A new natural hGH variant--17.5 kd--produced by alternative splicing. An additional consensus sequence which might play a role in branchpoint selection

Differential Diagnosis of the Short IGF-I-Deficient Child with Apparently Normal Growth Hormone Secretion

The current differential diagnosis for a short child with low insulin-like growth factor I (IGF-I) and a normal growth hormone (GH) peak in a GH stimulation test (GHST), after exclusion of acquired causes, includes the following disorders: (1) a decreased spontaneous GH secretion in contrast to a normal stimulated GH peak ("GH neurosecretory dysfunction," GHND) and (2) genetic conditions with a normal GH sensitivity (e.g., pathogenic variants of GH1 or GHSR) and (3) GH insensitivity (GHI). We present a critical appraisal of the concept of GHND and the role of 12- or 24-h GH profiles in the selection of children for GH treatment. The mean 24-h GH concentration in healthy children overlaps with that in those with GH deficiency, indicating that the previously proposed cutoff limit (3.0-3.2 μg/L) is too high. The main advantage of performing a GH profile is that it prevents about 20% of false-positive test results of the GHST, while it also detects a low spontaneous GH secretion in children who would be considered GH sufficient based on a stimulation test. However, due to a considerable burden for patients and the health budget, GH profiles are only used in few centres. Regarding genetic causes, there is good evidence of the existence of Kowarski syndrome (due to GH1 variants) but less on the role of GHSR variants. Several genetic causes of (partial) GHI are known (GHR, STAT5B, STAT3, IGF1, IGFALS defects, and Noonan and 3M syndromes), some responding positively to GH therapy. In the final section, we speculate on hypothetical causes.

Decreased Activity of the Ghrhr and Gh Promoters Causes Dominantly Inherited GH Deficiency in Humanized GH1 Mouse Models

Isolated growth hormone deficiency type II (IGHD2) is mainly caused by heterozygous splice-site mutations in intron 3 of the GH1 gene. A dominant-negative effect of the mutant GH lacking exon 3 on wild-type GH secretion has been proposed; however, the molecular mechanisms involved are elusive. To uncover the molecular systems underlying GH deficiency in IGHD2, we established IGHD2 model mice, which carry both wild-type and mutant copies of the human GH1 gene, replacing each of the endogenous mouse Gh loci. Our IGHD2 model mice exhibited growth retardation along with intact cellular architecture and mildly activated endoplasmic reticulum stress in the pituitary gland, caused by decreased GH-releasing hormone receptor (Ghrhr) and Gh gene promoter activities. Decreased Ghrhr and Gh promoter activities were likely caused by reduced levels of nuclear CREB3L2, which was demonstrated to stimulate Ghrhr and Gh promoter activity. To our knowledge, this is the first in vivo study to reveal a novel molecular mechanism of GH deficiency in IGHD2, representing a new paradigm that differs from widely accepted models.

Growth hormone: isoforms, clinical aspects and assays interference

The measurement of circulating concentrations of growth hormone (GH) is an indispensable tool in the diagnosis of both GH deficiency and GH excess. GH is a heterogeneous protein composed of several molecular isoforms, but the physiological role of these different isoforms has not yet been fully understood. The 22KD GH (22 K-GH) is the main isoform in circulation, followed by 20KD GH (20 K-GH) and other rare isoforms. Studies have been performed to better understand the biological actions of the different isoforms as well as their importance in pathological conditions. Generally, the non-22 K- and 20 K-GH isoforms are secreted in parallel to 22 K-GH, and only very moderate changes in the ratio between isoforms have been described in some pituitary tumors or during exercise. Therefore, in a diagnostic approach, concentrations of 22 K-GH accurately reflect total GH secretion. On the other hand, the differential recognition of GH isoforms by different GH immunoassays used in clinical routine contributes to the known discrepancy in results from different GH assays. This makes the application of uniform decision limits problematic. Therefore, the worldwide efforts to standardize GH assays include the recommendation to use 22 K-GH specific GH assays calibrated against the pure 22 K-GH reference preparation 98/574. Adoption of this recommendation might lead to improvement in diagnosis and follow-up of pathological conditions, and facilitate the comparison of results from different laboratories.

