Defective glucose-stimulated insulin secretion by pancreatic islet β cells could be cured with recombinant glucagon-like peptide 1 (GLP-1) or agonists of the GLP-1 receptor .Inhibition of Dipeptidylpeptidase IV (DP-IV) enzyme can decrease clearance GLP-1 ⁶.(Figure 1)

Figure 1: Novel Approaches for Diabetes Mellitus

Incretin: GIP and GLP-1:

Incretin hormones are intestinal hormones released in response to nutrient ingestion potentiate the glucose-induced insulin response. In humans, two peptide hormones are released glucose-dependent insulin releasing polypeptide (GIP), and glucagon-like peptide-1 (GLP 1) which govern the incretin effect⁷.

GLP-1 is a product of the proglucagon gene, which is located on the long arm of chromosome 2 that encodes GLP-1, glucagon, GLP-2 and other proglucagon-derived peptides⁸. GLP-1 is expressed in L-cell which is located in ileum and colon of the distal intestine⁹.It is also release in pancreatic alpha cell and neuron from brain areas like hypothalamus, pituitary, nucleus of the tractus solitarius and reticular nucleus. It is secreted from L –cell into two bioactive forms, GLP-1 and predominate circulating active form of GLP-1 amide¹⁰. Both peptide have same plasma half life and activity through same receptor¹¹. GIP is a single 42 amino acid peptide derived from the processing of a 153 amino acid precursor, whose 10 kilobase- spanning gene is located on chromosome 17 in humans⁸. It is secreted from k cell in single bioactive form duodenum and proximal jejunum of the upper small intestine. GIP receptors are located in the pancreatic islets, gut, adipose tissue, heart, pituitary, adrenal cortex and in several areas of the brain⁹.(Figure 2)

According to several trials on healthy subject and in type II diabetic patient, GLP-1 does not only stimulate insulin secretion in glucose dependent manner but also suppress glucagon, delay gastric emptying,¹² reduce appetite and also induces satiety,¹³ leading to weight loss if administered for weeks or months ¹⁴and may have an influence on insulin sensitivity¹⁵. Furthermore the data come from animal experiments suggest that GLP-1 induces differentiation of endocrine precursor cells into mature β-cells, stimulates replication of pancreatic β -cells as well as new islet formation after partial pancreatectomy¹⁶, and also inhibits apoptosis of β -cells in response to different toxic stimuli¹⁷. After long term administration of GLP-1 results into increase in β-cell mass¹⁸. Taken these finding together, all biological actions that have been described for GLP-1 in animal experiments or in healthy human volunteers could also be applied in Type II diabetic patients^16,18. Exceptions are those actions that lead to an enhanced β cell mass, in particular the differentiation of pancreatic endocrine β cells from precursor cells (pancreatic duct cells, islet-derived, nestin-positive progenitor cells,¹⁹ the neogenesis and growth (hyperplasia) of islets,²⁰ and the reduction in the rate of cells undergoing apoptosis¹⁸.

Figure 2: Secretion and pharmacological role of GLP-1 receptor in Different Organs

Exendin-4, the naturally occurring form of exenatide, was originally isolated from the salivary secretions of the lizard Heloderma suspectum²¹. Exenatide significantly improve glycemic control in patients with diabetes. Evidence suggests that these agents use a combination of mechanisms which may include glucose-dependent stimulation of insulin secretion, suppression of glucagon secretion, enhancement of β-cell mass, slowing of gastric emptying, inhibition of food intake²².(Figure 3)

Figure 3: Role of DPP-IV enzyme inhibitor on GIP and GLP-1

DPP-IV inhibitor in Diabetes mellitus:

Dipeptidyl peptidase (DPP)-IV inhibitors are one of the promising new approach to Type II diabetes. The introduction of DPP- IV inhibitors are targeted to understanding the physiological roles and metabolism of two intestinal hormones: Glucose-dependent insulinotropic polypeptide (Gastric inhibitory polypeptide; GIP) and Glucagon-like peptide-1 (GLP-1)²³collectively referred to as incretin. GIP and GLP-1 have very short half-lives following secretion due to their degradation by the enzyme, Dipeptidyl peptidase IV (DP IV) ²⁴. DP IV belong to member of the prolyl oligopeptidase family, was the major enzyme responsible for inactivating GIP and GLP-1, leading to the proposal that in vivo inhibition of its activity could lead to potentiating of endogenous incretin action during a meal and hence improve glucose tolerance in diabetics²⁵. In mammals, DP- IV is simultaneously expressed on the surface of endothelial and epithelial cells and highest levels in humans have been found in the intestine, bone marrow and kidney²⁶. DP IV acts by selectively removing N-terminal dipeptides from oligopeptidase with a strong preference for Proline (Pro) > Alanine (Ala) > Serine (Ser) as the penultimate amino acid²⁷. The enzyme is capable of less efficient cleavage of N-terminal dipeptides with hydroxyproline, dehydroproline, glycine, valine, threonine or leucine as the penultimate amino acid²⁸. In addition to its enzymatic activity, DP IV also contains several binding sites that associate with adenosine deaminase, HIV GP120 protein, fibronectin, collagen, chemokine receptor CXRC4 and tyrosine phosphatase CD45²⁹.These interactions are involved in functions such as immune regulation, extracellular matrix binding and cell–cell signaling³⁰.(Figure 4)

