Gastrin gene expression and regulation in rat islet cell lines

TCF7 is not essential for glucose homeostasis in mice

Objective

Glucose-dependent insulinotropic polypeptide (GIP) and Glucagon-like peptide-1 (GLP-1) are incretin hormones that exert overlapping yet distinct actions on islet β-cells. We recently observed that GIP, but not GLP-1, upregulated islet expression of Transcription Factor 7 (TCF7), a gene expressed in immune cells and associated with the risk of developing type 1 diabetes. TCF7 has also been associated with glucose homeostasis control in the liver. Herein we studied the relative metabolic importance of TCF7 expression in hepatocytes vs. islet β-cells in mice.

Methods

Tcf7 expression was selectively inactivated in adult mouse hepatocytes using adenoviral Cre expression and targeted in β-cells using two different lines of insulin promoter-Cre mice. Glucose homeostasis, plasma insulin and triglyceride responses, islet histology, hepatic and islet gene expression, and body weight gain were evaluated in mice fed regular chow or high fat diets. Tcf7 expression within pancreatic islets and immune cells was evaluated using published single cell RNA-seq (scRNA-seq) data, and in islet RNA from immunodeficient Rag2^−/−Il2rg^−/− mice.

Results

Reduction of hepatocyte Tcf7 expression did not impair glucose homeostasis, lipid tolerance or hepatic gene expression profiles linked to control of metabolic or immune pathways. Similarly, oral and intraperitoneal glucose tolerance, plasma insulin responses, islet histology, body weight gain, and insulin tolerance were not different in mice with targeted recombination of Tcf7 in insulin-positive β-cells. Surprisingly, islet Tcf7 mRNA transcripts were not reduced in total islet RNA containing endocrine and associated non-endocrine cell types from Tcf7^{βcell−/−} mice, despite Cre-mediated recombination of islet genomic DNA. Furthermore, glucose tolerance was normal in whole body Tcf7^−/− mice. Analysis of scRNA-seq datasets localized pancreatic Tcf7 expression to islet progenitors during development, and immune cells, but not within differentiated islet β-cells or endocrine lineages within mature islets. Moreover, the expression of Tcf7 was extremely low in islet RNA from Rag2^−/−Il2rg^−/− mice and, consistent with expression within immune cells, Tcf7 was highly correlated with levels of Cd3g mRNA transcripts in RNA from wild type mouse islets.

Conclusions

These findings demonstrate that Tcf7 expression is not a critical determinant of glucose homeostasis in mice. Moreover, the detection of Tcf7 expression within islet mRNA is attributable to the expression of Tcf7 RNA in islet-associated murine immune cells, and not in islet β-cells.

•Reduction of hepatocyte Tcf7 does not impair glucose homeostasis.

•Targeting beta cell Tcf7 using insulin-promoter-Cre does not reduce islet Tcf7 expression.

•RNA-seq localizes pancreatic Tcf7 to islet progenitors and lymphocytes.

•Tcf7 expression is markedly reduced in islet RNA from Rag2^−/−Il2rg^−/− mice.

Progastrin Peptides Increase the Risk of Developing Colonic Tumors: Impact on Colonic Stem Cells

Pre-neoplastic lesions (ACF, aberrant-crypt-foci; Hp, hyperplastic/dysplastic polyps) are believed to be precursors of sporadic colorectal-tumors (Ad, adenomas; AdCA, adenocarcinomas). ACF/Hp likely originate due to abnormal growth of colonic-crypts in response to aberrant queues in the microenvironment of colonic-crypts. Thus identifying factors which regulate homeostatic vs aberrant proliferation/apoptosis of colonocytes, especially stem/progenitor cells, may lead to effective preventative/treatment strategies. Based on this philosophy, role of growth-factors/peptide-hormones, potentially available in the circulation/microenvironment of colonic-crypts is being examined extensively. Since the time gastrins were discovered as trophic (growth) factors for gastrointestinal-cells, the effect of gastrins on the growth of normal/cancer cells has been investigated, leading to many discoveries. Seminal discoveries made in the area of gastrins and colon-cancer, as it relates to molecular pathways associated with formation of colonic tumors will be reviewed, and possible impact on diagnostic/preventative/treatment strategies will be discussed.

