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J P Whitehead

Publications and source records attributed to J P Whitehead.

At least 19 recordsLinked to original sources
PubMed2006

Adiponectin--a key adipokine in the metabolic syndrome.

Adiponectin is a recently described adipokine that has been recognized as a key regulator of insulin sensitivity and tissue inflammation. It is produced by adipose tissue (white and brown) and circulates in the blood at very high concentrations. It has direct actions in liver, skeletal muscle and the vasculature, with prominent roles to improve hepatic insulin sensitivity, increase fuel oxidation [via up-regulation of adenosine monophosphate-activated protein kinase (AMPK) activity] and decrease vascular inflammation. Adiponectin exists in the circulation as varying molecular weight forms, produced by multimerization. Recent data indicate that the high-molecular weight (HMW) complexes have the predominant action in the liver. In contrast to other adipokines, adiponectin secretion and circulating levels are inversely proportional to body fat content. Levels are further reduced in subjects with diabetes and coronary artery disease. Adiponectin antagonizes many effects of tumour necrosis factor-alpha(TNF-alpha) and this, in turn, suppresses adiponectin production. Furthermore, adiponectin secretion from adipocytes is enhanced by thiazolidinediones (which also act to antagonize TNF-alpha effects). Thus, adiponectin may be the common mechanism by which TNF-alpha promotes, and the thiazolidinediones suppress, insulin resistance and inflammation. Two adiponectin receptors, termed AdipoR1 and AdipoR2, have been identified and these are ubiquitously expressed. AdipoR1 is most highly expressed in skeletal muscle and has a prominent action to activate AMPK, and hence promote lipid oxidation. AdipoR2 is most highly expressed in liver, where it enhances insulin sensitivity and reduces steatosis via activation of AMPK and increased peroxisome-proliferator-activated receptor alpha ligand activity. T-cadherin, which is expressed in endothelium and smooth muscle, has been identified as an adiponectin-binding protein with preference for HMW adiponectin multimers. Given the low levels of adiponectin in subjects with the metabolic syndrome, and the beneficial effect of the adipokine in animal studies, there is exciting potential for adiponectin replacement therapy in insulin resistance and related disorders.

Adiponectin↗
PubMed2002

Genetic variants of insulin receptor substrate-1 (IRS-1) in syndromes of severe insulin resistance. Functional analysis of Ala513Pro and Gly1158Glu IRS-1.

AIMS: To define further the role of IRS-1 mutations in human syndromes of severe insulin resistance. METHODS: The IRS-1 gene was scanned for mutations in 83 unrelated affected subjects and 47 unaffected individuals using fluorescent single-strand conformation polymorphism (fSSCP) analysis. A novel heterozygous mutation, Gly1158Glu, was found in one affected subject. Four and two subjects were heterozygous for the previously reported variants Gly972Arg and Ala513Pro, respectively. The previously identified variant Gly819Arg was found in one affected and one unaffected subject. While Gly972Arg has been described to alter the signalling properties of IRS-1, no functional studies of Ala513Pro or Gly1158Glu have been reported. RESULTS: Chinese hamster ovary (CHO) cells stably over-expressing the insulin receptor were transiently transfected with vectors expressing either wild-type, Glu1158 or Pro513 IRS-1. A modest increase in insulin-stimulated tyrosine phosphorylation of Glu1158 IRS-1 was observed. However, this did not result in any significant change in the association of Grb2 or the p85 alpha subunit of PI3-kinase or of PI3-kinase activity. In parallel studies, the Pro513 IRS-1 variant was indistinguishable from wild-type IRS-1. CONCLUSIONS: While subtle effects of these variants cannot be excluded in this system, it is unlikely that these variants are responsible for the extreme insulin resistance seen in the subjects harbouring them. Although IRS proteins play a central role in insulin signalling, functionally significant mutations in the IRS-1 gene are a rare cause of human syndromes of severe insulin resistance.

Animals↗
PubMed2001

Class II phosphoinositide 3-kinase is activated by insulin but not by contraction in skeletal muscle.