Figure 4 : Mechanisam of exendine -4 inhibitor

Vildagliptin: DPP-IV inhibitor”:

Vildagliptin is an orally effective, selective inhibitor of Dipeptidyl peptidase IV (DPP-4) that increase meal-stimulated levels of biologically active Glucagon-like peptide-1 (GLP-1) and improves glucose tolerance in animal models of diabetes^31,32and in type II diabetic patient³³. Clinical trials confirmed that chronic treatment with vildagliptin monotherapy reduces postprandial glucose levels and produces a clinically meaningful reduction in HbA1c in Type 2 patients without causing hypoglycemia³⁴.

A single dose of 100 mg of Vildagliptin administered before the evening meal: Vildagliptin sustained inhibition of plasma DPP-IV activity and increased meal/post meal levels of GLP-1 and GIP³⁵.

Peroxisome Proliferator Activated Receptor:

Issemann and Green discover of the first peroxisome proliferator-activated receptor (PPAR) was the key to the present understanding of peroxisome proliferation and its growing medical significance³⁶. Subsequently, several PPAR isotypes (α, β or δ and γ) have been found in vertebrate species³⁷, e.g. Xenopus, mouse, hamster and human. Recently, there is an increased interest in PPAR γ research because they are

(A) Key regulators of adipocyte differentiation and energy source and

(B) Cellular targets of thiazolidinediones drugs, which are used to treat Type II diabetes by decreasing insulin resistance

A more predominate role of PPAR γ as it influences multiple fundamental pathways in the cell with wide-ranging biomedical implications³⁸. PPAR γ expression is found in the nucleus of many cells, but highest levels of PPAR γ mRNA and protein found in adipose tissue, large intestine and hematopoietic cells³⁹. However, PPAR γ mRNA has been identified in skeletal muscle and is found to be increased in obese subjects with insulin resistance⁴⁰.Under the influence of a number of metabolic and hormonal variables, expression of PPAR γ mRNA or protein or both in adipose tissue changes⁴¹. While short-term changes in food intake do not affect the expression of human PPAR γ, hypocaloric diets for a longer period result in its down regulation. The PPAR γ agonists induce a ‘fatty acid steal’ by the adipose tissue. Decreased in systemic availability of fatty acids and reduced fatty acid uptake by muscle will improve insulin resistance⁴².

A predominate hypothesis for regulation of insulin sensitivity by PPAR γ involves primary effects of PPAR γ on gene transcription in adipose tissue (where it is most abundantly expressed), which ultimately lead to improved insulin action in muscle and liver. Direct activation of PPAR γ leads to the induction of adipocyte genes such as those for lipoprotein lipase and fatty-acid transporter 1, which in turn contribute to lowering triglyceride and FFA levels, respectively⁴³.Similarly, suppression of TNF-α gene expression by PPAR γ in adipose tissue has been reported. As FFAs and TNF-α are both potential systemic mediators of insulin resistance, such effects are likely to contribute to the efficacy of PPAR γ activation in increasing insulin sensitivity. Due to reduced systemic lipid availability, muscle lipid levels can also be reduced⁴⁴.

Mechanisms of PPAR activation and regulation of target gene expression:

When PPAR γ is bound by natural ligand or synthetic molecules such as a thiazolidinediones, it becomes activated and complexed with another transcription factor known as the retinoid X receptor (RXR). Transcriptional regulation by PPARs is achieved through PPAR-RXR heterodimers which bind to DNA motifs termed peroxisome proliferative response elements (PPREs) in the promoters of target genes. The whole PPRE consensus sequence exhibits a pattern specific for PPAR-RXR heterodimers⁴⁵ and is distinguishable from the responsive elements of other nuclear receptors belonging to oestrogen, vitamin D or thyroid hormone. PPAR-mediated transcriptional control of genes is regulated by a new functional class of proteins called cofactors (co repressors and co activators). SMRT (silencing mediator for retinoid and thyroid hormone receptor) is one such co repressor reported to be involved in down-modulating PPAR γ -mediated gene transcription⁴⁶. Interestingly, a number of proteins have been identified and characterized as co activators of PPAR γ such as steroid receptor co activator (SRC-1)^47,48, PPAR binding protein (PBP) and PPAR γ coactivator-1 (PGC-1). PPAR interacting protein (PRIP) has been postulated that these co activators act as bridges to transmit the nuclear receptor regulatory signals to the cellular transcriptional machinery. In general, inactivated nuclear receptors are complexed with co repressors, which extinguish their transcriptional activity by the recruitment of histone deacetylases. (Figure 5)

Figure 5: Binding to target gene through PPRE consensus binding site

Activation of the receptor then induces a conformational change which results in the dissociation of co repressors and the recruitment of co activator complexes that contain proteins with histone acetyl transferase activity, which facilitates target gene transcription⁴⁹. Apart from these cofactors, activation of PPAR γ can also be depressed by phosphorylation of a seryl residue in its structural region, mediated by mitogen- activated protein (MAP) kinase⁵⁰. The final action of PPAR γ depends on a variety of factors such as the abundance of the relevant endogenous ligands/activators, numerous co-activators or co-repressors and the expression and function of RXRs, the companion nuclear receptors essential for formation of the active heterodimeric complex (PPAR γ + RXR)⁵¹.

