Stimulation of glucose transport in Clone 9 cells by insulin and thyroid hormone: role of GLUT-1 activation

TSH suppression aggravates arterial inflammation - an 18F-FDG PET study in thyroid carcinoma patients

Purpose

We aimed to investigate the influence of both hypothyroidism and thyroid-stimulating hormone (TSH) suppression on vascular inflammation, as assessed with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET)/computed tomography (CT).

Methods

Ten thyroid carcinoma patients underwent an ¹⁸F-FDG PET/CT during post-thyroidectomy hypothyroidism and during thyrotropin (TSH) suppression after ¹³¹I (radioiodine) ablation therapy. We analysed the ¹⁸F-FDG uptake in the carotids, aortic arch, ascending, descending, and abdominal aorta to investigate the effects of thyroid hormone status on arterial inflammation. Target-to-background ratios (TBRs) corrected for blood pool activity were established for all arterial territories. Results were further compared to euthyroid historic control subjects.

Results

In general, there was a trend towards higher vascular TBRs during TSH suppression than during hypothyroidism (TBR_max all vessels = 1.6 and 1.8, respectively, p = 0.058), suggesting a higher degree of arterial inflammation. In concurrence with this, we found increased C-reactive protein (CRP) levels after levothyroxine treatment (CRP = 2.9 mg/l and 4.8 mg/l, p = 0.005). An exploratory comparison with euthyroid controls showed significant higher TBRs during TSH suppression for the carotids, aortic arch, thoracic descending aorta, and when all vascular territories were combined (TBR_maxp = 0.013, p = 0.016, p = 0.030 and p = 0.018 respectively).

Conclusions

Arterial inflammation is increased during TSH suppression. This finding sheds new light on the underlying mechanism of the suspected increased risk of cardiovascular disease in patients with TSH suppression.

Electronic supplementary material

The online version of this article (10.1007/s00259-019-04292-w) contains supplementary material, which is available to authorized users.

GLUT3 is present in Clone 9 liver cells and translocates to the plasma membrane in response to insulin

Clone 9 cells have been reported to express only the GLUT1 facilitative glucose transporter; however, previous studies have not examined Clone 9 cells for GLUT3 content. The current study sought to profile the presence of glucose transporters in Clone 9 cells, H4IIE hepatoma cells, and L6 myoblasts and myotubes. While the other cell types contained the expected complement of transporters, Clone 9 cells had GLUT3 which was previously not reported. Interestingly, both GLUT3 mRNA and protein were detected in Clone 9 cells, but only mRNA for GLUT1 was detected. Glucose transport in Clone 9 cells was insulin-sensitive in a concentration-dependent manner, concomitant with the presence of GLUT3 in the plasma membrane after insulin treatment. Although basal glucose uptake was unaffected, insulin-stimulated glucose uptake was abolished with siRNA-mediated GLUT3 knockdown. These results contradict previous reports that Clone 9 cells exclusively express GLUT1 and suggest GLUT3 is a key insulin-sensitive glucose transporter required for insulin-stimulated glucose uptake by Clone 9 cells.

Angiotensin-induced EGF receptor transactivation inhibits insulin signaling in C9 hepatic cells

To investigate the potential interactions between the angiotensin II (Ang II) and insulin signaling systems, regulation of IRS-1 phosphorylation and insulin-induced Akt activation by Ang II were examined in clone 9 (C9) hepatocytes. In these cells, Ang II specifically inhibited activation of insulin-induced Akt Thr(308) and its immediate downstream substrate GSK-3alpha/beta in a time-dependent fashion, with approximately 70% reduction at 15 min. These inhibitory actions were associated with increased IRS-1 phosphorylation of Ser(636)/Ser(639) that was prevented by selective blockade of EGFR tyrosine kinase activity with AG1478. Previous studies have shown that insulin-induced phosphorylation of IRS-1 on Ser(636)/Ser(639) is mediated mainly by the PI3K/mTOR/S6K-1 sequence. Studies with specific inhibitors of PI3K (wortmannin) and mTOR (rapamycin) revealed that Ang II stimulates IRS-1 phosphorylation of Ser(636)/Ser(639) via the PI3K/mTOR/S6K-1 pathway. Both inhibitors blocked the effect of Ang II on insulin-induced activation of Akt. Studies using the specific MEK inhibitor, PD98059, revealed that ERK1/2 activation also mediates Ang II-induced S6K-1 and IRS-1 phosphorylation, and the impairment of Akt Thr(308) and GSK-3alpha/beta phosphorylation. Further studies with selective inhibitors showed that PI3K activation was upstream of ERK, suggesting a new mechanism for Ang II-induced impairment of insulin signaling. These findings indicate that Ang II has a significant role in the development of insulin resistance by a mechanism that involves EGFR transactivation and the PI3K/ERK1/2/mTOR-S6K-1 pathway.