Extragastric effects of gastrin gene knock-out mice

Gastrin is a peptide hormone that regulates both acid secretion and growth of the gastric oxyntic mucosa. Recent studies suggest that gastrin, in both its amidated, and less processed forms (glycine-extended gastrin and progastrin) may also exert biological activity in other organs in the gastrointestinal tract. This article will review the studies performed to date addressing the physiological role of gastrin outside of the gastric mucosa, with particular emphasis on the information gleaned from gastrin-deficient mice. Most of these studies address the potential role for the less processed forms of gastrin in regulating the proliferation of the colonic mucosa and colon cancers. There is also some data to support a potential role for gastrin in the regulation of the pancreas and the kidney, although the effects of gastrin deficiency on the function of these organs in mice have not yet been rigorously studied.

Transient transcriptional activation of gastrin during sodium butyrate-induced differentiation of islet cells

Transient expression of pancreatic gastrin corresponds to a period of rapid islet cell development. After birth gastrin expression silencing is coincidental with islet cell terminal differentiation, while persistent expression is accompanied with nesidioblastosis and reexpression observed in islet cell tumors. Experiments with transgenic animals suggested that gastrin might act synergistically with growth factors to stimulate islet cell development. The present study intended to establish an in vitro cell culture model to analyse the molecular events controlling gastrin gene activation and repression dependent on islet cell differentiation. Sodium butyrate, a proliferation-arresting compound has previously been shown to differentiate insulinoma cells while increasing insulin production. The present paper demonstrates concomitant transient increase in gastrin mRNA, intracellular and secreted gastrin during sodium butyrate treatment. Increased gastrin expression was due to activation or derepression of gastrin promoter activity as revealed by promoter analyses. This in vitro model mimics the expression pattern of gastrin and insulin observed during fetal islet cell development and provides an excellent tool to analyse the molecular mechanisms controlling gastrin gene activation and selective repression during islet cell differentiation.

Opposite effects of sodium butyrate on CCK mRNA and CCK peptide levels in RIN cells

The effects of the differentiation-inducing agent sodium butyrate on cholecystokinin (CCK) expression was investigated in the pancreatic islet tumor cell line RIN 1056E, which contains high levels of CCK-like immunoreactivity (CCK-LI). Exposure to butyrate for 24 h dose-dependently inhibited cell proliferation and increased the cell content in CCK-LI over the concentration range 0.1-8 mM. With 2 mM butyrate, cell proliferation was decreased by 50% and CCK-LI content was increased by 300%, whereas the level of steady-state CCK mRNA was reduced by 75%. Cycloheximide (10 μg/mL) abolished the sodium butyrate-induced increase in CCK-LI content. This article reports the novel finding that butyrate exerts opposite effects on CCK mRNA and immunoreactivity. The butyrate-induced increase in cellular CCK-LI content is entirely dependent on continuing protein synthesis.

Mutually exclusive interactions between factors binding to adjacent Sp1 and AT-rich elements regulate gastrin gene transcription in insulinoma cells