Gene therapy for diabetes mellitus:

Type 1 diabetes mellitus is result of deficiency insulin caused by the autoimmune destruction of insulin producing pancreatic β cells. Hyperglycemia causes a lot of long-term clinical problems, including renal failure, retinopathy, and neuropathy and heart disease⁵². However, the development of gene therapy has also generated a greater hope and excitement for a possible “cure” of diabetes since insulin gene was first cloned and expressed in cultured cells in the late 1970s⁵³. In severe condition of diabetes many attempts have been made, including islet transplantation, whole pancreas transplantation, regeneration of β cells and insulin gene therapy^54,55. Advances procedures in islet transplantation it means that patients with the disease can be cured by transplantation of primary human islets of Langerhans. The major drawbacks of these strategies are the insuffificient availability of donor islets, invasive procedure and high cost. Extensive proliferative ability of stem cell may provide a valuable source of islet progenitor cells .Several studies have shown that stem cells can be expanded in vitro to generate a large number of islet progenitor cells⁵⁶. Insulin gene therapy including any approach involving the introduction of a foreign gene into any cell type in the body can produce insulin⁵⁷.

Stem cells are those cells which are able to self-renewal and retaining a capacity to differentiate into specialized cell types under appropriate conditions. Adult pancreatic stem cells are presented in intra islet, nestin-positive cells, duct cells and oval cells which differentiate into pancreatic β-cells⁵⁸. Another advantage is that there is less chances of immune rejection because they behave as an autologous model whereby a patient’s own cells can be used⁵⁹. In rodent models and in human recipients of marrow or organ transplantation shows that bone marrow harbours cells converted into parenchymal cells after entering the liver, intestine, skin, lung, skeletal muscle, heart muscle, and central nervous system⁶⁰. In rodents study haemopoietic organs harbour cells that can differentiate into functional pancreatic endocrine cells⁶¹. One-Two months after bone-marrow transplantation, donor derived cells are found in pancreatic islets of recipient mice⁶². These cells express insulin and genetic markers of β cells. In culture, in response to glucose cells secrete insulin similar to normal β cells. However, only 1–3% of the islet cells originate from the transplanted marrow⁶³. A marrow-derived cell-type have a capacity to transdifferentiate into various phenotypes has been described⁶⁴. According to one of the experimental study on overtly diabetic mice whose β cells have been destroyed by streptozotocin, after bone-marrow transplantation, blood glucose and insulin concentrations were normal and survival was better⁶⁵. In islets, marrow-derived cells had differentiated into endothelial cells and occasionally into insulin expressing cells. Endothelial implant was speculated to stimulate the proliferation of local pancreatic progenitors, leading in turn to the increased insulin producing cell mass.

Pancreas resident progenitor cells might give rise to endocrine islet cells in isolated pancreatic tissue. Human and rodent pancreatic-duct cells, islet-derived cells, and exocrine tissue contain cells that can differentiate towards a pancreatic endocrine phenotype⁶⁶. These tissues, cultured and differentiated in vitro, have been transplanted and can reverse diabetes mellitus in rodents. Rodent-liver stem cells and human fetal-liver cells have been differentiated in vitro into insulin-secreting cells in culture method. In animal study when transplanted, these cells reverse diabetes mellitus⁶⁷.Cells within liver that can differentiate into insulin-secreting cells after introduction of ß-cell-specific genes have also been seen in vivo after adenoviral gene-delivery into rodents that have been rescued from diabetes for long periods^68,69. A bonefide pancreatic stem cell for β-cell regeneration remains elusive. A one of the study conducted in mice casts doubt on the existence of any β-cell progenitor cells and shows that β cells regenerate only by proliferation of existing β-cells⁷⁰. In human beings, early immunological intervention to stop β-cell destruction during the development of Type 1 diabetes mellitus allows recovery of pancreatic endocrine function⁷¹. This finding might in part be attributable to recovery in β-cell mass by recruitment of local pancreatic or extra pancreatic progenitor cells that differentiate into β-cells and/or proliferation of remaining β cells during protection from immune-mediated destruction

Sodium-glucose co-transporter inhibitors:

The kidney is important organ for the body’s energy control. Glucose filtered from the glomerulas is reabsorbed mainly in the S1 segment of the kidney’s proximal tubule⁷², but when the glucose reabsorption reaches saturation level, excess glucose is excreted in the urine⁷³.Two types of sodium glucose co transporters mediate reabsorption of the glucose, low affinity sodium glucose co transporter (SGLT2) and high-affinity sodium glucose co transporter (SGLT1).The low affinity sodium glucose co transporter is found in the kidney⁷⁴,and several mutations in the human SGLT2 gene can cause renal glucosoria⁷⁵. The high-affinity sodium glucose co transporter (SGLT1) is mainly found in the intestine and in some extent expressed in the kidney and contributes to glucose reabsorption⁷⁶. If there is genetic mutations in the SGLT1 gene occurs leading to a functional defect are responsible for glucose/galactose mal absorption⁷⁷. The SGLT2 molecule was cloned as a candidate sodium glucose co transporter, and its tissue distribution, substrate specificity, and affinities are reportedly very similar to those of the low-affinity sodium glucose co transporter in the renal proximal tubule⁷⁸.

