The Arg64 beta(3)-adrenergic receptor (beta(3)AR) variant is associated with an earlier age of onset of diabetes and lower levels of insulin secretion in humans. The aims of this study were to investigate whether beta(3)AR is expressed by islet cells, if receptor binding affects insulin secretion and, finally, if the beta(3)AR Arg64 variant induces abnormal insulin secretory activity. Human pancreas extracts were subjected to RT-PCR, Western blotting and immunostaining analyses. DNA sequencing and Western blotting demonstrated that the beta(3)AR gene is transcribed and translated in the human pancreas; immunostaining showed that it is expressed by the islets of Langerhans. Cultured rat beta-cells responded to human beta(3)AR agonists in a dose- and time-dependent manner. Transfection of cultured rat beta-cells with the wild-type human beta(3)AR produced an increased baseline and ligand-dependent insulin secretion compared with parental cells. On the other hand, cells transfected with the Arg64 variant of the beta(3)AR secreted less insulin, both spontaneously and after exposure to human beta(3)AR agonists. Furthermore, while transfection with the wild-type beta(3)AR preserved the glucose-dependent secretion of insulin, expression of the variant receptor rendered the host cells significantly less responsive to glucose. In summary, cells express the beta(3)AR, and its activation contributes to the regulation of insulin secretion. These findings may help explain the low levels of insulin secretion in response to an i.v. glucose tolerance test observed in humans carrying the Arg64 polymorphism.
R Perfetti, H Hui, K Chamie, S Binder, M Seibert, J McLenithan, K Silver, and JD Walston
Y Wang, HK Kole, C Montrose-Rafizadeh, R Perfetti, M Bernier, and JM Egan
Glucagon-like peptide-1 (7-36 amide) (GLP-1) is known to increase insulin release when given as a bolus in the fasted and fed state. GLP-1 also increases glucose uptake and lipid synthesis in cultured adipocytes. In this study we investigated the effects of GLP-1 on glucose uptake and on the levels of expression of the facilitative glucose transporters, GLUT1 and GLUT4, in fully differentiated 3T3-L1 adipocytes. Cells were incubated with GLP-1 (10 nM) with or without insulin (10 and 100 nM) for 24 h. Under these conditions, GLP-1 alone caused an increase in basal and acute insulin-stimulated glucose uptake along with an increase in GLUT1 and GLUT4 protein levels. However, there was no change in the expression of GLUT1 and GLUT4 mRNAs. In the absence of GLP-1, prolonged exposure to insulin caused a marked reduction in the levels of GLUT4 mRNA and protein, and an inhibition of glucose uptake after an acute exposure to insulin. This insulin-induced down-regulation of GLUT4 was prevented when GLP-1 was present during the 24-h treatment. In contrast, the acute insulin-stimulated glucose uptake could not be restored by GLP-1. GLP-1 is therefore the first gut hormone shown to be capable of modulating glucose transporter levels in cultured adipocytes.
A Bulotta, H Hui, E Anastasi, C Bertolotto, LG Boros, U Di Mario, and R Perfetti
The intestinal hormone glucagon-like peptide-1 (GLP-1) has been shown to promote an increase in pancreatic beta-cell mass via proliferation of islet cells and differentiation of non-insulin-secreting cells. In this study, we have characterized some of the events that lead to the differentiation of pancreatic ductal cells in response to treatment with human GLP-1. Rat pancreatic ductal (ARIP) cells were cultured in the presence of GLP-1 and analyzed for cell counting, cell cycle distribution, expression of cyclin-dependent-kinase (Cdk) inhibitors, transcription of beta-cell-specific genes, loss of ductal-like phenotype and acquisition of beta-cell-like gene expression profile. Exposure of ARIP cells to 10 nM GLP-1 induced a significant reduction in the cell replication rate and a significant decrease in the percentage of cells in S phase of the cell cycle. This was associated with an increase in the number of cells in G0-G1 phase and a reduction of cells in G2-M phase. Western blot analysis for the Cdk inhibitors, kinase inhibitor protein 1 (p27(Kip1)) and Cdk-interacting protein 1 (p21(Cip1)), demonstrated a significant increase in p27(Kip1) and p21(Cip1) levels within the first 24 h from the beginning of GLP-1 treatment. As cells slowed down their proliferation rate, GLP-1 also induced a time-dependent expression of various beta-cell-specific mRNAs. The glucose transporter GLUT-2 was the first of those factors to be expressed (24 h treatment), followed by insulin (44 h) and finally by the enzyme glucokinase (56 h). In addition, immunocytochemistry analysis showed that GLP-1 induced a time-dependent down-regulation of the ductal marker cytokeratin-20 (CK-20) and a time-dependent induction of insulin expression. Finally, GLP-1 promoted a glucose-dependent secretion of insulin, as demonstrated by HPLC and RIA analyses of the cell culture medium. The present study has demonstrated that GLP-1 induces a cell cycle re-distribution with a decrease in cell proliferation rate prior to promoting the differentiation of cells towards an endocrine-like phenotype.