While the role of the class IA phosphoinositide 3-kinase (PI 3-kinase) in insulin signaling is well established, little is known about the role of the class II PI 3-kinases. We show that insulin stimulation of intact rat soleus and epitrochlearis muscles causes a 3- to 4-fold increase in the activity of the wortmannin-resistant alpha isoform of the class II PI 3-kinase (PI3K-C2alpha). This activation is rapid and parallels the insulin-induced activation of the class IA PI 3-kinase associated with IRS-1 in these muscles. However, while contraction activated p38 Map kinase, it did not stimulate the activity of the class II PI 3-kinase. Therefore, activation of class II PI 3-kinase is unlikely to provide a mechanism that explains the fact that exercise-induced activation of glucose uptake is not blocked by wortmannin. However, the results suggest that activation of class II PI 3-kinase is likely to play a role in insulin signaling pathways in skeletal muscle.

Androstadienes↗
PubMed2001

Nocodazole inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes via a microtubule-independent mechanism.

Insulin stimulates glucose transport in adipocytes and muscle cells by triggering redistribution of the GLUT4 glucose transporter from an intracellular perinuclear location to the cell surface. Recent reports have shown that the microtubule-depolymerizing agent nocodazole inhibits insulin-stimulated glucose transport, implicating an important role for microtubules in this process. In the present study we show that 2 microm nocodazole completely depolymerized microtubules in 3T3-L1 adipocytes, as determined morphologically and biochemically, resulting in dispersal of the perinuclear GLUT4 compartment and the Golgi apparatus. However, 2 microm nocodazole did not significantly effect either the kinetics or magnitude of insulin-stimulated glucose transport. Consistent with previous studies, higher concentrations of nocodazole (10-33 microm) significantly inhibited basal and insulin-stimulated glucose uptake in adipocytes. This effect was not likely the result of microtubule depolymerization because in the presence of taxol, which blocked nocodazole-induced depolymerization of microtubules as well as the dispersal of the perinuclear GLUT4 compartment, the inhibitory effect of 10-33 microm nocodazole on insulin-stimulated glucose uptake prevailed. Despite the decrease in insulin-stimulated glucose transport with 33 microm nocodazole we did not observe inhibition of insulin-stimulated GLUT4 translocation to the cell surface under these conditions. Consistent with a direct effect of nocodazole on glucose transporter function we observed a rapid inhibitory effect of nocodazole on glucose transport activity when added to either 3T3-L1 adipocytes or to Chinese hamster ovary cells at 4 degrees C. These studies reveal a new and unexpected effect of nocodazole in mammalian cells which appears to occur independently of its microtubule-depolymerizing effects.

3T3 Cells↗
PubMed2001

The role of Ca2+ in insulin-stimulated glucose transport in 3T3-L1 cells.

We have examined the requirement for Ca2+ in the signaling and trafficking pathways involved in insulin-stimulated glucose uptake in 3T3-L1 adipocytes. Chelation of intracellular Ca2+, using 1,2-bis (o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra (acetoxy- methyl) ester (BAPTA-AM), resulted in >95% inhibition of insulin-stimulated glucose uptake. The calmodulin antagonist, W13, inhibited insulin-stimulated glucose uptake by 60%. Both BAPTA-AM and W13 inhibited Akt phosphorylation by 70-75%. However, analysis of insulin-dose response curves indicated that this inhibition was not sufficient to explain the effects of BAPTA-AM and W13 on glucose uptake. BAPTA-AM inhibited insulin-stimulated translocation of GLUT4 by 50%, as determined by plasma membrane lawn assay and subcellular fractionation. In contrast, the insulin-stimulated appearance of HA-tagged GLUT4 at the cell surface, as measured by surface binding, was blocked by BAPTA-AM. While the ionophores or ionomycin prevented the inhibition of Akt phosphorylation and GLUT4 translocation by BAPTA-AM, they did not overcome the inhibition of glucose transport. Moreover, glucose uptake of cells pretreated with insulin followed by rapid cooling to 4 degrees C, to promote cell surface expression of GLUT4 and prevent subsequent endocytosis, was inhibited specifically by BAPTA-AM. This indicates that inhibition of glucose uptake by BAPTA-AM is independent of both trafficking and signal transduction. These data indicate that Ca2+ is involved in at least two different steps of the insulin-dependent recruitment of GLUT4 to the plasma membrane. One involves the translocation step. The second involves the fusion of GLUT4 vesicles with the plasma membrane. These data are consistent with the hypothesis that Ca2+/calmodulin plays a fundamental role in eukaryotic vesicle docking and fusion. Finally, BAPTA-AM may inhibit the activity of the facilitative transporters by binding directly to the transporter itself.

