Differential regulation of GRP78 and GLUT1 expression in 3T3-L1 adipocytes

The parkinsonian LRRK2 R1441G mutation shows macroautophagy-mitophagy dysregulation concomitant with endoplasmic reticulum stress

Autophagy is a mechanism responsible for the degradation of cellular components to maintain their homeostasis. However, autophagy is commonly altered and compromised in several diseases, including neurodegenerative disorders. Parkinson's disease (PD) can be considered a multifactorial disease because environmental factors, genetic factors, and aging are involved. Several genes are involved in PD pathology, among which the LRRK2 gene and its mutations, inherited in an autosomal dominant manner, are responsible for most genetic PD cases. The R1441G LRRK2 mutation is, after G2019S, the most important in PD pathogenesis. Our results demonstrate a relationship between the R1441G LRRK2 mutation and a mechanistic dysregulation of autophagy that compromises cell viability. This altered autophagy mechanism is associated with organellar stress including mitochondrial (which induces mitophagy) and endoplasmic reticulum (ER) stress, consistent with the fact that patients with this mutation are more vulnerable to toxins related to PD, such as MPP⁺.

Calcium homeostasis and ER stress in control of autophagy in cancer cells

Autophagy is a basic catabolic process, serving as an internal engine during responses to various cellular stresses. As regards cancer, autophagy may play a tumor suppressive role by preserving cellular integrity during tumor development and by possible contribution to cell death. However, autophagy may also exert oncogenic effects by promoting tumor cell survival and preventing cell death, for example, upon anticancer treatment. The major factors influencing autophagy are Ca²⁺ homeostasis perturbation and starvation. Several Ca²⁺ channels like voltage-gated T- and L-type channels, IP3 receptors, or CRAC are involved in autophagy regulation. Glucose transporters, mainly from GLUT family, which are often upregulated in cancer, are also prominent targets for autophagy induction. Signals from both Ca²⁺ perturbations and glucose transport blockage might be integrated at UPR and ER stress activation. Molecular pathways such as IRE 1-JNK-Bcl-2, PERK-eIF2α-ATF4, or ATF6-XBP 1-ATG are related to autophagy induced through ER stress. Moreover ER molecular chaperones such as GRP78/BiP and transcription factors like CHOP participate in regulation of ER stress-mediated autophagy. Autophagy modulation might be promising in anticancer therapies; however, it is a context-dependent matter whether inhibition or activation of autophagy leads to tumor cell death.

Expression and activity of carbonic anhydrase IX is associated with metabolic dysfunction in MDA-MB-231 breast cancer cells

The expression of carbonic anhydrase IX (CAIX), a marker for hypoxic tumors, is correlated with poor prognosis in breast cancer patients. We show herein that the MDA-MB-231 cells, a "triple-negative," basal B line, express exclusively CAIX, while a luminal cell line (T47D) expresses carbonic anhydrase XII (CAXII). CAIX expression in the basal B cells is both density- and hypoxia-dependent and is correlated with carbonic anhydrase activity. Evidence is provided that CAIX contributes to extracellular acidification through studies on pH, lactic acid production, and CAIX inhibition. Together, these studies suggest that CAIX expression and activity is associated with metabolic dysfunction in MDA-MB-231 cells.

Stimulation of N-Linked Glycosylation and Lipid-Linked Oligosaccharide Synthesis by Stress Responses in Metazoan Cells

Endoplasmic reticulum (ER) stress responses comprising the unfolded protein response (UPR) are activated by conditions that disrupt folding and assembly of proteins inside the ER lumenal compartment. Conditions known to be proximal triggers of the UPR include saturation of chaperones with misfolded protein, redox imbalance, disruption of Ca2+ levels, interference with N-linked glycosylation, and failure to dispose of terminally misfolded proteins. Potentially, ER stress responses can reprogram cells to correct all of these problems and thereby restore ER function to normal. This article will review literature on stimulation of N-linked glycosylation by ER stress responses, focusing on metazoan systems. The mechanisms involved will be contrasted with those mediating stimulation of N-linked glycosylation by cytoplasmic stress responses. This information will interest readers who study the biological roles of stress responses, the functions of N-linked glycans, and potential strategies for treatment of genetic disorders of N-linked glycosylation.

