The observations that GLP-1R agonists including exendin-4 may play a role in islet neogenesis or differentiation based on observations from multiple preclinical studies have engendered considerable interest and excitement given the potential of these actions to enhance b-cell function in subjects with Type 2 diabetes. Furthermore, the possibility that GLP-1 receptor activation may enhance b-cell mass in patients with Type 1 diabetes is also under active investigation.
GLP-1 receptor signaling, cell survival and apoptosis
Human Islets
Complementary findings from several labs, using a variety of experimental rodent and cell models, indicate that GLP-1R agonists may modulate b-cell mass in part via reduction of b-cell apoptosis. These actions may be directly relevant to protection of human islet β cells as incubation of human islets with native GLP-1 for 5 days preserved islet morphology, reduced the expression of pro-apoptotic genes, decreased cell death, and improved glucose-stimulated insulin secretion. See Glucagon-like Peptide 1 inhibits cell apoptosis and improves glucose responsiveness of freshly isolated human islets. Endocrinology. 2003 Dec;144(12):5149-58
Tschen and colleagues identified a role for Skp2 (S phase kinase-associated protein 2, a component of the ubiquitin protein logase complex) as an essential downstream target required for the proliferative effect of GLP-1 in b-cells. Incubation of human and murine islets with GLP-1 increased protein levels of Skp2 and decreased levels of p27, in a LY294002-sensitive manner, without affecting cell proliferation, possibly due to persistent high expression of celll cycle inhibitors such as p16Ink4a. The ability of exendin-4 to increase Skp2 and reduce p27 was reduced following siRNA-mediated reduction of Irs2 levels in Min6 cells. Treatment of young 7-8 wk old Skp2-/- mice with exendin-4 for 7 days produced a 2.5-fold increase in b-cell mass and a 7.2-fold increase in Ki67+ b-cells in control +/- mice, but not in Skp2-/- mice, with reduced b-cell p27 expression observed by histochemical analysis. Treatmnet of 4 wk old db/db mice with exendin-4 for 7 days resulted in increased b-cell proliferation, associated with reduced levels of the cell cycle inhibitor p27 and increased Skp2 in b-cells; these proliferative actions were lost in 8 month old mice, possibly due to the increased expression of theCdk4 inhibitor p16Ink4a in b-cells. However, exendin-4 reduced levels of p27 in exocrine and ductal pancreatic tissue of older (7 months) mice, in association with low levels of p16Ink4a and a 3-fold increase in ductal proliferation. Skp2 Is Required for Incretin Hormone-Mediated beta-Cell Proliferation Mol Endocrinol. 2011 Oct 6.
What is the role of GLP-1 receptor signaling in the modulation of b cell differentiation in human β cells? A series of elegant experiments now demonstrates that a combination of GLP-1 receptor agonists (such as exendin-4), pdx-1 expression, and cell-cell contact, promotes the development of a more differentiated b cell in vitro. See b-Cell Differentiation from a Human Pancreatic Cell Line in Vitro and in Vivo. Mol Endocrinol. 2001 Mar 1;15(3):476-483. Similarly, incubation of the human ductal cell line Capan-1, which expresses the GLP-1R, with exendin-4 results in a significant increase in the number of cells exhibiting immunopositivity for insulin or glucagon, and induction of islet genes such as PDX-1, BETA2/NeuroD, and HNF-3b. Increased binding of HNF-3b to Pdx-1 promoter elements may represent one component of the differentiation mechanism activated by the GLP-1R in these cells. See Exendin-4 differentiation of a human pancreatic duct cell line into endocrine cells: Involvement of PDX-1 and HNF3beta transcription factors. J Cell Physiol. 2002 Sep;192(3):304-14
The observation that drugs used for immunosuppression in the context of islet transplantation may have deleterious effects on islet function and survival have prompted assessment whether GLP-1R activation may prove useful for survival of tacrolimus-treated human islets. Ex-4 restored Irs-2 expression and nearly normalized both human β-cell survival and rodent β-cell replication when co-administered with tacrolimus in rodent and human islets Calcineurin signaling regulates human islet {beta}-cell survival J Biol Chem. 2010 Dec 17;285(51):40050-9.