Is non-insulin dependent glucose uptake a therapeutic alternative? Part 1: physiology, mechanisms and role of non insulin-dependent glucose uptake in type 2 diabetes

Several decades of research for treating type 2 diabetes have yielded new drugs but the actual experience with the available oral antidiabetic compounds clearly shows that therapeutic needs are not matched. This highlights the urgent need for exploring other pathways. All cell types have the capacity to take up glucose independently of insulin, whereby basal but also hyperglycaemia-promoted glucose supply is ensured. Although poorly explored, insulin-independent glucose uptake might nevertheless represent a therapeutic target, as an alternative to the clear limits of actual drug treatments. This review not only critically examines some major pathways not requiring insulin (although they may be influenced by the hormone) but importantly, this analysis extends to the clinical applicability of these potential therapeutic principles by also considering their predictable tolerability for long-term therapy. In particular vascular safety (the ultimate problem linked with diabetes) will be envisaged because of the ubiquitous distribution of glucose transporters and some linked mechanisms. Several mechanisms can be identified which do not require insulin for their functioning. The first part of this review deals with the description, the regulation and the limits of some mechanisms representing potential pharmacological targets capable of having a highly significant impact on glucose uptake. These selected topics are: a) unmasking and/or activation of glucose transporters in cell plasma membranes, b) insulin mimetics acting at postreceptor level, c) activation of AMPK, d) increasing nitric oxide and e) increasing glucose-6P and glycogen stores.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

Insulin increases branched-chain alpha-ketoacid dehydrogenase kinase expression in Clone 9 rat cells

The branched-chain amino acids (BCAA) are committed to catabolism by the activity of the branched-chain alpha-ketoacid dehydrogenase (BCKD) complex. BCKD activity is regulated through the action of the complex-specific BCKD kinase that phosphorylates two serine residues in the E1alpha subunit. Greater BCKD kinase expression levels result in a lower activity state of BCKD and thus a decreased rate of BCAA catabolism. Activity state varies among tissues and can be altered by diet, exercise, hormones, and disease state. Within individual tissues, the concentration of BCKD kinase reflects the activity state of the BCKD complex. Here we investigated the effects of insulin, an important regulator of hepatic metabolic enzymes, on BCKD kinase expression in Clone 9 rat cells. Insulin effected a twofold increase in message levels and a twofold increase in BCKD kinase protein levels. The response was completely blocked by treatment with LY-294002 and partially blocked by rapamycin, thus demonstrating a dependence on phosphatidylinositol 3-kinase and mTOR function, respectively. These studies suggest that insulin acts to regulate BCAA catabolism through stimulation of BCKD kinase expression.

Glucose uptake and lactate production in cells exposed to CoCl(2) and in cells overexpressing the Glut-1 glucose transporter