Rescue of Isolated GH Deficiency Type II (IGHD II) via Pharmacologic Modulation of GH-1 Splicing

Isolated GH deficiency (IGHD) type II, the autosomal dominant form of GHD, is mainly caused by mutations that affect splicing of GH-1. When misspliced RNA is translated, it produces a toxic 17.5-kDa GH isoform that reduces the accumulation and secretion of wild-type-human GH (wt-hGH). Usually, isolated GHD type II patients are treated with daily injections of recombinant human GH in order to maintain normal growth. However, this type of replacement therapy does not prevent toxic effects of the 17.5-kDa GH isoform on the pituitary gland, which can eventually lead to other hormonal deficiencies. Here, we tested the possibility to restore the constitutive splicing pattern of GH-1 by using butyrate, a drug that mainly acts as histone deacetylase inhibitor. To this aim, wt-hGH and/or different hGH-splice site mutants (GH-IVS3+2, GH-IVS3+6, and GH-ISE+28) were transfected in rat pituitary cells expressing human GHRH receptor (GHRHR) (GC-GHRHR). Upon butyrate treatment, GC-GHRHR cells coexpressing wt-hGH and each of the mutants displayed increased GH transcript level, intracellular GH content, and GH secretion when compared with the corresponding untreated condition. The effect of butyrate was most likely mediated by the alternative splicing factor/splicing factor 2. Overexpression of alternative ASF/SF2 in the same experimental setting, indeed, promoted the amount of full-length transcripts thus increasing synthesis and secretion of the 22-kDa GH isoform. In conclusion, our results support the hypothesis that modulation of GH-1 splicing pattern to increase the 22-kDa GH isoform levels can be clinically beneficial and hence a crucial challenge in GHD research.

Growth hormone doping in sports: a critical review of use and detection strategies

GH is believed to be widely employed in sports as a performance-enhancing substance. Its use in athletic competition is banned by the World Anti-Doping Agency, and athletes are required to submit to testing for GH exposure. Detection of GH doping is challenging for several reasons including identity/similarity of exogenous to endogenous GH, short half-life, complex and fluctuating secretory dynamics of GH, and a very low urinary excretion rate. The detection test currently in use (GH isoform test) exploits the difference between recombinant GH (pure 22K-GH) and the heterogeneous nature of endogenous GH (several isoforms). Its main limitation is the short window of opportunity for detection (~12-24 h after the last GH dose). A second test to be implemented soon (the biomarker test) is based on stimulation of IGF-I and collagen III synthesis by GH. It has a longer window of opportunity (1-2 wk) but is less specific and presents a variety of technical challenges. GH doping in a larger sense also includes doping with GH secretagogues and IGF-I and its analogs. The scientific evidence for the ergogenicity of GH is weak, a fact that is not widely appreciated in athletic circles or by the general public. Also insufficiently appreciated is the risk of serious health consequences associated with high-dose, prolonged GH use. This review discusses the GH biology relevant to GH doping; the virtues and limitations of detection tests in blood, urine, and saliva; secretagogue efficacy; IGF-I doping; and information about the effectiveness of GH as a performance-enhancing agent.

Growth hormone (GH)-releasing hormone increases the expression of the dominant-negative GH isoform in cases of isolated GH deficiency due to GH splice-site mutations

An autosomal dominant form of isolated GH deficiency (IGHD II) can result from heterozygous splice site mutations that weaken recognition of exon 3 leading to aberrant splicing of GH-1 transcripts and production of a dominant-negative 17.5-kDa GH isoform. Previous studies suggested that the extent of missplicing varies with different mutations and the level of GH expression and/or secretion. To study this, wt-hGH and/or different hGH-splice site mutants (GH-IVS+2, GH-IVS+6, GH-ISE+28) were transfected in rat pituitary cells expressing human GHRH receptor (GC-GHRHR). Upon GHRH stimulation, GC-GHRHR cells coexpressing wt-hGH and each of the mutants displayed reduced hGH secretion and intracellular GH content when compared with cells expressing only wt-hGH, confirming the dominant-negative effect of 17.5-kDa isoform on the secretion of 22-kDa GH. Furthermore, increased amount of 17.5-kDa isoform produced after GHRH stimulation in cells expressing GH-splice site mutants reduced production of endogenous rat GH, which was not observed after GHRH-induced increase in wt-hGH. In conclusion, our results support the hypothesis that after GHRH stimulation, the severity of IGHD II depends on the position of splice site mutation leading to the production of increasing amounts of 17.5-kDa protein, which reduces the storage and secretion of wt-GH in the most severely affected cases. Due to the absence of GH and IGF-I-negative feedback in IGHD II, a chronic up-regulation of GHRH would lead to an increased stimulatory drive to somatotrophs to produce more 17.5-kDa GH from the severest mutant alleles, thereby accelerating autodestruction of somatotrophs in a vicious cycle.