The GLP-1R has also been localized to nestin positive islet-derived progenitor cells (NIPs) identified in islets and duct cells. About 60% of NIPs exhibit GLP-1R immunopositivity, and the GLP-1R is capable of transducing a functional [Ca2+]i response that is blocked by the GLP-1R antagonist exendin(9-39); this response was detected at normal but not elevated glucose concentrations. Incubation of human NIP cells with GLP-1 induced insulin expression in subsets of cells, and also produced changes in cellular morphology. GLP-1R agonists also induced insulin secretion in ~ 30% of NIP clones. Intriguingly, NIP cultures that approached confluence expressed the proglucagon gene and secreted GLP-1 into the culture medium, raising the possibility of an autocrine GLP-1-differentiation loop in these cells. See Multipotential nestin-positive stem cells isolated from adult pancreatic islets differentiate ex vivo into pancreatic endocrine, exocrine, and hepatic phenotypes Diabetes. 2001 Mar;50(3):521-33
GLP-1R activation also produces anti-inflammatory effects in human islets, notably when cAMP levels are stabilizied by concomitant treated with a PDE inhibitor. Exendin-4 suppressed induction of chemokine and cytokine expression in human and rodent islets in association with reduced STAT-1 expression Anti-inflammatory action of exendin-4 in human islets is enhanced by phosphodiesterase inhibitors: potential therapeutic benefits in diabetic patients Diabetologia. 2010 Nov;53(11):2357-68. Similarly, the GLP-1R agonist liraglutide also reduced apoptotic rates and improved the survival of transplanted islets Liraglutide, a long-acting human glucagon-like peptide 1 analogue, improves human islet survival in culture Transpl Int. 2010 Mar 1;23(3):259-65.
GLP-1 and GIP, alone and in combination also increased the expression of insulin, Bcl-2 and Pdx-1 genes in human islets The direct effects of GLP-1 and GIP, alone or in combination, on human pancreatic islets Regul Pept. 2010 Dec 10;165(2-3):129-32
The effects of exenatide have been examined in small non-randomized studies of human subjects with type 1 diabetes who were recipients of islet transplants. Exenatide treatment improvement glycemia and insulin responses but di not consistently effectively suppress glucagon levels in these patients over a sustained treatment period. Long-term metabolic and hormonal effects of exenatide on islet transplant recipients with allograft dysfunction Cell Transplant. 2009;18(10):1247-59
Similarly, the effects of acute GLP-1 administration have also been assessed in human subjects with islet vs. pancreas transplants. GLP-1 infusion increased insulin secretion to a greater extent in pancreas vs islet transplants, however glucagon suppression was defective in both groups Effect of glucagon-like peptide-1 on beta- and alpha-cell function in isolated islet and whole pancreas transplant recipients J Clin Endocrinol Metab. 2009 Jan;94(1):181-9.
GLP-1: Preclinical models
The anti-apoptotic properties of GLP-1R agonists have been demonstrated in Zucker diabetic rats and db/db mice. A 2 day infusion of GLP-1 increased islet size and b-cell mass, and reduced the numbers of apoptotic cells in the exocrine portion of the pancreas. The percentage of apoptotic b-cells in this study was surprisingly high at greater than 20%, and was significantly reduced by GLP-1 treatment. See Glucagon-Like Peptide-1 Promotes Islet Cell Growth and Inhibits Apoptosis in Zucker Diabetic Rats. Endocrinology. 2002 Nov 1;143(11): 4397-4408
Treatment of normoglycemic db/db mice with daily exendin-4 for 14 days prevented the progression to more frank diabetes, increased b-cell mass, and numbers of BrdU+ islet cells, and reduced numbers of Tunel+ apoptotic b-cells. Exendin-4-treated rats also exhibited increased levels of pancreatic Akt1, the prosurvival kinase, increased p44 MAP kinase, and reduced expression of activated caspase-3. See Glucagon-like peptide-1 treatment delays the onset of diabetes in 8 week-old db/db mice. Diabetologia. 2002 Sep; 45(9): 1263-73
