Type 1 diabetes results from autoimmune-mediated destruction of insulin-secreting β cells in the islets of Langerhans of the pancreas, whereas Type 2 diabetes is a disease of older individuals that is due to systemic insulin resistance and reduced insulin secretion by pancreatic β cells. Surgical resection of the pancreas may also cause insulin dependent diabetes depending on the size of the remaining pancreas. (Pancreatic stem/progenitor cells for the treatment of diabetes, 2007)
Despite intensive insulin therapy, however, most individuals with type 1 diabetes are unable to maintain a blood glucose level in the normal range at all times. Moreover, intensive glycemic control with insulin therapy is associated with an increased incidence of hypoglycemia. The successes achieved over the last few decades by the transplantation of whole pancreas and isolated islets suggest that diabetes can be cured by the replenishment of deficient β cells. It seems logical that replacement of the islet tissue itself offers a better approach than simply replacing insulin that has been lost. Islet allotransplantation can achieve insulin independence in patients with type 1 diabetes. Nonetheless, the promising results afforded by transplantation of whole pancreas and isolated islets, coupled with the shortage of cadaver pancreases relative to the potential demand, have lent strong impetus to the search for new sources of insulin-producing cells. Here we review possibilities for the development of stem cell therapy for diabetes. (The Diabetes Control and Complications Trial Research Group (1993) The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus, 2003) (Diabetes Mellitus: A Fundamental and Clinical Text, 3rd Edition)
Embryonic Stem Cells
Embryonic stem cells (ES cells) are derived from the inner mass of the mammalian blastocyst. ES cells can differentiate into all cell lineages, promising as a source of new β cells; however, this potential has proven more difficult than expected. Beginning in 2000, it has been reported by many groups that ES cells can differentiate into insulin-producing cells in vitro. (Stem cell therapy for type 1 diabetes mellitus) (Stem Cell Therapy to Treat Diabetes Mellitus, Rev Diabet Stud, 2008, 5(4):203-219)
Several approaches have been used to obtain enriched populations:
1. selection by manipulating culture conditions;
2. overexpression of key transcription factors such as paired box gene 4 (Pax4) and the pancreatic and duodenal homeobox factor-1 (PDX-1);
3. cell trapping with antibiotic resistance driven by the Nkx6.1 or insulin promoter o select cells.
(Stem Cell Therapy to Treat Diabetes Mellitus, Rev Diabet Stud, 2008, 5(4):203-219)
In vitro differentiation of putative β-cells from mouse embryonic stem cells (MESC). A: Genetic manipulation. i: Soria et al. (2001) used a 'cell-trapping' protocol to select for insulin-producing cells expressing the β-geo gene under the control of the human Insulin gene promoter [84]. This strategy was later refined by placing the β-geo gene under the control of the promoter of Nkx6.1 (ii). iii: Enforced expression of Pax4 or Pdx1 increased the frequency with which insulin-producing cells were isolated from differentiating MESC. B: Cell culture manipulation of ES cells. Lumelsky et al. (2001) developed a five-step protocol based on methods known to promote the generation of neural cell types from MESC [94]. Nestin-expressing cells were cultured in B27/N2 neurobasal medium went on to form cell clusters. Although the nature of insulin-staining cells derived by this method remains controversial, other groups have successfully used variations on this procedure to generate similar cell types. EB: embryoid bodies.
(Prospects of Stem Cell Therapy in Diabetes - Introduction to the RDS Special Issue, Rev Diabet Stud, 2010, 7(2):80-81
Strategies to obtain β cells from organ-specific stem or progenitor cells.
Schematic representation of the stepwise differentiation of embryonic stem (ES) and induced pluripotent stem (iPS) cells to insulin-producing cells. CYC: KAAD-cyclopamine; RA: All-trans retinoic acid; DAPT: γ-secretase inhibitor; Ex4: Exendin-4; FGF: Fibroblast growth factor.
