Mesenchymal stem cells for diabetes mellitus treatment: new advances

Mesenchymal stem cells (MSCs) are the most widely used stem cells of the human body due to ease of successful isolation and expansion for many years. In particular, from 2012 until now, MSCs have been widely clinically used to treat various diseases, including graft versus host disease (GVHD), Crohn’s disease, and knee osteoarthritis. In this review, the applications of MSCs in diabetes will be reviewed and discussed. Diabetes mellitus type  1, also known as Type 1 diabetes (T1DM), is an autoimmune disease in which immune cells attack the beta cells in islets of Langerhans (pancreatic islets). Although type 2 diabetes (T2DM) is considered to be a disease related to insulin resistance, several recent studies have shown some relation of immune dysfunction in this disease. Therefore, MSC transplantation may be a beneficial treatment for both T1DM and T2DM. MSC transplantation in preclinical trials and clinical trials for T1DM and T2DM have shown a moderate to significant improvement in diabetes without adverse side effects. In this review, we will discuss some of the updates from preclinical and clinical trials of MSC transplantation for diabetes.


Introduction
Diabetes mellitus (DM) is a metabolic disorder caused by deficient insulin secretion or insulin dysfunction leading to hyperglycemia as well as chronic metabolic change of carbohydrates, protein and fat (Bastaki, 2005). Chronic hyperglycemia leads to many serious complications, namely kidney failure, heart issue, and eye diseases. Thus, it considered a costly disease because a majority of countries spend 5% to 20% of their health expense on diabetes, according to the International Diabetes Federation (IDF) (IDF, 2015). Moreover, diabetes is a high-incidence disease. In 2004, there were more than 200 million DM patients; the predicted prevalence of DM is expected to double by 2025 (Halban, 2004). Also according to the IDF, of the approximate 7 billion people worldwide, about 415 million adults (aged 20-79) suffered from diabetes in 2015 and that number is expected to rise to 642 million people in 2040 (IDF, 2015). The Western Pacific region of the world has the highest prevalence of diabetes, with 153 million cases which account for about 37% of total worldwide cases.
Diabetes is classified into two main types (type 1 and type 2) which both lead to hyperglycemia (Moorefield, 2012). Diabetes mellitus type 1, or Type 1 diabetes (T1DM), is described as a genetic autoimmune disease. It is caused by a severe deficiency of insulin due to damage of pancreatic islet beta cells (Zhang et al., 2008). T1DM patients have to depend on exogenous insulin injection to stabilize their blood glucose. Diabetes mellitus type 2 (T2DM) accounts for 90-95% of diabetes cases, and is caused by insulin resistance in peripheral tissues. T2DM is related to many risk factors, such as obesity, hypertension, lifestyle, age, family history, past history of impaired glucose tolerance (IGT) and impaired fasting glucose (IFG), and gestational diabetes (GDM). Many of the risk factors can be preventable.
Diabetic specialists have been studying ways to optimize therapy for diabetes since current therapies have two major limitations. The first is the challenge of insulin injection; there is long-term dependency, difficulty in adjusting the exact amount of exogenous insulin appropriate for each moment, and potential of insulin resistance when used long-term. The second limitation is pancreas/ islet/ islet cell transplantation; there is the problem of donor shortage and nonavailability, potential of transplant rejection, and difficulty to functionally activate and prolong grafted materials. Therefore, improved treatments for diabetes which can address the aforementioned limitations will have valuable application to both diabetes research and management.

Mesenchymal stem cells
Mesenchymal stem cells (MSCs) have been demonstrated to be involved in the in vivo self-repair and self-regeneration processes of animal tissues. These cells can be isolated from different tissues such as bone marrow, adipose tissue, dental pulp, fetal appendages as well as umbilical cord blood, umbilical cord, and placenta. When cultured in vitro, they appear as spindle-shaped cells.
MSCs express a specific marker profile; they are positive for CD29, CD51, CD73, CD90, and CD105 expression, yet negative for hematopoietic markers such as CD31 and CD45 . Interestingly, they do not express MHC class II and only express MHC class I at low levels. Moreover, they do not express Fas ligand and costimulatory molecules such as B7 and CD40, thus they have been suggested to be hypoimmunogenic cells (Atoui and Chiu, 2012

Mesenchymal stem cell transplantation for diabetes mellitus
It is known that MSCs play a crucial role in healing damaged tissues. They can differentiate to replace the dead cells as well as secrete stimulant factors to activate surrounding cells in the microenvironment, enhancing the tissue repair process . Therefore, MSCs can be applied to treat tissues impaired by chronic hyperglycemia. For T1DM, MSC transplantation can theoretically increase beta cell mass via the following effects: (1) beta cell replacement through in vitro or in vivo differentiation; (2) local microenvironment modification by production of cytokines, chemokines and factors to stimulate endogenous regeneration; (3) reduction or prevention of autoimmunity to beta cells (Ezquer, 2014

