The somatostatin analog octreotide inhibits GH-stimulated, but not IGF-I-stimulated, bone growth in hypophysectomized rats

Comparative inhibition of the GH/IGF-I axis obtained with either the targeted secretion inhibitor SXN101959 or the somatostatin analog octreotide in growing male rats

Abnormally high GH/IGF-I levels, most often caused by adenomas arising from pituitary somatotrophs, generate deleterious effects. We recently described a targeted secretion inhibitor (SXN101742) comprising a GHRH domain and the endopeptidase domain of botulinum toxin serotype D (GHRH-light chain endopeptidase type D domain [LC/D] associated to a heavy chain translocation domain [HN]) able to down-regulate the GH/IGF-I axis. In the present study, we compared the effect of a single iv bolus of a related molecule developed for clinical studies (SXN101959, 1 mg/kg) with a sc infusion of the somatostatin analog octreotide (SMS201-995, 10 μg/kg · h) to lower GH/IGF-I activity in growing male rats. Ten days after administration of SXN101959 or initiation of the octreotide infusion, body and pituitary weights, body length, GH peaks, and IGF-I production were reduced by both treatments but to a greater extent with SXN101959. In contrast to unaltered GH gene expression and increased GH storage in pituitaries from octreotide-treated rats, the inhibition of GH secretion was associated with a collapse of both GH mRNA and protein level in pituitaries from SXN101959-treated rats, in line with a specific decrease in hypothalamic GHRH production, not observed with octreotide. SXN101959 did not induce major apoptotic events in anterior pituitary and exhibited a reversible mode of action with full recovery of somatotroph cell functionality 30 days after treatment. Octreotide infusion permanently decreased ghrelin levels, whereas SXN101959 only transiently attenuated ghrelinemia. Both treatments limited bone mass acquisition and altered specifically tissues development. In conclusion, SXN101959 exerts a powerful and reversible inhibitory action on the somatotropic axis. Specific features of SXN101959, including long duration of action coupled to a strong inhibition of pituitary GH synthesis, represent advantages when treating overproduction of GH.

Two phenotypes of arthropathy in long-term controlled acromegaly? A comparison between patients with and without joint space narrowing (JSN)

BACKGROUND

Arthropathy is an invalidating complication of acromegaly, also in long-term controlled patients, and is radiographically characterized by osteophytes and preserved joint spaces. However, joint space narrowing (JSN) is observed in the minority of patients. It is unknown whether JSN is the end-stage of acromegalic arthropathy or whether this feature develops independently of acromegaly.

OBJECTIVE

To gain insight into the pathophysiology of acromegalic arthropathy, and, more specifically, in the process of JSN, risk factors for radiographic JSN were studied in a cross-sectional study.

METHODS

We studied hips and knees of 89 well-controlled acromegaly patients (mean age 58.3 yr, 51% female). Joints were divided into two groups based on the presence of JSN, defined as an Osteoarthritis Research Society (OARSI) score ≥ 1. Potential risk factors for JSN were assessed, and its relationship to joint complaints. Individual knees and hips were analyzed in a Generalized Estimating Equations model, adjusted for age, sex, BMI and intra-patient effect.

RESULTS

In controlled acromegaly, JSN was found in, respectively, 10.3% and 15.4% of the hips and knees. Increasing age and female sex were associated with more JSN; acromegaly-specific risk factors for JSN were joint-site specific. In the hip, JSN was related to more active disease: higher pre-treatment GH/IGF-1, longer and more severe GH exposure and immediate postoperative cure was less frequently achieved. In the knee, especially previous knee surgery, not acromegaly-specific characteristics, was associated with JSN. The presence of JSN was associated with more joint complaints.

CONCLUSIONS

JSN is an infrequent finding in patients with acromegalic arthropathy, but it is associated with more symptoms. This study indicates that, at least in the hip, early and ongoing GH/IGF-1 activity play a role in JSN development.

Evaluation of four methods used to measure plasma insulin-like growth factor 1 concentrations in healthy cats and cats with diabetes mellitus or other diseases

OBJECTIVE

To evaluate 4 methods used to measure plasma insulin-like growth factor (IGF) 1 concentrations in healthy cats and cats with diabetes mellitus or other diseases.

ANIMALS

39 healthy cats, 7 cats with diabetes mellitus, and 33 cats with other diseases.

