,78,79]. Eotaxin [54], VEGF [54, 77], TIPP-1 [54], extracellular matrix (ECM) components, annexinA1 [78] and fibroblast growth factor 2 expression [77] in ADSCs seem to be upregulated as a result of paracrine communication between pancreatic islets and ADSCs.

In addition to pre-treating pancreatic islet grafts, studies have attempted co-transplantation of ADSCs and islets to explore the effectiveness of this approach [55, 76, 80,81,82]. Adipose-derived stem cells (ADSCs) can promote the generation of a new vascular network within the co-transplanted islets by secreting various angiogenic factors, including VEGF [77, 83], hepatocyte growth factor [83], kinase insert domain receptor [83], transforming growth factor-beta (TGF-β) [78], and IL-8 [84]. ADSCs can also significantly inhibit the production of pro-inflammatory cytokines such as IFN-γ [78], TNF-α [76, 85], IL-6β [76], and IL-17 [78]. ADSCs can suppress the infiltration of CD4+ and CD8+ T cells [80] and macrophages [85], thereby reducing the inflammatory response within the co-transplanted islets. Co-transplantation of ADSCs and islets into STZ-induced diabetic mice significantly increased vascularization of the transplanted islets and significantly suppressed infiltration of inflammatory cells, resulting in increased survival time of the co-transplanted islets [80].

Successfully differentiating ADSCs into IPCs requires a specific combination of culture media, including insulin, transferrin supplement, and nicotinamide. This means that in clinical applications, a feasible method needs to be found to provide these media components. Although ADSCs can differentiate into IPCs, their insulin secretion capacity is still relatively low. Therefore, further optimization of the differentiation process is needed to improve the efficiency and functionality of ADSCs differentiating into IPCs. Further research is also needed to determine the transplantation method and the number of transplanted cells, which will contribute to the translation of ADSCs into a clinical therapeutic approach.

Application of ADSCs in type 2 diabetes mellitus

T2DM is distinguished by insulin resistance in insulin-responsive tissues and impaired insulin secretion by pancreatic β cells. This type of diabetes accounts for 85–95% of all DM cases [86]. Additionally, excessive nutrition can lead to inflammation in adipose tissue, affecting multiple tissues and worsening insulin sensitivity and β cell function [86, 87].

Currently, there are few clinical trials on the treatment of T2DM with ADSCs. Most applications are still being tested in animal experiments, which indicate that injecting ADSCs through the tail vein, peritoneum, and renal capsule of mouse can improve hyperglycemia by restoring pancreatic β cells, reducing inflammation, and increasing insulin sensitivity [88, 89]. The exploration of some of these mechanisms has provided new directions for the clinical application of ADSCs [90,91,92,93]. See Table 3 for details.

Table 3 Animal experiments of application of ADSCs in T2DM
Full size table

Mechanism of action of ADSCs in T2DM

Improvement of insulin resistance

Transplantation of ADSCs in vivo was discovered to restore the count of glucose transporter 4 and insulin receptors on the cell membranes of skeletal muscle, liver, and adipose tissue, and increase the phosphorylation of insulin receptor substrate 1 in a high-fat diet/STZ-induced T2DM rat model. Consequently, this alleviated the state of hyperglycemia and insulin resistance [89]. Insulin resistance may be linked to systemic chronic inflammation related to obesity, where inflammatory factors can impede the phosphorylation of the insulin receptor substrate and PI3K in the insulin signaling pathway, resulting in obstructed signal transduction and insulin resistance [96]. Studies have shown that after ADSCs are injected, TNF-α, IL-6, and IL-1β in T2DM rats are significantly reduced [89]. Injection of ADSCs reduces liver weight and fat degeneration by inhibiting the expression of pro-inflammatory genes and reduces the level of insulin resistance by increasing the expression of insulin receptor substrate [88], indicating that injection of ADSCs has a favorable effect on liver fat degeneration.

