Abstract
Introduction
Diabetes mellitus is a fast growing problem worldwide. It is a chronic metabolic disorder that continues to worsen over time. Current treatment focuses on lowering blood glucose levels but it does not prevent the progression of the disease. TXNIP has been identified as an important factor contributing to the β cell death associated with diabetes.
Materials and methods
Multiple search engines and databases were used to conduct a comprehensive literature search in order to identify, studies showing TXNIP as a possible factor contributing to pancreatic β cell destruction in diabetes
Results
Most of the studies conducted were pre-clinical, only one pre-clinical study focused on the possibility of using the calcium channel blocker – verapamil hydrochloride as an oral medication to target the expression of TXNIP, and hence possibility preventing apoptosis of the pancreatic β cells.
Discussion and conclusion
TXNIP was revealed to be induced by glucose and elevated in diabetes. Furthermore verapamil hydrochloride proved to be effective in decreasing blood glucose levels and TXNIP expression in diabetes induced mice. Suggesting oral verapamil as a novel therapy to treat diabetes – halting the progression of the disease, by promoting β cell survival and function.
1 INTRODUCTION
Diabetes mellitus is a chronic metabolic disorder, characterised by chronic hyperglycaemia.(1) In diabetes mellitus there is either a defect in insulin synthesis and insulin secretion or the development of insulin resistance by receptors belonging to fat and muscle cells in the body that are responsible for lowering blood glucose levels(2). The two main types of diabetes are type 2 diabetes mellitus – formerly known as insulin independent diabetes mellitus (T2DM) and Type 1 diabetes mellitus (T1DM) – formerly known as juvenile onset diabetes or insulin dependent diabetes(2).
Diabetes Mellitus is a fast growing problem globally. At present, as stated by the international diabetes federation, 1 in 11 adults are living with diabetes and it is estimated that by the year 2040 the prevalence of diabetes will increase to 1 in 10 adults – that is approximately 642 million individuals(3). T1DM accounts for 10% of the all cases of diabetes mellitus and usually occurs in the younger population with a peak age of around 14yrs(4).
Diabetes often results in ill health and premature death. It increases ones risk of developing many severe health complications associated with hyperglycaemia. Complications such as, cardiovascular disease, neuropathy, nephropathy, retinopathy, diabetic foot and many more(5).
Type 2 Diabetes mellitus, is identified primarily by insulin resistance, the pancreas continues to secrete insulin, sometimes at levels higher than normal, but the body develops resistance to its effects(2). Unlike in type 1 diabetes, onset of the disorder usually occurs after the age of 30 years, but in may ensue in children and adolescents as well(2, 6). Type 2 diabetes mellitus is most times caused by ill health, but genetic and environmental factors play a role in the development of this chronic disorder as well(2). Type 2 Diabetes is allocated into two subgroups; diabetes with obesity and diabetes without obesity. In diabetes without obesity there is resistance to insulin in addition to insulin deficiency and secretion(2).
The pathogenesis behind T1DM is loss of insulin producing pancreatic β- cells. It is an autoimmune disorder, where activated CD4+ and CD8+ T cells and macrophages penetrate pancreatic islets causing destruction of the β cells(2). Both genetic and environmental factors contribute to the predisposition of this disorder(2).
The current treatment for T1DM involves external insulin injections and pens injected into subcutaneous tissue and insulin pumps that administers insulin through a catheter placed underneath the skin(7).
Insulin treatment can effectively lower blood glucose levels, however it fails to halt the process of pancreatic β cells destruction, which means that, the disease progressively worsens over time(8).
Recently, studies have shown that Thioredoxin interacting protein (TXNIP), a protein found in pancreatic β cells is one of the factors that contribute to the destruction of insulin producing cells and that high glucose levels exasperate the expression of TXNIP(8). Overexpression of TXNIP induces apoptosis of beta-cells and is essential for glucotoxicity induced beta-cell death, it does this by binding and inhibiting thioredoxin – a protein essential in controlling cellular redox state(9). Thus finding treatment that targets this protein on a genetic level could stop the destruction of these vital cells and halt the progression of the disease.
Verapamil hydrochloride an anti-hypertensive agent has been shown to successfully reduce the expression of TXNIP, therefore inhibiting β cell apoptosis (8, 10).
2 MATERIALS AND METHODS
There is currently a lack of data on human studies that are focused on TXNIP as a possible therapeutic target for the treatment of diabetes mellitus, hence search conditions were not limited to only clinical trial data, but included pre-clinical trial data, review articles and other relevant publications. Multiple search engines and databases were used to conduct a comprehensive literature search in order to identify, studies showing TXNIP as a possible factor contributing to pancreatic β cell destruction in diabetes, as well as verapamil hydrochloride as a possible future oral therapeutic agent to treat diabetes. The literature search was conducted on but not limited to the following search engines and databases; google scholar, University of Pretoria database, PubMed, clinical key, clinical evidence, Medline and Scopus. Titles and abstracts including but not limited to, the terms and keys words; “Thioredoxin interacting protein (TXNIP)”, “Type 1 Diabetes Mellitus”, “Type 2 Diabetes mellitus”, “pancreatic β cells”, “apoptosis”, were searched. The search results were then additionally constricted to articles and publications that were written in the English language. Initially the search was limited to articles published in the last 5 years, but due to an insufficient amount of data, the year span was increased to articles published in the last 20 years. References from related publications were also reviewed.
