Identification of a novel growth hormone binding protein mRNA in rat liver

Growth hormone receptors in the atherinid Odontesthes bonariensis: characterization and expression profile after fasting-refeeding and growth hormone administration

In order to improve the understanding of pejerrey Odontesthes bonariensis, growth hormone (Gh)-insulin-like growth factor-1(Igf1) axis, O. bonariensis growth hormone receptor type 1 (ghr1) and type 2 (ghr2) mRNA sequences were obtained. Both transcripts were ubiquitously expressed except in kidney, encephalon and anterior intestine. Alternative transcripts of both receptors were found in muscle. Interestingly, two different ghr2 transcripts with alternative polyadenylation (APA) sites located in the long 3' untranslated region (UTR-APA) were also found in liver. Hepatic ghr1, ghr2 and insulin-like growth factor type 1 (igf1) transcript levels were examined under two different metabolic conditions. In the first experimental condition, fish were fasted for 2 weeks and then re-fed for another 2 weeks. Despite igf1 mRNA relative expression did not show significant differences under the experimental period of time examined, both ghr transcripts decreased their expression levels after the fasting period and returned to their control levels after re-feeding. In the second treatment, recombinant O. bonariensis growth hormone (r-pjGh) was orally administered once a week. After 4 weeks of treatment, liver igf1, ghr1 and ghr2 mRNA relative expression increased (13, 4·5 and 2·1 fold, P < 0·05) compared to control values. These results add novel information to the growth hormone-insulin-like growth factor system in teleosts.

Weight loss increases soluble leptin receptor levels and the soluble receptor bound fraction of leptin

OBJECTIVE

Soluble leptin receptor (sOB-R) represents the main binding site for leptin in human blood. The aim of this study was to investigate the relationship between leptin and soluble leptin receptor and the bound/free ratio after pronounced weight reduction.

RESEARCH METHODS AND PROCEDURES

A total of 18 morbidly obese women participated in this prospective study. Subjects were examined for fat mass, leptin, and sOB-R concentrations before and 1 year after Swedish adjustable gastric banding.

RESULTS

Anthropomorphic measures displayed a significant reduction of body mass index [(42.9 +/- 5.6 to 32.9 +/- 6.0 kg/m(2) (mean +/- SD)]. Fat mass decreased from 56.3 +/- 9.0 to 33.9 +/- 12.5 kg. Plasma leptin concentration decreased from 44.6 +/- 18.0 to 20.0 +/- 13.1 ng/mL (p < 0.001), whereas the sOB-R levels increased from 11.1 +/- 3.6 to 16.6 +/- 6.0 U/mL after weight-reducing surgery. Thus, the sOB-R bound fraction of leptin increased from 7% to 33%.

DISCUSSION

This work demonstrates a relationship between weight loss, leptin, and sOB-R concentrations in vivo. During weight loss, leptin levels decreased, whereas sOB-R levels and the receptor bound fraction of leptin increased. Thus, sOB-R may negatively regulate free leptin.

In situ gene expression of growth hormone (GH) receptor and GH binding protein in adult male rat tissues

Pituitary growth hormone (GH) acts as a growth promoter in a wide range of tissues after binding to its specific GH receptor (GHR) or to a cytosolic circulating GH binding protein (GHBP). To further characterize GH target cells in the rat, in situ hybridization was used to investigate the tissue and cell distribution of mRNAs encoding GHR and GHBP, and their hepatic developmental expression was examined. Cryostat sections of adult male rat tissue were hybridized with [35S]dATP-labeled oligonucleotide antisense probes, one directed against a specific sequence of the intracellular domain of rat GHR mRNA, the other against the hydrophilic tail of rat GHBP mRNA. Several tests were carried out to validate the in situ detection of mRNA. Co-expression of the two transcripts in liver, spleen, thymus, kidney, adrenal, skin, muscle, heart, and pituitary was autoradiographically detected. However, relative expression levels, as demonstrated by computer-assisted microdensitometry, appeared to be variable. Both transcripts showed higher levels of expression in the liver, anterior and intermediate pituitary lobes, outer kidney medulla, adrenal cortex, skin epidermis, heart and muscle, but lower levels in spleen, thymus, hypodermis, adrenal medulla and posterior pituitary lobe. As a physiological control, hepatic levels of expression were examined during development, and the two forms of mRNA were found to be present at low levels in fetal liver, increasing considerably after birth. These results permit the identification in the adult male rat of cells that might be directly responsive to GH, and demonstrate the differential expression of rGHR and rGHBP transcripts.

Rat growth hormone receptor/growth hormone-binding protein mRNAs with divergent 5'-untranslated regions are expressed in a tissue-specific manner

In the rat, the growth hormone receptor (GH-R) gene generates two transcripts, one encoding the transmembrane GH-R, and a shorter one encoding the GH-binding protein (GH-BP). These transcripts exhibit a high degree of heterogeneity in their 5'-untranslated regions (5'-UTRs). Some of the exons encoding these 5'-UTR variants may be flanked by distinct promoter regions whose activity would result in the tissue-specific expression of the GH-R gene. To assess this possibility, we used single-sided polymerase chain reaction (PCR) amplification to characterize 5'-UTR variants in rat GH-R cDNAs, and by using 5'-UTR-specific probes, we determined their pattern of expression in several tissues. Besides two previously described variants (V1 and V2), three new 5'-UTR variants were identified, extending 56 nucleotides (V3), 135 nucleotides (V4), and 209 nucleotides (V5) upstream of the ATG translation initiation codon. The expression of GH-R and GH-BP transcripts was clearly tissue specific. In the liver, GH-BP mRNA was the predominant transcript, whereas in other tissues, there was equivalent expression of both transcripts or predominant expression of GH-R mRNA. With respect to the tissue distribution of the 5'-UTR variants in particular, variants V1 and V5 exhibited a pattern of expression closely resembling that seen with an exon 2 probe, with the overall expression of variant V1 being much higher than that of variant V5. The V2 species was exclusively expressed in liver. Variant V3 was expressed at low levels in liver, muscle, heart, and kidney; in muscle and heart, it was preferentially associated with GH-BP transcripts. Variant V4, although present in liver, was more abundant in extrahepatic tissues and predominantly found in GH-R mRNA transcripts. Southern blot analyses were consistent with exon 2 and the exons encoding the V1 and V2 sequences being in proximity, with the other 5'-UTR sequences being encoded by exons located further upstream of exon 2. These findings support the concept that different 5'-UTR variants are the result of the different promoters acting in a tissue-specific manner. The association of specific 5'-UTR variants with either GH-R or GH-BP transcripts raises the possibility that the alternative splicing process that generates GH-BP mRNA in the rat might be controlled by the 5'-flanking region regulating the expression of specific leader exons.

Serum binding proteins for growth hormone: origins, regulation of gene expression and possible roles

The above discussion highlights the heterogeneity of the family of GH receptors/GHBPs and their mRNAs. Considerable uncertainty still exists as to the interrelationships between the various forms, the specific mechanisms for their generation and their possible significance in terms of modulating GH action and receptor function. Collectively, the regulatory data indicate that while the mRNAs encoding the membrane and soluble GH receptors/GHBPs in the rat are expressed by the same broad distribution of tissues, they can be differentially regulated. Such regulated expression implies a functional basis for production of GHBP. The wide tissue distribution of GHBP mRNA also suggests a role for GHBP as a paracrine/autocrine effector molecule, perhaps in addition to an endocrine role. Additional studies, both in vitro and in vivo, perhaps utilizing highly purified recombinant GHBP, will be required to provide more definitive information as to the true physiological role(s) of the circulating GHBPs.