Promotion of insulin production

In addition to insulin resistance, the dysfunction of pancreatic β cell also plays a crucial role in the development of T2DM. Animal experiments have shown that after different sources of ADSCs are transformed into IPCs in vitro and transplanted into mice, they can effectively reduce the blood glucose levels compared with injecting undifferentiated ADSCs and blank groups [91, 94]. ADSCs can facilitate the restoration of remaining pancreatic islet function and enhance the quantity of pancreatic islet β cells. They repair pancreatic islet cells by decreasing caspase-3 activity and promote pancreatic islet vascularization by secreting angiogenic factors such as VEGF, insulin-like growth factor 1, hepatocyte growth factor (HGF), and vWF, thus contributing to the regeneration of pancreatic islet β cells [89].

Regulating liver glucose metabolism

Within 24 h after infusion of ADSCs, the hyperglycemic state of a T2DM rat model can be quickly relieved. This rapid action cannot be fully explained by improving β cell function and insulin resistance. The liver maintains normal blood glucose levels by regulating glycogen metabolism and gluconeogenesis. A 24-h period after ADSC infusion, T2DM rat models’ liver enzyme levels increased related to glucose metabolism, suggesting that ADSCs have a rapid effect on glucose homeostasis [93]. However, only this one research has reported similar phenomena, and the specific mechanism has not been clearly expounded.

However, it is worth noting that the clinical application of ADSCs in T2DM treatment is still in its early stages and requires further research to fully understand their safety and efficacy. Many technical and regulatory issues still need to be addressed. Despite these challenges, the potential benefits of ADSCs in T2DM treatment are significant, and their use represents a promising direction for future research and development in the field of regenerative medicine.

The potential mechanisms of ADSCs application in DM are summarized in Fig. 1.

Fig. 1
figure 1

The application and mechanism of ADSCs in Diabetes Mellitus. ADSCs adipose-derived stem cells, IPCs insulin-producing cells. All of the elements in the diagram were provided by Figdraw (http://www.fgdraw.com)

Full size image

Application of ADSCs in DM complications

Diabetic wounds

Diabetic wounds are a chronic complication of DM, which severely affect the quality of life of patients with diabetes. Diabetic foot ulcer is the most serious form of diabetic wounds [97], which is clinically manifested as peripheral neuropathy and lower limb ischemia, leading to sensory disorders, muscle atrophy, rest pain, and necrosis [98]. Moreover, if diabetic wounds are not treated properly, they may lead to amputation or even death [99]. Peripheral neuropathy can decrease the skin elasticity and secretion function of diabetic wounds, making patients with diabetes more prone to form wounds on the skin [100]. At the cellular level, DM damages macrophage function and prevents keratinocytes and fibroblasts from playing their roles in epithelial healing [101]. In addition, hyperglycemia causes endothelial damage in peripheral blood vessels, which further reduces skin perfusion and promotes the formation of skin ulcers [101].

Currently, numerous animal and clinical human experiments have been carried out on the application of ADSCs in the treatment of diabetic wounds, as shown in Table 4 and Table 5.

Table 4 Animal experiments of application of ADSCs in diabetic wounds
Full size table
Table 5 Clinical trials using ADSCs in diabetic wounds
Full size table

In one study, at 6 months, the diabetic foot ulcer closure rate was 100% for 51 subjects and ≥ 75% for 8 subjects. At 12 months, 100% of the DFUs in 50 subjects had healed, and ≥ 85% had healed in 4 subjects [122]. In another study, 59 patients with diabetic foot ulcers were randomly divided into an ADSCs treatment group and a polyurethane film control group. At week 8, the complete closure rate of wounds in the ADSCs treatment group was 73%, while in the control group was 47%. At week 12, the complete closure rate of wounds in the ADSCs treatment group was 82%, while in the control group was 53%. The median time for wound closure in the ADSCs treatment group and the control group was 28.5 days and 63.0 days, respectively [125]. The healing time of wounds in the group receiving allogeneic ADSC injection was 31 days, which was significantly shorter than that of the control group [126].

According to existing research, there are several possible mechanisms proposed for ADSCs therapy in the treatment of diabetic wounds. ADSCs have paracrine function and can secrete various cytokines, such as VEGF, fibroblast growth factor2, keratinocyte growth factor, TGF-β, platelet-derived growth factor, HGF, and collagen [127]. ADSCs also have the ability to directly differentiate into epithelial components and endothelial cells, playing an important role in dermal remodeling and wound healing [128]. In addition, ADSCs can inhibit the inflammatory response in diabetic wounds through paracrine function. After applying ADSCs, the expression of IL-6, IL-8 [129], and TNF-α [112] in diabetic wounds is significantly downregulated, and inflammatory cell infiltration is reduced [102].

