Elsevier

Brain Research

Volume 896, Issues 1–2, 30 March 2001, Pages 36-42
Brain Research

Research report
Effect of LPS on the permeability of the blood–brain barrier to insulin

https://doi.org/10.1016/S0006-8993(00)03247-9Get rights and content

Abstract

Insulin has emerged as an important neuropeptide. Central actions of insulin appear to oppose those in the periphery. Insulin is transported across the blood–brain barrier (BBB) by a saturable transport system. The permeability of the BBB to insulin is altered by various events, but no studies exist that have examined the permeability of the BBB to insulin during infection or inflammation, states which can induce peripheral insulin resistance. We looked at the effects of lipopolysaccharide (LPS), a bacterial endotoxin and a powerful cytokine releaser, on the permeability of the BBB to human insulin in CD-1 mice. Intraperitoneal injections of LPS significantly increased the uptake by the brain of 131I–insulin and disrupted the BBB to 125I–albumin. After subtraction of the brain/serum ratio for 125I–albumin, brain/serum ratios for insulin were increased: 10.38±0.70 μl/g (LPS) vs. 3.62±0.27 μl/g (no LPS), P<0.0001, showing that LPS increased the uptake of insulin independent of BBB disruption. This increase in insulin uptake was due to enhanced saturable transport. Pretreatment with indomethacin 10 min before LPS injections enhanced BBB disruption, but not insulin transport. Pretreatment with the nitric oxide (NO) synthase inhibitor aminoguanidine had no effect on insulin or albumin uptake, but pretreatment with NG-nitro-l-arginine methyl ester (l-NAME) enhanced insulin transport, but not BBB disruption. We conclude that LPS increases the saturable transport of insulin across the BBB independent of disruption and prostaglandins with potentiation by NO inhibition. Such increased transport could potentiate the central effects of insulin and so contribute to the peripheral insulin resistance seen with infection and inflammation.

Introduction

Insulin is secreted by the β cells in the pancreatic islets of Langerhans. Insulin consists of two peptide chains linked by two sulfhydryl bonds with a structure that is highly conserved across species. As a result, insulin from one species is often active in another. Human insulin has a molecular weight of 5786 and the human pancreas releases about 1 mg/24 h. Release of insulin is primarily regulated by levels of glucose and glucagon in the serum and is under cholinergic control, although α- and β-adrenergic tone can also affect secretion. About 50% of insulin is cleared by the liver where it inhibits glucose release. Insulin entering the circulation greatly facilitates the uptake of serum glucose by fat and muscle, but has little effect on the uptake of glucose by the brain. An absence of, or resistance to, insulin results in diabetes mellitus. Resistance to the peripheral effects of insulin occurs with inflammation and infection.

Insulin receptors are found throughout the brain, suggesting that insulin could have widespread effects within the central nervous system (CNS). The central action of insulin appears to counterbalance its activities in the periphery. Rather than promoting feeding and inducing hypoglycemia as it does in the periphery, insulin in the CNS encourages weight loss, probably through actions on neuropeptide Y [17], [18] and can induce hyperglycemia [17]. Centrally administered insulin also has been shown to affect sympathetic nerve activity [36], ischemic events [50], brain metabolism [23], and the production [48] as well as action [15] of other satiety hormones.

Unlike many regulatory proteins or small peptides produced by peripheral tissues, little or no insulin is synthesized by the brain. Therefore, to act at its central receptors, blood-born insulin must cross the blood–brain barrier (BBB). Insulin is transported across the BBB by a saturable transport system in the blood to brain [1], [6], but not the brain to blood [9], direction. Binding sites for insulin, which may represent the transporters, have been found on both the vascular endothelial and the choroid plexus epithelial cells which constitute the BBB [5], [20], [34]. The permeability of the BBB to insulin appears to be regulated by physiological events, such as fasting, starvation, or refeeding [16], [17], hibernation in marmots [17], [18], and brain development [12], [19]. Some pathophysiological states may also exert effects on the permeability of the BBB to insulin. For example, there is an increased uptake of insulin by the brain in diabetic mice [2].

Other work has suggested that substances such as tumor necrosis factor-α (TNF-α) [12], nitric oxide (NO) [7], and prostaglandins [35] may change the permeability of the BBB. These substances may also cross the BBB [22], exert effects on the CNS [21], and induce peripheral insulin resistance [28], [44]. Lipopolysaccharide (LPS), a bacterial wall endotoxin, is a powerful releaser of cytokines, prostaglandine E2, and NO [8] and can affect the permeability of the BBB [7], [30]. Here, we look at the effects of LPS on the permeability of the BBB to human insulin in mice.

