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Adrian J L Clark

Publications and source records attributed to Adrian J L Clark.

31 records · Page 2Linked to original sources
PubMed2003

Expression, desensitization, and internalization of the ACTH receptor (MC2R).

Research into the functions and mechanisms of action of the melanocortin 2 receptor (MC2R) has been severely hampered by difficulties in expressing this gene in heterologous cells. This probably arises because of the need for a cofactor for cell surface expression. Using either the Y1 cell line that expresses endogenous MC2R or the Y6 cell line that expresses this putative expression factor, we have explored the mechanisms of desensitization and internalization after agonist stimulation. Protein kinase A dependence of desensitization has been demonstrated, although internalization is apparently independent of this kinase and dependent on a G protein receptor kinase. Possible underlying reasons for this paradox are discussed.

Adrenocorticotropic Hormone↗
PubMed2003

Association between insulin-like growth factor I (IGF-I) polymorphisms, circulating IGF-I, and pre- and postnatal growth in two European small for gestational age populations.

The purpose of this study was to assess the association of IGF-I and birth size by studying small for gestational age (SGA) subphenotypes and undertaking more detailed analysis of IGF-I genetic markers. SGA subjects from Haguenau, France (n = 113), and Gothenburg, Sweden (n = 174), were studied. The Swedish subjects were subphenotyped according to postnatal growth (114 short SGA and 60 SGA catch-up). IGF-I dinucleotide repeat and single nucleotide polymorphism (SNP) markers were studied, and haplotypes were generated in the Swedish short SGA group by identity of state. Association analysis was undertaken using the Monte Carlo method of association analysis of multiallelic markers for dinucleotide repeat markers, by exact chi(2) analysis for SNPs and by ANOVA for serum IGF-I levels. IGF-I genotype was associated with the SGA phenotype, in particular with symmetrical SGA and low birth weight, and with IGF-I levels in SGA subjects. Association with postnatal growth was different in the two populations, which may reflect the power of the smaller subphenotype groups. Haplotype analysis in the Swedish short SGA subjects showed that the region of association lay between the promoter and intron 2 of the IGF-I gene. These studies validate the association of the IGF-I gene with birth size and refine the region of association in Swedish short SGA subjects.

Adolescent↗
PubMed2003

Fetal programming of perivenous glucose uptake reveals a regulatory mechanism governing hepatic glucose output during refeeding.

Increased hepatic gluconeogenesis maintains glycemia during fasting and has been considered responsible for elevated hepatic glucose output in type 2 diabetes. Glucose derived periportally via gluconeogenesis is partially taken up perivenously in perfused liver but not in adult rats whose mothers were protein-restricted during gestation (MLP rats)-an environmental model of fetal programming of adult glucose intolerance exhibiting diminished perivenous glucokinase (GK) activity. We now show that perivenous glucose uptake rises with increasing glucose concentration (0-8 mmol/l) in control but not MLP liver, indicating that GK is flux-generating. The data demonstrate that acute control of hepatic glucose output is principally achieved by increasing perivenous glucose uptake, with rising glucose concentration during refeeding, rather than by downregulation of gluconeogenesis, which occurs in different hepatocytes. Consistent with these observations, glycogen synthesis in vivo commenced in the perivenous cells during refeeding, MLP livers accumulating less glycogen than controls. GK gene transcription was unchanged in MLP liver, the data supporting a recently proposed posttranscriptional model of GK regulation involving nuclear-cytoplasmic transport. The results are pertinent to impaired regulation of hepatic glucose output in type 2 diabetes, which could arise from diminished GK-mediated glucose uptake rather than increased gluconeogenesis.

Animals↗
PubMed2002

Linkage of one gene for familial glucocorticoid deficiency type 2 (FGD2) to chromosome 8q and further evidence of heterogeneity.

In several cases of familial glucocorticoid deficiency (FGD), referred to as FGD type 1, mutations have been described in the coding exon of the adrenocorticotropin receptor (melanonocortin receptor type 2, MC2R) gene. However, for the majority of cases (FGD type 2), no mutations were found in this gene. In the more informative families, the involvement of the MC2R locus could be excluded by linkage or sequencing analysis and, as there was no obvious candidate gene, a genome linkage scan was performed. Fourteen families were studied in this report. Evidence of linkage was found with markers on chromosome 8q in three out of the 14 families (maximum heterogeneity LOD score of 2.81 at D8S1763). These three families were consanguineous and the gene could be located by homozygosity mapping between markers D8S285 and D8S1718 in a 8.8-cM region. No potential candidate genes were apparent in the region. Linkage to this region could be excluded in some families from our sample giving highly negative LOD scores with the markers of the region. This result suggests that at least one other gene, located on a different region, must be responsible for FGD in these families and provides new evidence of genetic heterogeneity of this disorder.

Adrenal Cortex Diseases↗
PubMed2002

Role of the proline-rich domain of dynamin-2 and its interactions with Src homology 3 domains during endocytosis of the AT1 angiotensin receptor.