Generation of 5 and 17 kDa human growth hormone fragments through limited proteolysis

INTRODUCTION

The reported presence of two fragments of 5 and 17 kDa originating from the 22 kDa human growth hormone (hGH) in blood and tissues, postulated as the sequences AA 1-43 and AA 44-191, has led to the hypothesis of a post-translational proteolytic origin with respect to the abundant 22 kDa variant (AA 1-191). To evaluate this hypothesis, the activity of several endo-proteases on the 22 kDa hGH protein has been evaluated.

METHODS

Proteolysis using pepsin, trypsin, V8-protease, proteinase K and thermolysin were explored under several conditions, including incubation time and pH. Results were monitored by MALDI-TOF and HPLC-ESI mass spectrometry. Proteolytic 5 and 17 kDa fragments were purified through reversed phase HPLC-UV, and their immuno-affinity properties evaluated by surface plasmon resonance.

RESULTS

Thermolysin was shown to target mainly the AA 43-44 bond of the 22 kDa sequence at physiological pH. Interaction studies of the purified fragments with anti-GH antibodies showed some reactivity for the 17 kDa fragment.

CONCLUSIONS

Thermolysin processes hGH generating 5 and 17 kDa fragments, demonstrating the feasibility of this reaction, although the enzyme responsible for this process in humans is still unknown. Specific antibodies should be used to detect these fragments in human specimens, and, at the same time, the 17 kDa fragment could constitute an interference in some hGH immunoassays.

Detection of GH abuse in sport: Past, present and future

Due to its considered performance enhancing effects, human growth hormone (hGH) is abused as a doping agent in sport. Its misuse also carries potentially serious side effects to a person's health. Consequently, hGH and its releasing factors are prohibited in sport, as established in the Prohibited List which is updated and published yearly by the World Anti-Doping Agency (WADA). In order to fight the menace that hGH doping poses to the spirit of sport and to the health of athletes, the sport movement and the anti-doping authorities, initially led by the International Olympic Committee (IOC) and later by WADA, have put substantial efforts into developing tests for its detection. Currently, a primary analytical approach, the isoform differential immunoassay, has been implemented in WADA-accredited laboratories. In parallel, a second, indirect approach for the detection of hGH abuse, based on the quantification of hGH-associated biological markers, has been developed. The final aim is to combine both methodologies to improve the sensitivity and expand the time window to detect doping with hGH. In addition, novel analytical procedures, based on proteomic and genomic technologies as well as the use of mass spectrometry-based methods of detection, are being investigated for future application in hGH anti-doping tests.

Growth hormone isoforms

Human growth hormone (GH) is a heterogeneous protein hormone consisting of several isoforms. The sources of this heterogeneity reside at the level of the genome, mRNA splicing, post-translational modification and metabolism. The GH gene cluster on chromosome 17q contains 2 GH genes (GH1 or GH-N and GH2 or GH-V) in addition to 2(-3) genes encoding the related chorionic somatomammotropin. Alternative mRNA splicing of the GH1 transcript yields two products: 22K-GH (the principal pituitary GH form) and 20K-GH. Post-translationally modified GH forms include N(alpha)-acylated, deamidated and glycosylated monomeric GH forms, as well as both non-covalent and disulfide-linked oligomers up to at least pentameric GH. GH fragments generated in the course of peripheral metabolism may be measured in immunoassays for GH. The GH-N gene is expressed in the pituitary, the GH-V gene in the placenta. Secretion of pituitary GH forms is pulsatile under control from the hypothalamus, whereas secretion of placental GH-V is tonic and rises progressively in maternal blood during the 2nd and 3rd trimester. Pituitary GH forms are co-secreted during a secretory pulse; no isoform-specific stimuli have been identified. There are minor differences in somatogenic and metabolic bioactivity among the GH isoforms, depending on species and assay system used. Both 20K-GH and GH-V have poor lactogenic activity. Oligomeric GH forms have variably diminished bioactivity compared to monomeric forms. GH isoforms cross-react in most immunoassays, but assays specific for 22K-GH, 20K-GH and GH-V have been developed. The metabolic clearance of 20K-GH and GH oligomers is delayed compared to that of 22K-GH. The heterogeneous mixture of GH isoforms in blood is further complicated by the presence of two GH-binding proteins, which form complexes with GH; isoform proportions also vary depending on the lag time from a secretory pulse because of different half-lives. GH forms excreted in the urine reflect monomeric GH isoforms in blood, but constitute only a minute fraction of the GH production rate. The heterogeneity of GH is one important reason for the notorious disparity among assay results. It also presents an opportunity for distinguishing endogenous from exogenous GH.