Similarly, treatment of mice with exendin-4 reduced b-cell apoptosis induced by streptozotocin (STZ), whereas Glp1r-/- mice exhibited increased susceptibility to STZ-induced b-cell apoptosis. Furthermore, exendin-4 directly reduced the extent of apoptotic cell death in purified rat b-cells exposed to a combination of cytotoxic cytokines, and GLP-1 or exendin-4 increased cell survival and reduced caspase activation in BHK fibroblasts expressing a transfected GLP-1 receptor. Hence these findings implicate a direct role for b-cell GLP-1 receptor signaling in regulation of the resistance to cellular apoptosis. See Glucagon-like Peptide-1 Receptor Signaling Modulates beta Cell Apoptosis. J Biol Chem. 2003 Jan 3;278(1):471-8
The anti-apoptotic and proliferative actions of exendin-4 have been examined in a model of type 1 diabetes, the NOD mouse. Initiation of exendin-4 alone after the development of diabetes had little therapeutic benefit. In contrast, exendin-4 together with lysophiline for 28 days, markedly improved glucose control in NOD mice, even 6-14 weeks after cessation of therapy. Furthermore, the combination therapy preserved the number of intact islets, and appeared to reduce the extent of inflammatory cell infiltration in the remaining islets. See Combined treatment with lisofylline and exendin-4 reverses autoimmune diabetes. Biochem Biophys Res Commun. 2006 Jun 9;344(3):1017-22.
Similarly, continuous administration of native GLP-1 to 8 week old female NOD mice for 4 or 8 weeks lowered blood glucose, increased formation of new b-cells and suppressed b-cell apoptosis and delayed the onset of diabetes as described in Continuous stimulation of human glucagon-like peptide-1 (7-36) amide in a mouse model (NOD) delays onset of autoimmune type 1 diabetes. Diabetologia. 2007 Sep;50(9):1900-9
In contrast, twice daily administration of exendin-4 at two different doses, 100 ng, and 2 ug twice daily, for up to 6 months, initiated prior to the development of clinical diabetes in NOD mice, produced only modest effects on reducing the numbers of disease-free mice, with detectable but small increased in beta cell mass and reductions in insulitis. Intriguingly, GLP-1 receptor expression was also detected in different immune compartments as outlined in Exendin-4 modulates diabetes onset in non obese diabetic mice Endocrinology 2008 Mar;149(3):1338-49.
Treatment of Min6 mouse islet cells with GLP-1 reduced the extent of hydrogen peroxide-induced apoptosis in vitro. The anti-apoptotic effects of GLP-1 were partially blocked by either the cAMP 'antagonist' Rp-cAMP, or the PI 3-kinase inhibitor LY294002. GLP-1 prevented cell death only when applied prior to exposure of cells to peroxide. See Glucagon-Like Peptide-1 Inhibits Apoptosis of Insulin-Secreting Cells via a Cyclic 5'-Adenosine Monophosphate-Dependent Protein Kinase A- and a Phosphatidylinositol 3-Kinase-Dependent Pathway. Endocrinology. 2003 Apr;144(4):1444-55. The ability of exendin-4 to improve islet survival in the presence of TNF-a in vitro correlates with inhibition of Jnk phosphorylation, likely through a PKA-mediated pathway Exendin-4 prevents c-Jun N-terminal protein kinase activation by tumor necrosis factor-alpha (TNFalpha) and inhibits TNFalpha-induced apoptosis in insulin-secreting cells Endocrinology. 2010 May;151(5):2019-29
Similarly, induction of fatty acid induced apoptosis in Rinm5F cells with palmitate was markedly reduced by agents that increased levels of cyclic AMP, including GLP-1 and exendin-4, actions partially reversed by the PKA antagonist H-89 Cyclic AMP dose-dependently prevents palmitate-induced apoptosis by both PKA- and cAMP- GEF- dependent pathways in beta -cells. J Biol Chem. 2004 Mar 5;279(10):8938-45. Related studies using rat INS-1 cells demonstrated that protein kinase B (Akt) is rapidly activated by exendin-4 and GLP-1 and the PI3K inhibitor wortmannin or a dominant negative Akt virus blocked both the proliferative and cytoprotective actions pursuant to GLP-1 receptor activation Glucagon-like peptide-1 regulates proliferation and apoptosis via activation of protein kinase B in pancreatic INS-1 beta cells. Diabetologia. 2004 Mar;47(3):478-87 Treatment of mouse b-cells or INS-1 cells with dexamethasone induces apoptosis, which was reduced in the presence of exendin-4 in a PKA-dependent manner as shown in Dexamethasone induces cell death in insulin-secreting cells, an effect reversed by exendin-4 Diabetes. 2006 May;55(5):1380-90.
