Transplantation of adipose derived stem cells in diabetes mellitus; limitations and achievements

Abstract

Objectives

Diabetes mellitus (DM) is a complex metabolic disease that results from impaired insulin secreting pancreatic β-cells or insulin resistance. Although available medications help control the disease, patients suffer from its complications. Therefore, finding effective therapeutic approaches to treat DM is a priority. Adipose Derived Stem Cells (ADSCs) based therapy is a promising strategy in various regenerative medicine applications, but its systematic translational use is still somewhat out of reach. This review is aimed at clarifying achievements as well as challenges facing the application of ADSCs for the treatment of DM, with a special focus on the mechanisms involved.

Methods

Literature searches were carried out on “Scopus”, “PubMed” and “Google Scholar” up to September 2022 to find relevant articles in the English language for the scope of this review.

Results

Recent evidence showed a significant role of ADSC therapies in DM by ameliorating insulin resistance and hyperglycemia, regulating hepatic glucose metabolism, promoting β cell function and regeneration, and functioning as a gene delivery tool. In addition, ADSCs could improve diabetic wound healing by promoting collagen deposition, inhibiting inflammation, and enhancing angiogenesis.

Conclusion

Overall, this literature review revealed the great clinical implications of ADSCs for translating into the clinical setting for the treatment of diabetes. However, further large-scale and controlled studies are needed to overcome challenges and confirm the safety and optimal therapeutic scheme before daily clinical application.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40200-023-01280-8.

Introduction

Diabetes mellitus is a non-communicable metabolic disease approaching epidemic proportions globally [1]. The most prevalent types of diabetes, type 1 and type 2, are characterized by selective autoimmune destruction of insulin-producing pancreatic β-cells with subsequent sudden and total deficiency of insulin and by a decrease in the sensitivity of target tissues to insulin [2]. Due to dietary habits and sedentary behaviors, the number of patients with diabetes is expected to rise globally [3]. According to an estimate by the International Diabetes Federation, the number of people with diabetes is about 463 million and is set to escalate to 700 million by the year 2045 [4].

Oral hypoglycemic agents and insulin therapy are the most prescribed treatments for diabetes. Although common interventions can decrease glycemic levels or temporarily ameliorate insulin sensitivity in insulin-responsive tissues, these can neither improve pancreatic β cell impairment nor insulin resistance [5]. Therefore, developing effective alternative approaches to curing diabetes is a priority. Islet cell transplantation has been proposed as a cell replacement approach for reestablishing glucose homeostasis since the 1970s [6]. However, efforts toward the routine use of this treatment have been hampered by islet source limitation, lack of oxygen and blood supply for transplanted islets, lifelong immunosuppression, and immune rejection of islets [7, 8].

Regenerative medicine, a relatively young branch of multidisciplinary research, has emerged as an important avenue that aims to treat a wide range of conditions by inducing the restoration of diseased or damaged tissue [9]. Stem cell therapies provide novel regenerative and immunomodulatory techniques for managing diabetes mellitus. Unique characteristics of mesenchymal stem cells (MSCs), including differentiation potential, immunoregulatory capacity, and paracrine effect, make them optimal for tissue regeneration [10]. Among the various sources of MSCs, adipose derived stem cells (ADSCs) have proved to be one of the most promising stem cell populations (Table 1) regarding their accessibility, abundance, efficacy, and minimally invasive collection procedure when compared to other sources [11].

Table 1.

Summary of the major clinical trials utilizing MSCs as a treatment for DM, extracted from the PubMed database, starting from the years 2017–2022

No	Source of MSC in the treatment of DM	Findings	References
1.	Pluripotent stem cell-derived pancreatic endoderm cells (PEC-01)	The progressive accumulation of functional, insulin-secreting cells occurs over a period of approximately 6–9 months from the time of implant.	Shapiro et al., 2021 [139]
2.	Allogenic Adipose Tissue-Derived Stromal/Stem Cells	ADSCs transplantation shows the short-term safety of infusions from healthy donors and daily oral cholecalciferol in patients with recent-onset T1D, as well as their potential therapeutic effect on glycemic control and pancreatic function. In most cases, mild and transient adverse events were observed.	Araujo et al., 2020 [140]
3.	Allogenic umbilical cord mesenchymal stromal cells	Subjects in the MSC-treated group achieved insulin independence and maintained it for 3 to 12 months.	Lu et al., 2021 [141]
4.	Autologous bone marrow stem cell	Combined therapy of intrapancreatic ASC infusion and HBOT showed increased metabolic control and reduced insulin requirements in patients with T2DM compared with standard treatment.	Estrada et al., 2019 [142];
5.	Allogeneic Wharton’s jelly-derived mesenchymal stem cells (WJ-MSCs)	The treatment of allogeneic WJ-MSCs to evaluate long-term safety and efficacy in treating diabetic patients was well-tolerated and safe. Most of the patients reported an improvement at all time points during follow-up. The maximum improvement was reported after six months, when all participants had received two injections.	Al Demour et al., 2021 [143]

