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

Recovery using "float" from high intensity stress on growth hormone-like molecules in resistance trained men

OBJECTIVE

The purpose of this study was to examine the influence of a novel "floatation-restricted environmental stimulation therapy" (floatation-REST) on growth hormone responses to an intense resistance exercise stress.

DESIGN

Nine resistance trained men (age: 23.4 ± 2.5 yrs.; height: 175.3 ± 5.4 cm; body mass: 85.3 ± 7.9 kg) completed a balanced, crossover-controlled study design with two identical exercise trials, differing only in post-exercise recovery intervention (i.e., control or floatation-REST). A two-week washout period was used between experimental conditions. Plasma lactate was measured pre-exercise, immediately post-exercise and after the 1 h. recovery interventions. Plasma iGH was measured pre-exercise, immediately-post exercise, and after the recovery intervention, as well as 24 h and 48 h after the exercise test. The bGH-L was measured only at pre-exercise and following each recovery intervention.

RESULTS

For both experimental conditions, a significant (P ≤ 0.05) increase in lactate concentrations were observed immediately post-exercise (~14 mmol • L-1) and remained slightly elevated after the recovery condition. The same pattern of responses was observed for iGH with no differences from resting values at 24 and 48 h of recovery. The bGH-L showed no exercise-induced changes following recovery with either treatment condition, however concentration values were dramatically lower than ever reported.

CONCLUSION

The use of floatation-REST therapy immediately following intense resistance exercise does not appear to influence anterior pituitary function in highly resistance trained men. However, the lower values of bGH suggest dramatically different molecular processing mechanisms at work in this highly trained population.

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.

Mutations of Pre-mRNA Splicing Regulatory Elements: Are Predictions Moving Forward to Clinical Diagnostics?

For more than three decades, researchers have known that consensus splice sites alone are not sufficient regulatory elements to provide complex splicing regulation. Other regulators, so-called splicing regulatory elements (SREs) are needed. Most importantly, their sequence variants often underlie the development of various human disorders. However, due to their variable location and high degeneracy, these regulatory sequences are also very difficult to recognize and predict. Many different approaches aiming to identify SREs have been tried, often leading to the development of in silico prediction tools. While these tools were initially expected to be helpful to identify splicing-affecting mutations in genetic diagnostics, we are still quite far from meeting this goal. In fact, most of these tools are not able to accurately discern the SRE-affecting pathological variants from those not affecting splicing. Nonetheless, several recent evaluations have given appealing results (namely for EX-SKIP, ESRseq and Hexplorer predictors). In this review, we aim to summarize the history of the different approaches to SRE prediction, and provide additional validation of these tools based on patients' clinical data. Finally, we evaluate their usefulness for diagnostic settings and discuss the challenges that have yet to be met.

Targeting GH-1 splicing as a novel pharmacological strategy for growth hormone deficiency type II

Isolated growth hormone deficiency type II (IGHD II) is a rare genetic splicing disorder characterized by reduced growth hormone (GH) secretion and short stature. It is mainly caused by autosomal dominant-negative mutations within the growth hormone gene (GH-1) which results in missplicing at the mRNA level and the subsequent loss of exon 3, producing the 17.5-kDa GH isoform: a mutant and inactive GH protein that reduces the stability and the secretion of the 22-kDa GH isoform, the main biologically active GH form. At present, patients suffering from IGHD II are treated with daily injections of recombinant human GH (rhGH) in order to reach normal height. However, this type of replacement therapy, although effective in terms of growth, does not prevent the toxic effects of the 17.5-kDa mutant on the pituitary gland, which may eventually lead to other hormonal deficiencies. As the severity of the disease inversely correlates with the 17.5-kDa/22-kDa ratio, increasing the inclusion of exon 3 is expected to ameliorate disease symptoms. This review focuses on the recent advances in experimental and therapeutic strategies applicable to treat IGHD II in clinical and preclinical contexts. Several avenues for alternative IGHD II therapy will be discussed including the use of small interfering RNA (siRNA) and short hairpin RNA (shRNA) constructs that specifically target the exon 3-deleted transcripts as well as the application of histone deacetylase inhibitors (HDACi) and antisense oligonucleotides (AONs) to enhance full-length GH-1 transcription, correct GH-1 exon 3 splicing and manipulate GH pathway.