3T3 Cells↗
PubMed2001

Electronic and structural variation among copper(II) complexes with substituted phenanthrolines.

A series of copper(II) complexes with substituted phenanthroline ligands has been synthesized and characterized electronically and structurally. The compounds that have been prepared include the monosubstituted ligand complexes of the general formula [Cu(5-R-phen)(2)(CH(3)CN)](BF(4))(2), where R = NO(2), Cl, H, or Me, and the disubstituted ligand complex [Cu(5,6-Me(2)-phen)(2)(CH(3)CN)](BF(4))(2). The complexes [Cu(5-NO(2)-phen)(2)(CH(3)CN](BF(4))(2) (1), [Cu(5-Cl-phen)(2)(CH(3)CN)](BF(4))(2) (2), [Cu(o-phen)(2)(CH(3)CN)](BF(4))(2) (3), and [Cu(5-Me-phen)(2)(CH(3)CN)](BF(4))(2) (4) each crystallize in the space group C2/c with compounds 1, 2, and 4 comprising an isomorphous set. The disubstituted complex [Cu(5,6-Me(2)-phen)(2)(CH(3)CN)](BF(4))(2) (5) crystallizes in the space group P2(1)/c. Each structure is characterized by a distorted trigonal bipyramidal arrangement of ligands around the central copper atom with approximate or exact C(2) symmetry. The progression from electron-withdrawing to electron-donating substituents on the phenanthroline ligands correlates with less accessible reduction potentials for the bis-chelate complexes.

Copper↗
PubMed2001

GLUT4--at the cross roads between membrane trafficking and signal transduction.

GLUT4 is a mammalian facilitative glucose transporter that is highly expressed in adipose tissue and striated muscle. In response to insulin, GLUT4 moves from intracellular storage areas to the plasma membrane, thus increasing cellular glucose uptake. While the verification of this 'translocation hypothesis' (Cushman SW, Wardzala LJ. J Biol Chem 1980;255: 4758-4762 and Suzuki K, Kono T. Proc Natl Acad Sci 1980;77: 2542-2545) has increased our understanding of insulin-regulated glucose transport, a number of fundamental questions remain unanswered. Where is GLUT4 stored within the basal cell? How does GLUT4 move to the cell surface and what mechanism does insulin employ to accelerate this process? Ultimately we require a convergence of trafficking studies with research in signal transduction. However, despite more than 30 years of intensive research we have still not reached this point. The problem is complex, involving at least two separate signal transduction pathways which feed into what appears to be a very dynamic sorting process. Below we discuss some of these complexities and highlight new data that are bringing us closer to the resolution of these questions.

Adipocytes↗
PubMed2001

Potentiation of glucose uptake in 3T3-L1 adipocytes by PPAR gamma agonists is maintained in cells expressing a PPAR gamma dominant-negative mutant: evidence for selectivity in the downstream responses to PPAR gamma activation.