PROCEDURES

4 assays preceded by different sample preparation methods were evaluated, including acid chromatography followed by radioimmunoassay (AC-RIA), acid-ethanol extraction followed by immunoradiometry assay (AEE-IRMA), acidification followed by immunochemiluminescence assay (A-ICMA), and IGF-2 excess followed by RIA (IE-RIA). Validation of the methods included determination of precision, accuracy, and recovery. The concentration of IGF-1 was measured with all methods, and results were compared among cat groups.

RESULTS

The intra-assay coefficient of variation was < 10% for AC-RIA, A-ICMA, and AEE-IRMA and 14% to 22% for IE-RIA. The linearity of dilution was close to 1 for each method. Recovery rates ranged from 69% to 119%. Five healthy cats had IGF-1 concentrations > 1,000 ng/mL with the AEE-IRMA, but < 1,000 ng/mL with the other methods. Compared with healthy cats, hyperthyroid cats had significantly higher concentrations of IGF-1 with the A-ICMA method, but lower concentrations with the IE-RIA method. Cats with lymphoma had lower IGF-1 concentrations than did healthy cats regardless of the method used.

CONCLUSIONS AND CLINICAL RELEVANCE

Differences in the methodologies of assays for IGF-1 may explain, at least in part, the conflicting results previously reported in diabetic cats. Disorders such as hyperthyroidism and lymphoma affected IGF-1 concentrations, making interpretation of results more difficult if these conditions are present in cats with diabetes mellitus.

Serum IGF-I is not a reliable pharmacodynamic marker of exogenous growth hormone activity in mice

Serum IGF-I is a well-established pharmacodynamic marker of GH administration in humans and has been used for this purpose in animal studies. However, its general suitability in wild-type laboratory mice has not been demonstrated. Here we show that treatment with recombinant human GH (rhGH) in four different strains of laboratory mice increases body weight, lean body mass, and liver weight but does not increase hepatic expression and release of IGF-I. In contrast and as expected, hypophysectomized rats show a rapid increase in serum IGF-I after rhGH administration. The lack of IGF-I up-regulation in mice occurs despite hepatic activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway and is not explained by GH dose, route of administration, origin of GH (i.e. recombinant human, bovine, and murine GH), treatment duration, genetic background, sex, or formation of neutralizing antibodies. Effects on other components of the GH/IGF pathway were highly influenced by genetic background and sex but not consistently affected by rhGH treatment. We conclude that IGF-I is not a reliable indicator of the biological effects of exogenous GH treatment in genetically and pharmacologically unmodified mice. We speculate that IGF-I release is already maximal in these animals and cannot be further increased by exogenous GH treatment. This is also suggested by the observation of restored IGF-I up-regulation in isolated murine hepatocytes after rhGH treatment. Total body weight, lean body mass, and liver weight may be more reliable phenotypic indicators in these models.

Tumor-induced hypophosphatemic rickets in an adolescent boy--clinical presentation, diagnosis, and histological findings in growth plate and muscle tissue

CONTEXT

The mechanism behind disabling muscle weakness in tumor-induced hypophosphatemic rickets is obscure. Histological investigation of growth plate tissue of patients with tumor-induced osteomalacia has so far not been reported.

PATIENT

A mesenchymal tumor was detected in the left distal fibula by (68)Ga-DOTATOC in a 17-yr-old boy with adolescent onset of severe hypophosphatemic rickets. Disabling muscle weakness improved within days after surgery, and normal mobility was restored within months.

METHODS AND RESULTS

The resected tissue included part of the growth plate allowing immunohistochemical investigation. Positive staining of FGF23 was found in the tumor cells and in hypertrophic chondrocytes, osteoblasts, and osteoclasts of the adjacent growth plate. This distribution matched that found in growth plate tissue of a healthy control. We found positive staining for the somatostatin receptor not only in the tumor but also within the growth plate and adjacent bony tissue in the patient and the healthy control. Muscle tissue provided evidence for a partial defect in respiratory chain complexes I-IV. Biochemical markers were nearly or completely restored to normal 12 months after surgery.

CONCLUSIONS

Hypertrophic growth plate chondrocytes are a target or source of FGF23 in tumor-induced osteomalacia. Low serum phosphate, FGF23, or other factors produced by the tumor may interfere with mitochondrial function.