The anti-apoptotic actions of GLP-1R agonists have been demonstrated in cultured fetal rat hippocampal neurons which exhibit GLP-1-dependent increases in cAMP formation. Both GLP-1 and exendin-4 significantly reduced the extent of glutamate-induced cell death in short term cultures of hippocampal neurons. Furthermore, both GLP-1 and exendin-4 reduced depletion of choline acetyltransferase immunoreactivity, a marker for cholinergic neurons in the basal forebrain, following administration of ibotenic acid. See Protection and reversal of excitotoxic neuronal damage by glucagon-like peptide-1 and exendin-4. J Pharmacol Exp Ther. 2002 Sep;302(3):881-8 and Glucagon-like peptide 1 modulates calcium responses to glutamate and membrane depolarization in hippocampal neurons. J Neurochem. 2003 Dec;87(5):1137-44
The calpain-10 molecule has also been linked to β cell apoptosis, and may be modified by GLP-1 receptor activation. GLP-1 decreased calpain activity, reversed ryanodine-induced calpain activation and apoptosis in β cells and MIN6 cells, as shown in RyR2 and calpain-10 delineate a novel apoptosis pathway in pancreatic islets. J Biol Chem. 2004 Jun 4;279(23):24794-802
GLP-1 and endoplasmic reticulum stress
Type 2 diabetes is associated with gradual loss of insulin secretion and a progressive reduction in b-cell mass. Insulin resistance produces a sustained increase in demand for insulin, and over time, the b-cell is unable to sustain augmented levels of insulin biosynthesis and secretion. GLP-1 and GIP appear to maintain insulin biosynthesis via interaction with ER stress pathways in the b-cell. The GLP-1R agonist exendin-4 significantly reduced biochemical markers of islet ER stress in islets from db/db mice in vivo and both exendin-4 and GIP attenuated translational downregulation of insulin and improved cell survival in purified rat b-cells and in INS-1 cells following induction of ER stress in vitro. The actions of GLP-1 to enhance translation are mediated via induction of ATF-4, and accelerated recovery from ER stress-mediated translational repression in islet b-cells in a PKA-dependent manner. Exendin-4 also reduced ER stress-associated b-cell death in a PKA-dependent manner. See GLP-1 receptor activation improves beta cell function and survival following induction of endoplasmic reticulum stress. Cell Metab. 2006 Nov;4(5):391-406 and the accompanying Editorial EXtENDINg beta cell survival by UPRegulating ATF4 translation. Cell Metab. 2006 Nov;4(5):333-4.
