Cryptic receptors for insulin-like growth factor II in the plasma membrane of rat adipocytes--a possible link to cellular insulin resistance

Postprandial gut microbiota-driven choline metabolism links dietary cues to adipose tissue dysfunction

The human body is an integrated circuit between microbial symbionts and our Homo sapien genome, which communicate bi-directionally to maintain homeostasis within the human meta-organism. There is now strong evidence that microbes resident in the human intestine can directly contribute to the pathogenesis of obesity and associated cardiometabolic disorders. In fact, gut microbes represent a filter of our greatest environmental exposure - the foods we consume. It is now clear that we each experience a given meal differently, based on our unique gut microbial communities. Biologically active gut microbe-derived metabolites, such as short chain fatty acids, secondary bile acids, and trimethylamine-N-oxide (TMAO), are now uniquely recognized as contributors to obesity and related cardiometabolic disorders. However, mechanistic insights into how microbe-derived metabolites promote obesity are largely unknown. Recent work has demonstrated that the meta-organismal production of the bacterial co-metabolite TMAO is linked to suppression of beiging of white adipose tissue in mice and humans. Furthermore, the TMAO pathway is becoming an increasingly attractive therapeutic target in obesity-associated diseases such as type 2 diabetes, kidney failure, and cardiovascular disease. In this commentary we discuss recent findings linking the TMAO pathway to obesity-associated disorders, and provide additional insights into potential mechanisms driving this microbe-host interaction.

Human immunodeficiency virus-1 uses the mannose-6-phosphate receptor to cross the blood-brain barrier

HIV-1 circulates both as free virus and within immune cells, with the level of free virus being predictive of clinical course. Both forms of HIV-1 cross the blood-brain barrier (BBB) and much progress has been made in understanding the mechanisms by which infected immune cells cross the blood-brain barrier BBB. How HIV-1 as free virus crosses the BBB is less clear as brain endothelial cells are CD4 and galactosylceramide negative. Here, we found that HIV-1 can use the mannose-6 phosphate receptor (M6PR) to cross the BBB. Brain perfusion studies showed that HIV-1 crossed the BBB of all brain regions consistent with the uniform distribution of M6PR. Ultrastructural studies showed HIV-1 crossed by a transcytotic pathway consistent with transport by M6PR. An in vitro model of the BBB was used to show that transport of HIV-1 was inhibited by mannose, mannan, and mannose-6 phosphate and that enzymatic removal of high mannose oligosaccharide residues from HIV-1 reduced transport. Wheatgerm agglutinin and protamine sulfate, substances known to greatly increase transcytosis of HIV-1 across the BBB in vivo, were shown to be active in the in vitro model and to act through a mannose-dependent mechanism. Transport was also cAMP and calcium-dependent, the latter suggesting that the cation-dependent member of the M6PR family mediates HIV-1 transport across the BBB. We conclude that M6PR is an important receptor used by HIV-1 to cross the BBB.

Amyloid insulin interaction with erythrocytes

Erythrocyte membrane interactions with insulin fibrils (amyloid) have been investigated using centrifugation, fluorescence spectroscopy, light scattering, and flow cytometric techniques. The results indicate that insulin fibrils are having moderate affinity to erythrocyte membrane. However, analysis of the apparent dissociation constants of human erythrocyte membranes (leaky and resealed vesicles) with amyloid insulin reveal that the insulin binding is drastically reduced on attaining the fibrillar state compared with native insulin. To understand the role of insulin receptors on erythrocytes binding to amyloid, we have studied the interaction of biotinylated forms of denatured and amyloidic insulin with erythrocytes. FITC-streptavidin was used as a counter staining in flow cytometry measurements. We found that insulin fibrils bind 10 times more with erythrocyte membranes than with amylin and denatured insulin.