In this review, we shall overview achievements as well as challenges facing the application of ADSCs for the treatment of DM, with a special focus on their role in diabetic wound healing and the underlying mechanisms involved.

Adipose derived stem cells

Adipose tissue has been widely acknowledged as a valuable origin of postnatal mesenchymal stem cells for the past four decades [12]. Zuk et al. first isolated a putative subset of adult stem cells from adipose tissue, initially called Processed Lipoaspirate or PLA cells [13]. Since then, these new multipotent stem cells have been renamed Adipose derived stem cells (ADSCs) and have made their mark as promising therapeutic cells in medicine and cell therapy [14]. In particular, adipose tissue that can be acquired as waste products of lipoaspiration or abdominoplasty offers significant therapeutic benefits, such as providing an easily accessible and ethically uncontroversial source of stem cells that yields high amounts of viable adult stem cells [15].

Adipose derived stem cells meet the criteria proposed by the International Society for Cellular Therapy (ISCT) to identify human mesenchymal stem cells (MSCs): (i) plastic adherent, [16] (ii) positive localization of CD73, CD90, and CD105 but no localization of CD45, CD34, CD14 or CD11b, CD79a or CD19, and HLA-DR surface molecules; (iii) trilineage mesenchymal differentiation potential [17]. Nevertheless, an exact immunophenotype characterization of ADSCs has been long debated due to their nonhomogeneous population [17]. Mildmay-White and Khan recommended a minimum panel of CD markers in ADSCs, including CD90+, CD44+, CD29+, CD105+, CD13+, CD34+, CD73+, CD166+, CD10+, CD49e+, CD59+, CD31-, CD45-, CD14-, CD11b-, CD19-, CD56- and CD146- [18]. Particularly, the expression of the markers varies based on the method of isolation, culture condition, passages, and donor variation [19, 20].

There have been numerous reports that ADSCs, which originate from mesodermal layers, can be differentiated into cells of the other two germ layers [21]. Actually, these cells can differentiate into adipogenic, osteogenic, chondrogenic, myogenic, angiogenic, cardiomyogenic, tenogenic, and periodontogenic lineages (11); in this respect, ADSCs and bone marrow stem cells (BMSCs) present large similarities in differentiation potential; however, several characteristics distinguish these two types of stem cells. For instance, ADSCs have an inferior potential for chondrogenic and osteogenic properties compared with BMSCs [16, 22], while being more prone to differentiate into cardiomyocytes and muscle cells [23]. In addition, ADSCs cellular activity is affected by cryopreservation [24] and long-term expansion [12]. Thus, it seems that further investigations should be carried out to optimize ADSC manipulation for clinical application.

Additionally, their expanded use is specifically due to paracrine effects through the secretion of soluble mediators, including anti-inflammatory cytokines, anti-apoptotic, neurotrophic, growth factors, and extracellular vesicles, collectively known as the secretome [25, 26]. The summary of the major clinical trials utilizing ADSCs as a treatment for various chronic diseases are listed in Table 2.

Table 2.

 A Summary of the major clinical trials utilizing ADSCs as a treatment for various chronic diseases, extracted from the PubMed database, starting from the years 2017–2022

