Glucose-responsive expression of the human insulin promoter in HepG2 human hepatoma cells

Generation of transgenic fibroblasts expressing pancreas‑specific and doxycycline-inducible ICER Iγ for the establishment of a porcine model of human diabetes mellitus

Inducible cyclic AMP (cAMP) early repressor (ICER) Iγ acts as an endogenous inhibitor and disrupts the transcriptional regulation of cAMP response element binding protein (CREBP) responsive genes. Since the overexpression of ICER Iγ induces severe diabetes in a transgenic mouse model, with characteristics similar to human diabetes mellitus, an ICER Iγ construct containing an adjustable pancreas tissue specific promoter was utilized in the present study. Using the human insulin promoter region, a doxycycline (dox)‑inducible ICER Iγ expression system was established using the tetracycline (tet)-controlled transactivator (tTA) with a TA response element (TRE) promoter. A unitary tet-on system that combined a tet-on activator cassette was also developed and was controlled by the human insulin promoter with a responder cassette containing genes encoding ICER Iγ regulated by the TRE promoter. To determine whether dox-enhanced ICER Iγ expression affected insulin production, the unitary tet-on ICER Iγ vector was introduced into a mouse pancreatic β-cell line and then the cells were treated with 0.1-1 mg/ml dox. The results revealed a robust increase in ICER Iγ expression and decreased insulin production. Therefore, this in vitro system may be useful for studying human diabetes mellitus and pre-diabetes using tissue-specific promoters and a dox-inducible transgene. In addition, porcine transgenic fibroblasts containing dox-inducible ICER Iγ were generated. These fibroblasts may serve as a cell source for somatic cell nuclear transfer to generate a porcine model of human diabetes mellitus.

Insulin gene therapy from design to beta cell generation

Despite the fact that insulin injection can protect diabetic patients from developing diabetes-related complications, recent meta-analyses indicate that rapid and long-acting insulin analogues only provide a limited benefit compared with conventional insulin regarding glycemic control. As insulin deficiency is the main sequel of type-1 diabetes (T1D), transfer of the insulin gene-by-gene therapy is becoming an attractive treatment modality even though T1D is not caused by a single genetic defect. In contrast to human insulin and insulin analogues, insulin gene therapy targets to supplement patients not only with insulin but also with C-peptide. So far, insulin gene therapy has had limited success because of delayed and/or transient gene expression. Sustained insulin gene expression is now feasible using current gene-therapy vectors providing patients with basal insulin coverage, but management of postprandial hyperglycaemia is still difficult to accomplish because of the inability to properly control insulin secretion. Enteroendocrine cells of the gastrointestinal track (K cells and L cells) may be ideal targets for insulin gene therapy, but cell-targeting difficulties have limited practical implementation of insulin gene therapy for diabetes treatment. Therefore, recent gene transfer technologies developed to generate authentic beta cells through transdifferentiation are also highlighted in this review.

Combinatorial insulin secretion dynamics of recombinant hepatic and enteroendocrine cells

One of the most promising cell-based therapies for combating insulin-dependent diabetes entails the use of genetically engineered non-β cells that secrete insulin in response to physiologic stimuli. A normal pancreatic β cell secretes insulin in a biphasic manner in response to glucose. The first phase is characterized by a transient stimulation of insulin to rapidly lower the blood glucose levels, which is followed by a second phase of insulin secretion to sustain the lowered blood glucose levels over a longer period of time. Previous studies have demonstrated hepatic and enteroendocrine cells to be appropriate hosts for recombinant insulin expression. Due to different insulin secretion kinetics from these cells, we hypothesized that a combination of the two cell types would mimic the biphasic insulin secretion of normal β cells with higher fidelity than either cell type alone. In this study, insulin secretion experiments were conducted with two hepatic cell lines (HepG2 and H4IIE) transduced with 1 of 3 adenoviruses expressing the insulin transgene and with a stably transfected recombinant intestinal cell line (GLUTag-INS). Insulin secretion was stimulated by exposing the cells to glucose only (hepatic cells), meat hydrolysate only (GLUTag-INS), or to a cocktail of the two secretagogues. It was found experimentally that the recombinant hepatic cells secreted insulin in a more sustained manner, whereas the recombinant intestinal cell line exhibited rapid insulin secretion kinetics upon stimulation. The insulin secretion profiles were computationally combined at different cell ratios to arrive at the combinatorial kinetics. Results indicate that combinations of these two cell types allow for tuning the first and second phase of insulin secretion better than either cell type alone. This work provides the basic framework in understanding the secretion kinetics of the combined system and advances it towards preclinical studies.

