Molecular Cloning of the Porcine Insulin cDNA Using a Monolayer Culture of Pancreatic Endocrine Cells

Potential roles of large mafs in cell lineages and developing pancreas

OBJECTIVES

Maf is a family of transcription factor proteins characterized by a typical bZip structure, and mafA, a member of the large-maf family, is a strong transactivator of insulin in cell lines. The present study investigated the expression profiles of the large-maf family proteins in porcine pancreatic tissue and in primary culture cells.

METHODS

Immunohistochemical staining was performed to localize each maf protein. Messenger RNA expression was quantitated by real-time polymerase chain reaction, and protein expression was assessed by Western blotting.

RESULTS

Islet formation was not as clear in newborn pancreatic tissue as in adult pancreatic tissue. MafA- and c-maf-positive cells were more diffusely localized in pancreatic tissue with fewer mafB-positive cell clusters scattered throughout. By contrast, islet formation was clearer, and positive staining for mafA and c-maf tended to be more prominent in the islets of adult pancreatic tissue. Messenger RNA and protein expressions were consistent with the immunohistochemical findings. MafA, mafB, and c-maf coexpressed with insulin-positive cells, and c-maf coexpressed with glucagon-positive cells in adult porcine pancreas based on the results of a double-staining study.

CONCLUSIONS

Large mafs were identified in normal porcine and human pancreas, and the expression levels and localizations of the large mafs in newborn and adult pancreatic tissues differed. Mafs may play important roles in establishing endocrine function during pancreatic cell differentiation.

Effect of GLP-1 (glucagon-like peptide 1:7-36 amide) on porcine pancreatic endocrine cell proliferation and insulin secretion

INTRODUCTION

We have established a method of isolation and primary monolayer culture of porcine pancreatic endocrine (PE) cells using nicotinamide. Recently, several insulin-stimulating factors have been recognized as modifying the function of endocrine cells. Glucagon-like peptide 1 (GLP-1), derived from intestinal cells, is one of the substances that may regulate insulin secretion and cell proliferation.

AIM

The aim of the current study was to examine the effect of GLP-1 on PE cell proliferation and insulin secretion in our culture system.

METHODOLOGY

The PE cells were prepared by nonenzymatic digestion and cultured in media with or without 10 nmol/L GLP-1 and maintained for 15 days. Basal insulin secretion and the glucose stimulated-response were observed by enzyme assay. PE-cell proliferation was assessed using 5-bromo 2' deoxyuridine (BrdU). beta-Cell specificity was confirmed by double staining with anti-BrdU and insulin antibody. mRNA expression in PE cells for insulin and pancreatic and duodenal homeobox gene 1 (PDX-1), a transcription factor, was semiquantitated by a real-time PCR method.

RESULTS

Neither baseline nor glucose-stimulated insulin secretion differed significantly with or without the addition of 10 nmol/L GLP-1. However, significant changes of insulin secretion were observed in doses up to 100 nmol/L of GLP-1. BrdU incorporation increased with 10 nmol/L GLP-1 from 2.35+/- 0.16% to 9.11+/- 0.53% as compared with the control (from 2.17 +/- 0.57% to 6.13 +/- 0.35%). This increase was observed after a 12-day culture period. Double staining identified well-preserved insulin-containing cells, but neither the number of positive cells nor staining intensity differed significantly between the 2 groups during the 15-day culture period. The level of mRNA expression for both insulin and PDX-1 increased slightly but significantly.

CONCLUSIONS

GLP-1 has a proliferative effect on PE cells and affects gene expression in short-term culture.

Analysis of gene expression and insulin secretion by monolayer-forming adult porcine pancreatic endocrine cells

INTRODUCTION

We recently established a method of isolation and primary monolayer culture of porcine pancreatic endocrine cells that involves the use of nicotinamide.

AIM

To obtain genetic information on cultured porcine endocrine cells and to examine cell function in relation to insulin secretion during long-term culture.

METHODOLOGY

Gene expression of insulin and several transcription factors, including PDX-1, Beta2/NeuroD, Pax6, and Nkx6.1, was assessed by reverse transcription-polymerase chain reaction analysis, and the insulin protein level was estimated by immunohistochemistry and enzyme assay during a 12-week period.

RESULTS

During the culture period, insulin accumulation in the medium at 5 weeks had decreased by almost half the level of accumulation in the first week. In contrast to the alteration of secretory function, insulin gene expression was maintained for at least 12 weeks, and regulatory transcription factors were expressed at the same levels until 9 weeks. These observations suggest that gene expression is not involved in the cause of decreased baseline insulin secretion. Moreover, although the insulin response to high glucose and potassium loading was maintained, the magnitude of the responses to both stimuli was attenuated in the late period of culture. Insulin secretion tended to decrease in our culture system, and the secretory response to pharmacological stimulation was attenuated despite maintenance of messenger RNA expression of insulin and other islet-specific genes for at least 9 weeks in vitro.

CONCLUSION

These findings indicate that cell integrity is maintained and that the alteration in insulin secretion must be explained by another mechanism.