Endocytosis is inhibited in hepatocytes from diabetic rats

Identification of potential lncRNAs and co-expressed mRNAs in gestational diabetes mellitus by RNA sequencing

AIM

Gestational diabetes mellitus is common during pregnancy, impacting maternal health and fetal development. The aim of this study was to identify potential long non-coding RNAs (lncRNAs) and mRNAs in gestational diabetes mellitus.

METHODS

The placenta tissues from four women patients with gestational diabetes mellitus and three healthy pregnant women were used for RNA sequencing. Differentially expressed lncRNAs and mRNAs were obtained. Then, interaction networks of lncRNA-nearby targeted mRNA and lncRNA-co-expressed mRNA were constructed, followed by functional annotation of co-expressed mRNAs. Third, GSE51546 dataset was utilized to validate the expression of selected co-expressed mRNAs. In addition, in vitro experiment was applied to expression validation of lncRNAs and mRNAs. Finally, GSE70493 dataset was utilized for diagnostic analysis of selected co-expressed mRNAs.

RESULTS

A total of 78 differentially expressed lncRNAs and 647 differentially expressed mRNAs in gestational diabetes mellitus were obtained. Several interaction pairs of lncRNA-co-expressed mRNA including LINC01504-CASP8, FUT8-AS1-TLR5/GDF15, GATA2-AS1-PQLC3/KIAA2026, and EGFR-AS1-HLA-G were identified. Endocytosis (involved HLA-G) and toll-like receptor signaling pathway (involved TLR5 and CASP8) were remarkably enriched signaling pathways of co-expressed mRNAs. It is noted that CASP8, TLR5, and PQLC3 had a significant prognosis value for gestational diabetes mellitus.

CONCLUSIONS

Our study identified several differentially expressed lncRNAs and mRNAs, and their interactions, especially co-expression, may be associated with gestational diabetes mellitus.

Bioinformatics analysis of hepatic gene expression profiles in type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is characterized by hyperglycemia. The liver has a critical role in regulating glucose homeostasis. The present study aimed to analyze hepatic gene expression profiles and to identify the key genes and pathways involved in T2DM. Gene expression profiles of 10 patients with T2DM and 7 subjects with normal glucose tolerance were downloaded from the Gene Expression Omnibus database. Subsequently, differentially expressed genes (DEGs) were identified and functional enrichment analysis was performed. In addition, a protein-protein interaction network was built and hub genes were identified. In total, 1,320 DEGs were identified, including 698 up- and 622 downregulated genes, and these were mainly enriched in positive regulation of transcription from RNA polymerase II promoter, cell adhesion, inflammatory response, positive regulation of apoptotic process, signal transduction and the Tolllike receptor signaling pathway. A total of 8 hub genes (G-protein subunit gamma transducin 2, ubiquitinconjugating enzyme E2 D1, glutamate metabotropic receptor 1, G-protein signaling modulator 1, C-X-C motif chemokine ligand 9, neurotensin, purinergic receptor P2Y1 and ring finger protein 41) were screened from the network. The present study may contribute to the elucidation of the hepatic pathology of T2DM.

Insulin Receptor Trafficking: Consequences for Insulin Sensitivity and Diabetes

Insulin receptor (INSR) has been extensively studied in the area of cell proliferation and energy metabolism. Impaired INSR activities lead to insulin resistance, the key factor in the pathology of metabolic disorders including type 2 diabetes mellitus (T2DM). The mainstream opinion is that insulin resistance begins at a post-receptor level. The role of INSR activities and trafficking in insulin resistance pathogenesis has been largely ignored. Ligand-activated INSR is internalized and trafficked to early endosome (EE), where INSR is dephosphorylated and sorted. INSR can be subsequently conducted to lysosome for degradation or recycled back to the plasma membrane. The metabolic fate of INSR in cellular events implies the profound influence of INSR on insulin signaling pathways. Disruption of INSR-coupled activities has been identified in a wide range of insulin resistance-related diseases such as T2DM. Accumulating evidence suggests that alterations in INSR trafficking may lead to severe insulin resistance. However, there is very little understanding of how altered INSR activities undermine complex signaling pathways to the development of insulin resistance and T2DM. Here, we focus this review on summarizing previous findings on the molecular pathways of INSR trafficking in normal and diseased states. Through this review, we provide insights into the mechanistic role of INSR intracellular processes and activities in the development of insulin resistance and diabetes.