Author, Year	Disease	Source	Method of delivery	Adverse events	Main result
Lee et al., 2019 [144]	Knee Osteoarthritis	Autologous ADSCs	intra-articular injection (1 × 108 cells)	No AEs at 6 months’ follow-up	ADSCs provided satisfactory functional improvement and pain relief.
Zhang et al., 2022 [145]	Knee Osteoarthritis	Autologous SVF	intra-articular injection (Not available)	No AEs at 12 months’ follow-up	ADSCs improved the clinical symptoms through cartilage regeneration.
Freitag et al., 2019 [146]	Knee Osteoarthritis	Autologous ADSCs	intra-articular injection (100 × 106 cells)	No AEs events at 12 months’ follow-up	ADSCs effectively improved function and relieved pain.
Zhou et al., 2020 [134]	Crohn’s fistula-in-ano	Autologous ADSCs	inner orifice and around the fistula injection (− 1 × 106 cells for fistula < 1 cm -2 × 106 cells for fistula > 1 cm)	No AEs events at 12 months’ follow-up	ADSCs accelerated the healing rate.
Araujo et al., 2020 [140]	Type 1 Diabetes Mellitus	Allogenic ADSCs	Intravenous injection (upper arm vein) (1 × 106 cells/kg)	Mild transient AEs	ADSCs treatment results in better glucose control and arrest β cell destruction.
Carstens et al., 2021 [147]	Chronic Diabetic Foot Ulcers	Autologous SVF	Subcutaneous Injection (30 × 106 cells)	No serious EVs	ADSCs treatment enhanced wound healing, reduced amputation, and improved quality of life.
Kumar et al., 2017 [148]	chronic discogenic low back pain		Intradiscal implantation of ADSCs and hyaluronic acid (Low dose: 2 × 107 cells/disc. High dose: 4 × 107 cells/disc)	No serious EVs in 1-year follow-up	The combined implantation of ADSCs and HA is safe and tolerable and has provided significant improvement in patient outcomes.
Kastrup et al., 2017 [149]	ischemic heart disease	Allogeneic ADSCs	Intramyocardial injection (Not available)	No AEs events at 6 months’ follow-up	ADSCs treatment improved cardiac function.
Huang et al., 2019 [150]	Liver Cirrhosis	Autologous ADSCs	Intrahepatic Injection (1 × 108 cells)	No AEs events at 24 weeks’ follow-up	ADSCs improved liver function and quality of life.
Carstens et al., 2020 [151]	Parkinson’s Disease	Autologous SVF	Facial injection 60 × 106 SVF	No AEs events at 12 months’ follow-up in one patient and 5 years in another	SVF treatment led to an improved long-term neurologic response.
Qayyum et al., 2019 [152]	refractory angina	Autologous ADSCs	intra-myocardial injections 72 ± 45 × 106 ASCs	3 years’ follow-up	ADSCs improved cardiac symptoms and unchanged exercise capacity.
Abumoawad et al., 2020 [153]	Atherosclerotic renovascular disease	autologous ADSCs	intra-arterial infusion of ADSCs (3 dose: 1, 2.5, and 5.0 × 105 cells/kg)	Have not been reported	ADSCs increase cortical and whole kidney blood flows.
Paitazoglou et al., 2019 [154]	Ischaemic heart failure	Allogenic ADSC	Intramyocardial injection (100 × 106)	No AEs events at 24 months’ follow-up	Have not been reported
Squassoni et al., 2021 [155]	Chronic Obstructive Pulmonary Disease	autologous ADSC and BMC	Co-infusion of ADSC and BMSC (5 × 107 ADSCs cells + 5 × 107 BMSC)	No AEs events at 12 months’ follow-up	Co-infusion therapy improved gas exchange and overall quality of life.

Therapeutic properties of ADSCs in diabetes

Nowadays, stem cell therapies are hot topics in the medical world, and this unique technique is just the beginning of a long story. Stem cells such as embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and mesenchymal stem cells (MSCs) can improve insulin production and also ameliorate insulin resistance in T2DM. The legal issues of ESCs and the tumorigenic complications of iPSCs limit their application in clinical settings. MSCs lack these disadvantages and are widely studied in T2DM. As mentioned above, adipose derived stem cells (ADSCs) are the most attractive source of MSCs for clinical cell therapy because of their self-renewal and expansion potential, easy accessibility, abundant sources, multipotency, and low immunogenicity. We tried to highlight the mechanisms and therapeutical properties of ADSCs in diabetes in Fig. 1.

Fig. 1.

Mechanisms and therapeutical properties of ADSCs in diabetes. Created with BioRender.com.

