Stem Cells as a Promising Therapeutic Strategy for Diabetes Mellitus: A Comprehensive Review

Fig.No. 1: Classification of Diabetes Mellitus into Type 1, Type 2, Gestational, and Secondary Diabetes Mellitus

Late hypoglycemia of occult diabetes may develop in some patients with impaired glucose tolerance, or early type 1 or type 2 diabetes. After a high-carbohydrate meal, the patient experiences hypoglycemia [14].

T1DM, also known as type 1A DM or as per the previous nomenclature as insulin-dependent diabetes mellitus (IDDM) or juvenile-onset diabetes, constitutes about 5–10% of all the cases of diabetes. It is an autoimmune disorder characterized by T-cell-mediated destruction of pancreatic β-cells, which results in insulin deficiency and ultimately hyperglycemia [12]. In this condition the immune system attacks and destroys the insulin producing beta cells of the pancreas. There is beta cell deficiency leading to complete insulin deficiency. Thus is it termed an autoimmune disease where there are anti insulin or anti-islet cell antibodies present in blood. These cause lymphocytic infiltration and destruction of the pancreas islets. The destruction may take time but the onset of the disease is rapid and may occur over a few days to weeks.

There may be other autoimmune conditions associated with type 1 diabetes including vitiligo and hypothyroidism. Type 1 diabetes always requires insulin therapy, and will not respond to insulin-stimulating oral drugs [13].

Type 2 diabetes mellitus

Insulin resistance (IR) and metabolic syndrome (MS) are often present in type 2 diabetes (T2D), which is defined by a nonautoimmune, heterogeneously increasing lack of sufficient islet β cell insulin production [15]. T2DM, also known as non-insulin-dependent diabetes mellitus (NIDDM) or adult-onset diabetes, as per the previous nomenclature, constitutes about 90–95% of all the cases of diabetes. This type of diabetes is characterized by two main insulin-related anomalies: insulin resistance and β-cell dysfunction [12]. The pathophysiology of type 2 diabetes mellitus is distinguished by insulin deficiency and insulin resistance, which have been linked to inflammatory cytokines in the plasma and high levels of fatty acids, leading to deficient glucose transport into target cells, elevated breakdown of fat, and increased hepatic glucose production . Consequent hyperglycemia is caused by the over secretion of glucagon and a deficiency of insulin by the glucagon-secreting alpha cell and the insulin-secreting beta cell, respectively. In the case of type 2 diabetes mellitus, the disease is diagnosed because patients cannot increase insulin secretion to make up for their insulin resistance, thereby causing a high level of glycemic value [16].

Gestational diabetes mellitus

GDM is defined as any degree of glucose intolerance or diabetes diagnosed at the outset or during pregnancy, usually the second or third trimester. This definition earlier also included any undetected T2DM which may begin prior to or occur at the time of pregnancy onset [12]. At the onset of pregnancy, it is noted that fasting and random blood are lower than the normal blood concentration value, and an exponential increase in blood glucose levels in the third trimester confirms gestational diabetes mellitus [16].

Fig.No. 2: Pathophysiology of Diabetes Mellitus and Its Metabolic Consequences

Insulin sensitivity changes during pregnancy in order to meet the metabolic demand of energy for the growing fetus. Insulin sensitivity rises in early pregnancy, facilitating glucose uptake into adipose cells and preparing to meet the higher energy requirements later during pregnancy. The cause of β-cell dysfunction is thought to be long-term, resulting in uncontrolled insulin synthesis in response to excess of chronic fuel. However, as pregnancy progresses, a surge in local and placental hormones such as estrogen, progesterone, leptin, cortisol, placental lactogen, and placental growth hormone promote insulin resistance . The alterations in β-cells can occur at any stage of the insulin signaling cascade: pro-insulin synthesis, post-translational modifications, and gene alterations associated with insulin signaling . Altered insulin signaling hampers the translocation of GLUT4, the primary translocator of glucose molecules in the plasma membrane of cells. In pregnancies with GDM, the insulin-stimulation rate by glucose uptake into cells is reduced by 54% compared to normal pregnancies [17].