Existing studies have confirmed that ADSCs can promote regulation, neovascularization, and fibrosis, and can be used as a potential therapy for the treatment of diabetic wounds. However, there are still relatively few clinical experiments on the application of ADSCs in the human body [122,123,124,125,126]. Further research is needed to determine more efficient methods of utilizing ADSCs for the treatment of trauma or surgical wounds in diabetic patients, in order to achieve the goal of treating diabetic wounds.

Diabetic retinopathy

Diabetic retinopathy is a microvascular disease of the retina caused by retinal ischemia [130]. Increasing evidence suggests that diabetes-related neurodegeneration occurs prior to retinal vascular endothelial changes, indicating that diabetic retinopathy should be considered a neurovascular degenerative disease [131, 132].

The self-renewal ability of pericytes and endothelial cells in the eyes of patients with diabetes is impaired, and the repair ability of these two cell types is continuously depleted [133]. Subsequently, the blood flow of capillaries decreases, resulting in hypoxia in adjacent areas of the retina. This hypoxic environment causes upregulation of VEGF, leading to increased vascular permeability [134] and the development of diabetic macular edema, ultimately resulting in loss of visual function [135]. Continuous hyperglycemia leads to abnormal function of ganglion cells, resulting in changes in retinal electrical activity before vascular endothelial changes occur [136]. In addition, hypoxia-inducible factor-1α is induced by hypoxia in the retina, which increases the expression of VEGF regulated by hypoxia, causing intraretinal microvascular abnormalities in the retina [137]. Proliferation and migration of vascular endothelial cells can eventually lead to the formation of neovascularization in the retina, characterized by proliferative diabetic retinopathy.

Currently, the primary treatment for diabetic retinopathy is still aimed at controlling blood sugar to slow down the progression of the disease [138,139,140,141]. When the disease progresses to macular edema or proliferative diabetic retinopathy that threatens vision, laser therapy can be used clinically to destroy the surrounding retina and reduce oxygen demand to help alleviate the disease [142]; however, laser therapy may cause many complications, such as decreased visual acuity, thickening of the retina, and loss of visual field [143].

Recently, some studies explored a new method for treating diabetic retinopathy by using ADSCs. This method is based on the ability of ADSCs to differentiate into pericytes, which can prevent neurovascular damage and promote the regeneration of damaged retinas, thereby achieving treatment for diabetic retinopathy [144, 145].

Thomas A. Mendel et al. founded in animal experiments that ADSCs injected into the vitreous body of OIR mice can differentiate into pericytes and integrate into retinal blood vessels, delaying the breakdown of the blood-retinal barrier. After two months of injection, approximately 80% of capillary loss can be prevented. Injection of ADSCs before vascular instability in OIR mice can reduce capillary loss by approximately 50% [146]. In another study, the histopathology of retinal tissue in T1DM nude mice showed a significant reduction in vascular leakage and apoptosis of retinal vascular cells around the eyes that received ADSCs injection compared to those that received saline injection. Additionally, the expression of inflammatory genes related to diabetic retinopathy was downregulated. Furthermore, in vitro experiments confirmed that co-culturing ADSCs with retinal endothelial cells can improve the survival rate of endothelial cells. These findings suggest that ADSCs have a protective effect against retinal damage caused by diabetes [147].

Overall, based on current perspectives, ADSCs can be a potential method for the future treatment of diabetic retinopathy. However, determining the optimal transplantation method and localization of ADSCs remains a challenge. Current research mainly focuses on local or intravenous injection of ADSCs, and further studies are needed to ensure the accurate location of cells in the damaged retinal area after injection into the eye. Additionally, although current research suggests that adipose-derived stem cells can protect retinal blood vessels, the specific therapeutic mechanisms are still unclear and require further investigation. Furthermore, the current research on the use of ADSCs for the treatment of diabetic retinopathy is limited to animal experiments, and there is still a long way to go before ADSCs can be used as an actual treatment method.