Islet Cell Proliferation
GLP-1 receptor activation couples to multiple signal transduction pathways important for control of b-cell proliferation. Wnt signaling appears to be an important downstream target for GLP-1R-dependent pathways as GLP-1R agonists increase wnt signaling in b-cells and inhibition of wnt signaling reduces both basal and GLP-1R-dependent regulation of b-cell proliferation, as described in Glucagon-like peptide-1 activation of TCF7L2-dependent Wnt signaling enhances pancreatic beta-cell proliferation J Biol Chem. 2008 Mar 28;283(13):8723-35
Additional evidence that GLP-1 stimulates islet cell proliferation in vitro is illustrated in Glucose and glucoincretin peptides synergize to induce c-fos, c-jun, junB, zif-268, and nur-77 gene expression in pancreatic beta(INS-1) cells. FASEB J. 1998 Sep;12(12):1173-82 and in Glucagon-like peptide-1 promotes DNA synthesis, activates phosphatidylinositol 3-kinase and increases transcription factor pancreatic and duodenal homeobox gene 1 (PDX-1) DNA binding activity in beta (INS-1)-cells. Diabetologia. 1999 Jul;42(7):856-64
The signal transduction system activated by the GLP-1R signaling system which leads to cell growth depends on the islet cell model under study, with cAMP, PKC and PI-3-kinase activated by the GLP-1R in several studies. Protein Kinase Czeta Activation Mediates Glucagon-Like Peptide-1-Induced Pancreatic beta-Cell Proliferation. Diabetes. 2001 Oct;50(10):2237-2243. The transcription factor FoxO1 is an essential mediator required for the proliferative and anti-apoptotic actions of GLP-1on the b-cell. GLP-1 stimulated the nuclear to cytoplasmic translocation of Foxo1, in an EGFR-and -PI-3K-dependent manner. A constitutively active mutant of Foxo1 blocked the actions of GLP-1 on b-cell proliferation. FoxO1 may exert its effects via repression of Pdx-1 and Foxa2 expression as described in Transcription Factor FoxO1 Mediates Glucagon-Like Peptide-1 Effects on Pancreatic b-cell mass Diabetes. 2006 May;55(5):1190-6. Adenylate cyclase 8 seems particularly important for maintaining the responsivity of islets to glucose and GLP-1 Adenylyl cyclase 8 is central to glucagon-like peptide 1 signalling and effects of chronically elevated glucose in rat and human pancreatic beta cells Diabetologia. 2011 Feb;54(2):390-402.
The importance of Pdx-1 as a downstream target for multiple aspects of GLP-1 action, including insulin secretion and control of b-cell apoptosis is consistent with findings made using mice with b-cell-specific inactivation of the Pdx-1 gene as shown in beta-Cell Pdx1 expression is essential for the glucoregulatory, proliferative, and cytoprotective actions of glucagon-like peptide-1. Diabetes. 2005 Feb;54(2):482-91.
Provocative experiments using the INS-1 cell line demonstrate the potential involvement of the EGFR and EGFR ligands such as betacellulin in the direct and indirect GLP-1R-dependent activation of cell proliferation as shown in Glucagon-Like Peptide 1 Induces Pancreatic beta-Cell Proliferation Via Transactivation of the Epidermal Growth Factor Receptor. Diabetes. 2003 Jan; 52(1): 124-32
The cyclic AMP-dependent transcription factor CREB has been linked to GLP-1-mediated cell growth and survival, as outlined in cAMP promotes pancreatic beta-cell survival via CREB- mediated induction of IRS2. Genes Dev. 2003 Jul 1;17(13):1 575-80
GLP-1 and islet cell proliferation in vivo
Several lines of evidence support a role for GLP-1 in the control of islet proliferation and regeneration in rodent studies in vivo. Administration of subcutaneous GLP-1 for 2 days to lean mice increased the islet labeling index, as shown in Initiation of increased pancreatic islet growth in young normoglycemic mice (Umea +/?). Endocrinology 1999 Feb;140(2):778-83. Furthermore, exendin-4 attenuates glucose intolerance following partial pancreatectomy in the rat. Exendin-4 stimulates both beta-cell replication and neogenesis, resulting in increased beta-cell mass and improved glucose tolerance in diabetic rats. Diabetes 1999 Dec;48(12):2270-6 Similarly, infusion of GLP-1 increased β cell mass and promoted enhanced pdx-1 expression and islet neogenesis Glucagon-like peptide-1 induces cell proliferation and pancreatic-duodenum homeobox-1 expression and increases endocrine cell mass in the pancreas of old, glucose-intolerant rats. Endocrinology 2000 Dec;141(12):4600-5
If administered to young 6 week old prediabetic db/db mice, daily exendin-4 treatment for 2 weeks prevented the progression to more severe diabetes, in association with expansion of islet mass, stimulation of b-cell proliferation and improved glucose tolerance and insulin secretion. See Glucagon-like peptide-1 treatment delays the onset of diabetes in 8 week-old db/db mice. Diabetologia. 2002 Sep;45(9):1263-73. Similarly, NN2211 and exendin-4 increased β cell proliferation in db/db mice, but not in ob/ob mice, as shown in The long-acting GLP-1 derivative NN2211 ameliorates glycemia and increases beta-cell mass in diabetic mice. Am J Physiol Endocrinol Metab. 2002 Oct;283(4):E745-52