(Representation of the stepwise differentiation of embryonic stem (ES), World J Stem Cells 2009 December 31; 1(1): 36-42)
Bone Marrow Cells/Spleen Cells
An appealing report suggested that cells derived from bone marrow have the capacity to differentiate into functionally competent pancreatic endocrine β cells in vivo without evidence of cell fusion. On the other hand, several other laboratories showed no evidence in this model that bone marrow-derived cells differentiated into insulin expressing cells.
The infusion of bone marrow cells facilitates islet regeneration and/or replication, although the mechanism is unknown. β cell neogenesis from only bone marrow-derived cells might be rare, but the existence of multipotent adult stem cells that can differentiate into cells has not been disproved.
Adult Pancreatic Stem/Progenitor Cells
Islet neogenesis, the budding of new islets from pancreatic stem/progenitor cells located in or near ducts, has long been assumed to be an active process in the postnatal pancreas. Several in vitro studies have shown that insulin-producing cells can be generated from adult pancreatic ductal tissues.
Induction of Insulin-producing Cells from stem Cells
Some agents are shown to stimulate islet neogenesis :
1. GLP-1;
2. GLP-1 analog exendin-4;
3. INGAP;
4. the combination of betacellulin and activin A;
5. conophylline;
6. the combination of EGF and gastrin
Possible approach for generation of insulin-producing cells from progenitor cells. Stem cells are instructed to develop into cells with a β-like phenotype by expression of transcription factors known to promote pancreas and islet development and incubation with additional soluble inducers. Genes that allow selection (such as neomycin resistance or green fluorescence protein) are also introduced into the cells under control of the insulin promoter, facilitating purification of cells expressing the insulin gene. The selected cells can be further engineered with immunoprotective genes and encapsulated to increase their survival following transplantation into diabetic patients. (Reproduced from Efrat S. Cell replacement therapy for type 1 diabetes. Trends Mol Med 2002;8:334, with permission.
GLP-1/exendin-4 have incretin effects, enhancing insulin secretion; they also stimulate β cell replication and neogenesis and have anti-apoptotic effects. INGAP, a peptide fragment of pancreatic REG protein, has been associated with regeneration in rodents. Betacellulin, a member of the EGF family, stimulates β cell proliferation. Conophylline is useful in inducing the differentiation of pancreatic β cells both in vivo and in vitro. Both activin A, a member of the TGFβ family, and gastrin are thought to promote β cell differentiation. In particular, exendin-4 has recently received FDA approval for clinical use since successful trials of exendin-4 treatment in type 2 diabetes have been reported, and patients had a significant improvement in glycemic control associated with weight loss. Trials are also being initiated with various combinations of gastrin, EGF and GLP-1 agonists, and no doubt other approaches to increasing β cell mass will soon be developed. It has also been shown that overexpression of embryonic transcription factors, such as PDX-1, Ngn3, BETA2/NeuroD, Pax4, in stem cells could efficiently induce their differentiation into insulin-expressing cells. (Therapeutic Approaches to Preserve Islet Mass in Type 2 Diabetes, Annual Review of Medicine Vol. 57: 265-281 (Volume publication date February 2006) DOI: 10.1146/annurev.med.57.110104.115624)
Conclusions
The most difficult and yet unsolved issue is how to differentiate stem/progenitor cells and acquire fully functional islets. The possibility that pharmacological agents might increase β cell mass is tantalizing because a decrease in β cell mass is the root cause of both types of diabetes. Because of the intensity with which new agents are being tested in clinical trials, answers should emerge in the very near future. Protein transduction technology could also offer a novel therapeutic approach for diabetes. Moreover, the internalization of PDX-1 and BETA2/NeuroD proteins is extremely interesting evidence because endogenous transcription factors, such as TAT and neural homeoproteins, have the activity of intercellular transfer. PDX-1 and BETA2/NeuroD proteins may, in some circumstances, have paracrine activities as part of the physiological process, in that they are released by one cell and then internalized by other cells. Further investigations to understand the regenerative process of the adult pancreas and the appropriate induction of stem/progenitor cell differentiation will help to establish cell-based therapy in diabetes.
The ethical issue is another major obstacle to the clinical use of ES cells.
