Search

Background/aims: Circulating growth hormone-binding protein (GHBP), in humans, is the proteolytic product of the growth hormone receptor (GHR). We investigated a prepubertal male subject who was of short stature, but who had a markedly elevated serum level of GHBP., Methods: Serum and DNA from the patient and his mother were analyzed., Results: Both the patient and mother had serum GHBP concentrations over 100-fold higher than normal, by assays, and Western and ligand blot analysis. Sequencing of the GHR gene revealed a novel heterozygous C>A transversion at position 785-3 in the acceptor splice site of intron 7., Conclusion: In silico analysis of the altered sequence suggested that 785-3(C>A) is a splicing mutation, with either retention of intron 7 or the skipping of exon 8. The consequence is a truncated GHR lacking the transmembrane domain (encoded by exon 8) and the cytoplasmic domain. We hypothesize that this GHR variant cannot anchor to the cell membrane, and the continual secretion into the circulation explains the elevated levels of serum GHBP detected in the patient and his mother. Despite this mutation, the presence of the wild-type GHR allele, presumably, permits some normality in GH-induced action., (Copyright 2009 S. Karger AG, Basel.)

Context: Among more than 250 cases of GH insensitivity syndrome (GHIS) reported to date, the largest cohort was identified in southern Ecuador. In the Ecuadorian GHIS cohort, a sense mutation (GAA>GAG) at codon E180 of GH receptor [GHR (E180sp)] results in deletion of codons 181-188. No functional studies of this mutation have been performed, nor have different mutations at codon 180 been reported., Objective: We now report identification of a novel GHR mutation, also within codon E180, in two distantly related GHIS subjects of Inuit origin and provide mechanistic insights into the defects caused by the Inuit and Ecuadorian GHR mutations., Patients: The two Inuit subjects, with heights of -5 sd score and -7 sd score, respectively, had elevated circulating levels of GH but low levels of GH-binding protein, IGF-I, and IGF-binding protein-3., Results: Both Inuit subjects carry the same novel nonsense homozygous GHR mutation at codon E180 (GAA->TAA, E180X). In vitro reconstitution experiments demonstrated that GHR (E180sp), but not GHR (E180X), could be stably expressed. GHR (E180sp), however, could not bind GH and could neither activate signal transducer and activator of transcription-5b nor induce -5b-dependent gene expression on GH treatment. Furthermore, the GHR (E180sp), which has a deletion of eight amino acid residues within the GHR dimerization domain, although retaining the ability to homodimerize, was defective in trafficking to the cell surface., Conclusions: The E180X mutation identified in two Inuit patients resulted in a truncated, unstably expressed GHR variant, whereas the E180 splicing mutation previously identified in the Ecuadorian cohort, affected both GH binding and GHR trafficking and rendered the abnormal GHR nonfunctional.

Objective: Insulin-like growth factor binding protein-3 (IGFBP-3) has been shown to exhibit diverse biological actions, including IGF-independent effects on cell growth and cell death. Here we report that IGFBP-3 sensitizes prostate cancer cells to interferon-gamma (IFN-gamma)-induced apoptosis and inhibition of cell proliferation., Design: The cell growth or cell death of prostate cells in response to the treatments of IGFBPs and/or IFN-gamma was measured, and the signaling pathways mediating these actions assessed., Results: Cell proliferation was minimally affected when M12 prostate cancer cells were treated with exogenous IGFBP-3 (1-5 microg/ml), IGFBP-1 (1-5 microg/ml) or IFN-gamma (20 U/ml). However, strong inhibition of cell growth and significant apoptosis were observed when M12 cells were co-treated with IGFBP-3 and IFN-gamma, but not with IGFBP-1 and IFN-gamma. These effects were IGF-independent and appear not to require intracellular localization of IGFBP-3, as similar results were obtained with mutants of IGFBP-3 that either could not bind IGF or has impaired ability to be internalized. Further analyses revealed that IGFBP-3, but not IGFBP-1, could significantly enhance the weak tyrosine phosphorylation of STAT1 induced by IFN-gamma (20 U/ml) alone. The IGFBP-3-promoted apoptosis in the presence of IFN-gamma could also be abrogated by blockade of the mTOR pathway with its pharmacological inhibitors, LY294002 or rapamycin., Conclusions: These results demonstrated that in a cancer cell line not responsive to exogenous IGFBP-3 alone, IGFBP-3 sensitized the cells to the anti-proliferative, proapoptotic actions of IFN-gamma through an IGF-independent, STAT1- and mTOR-dependent mechanism.