Membrane fluidity is regulated by the C. elegans transmembrane protein FLD-1 and its human homologs TLCD1/2

Dietary fatty acids are the main building blocks for cell membranes in animals, and mechanisms must therefore exist that compensate for dietary variations. We isolated C. elegans mutants that improved tolerance to dietary saturated fat in a sensitized genetic background, including eight alleles of the novel gene fld-1 that encodes a homolog of the human TLCD1 and TLCD2 transmembrane proteins. FLD-1 is localized on plasma membranes and acts by limiting the levels of highly membrane-fluidizing long-chain polyunsaturated fatty acid-containing phospholipids. Human TLCD1/2 also regulate membrane fluidity by limiting the levels of polyunsaturated fatty acid-containing membrane phospholipids. FLD-1 and TLCD1/2 do not regulate the synthesis of long-chain polyunsaturated fatty acids but rather limit their incorporation into phospholipids. We conclude that inhibition of FLD-1 or TLCD1/2 prevents lipotoxicity by allowing increased levels of membrane phospholipids that contain fluidizing long-chain polyunsaturated fatty acids.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Revisiting the membrane-centric view of diabetes

Fundamental questions remain unresolved in diabetes: What is the actual mechanism of glucose toxicity? Why is there insulin resistance in type 2 diabetes? Why do diets rich in sugars or saturated fatty acids increase the risk of developing diabetes? Studying the C. elegans homologs of the anti-diabetic adiponectin receptors (AdipoR1 and AdipoR2) has led us to exciting new discoveries and to revisit what may be termed “The Membrane Theory of Diabetes”. We hypothesize that excess saturated fatty acids (obtained through a diet rich in saturated fats or through conversion of sugars into saturated fats via lipogenesis) leads to rigid cellular membranes that in turn impair insulin signalling, glucose uptake and blood circulation, thus creating a vicious cycle that contributes to the development of overt type 2 diabetes. This hypothesis is supported by our own studies in C. elegans and by a wealth of literature concerning membrane composition in diabetics. The purpose of this review is to survey this literature in the light of the new results, and to provide an admittedly membrane-centric view of diabetes.

A resealed-cell system for analyzing pathogenic intracellular events: perturbation of endocytic pathways under diabetic conditions

Cell-based assay systems that can serve as cellular models of aberrant function in pathogenic organs would be novel and useful tools for screening drugs and clarifying the molecular mechanisms of various diseases. We constructed model cells that replicated the conditions in diabetic hepatocytes by using the cell resealing technique, which enables the exchange of cytosol. The plasma membrane of HeLa cells was permeabilized with the streptococcal toxin streptolysin O, and cytosol that had been prepared from wild-type or db/db diabetic mice was introduced into the resulting semi-intact cells. By resealing the plasma membrane by exposure to Ca²⁺, we created WT or Db model cells, in which the cytosolic conditions replicated those of healthy or diabetic liver. Interestingly, phosphorylation of p38 MAPK was promoted, whereas the level of endosomal phosphatidylinositol-3-phosphate was decreased, in Db cells. We investigated several endocytic pathways in WT and Db cells, and found that retrograde endosome-to-Golgi transport was delayed in a p38 MAPK-dependent manner in Db cells. Furthermore, the degradation pathway of the EGF receptor from endosomes to lysosomes was enhanced in Db cells, and this did not depend on the activation of p38 MAPK. The disease model cell system should become a powerful tool for the detection of aberrant processes in cells under pathogenic conditions and for therapeutic applications.