Diabetic nephropathy

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease and the main cause of death for patients with T1DM and T2DM [148]. The main feature of DN is the abnormality of kidney function and morphology. Abnormalities in the morphology of glomeruli include increased glomerular size, podocyte injury, gradual accumulation of ECM, mesangial matrix expansion, thickening of the glomerular basement membrane, and the appearance of glomerulosclerosis and interstitial fibrosis. Functional abnormalities include proteinuria, decreased glomerular filtration rate, and increased glomerular perfusion and filtration [149]. Long-term high blood sugar, high blood pressure, and local inflammation can lead to progressive and irreversible damage to the glomeruli and renal tubulointerstitium, ultimately resulting in renal dysfunction and eventually progressing to renal failure [148].

Under special conditions, podocytes and mesangial cells release various mediators to promote functional and morphological changes in glomeruli [150]. Mediators such as VEGFA, TGF-β1, angiotensin II, angiotensin-converting enzyme, inflammatory cytokines, and glomerular capillary remodeling cytokines can induce pathological changes in the kidneys. They activate cell remodeling signaling pathways, increase ECM synthesis, or activate NADPH oxidase, leading to increased oxidative stress levels. These changes can alter cell morphology and contribute to the development of kidney disease [151]. Furthermore, persistent hyperglycemia can generate advanced glycation end products (AGEs) in plasma and tissues, which can exacerbate DN via two mechanisms. AGEs can bind to matrix proteins like laminin and type IV collagen, inhibiting their breakdown by matrix metalloproteinases. This leads to an accumulation of excessive ECM proteins and fibrosis [152]. AGEs can bind to receptors on podocytes and mesangial cells, causing the secretion of fibrosis-promoting factors like VEGF, connective tissue growth factor, and TGF-β1, and an increase in NADPH oxidase expression. These factors promote the proliferation, expansion, and hypertrophy of glomerular cells [153]. Inflammation plays a significant role in the development of DN. As glomerular function deteriorates, inflammatory cells infiltrate the renal interstitium and release factors that worsen the progression of DN, including TNF-α, IFN-γ, IL-1, IL-6, and MCP-1. Inflammatory cells can also activate NADPH oxidase, leading to local oxidative stress responses [154].

Increasing evidence suggests that exosomes derived from stem cells are relatively safe and effective in treating kidney diseases in rat or mouse models [155]. The exosomes secreted by MSCs play a significant protective role in acute kidney injury and chronic kidney disease [156]. Exosomes are nanoscale membrane vesicles released by various types of cells, including mesenchymal stem cells [157]. The microRNAs can be enclosed in exosomes and serve as potential paracrine regulatory factors involved in the regulation of many diseases, such as ischemic diseases and degenerative eye diseases [158, 159]. The microRNAs produced by MSCs, such as miR-150 and miR-134, play a crucial role in the treatment of DN [160]. Exosomes secreted by human urine-derived stem cells alleviate DN and high glucose-induced podocyte injury through the transfer of miR16-5p [161]. Therefore, some researchers have attempted to use ADSCs-Exos to achieve the goal of treating DN, and have successfully improved the functional impairments of foot cells and symptoms of DN to varying degrees.

In vivo studies have also shown that ADSC-Exo can inhibit high glucose-induced podocyte apoptosis in mice [162]. According to the research of Duan Y et al., exosomes produced by ADSCs contain miR-26a-5p, which can be transferred to glomerular podocytes and improve DN in diabetic mice. In vitro studies have shown that ADSCs-Exo-miR-26a-5p can prevent podocyte apoptosis caused by high glucose by targeting TLR4, reducing the expression of VEGFA, inhibiting the pathway of NF-κB, and suppressing oxidative stress reactions [163]. In the experiment conducted by Jin et al. ADSC-Exo-miR-486 can inhibit high glucose-induced podocyte apoptosis by targeting Smad1, downregulating its expression, and suppressing the mTOR pathway which promotes autophagy flux and reduces podocyte apoptosis [162]. Besides they also founded that ADSCs-Exo-miR-215-5P can inhibit the expression of zinc finger E-box-binding homeobox 2, alleviate the progression of epithelial-mesenchymal transition, and foot cell migration [164].