R Perfetti, M Raygada, Y Wang, M E Zenilman, J M Egan, K M Denno, T W Sadler, and A R Shuldiner
The pancreatic regenerating (reg) gene is proposed to be involved in pancreatic β-cell growth. Up- or down-regulation of reg gene expression has been shown to parallel variations in β-cell mass and function in the adult pancreas. In several species at least two nonallelic reg genes have been identified. In this study we investigated the expression of each individual reg gene (reg-I and reg-II) during embryogenesis in the mouse. Single mouse embryos were harvested at 8·5, 9, 10, and 12 days of development, homogenized and subjected individually to reverse transcription (RT)-PCR, with a single primer pair to amplify both reg-I and -II mRNAs. Southern blot analysis of the RT-PCR products revealed the presence of reg mRNA at day 9 of embryogenesis, just before the beginning of pancreatic organogenesis. Slot-blot analysis with internal oligonucleotide probes that specifically recognize reg-I or -II sequences demonstrated that only reg-I mRNA was present in day 9 and day 10 prepancreatic embryos. Reg-II mRNA was not detected until day 12, a stage corresponding to late organogenesis. RT-PCR for insulin mRNA from the same samples used for the amplification of reg mRNA showed that the earliest insulin expression occurred at day 8·5, and coincided with the onset of reg-I expression. Hybridization with gene-specific oligonucleotide probes revealed that only insulin-II mRNA was detectable at this time. Insulin-I mRNA was not detectable until day 12 and coincided with early reg-II expression. These results suggest that the two nonallelic reg genes and the two insulin genes are expressed differentially during early embryogenesis. Differential expression of reg-I and -II suggests that they may be induced by different and independent stimuli and have distinct functions.
E Anastasi, C Santangelo, A Bulotta, F Dotta, B Argenti, C Mincione, A Gulino, M Maroder, R Perfetti, and U Di Mario
The elucidation of mechanisms regulating the regeneration and survival of pancreatic beta cells has fundamental implications in the cell therapy of type 1 diabetes. The present study had the following three aims: 1. to investigate whether pancreatic ductal epithelial cells can be induced to differentiate into insulin-producing cells by exposing them to hepatocyte growth factor (HGF); 2. to characterize some of the molecular events leading to their differentiation toward a beta-cell-like phenotype; 3. to evaluate the susceptibility of newly differentiated insulin-secreting cells to cytokine-induced apoptosis, a mechanism of beta-cell destruction occurring in type 1 diabetes. We demonstrated that HGF-treated rat pancreatic ductal cell line (ARIP) cells acquired the capability to transcribe the insulin gene and translate its counterpart protein. HGF-treated cells also exhibited a glucose-dependent capability to secrete insulin into the cultured medium. Expression analysis of some of the genes regulating pancreatic beta-cell differentiation revealed a time-dependent transcription of neurogenin-3 and Neuro-D in response to HGF. Finally, we determined the susceptibility to proinflammatory cytokine (PTh1)-induced apoptosis by incubating HGF-treated and untreated ARIP cells with a cocktail of interleukin-1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ). Such treatment induced apoptotic death, as determined by the TUNEL technique, in about 40% of HGF-treated, insulin-secreting ARIP cells, while untreated ARIP cells were resistant to PTh1-induced apoptosis. In conclusion, we showed that HGF promotes the differentiation of ARIP cells into pancreatic beta-cell-like cells, and that the differentiation toward an insulin-secreting phenotype is associated with the appearance of susceptibility to cytokine-induced apoptosis.