Identification of changes in gene expression in dorsal root ganglia in diabetic neuropathy: correlation with functional deficits

This study aimed to correlate the onset of functional deficits in diabetic neuropathy with changes in gene expression in rat dorsal root ganglia (DRG). After 1, 4, or 8 weeks of streptozotocin-induced diabetes, sensory and motor nerve conduction velocities (NCV) were measured as an indicator of neuropathy and changes in gene expression were measured using Affymetrix oligonucleotide microarrays. No significant changes in NCV were found after 1 week of diabetes, but after 4 and 8 weeks, there was a significant reduction in both sensory and motor NCV. Global gene expression changes in diabetic rat DRG were evident from principal component analysis of microarray data after 1, 4, and 8 weeks. Expression changes in individual genes were relatively small in line with a gradual degenerative neuropathy indirectly resulting from diabetes. Sets of differentially expressed genes have been identified and quantitative reverse transcriptase-polymerase chain reaction has been used to confirm the microarray data for several genes. Gene ontology overrepresentation analysis was performed on the microarray data to identify biologic processes altered in diabetic DRG. The genes identified in this study may be responsible for causing the functional deficits and suggest pathways/processes that require further investigation as possible targets for therapeutic intervention.

Thyroid hormone regulation of hepatic genes in vivo detected by complementary DNA microarray

The liver is an important target organ of thyroid hormone. However, only a limited number of hepatic target genes have been identified, and little is known about the pattern of their regulation by thyroid hormone. We used a quantitative fluorescent cDNA microarray to identify novel hepatic genes regulated by thyroid hormone. Fluorescent-labeled cDNA prepared from hepatic RNA of T3-treated and hypothyroid mice was hybridized to a cDNA microarray, representing 2225 different mouse genes, followed by computer analysis to compare relative changes in gene expression. Fifty five genes, 45 not previously known to be thyroid hormone-responsive genes, were found to be regulated by thyroid hormone. Among them, 14 were positively regulated by thyroid hormone, and unexpectedly, 41 were negatively regulated. The expression of 8 of these genes was confirmed by Northern blot analyses. Thyroid hormone affected gene expression for a diverse range of cellular pathways and functions, including gluconeogenesis, lipogenesis, insulin signaling, adenylate cyclase signaling, cell proliferation, and apoptosis. This is the first application of the microarray technique to study hormonal regulation of gene expression in vivo and should prove to be a powerful tool for future studies of hormone and drug action.

Carrier-mediated uptake of lucifer yellow in skate and rat hepatocytes: a fluid-phase marker revisited

Uptake of lucifer yellow (LY), a fluorescent disulfonic acid anionic dye, was studied in isolated skate (Raja erinacea) perfused livers and primary hepatocytes to evaluate its utility as a fluid-phase marker in these cells. However, our findings demonstrated that LY is transported across the plasma membrane of skate hepatocytes largely via carrier-mediated mechanisms. Isolated perfused skate livers cleared 50% of the LY from the recirculating perfusate within 1 h of addition of either 22 or 220 microM LY, with only 4.5 and 9% of the LY remaining in the perfusate after 7 h, respectively. Most of the LY was excreted into bile, resulting in high biliary LY concentrations (1 and 10 mM at the two doses, respectively), indicating concentrative transport into bile canalicular lumen. LY uptake by freshly isolated skate hepatocytes was temperature sensitive, exhibited saturation kinetics, and was inhibited by other organic anions. Uptake was mediated by both sodium-dependent [Michaelis-Menten constant (K(m)), 125 +/- 57 microM; maximal velocity (V(max)), 1.5 +/- 0.2 pmol. min(-1). mg cells(-1)] and sodium-independent (K(m), 207 +/- 55 microM; V(max), 1.7 +/- 0.2 pmol. min(-1). mg cells(-1)) mechanisms. Both of these uptake mechanisms were inhibited by various organic anions and transport inhibitors, including furosemide, bumetanide, sulfobromophthalein, rose bengal, probenecid, N-ethylmaleimide, taurocholate, and p-aminohippuric acid. Fluorescent imaging techniques showed intracellular vesicular compartmentation of LY in skate hepatocyte clusters. Studies in perfused rat livers also indicated that LY is taken up against a concentration gradient and concentrated in bile. LY uptake in isolated rat hepatocytes was saturable, but only at high concentrations, and demonstrated a K(m) of 3.7 +/- 1.0 mM and a V(max) of 1.75 +/- 0.16 nmol. min(-1). mg wet wt(-1). These results indicate that LY is transported into skate and rat hepatocytes and bile largely by carrier-mediated mechanisms, rather than by fluid-phase endocytosis.