Growth hormone deficiency and splicing fidelity: two serine/arginine-rich proteins, ASF/SF2 and SC35, act antagonistically

The majority of mutations that cause isolated growth hormone deficiency type II are the result of aberrant splicing of transcripts encoding human growth hormone. Such mutations increase skipping of exon 3 and encode a 17.5-kDa protein that acts as a dominant negative to block secretion of full-length protein produced from unaffected alleles. Previously, we identified a splicing regulatory element in exon 3 (exonic splicing enhancer 2 (ESE2)), but we had not determined the molecular mechanism by which this element prevents exon skipping. Here, we show that two members of the serine/arginine-rich (SR) protein superfamily (ASF/SF2 and SC35) act antagonistically to regulate exon 3 splicing. ASF/SF2 activates exon 3 inclusion, but SC35, acting through a region just downstream of ESE2, can block such activation. These findings explain the disease-causing mechanism of a patient mutation in ESE2 that creates a functional SC35-binding site that then acts synergistically with the downstream SC35 site to produce pathological levels of exon 3 skipping. Although the precedent for SR proteins acting as repressors is established, this is the first example of a patient mutation that creates a site through which an SR protein represses splicing.

Expression, translation, and localization of a novel, small growth hormone variant

A novel transcript of the GH gene has been identified in ocular tissues of chick embryos. It is, however, unknown whether this transcript (small chicken GH, scGH) is translated. This possibility was therefore assessed. The expression of scGH mRNA was confirmed by RT-PCR, using primers that amplified a 426-bp cDNA of its coding sequence. This cDNA was inserted into an expression plasmid to transfect HEK 293 cells, and its translation was shown by specific scGH immunoreactivity in extracts of these cells. This immunoreactivity was directed against the unique N terminus of scGH and was associated with a protein of 16 kDa, comparable with its predicted size. Most of the immunoreactivity detected was, however, associated with a 31-kDa moiety, suggesting scGH is normally dimerized. Neither protein was, however, present in media of the transfected HEK cells, consistent with scGH's lack of a signal sequence. Similar moieties of 16 and 31 kDa were also found in proteins extracted from ocular tissues (neural retina, pigmented epithelium, lens, cornea, choroid) of embryos, although they were not consistently present in vitreous humor. Specific scGH immunoreactivity was also detected in these tissues by immunocytochemistry but not in axons in the optic fiber layer or the optic nerve head, which were immunoreactive for full-length GH. In summary, we have established that scGH expression and translation occurs in ocular tissues of chick embryos, in which its localization in the neural retina and the optic nerve head is distinct from that of the full-length protein.

Laboratory measurement of growth hormone

Growth hormone (GH) measurements are complicated by the heterogeneous nature of GH, as well as by the presence of the GH binding protein in plasma. Several isoforms of GH exist, and specific assays for each are currently either unavailable, impractical, or not clinically indicated. Bioassays include the in vivo assays based on rat weight gain, tibial line widening, or IGF-I generation. In vitro bioassays, based on the proliferation of cell lines expressing the prolactin receptor or GH receptor, are sensitive but prone to nonspecific interference by factors present in serum. Immunoassays (RIA, IRMA, ELISA, and immunofunctional assay design) are widely used in the clinical laboratory because of speed, sensitivity, and convenience. Discrepancies among results rendered by different immunoassays have become more apparent as monoclonal assays have superseded polyclonal assays, presumably because different antibodies recognize different epitopes among the heterogeneous mixture of GH isoforms in serum. Some assays, especially those with short, nonequilibrium incubation times are vulnerable to interference by the GH binding protein present in serum. Recommendations are given for strategies designed to minimize disparity of results obtained by different GH immunoassays applied to serum. Urinary GH measurements, while technically feasible, are of limited clinical utility because of biological variation in urinary GH excretion.