Differentiation potential of ADSCs

The enhanced multidifferentiation potential of ADSCs adds a lot to their therapeutic capacity for a wide range of diseases, including DM. In fact, adipose derived stem cells have the potential to differentiate into nonmesodermal cells such as insulin producing cells (IPCs), which is the earliest therapeutic approach to curing diabetes and results in an increase in insulin levels in T2DM patients. Chandra et al. first developed a differentiation protocol for murine ADSCs to differentiate into IPCs, which could effectively bring about normoglycemia in rat models within 2 weeks [27]. In line with this study, Nam et al. differentiated human eyelid adipose tissue-derived stem cells into insulin-secreting cells. Transplantation of differentiated cells into a type 2 diabetes mouse model resulted in an increased circulating insulin level and lowered levels of IL-6, triglyceride, and free fatty acids [28]. The generation of insulin-producing cells derived from ADSCs has been successfully developed by other researchers [29–33], which can effectively ameliorate hyperglycemia. However, several uncertain factors in the differentiation of ADSCs into IPCs still remain, including immune rejection and graft hypertrophy, the effect of the diabetic microenvironment on the differentiated insulin producing cells, and the tumorigenic risks associated with virus-mediated differentiation [34]. These variables are the crucial elements that affect the success of ADSC therapy. The exact dosage of cells administered has been demonstrated as another uncertain issue influencing treatment effectiveness. Moreover, problems in techniques and methodologies used in ADSC isolation, expansion, and differentiation must be resolved for promising reproducibility in the various clinical applications [34, 35].

Amelioration of insulin resistance

In addition to diminished β-cells function, insulin resistance [7] also plays a crucial role in the pathogenesis of type 2 diabetes and high-risk states [36]. Improving insulin resistance is essential for successfully alleviating hyperglycemia. Advances in research showed that MSCs lower blood sugar not only by increasing insulin production but also by reversing insulin resistance through various mechanisms. Si et al. found that the infusion of MSC into a high-fat diet/STZ-induced T2DM rat model improved insulin sensitivity by increasing glucose transporter type 4 (GLUT4) expression with increased phosphorylation of insulin receptor substrate 1 (IRS-1) and Akt (protein kinase B) in insulin target tissues [37]. Like other MSCs, ADSCs have the potential to ameliorate insulin sensitivity. The effects are due to a significant reduction of inflammatory cytokines and chemokines [38], promoting the transition of macrophages from pro-inflammatory (M1) to anti-inflammatory (M2) [39], and suppressing the expression of IL-6 [40], which is implicated in insulin resistance [41].

Inflammatory conditions and biomarkers of inflammation are major contributors to the development of IR and DM through various signaling pathways [42, 43]. Pro-inflammatory cytokines, including TNF-α (tumor necrosis factor-α) and IL-6 (interleukin-6) are involved in the activation of the NF-κB pathway leading to serine phosphorylation of IRS (insulin receptor substrate), resulting in the IR and development of DM [43, 44]. ADSC infusion in T2DM-induced Streptozotocin induced rats significantly decreased the concentration of inflammatory cytokines such as TNF-α, IL-6 and IL-1β (Interleukin-1β), which resulted in improved insulin sensitivity in insulin target tissues [45].

Accumulating evidence has revealed the key functional roles of the ADSC secretome as a cell free therapy for the treatment of diabetes and its complications [46]. Shree N. and Bhonde R. demonstrated the important role of the ADSC secretome for reversal of insulin resistance for the first time [47]. They observed an enhancement of GLUT4 and phospho Akt protein expression, a significant decrease of intramuscular triglyceride, and adipogenesis inhibition in C2C12 cells under the influence of the ADSC secretome. It has also been established that the secretome of ADSCs down regulates inflammatory markers such as IL6 and PAI1, which are significant in ameliorating IR [47]. Moreover, Sanap et al. discovered that ADSCs condition medium significantly reduced reactive oxygen species generation in insulin resistant 3T3-L1 and C2C12 cells [48]. This finding is of interest, taking into consideration the involvement of ADSC condition medium as insulin sensitizer to combat insulin resistance.

Emerging evidence has shown the therapeutic roles of EVs in modulating different singling pathways due to their delivery of bioactive molecules into target cells [46]. Extracellular vesicles participate in ameliorating insulin resistance by regulating inflammation, which is an important factor in DM pathogenesis [49]. Zhao et al. treated obese mice with ADSC-derived exosomes and found that these EVs activated M2 macrophage polarization into anti-inflammatory type 2 macrophage (M2) phenotypes and subsequently improved insulin sensitivity [50]. These results suggest that ADSC exosomes can act as a prospective insulin sensitizer for DM therapy.

Regulating hepatic glucose metabolism

The liver plays pivotal roles in the control of glucose hemostasis. An in vivo study conducted by Xie et al. confirmed that intravenous infusion of ADSCs into T2DM rats rapidly reduced blood glucose levels [51]. Xie et al. further demonstrated that this role of ADSCs is related to activation of AMP-activated protein kinase (AMPK) signaling pathway that takes place in hepatic cells, contributing to gluconeogenesis inhibition [51].