The gastrin gene is transiently expressed in fetal pancreatic islets during islet neogenesis but then switched off after birth when islet cells become fully differentiated. Previous studies identified a cis-regulatory sequence between -109 and -75 in the human gastrin promoter which binds islet cell-specific activators and a nonspecific repressor and thus may act as a molecular switch. The present study identified another cis-regulatory sequence (-163ACACTAAATGAAAGGGCGGGGCAG-140) which bound two islet nuclear proteins in a mutually exclusive manner, as defined by gel shift competition, methylation interference, and DNase I foot-printing assays. The general transactivator Sp1 recognized the downstream GGGCGGGG sequence, but Sp1 binding was prevented when another islet factor bound to the adjacent AT-rich sequence (CTAAATGA). This gastrin AT-rich element is nearly identical to the binding site (ATAAATGA) for the islet-specific transcription factor beta TF-1. However, the gastrin AT-binding factor appeared to differ from beta TF-1 in its gel mobility shift pattern. Transfections of rat insulinoma cells revealed that mutations which blocked binding to the AT-rich element but allowed Sp1 binding up-regulated transcriptional activity. These results suggest that the gastrin AT-binding factor blocks transactivation by Sp1 and may have a role in the repression of gastrin transcription seen at the end of islet differentiation.

Effects of carbamylcholine chloride on human antral gastrin mRNA levels

The effects of the muscarinic receptor agonist, carbamylcholine chloride (carbachol), on gastrin release and gastrin mRNA levels in human antral mucosa (n = 15) were determined. During a-2-h incubation period, carbachol (10(-6)-10(-4) M) decreased gastrin mRNA levels to 71 +/- 8% (10(-6) M), 40 +/- 8% (10(-5) M), and 33 +/- 5% (10(-4) M) of control levels. Carbachol (10(-5) M) decreased intracellular gastrin (from 1634 +/- 103 to 1272 +/- 126 pg/mg tissue protein), while it increased gastrin release into the medium (from 609 +/- 48 to 918 +/- 68 pg/ml per mg tissue protein). After 6- and 9-h culture, carbachol gradually increased gastrin mRNA levels, by 96 +/- 12% and 126 +/- 23%, respectively. Atropine sulfate (10(-5) M) completely inhibited the carbachol-induced changes. Cycloheximide markedly decreased tissue gastrin concentration, but increased gastrin mRNA levels, whereas it had no effects on gastrin release. These findings suggested that carbachol may have a time-related biphasic action on human antral gastrin biosynthesis.

Activation of gastrin gene transcription in islet cells by a RAP1-like cis-acting promoter element

Gastrin transcription in islet cells is activated by a cis-regulatory sequence containing a binding site for the yeast transcription factor RAP1. The DNA-protein interactions between RAP1 protein and the gastrin DNA element determined by methylation interference assays are identical to those of RAP1 and yeast genes. Point mutations in the gastrin RAP1 binding site, which abolished RAP1 binding, decreased transcriptional activation by this sequence. Islet cells revealed a DNA binding protein with RAP1-like binding specificity. These findings support the conclusion that gastrin transcription is activated in mammalian cells by a RAP1-like transcription factor.

Initiation of gastrin expression during the development of the mouse pancreas

Gastrin expression occurs in the pancreas in only two situations: (1) in cases of gastrinoma and (2) in the embryonic/fetal pancreas. The initiation of gastrin expression in the embryonic pancreas may be recapitulated during gastrinoma tumorigenesis. For this reason, we have tried to identify the point of onset of gastrin expression in the developing pancreas. Previously, determining the point of onset for genes in embryonic tissues has been difficult because of low expression levels and small tissue samples. Using the sensitive polymerase chain reaction assay, we were able to determine, with a sensitivity of 10 molecules of mRNA, the earliest expression of gastrin in the developing pancreas. This expression occurred at 30 somites, or at a gestation of 9.5 to 10 days.