Morphogenic and tumorigenic potentials of the mammary growth hormone/growth hormone receptor system

Due to the characteristics of the luteal phase of the ovarian cycle in the dog, which spans a prolonged time period, this species is a suitable model to study the role of progestins in both normal morphogenic and abnormal tumorigenic processes in the mammary gland. It has been convincingly shown that progestins, including endogenous progesterone, induce the synthesis of growth hormone (GH) in the normal and the tumorous canine mammary gland. The growth hormone receptor (GHR) is also expressed in normal and tumorous canine mammary tissues and in this concise overview we highlight recent advances in our understanding of the significance of the GH/GHR system for mammary gland (patho)biology. In an attempt to unravel the cellular and molecular mechanisms associated with the GH/GHR system, we were able to show that both GH and GHR are differentially expressed in normal canine mammary tissues. Maximum expression of both GH and GHR occurs during the proliferation phase of the tissue, which links the progestin-induced mammary GH synthesis to the progestin-associated proliferation of epithelial cells in the mammary gland. Expression of the GH/GHR system is also present in most canine mammary tumors, albeit that GHR expression may be downregulated in undifferentiated mammary carcinomas. Upon GH stimulation of the GHR-positive CMT-U335 canine mammary tumor cell line, the transcription factors STAT5A and STAT5B become phosphorylated on their tyrosine residues, which is likely to reflect the significance of mammary GH in vivo. Molecular analysis of the canine mammary GHR transcripts by RT-PCR provided evidence for normal and alternative processing of the GHR primary transcript encoding the full-length plasma membrane GHR and at least four putative GH binding proteins (GHBPs), respectively. The translation products from the alternatively spliced GHR transcripts indicate an intact N-terminal ligand binding domain and an unique C-terminal portion, lacking the transmembrane domain and cytoplasmic tail. Thus, these proteins are considered to be able to bind GH, but have lost their signaling potential. The exact biological role of these GHBPs remains to be established, but GHBPs may have a transport function in the endocrine route, regulate the level of biologically available GH locally, or dominant-negatively influence the full-length plasma membrane GHR. In dog mammary cancer specimens strongly reduced levels of alternatively spliced GHR transcripts were found compared to the non-malignant mammary tissue. Notably, expression of both GH and GHR in mammary cancer cells is not restricted to dogs. Recent experiments generated evidence for GH and GHR expression in human breast cancer cells, and also in human prostate cancer cells, which represents another highly prevalent hormone-sensitive human malignancy. In agreement with our findings in the dog, the expression of the hGH-N gene in human mammary cancer cells seemed to correlate positively with their progesterone receptor status, which warrants, in our opinion, a reconsideration of the role of progestins in breast cancer of women. In human prostate cancer cells four different hGH-N transcripts were detected, which encode classical 22 kDa GH and GH-related proteins. Consistent with the findings on the canine GHR, different GHR transcripts in human mammary cancer cells and prostate cancer cells were detected encoding the full-length plasma membrane GHR and putative GHBPs.

Reduced human growth hormone (hGH) bioactivity without a defect of the GH-1 gene in three patients with rhGH responsive growth failure

BACKGROUND

A GH deficiency-like phenotype with normal or high hGH secretion, pathologically low IGF-I serum levels, and catch-up growth under treatment with recombinant hGH is suggestive of the presence of biologically inactive hGH syndrome, whose presumably heterogenous molecular basis is substantially unknown.

DESIGN

Serum samples from patients who fulfilled the above criteria and from controls with idiopathic short stature were measured by polyclonal hGH-RIA, Nb2 rat lymphoma proliferation assay, and hGH immunofunctional assay (IFA). If assays were suggestive of the presence of bioinactive GH, mutational analysis of the hGH-1 gene was performed.