Conversely, age appears to be an important determinant regulating b-cell proliferation, including the response to GLP-1R agonists. Using the partial pancreatectomy or low dose STZ injury model, Rankin and Kushner demonstrated b-cell regeneration was robustly detected in young (2 months) or 8 month old mice. Conversely, no b-cell proliferation was observed in 19-month old mice, although acinar cell regeneration was detected in the same experiments. Similarly 14-month old mice failed to exhibit a proliferative response to exendin-4. Hence adaptive changes in the capacity for b-cell proliferation may restrict the adaptive b-cell response to older islets. However the response to exendin-4 was not studied in mice that also received a PPx or STZ. See ADAPTIVE BETA CELL PROLIFERATION IS SEVERELY RESTRICTED WITH ADVANCED AGE. Diabetes. 2009 Jun;58(6):1365-72
Administration of the GLP-1R agonist liraglutide (NN2211) to normal and ZDF rats as well as to 60% pancreatectomized rats demonstrated that the ability of liraglutide to expand β cell mass was correlated with the metabolic milieu, and the duration of treatment, with normoglycemic normal animals failing to show an increase in β cell mass, whereas hyperglycemic animals responded to liraglutide with an increase in β cell mass. Similarly, β cell mass was increased in non-diabetic Sprague-Dawley rats treated with for 1 week, but after 6 weeks of treatment, β-cell mass was comparable in treated vs control rats, as described in The endocrine pancreas in non-diabetic rats after short-term and long-term treatment with the long-acting GLP-1 derivative NN2211. APMIS. 2003 Dec;111(12):1117-1124. Hence, the trophic effects of GLP-1R agonists in the islet, like their insulinotropic properties, are probably coupled to the presence of hyperglycemia. See GLP-1 derivative liraglutide in rats with b-cell deficiencies: influence of metabolic state on b-cell mass dynamics. Br J Pharmacol. 2003 Sep;140(1):123-132.
Complementary studies of GLP-1 or exendin-4 administration for 5 days in the neonatal GK rat demonstrate persistent improvement in glucose homeostasis, enhanced pancreatic insulin content and total beta-cell mass due to stimulation of beta-cell neogenesis and regeneration. Follow-up from day 7 to adult age (2 months) demonstrated GLP-1 or Ex-4 treated rats maintained enhanced beta-cell mass and improved glycemic control at adult (2 months) age. See Persistent Improvement of Type 2 Diabetes in the Goto-Kakizaki Rat Model by Expansion of the beta-Cell Mass During the Prediabetic Period With Glucagon-Like Peptide-1 or Exendin-4. Diabetes. 2002 May;51(5):1443-1452
Is GLP-1 receptor activation essential for stimulation of islet neogenesis and proliferation? The finding of only modest and subtle abnormalities in islet development in Glp1r-/- mice suggest that islets develop normally in the absence of Glp1r signaling. Furthermore, the development of islet hyperplasia and hyperinsulinemia in the setting of insulin resistance, hyperglycemia and leptin deficiency is not compromised in the ob/ob:Glp1r-/- mutant mouse. Hence it appears that although activation of GLP-1R signaling stimulates islet neogenesis and proliferation, the GLP-1R signaling pathway is not required for islet adaptation in the mouse in vivo. See Elimination of glucagon-like peptide 1R signaling does not modify weight gain and islet adaptation in mice with combined disruption of leptin and GLP-1 action Diabetes 2000 Sep; 49(9): 1552-6
A complementary approach to address the importance of endogenous GLP-1 receptor expression for β cell regeneration involves analysis of the islet regenerative responsive to partial pancreatectomy. Wildtype mice subjected to partial pancreatectomy were infused with the antagonist exendin(9-39); partial pancreatectomy was also carried out in Glp1r-/- mice. In Ex(9-39)-treated sham-operated mice, persistent fasting hyperglycemia was observed, but β cell mass was not diminished. In pancreatectomized mice, persistent glucose intolerance was noted, but this was not further exacerbated by Ex(9-39) and recovery of β cell mass in Ppx mice was not impaired. In contrast, Glp1r-/- mice exhibited greater hyperglycemia after pancreatectomy compared with wild-type mice, and this correlated with a significant defect in regeneration of β cell mass. Hence, the importance of endogenous GLP-1R signaling following partial pancreatectomy depends on the experimental context. See Role of Endogenous Glucagon-Like Peptide-1 in Islet Regeneration After Partial Pancreatectomy. Diabetes. 2003 Feb;52(2): 365-371.