Promotion of β cell function and regeneration

ADSCs also promote the regeneration and repair of islet β cells. One of the first studies on the use of ADSCs in diabetes mellitus dates to 2010, when Abu-Abeeleh injected two million human ADSCs into Streptozotocin (STZ) induced diabetic rats. The results of their study indicated that intravenously injection of ADSCs normalized the C-peptide level in the blood. They considered the possible role of ADSCs in the restoration of pancreatic islet cells and the return of their function [52]. ADSCs infusion can restore β cell function by reducing the apoptosis of islet cells through decreasing caspase-3 activity. In addition, ADSCs facilitate regeneration by promoting islet vascularization via the release of angiogenic factors such as VEGF, HGF, and vWF [45]. Animal experiments have shown that in a mouse model of T1DM, intravenously injected ADSC can support the survival of β cell progenitors and reduce the levels of blood glucose. The positive effects of ADSCs on the protection of endogenous islet cells can be attributed to their immunoregulatory capacity and direct contact with immune cells [53]. Similarly, a study by Khatri et al. found that intrapancreatic administration of ADSCs to STZ-induced diabetic NMRI nude mice exerted a restorative effect on damaged islet cells [54].

Recent studies have revealed that β cells dedifferentiation is an important mechanism of the pathophysiology of type 2 diabetes (T2DM), which leads to the loss of β cells and dysfunctional insulin resistance [55, 56]. Several previous studies have demonstrated that mesenchymal stem cells (MSCs) can inhibit pancreatic β-cell dedifferentiation. Wang et al. demonstrated that MSC treatment can reverse β cell dedifferentiation in human T2DM islets in an IL-1Ra-mediated manner [57]. However, to date, there have been no reports specifically on the effect of adipose derived stem cells on diabetic pancreatic β cells.

As a gene delivery tool

Since MSCs are the ideal gene delivery vehicles, the combination of MSCs and gene therapy opened up a new chapter in the therapeutic strategy for the treatment of diabetes. Sun et al. used an approach involving lentivirally transduced adipose derived stem cells to deliver betatrophin to the pancreas of Streptozotocin (STZ)-induced diabetic mouse. Infusion of these ADSCs overexpressing betatrophin increased human β cell proliferation and insulin secretion [58]. Most recently, an interesting study transduced human adipocyte-derived stem cells with lentiviral vectors expressing a furin-cleavable insulin gene to treat type 1 diabetes in a mouse model [59]. These reports imply the strong potential of ADSCs as a good candidate vector for cell-based therapy.

Diabetes and chronic wound

One of the main complications of diabetes is chronic wound healing problems, which affect approximately 25% of all patients with DM [60]. In recent years, adipose derived stem cell-based therapy demonstrated efficacy in wound healing, providing a potentially therapeutic tool by addressing multifactorial etiology [61]. The impairment of self-repairing abilities in diabetic patients is due to defects in each phase of wound healing, i.e., haemostasis, inflammation, proliferation, and remodeling phase [62]. Hyperglycemia is associated with higher inflammation indices which lead to an abundance of neutrophils, altered macrophage phenotype, and excessive levels of pro-inflammatory cytokines such as IL-6, IL-8, and TNF-α [63]. Consequently, prolonged inflammatory phase in the wound creates a cascade that maintains the chronic wound state [64].

Mechanisms of ADSCs action on diabetic wound healing

Tissue repair

• Extracellular matrix remodeling.

Several studies demonstrated that ADSC therapy can accelerate skin reconstruction by stimulating collagen synthesis, which is essential for ECM formation [65]. Liu K. et al. observed enhanced wound reepithelization in ADSC treated wounds [66].

With the treatment of ADSC, expression of collagen III/I, TGF-β3/TGF-β1, and MMP-3/tissue inhibitors of MMP-1 (TIMP-1) increased at the wound site [67]. Enhanced transforming growth factor-β1 (TGF-β1) expression is often accompanied by collagen secretion and differentiation of fibroblasts to myofibroblasts, while an elevated level of TGF-β3 leads to collagen matrix reorganization and wound reepithelization [68, 69]. A study using ADSC-derived exosomes demonstrated that ADSCs promoted the migration and proliferation of fibroblasts by modulating PI3K/Akt signaling [70].