PATIENTS

Three patients were selected because of clinical and biochemical evidence. At the time of diagnosis mean age was 3.4 (2.2, 3.5, 4.5) years, mean height -3.5 (-2.8, -3.6, -4.2) SD score (SDS) and mean growth rate -1.5 (-1.4, -1.5, -1.6) SDS. Mean IGF-I serum levels were -1.9 (-0.7, -2.4, -2.5) SDS and mean IGFBP-3 serum levels -1.2 (-1.1, -1.2, -1.2) SDS. Stimulated and spontaneous GH peaks measured by a polyclonal radioimmunoassay were all above 14 microg/l. GHBP serum levels were normal, and antihGH antibodies were not detected. Therapy with rhGH was effective in causing catch-up growth of the three children with an initial mean growth rate of + 2.9 (+ 1.7, + 2.1, + 5.0) SDS, and normalization of IGF-I (mean: -0.66 SDS: -1.8, - 1.2, + 1.1 SDS) and IGFBP-3 serum levels (mean: + 0.81 SDS: -0.2, + 0.8, + 1.8 SDS).

RESULTS

In comparison to controls, the patients' serum hGH levels were much lower when measured by Nb2 rat lymphoma cell proliferation bioassay (mean: -2.3 SDS, range: -1.7- -4.1) and by the immunofunctional assay (IFA) (-1.5 SDS, range: -0.2- -3.1) than by RIA. Retesting of two of the three patients including an one year break of therapy confirmed the rhGH dependence of growth in spite of normal endogenous GH secretion. Radioactive direct sequencing of both strands of PCR-amplified genomic DNA and cDNA excluded a GH-1 gene mutation in all three children.

CONCLUSION

Mutations of the GH-1 gene are probably not the main genetic defect in children with biologically inactive hGH syndrome. Posttranslational processing of hGH might reduce the biological activity of the normal translation product.

Species-specific alternative splicing of transgenic RNA in the mammary glands of pigs, rabbits, and mice

Gene-farming techniques provide an effective tool for the production of recombinant proteins in livestock. Transgenes consisting of genomic DNA sequences are as a rule more efficiently expressed than those in which the product of interest is encoded by a cDNA. However, the processing of pre-mRNA from genomic constructs may yield unexpected messenger RNAs and subsequently protein variants. We describe the appearance of different alternative mRNA splice patterns of a gene construct in which a mutant human growth hormone (hGH-N) gene is transcriptionally controlled by 2.5 kb of mouse whey acidic protein (WAP2) regulatory sequences in the mammary gland of different livestock species. Compared to the transcription products in transgenic mice harboring the same gene construct and to cell transfection experiments, expression analysis in transgenic pigs and rabbits revealed different mRNA splice patterns with regard to the proportion of the processed transcripts. Apart from already-known physiological mRNA splice products, previously undescribed processed hGH transcripts were observed in these species. Sequence analysis of the transgenes suggests that the species-specific hGH mRNA patterns may be caused by species- and tissue-specific differences in trans-acting splice factors.

Splicing variants of the human growth hormone mRNA: detection in pituitary, mononuclear cells and dermal fibroblasts

The human growth hormone/human chorionic somatomammotropin (hGH/hCS) gene cluster contains five genes: hGH-N, hGH-V, hCS-B, and hCS-L. In this study, the nature of splicing products of their primary transcripts (except hCH-L) was analyzed by nuclease mapping as well as by reverse transcription-polymerase chain reaction (RT-PCR) experiments All the previously described hGH-N mRNAs encoding the normal 22-K growth hormone, the 20-K variant as well as a transcript lacking the third exon were found in pituitary tissue and pituitary tissue and in transiently transfected human 293-S cells. In addition, splicing products lacking either exons 3 and 4 exons 2,3 and 4 were found in both tissues. In accordance to previously reported data, the hGH-V, the hCS-A and the hCS-B genes which are expressed in placental tissue give rise to the 22-K mRNA but not to 20-K mRNA. Furthermore, no hCS mRNA arising from skipping of exon 3 was present, whereas mRNAs arising from ligation of exon 2 to exon 5 and of exon 1 to exon 5 were clearly detectable. The various hGH cDNas were expressed in vivo and screened for lactogenic activity. Only the 22-K and the 20-K variant were active in this assay. All of the hGH-N-derived differentially processed RNAs were found in cell lines of lymphoid (Hut-78) and of myelomonocytic type (U937), which had been recently described to secrete growth hormone. Interestingly, RT-PCR analysis allowed the determination of hGH-N transcripts in dermal fibroblasts. This finding underlines the importance of growth hormone in influencing immune system development and further suggests possible autocrine/paracrine regulatory loops in skin tissue.