Similarly, b-cell regeneration that develops in neonatal rats following administration of streptozotocin appears to be partially GLP-1-dependent, as the extent of islet and b-cell regeneration was diminished in the presence of the GLP-1R antagonist exendin (9-39), as outlined in ONTOGENY OF REGENERATION OF {beta}-CELLS IN THE NEONATAL RAT FOLLOWING TREATMENT WITH STREPTOZOTOCIN. Endocrinology. 2006 May;147(5):2346-56.
These new findings add yet another important mechanism to the broad spectrum of GLP-1 actions which converge on lowering blood glucose in vivo. Furthermore, the possibility that activation of GLP-1R signaling might be therapeutically useful for regeneration of b-cell mass has important implications for the treatment of Type 1 and Type 2 Diabetes, although this has been very difficult to prove in clinical studies.
Islet Cell Differentiation
GLP-1 or exendin-4 promotes differentiation of the AR42J cell line to an endocrine phenotype. Differentiated cells exhibited increased expression of β cell genes and the capacity for glucose-dependent insulin secretion Glucagon-like peptide 1 and exendin-4 convert pancreatic AR42J cells into glucagon-and insulin-producing cells Diabetes 1999 Dec;48(12):2358-66 ; and b-Cell Differentiation from a Human Pancreatic Cell Line in Vitro and in Vivo. Mol Endocrinol. 2001 Mar 1;15(3):476-483.
Although the mechanisms underlying the GLP-1 dependent-stimulation of exocrine-endocrine differentiation are poorly understood, Yew and colleagues demonstrated that very low doses of GLP-1 or exendin-4 are sufficient to induce an endocrine differentiation program in AR42J cells. Furthermore, activation of GLP-1R signaling was associated with release of TGF-b and changes in expression of SMAD proteins. Moreover, modulation of SMAD expression significantly impacted the extent of endocrine differentiation. See Interplay of Glucagon-Like Peptide-1 and Transforming Growth Factor-{beta} Signaling in Insulin-Positive Differentiation of AR42J Cells. Diabetes. 2004 Nov;53(11):2824-2835.
Similarly, treatment of fetal (75-90 day gestational age) pig islet clusters with GLP-1 promoted glucose-dependent insulin secretion and after several days of treatment with GLP-1R agonists, the cells exhibited increased Pdx-1 expression and enhanced differentiation along the β cell pathway. Furthermore, transplantation of the GLP-1-treated islet clusters into SCID mice revealed a significantly increased number of functionally mature β cells after 8 weeks in vivo. See Functional maturation of fetal porcine beta-cells by glucagon-like peptide 1 and cholecystokinin. Endocrinology. 2002 Sep;1 43(9) :3505-14.
Exendin-4 has also been shown to promote transdifferentiation of liver cells in the context of ectopic pdx-1 expression. Although expression of the known GLP-1R was not identified in liver cells, either prior to or following the transdifferentiation process, Ex-4 increased levels of cAMP in human liver cells, and increased levels of phosphoCREB, Akt, PKC, and ERK1/2. Following Pdx-1 transduction, Ex-4 increased insulin promoter activity and insulin gene expression in transduced liver cells. Similarly, Ex-4 induced expression of a broad number of transcription factors and proteins associated with specialized beta cell function. Ex-4 was also found to increase liver cell proliferation and likely increases the proportion of cells susceptible to transduction by Pdx-1. Remarkably, mRNA transcripts for the known GLP-1R were not detected at any point in human liver cells Exendin-4 promotes liver cell proliferation and enhances PDX-1-induced liver to pancreas transdifferentiation J Biol Chem. 2009 Sep 15. 33509-20
GLP-1(1-37) and Intestinal Endocrine Differentiation
The vast majority of GLP-1 produced in gut endocrine cells is either GLP-1(7-36)amide or GLP-1(7-37). Very little is known about the generation and biological activity of GLP-1(1-37). . Suzuki et al demonstrate that GLP-1(1-37) promotes β cell-like differentiation, in association with development of glucose-dependent insulin secretion, in cultured embryonic gastrointestinal tissues. These effects were observed using organ culture in vitro, blocked by the GLP-1R antagonist exendin(9-39) in vivo and the functional capacity of the cells to secrete insulin was demonstrated using transplant experiments in STZ-induced diabetic mice. GLP-1(1–37) injecte dinto 10-wk-old adult mice everyday for 9 days also produced rare scatterd insulin+ cells in the ileum in vivo See Glucagon-like peptide 1 (1-37) converts intestinal epithelial cells into insulin-producing cells. Proc Natl Acad Sci U S A. 2003 Apr 29;100(9):5034-9.