Keratinocytes assume an urgent part in the recovery of wounds, tissue remodelling, and deposition of extracellular matrix [71]. ADSCs advance the expression of two epidermal keratinocyte marker proteins, cytokeratins CK5 and CK14 [72] enhancing the proliferation and migration of keratinocytes thus stimulating the re-epithelialization process in wound healing [73]. Other than keratinocytes, fibroblasts are vital in wound repair as well and can emit different bioactive variables to advance wound healing. ADSC transplantation can initiate the fibroblast aggregate by expanding the enrollment of endothelial cells and macrophages and advancing the development of granulation tissue [73, 74].

• Site specific differentiation.

ADSCs are pluripotent stem cells with the ability to differentiate into various cell lineages [75]. One mechanism of ADSCs to accelerate wound healing is through differentiation into skin cells in a specific microenvironment [76]. An et al. showed improved wound healing in streptozotocin-induced diabetic mice by differentiation of green fluorescent protein-positive ddADSCs into fibroblasts and endothelial cells [77]. This finding was consistent with other studies in the mouse chronic wound models [76, 78].

ADSCs and immunomodulation

In a diabetic environment, high glucose levels often lead to elevated activity of inflammatory factors such as TNF-α, IL-6, and monocyte chemotactic protein-1 (MCP-1) [79]. ADSCs can attenuate inflammatory responses at wound sites by modulating cytokine expression. It has been reported that ADSCs can suppress the expression of IFN-γ, IL-1, TNF-α, and IL-6 [80, 81]. Particularly, it has been demonstrated that TNF- substantially increases the production of prostaglandin-E2 by ADSCs, increasing their immunosuppressive effect [82]. IFN-γ directly induces the synthesis of indoleamine-pyrrole 2,3-dioxygenase (IDO), which aids in the immunomodulatory effects of ADSCs [83]. Additionally, ADSCs can also increase the expression of anti-inflammatory factors, such as IL10 and IL4, thus controlling cell survival, apoptosis, and reducing inflammation [80, 84].

Other studies have shown that ADSCs can control the number, type, and grouping of macrophages. This means that macrophages can be controlled to help wounds heal. M1 macrophages begin to change into M2-type anti-inflammatory macrophages as wound healing progresses [85]. However, in a high glucose environment, M1 macrophages are more active, and they are more difficult to polarize into M2 macrophages. ADSCs can selectively activate M2 macrophages, thereby exerting immunomodulatory effects [86].

Prostaglandin E2 (PGE2) is a major regulatory factor of ADSC immunomodulation, having multiple functions [87]. One of these is inhibition of NK cells and T helper cells, thereby attenuating the inflammatory response [88].

Because of these findings, ADSCs are now a viable option in regenerative medicine and a powerful tool in cell-based therapy for decreasing inflammatory/immune response and promoting wound healing.

ADSCs and angiogenesis

One of the main causes of diabetic foot is damage to blood vessels and sores on the skin caused by high blood sugar and endothelial dysfunction. The new blood vessels formation results in the delivery of nutrients and oxygen to the wound sites; therefore, it is especially important for wound healing [89]. ADSCs promote angiogenesis through paracrine effects. Studies have shown that ADSCs can secrete a variety of cytokines, including VEGF, stromal cell-derived factor-1 (SDF-1), epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF-β), and platelet-derived growth factor (PDGF). These factors can promote angiogenesis and facilitate the regeneration of damaged tissues [90, 91]. One of the most important factors involved in angiogenesis is VEGF. The expression level of the VEGF protein was considerably lower in diabetic rats compared to non-diabetic rats [92]. This factor is upregulated following ADSC transplantation in rat studies [93–95]. The theoretical basis for this process in diabetic foot ulcers was found by Zhou et al. illustrating that ADSCs regulate VEGF-C expression via the METTL3/IGF2BP2-m6A pathway in diabetic mice [96]. Further research revealed that the tissue around implanted ADSCs exhibits noticeably greater levels of VEGF expression [97]. In studies using human ADSCs, VEGF levels in the tissue around wounds were found to be markedly greater, regardless of whether the ADSCs came from diabetic or healthy donors [95, 98, 99]. Chen l. et al. found that ADSCs may recruit and differentiate endothelial progenitor cells (EPCs) through VEGF-PLCγ-ERK signaling pathway, thereby accelerating diabetic wound healing [100]. Furthermore, Diao et al. demonstrated that VEGF could activate transcription factors in EPCs, recruit EPCs to the bone marrow, and prevent EPC apoptosis in addition to directly promoting angiogenesis [101]. In addition, SDF-1 (CXC chemokine ligand-12) is a powerful chemokine secreted by ADSCs that plays a central role in biological functions associated with wound healing [102]. SDF-1 exerts its biological effect by activating CXC chemokine receptor (CXCR) 4 + and CXCR7 + cells [102]. The SDF-1/CXCR4 axis is perceived to be critical for cell proliferation, chemoattraction of inflammatory cells, and enhancing angiogenesis at the damage sites for wound healing [102]. The secretion of SDF-1 is reduced in diabetic patients. ADSCs have proven to overexpress SDF-1 to be recruited into the damaged site [103, 104]. Di-Rocco et al. showed that topical administration of genetically engineered ADSCs with the SDF-1 gene improved wound healing in diabetic mice (Fig. 2) [105].