Is the osteopetrotic (op/op mutant) mouse completely deficient in expression of macrophage colony-stimulating factor?

The op/op mouse has a mutation in the macrophage colony-stimulating (CSF-1) gene. The phenotype of gross deficiency in the macrophage and osteoclast lineages corrects significantly with age, suggesting that other factors can substitute for CSF-1. This review examines the evidence that the op/op mouse is completely CSF-1 deficient and considers the possibility that alternative splicing within the CSF-1 gene might bypass the mutation, yielding an incompletely penetrant phenotype.

Nitric oxide signaling to iron-regulatory protein: direct control of ferritin mRNA translation and transferrin receptor mRNA stability in transfected fibroblasts

Iron-regulatory protein (IRP) is a master regulator of cellular iron homeostasis. Expression of several genes involved in iron uptake, storage, and utilization is regulated by binding of IRP to iron-responsive elements (IREs), structural motifs within the untranslated regions of their mRNAs. IRP-binding to IREs is controlled by cellular iron availability. Recent work revealed that nitric oxide (NO) can mimic the effect of iron chelation on IRP and on ferritin mRNA translation, whereas the stabilization of transferrin receptor mRNA following NO-mediated IRP activation could not be observed in gamma-interferon/lipopolysaccharide-stimulated murine macrophages. In this study, we establish the function of NO as a signaling molecule to IRP and as a regulator of mRNA translation and stabilization. Fibroblasts with undetectable levels of endogenous NO synthase activity were stably transfected with a cDNA encoding murine macrophage inducible NO synthase. Synthesis of NO activates IRE binding, which in turn represses ferritin mRNA translation and stabilizes transferrin receptor mRNA against targeted degradation. Furthermore, iron starvation and NO release are shown to be independent signals to IRP. The posttranscriptional control of iron metabolism is thus intimately connected with the NO pathways.

Growth Hormone What is it and what does it do?

The evidence is now irrefutable that growth hormone (GH), long thought to be a single substance, is actually a mixture of several different forms. These multiple forms must be a consideration in any physiologic study if an accurate evaluation of the actions of GH is to be made.

Structure of the bovine placental lactogen gene and alternative splicing of transcripts

Preliminary evidence for heterogeneity among bovine placental lactogen (bPL) transcripts prompted characterization of additional cDNA clones and isolation of the bPL gene. Nucleotide replacements were detected among sequenced cDNAs isolated from different animals at a total of 11 positions. Four of these predict amino acid substitutions, which are generally conservative in nature. In addition, truncated forms of bPL are predicted by the sequences of two cDNAs in which alternative splicing is evident. In one case, exon III is deleted with no effect on reading frame. However, in the other instance, a shifted reading frame resulting in a novel carboxyl terminus is generated by use of an alternative 5' splice donor site within exon IV. Nuclease protection analysis demonstrated that these variant transcripts comprise about 13% of the total bPL mRNA present in the midgestation placenta. Characterization of the bPL gene revealed that it is similar in structure to other members of this gene family. Sequence analysis demonstrated that the 5'-flanking region of the bPL gene has diverged considerably from the bovine prolactin and growth hormone genes, but shares homology with the previously characterized gene corresponding to bovine prolactin-related cDNAI (bPRCI). Primer extension as well as nuclease protection analysis indicated that a single transcription start site was utilized in the fetal placenta at midgestation. Exact matches to the consensus sequences for response elements for thyroid hormone and transcription factor AP-2 were located 50 and 70 bp, respectively, upstream from the transcription start site in cloned genomic 5'-flanking sequences. We conclude that the bovine placenta may express more than a single placental lactogen product, raising the possibility of alternative hormones with distinct biological properties, and that the bPL gene may share regulatory elements with the gene for the distinct prolactin-related product, bPRCI, based on similarities in the 5' regions of the corresponding genes.