Other types of diabetes

Besides T1DM, T2DM, and GDM, diabetes in various other forms, though in smaller percentages with respect to overall diabetic incidence scenario, has been found to be associated with some specific conditions including various pathologies and/or several disorders. The prominent among these types of diabetes include diabetes resulting from the monogenic defects in β-cell function and those due to genetic abnormalities in insulin action, endocrinopathies, exocrine pancreatic pathologies, and several other specific conditions.

Diabetes caused due to the monogenic defects in β-cell function

Diabetes resulting from monogenic defects in β-cell function constitutes only 0.6–2% of all the cases of diabetes and mainly includes maturity-onset diabetes of the young (MODY) and neonatal diabetes, besides other but rare types.

Maturity-onset diabetes of the young and Neonatal diabetes mellitus

MODY is a genetically, metabolically, and clinically heterogeneous group of mostly non-insulin-dependent diabetes, resulting from mutations in several specific genes involved in pancreatic β-cell function, which affects glucose sensing and subsequent insulin secretion with no or minimal defects, if any, in insulin action [12].

The genetic abnormalities lead to β-cell dysfunction and decreased β-cell mass due to increased apoptotic or non-apoptotic β-cell death. These defects also result in developmental abnormalities of pancreas and/or its islets or in very rare cases their complete absence leading to decreased production and secretion of insulin or hypoinsulinaemia and in the latter case an absolute insulin deficiency [12].

Endocrinopathies

Several endocrinopathies resulting in or from abnormal functioning of various hormones can lead to diabetes. Diabetes associated with these endocrine disorders usually occurs when a defect in insulin secretion and/or action is already present. Some endocrinopathies induce diabetes through inhibition of insulin secretion and these include somatostatinoma, which leads to the excessive secretion of somatostatin and primary hyperaldosteronism, which involves the hypersecretion and hyperactivity of the hormone, aldosterone [18].

Exocrine pancreatic pathologies

Several diseases of the exocrine pancreas have been found to cause diabetes but the contribution of these diseases to the overall incidence of diabetes is minimal with less than 0.5% of all the cases of diabetes resulting from the diseases of the exocrine pancreas. the exception of pancreatic neoplasia, lead to diabetes only when they are severe enough to cause extensive pancreatic damage, involving the endocrine pancreas, including the islets of Langerhans, which leads to a considerable reduction in the β-cell mass and impairment of β-cell function [19].

Fig.No. 3: Major Risk Factors Associated with Diabetes Mellitus

Stem Cells: An Overview

Stem cells are undifferentiated cells with the ability to self-renew and differentiate into specialized cell types. They play a crucial role in tissue repair and regeneration [20].  These cells can replicate, producing a pool of stem cells with the potential, under specific conditions (in vivo and in vitro), to differentiate and mature along multiple lineages, resulting in a range of tissue-specific cell phenotypes and morphotypes. Importantly, stem cells have the capacity for prolonged self-renewal and/or differentiation controlled by a myriad of intrinsic mechanisms that, in turn, are regulated by the local niche environment of each stem cell [21].

Stem cell therapy for diabetes is a regenerative medicine approach that uses mesenchymal stem cells to regenerate insulin-producing beta cells, reverse insulin resistance, and repair diabetic complications including neuropathy, nephropathy, and retinopathy [20]. The term ‘stem cell’ encompasses a diverse group of cell types extending beyond fetal development and even into adult life, when they may lose the ability to differentiate into any body cell type but can give rise to specialized cells. Bone marrow stem cells are examples of multipotent stem cells because they can differentiate into the several cell types of the haemopoietic system. Current knowledge of stem cells and how they might be used for treatment of diseases including Parkinson’s, diabetes, ischaemic injury and muscular disease are summarized in this review [22].

Stem cells are broadly classified based on their source and differentiation potential into embryonic stem cells, adult stem cells, and induced pluripotent stem cells. The regenerative and immunomodulatory properties of stem cells make them suitable candidates for treating chronic diseases like diabetes mellitus, where tissue damage is progressive and irreversible [20].

Fig.No. 4: Development and Sources of Pluripotent Stem Cells

Types of Stem Cells Used in Diabetes Therapy

1.Embryonic Stem Cells (ESCs)

Embryonic stem cells are derived from the inner cell mass of blastocysts and possess pluripotent differentiation potential [23]. During days 3-5 following fertilization and prior to implantation, the embryo (at this stage, called a blastocyst), contains an inner cell mass that is capable of generating all the specialized tissues that make up the human body [24].