Fig. 2.

Mechanisms of ADSCs in the diabetic wound healing process. Created with BioRender.com.

In a diabetic foot ulcer rat model, Li et al. showed that ADSCs can potentially accelerate wound healing by promoting angiogenesis [106]. In fact, ADSCs are believed to contribute to tissue repair and regeneration by both paracrine signaling and differentiation [107]. Immunofluorescence tracing of the injected ADSCs confirmed that they could migrate to the targeted injury sites and differentiate into endothelial cells to participate in new vessel formation [98, 99, 108, 109]. Wounds treated with ADSCs generally have much higher blood vessel density compared to non-treated wounds [99, 110, 111]. Newly formed vessel density in ADSC-treated tissues is measured to be 2.5 times higher than those of non-treated groups by immunofluorescence localization [111]. Overall, the ability of ADSCs to migrate and differentiate into endothelial cells to participate in the formation of new vessels and to promote this process can be demonstrated by tracing the ADSCs that are injected into wounds’ subdermal layers [96].

Additionally, ADSCs can enhance angiogenesis through macrophage polarization. Wang X. et al. demonstrated that ADSC-Exosome-induced M2-phenotype polarization of macrophages via the JAK/STAT6 signaling pathway can promote the angiogenesis and revascularization of ischemic lower limbs in type 2 diabetic mice [112].

In diabetic ulcers, exosomes derived from MSCs may promote angiogenesis and ischemic tissue repair through the use of microRNAs. A number of microRNAs regulate angiogenesis by targeting VEGF and activating the PI3K/AKT and MAPK pathways [113]. Huang C. et al. showed that miRNA-21-5p could potently promote angiogenesis through activating serine/threonine kinase (AKT), mitogen-activated protein kinase (MAPK), and vascular endothelial growth factor receptor (VEGFR) [114]. In addition, miRNA-210-3p could enhance microcirculation and support angiogenesis by upregulating VEGF via activating a number of proangiogenic proteins, such as SRC, AKT, and ERK [115]. Exosomal miRNA-221-3p derived from MSCs enhanced the biological function of endothelial cells via the AKT/eNOS pathway, thereby facilitating the repair of diabetic wounds [116].

An Y. et al. used ADSCs overexpressing miR-21 to promote endothelial cell vascularization for regenerative purposes. They demonstrated that mir-21 secreted by engineered ADSCs could possibly be used to assist wound healing by enhancing HIF-1α and VEGF expression [117].

These studies show that paracrine growth factors, and microRNAs may help ADSCs promote angiogenesis at the injury site in a direct or indirect manner.

Safety of using adipose derived stem cells for transplantation

The emergence of ADSC therapy over the last decade has changed the therapeutic landscape for tissue engineering and organ regeneration. Numerous studies have demonstrated the significant role of ADSCs not only in DM but also in other clinical situations [118]. However, numerous uncertainties remain about ADSC therapy, which need to be addressed before daily clinical application.

The contribution of MSCs to tumor pathogenesis is widely discussed in the literature [118]. While various studies have shown that MSCs inhibit tumor progression and metastasis in multiple types of cancer [61, 63, 107, 119, 120], a large body of research has explored the capability of MSCs to contribute to tumorigenesis. It is postulated that MSCs have the capacity to differentiate into tumor-associated fibroblasts in the tumor microenvironment, providing both a functional and structural supportive environment through the production of tumor-stimulating factors and immunomodulatory mechanisms [121]. Moreover, MSCs appear to modulate immune responses and suppress both innate immune cells (neutrophils, macrophages, and NK cells) and adaptive immune system cells (DCs, B lymphocytes, and T lymphocytes) [122]. In addition, several studies have shown that MSCs can promote tumor angiogenesis by secreting proangiogenic factors (angiopoietins, EGF, galectin-1, IGF‐1, KGF, and VEGF) as well as through their potential to differentiate into pericytes, stromal fibroblasts, or endothelial-like cells [123, 124]. In addition to the abovementioned mechanisms, MSCs exert their supportive effect on tumors through inhibition of tumor cell apoptosis, stimulation of epithelial-mesenchymal transition (EMT), and promotion of tumor metastasis [125].