Insulin can enhance GLUT4 gene expression in 3T3-F442A cells and this effect is mimicked by vanadate but counteracted by cAMP and high glucose--potential implications for insulin resistance

It is well-established that high levels of cAMP or glucose can produce insulin resistance. The aim of this study was to characterize the interaction between these agents and insulin with respect to adipose tissue/muscle glucose transporter isoform (glucose transporter 4, GLUT4) gene regulation in cultured 3T3-F442A adipocytes and to further elucidate the GLUT4-related mechanisms in insulin resistance. Insulin (10(4) microU/ml) treatment for 16 h clearly increased GLUT4 mRNA level in cells cultured in medium containing 5.6 mM glucose but not in cells cultured in medium with high glucose (25 mM). 8-Bromo-cAMP (1 or 4 mM) or N(6)-monobutyryl cAMP, a hydrolyzable and a non-hydrolyzable cAMP analog, respectively, markedly decreased the GLUT4 mRNA level irrespective of glucose concentrations. In addition, these cAMP analogs also inhibited the upregulating effect of insulin on GLUT4 mRNA level. Interestingly, the tyrosine phosphatase inhibitor vanadate (1-50 microM) clearly increased GLUT4 mRNA level in a time- and concentration-dependent manner. Furthermore, cAMP-induced inhibition of the insulin effect was also prevented by vanadate. In parallel to the effects on GLUT4 gene expression, both insulin, vanadate and cAMP produced similar changes in cellular GLUT4 protein content and cAMP impaired the effect of insulin to stimulate (14)C-deoxyglucose uptake. In contrast, insulin, vanadate or cAMP did not alter insulin receptor (IR) mRNA or the cellular content of IR protein.

IN CONCLUSION

(1) Both insulin and vanadate elicit a stimulating effect on GLUT4 gene expression in 3T3-F442A cells, but a prerequisite is that the surrounding glucose concentration is low. (2) Cyclic AMP impairs the insulin effect on GLUT4 gene expression, but this is prevented by vanadate, probably by enhancing the tyrosine phosphorylation of signalling peptides and/or transcription factors. (3) IR gene and protein expression is not altered by insulin, vanadate or cAMP in this cell type. (4) The changes in GLUT4 gene expression produced by cAMP or vanadate are accompanied by similar alterations in GLUT4 protein expression and glucose uptake, suggesting a role of GLUT4 gene expression for the long-term regulation of cellular insulin action on glucose transport.

Insulin promotes and cyclic adenosine 3',5'-monophosphate impairs functional insertion of insulin receptors in the plasma membrane of rat adipocytes: evidence for opposing effects of tyrosine and serine/threonine phosphorylation

The aim of the present study was to elucidate events in the plasma membrane (PM) associated with the previously described effect of insulin to rapidly enhance the number of cell surface insulin binding sites in rat adipocytes. [125I]insulin was cross-linked to cell surface insulin receptors of intact cells that had been preincubated with or without insulin. Subsequently prepared PM displayed a approximately 3-fold increase in bound [125I]insulin when cells had been pretreated with 6 nM insulin for 20 min compared to membranes from control cells, and SDS-PAGE with autoradiography showed that this occurred at the insulin receptor alpha-subunit. The magnitude of the effect was similar to that found for insulin binding to intact cells that had been preincubated with insulin. In contrast, the insulin binding capacity in the PM was not affected by prior treatment of cells with insulin when assessed with the addition of [125I]insulin directly to solubilized PM; this suggests an unchanged total number of PM receptors. Thus, the enhancement of cell surface insulin binding capacity produced by insulin is not due to the translocation of receptors, but instead appears to be confined to receptors already present in the PM. The addition of phospholipase C (from Clostridium perfringens), which cleaves PM phospholipids, mimicked the effect of insulin to enhance cell surface binding in adipocytes, and this suggests a pool of cryptic PM receptors. Both the nonmetabolizable cAMP analog N6-monobutyryl cAMP (N6-mbcAMP) and the serine/threonine phosphatase inhibitor okadaic acid abolished the effect of concomitant insulin treatment to increase binding capacity. In contrast, the tyrosine phosphatase inhibitor vanadate increased insulin binding even in the presence of okadaic acid or N6-mbcAMP. The effect of N6-mbcAMP to impair cell surface insulin binding was also evident in the presence of a peptide derived from the major histocompatibility complex type I that effectively impairs receptor internalization, but the amount of PM receptors assessed by immunoblot was unaltered. Taken together, the data suggest that insulin exposure leads to the uncovering of cryptic receptors associated with the PM. It is also suggested that tyrosine phosphorylation promotes this process, whereas enhanced serine phosphorylation, e.g. produced by cAMP, impairs the functional insertion of the receptors, rendering them unable to bind insulin.