Neuromodulatory activities of CD4+CD25+ regulatory T cells in a murine model of HIV-1-associated neurodegeneration

HIV-1-associated neurocognitive impairments are intrinsically linked to microglial immune activation, persistent viral infection, and inflammation. In the era of antiretroviral therapy, more subtle cognitive impairments occur without adaptive immune compromise. We posit that adaptive immunity is neuroprotective, serving in both the elimination of infected cells through CD8(+) cytotoxic T cell activities and the regulation of neuroinflammatory responses of activated microglia. For the latter, little is known. Thus, we studied the neuromodulatory effects of CD4(+) regulatory T cells (Treg; CD4(+)CD25(+)) or effector T cells in HIV-1-associated neurodegeneration. A newly developed HIV-1 encephalitis mouse model was used wherein murine bone marrow-derived macrophages are infected with a full-length HIV-1(YU2)/vesicular stomatitis viral pseudotype and injected into basal ganglia of syngeneic immunocompetent mice. Adoptive transfer of CD3-activated Treg attenuated astrogliosis and microglia inflammation with concomitant neuroprotection. Moreover, Treg-mediated anti-inflammatory activities and neuroprotection were associated with up-regulation of brain-derived neurotrophic factor and glial cell-derived neurotrophic factor expression and down-regulation of proinflammatory cytokines, oxidative stress, and viral replication. Effector T cells showed contrary effects. These results, taken together, demonstrate the importance of Treg in disease control and raise the possibility of their utility for therapeutic strategies.

Cellular origins of adult human islet in vitro dedifferentiation

Cultured human islets can be dedifferentiated to duct-like structures composed mainly of cytokeratin+ and nestin+ cells. Given that these structures possess the potential to redifferentiate into islet-like structures, we sought to elucidate their specific cellular origins. Adenoviral vectors were engineered for beta-, alpha-, delta- or PP-cell-specific GFP expression. A double-stranded system was designed whereby cultures were infected with two vectors: one expressed GFP behind the cumate-inducible promoter sequence, and the other expressed the requisite transactivator behind the human insulin, glucagon, somatostatin or pancreatic polypeptide promoter. This system labels hormone+ cells in the islet in a cell-specific manner, allowing these cells to be tracked during the course of transformation from islet to duct-like structure. Post-infection, islets were cultured to induce dedifferentiation. Fluorescence microscopy demonstrated that alpha-, delta- and PP-cells contributed equally to the cytokeratin+ population, with minimal beta-cell contribution, whereas the converse was true for nestin+ cells. Complementary targeted cell ablation studies, using streptozotocin or similar adenoviral expression of the Bax (Bcl2-associated X protein) toxigene, validated these findings and suggested a redundancy between alpha-, delta- and PP-cells with respect to cytokeratin+ cell derivation. These results call into question the traditional understanding of islet cells as being terminally differentiated and provide support for the concept of adult islet morphogenetic plasticity.

Tissue-specific and glucose-responsive expression of the pancreatic derived factor (PANDER) promoter

Pancreatic derived factor (PANDER) is a recently identified cytokine-like protein that is dominantly expressed in the islets of Langerhans of the pancreas. To investigate the mechanism of tissue-specific regulation of PANDER, we identified and characterized the promoter region. The transcriptional start site was identified 520 bp upstream of the translational start codon by 5'-RLM-RACE. Computer algorithms identified several islet-associated and glucose-responsive binding motifs that included A and E boxes, hepatocyte nuclear factors 1 and 4, Oct-1, and signal transducer and activator of transcription 3, and 5. Reporter gene analysis revealed cell type-specific PANDER promoter expression in islet and liver-derived cell lines. Levels of PANDER mRNA were directly concordant to the observed cell type-specific PANDER promoter gene expression. The minimal element was mapped to the 5'-UTR and located between +200 and +491 relative to the transcriptional start site and imparted maximal gene expression. In addition, several putative glucose-responsive binding sites were further functionally characterized to reveal critical regulatory elements of PANDER. The PANDER promoter was demonstrated to be glucose-responsive in a dose-dependent manner in murine insulinoma beta-TC3 cells and primary murine islets, but unresponsive in glucagon-secreting alpha-TC3 cells. Our findings revealed that the 5'-UTR of PANDER contains the minimal element for gene expression and imparts both tissue-specificity and glucose-responsiveness. The regulation of PANDER gene expression mimics that of insulin and suggests a potential biological function of PANDER involved in metabolic homeostasis.