Glut-1-mediated glucose transport is augmented in response to a variety of conditions and stimuli. In this study we examined the metabolic fate of glucose in cells in which glucose transport is stimulated by exposure to CoCl(2), an agent that stimulates the expression of a set of hypoxia-responsive genes including several glycolytic enzymes and the Glut-1 glucose transporter. Similarly, we determined the metabolic fate of glucose in stably transfected cells overexpressing Glut-1. Exposure of Clone 9 liver cell line, 3T3-L1 fibroblasts, and C(2)C(12) myoblasts to CoCl(2) resulted in an increase glucose uptake and in the activity of glucose phosphorylation ("hexokinase") and lactate dehydrogenase. In cells treated with CoCl(2), the net increase in glucose taken up was accounted for by its near-complete conversion to lactate. Cells stably transfected to overexpress Glut-1 also exhibited enhanced net uptake of glucose with the near-complete conversion of the increased glucose taken up to lactate; however, the effect in these cells was observed in the absence of any change in the activity of two glycolytic enzymes examined. These findings suggest that in cells in which glucose transport is rate-limiting for glucose metabolism, enhancement of the glucose entry step per se results in a near-complete conversion of the extra glucose to lactate.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Cytosolic [Ca(2+)] modulates basal GLUT1 activity and plays a permissive role in its activation by metabolic stress and insulin in rat epithelial cells

The aim of this work was to investigate the role of cytosolic free calcium ([Ca(2+)]c) in the stimulation of GLUT1 by metabolic stress and insulin. Chelation of [Ca(2+)]c with bapta, introduced in rat liver epithelial Clone 9 cells in the acetoxymethyl (AM) form, decreased their basal rate of 2-deoxyglucose uptake in a dose-dependent fashion. Maximal inhibition at 75 microM bapta was by 38 +/- 8% (n = 8). The effect was partially reversed by ionomycin. Basal sugar uptake was also decreased by lowering extracellular [Ca(2+)] in ionomycin-permeabilized cells. Increasing [Ca(2+)]c over its resting level of 168 +/- 32 (n = 27) had no affect on sugar uptake. Chelation of [Ca(2+)]c did not change the abundance of surface GLUT1 and had no significant effect on the affinity of GLUT1 for sugars. In addition, calcium chelation abolished the activation of GLUT1 by azide, arsenate, 2,4-dinitrophenol and insulin. However, [Ca(2+)]c did not increase in the presence of azide. We conclude that [Ca(2+)]c, near or below its resting level, modulates GLUT1 activity over a considerable range and plays a permissive role in the activation of the carrier by metabolic stress and insulin.

Regulation of glucose transport by hypoxia

Transport of glucose into most mammalian cells and tissues is rate-controlling for its metabolism. Glucose transport is acutely stimulated by hypoxic conditions, and the response is mediated by enhanced function of the facilitative glucose transporters (Glut), Glut1, Glut3, and Glut4. The expression and activity of the Glut-mediated transport is coupled to the energetic status of the cell, such that the inhibition of oxidative phosphorylation resulting from exposure to hypoxia leads to a stimulation of glucose transport. The premise that the glucose transport response to hypoxia is secondary to inhibition of mitochondrial function is supported by the finding that exposure of a variety of cells and tissues to agents such as azide or cyanide, in the presence of oxygen, also leads to stimulation of glucose transport. The mechanisms underlying the acute stimulation of transport include translocation of Gluts to the plasma membrane (Glut1 and Glut4) and activation of transporters pre-exiting in the plasma membrane (Glut1). A more prolonged exposure to hypoxia results in enhanced transcription of the Glut1 glucose transporter gene, with little or no effect on transcription of other Glut genes. The transcriptional effect of hypoxia is mediated by dual mechanisms operating in parallel, namely, (1) enhancement of Glut1 gene transcription in response to a reduction in oxygen concentration per se, acting through the hypoxia-signaling pathway, and (2) stimulation of Glut1 transcription secondary to the associated inhibition of oxidative phosphorylation during hypoxia. Among the various hypoxia-responsive genes, Glut1 is the first gene whose rate of transcription has been shown to be dually regulated by hypoxia. In addition, inhibition of oxidative phosphorylation per se, and not the reduction in oxygen tension itself, results in a stabilization of Glut1 mRNA. The increase in cell Glut1 mRNA content, resulting from its enhanced transcription and decreased degradation, leads to increased cell and plasma membrane Glut1 content, which is manifested by a further stimulation of glucose transport during the adaptive response to prolonged exposure to hypoxia.