150-kD oxygen-regulated protein is an essential factor for insulin release

OBJECTIVE

This study was designed to show the induction and function of 150-kD oxygen-regulated protein (ORP150) in insulin secretion in vitro.

METHODS

A mouse beta-cell line, MIN6 cells, was cultured in medium containing potassium channel openers or various concentrations of glucose (2-25 mmol/L). ORP150 expression was studied by reporter assay, Western blot, or Northern blot analysis, concomitantly with insulin secretion. In addition, MIN6 cells infected with ORP150 recombinant adenovirus were adopted to show the function of ORP150 in insulin release.

RESULTS

ORP150 expression in MIN6 cells was suppressed dose-dependently by the potassium channel opener diazoxide. Both low glucose (<2 mmol/L) and high glucose concentrations (25 mmol/L glucose) significantly induced more ORP150 expression compared with 10 mmol/L glucose. The treatment with diazoxide or infection of ORP150 antisense adenovirus suppressed ORP150 expression, and glucose-stimulated insulin secretion was effectively prevented.

CONCLUSION

These findings show the involvement of ORP150 in insulin secretion in MIN6 cells.

Glucose deprivation enhances targeting of GLUT1 to lipid rafts in 3T3-L1 adipocytes

Glucose deprivation dramatically increases glucose transport activity in 3T3-L1 adipocytes without changing the concentration of GLUT1 in the plasma membrane (PM). Recent data suggest that subcompartments within the PM, specifically lipid rafts, may sequester selected proteins and alter their activity. To evaluate this possibility, we examined the distribution of GLUT1 in Triton X-100-soluble and -insoluble fractions. Our data show that 77% of the GLUT1 pool in PMs isolated from control 3T3-L1 adipocytes was extracted by 0.2% Triton X-100. After glucose deprivation for 12 h, only 56% of GLUT1 was extracted by detergent. In contrast, there was a twofold increase in the GLUT1 content of the detergent-resistant fraction. To evaluate whether GLUT1 interacts with a specific protein within lipid rafts, we focused on stomatin, recently shown to interact with and inhibit GLUT1 activity. Stomatin is distributed about equally between the PM and the biosynthetic compartments, and its expression is not affected by glucose deprivation. Nearly 90% of the PM pool of stomatin is in detergent-resistant lipid rafts. In normal 3T3-L1 adipocytes, we were unable to demonstrate an interaction between GLUT1 and stomatin in coimmunoprecipitation experiments. However, in stomatin-overexpressing cells, there was clear coprecipitation of stomatin with GLUT1 antibodies. Glucose deprivation increased this interaction threefold, which may reflect the increase of GLUT1 in lipid rafts. Despite this, there was little change in transport activity in glucose-deprived, stomatin-overexpressing cells vs. that in control cells. Thus GLUT1 interacts with stomatin in lipid rafts, but this interaction per se does not alter transport activity. Rather, stomatin may serve as an anchor for GLUT1 in lipid rafts, the environment of which favors activation.

Agent and cell-type specificity in the induction of insulin resistance by HIV protease inhibitors

OBJECTIVE

To test agent and cell-type specificity in insulin resistance induced by prolonged exposure to HIV protease inhibitors (HPI), and to assess its relation to the direct, short-term inhibition of insulin-stimulated glucose uptake.

METHODS

Following prolonged (18 h) and short (5-10 min) exposure to HPI, insulin-stimulated glucose transport, protein kinase B (PKB) phosphorylation, and GLUT4 translocation were evaluated in 3T3-L1 adipocytes, fibroblasts, L6 myotubes, and L6 cells overexpressing a myc tag on the first exofacial loop of GLUT4 or GLUT1.