The first SGLT inhibitor was phlorizin, which was found from the root bark of the apple tree. Phlorizin lowering plasma glucose concentration and improving insulin resistance by increasing renal glucose excretion (via an inhibition of renal glucose transporter) .However phlorizin was poorly absorbed in the intestine that’s why it is not developing as a drug for the treatment of diabetes because it was easily hydrolyzed by lactase phlorizin hydrolase⁷⁹.

Pharmacology of SGTLT2 inhibitors:

In the kidney most of the glucose filtered through the glomerulas and it is reabsorbed by SGLT2 in the S1 segment of the proximal tubule, and remaining glucose reabsorbed by SGLT1 in the S3 segment, excess glucose is excreted in the urine when both SGTL2 and SGTL1 are saturated⁸⁰.

Phlorizin was inhibitor of SGLT2 and SGLT1, it suppress renal glucose reabsorption and lowering plasma glucose result in increase glucose excretion in the urine⁷⁹.Although phlorizin was used as a research tool to demonstrate that hyperglycemia contribute to the insulin resistance involve in the etiology of Type II diabetes⁸¹.

Sergliflozin is a promising SGLT2 inhibitor. Sergliflozin is 296-fold more selective for SGLT2 than for SGLT1, and it appear to increase renal glucose excretion by reducing the transport maximum for glucose (with consequent improvement in hyperglycemia) in STZ-induced rats⁸².SGLT2 plays a vital role in renal glucose reabsorption and shows that SGLT2 is a main molecular target among SGLTS for the suppression of renal glucose reabsorption.

Potential benefit and safety of SGLT2 inhibitor:

Lifestyle interevation is the most important management for Type II diabetes which promotes body weight loss leading to improvement in glycemic control. This management depends on energy control of diet and exercise⁸³. However it is difficult to maintaining the restricted lifestyle in the long term, and this deficiency in negative energy partitioning account for the progressive nature of diabetes condition. In such cases SGLT2 inhibitor can alongside lifestyle intervention, shift the boy’s energy in a negative direction .Indeed, SGLT2 inhibitor exhibit sufficient therapeutic potency to control glycemia effectively and adequate safety and tolerability profile for the treatment to be easily maintained long term⁸⁴.

Jun n terminal kinase (JNK) pathway:

The development of Type II diabetes is usually caused by combination of pancreatic β-cell dysfunction and insulin resistance⁸⁵.Chronic hyperglycemia is a cause of impairment of insulin biosynthesis and secretion; once hyperglycemia becomes apparent, β -cell function gradually deteriorates and insulin resistance aggravates⁸⁶.This process is called “glucose toxicity”. In diabetic conditions, reactive oxygen species (ROS) are increased in various tissues⁸⁷.Recently, pancreatic β-cells emerged as a target of oxidative stress-mediated tissue damage⁸⁸. β-cells display highly efficient glucose uptake when exposed to high glucose concentration. If extracellular hyperglycemia occurs it causes intracellular hyperglycemia in β-cells, leading to the induction of ROS in pancreatic islets of diabetic animals. Indeed, it was shown that expression of oxidative stress markers such as 8-hydroxy-2'- deoxyguanosine (8-OHdG) and 4-hydroxy-2,3-nonenal (4- HNE) are increased in islets under diabetic conditions⁸⁹.β-cells are rather vulnerable to oxidative stress due to the relatively low expression of antioxidant enzymes such as catalase, and glutathione peroxidase⁹⁰. Thus, it is likely that oxidative stress plays a major role in β-cell deterioration in Type II diabetes. Non-enzymatic glycosylation reaction in the β transport chain in mitochondria and the hexosamine pathway are several sources of ROS productions in cells⁸⁸. The β transport chain in mitochondria, and the hexosamine pathway⁹¹.Among those, the glycation reaction have shows pathological significance in diabetic complications, because under hyperglycemic conditions the production of various reducing sugars such as glucose, glucose- 6-phosphate, and fructose, increases through glycolysis and the polyol pathway. All of these reducing sugars are known to promote glycation reactions of various proteins during the reaction which in turn produces Schiff base, Amadori product and advanced glycosylation end products (AGE), ROS are also produced⁸⁸. To produce ROS Electron transport chain in mitochondriais likely to be an important pathway. But it was reported that mitochondrial overwork, which causes induction of ROS, leads to impaired first-phase of glucose stimulated insulin secretion found in the primary stage of diabetes⁸⁹. Pancreatic and duodenal homeobox factor-1 (PDX-1)is a member of the homeodomain-containing transcription factor family⁹¹. PDX-1 is mainly found in the pancreas and duodenum and plays a vital role in pancreas development⁹², β-cell differentiation / regeneration, and in maintaining normal β-cell function by regulating various β-cell-related genes⁹³. In addition, mutations in PDX-1 are known to cause some cases of maturity-onset diabetes of the young (MODY)⁹⁴. As a possible cause of the reduction in the insulin gene promoter activity by oxidative stress,