Pharmacological agonists for the nuclear receptor PPAR gamma enhance glucose disposal in a variety of insulin-resistant states in humans and animals. The precise mechanisms whereby activation of PPAR gamma leads to increased glucose uptake in metabolically active cells remain to be determined. Notably, certain novel, synthetic PPAR gamma ligands appear to antagonize thiazolidinedione-induced adipogenesis yet stimulate cellular glucose uptake. We have explored the molecular mechanisms underlying the enhancement of glucose uptake produced by PPAR gamma agonists in 3T3-L1 adipocytes. Rosiglitazone treatment for 48 h significantly increased basal and insulin-stimulated glucose uptake and markedly increased the cellular expression of GLUT1 but not GLUT4. Rosiglitazone increased plasma membrane levels of GLUT1, but not GLUT4, both basally and after insulin stimulation. Surprisingly, adenoviral expression of a dominant-negative mutant PPAR gamma, which was demonstrated to strongly inhibit adipogenesis, completely failed to inhibit rosiglitazone-stimulated glucose uptake. Similar findings were obtained with the non-thiazolidinedione PPAR gamma agonists, GW1929 and GW7845. The insensitivity of PPAR gamma agonist-stimulated glucose uptake to expression of a dominant-negative mutant, compared with the latter's marked inhibitory effects on preadipocyte differentiation, suggests that, as is the case for other nuclear receptors, the precise molecular mechanisms linking PPAR gamma activation to downstream events may differ depending on the nature of the biological response. The growing evidence that the effects of PPAR gamma on adipogenesis and glucose uptake can be dissociated may have important implications for the development of improved antidiabetic drug treatments.

3T3 Cells↗
PubMed2000

Arachidonic acid stimulates glucose uptake in 3T3-L1 adipocytes by increasing GLUT1 and GLUT4 levels at the plasma membrane. Evidence for involvement of lipoxygenase metabolites and peroxisome proliferator-activated receptor gamma.

Exposure of insulin-sensitive tissues to free fatty acids can impair glucose disposal through inhibition of carbohydrate oxidation and glucose transport. However, certain fatty acids and their derivatives can also act as endogenous ligands for peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor that positively modulates insulin sensitivity. To clarify the effects of externally delivered fatty acids on glucose uptake in an insulin-responsive cell type, we systematically examined the effects of a range of fatty acids on glucose uptake in 3T3-L1 adipocytes. Of the fatty acids examined, arachidonic acid (AA) had the greatest positive effects, significantly increasing basal and insulin-stimulated glucose uptake by 1.8- and 2-fold, respectively, with effects being maximal at 4 h at which time membrane phospholipid content of AA was markedly increased. The effects of AA were sensitive to the inhibition of protein synthesis but were unrelated to changes in membrane fluidity. AA had no effect on total cellular levels of glucose transporters, but significantly increased levels of GLUT1 and GLUT4 at the plasma membrane. While the effects of AA were insensitive to cyclooxygenase inhibition, the lipoxygenase inhibitor, nordihydroguaiaretic acid, substantially blocked the AA effect on basal glucose uptake. Furthermore, adenoviral expression of a dominant-negative PPARgamma mutant attenuated the AA potentiation of basal glucose uptake. Thus, AA potentiates basal and insulin-stimulated glucose uptake in 3T3-L1 adipocytes by a cyclooxygenase-independent mechanism that increases the levels of both GLUT1 and GLUT4 at the plasma membrane. These effects are at least partly dependent on de novo protein synthesis, an intact lipoxygenase pathway and the activation of PPARgamma with these pathways having a greater role in the absence than in the presence of insulin.

3T3 Cells↗
PubMed2000

Contraction inhibits insulin-stimulated insulin receptor substrate-1/2-associated phosphoinositide 3-kinase activity, but not protein kinase B activation or glucose uptake, in rat muscle.

The initial stages of insulin-stimulated glucose uptake are thought to involve tyrosine phosphorylation of insulin receptor substrates (IRSs), which recruit and activate phosphoinositide 3-kinase (PI 3-kinase), leading to the activation of protein kinase B (PKB) and other downstream effectors. In contrast, contraction stimulates glucose uptake via a PI 3-kinase-independent mechanism. The combined effects of insulin and contraction on glucose uptake are additive. However, it has been reported that contraction causes a decrease in insulin-stimulated IRS-1-associated PI 3-kinase activity. To investigate this paradox, we have examined the effects of contraction on insulin-stimulated glucose uptake and proximal insulin-signalling events in isolated rat epitrochlearis muscle. Stimulation by insulin or contraction produced a 3-fold increase in glucose uptake, with the effects of simultaneous treatment by insulin and contraction being additive. Wortmannin completely blocked the additive effect of insulin in contracting skeletal muscle, indicating that this is a PI 3-kinase-dependent effect. Insulin-stimulated recruitment of PI 3-kinase to IRS-1 was unaffected by contraction; however, insulin produced no discernible increase in PI 3-kinase activity in IRS-1 or IRS-2 immunocomplexes in contracting skeletal muscle. Consistent with this, contraction inhibited insulin-stimulated p70(S6K) activation. In contrast, insulin-stimulated activation of PKB was unaffected by contraction. Thus, in contracting skeletal muscle, insulin stimulates glucose uptake and activates PKB, but not p70(S6K), by a PI 3-kinase-dependent mechanism that is independent of changes in IRS-1- and IRS-2-associated PI 3-kinase activity.