3 RESULTS
Results obtained using the above mentioned search methods indicated no data on human studies conducted with TXNIP as the focus of the study to treat either type 1 or type 2 diabetes mellitus. Most of the studies conducted were pre-clinical, and only one pre-clinical study focused on the possibility of using the calcium channel blocker – verapamil hydrochloride as an oral medication to target the expression of TXNIP, and hence possibly preventing apoptosis of the pancreatic β cells. Most of the studies conducted, showed a link between glucotoxicity induced β cells death and thioredoxin interacting protein. The search did however identify a currently ongoing clinical trial testing the use of verapamil hydrochloride as adjunct therapy to treat type 1 diabetes, with the focus of the study directed towards using the antihypertensive agent to suppress the expression of TXNIP.
4 DISCUSSION AND CONCLUSION
4.1 TXNIP EXPRESSION IS INDUCED BY GLUCOSE
In a human oligonucleotide microarray study, TXNIP was shown to be the strongest glucose induced gene and the expression of this protein was increased in diabetes(11). This was supported in a study conducted by Alexandra H. Minn and associates where expression of the TXNIP gene was also found to be prompted by glucose and raised in diabetes (12).
These findings were furthermore supported by those obtained in a preclinical study conducted by J Chen and associates on mice models of diabetes. In this study TXNIP was shown to be an important factor for glucotoxixity induced β cell death. Overexpression of TXNIP resulted in the activation of the intrinsic mitochondrial pathway of apoptosis. Moreover this study revealed that the physiological levels of the β cell TXNIP protein were drastically raised in response to elevated glucose levels. This proposes that TXNIP may contribute to the advancement of diabetes and the associated continued destruction of pancreatic β cells (13).
4.2 VERAPAMIL HYDROCHLORIDE COULD PREVENT Β CELL APOPTOSIS
Verapamil hydrochloride a calcium channel blocker indicated for the treatment of ailments such as, cardiovascular disease, hypertension and migraine prophylaxis was shown to prevent β cell apoptosis and possibly reverse diabetes(10).In another pre- clinical study conducted by J Chen and associates, to test the efficacy of verapamil in treating diabetes, model mice were injected with diabetes model of multiple low dose streptozotocin (STZ), given verapamil in their drinking water and had their blood glucose levels monitored for 10 days. The control group mice became overtly diabetic reaching blood glucose levels above 400mg/dL, whilst mice treated with verapamil retained normal blood glucose levels of under 250mg/dL(8).
This protection was complemented by a drastic 80% decline in TXNIP levels in secluded islets of verapamil treated mice (8).
In addition to that, immunohistochemistry of the pancreas cross sections showed severely disrupted islets in the pancreas of control mice and standard insulin encompassing islets were seen in verapamil treated mice(8).
To further address the assumption whether verapamil could reverse diabetes and not just avert it, model mice were induced with diabetes with multiple-low dose streptozocin injections, and only those that had become overtly diabetic after 15 days (blood glucose >250mg/dL) were selected, started on verapamil and followed for an added 10 days. All the experimental groups were severely diabetic, with blood glucose levels reaching 300mg/dL by day 15 and there was no significant difference between the groups. However, whilst diabetes continued to progress in the control mice group, with blood glucose levels reaching 350mg/dL, 25 days after being injected with streptozotocin, low-dose verapamil considerably lowered blood glucose levels to approximately 200mg/dL, indicating that verapamil can improve overt diabetes. Verapamil also led to a significant rise in serum insulin levels, and once again while severe islet destruction was seen in the pancreas of the control group mice, healthy insulin containing islets were seen in pancreas cross sections of mice treated with verapamil(8).
Additionally in the same study, Verapamil hydrochloride was shown to decrease the expression of TXNIP and inhibit β cell apoptosis. Interestingly, it was also shown that verapamil only reduced TXNIP expression when glucose levels were raised, which would permit for normalisation of TXNIP levels in diabetes but could avoid unwarranted effects caused by excessive decline of TXNIP below normal levels(8).
4.2.1 Mechanism of action of Verapamil hydrochloride in reducing TXNIP expression
In two pre-clinical studies conducted on mice, verapamil was shown to decrease the expression of TXNIP by inhibiting Carbohydrate responsive element binding protein (ChREBP) – a transcription factor – from binding onto the TXNIP promoter, therefor inhibiting the transcription of the TXNIP gene (9, 12). Under normal circumstances the gene would be translated to the TXNIP protein, which binds to thioredoxin, inhibiting its activity. Thioredoxin is an antioxidant and it protects cells such as pancreatic β cells from apoptosis, without it the result would be an increase in free radicals, resulting in a change in the cellular redox state. These studies proved that the downregulation of TXNIP could prevent the inhibition of thioredoxin, thus protecting cells from oxidative stress and ultimately apoptosis (8).
In conclusion, with the results obtained from pre-clinical studies, TXNIP was proven to be a critical factor contributing to the advancement of diabetes, as it induces pancreatic β cell apoptosis. Furthermore, the common anti-hypertensive verapamil hydrochloride has shown to be a promising possible oral therapeutic agent for the treatment and improvement of patients with diabetes. These results provide the basis for future human studies, using verapamil hydrochloride to enhance β cell survival and function. Leading to better therapies for patients with Diabetes mellitus.