Antioxidant Treatment for β-Cell Glucose Toxicity:

Oxidative stress is produced under diabetic conditions and is possibly involved in pancreatic β-cell dysfunction found in diabetes. According to animal study it was proved that antioxidants like N-acetyl-L-cysteine plus vitamin C and E is preserved glucose-stimulated insulin secretion and moderately decreased blood glucose level⁹⁵.

The JNK pathway is known to be activated under diabetic conditions and to possibly be involved in the progression of insulin resistance. According to animal study on mice shows that dominant-negative type JNK in the liver of diabetic dramatically improved insulin resistance and markedly decreased blood glucose levels⁹⁶. Some of the trial proves that suppression of the JNK pathway in the liver exerts greatly beneficial effects on insulin resistance status and glucose tolerance in both genetic and dietary models of diabetes⁹⁷.

CONCLUSION:

The increasing knowledge on the biochemical and cellular alterations occurring in diabetes mellitus has promoted the development of novel and potentially more-effective novel approaches to treat the disease. In a few of these approaches, as is the case for GLP-1 receptor agonist, inhibitors of Dipeptidylpeptidase IV, PPAR γ , Sodium glucose co-transporter and reduction in oxidative stress by inactivating JNK pathway and gene therapy may be new promising approaches for the treatment of diabetes mellitus. Studies in animal models and in humans have been carried out, and it is possible that these agents will be available as therapeutic treatments in the near future. Most approaches are still at early stages of development and the data available have just provided the proof of concept. Thus, further research is necessary to confirm their therapeutic value, as well as to show that the strategy is safe for treatment of diabetes mellitus patients.

REFERNCES:

1. Amos A, McCarty D and Zimmet P. The rising global burden of diabetes and its complications: estimates and projections by 2010. Diabet. Med. 1997; 5: S5–S85.

2. Nagappa A. 16-Novel strategies for the therapeutic management of type 2 diabetes. 2000; 1-2: 58-68.

3. Shaw J et al. Type 2 Diabetes Worldwide According to the New Classification and Criteria. Diabetes Care 23 (Suppl. 2). 2000; 23: B5–B10.

4. Curtis L et al.Diabetes Mellitus. In DiPiro JT et al,Pharmacotherapy, Edited by McGraw-Hill, New York: 2005;6 th Edi: pp,1335-36

5. Harris M. National institute of health, diabetes and digestive and kidney diseases, "diabetes in america".Diabetes care.1995; 95-1468.

6. Moller DE. New drug targets for type 2 diabetes and the metabolic syndrome Nature. 2001; 414: 820-27.

7. Gautier JF, Choukem SP, Girard J, Physiology of incretins (GIP and GLP-1) and abnormalities in type 2 diabetes, Elsevier.Diabetes Metab.2008; 34: S65-S72.

8. Fehmann HC, Goke R, Goke B. Cell and molecular biology of the incretin hormones glucagon-like peptide-I and glucose-dependent insulin releasing polypeptide. Endocr Rev.1995; 16: 390-410.

9. Baggio LL, Drucker DJ. Biology of incretins: GLP-1 and GIP. Gastroenterology 2007; 132: 2131-57.

10. Egan JM et al. GLP-1 receptor agonists are growth and differentiation factors for pancreatic islet beta cells. Diabetes Metab Res Rev. 2003; 19: 115-23.

11. Naslund E et al. Energy intake and appetite are suppressed by glucagon-like peptide-1 (GLP-1) in obese men. Int J Obes Relat Metab Disord. 1999; 23: 304-11.

12. Gutzwiller JP et al. Glucagon-like peptide-1 promotes satiety and reduces food intake in patients with diabetes mellitus type 2. Am J Physiol. 1999; 276: R1541-4.

13. Nauck MA et al. Additive insulinotropic effects of exogenous synthetic human gastric inhibitory polypeptide and glucagon-like peptide-1-(7–36) amide infused at near-physiological insulinotropic hormone and glucose concentrations. J Clin Endocrinol Metab. 1993; 76: 912.

14. Kreymann B et al. Glucagon-like peptide-1 (7–36): a physiological incretin in man. Lancet. 1987; 2: 1300–4.

15. Zander M et al. Effect of 6-week course of glucagon-like peptide 1 on glycaemic control, insulin sensitivity, and beta-cell function in type 2 diabetes: a parallel-group study. Lancet 2002; 359: 824– 30.

16. Hui H, Wright C, Perfetti R. Glucagon-like peptide 1 induces differentiation of islet duodenal homeobox-1-positive pancreatic ductal cells into insulin-secreting cells. Diabetes. 2001; 50: 785–96.