Context: Primary GH insensitivity (GHI) or Laron syndrome, caused by mutations of the GH receptor (GHR) gene, has a clinical phenotype of postnatal growth failure associated with normal elevated serum concentrations of GH and low serum levels of IGF-I., Objective: We investigated the clinical and biochemical implications of molecular defects in the GHR gene in an Austrian family with two daughters who were GHI., Patients: Patient 1 [height, -4.8 sd score (SDS)] and patient 2 (height, -5.0 SDS) had elevated circulating levels of GH, low-normal levels of GH-binding protein, and abnormally low IGF-I (-5.0 SDS and -2.6 SDS, respectively) and IGF-binding protein-3 (-2.6 SDS and -2.0 SDS, respectively)., Results: Both patients carry novel compound, missense, heterozygous GHR mutations, C94S and H150Q. In vitro reconstitution experiments demonstrated that whereas each of the mutants could be stably expressed, GHR(C94S) lost its affinity for GH and could neither activate signal transducer and activator of transcription (STAT)-5b nor drive STAT5b-dependent gene transcription in response to GH (1-100 ng/ml). GHR(H150Q) showed normal affinity for GH but impaired capacity for signal transduction. The compound heterozygote and C94S heterozygote, but not the H150Q heterozygote, showed significant deficiency in activating GH-induced gene expression, corroborating diminished GH-induced STAT5b activation in fibroblasts carrying GHR(C94S) as either a compound heterozygote (in the patients) or a simple heterozygote (in one parent)., Conclusions: Each of the compound heterozygous mutations contributed additively to the pathological condition seen in the patients, and the more detrimental of the two mutations, C94S, may cause (partial) primary GHI, even in a heterozygous state.

Background/aims: Growth hormone insensitivity (GHI) is characterized by severe short stature, high serum growth hormone (GH), low serum IGF-I and IGFBP-3 levels and is classically associated with genetic defects of the GH receptor (GHR). Recently, mutations of the STAT5b gene have been identified and shown to be associated with GHI and severe IGF deficiency. We investigated 2 sisters from a consanguineous family from Kuwait, with clinical and biochemical features of GHI, in whom no molecular defects in the GHR were identified., Methods: Serum and DNA were analyzed., Results: In addition to GHI, siblings 2 and 1 presented with, respectively, a diagnosis of juvenile idiopathic arthritis and recurrent pulmonary infections. Molecular analysis of the STAT5b gene revealed a novel homozygous deletion of a G at the junction of exon 13-intron 13. The parents, who are of normal height, were heterozygous for the mutation., Conclusions: This is the first STAT5b defect to be identified in siblings, further supporting the autosomal recessive mode of transmission of STAT5b deficiency. The results affirm that defective STAT5b is an etiology for IGF deficiency and the GHI phenotype, and emphasize the importance of considering this diagnosis in patients with IGF deficiency, especially when associated with diverse immunological problems., ((c) 2007 S. Karger AG, Basel.)