Glucose transporter-2 (GLUT2) promoter mediated transgenic insulin production reduces hyperglycemia in diabetic mice

Insulin production afforded by hepatic gene therapy (HGT) retains promise as a potential treatment for type 1 diabetes, but successful approaches have been limited. We employed a novel and previously untested promoter for this purpose, glucose transporter-2 (GLUT2) to drive insulin production via delivery by recombinant adeno-associated virus (rAAV). In vitro, the GLUT2 promoter was capable of robust glucose-responsive expression in transduced HepG2 human hepatoma cells. Therefore, rAAV constructs were designed to express the furin-cleavable human preproinsulin B10 gene, under the control of the murine GLUT2 promoter and packaged for delivery with rAAV expressing the type 5 capsid. Streptozotocin-induced diabetic mice were subjected to hepatic portal vein injection immediately followed by implantation of a sustained-release insulin pellet to allow time for transgenic expression. All mice injected with the rAAV5-GLUT2-fHPIB10 virus remained euglycemic for up to 35 days post-injection, with 50% euglycemic after 77 days post-injection. In contrast, mock-injected mice became hyperglycemic within 15 days post-injection following dissolution of the insulin pellet. Serum levels of both human insulin and C-peptide further confirmed successful transgenic delivery by the rAAV5-GLUT2-fHPIB10 virus. These findings indicate that the GLUT2 promoter may be a potential candidate for regulating transgenic insulin production for hepatic insulin gene therapy in the treatment of type I diabetes.

Localized Gene Expression Following Administration of Adeno-associated Viral Vectors via Pancreatic Ducts

Gene transfer into pancreatic cells in vivo could be of immense therapeutic benefit in cases of type 1 diabetes (T1D) through the production of molecules capable of interrupting the progression of autoimmunity or promoting regeneration of insulin-secreting beta cells. We adapted a clinically relevant surgical technique (endoscopic retrograde cholangiopancreatography) to deliver rAAV encoding human alpha1-antitrypsin (approved gene symbol SERPINA1) to the pancreas of 3-week-old Fisher 344 rats and C57BL/6 mice. We compared natural as well as bioengineered serotypes of rAAV (rAAV1, rAAV2/Apo, rAAV8) as well as different promoters (chicken beta-actin, human insulin) for their expression in vivo. Rats injected with rAAV1 showed the highest hAAT expression (week 2, rAAV1/CB-AT, 579 +/- 457 ng/ml). In mice, rAAV8 vector delivered the highest serum concentration of hAAT (week 2, rAAV8/CB-AT, 19 +/- 6 microg/ml). The chicken beta-actin promoter provided the highest expression in both rodent experiments. Immunohistochemical staining indicated transduction primarily of pancreatic acinar cells with either the rAAV1/CB-AT vector in the rat or the rAAV8/CB-AT vector in the mouse. This study demonstrates that rAAV vectors can be designed to deliver therapeutic genes efficiently to the pancreas and achieve high levels of gene expression and may be useful in treating pancreatic disorders, including T1D.

Heterogeneity in predisposition of hepatic cells to be induced into pancreatic endocrine cells by PDX-1

AIM: The role of Pancreatic and Duodenal Homeobox-1 (PDX-1) as a major regulator of pancreatic development determines the function and phenotype of β cell. In this study, potential plasticity of liver cells into pancreatic endocrine cells induced by PDX-1 was evaluated.

METHODS: Human hepatoma cell line HepG₂ was stably transfected with mammalian expression plasmid pcDNA3-PDX encoding human PDX-1 gene. Ectopic expression of PDX-1 and insulin were detected by RT-PCR, Western blot and/or immunostaining. PDX-1⁺ HepG₂ cells were transplanted under renal capsule of STZ-induced diabetic nude mice (n = 16) to examine the inducing effect in vivo.