Insulin-like growth factor I (IGF-I) and its receptor (IGF-1R) in the rat anterior pituitary

Few and controversial results exist on the cellular sites of insulin-like growth factor (IGF)-I synthesis and the type 1 IGF receptor (IGF-1R) in mammalian anterior pituitary. Thus, the present study analysed IGF-I and the IGF-1R in rat pituitary. Reverse transcription-polymerase chain reaction revealed IGF-I and IGF-1R mRNA expression in pituitary. The sequences of both were identical to the corresponding sequences in other rat organs. In situ hybridization localized IGF-I mRNA in endocrine cells. The majority of the growth hormone (GH) cells and numerous adrenocorticotropic hormone (ACTH) cells exhibited IGF-1R-immunoreactivity at the cell membrane. At lower densities, IGF-1 receptors were also present at the other hormone-producing cell types, indicating a physiological impact of IGF-I for all endocrine cells. IGF-I-immunoreactivity was located constantly in almost all ACTH-immunoreactive cells. At the ultrastructural level, IGF-I-immunoreactivity was confined to secretory granules in co-existence with ACTH-immunoreactivity, indicating a concomitant release of both hormones. Occasionally, IGF-I-immunoreactivity was detected in an interindividually varying number of GH cells. In some individuals, weak IGF-I-immunoreactions were also detected also in follicle-stimulating hormone and luteinizing hormone cells. Thus, IGF-I seems to be produced as a constituent in ACTH cells, possibly indicating its particular importance in stress response. Generally, IGF-I from the endocrine cells may regulate synthesis and/or release of hormones in an autocrine/paracrine manner as well as prevent apoptosis and stimulate proliferation. Production of IGF-I in GH cells may depend on the physiological status, most likely the serum IGF-I level. IGF-I released from GH cells may suppress GH synthesis and/or release by an autocrine feedback mechanism in addition to the endocrine route.

The effect of hypophysectomy on pancreatic islet hormone and insulin-like growth factor I content and mRNA expression in rat

The growth arrest after hypophysectomy in rats is mainly due to growth hormone (GH) deficiency because replacement of GH or insulin-like growth factor (IGF) I, the mediator of GH action, leads to resumption of growth despite the lack of other pituitary hormones. Hypophysectomized (hypox) rats have, therefore, often been used to study metabolic consequences of GH deficiency and its effects on tissues concerned with growth. The present study was undertaken to assess the effects of hypophysectomy on the serum and pancreatic levels of the three major islet hormones insulin, glucagon, and somatostatin, as well as on IGF-I. Immunohistochemistry (IHC), in situ hybridization (ISH), radioimmunoassays (RIA), and Northern blot analysis were used to localize and quantify the hormones in the pancreas at the peptide and mRNA levels. IHC showed slightly decreased insulin levels in the beta cells of hypox compared with normal, age-matched rats whereas glucagon in alpha cells and somatostatin in delta cells showed increase. IGF-I, which localized to alpha cells, showed decrease. ISH detected a slightly higher expression of insulin mRNA and markedly stronger signals for glucagon and somatostatin mRNA in the islets of hypox rats. Serum glucose concentrations did not differ between the two groups although serum insulin and C-peptide were lower and serum glucagon was higher in the hypox animals. These changes were accompanied by a more than tenfold drop in serum IGF-I. The pancreatic insulin content per gram of tissue was not significantly different in hypox and normal rats. Pancreatic glucagon and somatostatin per gram of tissue were higher in the hypox animals. The pancreatic IGF-I content of hypox rats was significantly reduced. Northern blot analysis gave a 2.6-, 4.5-, and 2.2-fold increase in pancreatic insulin, glucagon, and somatostatin mRNA levels, respectively, in hypox rats, and a 2.3-fold decrease in IGF-I mRNA levels. Our results show that the fall of serum IGF-I after hypophysectomy is accompanied by a decrease in pancreatic IGF-I peptide and mRNA but by partly discordant changes in the serum concentrations of insulin and glucagon and the islet peptide and/or mRNA content of the three major islet hormones. It appears that GH deficiency resulting in a "low IGF-I state" affects translational efficiency of these hormones as well as their secretory responses. The maintenance of normoglycemia in the presence of reduced insulin and elevated glucagon serum levels, both of which would be expected to raise blood glucose, may result mainly from the enhanced insulin sensitivity, possibly due to GH deficiency and the subsequent decrease in IGF-I production.