RESULTS

Prolonged exposure of 3T3-L1 adipocytes to nelfinavir, but not to indinavir or saquinavir, resulted in increased basal lipolysis but decreased insulin-stimulated glucose transport and PKB phosphorylation. In addition, impaired insulin-stimulated glucose uptake and PKB phosphorylation were also observed in the skeletal muscle cell line L6, and in 3T3-L1 fibroblasts. Interestingly, this coincided with increased basal glucose uptake as well as with elevated total-membrane glucose transporter GLUT1 protein content. In contrast to these unique effects of nelfinavir, the mere presence of any of the agents in the 5 min transport assay inhibited insulin-stimulated glucose-uptake activity. This appeared to be caused by direct and specific interaction of the drugs with GLUT4 fully assembled at the plasma membrane, since insulin-stimulated cell-surface exposure of an exofacial myc epitope on GLUT4 was normal.

CONCLUSIONS

Independent mechanisms for HPI-induced insulin resistance exist: prolonged exposure to nelfinavir interferes with insulin signaling and alters cellular metabolism of adipocytes and muscle cells, whereas a direct inhibitory effect on insulin-stimulated glucose uptake may occurs through specific interaction of HPI with GLUT4.

Glycogen phosphorylase is activated in response to glucose deprivation but is not responsible for enhanced glucose transport activity in 3T3-L1 adipocytes

We have previously shown that glucose deprivation activates glucose transport in a time- and protein synthesis-dependent fashion in 3T3-L1 adipocytes, a mouse cell line. Coincident with this is loss of glycogen. Because glycogen phosphorylase (GP) is responsible for glycogen degradation, we have examined its regulation to determine the relationship between transport activation and glycogen turnover. We first cloned the adipose GP cDNA and found sequence similarity to rat and human liver GP. Because the mouse liver GP cDNA sequence was unavailable, we cloned this cDNA as well and showed 100% identity between mouse adipose and liver sequences. A 3.1 kb transcript was readily observed in total RNA isolated from mouse liver or adipose by Northern blot analysis but, surprisingly, not in either total or poly(A) selected RNA from 3T3-L1 adipocytes. To evaluate regulation in 3T3-L1 adipocytes, we amplified GP mRNA from total RNA using multiplex, semi-quantitative PCR but found that expression did not change in response to deprivation. GP protein levels did not change either. However, endogenous GP activity from glucose-deprived cells was significantly elevated relative to controls, due to an increase in the phosphorylated form of GP (GPa). Finally, we overexpressed GP to determine its direct influence on the glucose transport system. These results were negative, which suggests that the nutrient control of glucose transport and GP occurs independently despite kinetic similarities in transport activation and glycogen turnover.

An unusual subcellular localization of GLUT1 and link with metabolism in oocytes and preimplantation mouse embryos

Although mouse oocytes and cleavage-stage embryos prefer pyruvate and lactate for metabolic fuels, they do take up and metabolize glucose. Indeed, presentation of glucose during the cleavage stages is required for subsequent blastocyst formation, which normally relies on uptake and metabolism of large amounts of glucose. Expression of the facilitative glucose transporter GLUT1 was examined using immunohistochemistry and Western blotting, and in polyspermic oocytes, metabolism of glucose was measured and compared with that of pyruvate and glutamine. GLUT1 was observed in all oocytes and embryos, and membrane and vesicular staining was present. Additionally, however, in polyspermic oocytes, the most intense staining was in the pronuclei, and this nuclear staining persisted in cleaving normal embryos. Furthermore, GLUT1 expression appeared to be up-regulated both in nuclei and plasma membranes following culture of oocytes in the absence of glucose. In polyspermic oocytes, the metabolism of glucose, but not of pyruvate or glutamine, was directly proportional to the number of pronuclei formed. After compaction, nuclear staining diminished, and GLUT1 localized to basolateral membranes of the outer cells and trophectoderm. In blastocysts, a weak but uniform staining of inner-cell-mass plasma membranes was apparent. The results are discussed in terms of potential roles for GLUT1 in pronuclei of oocytes and zygotes, nuclei of cleavage-stage embryos, and a transepithelial transport function for GLUT1, probably coupled with GLUT3, in compacted embryos and blastocysts.