RESULTS: Exogenous PDX-1 transgene was proved to express effectively in HepG₂ cell at both mRNA and protein levels. The expression of endogenous insulin and some β cell-specific differentiation markers and transcription factors were not induced in PDX-1⁺ HepG₂ cells. When transplanted under renal capsule of STZ-induced diabetic nude mice, PDX-1⁺ HepG₂ cells did not generate insulin-producing cells. These data indicated that stable transfected PDX-1 could not convert hepatoma cell line HepG₂ to pancreatic cells in vitro or in vivo. Mature hepatocytes might need much more complicated or rigorous conditions to be shifted to insulin-producing cells.

CONCLUSION: The expression of exogenous PDX-1 is not sufficient to induce relatively mature hepatocytes differentiating into insulin-producing cells.

Nitric oxide stimulates insulin release in liver cells expressing human insulin

The establishment of surrogate islet beta cells is important for the treatment of diabetes. Hepatocytes have a similar glucose sensing system as beta cells and have the potential to serve as surrogate beta cells. In this report, we demonstrate that infection of Hepa1-6 liver cells with a lentivirus expressing the human insulin cDNA results in expression and secretion of human insulin. Furthermore, we show that l-arginine at low levels of glucose significantly stimulates the release of insulin from these cells, compared to exposure to high concentration of glucose. The arginine-induced insulin release is via the production of nitric oxide, since treatment with N(G)-nitro-l-arginine, an inhibitor of nitric oxide synthase, blocks insulin secretion induced by l-arginine. These results indicate that nitric oxide plays a role in l-arginine-stimulated insulin release in hepatocytes expressing the human insulin gene, and provides a new strategy to induce insulin secretion from engineered non-beta cells.

Effect of polyethylenimine on recombinant adeno-associated virus mediated insulin gene therapy

BACKGROUND

Recombinant adeno-associated virus (rAAV) is becoming a promising vector for gene therapy for type I diabetes. The objective of this study was to investigate the effect of incorporation of polyethylenimine (PEI) on rAAV-mediated insulin gene therapy in vitro and in vivo.

METHODS

Recombinant AAV vector, harboring the furin-mutated human insulin and enhanced green fluorescent protein (EGFP) genes, was constructed. The effect of complexation with PEI on rAAV-mediated gene transfer was examined in Huh7 human hepatoma cells. The transgene expression was also examined in streptozotocin (STZ)-induced diabetic C57BL/6J mice by direct administration of rAAV into the livers of the animals, followed by monitoring changes in body weight and blood glucose levels. Secretion of human insulin was determined by radioimmunoassay (RIA) and immunohistochemical staining in the livers.

RESULTS

Complexation with PEI was shown to enhance rAAV-mediated transgene expression in Huh7 cells, resulting in higher transduction efficiency and enhanced production of immunoreactive human insulin. Heparin competition assay demonstrated that endocytosis of rAAV-PEI was partially inhibited by heparin. The enhancement of rAAV-mediated transgene expression was also demonstrated in the animals, showing lowering of blood glucose and longer duration of normoglycemia. Immunofluorescent staining of the liver sections demonstrated that PEI increased the uptake of rAAV and enhanced insulin secretion. The enhancement of PEI on rAAV-mediated insulin gene therapy was further confirmed by glucose challenge and a 10-h fasting blood glucose test.

CONCLUSIONS

Results obtained in this study demonstrated that incorporation of PEI augmented rAAV-mediated insulin gene transfer and enhanced amelioration of hyperglycemia in the STZ-induced diabetic animals.

Gene Therapy Progress and Prospects: Gene therapy for diabetes mellitus

Diabetes mellitus has long been targeted, as yet unsuccessfully, as being curable with gene therapy. The main hurdles have not only been vector-related toxicity but also the lack of physiological regulation of the expressed insulin. Recent advances in understanding the developmental biology of beta-cells and the transcriptional cascade that drives it have enabled both in vivo and ex vivo gene therapy combined with cell therapy to be used in animal models of diabetes with success. The associated developments in the stem cell biology and immunology have opened up further opportunities for gene therapy to be applied to target autoimmune diabetes.