Androstadienes↗
PubMed2000

Natural variants of human p85 alpha phosphoinositide 3-kinase in severe insulin resistance: a novel variant with impaired insulin-stimulated lipid kinase activity.

AIMS/HYPOTHESIS: Phosphoinositide 3-kinase (PI 3K) plays a central part in the mediation of insulin-stimulated glucose disposal. No genetic studies of this enzyme in human syndromes of severe insulin resistance have been previously reported. METHODS: Phosphoinositide 3-kinase p85 alpha regulatory subunit cDNA was examined in 20 subjects with syndromes of severe insulin resistance by single strand conformational polymorphism and restriction fragment length polymorphism analyses. Insulin-stimulated phosphoinositide 3-kinase activity and recruitment into phosphotyrosine complexes of variants of p85 alpha were studied in transiently transfected HEK293 cells. Phosphopeptide binding characteristics of wild-type and mutant p85 alpha-GST fusion proteins were examined by surface plasmon resonance. RESULTS: The common p85 alpha variant, Met326I1e, was identified in 9 of the 20 subjects. Functional studies of the Met326Ile variant showed it to have equivalent insulin-stimulated lipid kinase activity and phosphotyrosine recruitment as wild-type p85 alpha. A novel heterozygous mutation, Arg409Gln, was detected in one subject. Within the proband's family, carriers of the mutation had a higher median fasting plasma insulin (218 pmol/l) compared with wild-type relatives (72 mol/l) (n = 8 subjects, p = 0.06). The Arg409Gln p85 alpha subunit was associated with lower insulin-stimulated phosphoinositide 3-kinase activity compared with wild-type (mean reduction 15%, p < 0.05, n = 5). The recruitment of Arg409Gln p85 alpha into phosphotyrosine complexes was not significantly impaired. GST fusion proteins of wild-type and mutant p85 alpha showed identical binding to phosphopeptides in surface plasmon resonance studies. CONCLUSION/INTERPRETATION: Mutations in p85 alpha are uncommon in subjects with syndromes of severe insulin resistance. The Met326Ile p85 alpha variant appears to have no functional effect on the insulin-stimulated phosphoinositide 3-kinase activity. The impaired phosphoinositide 3-kinase activity of the Arg409Gln mutant suggests that it could contribute to the insulin resistance seen in this family.

Adult↗
PubMed2000

Signalling through the insulin receptor.

The insulin receptor substrates function at the heart of the insulin signalling network. It has recently become apparent that the intracellular localisation of these molecules is regulated in a precise manner that is critical for both the generation and the termination of the insulin signal. Some insulin receptor substrate isoforms appear to be associated with an insoluble matrix that resembles the cytoskeleton. When inappropriately dissociated from this matrix the signalling network collapses concomitant with loss of insulin sensitivity.

Animals↗
PubMed2000

Depot-related and thiazolidinedione-responsive expression of uncoupling protein 2 (UCP2) in human adipocytes.

