Brian M. Little

Role of leptin and melanocortin signaling in uremia-associated cachexia.

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.

THE DISAPPEARANCE OF 7-H3-d-ALDOSTERONE IN THE PLASMA OF NORMAL SUBJECTS*

The study of the disappearance of the radioactivity in plasma after the injection of labeled aldosterone should elucidate the transport and metabolism of this steroid in man. In particular this would allow the calculation of the turnover rate in various clinical conditions. This value, together with a concomitant determination of the secretion rate estimated from the specific activity of a urinary metabolite, would enable the mean blood concentration of aldosterone to be calculated. This is particularly important at the present time, as no practical direct method has yet been reported for the analysis of aldosterone in peripheral blood. After the administration of 16-H13-aldosterone to one normal subject, the labeled hormone had a half-life in plasma of 20 minutes, as calculated from the change in concentration of radioactivity measured specifically as aldosterone between 2 and 3 hours after the injection (1). This indicated that the rate of metabolism of aldosterone was greater than that of cortisol and corticosterone. The data obtained were not sufficient to give a relialle estimate of volumes of distribution. Also, the low specific activity of aldosterone then available made it necessary to administer 2 jug of hormone. This amount represents about 25 per cent of total body pool of hormone. Peterson reported a corresponding half-life of 0.5 to 0.8 hour in normal subjects after injection of tritiated d,l-aldosterone (2). Again in this study large quantities of the hormone were injected. Also it is not known whether the optical isomers of aldosterone are metabolized in an identical manner. The natural hormone is in the d form exclusively.

The metabolic clearance rate of progesterone in males and ovariectomized females.

The metabolic clearance rate (MCR) of a steroid hormone, which is probably most generally defined as the volume of blood cleared completely and irreversibly of steroid in unit time, determines the relationship between plasma production rate and plasma concentration (1). A knowledge of its value may also be important in predicting some of the characteristics of any control system (1-3). It can be shown that after a single injection or continuous infusion of radioactive steroid, the metabolic clearance rate can be calculated as the reciprocal of the integrated plasma concentration of radioactivity measured specifically as the hormone (1). If this integration is performed correctly, the two methods should give the same value for MCR, and a comparison of the values obtained therefore serves as a check on this and other technical requirements for the validity of the calculation (1, 4). In the studies described here, the MCR of progesterone has been measured by application of the single injection and continuous infusion methods in ovariectomized females and in males. Information on volumes of distribution has also been obtained after single injection.

Growth hormone insensitivity and severe short stature in siblings : A novel mutation at the exon 13-intron 13 junction of the STAT5b gene

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.

Extreme Elevation of Serum Growth Hormone-Binding Protein Concentrations Resulting from a Novel Heterozygous Splice Site Mutation of the Growth Hormone Receptor Gene

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.

Identification of the first patient with a confirmed mutation of the JAK-STAT system

Growth hormone insensitivity (GHI) has been attributable, classically, to mutations in the gene for the GH receptor. After binding to the GH receptor, GH initiates signal transduction through a number of pathways, including the JAK-STAT pathway. We describe the first patient reported with a mutation in the gene for STAT5b, a protein critical for the transcriptional regulation of insulin-like growth factor-I.