Context: Primary IGF deficiency (IGFD) describes the condition in which serum concentrations of IGF-I are low in the face of normal to elevated GH production. Because IGF-I, which circulates as part of a ternary complex with IGF binding protein (IGFBP)-3 and acid-labile subunit (ALS), mediates the growth-promoting effects of GH, IGFD is associated with severe growth failure in humans., Objective: We investigated a case of IGFD in which serum IGF-I and IGFBP-3 were abnormally low, yet growth failure was modest (-2.1 sd score at 15.5 yr of age)., Results: The young male subject, from a consanguineous pedigree, had a postnatal growth profile consistently below the third percentile. The subject had a normal fasting GH level of 3.7 muU/ml and normal serum GH binding protein level (1258 pmol/liter; normal range 431-1892 pmol/liter), but serum IGF-I and IGFBP-3 were profoundly reduced (-5.8 and -7.2 sd score, respectively, at age 12.3 yr), even through puberty. A novel homozygous missense mutation was subsequently identified in the ALS gene, which resulted in severe deficiency of serum ALS (undetectable)., Conclusions: ALS is critical for maintaining normal serum concentrations of IGF-I and IGFBP-3, most likely by prolonging the half-lives of both proteins. ALS deficiency can be associated with moderate growth failure, but in this patient, the onset and progression of puberty appear to be normal. Altogether the results support a modest role for the ternary complex in the regulation of stature.

Context: A natural missense mutation in the signal transducer and activator of transcription (STAT) 5b gene was recently identified in association with a female patient presenting with severe growth failure and immune dysfunction. The mutation results in an alanine to proline substitution at residue 630 (A630P) in the src-homology-2 domain, a region essential for docking of STATs to phospho-tyrosines on activated receptors, STAT dimerization, and stabilization of phospho-STAT-DNA interactions., Objective: The purpose of this study was to explore the molecular mechanisms underlying the GH insensitivity and IGF-I deficiency caused by the A630P-mutated STAT5b., Results: In reconstitution experiments using HEK293 cells, both GH and interferon-gamma were unable to activate mutant STAT5b (A630P), as demonstrated by lack of immunodetectable phospho-tyrosyl-STAT5b (A630P) and inability to drive luciferase reporter activity. However, the Src family of nonreceptor kinases [constitutively active v-src and epithelial growth factor-induced c-src] tyrosine-phosphorylated STAT5b(A630P). The v-src-induced phospho-STAT5b(A630P) translocated to the nucleus but, unlike wild-type Stat5b, was unable to bind DNA., Conclusions: The A630P mutation disrupts the src-homology-2 architecture such that: 1) mutant STAT5b most likely cannot dock to phospho-tyrosines on ligand-activated receptors; and 2) stable interactions with DNA are prevented. Because STAT5b (A630P) is an inefficient signal transducer and transcription factor, the detrimental impact on signaling pathways important for normal growth and immunity explains, in part, the complex clinical phenotype of GH insensitivity and immune dysfunction.

The pathogenesis of cachexia in patients with uremia is unknown. We tested the hypothesis that uremia-associated cachexia is caused by leptin signaling through the hypothalamic melanocortin receptor 4 (MC4-R). We performed either subtotal nephrectomy (N) or sham operations in WT, leptin receptor-deficient (db/db), and MC4-R knockout (MC4-RKO) mice. The animals were on 17% protein diets, and none of the uremic animals were acidotic. WT-N mice produced a classic syndrome of cachexia characterized by decreased food intake, increased metabolic rate, and loss of lean body mass. Corrected leptin levels were elevated. db/db mice and MC4-RKO mice resisted the cachexic effects of uremia on weight gain, body composition, and metabolic rate. Likewise, treatment of WT mice with intracranial agouti-related peptide reversed the cachexic effects of uremia on appetite, weight gain, body composition, and metabolic rate. Gene expression of ubiquitin C and proteasome subunits C2, C3, and C9 was not changed in the uremic animals, suggesting that other pathways are involved in this model of nonacidotic uremic cachexia. The results of this study suggest that elevated circulating levels of cytokines such as leptin may be an important cause of uremia-associated cachexia via signaling through the central melanocortin system.