OBJECTIVES: Uncoupling protein 2 (UCP2) is a recently described homologue of the uncoupling protein of brown adipocytes (UCP1), which is expressed at high levels in human white adipose tissue. Studies were undertaken (1) to establish whether the expression of UCP2 mRNA varies in a depot-related manner in isolated human adipocytes, (2) to determine whether thiazolidinedione exposure influences the expression of UCP2 mRNA in cultured human pre-adipocytes, and (3) to determine whether human UCP2 is targeted to mitochondria when transfected into mammalian cells. SUBJECTS: Abdominal subcutaneous and omental adipose tissue biopsies were obtained from adult patients undergoing elective intra-abdominal surgical procedures. MEASUREMENTS: A competitive reverse transcriptase-polymerase chain reaction (RT-PCR) was used to quantify UCP2 mRNA expression in human omental and subcutaneous adipocytes, and in cultured human preadipocytes differentiated in vitro using the thiazolidinedione, BRL49653. Chinese hamster ovary cells were transfected with a vector expressing human UCP2, and its cellular localization was determined by confocal immunofluorescence microscopy. RESULTS: Adipocytes isolated from human omentum consistently expressed more UCP2 mRNA than did subcutaneous adipocytes from the same subjects (mean fold difference 2.92+/-0.44 P<0.001, n=11) with no effect of gender or body mass index being seen. BRL49653 treatment of subcutaneously, but not omentally, derived preadipocytes stimulated expression of UCP2 mRNA (5.1+/-1.1 fold). Transfected human UCP2 was detected exclusively in mitochondria of CHO cells. CONCLUSIONS: Increased expression of UCP2 in human omental adipose tissue relative to subcutaneous adipose tissue is related to the expression levels in adipocytes per se, a finding which may relate to the particular functional attributes of this sub-population of adipocytes. Furthermore, BRL 49653 has site-specific effects of on the expression of UCP2 in human preadipocytes, a finding which may be relevant to the therapeutic effects of such compounds. Finally we present evidence for the mitochondrial localisation of human UCP2.

Adipocytes↗
PubMed2000

Naturally occurring amino acid substitutions at Arg1174 in the human insulin receptor result in differential effects on receptor biosynthesis and hybrid formation, leading to discordant clinical phenotypes.

Missense mutations in the tyrosine kinase domain of the human insulin receptor frequently result in a dominantly inherited form of insulin resistance. We noted a marked disparity in the clinical phenotypes of our study subjects with different missense mutations at the same residue (Arg1174) of the insulin receptor. Subjects with a tryptophan substitution (W) were only moderately hyperinsulinemic, whereas those with a glutamine substitution (Q) had severe clinical and biochemical insulin resistance. Studies were undertaken to explore the molecular mechanisms underlying these differences. Both W and Q mutant receptors bound insulin normally but were kinase inactive. The W mutation resulted in more rapid degradation of newly synthesized mutant receptor, which contrasted with the near-normal biosynthesis of the Q receptor. The propensity of the W receptor to form hybrids with the cotransfected wild-type (WT) receptor was also markedly impaired compared with the Q receptor, to an extent greater than could be explained by lower steady-state expression. Thus, the more clinically benign consequences of the heterozygous W mutant receptor are likely to relate to its impaired biosynthesis and/or reduced capacity to form hybrids with WT receptors. In addition to providing an explanation for the milder phenotype of 1174W versus 1174Q carriers, these studies provide further support for the notion that the dominant-negative effect of insulin receptor tyrosine kinase mutations involves the competition between inactive mutant homodimers and WT/mutant hybrids with active WT homodimers for both ligands and intracellular substrates.

Adolescent↗
PubMed1999

Differences in signaling properties of the cytoplasmic domains of the insulin receptor and insulin-like growth factor receptor in 3T3-L1 adipocytes.

Insulin and insulin-like growth factors (IGFs) elicit distinct but overlapping biological effects in vivo. To investigate whether differences in intrinsic signaling capacity of receptors contribute to biological specificity, we constructed chimeric receptors containing the extracellular portion of the neurotrophin receptor TrkC fused to the intracellular portion of the insulin or IGF-I receptors. Chimeras were stably expressed in 3T3-L1 adipocytes at levels comparable to endogenous insulin receptors and were efficiently activated by neurotrophin-3. The wild-type insulin receptor chimera mediated approximately 2-fold greater phosphorylation of insulin receptor substrate 1 (IRS-1), association of IRS-1 with phosphoinositide 3-kinase, stimulation of glucose uptake, and GLUT4 translocation, compared with the IGF-I receptor chimera. In contrast, the IGF-I receptor chimera mediated more effective Shc phosphorylation, association of Shc with Grb2, and activation of mitogen-activated protein kinase compared with the insulin receptor chimera. The two receptors elicited similar activation of protein kinase B, p70S6 kinase, and glycogen synthesis. We conclude that the insulin receptor mediates some aspects of metabolic signaling in adipocytes more effectively than the IGF-I receptor, as a consequence of more efficient phosphorylation of IRS-1 and greater recruitment/activation of phosphoinositide 3-kinase.