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.

Butyrate increases intracellular calcium levels and enhances growth hormone release from rat anterior pituitary cells via the G-protein-coupled receptors GPR41 and 43

Butyrate is a short-chain fatty acid (SCFA) closely related to the ketone body ß-hydroxybutyrate (BHB), which is considered to be the major energy substrate during prolonged exercise or starvation. During fasting, serum growth hormone (GH) rises concomitantly with the accumulation of BHB and butyrate. Interactions between GH, ketone bodies and SCFA during the metabolic adaptation to fasting have been poorly investigated to date. In this study, we examined the effect of butyrate, an endogenous agonist for the two G-protein-coupled receptors (GPCR), GPR41 and 43, on non-stimulated and GH-releasing hormone (GHRH)-stimulated hGH secretion. Furthermore, we investigated the potential role of GPR41 and 43 on the generation of butyrate-induced intracellular Ca²⁺ signal and its ultimate impact on hGH secretion. To study this, wt-hGH was transfected into a rat pituitary tumour cell line stably expressing the human GHRH receptor. Treatment with butyrate promoted hGH synthesis and improved basal and GHRH-induced hGH-secretion. By acting through GPR41 and 43, butyrate enhanced intracellular free cytosolic Ca²⁺. Gene-specific silencing of these receptors led to a partial inhibition of the butyrate-induced intracellular Ca²⁺ rise resulting in a decrease of hGH secretion. This study suggests that butyrate is a metabolic intermediary, which contributes to the secretion and, therefore, to the metabolic actions of GH during fasting.

Experimental assessment of splicing variants using expression minigenes and comparison with in silico predictions

Assessment of the functional consequences of variants near splice sites is a major challenge in the diagnostic laboratory. To address this issue, we created expression minigenes (EMGs) to determine the RNA and protein products generated by splice site variants (n = 10) implicated in cystic fibrosis (CF). Experimental results were compared with the splicing predictions of eight in silico tools. EMGs containing the full-length Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) coding sequence and flanking intron sequences generated wild-type transcript and fully processed protein in Human Embryonic Kidney (HEK293) and CF bronchial epithelial (CFBE41o-) cells. Quantification of variant induced aberrant mRNA isoforms was concordant using fragment analysis and pyrosequencing. The splicing patterns of c.1585-1G>A and c.2657+5G>A were comparable to those reported in primary cells from individuals bearing these variants. Bioinformatics predictions were consistent with experimental results for 9/10 variants (MES), 8/10 variants (NNSplice), and 7/10 variants (SSAT and Sroogle). Programs that estimate the consequences of mis-splicing predicted 11/16 (HSF and ASSEDA) and 10/16 (Fsplice and SplicePort) experimentally observed mRNA isoforms. EMGs provide a robust experimental approach for clinical interpretation of splice site variants and refinement of in silico tools.

Isolated growth hormone deficiency (GHD) in childhood and adolescence: recent advances

The diagnosis of GH deficiency (GHD) in childhood is a multistep process involving clinical history, examination with detailed auxology, biochemical testing, and pituitary imaging, with an increasing contribution from genetics in patients with congenital GHD. Our increasing understanding of the factors involved in the development of somatotropes and the dynamic function of the somatotrope network may explain, at least in part, the development and progression of childhood GHD in different age groups. With respect to the genetic etiology of isolated GHD (IGHD), mutations in known genes such as those encoding GH (GH1), GHRH receptor (GHRHR), or transcription factors involved in pituitary development, are identified in a relatively small percentage of patients suggesting the involvement of other, yet unidentified, factors. Genome-wide association studies point toward an increasing number of genes involved in the control of growth, but their role in the etiology of IGHD remains unknown. Despite the many years of research in the area of GHD, there are still controversies on the etiology, diagnosis, and management of IGHD in children. Recent data suggest that childhood IGHD may have a wider impact on the health and neurodevelopment of children, but it is yet unknown to what extent treatment with recombinant human GH can reverse this effect. Finally, the safety of recombinant human GH is currently the subject of much debate and research, and it is clear that long-term controlled studies are needed to clarify the consequences of childhood IGHD and the long-term safety of its treatment.