In fact, several discrepant results have been reported regarding the multifaceted roles of MSCs in tumor support or suppression. Thus, further studies characterizing the mechanisms of pro- or anti-tumorigenic effects of MSCs may increase the utilization of MSCs without increasing the risk of tumorigenicity. Although DSCs are typically considered stable after in vitro manipulation and expansion compared with cells derived from other sources, chromosomal instability in long-term cultures is still a major issue in the advanced application of ADSCs [102].

Another major safety issue of note is the need for 10–20% FBS during ADSC culture; actually, FBS comes with safety concerns such as transmitting unknown viral diseases and provoking immunoreactivity. Therefore, studies to develop and measure the safety of serum-free or xeno-free culture media without animal serum are necessary [71, 126].

An important issue defining any new efficient therapeutic application at a large scale is to establish the optimal dosing regimens and route of administration [72]. Further, large scale randomized placebo-controlled studies to optimize pharmacologically effective dose range, dosing schedule and rout of administration are strongly encouraged. Furthermore, another safety concern regarding using ADSCs for clinical applications is product purity.

Future Perspectives

ADSCs based therapy is a promising strategy for the clinical treatment of numerous diseases, such as diabetes mellitus, but its systematic translational use is still somewhat out of reach. In order to increase ADSC’s therapeutic efficacy and expand their application, various strategies have been examined.

Notably, priming or preconditioning of ADSCs with cytokines, hypoxia, or pharmacological drugs in vitro is one of the foremost applicable strategies to improve cell function, survival, and therapeutic potential [127]. However, several limitations, including high cost, induction of immunogenicity, and difficulty in providing good manufacturing practices (GMP) grade quality assurance for clinical application, still need to be addressed in further research [127, 128].

Genetic engineering of ADSCs is another promising approach to improve survival, migration, and immunomodulatory properties [129]. ADSCs transfected with C-X-C chemokine receptor type 4 (CXCR4) showed enhanced homing and engraftment in diabetic mice with hindlimb ischemia [130]. Sen et al. showed that ADSCs overexpressing superoxide dismutase 2 (SOD2) exhibited beneficial outcomes in reducing inflammation and improving glucose tolerance in obese diabetic mice model [131]. However, viral transduction and genetic modification are closely associated with safety concerns such as high immunogenicity, toxicity, and insertional mutagenesis [132, 133].

Recent studies have shown significant therapeutic effects of MSC-cell free therapy under a variety of experimental conditions, including metabolic disorders [134, 135]. Indeed, MSCs exert their therapeutic effects by releasing secretome, including growth factors and extracellular vesicles (EVs) [74, 136]. Recently, MSC-derived EVs have attracted significant attention as a cell free therapeutic option [137]. ADSC-derived EVs emerged as direct mediators of diabetes [134]. MiRNAs carried by adipocyte-derived EVs have been shown to have hormone-like activity, communicating with other organs and tissues to control metabolic homeostasis as well as systemic insulin resistance [104, 138]. Given the potential of adipocyte-derived EVs in regenerative therapies, future research should examine their exact therapeutic capabilities in diabetes.

Conclusions

Until now, the potential therapy effect of ADSCs in DM has been demonstrated in several studies, which increases the expectation for utilizing them in clinical application. ADSCs are able to improve insulin production and also ameliorate insulin resistance in T2DM. In addition to being able to differentiate into IPCs, ADSCs can also transport genes, increase hepatic glucose metabolism, and regenerate islet cells. Specifically, they can accelerate diabetic wound healing by synergistic effects, such as; enhancing proliferation and re-epithelialization of skin cells, site specific differentiation, immunomodulation, and promoting angiogenesis.

However, many safety challenges need to be considered, from the preparation of ADSCs to their application. Further large-scale and controlled studies are needed to overcome those issues and confirm the safety and optimal therapeutic scheme before daily clinical application.

Electronic supplementary material

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Author contributions

All authors have contributed and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Declarations

Conflict of interest

The authors declare that there is no conflict of interest regarding the publication of this paper.