In summary, according to the current results, ADSCs-Exo have potential therapeutic effects in the treatment of diabetic nephropathy, and in the future, they may be a relatively good choice for the treatment of DN. However, there are still some issues that need to be addressed, such as optimizing the preparation methods of ADSC extracellular vesicles, determining the active molecules in the extracellular vesicles, and exploring methods to accurately deliver the extracellular vesicles to the kidneys. These will contribute to the translation of adipose-derived stem cells into clinical applications.

Recent studies have shown that liver changes is another complication of DM [165]. Hyperglycemia caused by DM increases the risk of liver damage and liver fibrosis [166], severely affecting the health and quality of life of patients. DM is closely associated with liver diseases [165, 167], but the pathological and physiological basis and progression of liver changes in DM are not yet fully understood, and effective early intervention is lacking. Some studies have attempted to transplant ADSCs into animals to alleviate diabetes-induced liver damage and fibrosis, and have achieved certain positive results [168, 169]. This provides an important theoretical and experimental basis for further research and development of ADSCs for the treatment of DM-related liver diseases. However, clinical trials have not yet been conducted, and the explanation of the therapeutic mechanisms and pathways of ADSCs is not sufficiently detailed, requiring further research for clarification.

The potential mechanisms of ADSCs application in complication of DM are summarized in Fig. 2.

Fig. 2
figure 2

The application and mechanism of ADSCs in the complications of Diabetes Mellitus. ADSCs adipose-derived stem cells. All of the elements in the diagram were provided by Figdraw (http://www.fgdraw.com)

Full size image

Current challenges

Although ADSCs have broad prospects for application in disease treatment and tissue engineering, their application still faces some challenges. The following are some possible issues:

Complications issues There are certain difficulties and risks in obtaining and processing adipose tissue, such as the risk of wound infection due to improper handling and the possibility of blood clots from excessive intravenous infusion of ADSCs. Further research and clinical observation are needed to ensure the long-term effectiveness and safety of ADSCs therapy.

Standardization issues ADSCs are derived from various tissues, such as subcutaneous adipose tissue, breast tissue, and bone marrow. The differences in preparation and culture conditions of ADSCs mean that it is not guaranteed to obtain the same cell population in different laboratories. ADSCs from different sources have differences in biological characteristics, differentiation ability, and immunogenicity, which poses a challenge to the stability of ADSCs application. ADSCs have different abilities and functions, and standardized methods have not been established. More researches are needed to develop good quality control standards to ensure the consistency and stability of cells and achieve the desired therapeutic effects. In addition, it is also necessary to determine the number of cells needed for transplantation to cure diabetes mellitus and its complications in order to reduce the number of transplantations and patient suffering.

Transplantation efficiency issues During the in vitro culture process, some ADSCs may be lost, and similarly, some ADSCs may be lost during the transplantation process, which may affect the transplantation effect. The survival rate of adipose-derived stem cells after transplantation is an important issue, and optimization of long-term preservation and storage conditions of ADSCs needs to be addressed. More effective methods need to be explored to ensure the purity and stability of cells during the cell culture and expansion process.

Plasticity issues ADSCs need to undergo differentiation to generate insulin-secreting cells. ADSCs may be unstable during the differentiation process, leading to inconsistent results in differentiation products. The efficiency and stability of the differentiation process are key issues, therefore, further research and exploration are needed to optimize the stability of ADSCs differentiation.

It should be noted that the above issues are just some potential challenges mentioned in this article, and there may be other issues in actual applications.

Conclusions

In summary, an increasing amount of research suggests that ADSCs may serve as a new therapeutic approach for DM. Treatment with ADSCs has the potential to improve high blood glucose levels and alleviate symptoms of related complications in both animals and humans. However, there is still much work to be done in order to translate ADSCs into practical clinical applications. Further research and clinical observation are needed to assess the long-term effects of ADSCs treatment and minimize potential risks associated with their usage, in order to achieve more reliable and effective benefits in future clinical applications.