GLP-1 and human b-cell differentiation in vivo?
Will gut hormones such as GLP-1 promote proliferation of human islet b-cells? There is insufficient information to answer this question. Intriguingly, six patients with hyperinsulinemic hypoglycemia detected 0.5-8 years following Roux-en-Y gastric bypass surgery were initially reported to exhibit histological evidence for nesidioblastosis following resection of pancreatic tissue-one patient was found to have multiple insulinomas as described in the July 21 2005 New England Journal of Medicine Hyperinsulinemic hypoglycemia with nesidioblastosis after gastric-bypass surgery. N Engl J Med. 2005 Jul 21;353(3):249-54. The authors speculated, as further discussed in an accompanying editorial, that excessive secretion of gut hormones such as GLP-1 may have contributed to the development of islet proliferation in these human subjects. A similar clinical picture was reported in 3 subjects following gastric bypass, and plasma levels of GLP-1 were markedly elevated in these studies Severe hypoglycaemia post-gastric bypass requiring partial pancreatectomy: evidence for inappropriate insulin secretion and pancreatic islet hyperplasia. Diabetologia. 2005 Nov;48(11):2236-40 . However, a subsequent re-analysis of the pancreas histology, together with control slides obtained from 31 obese subjects and 16 lean control subjects, yielded somewhat modified conclusions. Meier and colleagues reported that b-cell area was not increased in the subjects with gastric bypass-associated hypoglycemia and no evidence of increased islet neogenesis or b-cell proliferation was detected in this group. These findings further emphasis the importance of functional defects, namely changes in gut motility and the acute b-cell response to nutrients/gut hormones in the pathogenesis of the hyperinsulinemic hypoglycemia syndrome seen in some subjects. See Direct evidence of attempted beta cell regeneration in an 89-year-old patient with recent-onset type 1 diabetes. Diabetologia. 2006 Aug;49(8):1838-44
Indeed, different types of intestinal surgery have been associated with increased secretion of gut hormones in experimental preclinical models, as described in Weight loss through ileal transposition is accompanied by increased ileal hormone secretion and synthesis in rats. Am J Physiol Endocrinol Metab. 2005 Feb;288(2):E447-53. Although there is limited human data in patients undergoing this type of surgery, the available evidence does not support a uniform robust increase in plasma levels of gut hormones such as GLP-1 in most patients as reviewed in Hormonal changes after Roux-en Y gastric bypass for morbid obesity and the control of type-II diabetes mellitus. Am Surg. 2004 Jan;70(1):1-4; and The early effect of the Roux-en-Y gastric bypass on hormones involved in body weight regulation and glucose metabolism. Ann Surg. 2004 Aug;240(2):236-42. In contrast, patients studied 20 years after jejunoilealbypass did exhibit increased circulating levels of GLP-1 and GIP as shown in Importance of small bowel peptides for the improved glucose metabolism 20 years after jejunoileal bypass for obesity. Obes Surg. 1998 Jun;8(3):253-60. Hence, there is a possibility that in some subjects, sustained increased secretion of multiple gut hormones may contribute to prolonged stimulation of b-cell growth or enhancement of b-cell function.
For an overview of the effects of gastric bypass surgery on plasma levels of gut hormones, see Gut hormones and bariatric surgery