3T3 Cells↗
PubMed1999

Truncated human leptin (delta133) associated with extreme obesity undergoes proteasomal degradation after defective intracellular transport.

We recently described a homozygous frameshift mutation in the human leptin (ob) gene associated with undetectable serum leptin and extreme obesity in two individuals. This represented the first identified genetic cause of morbid obesity in humans. Preliminary data suggested a defect in the secretion of this truncated (delta133) mutant leptin. In the present investigation, we have examined the mechanisms underlying the defective secretion of the delta133 leptin in transient transfection studies in Chinese hamster ovary and monkey kidney epithelium cells. Consistent with our previous observations, only immunoreactive wild-type (wt) leptin was secreted. In pulse chase experiments, intracellular wt leptin levels decreased, concomitant with secretion into the medium. In contrast, though immunoreactive delta133 leptin disappeared from cell lysates with kinetics similar to those of wt leptin (half-life, 45 min), it was not detected in the medium. Inhibition of the proteasome, using the inhibitor clastolactacystin beta-lactone, led to a significant increase in the intracellular levels of delta133 leptin, indicating a role for the proteasome in the degradation pathway. Although intracellular immunoprecipitated wt and delta133 leptin levels were comparable, analysis of total cell lysates revealed a 7-fold increase in total intracellular delta133 leptin, compared with wt leptin. Size-exclusion membrane filtration demonstrated that intracellular delta133 leptin accumulated in an aggregated form, presumably as a result of misfolding in the endoplasmic reticulum. Consistent with this, an endoplasmic reticulum-like localization for delta133 leptin was detected by immunofluorescence microscopy. In conclusion, the delta133 mutant leptin is not secreted but accumulates intracellularly, as a consequence of misfolding/aggregation, and is subsequently degraded by the proteasome. These studies further define the genotype/phenotype correlation in this paradigmatic case of human leptin deficiency.

Animals↗
PubMed1998

Impaired activation of phosphoinositide 3-kinase by insulin in fibroblasts from patients with severe insulin resistance and pseudoacromegaly. A disorder characterized by selective postreceptor insulin resistance.

Some patients with severe insulin resistance develop pathological tissue growth reminiscent of acromegaly. Previous studies of such patients have suggested the presence of a selective postreceptor defect of insulin signaling, resulting in the impairment of metabolic but preservation of mitogenic signaling. As the activation of phosphoinositide 3-kinase (PI 3-kinase) is considered essential for insulin's metabolic signaling, we have examined insulin-stimulated PI 3-kinase activity in anti-insulin receptor substrate (IRS)-1 immunoprecipitates from cultured dermal fibroblasts obtained from pseudoacromegalic (PA) patients and controls. At a concentration of insulin (1 nM) similar to that seen in vivo in PA patients, the activation of IRS-1-associated PI 3-kinase was reduced markedly in fibroblasts from the PA patients (32+/-7% of the activity of normal controls, P < 0.01). Genetic and biochemical studies indicated that this impairment was not secondary to a defect in the structure, expression, or activation of the insulin receptor, IRS-1, or p85alpha. Insulin stimulation of mitogenesis in PA fibroblasts, as determined by thymidine incorporation, was indistinguishable from controls, as was mitogen-activated protein kinase phosphorylation, confirming the integrity of insulin's mitogenic signaling pathways in this condition. These findings support the existence of an intrinsic defect of postreceptor insulin signaling in the PA subtype of insulin resistance, which involves impairment of the activation of PI 3-kinase. The PA tissue growth seen in such patients is likely to result from severe in vivo hyperinsulinemia activating intact mitogenic signaling pathways emanating from the insulin receptor.

Acromegaly↗