In nonneural tissues, the dynamin-2 isoform participates in the formation of clathrin-coated vesicles during receptor endocytosis. In this study, the mechanism of dynamin-2 action was explored during endocytosis of the G protein-coupled AT1A angiotensin receptor expressed in Chinese hamster ovary cells. Dynamin-2 molecules with mutant pleckstrin homology domains or deleted proline-rich domains (PRD) exerted dominant negative inhibition on the endocytosis of radiolabeled angiotensin II. However, only the PRD mutation interfered with the localization of the dynamin-2 molecule to clathrin-coated pits and reduced the inhibitory effect of the GTPase-deficient K44A mutant dynamin-2. Green fluorescent protein-tagged Src homology 3 (SH3) domains of endophilin I and amphiphysin II, two major binding partners of dynamins, also inhibited AT1A receptor-mediated endocytosis of angiotensin II. These effects were partially or fully, respectively, restored by the overexpression of dynamin-2. Transient overexpression of these SH3 domains also reduced the localization of dynamin-2 to clathrin-coated pits. These data indicate that, similar to the recruitment of dynamin-1 during the recycling of synaptic vesicles, interaction of the dynamin-2 PRD with SH3 domains of proteins such as the amphiphysins and endophilins is essential for AT1A receptor endocytosis. This mechanism could be of general importance in dynamin-dependent endocytosis of other G protein-coupled receptors in nonneural tissues.

Animals↗
PubMed2002

Spontaneous growth hormone secretory characteristics in children with partial growth hormone insensitivity.

OBJECTIVE: To investigate the characteristics of spontaneous GH secretion in four male children with short stature due to partial GH insensitivity. Their molecular defect consists of inclusion of a mutant intronic pseudoexon in the region of the GH receptor involved in homodimerization. SUBJECTS: The subjects were two pairs of brothers who were first cousins, aged 10.4-14.2 years, heights -3.3 to -5.6 SDS, from a consanguineous Pakistani family. Basal serum IGF-I levels were extremely low (20-29 mg/l; NR > 50), with absent or minimal response to human recombinant GH (hGH) stimulation. Serum IGFBP-3 SDS levels were also low (-2.9 to -8.9). GH binding protein (GHBP) levels were normal (28.1-51.7%). METHODS: Spontaneous GH secretion was studied by intermittent (20 min) venous sampling from 2000 to 0800 h. The secretion profiles were analysed using the Pulsar programme and compared to data from a reference population of 76 prepubertal Swedish children [median age 10.7 years, median height -1.1 SDS (-2.0 to 1.4)] according to Swedish growth standards. RESULTS: Median (range) Pulsar-derived values in the four patients and controls were, respectively: GHmax (mU/l) 276.6 (178.7-325.8) and 27.2 (13.1-94.9), mean GH (mU/l) 64.5 (41.9-77.8) and 5.8 (3.2-20.6), baseline (mU/l) 12.3 (11.7-20.1) and 1.1 (0.2-6.1), AUCb (mU/l x 24 h) 1210 (684-1555) and 112.5 (60.6-316.4), i.e. all parameters of GH secretion in the four patients were markedly elevated compared with the control population. CONCLUSIONS: Spontaneous GH secretion is elevated in partial GH insensitivity. This investigation could be of diagnostic value in children with short stature.

Adolescent↗
PubMed2002

Impaired desensitization of a mutant adrenocorticotropin receptor associated with apparent constitutive activity.

A naturally occurring ACTH receptor [melanocortin 2 receptor (MC2R)] mutation (F278C) has been identified in a subject with ACTH-independent Cushing's syndrome. Functional characterization of this mutant receptor reveals that it is associated with elevated basal cAMP accumulation when compared with wild-type receptor-expressing cell lines. Dose responsiveness is similar between wild-type and mutant receptors in cell lines expressing similar numbers of binding sites. In view of the location of this mutation in the C-terminal tail of the MC2R, desensitization and internalization were investigated and found to be impaired. Inhibition of protein kinase A by H89 blocks wild-type MC2R desensitization and also results in increased basal activity, as does alanine substitution of Ser 280 in the C-terminal tail. Alanine substitution of Ser 208, the consensus protein kinase A phosphorylation target in the third cytoplasmic loop also results in a reduction in desensitization without significant change in basal activity or internalization. These findings suggest a novel mechanism is involved in the apparently constitutive activation of the MC2R in which failure of desensitization appears to be associated with enhanced basal receptor activity.

Adrenal Cortex↗
PubMed2002

Advances in the understanding of the genetic basis of adrenal insufficiency.

Primary adrenal failure in a pediatric population most frequently results from autoimmune destruction of the gland. Recent discoveries regarding the genetic factors underlying this and other inherited causes of adrenocortical failure provide valuable insights into the molecular basis of adrenal development and function. Among these are the clinical syndromes resulting from defects in DAX-1 and SF-1 and the functional defects caused by mutations in the ACTH receptor and the StAR protein. Adrenoleukodystrophy and the triple A syndrome result from the degenerative effects induced by failure of very long chain fatty acid metabolism in the former case and the unknown effects of a novel protein that is mutated in the latter case. This review summarizes these recent findings and briefly describes the possible pathogenetic mechanisms that they reveal.

Adrenal Insufficiency↗