17. Abraham EJ et al. Insulinotropic hormone glucagon-like peptide-1 differentiation of human pancreatic islet-derived progenitor cells into insulin-producing cells. Endocrinology. 2002; 143: 3152– 61.

18. Hui H et al. Glucagon-like peptide-1 inhibits apoptosis of insulin-secreting cells via a cyclic 5V-adenosine monophosphate-dependent protein kinase A- and a phosphatidylinositol 3-kinase-dependent pathway. Endocrinology. 2003; 144: 1444– 55.

19. Brubaker PL, Drucker DJ. Minireview: glucagon-like peptides regulate cell proliferation and apoptosis in the pancreas, gut, and central nervous system. Endocrinology. 2004; 145: 2653– 9 .

20. Xu G et al. Exendin-4 stimulates both beta-cell replication and neogenesis, resulting in increased beta-cell mass and improved glucose tolerance in diabetic rats. Diabetes. 1999; 48: 2270– 6.

21. Eng J et al. Isolation and characterization of exendin-4, an exendin-3 analogue from Heloderma suspectum venom. J Biol Chem. 1992; 267: 7402 –5.

22. Loretta L et al. Pharmacology of exenatide (synthetic exendin-4): a potential therapeutic for improved glycemic control of type 2 diabetes , Regul. Pept 2004;117 :77–88.

23. Holz GG, Chepurny OG. Glucagon-like peptide-1 synthetic analogs: new therapeutic agents for use in the treatment of diabetes mellitus. Curr Med Chem. 2003; 10: 2471–83.

24. Mentlein R, Gallwitz B, Schmidt W. Dipeptidyl peptidase IV hydrolyzes gastric inhibitory polypeptide, glucagon-like peptide-1(7–36), peptide histidine methionine and is responsible for their degradation in human serum. Eur J Biochem. 1993; 214: 829– 35.

25. Christopher HS, Demuth HU, Kim SJ. Applications of dipeptidyl peptidase IV inhibitors in diabetes mellitus, Int J Biochem Cell Bio. 2006; 38: 860–872.

26. Low SH et al. Apical cell surface expression of rat dipeptidyl peptidase IV in transfected Madin –Darby canine kidney cells. J Biol Chem. 1991; 266: 13391– 6.

27. Lambeir AM et al. Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions and clinical aspects of the enzyme DPP IV. Crit Rev Clin Lab Sci. 2003; 40: 209–94.

28. Meester DI et al. Dipeptidyl peptidase IV substrates. Adv Exp Med Biol. 2003; 524: 3 – 13.

29. Mentlein R. Dipeptidyl-peptidase IV (CD-26)—role in the inactivation of regulatory peptides. Regul Pept. 1999; 85: 9–24.

30. Augustyns K et al. The unique properties of dipeptidyl peptidase IV (DPP IV/ CD26) and the therapeutic potential of DPP IV inhibitors. Curr Med Chem. 1999; 6: 311 –27.

31. Villhauer EB et al. 1-[[(3-Hydroxy-1-adamantyl)amino] acetyl]2-cyano-(S)-pyrrolidine: a potent, selective, and orally bioavailable dipeptidyl peptidase IV inhibitor with antihyperglycemic properties. J Med Chem. 46: 2774–2789.

32. Dardik B et al. A dipeptidyl peptidase IV inhibitor, improves glucose tolerance and delays gastric emptying in obese insulin resistant cynomolgus monkeys. Diabetes. 2003 ;52: A322.

33. Ahren B et al. Inhibition of dipeptidyl peptidase-4 reduces glycemia, sustains insulin levels, and reduces glucagon levels in type 2 diabetes. J Clin Endocrinol Metab.2004; 89: 2078–2084.

34. Ahren B et al . Twelve- and 52-week efficacy of the dipeptidyl peptidase IV inhibitor LAF237 in metformin-treated patients with type 2 diabetes.Diabetes Care.2004; 27:2874–2880.

35. Balas BN et al. The Dipeptidyl Peptidase IV Inhibitor Vildagliptin Suppresses Endogenous Glucose Production and Enhances Islet Function after Single-Dose Administration in Type 2 Diabetic Patients; The Journal of Clinical Endocrinology & Metabolism. 2009; 92(4): 1249–1255.

36. Issemann I and Green S. An oxysterol signalling pathway mediated by the nuclear receptor LXR α .Nature. 1990; 347: 645–650.

37. Lemberger T, Desvergne B, Wahli W. Peroxisome proliferator-activated receptors: A Nuclear Receptor Signaling Pathway In Lipid Physiology. Annu Rev Cell Dev Biol. 1996;12:335–363.

38. Vamecq J and Latruffe N. Medical significance of peroxisome proliferator-activated receptors. Diabetes. 1999; 354: 141–148.

39. Auboeuf, D et al. Tissue distribution and quantification of the expression of mRNAs of peroxisome proliferator-activated receptors and liver X receptor- in humans. No alteration in adipose tissue of obese and NIDDM patients.Diabetes.1997; 48: 1319–1327.