Angiotensin AT1 receptors in Clone 9 rat liver cells: Ca2+ signaling and c-fos expression

In C9 (Clone 9) liver cells, angiotensin 11 increased the intracellular Ca2+ content, inositol phosphate production and c-fos mRNA expression. Other angiotensins were also active with the order of potency being angiotensin II = angiotensin III >> angiotensin I > angiotensin IV. Losartan, but not PD 123177 (1-(4-amino-3-methyl)-5-diphenylacetyl-4,5,6,7-tetrahydro-1H-imida zo [4,5c]pyridine-6-carboxylic acid), blocked the effects of angiotensin II. Pertussis toxin did not alter these actions of angiotensin II. These data indicate that the effects were mediated through angiotensin AT1 receptors involving pertussis toxin-insensitive G-proteins. Phorbol myristate acetate was also able to increase c-fos mRNA expression. The action of angiotensin II was consistently greater than that of the active phorbol ester. Staurosporine but not genistein inhibited this effect of angiotensin II. Angiotensin II- and phorbol myristate acetate-induced proto-oncogene mRNA expression was attenuated in cells incubated overnight with the active phorbol ester, which suggests a major role of protein kinase C.

Glycaemic regulation of glucose transporter expression and activity in the human placenta

To determine whether the expression and activity of glucose transporters in human trophoblast are regulated by glucose, syncytiotrophoblast cells, choriocarcinoma cells, and villous fragments were incubated with a range of glucose concentrations (0-20 mM, 24 h). Expression of GLUT1 and GLUT3 glucose transporters was measured by immunoblotting, while glucose transporter activity was determined by [3H]2-deoxyglucose uptake in the cultured cells. GLUT1 expression in syncytial cells was enhanced following incubation in absence of glucose, reduced by incubation in 20 mM glucose but was not altered by incubation at 1 or 12 mM glucose. Transporter activity was inversely related to extracellular glucose over the entire range of concentrations tested (0-20 mM). Incubation of villous fragments in 20 mM glucose produced a limited suppression of GLUT1 expression, but no effects were noted following incubation at 0 or 1 mM glucose. Neither GLUT1 expression in JAr and JEG-3 choriocarcinoma cells nor transport activity in JEG-3 cells was affected by extracellular glucose concentration. Unlike syncytial cells, JAr, JEG-3 and BeWo all expressed GLUT3 protein in addition to GLUT1. These results show that while syncytiotrophoblast GLUT1 expression is altered at the extremes of extracellular glucose concentration, it is refractory to glucose alone at lower concentrations. By contrast, an inverse relationship exists between glucose transporter activity and extracellular glucose. This suggests that there are post-translational regulatory mechanisms which may respond to changes in extracellular glucose concentration.

Glycemia-lowering effect of cobalt chloride in the diabetic rat: increased GLUT1 mRNA expression

We have recently shown that expression of the GLUT1 glucose transporter isoform is augmented in cells exposed to cobalt chloride [Co(II)], an agent that stimulates the expression of hypoxia-responsive genes (Behrooz, A., Ismail-Beigi, F., 1997. J. Biol. Chem. 272, 5555-5562.). Here, we examine the effect of Co(II) on glycemia and tissue GLUT1 mRNA content of normal and diabetic rats. The addition of 2 mM Co(II) in the drinking water reduced the glycemia of streptozotocin-induced diabetic rats by day 3 from 32.3 +/- 2.1 to 21.0 +/- 1.9 mM (non-fasting). Co(II) resulted in no change in serum insulin levels of normal or diabetic rats. Treatment with 4 mM Co(II) was more effective than 2 mM Co(II) in reducing the glycemia of diabetic rats, while 6 mM Co(II) was associated with severe toxicity. GLUT1 mRNA content increased significantly in ventricular myocardium, renal cortex, skeletal muscle, cerebrum and liver of normal and diabetic rats treated with 2 mM cobalt chloride (ranging from 1.3- to 2.9-fold in the different tissues). It is concluded that: (1) treatment with Co(II) decreases the glycemia of diabetic rats, and (2) the glycemia-lowering effect of Co(II) is associated with, and may be mediated by, enhanced expression of GLUT1 mRNA.