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Biomedical subjects

M J Low

Publications and source records attributed to M J Low.

87 records · Page 5Linked to original sources
PubMed1988

A Moloney MLV-rat somatotropin fusion gene produces biologically active somatotropin in a transgenic pig.

Expression of a Moloney murine leukemia virus (MLV) rat somatotropin fusion gene was examined in a transgenic pig. The fusion gene was integrated in a single site within the genome in a tandem array with approximately eight copies per cell. The integrated in a single site within the genome in a tandem array with approximately eight copies per cell. The integrated MLV-rat somatotropin fusion gene produced high levels of circulating rat somatotropin and resulted in an elevation in the circulating levels of insulin-like growth factor I. Although there was no increase in the rate of growth of the transgenic animal during the rapid growth phase, several phenotypic changes were evident. Skeletal growth was markedly increased and fat deposition was reduced throughout the animal. Blood glucose levels were elevated without ketosis. Northern blot analyses of rat somatotropin RNA revealed that expression of the fusion gene was highest in the spleen, lung, intestine, lymph nodes, and bone marrow. These results show that the MLV promoter can be used to express high levels of biologically active rat somatotropin in transgenic swine.

Animals↗
PubMed1987

Cell-specific processing of preprosomatostatin in cultured neuroendocrine cells.

We have previously found that preprosomatostatin is processed accurately to both somatostatin-14 and somatostatin-28 in pituitary gonadotrophs of transgenic mice. The foreign somatostatin peptides have been shown to enter the regulated secretory pathway of these cells. To determine whether accurate preprosomatostatin processing can occur in any neuroendocrine cell, we introduced preprosomatostatin cDNA expression vectors into several different neuroendocrine cell lines. We found that prosomatostatin was cleaved efficiently to somatostatin-14 and somatostatin-28 in RIN 5F and AtT20 cells, but not in GH4 or PC12 cells. The ability of a particular cell type to process prosomatostatin did not correlate with cellular storage capacity and was independent of the level of biosynthesis of the precursor. These data suggest that prosomatostatin processing requires specific pathways which are present in some neuroendocrine cells, but not in others.

Adrenal Gland Neoplasms↗
PubMed1986

Somatostatin is targeted to the regulated secretory pathway of gonadotrophs in transgenic mice expressing a metallothionein-somatostatin gene.

The pituitaries of transgenic mice that express a metallothionein-somatostatin fusion gene contain high concentrations of somatostatin-14 exclusively in the gonadotrophic cells. The purpose of this study was to determine whether somatostatin expressed from the foreign fusion gene enters the normal secretory pathway within these cells. Immuno-gold labeling of serial thin sections localized somatostatin to the secretory granules of gonadotropin-producing cells. The gonadotroph-specific hypophysiotropic factor, luteinizing hormone-releasing hormone caused a dose-dependent secretion of somatostatin when applied to primary pituitary cultures from these mice. Growth hormone-releasing hormone, thyrotropin-releasing hormone, corticotropin releasing factor, and dopamine did not affect somatostatin secretion. These experiments demonstrate that a neurosecretory peptide encoded by a foreign gene can enter the regulated secretory pathway of pituitary cells from transgenic mice.

Animals↗
PubMed1986

Gonadotroph-specific expression of metallothionein fusion genes in pituitaries of transgenic mice.

Transgenic mice expressing a metallothionein-somatostatin fusion gene contain high concentrations of somatostatin in the anterior pituitary gland, a tissue that does not normally produce somatostatin. Immunoreactive somatostatin within the anterior pituitaries was found exclusively within gonadotrophs. Similarly, a metallothionein-human growth-hormone fusion gene was also expressed selectively in gonadotrophs. It is proposed that sequences common to the two fusion genes are responsible for the gonadotroph-specific expression.

Animals↗
PubMed1986

Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells.

Although the factors controlling the secretion of the neuropeptide somatostatin have been extensively studied, little is known about the mechanisms that control somatostatin biosynthesis. Somatostatin secretion is regulated by numerous agents that increase intracellular levels of cAMP. We sought to determine whether cAMP also regulates somatostatin mRNA accumulation. We found that forskolin elicited an increase in somatostatin secretion and mRNA levels in primary cultures of rat diencephalic cells. Another secretagogue, KCl, was as effective as forskolin in causing somatostatin secretion but had no effect on mRNA accumulation. Somatostatin expression in fibroblast cells transfected with the somatostatin gene was also regulated by forskolin. These results demonstrate that somatostatin mRNA accumulation can be regulated through a cAMP-dependent pathway, that this pathway is operative in heterologous cells transfected with the somatostatin gene, and that stimulation of somatostatin secretion and mRNA accumulation can be uncoupled from one another.

Animals↗
PubMed1985

Biosynthesis of rat preprosomatostatin.

