Effects of a leucine analog on growth hormone processing and secretion by cultured cells

Bovine and rat growth hormones (bGH and rGH, respectively) possess signal peptides that direct the hormone to the secretory pathway and are proteolytically cleaved prior to secretion. Previous in vitro translation studies indicated that incorporation of the polar leucine analog beta-hydroxyleucine into de novo synthesized polypeptides inhibits signal peptide function. To test the effects of this analog on GH secretion by cultured animal cells, transfections of mouse L-cells with a bGH expression plasmid or metabolic labeling of endogenous rGH in anterior pituitary cells was performed in the absence or presence of beta-hydroxyleucine. Transient expression of bGH in mouse L-cells or endogenous expression of rGH in anterior pituitary cells resulted in an accumulation of GH in the culture medium. Treatment with beta-hydroxyleucine resulted in a block in secretion as evidenced by an accumulation of GHs within these cells. Amino-terminal sequencing of the intracellular form of the analog-substituted GHs demonstrated accurate signal peptide cleavage. In contrast, in vitro translations of bGH RNA performed in the presence of beta-hydroxyleucine and microsomal membranes resulted in the inhibition of signal peptide cleavage. The results suggest that beta-hydroxyleucine can uncouple signal peptide processing and protein secretion in cultured cells.     

Conversion of bovine growth hormone cysteine residues to serine affects secretion by cultured cells and growth rates in transgenic mice.

GHs have been found to possess two disulfide bonds. We set out to determine the importance of bovine (b) GH's disulfide bonds relative to the ability of the hormone to be secreted by cultured cells in vitro and to promote growth in transgenic mice. We have generated six mutated bGH genes that encode serine (Ser) substitutions for cysteines (Cys). These mutated genes were used to generate bGH analogs in which either one or both disulfide bonds are destroyed. When the small loop of bGH was destroyed (Cys181-Ser or Cys189-Ser), the bGH analogs were found to be secreted by mouse L-cells at levels comparable to those of wild-type bGH. However, secretion was drastically reduced when the large loop was abolished (Cys53-Ser or Cys164-Ser). An immunofluorescence study of these bGH analogs revealed two distinct patterns of subcellular localization. Bovine GH analogs with mutations in the small loop demonstrated a perinuclear distribution similar to that of wild-type bGH, but analogs containing a disrupted large loop revealed a uniform cytoplasmic distribution pattern. When these mutated bGH genes were individually introduced into transgenic mice, only those animals that expressed bGH analogs with the large loop intact demonstrated a growth-enhanced phenotype. Transgenic mice that expressed bGH analogs lacking the large loop showed growth rates similar to those of nontransgenic mice. These results suggest that the integrity of the large loop, but not that of the small loop, is essential for the growth-enhancing activity of bGH in transgenic mice.     

Multihormonal regulation of the human, rat, and bovine growth hormone promoters: differential effects of 3',5'-cyclic adenosine monophosphate, thyroid hormone, and glucocorticoids

We have analyzed the effects of a variety of hormones on activity of the rat GH (rGH), human GH, (hGH), and bovine GH (bGH) promoters. After transient transfection of rat pituitary tumor cells, all three promoters are induced by addition of 8-bromo-cAMP. Sequences required for the cAMP responsiveness of the hGH and rGH promoter lie within 183 base pairs of the mRNA start site. Although the rGH promoter is thyroid hormone (T3) responsive in this system, a construct containing 2.7 kilobases of the hGH promoter 5'-flanking sequences is not. Since we also found that the bGH promoter is T3 responsive in these cells, the hGH results are not likely to be due to a species specific factor required for induction in rat pituitary cells. The hGH promoter is weakly induced by dexamethasone whereas the rGH promoter does not respond to glucocorticoids. The hGH and rGH promoters are not responsive to TRH. These results illustrate the potential heterogeneity in hormonal responses of the same gene in different species.     

Synthesis of bovine growth hormone in primates by using a herpesvirus vector.