Prolactin and growth hormone: molecular heterogeneity and measurement in serum

The pituitary hormones prolactin and growth hormone are related single-chain polypeptides. Both hormones exist in the circulation in several molecular forms, and this heterogeneity may account for some of the complex and sometimes contradictory actions, in vivo and in vitro, of both hormones. It may also lead to problems with quantitation by immunoassays and discrepancies between the results given by assays using different antibodies. Modified forms of the hormones may have markedly different activity in bioassays from that of the parent hormone, but the clinical significance of this is unclear. In this review we summarize what is known about the molecular heterogeneity of the hormones and briefly discuss the implications for clinical biochemists.

Antibodies to newly recognized murine 13-18 KDa pituitary peptides crossreact with growth hormone and prolactin from several species, including man

Recently we identified five novel peptides of Mr 13,000 to 18,000 (designated P13, P14, P16, P17, and P18 according to approximate Mr) in the anterior pituitary gland of rat and man that appeared related to GH and PRL in regulation and structure. We have now raised polyclonal antibodies in the rabbit to four of these peptides--P13, P14, P17 and P18--isolated from rat anterior pituitary; the rabbit injected with P16 did not produce antibodies. Besides reacting with their respective immunogens, antisera to all four peptides crossreacted, quite unexpectedly, with human GH and with human, porcine, and ovine PRL. Antiserum to P17, in addition, crossreacted very strongly with rat PRL, while P18 antiserum crossreacted not only with human GH but also with its 20K and cleaved variants. These results provide strong evidence for the structural relatedness of these peptides to GH and PRL, and raise the possibility that they may be related functionally as well.

Expression of the rat growth hormone gene in transfected CV-1 cells

Expression of the cloned rat growth hormone gene was studied in transfected CV-1 cells. Cell lines expressing rat growth hormone synthesized and secreted the 22 kilodalton form and the 20 kilodalton variant. The results confirm that the 20 kilodalton variant arises from an alternatively spliced mRNA rather than expression from a variant gene in rat. The 5' end of the mRNA from expressing cell lines was located upstream of the normal initiation site in rat. Transcription initiated within a 5' flanking AT rich sequence. Results presented indicate that transcription from normally silent promoter or promoter-like sequences dictated rat growth hormone gene expression in transfected cells. Finally, no hormone was expressed by CV-1 cells transfected with a plasmid containing the rat growth hormone gene in the same transcriptional reading frame as the neomycin resistance gene of pSV2neo indicating an effect of cloning orientation on expression in CV-1 cells.

Pairing properties of bromouracil and repair of bromouracil-containing DNA. Possible utilization of bromodeoxyuridine triphosphate for site-directed mutagenesis

5-Bromo-2'-deoxyuridine triphosphate (Br-dUTP) and dTTP are used interchangeably for DNA synthesis in vitro by the Klenow fragment of Escherichia coli DNA polymerase I. When DNA containing Br-dUMP instead of dTMP at a few preselected sites is transfected into competent bacteria, no mutation occurs, indicating that in vivo E. coli DNA polymerase always places a dAMP residue in front of any unrepaired Br-dUMP residue. On the other hand, in vitro Br-dUTP can also replace dCTP, but only with difficulty: when dCTP is absent, Br-dUMP can be forced in front of a dGMP residue, but the Klenow polymerase pauses before and after addition of Br-dUMP. Transfection into E. coli of the substituted DNA leads to the expected G----A transitions. These mutations can easily be targeted by using a suitable primer and the correctly chosen mix of deoxynucleoside triphosphates containing Br-dUTP. When Br-dUMP has been placed in front of a dGMP residue, the mutation yield is not 100%, showing a partial repair of the transfected DNA before it is replicated. Advantage can be taken of this partial repair to prepare a set of different mutations within a target region in a single experiment.

Molecular cloning of cDNA encoding 20 kDa variant human growth hormone and the alternative splicing mechanism

cDNA encoding the 20 kDa variant form of human growth hormone has been cloned, and its sequence analysis verified the alternative splicing mechanism for the mRNA synthesis. The cDNA sequence lacked 45 nucleotides corresponding to the sequence of 15 amino acids in the 22 kDa form of growth hormone. The cDNA clones for the 20 kDa variant hormone had the homogeneous 5'-ends, while the clones for the 22 kDa form showed a minor heterogenity, having two transcription initiation sites. The percentage of 20 kDa variant cDNA clones was approx. 7.7% of the total human growth hormone cDNA clones, consistent with the contents of 20 kDa hormone protein in human anterior pituitary and plasma.