The blastocyst is 3-5 days old and is a cell cluster under a hollow microscope. The differentiation of ESCs occurs when they come together to form embryoid bodies [25]. The differentiation of embryonic stem cells is spontaneous and uncontrolled. Therefore, they are defined by unlimited self-renewal and pluripotency. These pluripotent stem cells have the potential to become almost any cell type and are only found during the first stages of development [23].

ESCs can differentiate into insulin-producing pancreatic β-like cells under controlled laboratory conditions. However, ethical concerns, immune rejection, and risk of tumor formation limit their clinical application [24].

2. Adult Stem Cells

Adult stem cells (ASCs) can be derived from a series of adult tissues, such as skin, bone marrow, blood vessels, skin, muscle and bone. These stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain and repair tissues [23]. They have the ability of multipotential differentiation, indicating their tissue origin. For example, hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) exist in bone marrow, while neural stem cells (NSCs) exist in the subventricular zone and hippocampus [26].

 Adult stem cells are multipotent cells found in various tissues and are widely studied for diabetes therapy. Hematopoietic stem cells (HSCs) have been explored for immune modulation in Type 1 diabetes. Mesenchymal stem cells (MSCs), derived from bone marrow, adipose tissue, umbilical cord, and placenta, are the most extensively studied due to their safety profile and immunomodulatory effects. MSCs can improve insulin sensitivity, reduce inflammation, and promote β-cell regeneration [27].

The main role of ASCs in organisms is to maintain and repair the tissues in which they are located. They appear in the body after embryonic development [24].

3. Induced Pluripotent Stem Cells (iPSCs)

Induced pluripotent stem cells are generated by reprogramming adult somatic cells into a pluripotent state. Induced pluripotent stem cells are stem cells that are created in the laboratory, a happy medium between adult stem cells and embryonic stem cells [28]. iPSCs are created through the introduction of embryonic genes into a somatic cell (a skin cell for example) that cause it to revert back to a “stem cell like” state. For example, Gifford CA et al. reprogrammed patient-derived fibroblasts into pluripotent stem cells through the expression of OCT3/4, SOX2 and KLF4 [23].

iPSCs overcome ethical issues associated with ESCs and allow autologous transplantation, reducing immune rejection. However, genetic instability and tumorigenicity remain major concerns [24].

Fig.No. 5: Differentiation Potential of Stem Cells into Various Cell Types

Mechanisms of Action of Stem Cell Therapy in Diabetes

The mechanisms underlying stem cell therapy in diabetes mellitus include a combination of regenerative, immunological, and metabolic mechanisms that are complementary in restoring pancreatic function and enhancing insulin secretion and glycaemic control [29].

Differentiation into insulin-producing β-cells:

The most essential mechanism of action of stem cell therapy is the differentiation of stem cells into β-cells, which produce insulin. Under specific conditions in vitro and in vivo, stem cells (especially ES and iPS) can be directed to differentiate into β-like cells that perform similar functions as natural pancreatic cells by sensing blood glucose and producing insulin [29].

Stimulation of endogenous β-cell regeneration:

Besides the process of differentiation, stem cells increase proliferation and survival of the endogenous pancreatic β-cells due to secretion of bioactive molecules. This mechanism may restore the ability of the body to regenerate β-cells and thus, restore endogenous insulin production especially at an early stage of diabetes development [30].

Immunomodulation and suppression of autoimmune responses:

MSCs have very strong immunomodulating properties due to their influence on the function of both innate and adaptive immunity. In particular, they inhibit activation of autoreactive T-cells, decrease production of pro-inflammatory cytokines, and stimulate regulatory T-cells. This mechanism works especially efficiently in Type 1 diabetes, where β-cells are damaged due to autoimmune reactions [29].

Anti-inflammatory and anti-apoptotic effects:

Chronic inflammation and oxidative stress cause impairment of the functions of pancreatic β-cells. Therefore, secretion of anti-inflammatory cytokines and growth factors helps to protect β-cells from apoptosis and to maintain their functions [31].

Paracrine secretion of growth factors and cytokines:

One of the main mechanisms of stem cell therapy is associated with paracrine effects of stem cells. They secrete various growth factors, cytokines, and extracellular vesicles, which promote angiogenesis, repair tissue damage and provide for increased survival of islet cells [32].