Safety of human mesenchymal stem cell transplantation
The first concern of any therapy is the risk of mortality. Similar to modern generation drugs, MSC-based therapies should be controlled and monitored for safety before their positive effects are determined. In diabetic animal models, graft rejection as well as acute adverse responses or sudden death were not  Table 1).
Human MSC transplantations lead to alleviated blood glucose levels in diabetic animals. It was confirmed that hMSC infusion could improve blood glucose homeostasis in both type 1 and type 2 diabetic animals. The glucose levels decreased markedly from a few days to 2 weeks after hMSC infusion and this decrease was maintained until 20 days to 10

Clinical applications
Although many preclinical studies have shown evidence that hMSC therapy has beneficial effects in the treatment of diabetes mellitus, there are still not many clinical applications of MSC therapy for T1DM or T2DM in the world. The first problem is due to the safety issue of MSC transplantation ( Table 2). However, this can be alleviated by understanding the special characteristics as well as in vitro and in vivo behavior of MSCs through experimental studies and evidence. MSCs themselves show unchanged morphology and phenotype as well as the normal karyotype. They also express tumor suppressors and oncogenes at normal levels even after they undergo long-term culture. There is no evidence that tumor formation is associated with MSC transplantation. Additionally, most of the clinical trials have proven that MSC transplantation for the treatment of diabetes is safe although there have been a few reported cases of fever.
However, it is difficult to elucidate whether the fever was caused by the cell transplantation or from diabetic symptoms or certain infections.
The second problem or issue is whether MSC therapy is effective for the treatment of DM in humans. Although the mechanism has not been clearly demonstrated, MSC transplantation is capable of reducing blood glucose in various periods of follow-up time from a few months to several years. Moreover, it has been suggested that MSC transplantation can normalize or maintain the ameliorated blood glucose levels as well as improve serum insulin levels, Cpeptide, HbA1C, and the daily insulin requirement for a long period. Beside evidence of these systemic effects, MSC transplantation has the potential to treat diabetic complications such as foot ulcers, thrombosis, heart failure, kidney failure, and blindness. Therefore, it is meaningful that MSC therapy can be applied to treat early and/or late stages of diabetes as well as relieve the pain of complications and delay or cease the need for amputations. Moreover, based on the successful clinical trials, many MSC therapies continue to be developed to improve the efficiencies of the following: (1) prolonged time effect; (2) reduced expenditure on treatment and increase in patients treated; and (3) upgrade in cell products, from abundance and availability (commercial distribution) to development of diverse sources of MSCs and improvement of identifiable therapeutic characteristics of MSCs (e.g. immunomodulatory potential).
Moreover, MSCs have been suggested as universal therapeutic cells (Atoui and Chiu, 2012) based on their immune privilege and are being developed as ready-to-use products (e.g. Prochymal). Finally, the mechanisms related to MSC therapy, while still unclear, are gradually being discovered through more research findings. As of now, pertinent questions remain such as:

Differentiation potential and cell replacement
It has been shown that MSC transplantation can ameliorate blood glucose levels within the follow-up time from few weeks to several years. However, the underlying mechanism of this effect is still unclear. One proposed mechanism is the replacement potential of transplanted MSC-derived cells. It has been found that MSCs can produce many cytokines and factors which improve and modulate the surrounding microenvironment. These components include inflammatory cytokines, immunosuppressive molecules and growth factors responsible for the tissue repair process ( . Therefore, it is necessary to determine inflammation and cytokine levels as well as immunosuppressants to optimize the procedure of MSC transplantation. Notably, it is perceived that MSCs derived from various tissues have different capabilities of immunomodulation (Mattar and Bieback, 2015). This difference may derive from their origins or different culture conditions (Menard et al., 2013).

Conclusion
Mesenchymal stem cells possess unique properties which make them suitable candidates for use in diabetes mellitus treatment strategies. Besides their potential to differentiate into various types of cells (e.g. beta cells), they also possess the ability to modulate immunity and angiogenesis via secreted paracrine factors. For Type 1 diabetes, the effects of MSCs on immune modulation are clearly evident; for Type 2 diabetes, some of the mechanisms exerted by MSCs are still unclear. However, for both T1DM and T2DM, MSC transplantation can dramatically improve the blood glucose levels, while reducing the insulin dose and side effects associated with DM. More importantly, both in preclinical trials and clinical trials, MSC transplantation has been demonstrated to be safe; there have been no observed adverse side effects or tumorigenesis. The data thus far have suggested that MSC transplantation is a promising therapy for DM. More controlled and randomized clinical trials are needed to further optimize the stem cell transplantation process.