Nef-mediated clathrin-coated pit formation

The sequence of events leading to clathrin-coated pit (CCP) nucleation on the cell surface and to the incorporation of receptors into these endocytic structures is still imperfectly understood. In particular, the question remains as to whether receptor tails initiate the assembly of the coat proteins or whether receptors migrate into preformed CCP. This question was approached through a dissection of the mechanisms implemented by Nef, an early protein of human and simian immunodeficiency virus (HIV and SIV, respectively), to accelerate the endocytosis of cluster of differentiation antigen type 4 (CD4), the major receptor for these viruses. Results collected showed that: (a) Nef promotes CD4 internalization via an increased association of CD4 with CCP; (b) the Nef-mediated increase of CD4 association with CCP is related to a doubling of the plasma membrane area occupied by clathrin-coated structures; (c) this increased CCP number at the plasma membrane has functional consequences preferentially on CD4 uptake and does not significantly affect transferrin receptor internalization or fluid-phase endocytosis; (d) the presence of a CD4 cytoplasmic tail including a critical dileucine motif is required to induce CCP formation via Nef; and (e) when directly anchored to the cytoplasmic side of the plasma membrane, Nef itself can promote CCP formation. Taken together, these observations lead us to propose that CD4 can promote CCP generation via the connector molecule Nef. In this model, Nef interacts on one side with CD4 through a dileucine-based motif present on CD4 cytoplasmic tail and on the other side with components of clathrin-coated surface domain (i.e., adaptins). These Nef-generated complexes would then initiate the nucleation of CCP.

Second-messenger regulation of receptor association with clathrin-coated pits: a novel and selective mechanism in the control of CD4 endocytosis

CD4, a member of the immunoglobulin superfamily, is not only expressed in T4 helper lymphocytes but also in myeloid cells. Receptor-mediated endocytosis plays a crucial role in the regulation of surface expression of adhesion molecules such as CD4. In T lymphocytes p56lck, a CD4-associated tyrosine kinase, prevents CD4 internalization, but in myeloid cells p56lck is not expressed and CD4 is constitutively internalized. In this study, we have investigated the role of cyclic AMP (cAMP) in the regulation of CD4 endocytosis in the myeloid cell line HL-60. Elevations of cellular cAMP were elicited by 1) cholera toxin, 2) pertussis toxin, 3) forskolin and IBMX, 4) NaF, or 5) the physiological receptor agonist prostaglandin E1. All five interventions led to an inhibition of CD4 internalization. Increased cAMP levels did not inhibit endocytosis per se, because internalization of insulin receptors and transferrin receptors and fluid phase endocytosis were either unchanged or slightly enhanced. The mechanism of cAMP inhibition was further analyzed at the ultrastructural level. CD4 internalization, followed either by quantitative electron microscopy autoradiography or by immunogold labeling, showed a rapid and temperature-dependent association of CD4 with clathrin-coated pits in control cells. This association was markedly inhibited in cells with elevated cAMP levels. Thus these findings suggest a second-messenger regulation of CD4 internalization through an inhibition of CD4 association with clathrin-coated pits in p56lck-negative cells.