40. Park, KS et al. Troglitazone Effects on Gene Expression in Human Skeletal Muscle of Type II Diabetes Involve Up- Regulation of Peroxisome Proliferator-Activated Receptor-g. Diabetes. 1997; 46: 1230– 1234.

41. Loviscach M et al. Distribution of peroxisome proliferator-activated receptors (PPARs) in human skeletal muscle and adipose tissue: relation to insulin action. Diabetologia. 2000;43: 304–311.

42. Martin, G et al. PPARγ activators improve glucose homeostasis by stimulating fatty acid uptake in the adipocytes. Atherosclerosis J. Biol. Chem.1997;272: 28210–28217.

43. Berger, J, Moller, DE. Mechanism of PPAR action. Annu. Rev. Med. 2002;53:409-35.

44. Oakes ND et al. A new antidiabetic agent, BRL 49653, reduces lipid availability and improves insulin action and gluco regulation in the rat. Diabetes. 1994; 43: 1203–1210.

45. Osada, T et al. Identification of an extended half-site motif required for the function of peroxisome proliferator-activated receptor alpha Genes Cells. 1997; 2:315–327.

46. Lavinsky RM et al. Diverse singaling pathway modulate nuclear receptor recruitment of N-CR and SMART complex Proc. Natl Acad. Sci.1998; 95: 2920–2925.

47. Glass CK, Rose DW and Rosenfeld, M. G., Nuclear receptor co activators and corepressors. Curr. Opin. Cell. Biol.1997; 9: 222–232.

48. Esterbauer H et al. Human peroxisome proliferator activated receptor gamma co activator 1 (PPARGC1) gene: c DNA sequence, genomic organization, chromosomal localization, and tissue expression. Genomics. 1999; 62: 98–102.

49. Pazin MJ and Kadonaga, JT, What's up and down with histone deacetylation and transcription? Cell.1997; 89: 325–328.

50. Shao D et al. Interdomain communication regulating ligand binding by PPAR-gamma. Nature.1998; 396: 377–380.

51. Camp HS, Tafuri SR.Regulation of peroxisome proliferator-activated receptor gamma activity by mitogen-activated protein kinase J. Biol. Chem.1997; 272: 10811–10816.

52. Dieterle C et al. Therapy of diabetes mellitus. Pancreas transplantation, islet transplantation, stem cell and gene therap. Internist (Berl). 2006; 47: 489-496.

53. Samson SL, Chan L. Gene therapy for diabetes: reinventing the islet. Trends .Endocrinol Metab. 2006; 17: 92-100.

54. Fukushima M et al. Adiponectin gene therapy of streptozotocin-induced diabetic mice using hydrodynamic injection. J Gene Med. 2007; 9: 976-985.

55. Lu YC et al. Release of transgenic human insulin from gastric g cells: a novel approach for the amelioration of diabetes. Endocrinology. 2005; 146: 2610-2619.

56. Cho YM et al. Betacellulin and nicotinamide sustain PDX1 expression and induce pancreatic beta-cell differentiation in human embryonic stem cells. Biochem Biophys Res Commun. 2008; 366: 129-134.

57. Serafimidis I et al. Novel effectors of directed and Ngn3-mediated differentiation of mouse embryonic stem cells into endocrine pancreas progenitors. Stem Cells. 2008; 26: 3-16.

58. Körbling M. Estrov, Z. Adult stem cells for tissue repair – a new therapeutic concept? N. Engl. J. Med. 2003; 349: 570–582.

59. Street CN. Stem cell-based approaches to solving the problem of tissue supply for islet transplantation in Type 1 diabetes. Int. J. Biochem. Cell Biol. 2004; 36: 667–683.

60. Herzog EL, Chai L, Krause DS. Plasticity of marrow-derived stem cells. Blood. 2003; 102: 3483–93.

61. Theise ND et al. Liver from bone marrow in humans. Hepatology. 2000; 32: 11–16.

62. Korbling M et al. Hepatocytes and epithelial cells of donor origin in recipients of peripheral blood stem cells. N Engl J Med. 2002; 346: 738–46.

63. Ianus A et al. In vivo derivation of glucose-competent pancreatic endocrine cells from bone marrow without evidence of cell fusion. J Clin Invest. 2003; 111: 843–50.

64. Jiang Y et al. Pluripotency of mesenchymal stem cells derived from adult marrow. Nature .2002; 418: 41–49.

65. Hess D et al. Bone marrow-derived stem cells initiatepancreatic regeneration. Nat Biotechnol. 2003; 21: 763–70.

66. Teitelman, G. Induction of beta-cell neogenesis by islet injury. Diab Metab. Rev 1996; 12: 91–102.

67. Zalzman M et al. Reversal of hyperglycemia in mice by using human expandable insulin-producing cells differentiated from fetal liver progenitor cells. Proc Natl Acad Sci USA .2003; 100: 7253–58.

68. Meivar LI, Ferber S. New organs from our own tissues: liver-to-pancreas transdifferentiation. Trends Endocrinol Metab. 2003; 14: 460–66.