Insulin sensitivity in peripheral tissues is increased:

The stem cells improve insulin sensitivity in peripheral tissues like muscles, liver, and adipose tissue by lowering inflammation in the body and improving glucose absorption. This helps lower insulin resistance and results in good blood sugar management, particularly for type 2 diabetes mellitus [32].

Overall effect of therapy:All the above-mentioned ways contribute to better glycemic regulation, re-establishment of pancreatic beta-cell mass, and prevention of complications. Therefore, stem cell therapy does not just treat the symptoms but works on the cause of the disease [32].

Fig.No. 6: Stem Cell-Based Therapeutic Approaches for Type 1 Diabetes Mellitus

MSCs might initiate endogenous insulin production and stimulate pancreatic islet β-cell regeneration. Under selective conditions, MSCs may differentiate into insulin-producing cells (IPCs) by directed differentiation . Yet, MSCs implement most of their reparative properties through release of a rich secretome comprising extracellular vesicles (EVs), cytokines, and myriad growth factors, including vascular endothelial growth factor, insulin-like growth factor, platelet-derived growth factor, and transforming growth factor -β, thereby stimulating intrinsic pancreatic islet regeneration and restoring β-cell function. MSCs can alleviate IR in insulin target tissues by increasing the function of glucose transporter-4, phosphorylation of insulin receptor substrate-1 and protein kinase-B, and inhibiting NOD-like receptor protein-3 inflammasome formation through immune regulation . While the mechanism by which MSCs alleviate IR remains incompletely understood, increasing evidence implicates immune dysfunction in T2DM and IR. Hence, the ability of MSCs to modulate multiple immune cells via their secretome might in turn improve pancreatic β-cell function and alleviate IR [33].

Preclinical Evidence from Animal Studies

Numerous preclinical studies using animal models of diabetes induced by streptozotocin have proven the effectiveness of stem cell therapy. Treatment with mesenchymal stem cells in diabetic rats has proved successful, as it resulted in a decrease in blood glucose levels, increase in insulin secretion and pancreatic islets regeneration [34].

There have been reports of improvement of diabetes complications such as nephropathy and neuropathy in animal studies. However, differences in animal models and human physiology prevent direct translation of these results [35].

In preclinical studies, a wide variety of researches have been carried out about the use of MSCs as a means of control over the development and progression of several autoimmune and inflammatory diseases [34]. In T1D rodent models, transplantation of MSCs resulted in reversal of hyperglycemia, restoration of pancreatic islets, increase in insulin production and creation of beneficial immunologic changes. On the basis of the obtained experimental results, some clinical trials have been performed worldwide to evaluate the safety and efficacy of MSCs derived from bone marrow, umbilical cord or menstrual blood in T1D patients treatment [35].

The therapeutic mechanisms of MSCs in patients with diabetes are mainly based on angiogenesis induction, immunomodulation, islet β cell regeneration and islet β cell apoptosis inhibition [36]. In agreement with published results, treatment with MenSC had similar therapeutic effects on STZ-induced T1D mice and resulted in significant reduction of FBG levels in them. Histological studies showed that MSC transplantation in T1D mice significantly increased insulin+ islet β cells in T1D mouse pancreas, and angiogenesis stimulation, antiapoptotic effect and islet β cell regeneration played an important role in it. Moreover, subsequent protein assays showed systemically reduced inflammation (decrease of IL-1β and TNFα expression) and increase of angiogenesis potential (increase in VEGF expression) in MSC treated mice [35]. Also, MSC treatment caused up-regulation of IL-6 expression in sera of T1D mice, and a low dose of IL-6 reversed the cytotoxic effects of IL-1β on islet β cells and increased insulin secretion by islet β cells. Moreover, MSC treatment increased islet β cell viability through anti-apoptosis and immunomodulation. Additionally, MenSCs are expected to become a promising alternative for the treatment of diabetes in the clinic because of their advantages, which include non-invasive isolation, abundance and high proliferative capacity [36].

Clinical Studies and Current Status

Several studies have already evaluated the safety and efficacy of stem cell therapy in diabetes. The treatment resulted in improvement in HbA1c levels, reduced insulin requirement, and improved β-cell function [37]. Although most of the studies demonstrate favorable results in terms of safety, the effectiveness and standardization of therapy are still lacking. Large scale randomized controlled trials are necessary before introduction to routine clinical practice.