Role of the transmembrane domain and flanking amino acids in internalization and down-regulation of the insulin receptor

We have characterized the internalization and down-regulation of the insulin receptor and nine receptors with mutations in the transmembrane (TM) domain and/or flanking charged amino acids to define the role of this domain in receptor cycling. When expressed in Chinese hamster ovary cells, all had normal tetrameric structure and normal insulin-stimulated autophosphorylation/kinase activity. Replacement of the TM domain with that of the platelet-derived growth factor receptor, insertion of 3 amino acids, and substitution of Asp for Val938 or of Ala for either Gly933 or Pro934 had no effect on internalization. Replacement of the TM domain with that of c-neu or conversion of the charged amino acids on the cytoplasmic flank to uncharged amino acids, on the other hand, resulted in a 40-60% decrease in insulin-dependent internalization rate constants. By contrast, substitution of Ala for both Gly933 and Pro934 increases lateral diffusion mobility and accelerates internalization rate. These changes in internalization were due to decreased or increased rates of redistribution of receptors from microvilli to the nonvillous cell surface. In all cases, receptor down-regulation and receptor-mediated insulin degradation paralleled the changes in internalization. Thus, the structure of the transmembrane domain of the insulin receptor and flanking amino acids are major determinants of receptor internalization, insulin degradation, and receptor down-regulation.

Insulin receptor internalization: molecular mechanisms and physiopathological implications

The initial interaction between insulin and its receptor on target cell surface is followed by a series of surface and intracellular steps which participate in the control of insulin action. Abnormalities of any of these steps could result in mishandling of the receptor leading to defective modulation of receptor number on the cell surface and to inappropriate cell sensitivity to the hormone. Thus, the identification of each of these steps as well as understanding the mechanisms governing them is obligatory to unravel some aspects of the pathogenesis of insulin resistance states. This was the goal of the studies we have carried out during recent years using combined molecular and cellular biology as well as biochemical techniques. These studies allowed us to propose the following ordered sequence of events: 1) insulin binds to receptors preferentially associated with microvilli on the cell surface; 2) insulin triggers receptor kinase activation and autophosphorylation which not only results in initiation of the various biological signals leading to insulin action but also in redistribution of the hormone-receptor complex in the plane of the membrane; 3) on the non-villous domain of the cell surface, insulin receptors anchor to clathrin-coated pits through specific "internalization sequences" present in their cytoplasmic juxtamembrane domain; 4) insulin-receptor complexes are internalized together with other receptors present in the same clathrin-coated pits through the formation of clathrin-coated vesicles; 5) the complexes are delivered to endosomes, the acidic pH of which induces the dissociation of insulin molecules from insulin receptors and their sorting in different directions; 6) insulin molecules are targetted to late endosomes and lysosomes where they are degraded; 7) receptors are recycled back to the cell surface in order to be reused.

Robert Feulgen Prize Lecture 1993. The journey of the insulin receptor into the cell: from cellular biology to pathophysiology

The data that we have reviewed indicate that insulin binds to a specific cell-surface receptor. The complex then becomes involved in a series of steps which lead the insulin-receptor complex to be internalized and rapidly delivered to endosomes. From this sorting station, the hormone is targeted to lysosomes to be degraded while the receptor is recycled back to the cell surface. This sequence of events presents two degrees of ligand specificity: (a) The first step is ligand-dependent and requires insulin-induced receptor phosphorylation of specific tyrosine residues. It consists in the surface redistribution of the receptor from microvilli where it preferentially localizes in its unoccupied form. (b) The second step is more general and consists in the association with clathrin-coated pits which represents the internalization gate common to many receptors. This sequence of events participates in the regulation of the biological action of the hormone and can thus be implicated in the pathophysiology of diabetes mellitus and various extreme insulin resistance syndromes, including type A extreme insulin resistance, leprechaunism, and Rabson-Mendehall syndrome. Alterations of the internalization process can result either from intrinsic abnormalities of the receptor or from more general alteration of the plasma membrane or of the cell metabolism. Type I diabetes is an example of the latter possibility, since general impairment of endocytosis could contribute to extracellular matrix accumulation and to an increase in blood cholesterol. Thus, better characterization of the molecular and cellular biology of the insulin receptor and of its journey inside the cell definitely leads to better understanding of disease states, including diabetes.