Endoplasmic reticulum stress and apoptosis contribute to the pathogenesis of dominantly inherited isolated GH deficiency due to GH1 gene splice site mutations

Dominantly inherited isolated GH deficiency is mainly caused by a heterozygous donor site mutation of intron 3 in the GH1 gene. An exon 3 deletion in GH (del32-71 GH) is produced from a mutant allele, whereas wild-type GH is produced from the other allele. Several studies have demonstrated a dominant negative effect of del32-71 GH on wild-type GH secretion, but the precise molecular mechanisms remain unclear. We hypothesized that unfolded del32-71 GH accumulates in the endoplasmic reticulum (ER) and causes ER stress and apoptosis in somatotrophs, promoting GH deficiency. To evaluate del32-71 GH-mediated ER stress, we established GH4C1 cell lines with doxycycline (dox)-controlled del32-71 GH expression. In 20 of 23 dox-controlled cell lines, the concentration of wild-type GH in the culture medium significantly decreased with del32-71 GH induction, demonstrating the dominant negative effect of this mutant. Cell viability, mRNA abundance of ER stress-response genes, caspase activation, and DNA fragmentation were evaluated in 5 dox-controlled cell lines selected as cellular models. In 4 of the 5 cell lines, del32-71 GH induction decreased cell viability, increased expression of 3 major ER stress response pathways (PRKR-like endoplasmic reticulum kinase [PERK], activating transcription factor-6 [ATF6], and inositol requirement 1 [IRE1]), and induced caspase-3 and caspase-7 activation. In 1 of the 4 cell lines, DNA fragmentation was demonstrated. Finally, overexpression of XBP1(S), a nuclear transcription factor downstream of IRE1, completely reversed the observed caspase activation. These data suggested that del32-71 GH-mediated ER stress and apoptosis contributed to the decrease in wild-type GH secretion observed in GH deficiency due to the GH1 gene slice-site mutations.

Cellular and intracellular distribution of growth hormone in the adult chicken testis

Endocrine actions of growth hormone (GH) have been implicated during the development of adult testicular function in several mammalian species, and recently intracrine, autocrine, and paracrine effects have been proposed for locally expressed GH. Previous reports have shown the distribution of GH mRNA and the molecular heterogeneity of GH protein in both adult chicken testes and vas deferens. This study provides evidence of the presence and distribution of GH and its receptor (GHR) during all stages of spermatogenesis in adult chicken testes. This hormone and its receptor are not restricted to the cytoplasm; they are also found in the nuclei of spermatogonia, spermatocytes, and spermatids. The pattern of GH isoforms was characterized in the different, isolated germ cell subpopulations, and the major molecular variant in all subpopulations was 17 kDa GH, as reported in other chicken extra-pituitary tissues. Another molecular variant, the 29 kDa moiety, was found mainly in the enriched spermatocyte population, suggesting that it acts at specific developmental stages. The co-localization of GH with the proliferative cell nuclear antigen PCNA (a DNA replication marker present in spermatogonial cells) was demonstrated by immunohistochemistry. These results show for the first time that GH and GHR are present in the nuclei of adult chicken germinal cells, and suggest that GH could participate in proliferation and differentiation during the complex process of spermatogenesis.

Genetic causes and treatment of isolated growth hormone deficiency-an update

Isolated growth hormone deficiency is the most common pituitary hormone deficiency and can result from congenital or acquired causes, although the majority of cases are idiopathic with no identifiable etiology. Known genes involved in the genetic etiology of isolated growth hormone deficiency include those that encode growth hormone (GH1), growth-hormone-releasing hormone receptor (GHRHR) and transcription factor SOX3. However, mutations are identified in a relatively small percentage of patients, which suggests that other, yet unidentified, genetic factors are involved. Among the known factors, heterozygous mutations in GH1 remain the most frequent cause of isolated growth hormone deficiency. The identification of mutations has clinical implications for the management of patients with this condition, as individuals with heterozygous GH1 mutations vary in phenotype and can, in some cases, develop additional pituitary hormone deficiencies. Lifelong follow-up of these patients is, therefore, recommended. Further studies in the genetic etiology of isolated growth hormone deficiency will help to elucidate mechanisms implicated in the control of growth and may influence future treatment options. Advances in pharmacogenomics will also optimize the treatment of isolated growth hormone deficiency and other conditions associated with short stature, for which recombinant human growth hormone is a licensed therapy.