The results of clinical trials suggest that stem cell therapy may be considered a promising method of diabetes treatment. Some of the patients treated with stem cells no longer need insulin injections. One of the clinical trials performed by Vertex Pharmaceuticals demonstrated that a dozen patients suffering from type 1 diabetes started producing insulin on their own after treatment with embryonic stem cells. Another study conducted in Brazil revealed that 21 adults with type 1 diabetes no longer needed insulin injections several years after treatment. However, in some patients, there was no improvement in their condition [38].

With an obesity epidemic and fast growing number of DM cases around the world, new ways to cure diabetes or obtain long-lasting therapeutic effect should be sought. So far, such methods as islet cell transplantation, pancreas transplantation, and the administration of anti-CD3 mAb have been approved for clinical use [39]. According to the United Network for Organ Sharing (UNOS) Data Registry Analysis, 60% of people achieved insulin independence after 4 years following the transplantation of the pancreas. Nevertheless, the surgery itself entails considerable mortality (78% survival rate in 1 year). The results of the Collaborative Islet Transplant Registry (CITR) revealed that 44% of patients became insulin independent within 3 years of transplant from 2007 to 2010, which is 27% more than for patients undergoing clinical islet transplant from 1999 to 2002. The increase of success rates of transplantation may be associated with better immunosuppressive strategies. On the other hand, the therapy with anti-CD3 mAb, despite being relatively safe, resulted in insulin independence in only 5% of subjects in 2-year period [37]. Nevertheless, islet cell transplantation is very effective in stopping the development of complications related to both short-term and long-term effects of DM. However, islet cell transplantation faces a number of difficulties; the problem of low amount of donors and their variability is particularly acute [38].

This suggests that the kind of cells injected plays an important role in the success of treatment. Intravenous infusion of CD34+ BM-HSC, obtained via leukapheresis of peripheral blood after G-CSF mobilization had the best result, since 58.9% of the treated T1DM patients were insulin-independent for a mean time span of 16 months. 7.53% of the T1DM patients also had their insulin requirement reduced by more than 50%. This shows that CD34+ BM-HSCs can be successfully used for the treatment of DM. On the contrary, intravenous infusion of UCB even though having the same amount of CD34+ cells as in BM-HSC, did not have any effect on the C-peptide levels, HbA1c levels, and insulin requirement levels of T1DM patients. This information has been derived from few studies, and needs verification through larger randomized trials [40].

Advantages of Stem Cell Therapy in Diabetes Mellitus

The use of stem cell therapy is a revolution in modern science. It exploits natural abilities of our bodies and is able to help treat several chronic diseases like diabetes, organ damage, and neuropathy [10].

Pluripotent stem cells, obtained from donated blastocysts, are the most effective method of treatment of diabetes and other chronic illnesses. Such stem cells activate repair in more than 220 types of cells, thus being able to repair organs and nerves [41].

Using stem cells to recover damaged parts of the pancreas will allow increasing insulin production and distributing it through the body. Stem cells can regenerate pancreatic beta cells, responsible for producing and secreting insulin. Restoration of natural insulin will reduce the need for insulin injections and can help to prevent many complications caused by diabetes, like organs failure [40].

It will allow people to improve their quality of life and return to an active way of life, which will help to regulate their glucose levels naturally [41].

Stem cells can be produced in many ways and from many sources. Different types of stem cells have different applications and clinical limitations. Embryonic stem cells (ESCs, isolated from inner cell mass of pre-implanted embryo) have several limitations: high tumorigenic risk, obvious host immune rejection and ethical controversy [39]. Thus, the clinical application of ESCs is questionable. The biological characteristics of induced pluripotent stem cells (IPSCs, derived from embryonic gonadal ridge or postnatal testes) are highly similar to ESCs, and their main advantage is the possibility of avoiding immune rejection and ethical controversies in ESCs transplantation by getting specific IPSCs from patients themselves. IPSCs-derived β cells are considered as one of the most promising sources of β cells for T1DM. However, IPSCs technology still faces the following problems: (1) high genetic variability between individual cell lines can lead to immature function of derived β cells. This mutation has been found to be repairable using genome editing tools such as CRISPR-Cas9 [41].

Table No.1: Advantages and disadvantages of different types of stem cells in Diabetes Mellitus