The biologically active forms of somatostatin, somatostatin-14 (SS-14) and somatostatin-28 (SS-28) arise by post-translational cleavage of prosomatostatin. Prosomatostatin in turn is derived from a larger precursor, preprosomatostatin. We have previously reported the structure of a complementary DNA molecule encoding rat preprosomatostatin. The nucleotide sequence of this cDNA indicated that SS-14 and SS-28 are located at the carboxy-terminus of a 116 amino acid precursor. At the amino-terminus of the precursor is a hydrophobic region characteristic of a leader or pre-sequence. Sequential Edman degradations of cell-free translation products synthesized in the presence of microsomal membranes indicate that preprosomatostatin is cleaved within the endoplasmic reticulum to form prosomatostatin, a precursor of 92 amino acids. To begin to elucidate the factors which regulate the expression of the rat somatostatin gene, we have determined the sequence of the gene isolated from recombinant bacteriophage libraries. The gene spans 1.2 kilobases in length and is interrupted within the coding sequence of prosomatostatin by a single intron of 630 bases. A variant of the Goldberg-Hogness promotor, TTTAAA, is located 31 bases upstream from the transcriptional start point. A repetitive sequence was identified in the 5' region of the gene within 650 bases of the promoter. The nucleotide sequence of this region reveals an alternating GT sequence 42 bases in length characteristic of DNA with Z-forming potential. Such sequences are thought to influence the expression of other eukaryotic genes.

Amino Acid Sequence↗
PubMed1985

Tissue-specific posttranslational processing of pre-prosomatostatin encoded by a metallothionein-somatostatin fusion gene in transgenic mice.

The somatostatins are neuropeptides of 14 and 28 amino acids that inhibit the release of growth hormone and other hypophyseal and gastrointestinal peptides. These neuropeptides are cleaved posttranslationally from a common precursor, pre-prosomatostatin. We report here the production and processing of pre-prosomatostatin by transgenic mice carrying a metallothionein-somatostatin fusion gene. The most active site of somatostatin production, as determined by hormone concentrations in the tissues, is the anterior pituitary, a tissue that does not normally synthesize somatostatin-like peptides. Anterior pituitary processed pre-prosomatostatin almost exclusively to the two biologically active peptides, somatostatin-14 and somatostatin-28, whereas the liver and kidney synthesized much smaller quantities of predominantly a 6000 dalton somatostatin-like peptide. The growth of the transgenic mice was normal despite high plasma levels of the somatostatin-like peptides. These studies indicate that proteases which cleave prosomatostatin to somatostatin-28 and somatostatin-14 are not specific to tissues that normally express somatostatin.

Animals↗
PubMed1984

High plasma levels of immunoreactive somatostatin in transgenic mice expressing a metallothionein-somatostatin fusion gene.

To test the hypothesis that processing of pre-prosomatostatin (pre-proSS) can be accomplished by cells that do not normally synthesize the precursor, we have introduced the rat pre-proSS gene under control of the mouse metallothionein promoter into the germ line of mice. Four of the 11 resultant transgenic mice had markedly elevated plasma levels of somatostatin-like immunoreactivity (SLI); however, their growth was identical to control littermates. Liver contained 263 +/- 89 pg of SLI/mg of protein and kidney had 152 +/- 19 pg/mg. Gel filtration chromatography of tissue extracts resolved one major 6000-dalton peak of SLI and three minor peaks of 8500, 3000, and 1600 daltons. The latter two corresponded in elution position to synthetic somatostatin-28 (S-28) and somatostatin-14 (S-14). Almost all of the plasma SLI corresponded in size to the 6000-dalton peptide. These findings indicate that a metallothionein-somatostatin fusion gene was successfully integrated into the mouse genome and was expressed in tissues that do not normally synthesize pre-proSS. Pre-proSS was processed to S-28 and S-14 but atypical processing to a 6000-dalton peptide also occurred.

Animals↗
PubMed1980

Effects of immunizing rabbits against oestradiol-6-oxime-BSA on ovarian follicular development.

Female rabbits were actively immunized either against bovine serum albumin (BSA) or oestradiol-6-oxime BSA. When the antibody titres were elevated they were sacrificed and various parameters measured. Uterine weights were significantly lower, and a large number of haemorrhagic and some cystic follicles were present in the ovaries of the oestradiol immunized group. Increased serum gonadotrophins, testosterone, oestradiol, and progesterone levels as well as greater serum binding of oestradiol were also noted. Examination of the response of isolated follicles to gonadotrophin treatment in vitro indicated that oestradiol immunization decreased the ability of isolated follicles to form androgen and oestrogen following gonadotrophin treatment; however, progesterone levels were high regardless of the presence or absence of gonadotrophins. These data suggest that oestradiol plays an important role in follicular development.

Animals↗
PubMedDate not supplied

Biosynthesis of pancreatic islet hormones.

We have outlined the various strategies used to characterize the precursors of three pancreatic islet hormones--somatostatin, pancreatic polypeptide and VIP. In each case, isolation of the cDNA clones was facilitated by the use of gastrointestinal tissues that were extremely rich in specific mRNA. Characterization of the structures of the precursors is clearly only the first step in understanding the regulation of pancreatic hormone biosynthesis. It is likely that the availability of the cDNA clones will allow us to define the actual mechanisms underlying hormone production within the pancreas.

Animals↗
PubMedDate not supplied

Expression of IGF system genes during T-antigen driven pituitary tumorigenesis.

Expression of IGF-I, IGF-II, the Type-I IGF receptor and six IGF binding proteins were examined in three different T-ag-driven mouse tumors. Unlike the widespread expression of IGF-II in pancreatic beta-cell tumors, IGF-II was not widely expressed in the two different pituitary tumors examined indicating that a mechanism independent of focal IGF-II expression can also drive T-antigen tumorigenesis. In addition, multiple IGF binding proteins were expressed in all three tumor types. This expression, however, was generally heterogeneous with no specific changes to indicate a required role for any IGF binding protein in T-antigen tumorigenesis.

Animals↗