Bovine and feline gastrin cDNA sequences and the amino acid and nucleotide sequence homologies among mammalian species

The complete nucleotide sequences of cDNAs encoding bovine and feline preprogastrins have been cloned from the antral mucosa mRNA. The gastrin mRNA of each animal encodes a preprogastrin of 104 amino acids consisting of a signal peptide, a prosegment of 37 amino acids, and a gastrin 34 sequence, followed by a glycine (the amide donor). The cleavage following a pair of lysine residues yields gastrin 17. We found that pairs of arginine residues flanking gastrin 34, the typical processing site sequence of all other preprogastrins and many peptide hormones, were arginines in the bovine preprogastrin, but the first basic amino acid pair had changed to Arg-Trp (57-58 residues) instead of Arg-Arg in the feline preprogastrin. Comparison of these amino acid and nucleotide sequences with published mammalian sequences showed extensive homology in the coding (63 to 73% amino acid identity) and in the untranslated regions (67 to 89% identity). Prosequence, the most variable region, shows greater amino acid difference between bovine and human preprogastrin (54% identity), and between bovine and rat preprogastrin (54% identity) than between other species (62 to 82% identity).

Expression of the cholecystokinin gene in a human (small-cell) lung carcinoma cell-line

Expression of the cholecystokinin (CCK), gastrin and enkephalin A genes were studied by Northern blot analysis and a library of sequence-specific radioimmunoassays in human cell lines. The human small-cell lung carcinoma line (SCLC) U-1690 expressed moderate levels of CCK mRNA as compared to the human neuroepithelioma cell line SK-N-MC. Neither gastrin nor (pro)enkephalin A mRNAs were detectable in the U-1690 cell line. In contrast, the SCLC-line H-69 expressed Enk A but no CCK mRNA. The radioimmunoassays showed that the CCK mRNA transcript in the SCLC line U-1690 also is translated, and that preproCCK is processed into bioactive, carboxyamidated CCK peptides. Thus, the human small cell carcinoma cell line U-1690 is a useful model for studies of cell-specific CCK gene expression.

Characterization of muscarinic acetylcholine receptors on the rat pancreatic gastrin-producing cell line B6 RIN

The mechanisms of cholinergic stimulation of gastrin cells were studied in the rat pancreatic cell line B6 RIN. Carbachol induced an increase in intracellular Ca2+ and stimulated gastrin release in a dose-dependent manner over the range 10(-5)-10(-3) M. These effects were completely abolished by atropine, suggesting the implication of muscarinic cholinergic receptors. The binding properties of these receptors were investigated. [N-Methyl-3H]scopolamine [( 3H]NMS) binding on cell homogenates was time-dependent, saturable and consistent with a single high-affinity binding class (Kd = 39.5 pM, and Bmax = 7.9 fmol/mg DNA). Carbachol competitively inhibited [3H]NMS binding. The potency of inhibition of [3H]NMS binding by subtype selective antagonists was hexahydrodifenidol greater than pirenzepine greater than AF-DX 116. These results suggest the M3 muscarinic receptors may be involved in the carbachol-induced gastrin release from B6 RIN cells.

Studies of regulation of gastrin synthesis and post-translational processing by molecular biology approaches

We have examined the regulation of gene expression and post-translational processing of progastrin by starvation and feeding in rats. An oligonucleotide complementary to rat preprogastrin cDNA was used in RNA blot and hybridization analysis to measure gastrin mRNA levels. A region-specific antibody raised against the predicted amino acid sequence of the carboxyl terminal extension of progastrin was used for quantitation of progastrin peptides. The effects of starvation and of refeeding on rat antral gastrin mRNA and pro-hormone peptide levels were examined in rats starved for 48 h and after refeeding with a solid meal. Antral gastrin mRNA concentrations decreased to a plateau level (30% of the nonstarved control) after 48 h of starvation. Immunoreactive gastrin concentration decreased threefold, but progastrin processing intermediates did not decrease significantly during fasting. Following refeeding, significant increases in antral mRNA level were detected in 1 h, and peak levels were reached by 2 h (more than two times higher than starved control). There was a rapid and significant decrease in progastrin immunoreactivity within 30 min, followed by a significant increase in gastrin immunoreactivity 2 h after refeeding. These data suggest that rapid increases of blood and tissue gastrin levels in response to food may be associated with increases in both gastrin gene expression and post-translational processing of progastrin.