A strain of herpesvirus saimiri containing a bovine growth hormone (bGH) gene under the control of the simian virus 40 (SV40) late-region promoter was constructed. This strain, bGH-Z20, was replication competent and stably harbored the bGH gene upon serial passage. Nonpermissive marmoset T cells persistently infected with bGH-Z20 produced a 0.9-kilobase RNA which contained all of the bGH exon sequences and appeared to initiate within the SV40 promoter region. However, in permissively infected owl monkey kidney cells, RNAs containing growth hormone sequences appeared to initiate from herpesvirus saimiri promoters positioned upstream from the SV40-growth hormone gene. Persistently infected T cells in culture secreted 500 ng of bGH protein per 10(6) cells per 24 h during the several months of testing. The secreted protein was 21 kilodaltons, the size of authentic bGH. New World primates experimentally infected with bGH-Z20 produced circulating bGH and developed immunoglobulin G antibodies directed against bGH. Because herpesviruses characteristically remain latent in the infected host, these observations suggest a means for replacing gene products missing or defective in hereditary genetic disorders.     

Analysis of glucocorticoid unresponsive cell variants using a mouse glucocorticoid receptor complementary DNA clone

We have used a glucocorticoid receptor cDNA isolated from a mouse lymphoma cell line to characterize receptor mRNA and genomic sequences present in wild type and mutant rat hepatoma (HTC) and mouse thymoma (S49 and WEHI7) cells. Wild type rat and mouse cell lines contain two receptor mRNAs, 5 and 7 kilobase pairs (kb) in length, which differ in the length of their 3'-untranslated regions. Levels of receptor mRNA present in mutant HTC, WEHI7, and S49 cells of the r- (receptorless) phenotype are decreased compared to wild type cells. This decreased level of receptor mRNA parallels the decreased level of total immunoreactive receptor protein found in these cells. S49 nt- (nuclear transfer minus) cells contain receptor mRNA levels which parallel their hormone binding and immunoreactive receptor levels. Cells of the r- and nt- phenotype contain no detectable deletions or rearrangements of the receptor gene. We conclude that r- cells have lesions which affect the expression of receptor mRNA. Surprisingly, HTC cells of the r- phenotype differ from WEHI7 and S49 r- cells in that HTC r- cells contain a lower level of receptor DNA than does their parental wild type cell line. Although these cells may contain multiple lesions, it appears that loss of receptor genomic sequences is responsible, in part, for the phenotype of the HTC r- cells. The S49 nti (nuclear transfer increase) cells produce glucocorticoid receptors of molecular weights 40,000 and 94,000. These cells produce, in addition to the wild type 5- and 7-kb receptor mRNAs, two other receptor messages 5.5 and 3.5 kb in length. RNA blot analysis using various portions of our receptor cDNA indicates that these are 5' truncated messages and suggests that the 40-kDa nti receptor is truncated at its NH2-terminal end. These data also indicate that the hormone and DNA-binding regions of the receptor are located in the COOH-terminal half of the protein.     

Human insulin-like growth factor I receptor 950tyrosine is required for somatotroph growth factor signal transduction.

Insulin-like growth factor I (IGF-I), a growth hormone (GH)-dependent growth factor exerts feedback regulation of GH by inhibiting GH gene expression. IGF-I inhibition of GH secretion is enhanced 3-5-fold in GC rat pituitary cells overexpressing the wild type 950Tyr human IGF-I receptor which autophosphorylates appropriately. To determine the critical amino acid sequence responsible for IGF-I signaling, insertion, deletion, and site-directed mutants were constructed to substitute for 950Tyr in exon 16 of the human IGF-I receptor beta-subunit transmembrane domain. All mutant transfectants bound IGF-I with a similar Kd to untransfected cells but had markedly increased (7-34-fold) IGF-I-binding sites. GH responsiveness to IGF-I was tested in mutant transfectants. Overexpressed site-directed and insertion mutant IGF-I receptors exhibited a modest suppressive effect on GH in response to the IGF-I ligand, similar to that observed in untransfected cells. Deletion mutant (IG-FIR delta 22) (amino acid 944-965) did not transduce the IGF-I signal to the GH gene. Site-directed and insertion mutants therefore did not enhance the IGF-I response of the endogenous rat receptor, unlike the 950Tyr wild type transfectants which enhanced the IGF-I signal. All mutant transfectants, except the deletion mutant, internalized radioactive ligand similarly to 950Tyr wild type transfectants. 950Tyr of the human IGF-I receptor is therefore required for IGF-I signal transduction in the pituitary somatotroph, but not for IGF-I-mediated internalization.     