Differential localization of the type II cAMP-dependent protein kinase (PKA) is achieved by interaction of the regulatory subunit (RII) with A-kinase anchor proteins (AKAPs). Anchoring is a likely means to adapt PKA for regulation of cAMP-responsive events through colocalization of the kinase with preferred substrates. Using an interaction cloning strategy with an RII alpha protein probe, we have identified a 655-amino acid protein (named AKAP100). Recombinant AKAP100, expressed in Escherichia coli, binds RII alpha in a solid-phase overlay assay. The cellular and subcellular distribution of AKAP100 was analyzed by various methods. Northern blot analysis with the AKAP100 cDNA as a probe detected an 8-kilobase message in some human tissues including various brain regions; however, the message was predominately expressed in cardiac and skeletal muscle. Anti-AKAP100 antibodies confirmed expression in the rat cardiac and skeletal muscle cell lines, H9c2 and L6P, whereas immunohistochemical analysis revealed that AKAP100 was localized to the sarcoplasmic reticulum of both cell types. RII was also detected in these regions. AKAP100 was detected in preparations of RII purified from L6P cell extracts by cAMP-agarose affinity chromatography. Collectively, these results suggest that AKAP100 functions to maintain the type II PKA at the sarcoplasmic reticulum.

Leishmania are parasitic protozoa with two major stages in their life cycle: flagellated promastigotes that live in the gut of the insect vector and nonflagellated amastigotes that live inside the lysosomes of the vertebrate host macrophages. The Pro-1 glucose transporter of L. enriettii exists as two isoforms, iso-1 and iso-2, which are both expressed primarily in the promastigote stage of the life cycle. These two isoforms constitute modular structures: they differ exclusively and extensively in their NH2-terminal hydrophilic domains, but the remainder of each isoform sequence is identical to that of the other. We have localized these glucose transporters within promastigotes by two approaches. In the first method, we have raised a polyclonal antibody against the COOH-terminal hydrophilic domain shared by both iso-1 and iso-2, and we have used this antibody to detect the transporters by confocal immunofluorescence microscopy and immunoelectron microscopy. The staining observed with this antibody occurs primarily on the plasma membrane and the membrane of the flagellar pocket, but there is also light staining on the flagellum. We have also localized each isoform separately by introducing an epitope tag into each protein sequence. These experiments demonstrate that iso-1, the minor isoform, resides primarily on the flagellar membrane, while iso-2, the major isoform, is located on the plasma membrane and the flagellar pocket. Hence, each isoform is differentially sorted, and the structural information for targeting each transporter isoform to its correct membrane address resides within the NH2-terminal hydrophilic domain.

The parasitic protozoan Leishmania enriettii contains a family of tandemly repeated genes, designated Pro-1, that encode proteins with significant sequence similarity to mammalian facilitative glucose transporters. Pro-1 mRNAs are expressed almost exclusively in the promastigote or insect stage of the parasite life cycle. The Pro-1 tandem repeat encodes two isoforms of the putative transporter, iso-1 and iso-2, which have identical predicted amino acid sequences except for their NH2-terminal hydrophilic domains. We have now expressed both iso-1 and iso-2 by microinjecting their RNAs into Xenopus oocytes and assaying these oocytes for transport of various radiolabeled ligands. Both iso-1 and iso-2 transport [3H]2-deoxy-D-glucose, confirming that each protein is a bona fide glucose transporter. Each isoform also transports fructose and, to a much lesser degree, mannose. Compounds which inhibit 2-deoxy-D-glucose transport in L. enriettii promastigotes also inhibit transport in the microinjected oocytes expressing each isoform, indicating that the substrate specificities and pharmacological properties of each isoform are similar to those measured for 2-deoxy-D-glucose transport in intact parasites. The Km for transport of 2-deoxyglucose in oocytes expressing iso-1 is similar to that for oocytes expressing iso-2. These results reveal that both transporter isoforms have closely related functional properties and that the difference in their structures may serve some other purpose such as differential subcellular localization.