Proglucagon mRNA transcripts are transcribed in the pancreas, bowel, and brain, after which posttranslational processing results in the liberation of a different profile of biologically active peptides in each tissue. The receptors for two of these peptides, glucagon and glucagon-like peptide-1 (GLP-1), have recently been identified, but only limited information is available concerning the tissue- and age-specific distribution of these receptors in vivo. We have investigated the expression of these receptors in the mouse using a combination of Northern blot analysis and reverse transcription-polymerase chain reaction. DNA sequence analysis of a partial mouse glucagon receptor cDNA demonstrated a high degree of sequence conservation across rodent species. Glucagon receptor mRNA transcripts were detectable by Northern blotting in poly(A)+ RNA from liver and kidney. Reverse transcription-polymerase chain reaction also detected glucagon receptor mRNA transcripts in both fetal and adult pancreas and lung, jejunum, and ileum, but not in the large intestine. In contrast, mRNA transcripts for the GLP-1 receptor were detected in both small and large intestine as well as in pancreas, liver, lung, and kidney. Both glucagon and GLP-1 receptor mRNA transcripts were identified in different regions of the fetal and adult mouse brain, but the relative levels of GLP-1 receptor mRNA transcripts were much greater in the central nervous system. Furthermore, regulation of the GLP-1 receptor (but not the glucagon receptor) gene in the brain resembled the pattern of region-specific gene expression recently defined for the mouse proglucagon gene. Taken together, these studies define novel sites for both glucagon and GLP-1 receptor gene expression in the mouse and suggest that different regulatory mechanisms have evolved for tissue-specific and developmental control of receptor gene expression.

Proglucagon mRNA transcripts are transcribed in the pancreas, bowel, and brain, after which posttranslational processing results in the liberation of a different profile of biologically active peptides in each tissue. The receptors for two of these peptides, glucagon and glucagon-like peptide-1 (GLP-1), have recently been identified, but only limited information is available concerning the tissue- and age-specific distribution of these receptors in vivo. We have investigated the expression of these receptors in the mouse using a combination of Northern blot analysis and reverse transcription-polymerase chain reaction. DNA sequence analysis of a partial mouse glucagon receptor cDNA demonstrated a high degree of sequence conservation across rodent species. Glucagon receptor mRNA transcripts were detectable by Northern blotting in poly(A)+ RNA from liver and kidney. Reverse transcription-polymerase chain reaction also detected glucagon receptor mRNA transcripts in both fetal and adult pancreas and lung, jejunum, and ileum, but not in the large intestine. In contrast, mRNA transcripts for the GLP-1 receptor were detected in both small and large intestine as well as in pancreas, liver, lung, and kidney. Both glucagon and GLP-1 receptor mRNA transcripts were identified in different regions of the fetal and adult mouse brain, but the relative levels of GLP-1 receptor mRNA transcripts were much greater in the central nervous system. Furthermore, regulation of the GLP-1 receptor (but not the glucagon receptor) gene in the brain resembled the pattern of region-specific gene expression recently defined for the mouse proglucagon gene. Taken together, these studies define novel sites for both glucagon and GLP-1 receptor gene expression in the mouse and suggest that different regulatory mechanisms have evolved for tissue-specific and developmental control of receptor gene expression.

A cAMP response element (CRE) has been identified in the proximal 5'-flanking region of the rat glucagon gene, and activation of the cAMP-dependent pathway in fetal rat intestinal cells leads to an increase in the levels of glucagon mRNA transcripts. In contrast, the human glucagon gene does not contain a similar CRE, and the results of studies using immortalized rat and hamster islet cell lines have suggested that glucagon gene expression may not be regulated by cAMP. To reconcile these observations, we have studied the control of glucagon gene expression. Incubation of primary rat islet cell cultures with forskolin in the presence of low (0.5 g/liter) or high (2.0 g/L) glucose resulted in a 2- to 3-fold increase in the levels of glucagon mRNA transcripts. Forskolin also stimulated the secretion and synthesis of immunoreactive glucagon. The importance of the protein kinase-A-dependent pathway in the regulation of glucagon gene expression was also examined in hamster islet InR1-G9 cells. Cotransfection of a glucagon-chloramphenicol acetyltransferase (CAT) fusion gene containing the glucagon CRE and a cDNA encoding the catalytic subunit of protein kinase-A resulted in stimulation of glucagon-CAT activity in hamster islet cells. Catalytic subunit cotransfection also activated somatostatin-CAT, but no activation of RSVCAT was detected. The results of these experiments suggest that the rat glucagon gene is regulated by a protein kinase-A-dependent pathway in the endocrine pancreas.

Various cholestatic liver diseases as well as regeneration after submassive necrosis are accompanied by a striking increase in the number of bile ductules. These reactive bile ductules are thought to arise either from proliferation of pre-existing bile ductules or bile ductule-related facultative stem cells, or from ductular metaplasia of hepatocytes. Recently, we found that reactive bile ductules display neuro-endocrine features, and speculated that the substance(s), produced in the neuro-endocrine granules, might play a role in their growth and/or differentiation through an autocrine or paracrine pathway. Parathyroid hormone-related peptide has been shown to be encoded by a growth factor-regulated gene that may play a role in cell growth and differentiation. We studied the immunohistochemical expression of this peptide in human liver, including three normal biopsies, 11 cases of cholestatic liver disease, six cases of focal nodular hyperplasia and three cases of regenerating liver. In regenerating liver, primary biliary cirrhosis, primary sclerosing cholangitis and partial or intermittent obstruction, the majority of reactive ductular cells expressing neuro-endocrine markers also expressed parathyroid hormone-related peptide. In focal nodular hyperplasia, a smaller number of bile ductular cells expressed the peptide. These findings suggest that parathyroid hormone-related peptide is localized in bile ductular cells and may indicate a role for this hormone in the growth and/or differentiation of human reactive bile ductules.