69. Kojima H et al. NeuroD-betacellulin gene therapy induces islet neogenesis in the liver and reverses diabetes in mice. Nat Med. 2003; 9: 596–603.

70. Dor Y et al. Adult pancreatic beta-cells are formed by self-duplication rather than stem-cell differentiation. Nature. 2004; 429: 41–46.

71. Herold KC et al. Anti-CD3 monoclonal antibody in new-onset type 1 diabetes mellitus. N Engl J Med. 2002; 346: 1692–98.

72. Bishop JH, Green R, and Thomas S. Free-flow reabsorption of glucose, sodium, osmoles and water in rat proximal convoluted tubule. J Physiol (Lond) .1979; 288:331– 351.

73. Silverman M and Turner RJ Glucose transport in the renal proximal tubule, in Handbook of Physiology. Section 8. Renal Physiology.1992; 2017–2038.

74. Kanai Y et al. The human kidney low affinity Na_/glucose cotransporter SGLT2: delineation of the major renal reabsorptive mechanism for D-glucose. J Clin Investig.1992; 93:397–404.

75. Van DH et al. Autosomal recessive renal glucosuria attributable to a mutation in the sodium glucose cotransporter (SGLT2). Hum Genet.2003; 111:544–547.

76. Pajor A and Wright EM. Cloning and functional expression of a mammalian Na_/nucleoside cotransporter. A member of the SGLT family. J Biol Chem.1992; 267: 3557–3560.

77. Turk E et al. Glucose/galactose malabsorption caused by a defect in the Na_/glucose cotransporter. Nature (Lond).1991; 350:354–356.

78. Wells RG et al. Cloning of a human kidney cDNA with similarity to the sodium-glucose cotransporter. Am J Physiol.1992; 263:F459–F465.

79. Ehrenkranz JR et al. A review.Diabetes Metab Res Rev;2005 21(1):31-38.

80. Thorens B. Glucose transporter in the regulation of intestinal,renal and liver glucose fluxes. Am J Physiol-gastrointest. Liver Physiol. 1996; 270(33): G541-G543.

81. Rossetti L et al.Correction Of Hyperglycemia With Phlorizin Normalizes Tissue Sensitivity To Insuline In Diabeetic Rat.1987 ;79 :1510-15.

82. Katsuno et al. Sergliflozin A Novel Selective Inhibitor Of Low Affinity Sodium Glucose Cotransporter (Sglt2) Validates The Critical Role Of Sglt2 In Renal Glucose Reabsorption And Modulates Plasma Glucose.Level.J Pharmacol Exp Ther. 2007;320:323-330.

83. Wright Em,Turk E.The Sodium Glucose Cotransporter Family slc5 Pflugersarch;2004 447:510-18.

84. Nishimura S.Tissue Specific M-Rna Expression Profile Of Human Atp-Binding Cassette And Solute Carrier Transporter Superfamillies.Drug Meteb Pharmacokinetic ;2005 20:452-77.

85. Saltiel AR, Kahn CR. Insulin signalling and the regulation of glucose and lipid metabolism. Nature.2001; 414: 799-806.

86. Weir GC et al. β- Cell adaptation and decompensation during the progression of diabetes. Diabetes. 2001; 50: S154-159.

87. Dandona P et al. Oxidative damage to DNA in diabetes mellitus. Lancet 1996; 347: 444-445.

88. Brownlee M. Biochemistry and molecular cell biology of diabetic complications. Nature. 2001; 414: 813-820.

89. Kaneto H et al. Reducing sugars trigger oxidative modification and apoptosis in pancreatic b-cells by provoking oxidative stress through the glycation reaction. Biochem J. 1996; 320: 855-863.

90. Gorogawa S et al. Probucol preserves pancreatic b-cell function through reduction of oxidative stress in type 2 diabetes. Diabetes Res Clin Prac. 2002; 57: 1-10.

91. Sakai K et al. Mitochondrial reactive oxygen species reduce insulin secretion by pancreatic b-cells. Biochem Biophys Res Commun. 2003; 300: 216-222.

92. Miller CP, McGehee RE, Habener JF. IDX-1: a new homeodomain transcription factor expressed in rat pancreatic islets and duodenum that transactivates the somatostatin gene. EMBO J. 1994; 13: 1145-1156.

93. Dutta S.et al. Regulatory factor linked to late-onset diabetes? Nature. 1998; 392: 560.

94. Ahlgren U et al. β-Cellspecific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the β-cell phenotype and maturity onset diabetes. Genes Dev. 1998; 12: 1763-1768.

95. Stoffers DA et al. Early-onset type- II diabetes mellitus (MODY4) linked to IPF1. Nat Genet .1997; 17: 138-139.

96. Nakatani Y et al. Modulation of the JNK pathway in liver affects insulin resistance status. J Biol Chem .2004; 279: 45803-45809.

97. Kaneto H et al. Oxidative Stress and the JNK Pathway in Diabetes Current Diabetes Reviews, 2005; 1: 65-72.

Received on 13.05.2009

Accepted on 10.06.2009

Research J. Pharmacology and Pharmacodynamics 2(2): March –April 2010: 141-147