Section snippets

Radioactive labeling of insulin and albumin

Human insulin was obtained from Sigma Chemical Co. The insulin (5 μg) and bovine serum albumin (5 μg) were radioactively labeled with 131I and 125I, respectively, by the chloramine T method and purified by filtration on a column of Sephadex G-10. Incorporations of 131I to insulin and 125I by albumin, as determined by trichloroacetic acid (TCA) precipitation, were each greater than 90%. The 131I–insulin had a specific activity of about 55 Ci/g.

Measurement of BBB permeability

Male CD-1 mice (Charles River, Wilmington, MA; 25∼35

Results

The initial pilot study suggested that the peak uptake of 131I–insulin by the brain was at the time between 16 and 24 h after a single i.p. injection of LPS (data not shown). Based upon this, 131I–insulin uptake by the brain was determined 24 h after injections of LPS in subsequent studies. Statistically significant increases in 131I–insulin uptake by the brain were found [F(3,27)=4.840] 24 h after one (P=0.0019, n=10/group), three (P<0.0001, n=10/group), and five (P<0.0001, n=10/group)

Discussion

In this study, we showed that intraperitoneal LPS enhanced the permeability of the BBB to insulin through two distinct mechanisms: enhanced saturable transport (Fig. 3) and disruption of the BBB (Fig. 2). The ability of bacterial LPS or cytokines released by LPS to enhance the permeability of the BBB to insulin suggests that systemic bacterial infections or other inflammation states could facilitate the transport of insulin into the CNS, which in turn, could alter insulin’s central effects.

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References (50)

  • N Henneberg et al.

    Short-term or long term intracerebroventricular (i.c.v.) infusion of insulin exhibits a discrete anabolic effect on cerebral energy metabolism in the rat

    Neurosci. Lett.

    (1994)
  • P Kara et al.

    Dynamic modulation of cerebral cortex synaptic function by nitric oxide

    Prog. Brain Res.

    (1998)
  • G Kwon et al.

    Tumor necrosis factor-alpha-induced pancreatic beta-cell insulin resistance is mediated by nitric oxide and prevented by 15-deoxy-Delta12, 14-prostaglandin J2 and aminoguanidine. A role for peroxisome proliferator-activated receptor gamma activation and inos expression

    J. Biol. Chem.

    (1999)
  • S.A Lipton et al.

    Nitric oxide in the central nervous system

    Prog. Brain Res.

    (1994)
  • W.G Mayhan

    Effect of lipopolysaccharide on the permeability and reactivity of the cerebral microcirculation: role of inducible nitric oxide synthase

    Brain Res.

    (1998)
  • W.G Mayhan

    Role of nitric oxide in histamine-induced increases in permeability of the blood–brain barrier

    Brain Res.

    (1996)
  • W.G Mayhan

    Role of nitric oxide in disruption of the blood–brain barrier during acute hypertension

    Brain Res.

    (1995)
  • T Minami et al.

    Penetration of cisplatin into mouse brain by lipopolysaccharide

    Toxicology

    (1998)
  • J Ou et al.

    Differential effects of nonselective nitric oxide synthase (NOS) and selective inducible NOS inhibition on hepatic necrosis, apoptosis,ICAM-1 expression, and neutrophil accumulation during endotoxemia

    Nitric Oxide

    (1997)
  • E.R Pettipher et al.

    Cyclooxygenase inhibitors enhance tumor necrosis factor production and mortality in murine endotoxic shock

    Cytokine

    (1994)
  • M.W Phelan et al.

    Hypoxia increases thrombospondin-1 transcript and protein in cultured endothelial cells

    J. Lab. Clin. Med.

    (1998)
  • T Polte et al.

    Cyclic AMP mediates endothelial protection by nitric oxide

    BBRC

    (1998)
  • G.A Rosenberg et al.

    Tumor necrosis factor-α-induced gelatinase B causes delayed opening of the blood–brain barrier: an expanded therapeutic window

    Brain Res.

    (1995)
  • D.I Utepbergenov et al.

    Nitric oxide protects blood–brain barrier in vitro from hypoxia/reoxygenation-mediated injury

    FEBS Lett.

    (1998)
  • W.A Banks et al.

    Physiological consequences of the passage of peptides across the blood–brain barrier

    Rev. Neurosci.

    (1993)
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