Characterization of wild type and mutant glucocorticoid receptors from rat hepatoma and mouse lymphoma cells

Using a combination of immunological blotting techniques and hormone affinity labeling, we have characterized the glucocorticoid receptors present in wild type and mutant rat hepatoma (HTC) and mouse thymoma (S49 and WEHI7) cells. Mutant HTC and WEHI7 cells of the receptorless phenotype, which contain greatly reduced amounts of glucocorticoid hormone binding activity, show parallel decreases in immunoreactive material using a monoclonal antibody raised against the rat liver glucocorticoid receptor. This indicates that these receptorless mutant cells harbor defects in either the production or accumulation of receptor protein. Quantitation of immunoreactivity and hormone binding activity present in wild type and mutant S49 cells indicates that these cells contain significantly more immunoreactive material than hormone binding activity. We conclude that S49 cells produce, in addition to their well characterized wild type or mutant receptors, a mutant receptor from a second allele which is of wild type size, is immunologically reactive, but is unable to bind hormone. The S49 mutant cell line nti (nuclear transfer increase) contains a glucocorticoid receptor which has a molecular weight of 40,000, while the wild type receptor has a molecular weight of 94,000. Affinity labeling of glucocorticoid receptors in nti cells with [3H]dexamethasone mesylate indicates that nti cells do not contain wild type sized precursor molecules which bind hormone, nor do they contain immunoreactive fragments of a molecular mass smaller than 94 kDa. It is proposed that the 40-kDa nti receptor is produced as a truncated protein most likely resulting from a nonsense mutation or from a truncated messenger RNA.     

Ontogenesis of hepatic growth hormone receptors in the rat

Detailed ontogenic studies of the binding of human (hGH) and bovine growth hormone (bGH) have been performed in liver preparations from male and female rats during the neonatal, weanling, pre- and post-pubertal periods. Specific binding of both hormones was readily detected at all ages, with no apparent interference due to occupancy by endogenous hormones. No sex difference in binding was observed prior to weaning (22 days) for hGH, which binds to both somatotrophic and lactogenic sites. However, after weaning a marked sex-related dissociation in the pattern of binding did occur, with female rats binding 3-4 times more hGH than in the pre-weaning period and male rats binding hGH to only half their pre-weaning levels. A very similar pattern was seen for binding of bGH (which binds only to somatotrophic sites) except that in male rats, the post-weaning levels did not fall. Binding patterns for either hGH or bGH prior to weaning did not mirror the known age-related pattern of circulating rat GH levels, suggesting the absence of a definitive auto-regulation system for the GH-GH receptor system under normal circumstances in vivo. The possible role of the weaning process per se in the post-weaning changes of GH binding seen in male and female rats still requires elucidation.     

Growth hormone gene expression in eukaryotic cells directed by the Rous sarcoma virus long terminal repeat or cytomegalovirus immediate-early promoter

The cytomegalovirus (CMV) immediate-early (IE) gene-regulatory region was found to be three- to fourfold more efficient than the Rous sarcoma retroviral long terminal repeat (LTR) in promoting expression of the bovine growth hormone (bGH) gene by rat GH3 cells.     

Molecular cloning of DNA complementary to bovine growth hormone mRNA

We have cloned DNA complementary to mRNA coding for bovine growth hormone (bGH). Double-stranded DNA complementary to bovine pituitary mRNA was inserted into the Pst I site of plasmid pBR322 by the dC x dG tailing technique and amplified in E. coli x 1776. A recombinant plasmid containing bGH cDNA ws identified by hybridization to cloned rat growth hormone cDNA. It contains the entire coding and 3'-untranslated regions and 31 bases in the 5'-untranslated region. Nucleotide sequence analysis determined the sequence of the 26-amino acid signal peptide and confirmed the published amino acid sequence of the secreted hormone at all but 2 residues. Codon usage is nonrandom, with 81.7% of the codons ending in G or C. The nucleotide sequence of bGH mRNA is 83.9% homologous with rat GH mRNA and 76.5% homologous with human GH mRNA, while the respective amino acid sequence homologies are 83.5% and 66.8%.