Alternative glycosylation of the insulin receptor prevents oligomerization and acquisition of insulin-dependent tyrosine kinase activity

Glucose deprivation leads to the synthesis of an aberrantly glycosylated ('alternative') and inefficiently processed form of the insulin proreceptor in 3T3-L1 adipocytes. To further explore the effect of aberrant (rather than absent) N-linked glycosylation of the insulin receptor, we examined the relationship of processing to function. Our studies show that the alternative form of the proreceptor does not oligomerize nor does it acquire the ability to undergo insulin-sensitive autophosphorylation. This along with an interaction with the glucose-regulated stress protein GRP78/BiP implies inappropriate folding/dimerization and retention in the ER. Glucose refeeding causes the post-translational modification of the alternative form of the proreceptor to a novel 'intermediate' form which is independent of new protein synthesis. As little as 100 microM glucose (or mannose) can induce this modification. In vitro digestion of the alternative and intermediate proreceptors with SPC1/furin shows that both the alpha- and beta-subunit domains are glycosylated, albeit aberrantly. This implies that the aberrantly glycosylated proreceptor could serve as a substrate for SPC1 in a physiological setting if the receptor was able to interact with the enzyme in the appropriate compartment (i.e., the trans-Golgi network). Based on inhibitor studies, however, both the alternative and intermediate forms of the proreceptor appear to be primarily targeted to the proteasome for degradation.

Effect of alternative glycosylation on insulin receptor processing

The mature insulin receptor is a cell surface heterotetrameric glycoprotein composed of two alpha- and two beta-subunits. In 3T3-L1 adipocytes as in other cell types, the receptor is synthesized as a single polypeptide consisting of uncleaved alpha- and beta-subunits, migrating as a 190-kDa glycoprotein. To examine the importance of N-linked glycosylation on insulin receptor processing, we have used glucose deprivation as a tool to alter protein glycosylation. Western blot analysis shows that glucose deprivation led to a time-dependent accumulation of an alternative proreceptor of 170 kDa in a subcellular fraction consistent with endoplasmic reticulum localization. Co-precipitation assays provide evidence that the alternative proreceptor bound GRP78, an endoplasmic reticulum molecular chaperone. N-Glycosidase F treatment shows that the alternative proreceptor contained N-linked oligosaccharides. Yet, endoglycosidase H insensitivity indicates an aberrant oligosaccharide structure. Using pulse-chase methodology, we show that the synthetic rate was similar between the normal and alternative proreceptor. However, the normal proreceptor was processed into alpha- and beta-subunits (t((1)/(2)) = 1.3 +/- 0.6 h), while the alternative proreceptor was degraded (t((1)/(2)) = 5.1 +/- 0.6 h). Upon refeeding cells that were initially deprived of glucose, the alternative proreceptor was processed to a higher molecular weight form and gained sensitivity to endoglycosidase H. This "intermediate" form of the proreceptor was also degraded, although a small fraction escaped degradation, resulting in cleavage to the alpha- and beta-subunits. These data provide evidence for the first time that glucose deprivation leads to the accumulation of an alternative proreceptor, which can be post-translationally glycosylated with the readdition of glucose inducing both accelerated degradation and maturation.

Glucose induces expression of stearoyl-CoA desaturase in 3T3-L1 adipocytes

Stearoyl-CoA desaturase (SCD; EC 1.14.99.5) is a key enzyme in the synthesis polyunsaturated fatty acids. Liver and ose tissue are the predominant sites of SCD expression. Regulation of tic SCD by various nutritional and hormonal ors, such as insulin, dietary carbohydrates and polyunsaturated fatty s, has been well documented. Little is known, ver, about adipocyte SCD regulation despite high levels of SCD activity adipose tissue. The present study was gned to investigate SCD regulation in adipocytes by examining the cts of glucose and insulin on SCD expression. We rt here that glucose availability directly increased SCD gene scription in 3T3-L1 adipocytes. This response was pendent of insulin, and insulin alone in the absence of glucose had no ct on SCD mRNA levels. SCD thus represents a l model in which to investigate the mechanisms of direct regulation of expression by glucose in adipose cells.