Objectives  To determine the effects of induced hyperinsulinemia with euglycemia on A, transthyretin, and inflammatory markers and modulators in plasma and cerebrospinal fluid (CSF).

Design  Randomized crossover trial.

Setting  Veterans Affairs hospital clinical research unit.

Participants  Sixteen healthy adults ranging from 55 to 81 years of age (mean age, 68.2 years).

Interventions  On separate mornings, fasting participants received randomized infusions of saline or insulin (1.0 mU · kg^-1 · min^-1) with variable dextrose levels to maintain euglycemia, achieving plasma insulin levels typical of insulin resistance. Plasma and CSF were collected after an approximately 105-minute infusion.

Main Outcome Measures  Plasma and CSF levels of interleukin 1, interleukin 1, interleukin 6, tumor necrosis factor , F₂-isoprostane (CSF only), A, norepinephrine, transthyretin, and apolipoprotein E.

Results  Insulin increased CSF levels of F₂-isoprostane and cytokines (both P<.01), as well as plasma and CSF levels of A42 (both P<.05). The changes in CSF levels of A42 were predicted by increased F₂-isoprostane and cytokine levels (both P<.01) and reduced transthyretin levels (P = .02). Increased inflammation was modulated by insulin-induced changes in CSF levels of norepinephrine and apolipoprotein E (both P<.05).

Conclusion  Moderate hyperinsulinemia can elevate inflammatory markers and A42 in the periphery and the brain, thereby potentially increasing the risk of Alzheimer disease.

In the periphery, insulin modulates many aspects of the inflammatory network. Low doses of insulin exert anti-inflammatory effects⁸; however, excessive hyperinsulinemia exacerbates inflammation and increases the levels of markers of oxidative stress.⁹ In humans, plasma concentrations of the proinflammatory cytokines IL-6 and tumor necrosis factor (TNF-) increase synergistically when insulin is administered with the endotoxin lipopolysaccharide.¹⁰ Insulin may also contribute to inflammation in the central nervous system (CNS), partially through effects on A. Insulin promotes the release of A from intracellular neuronal compartments and inhibits its degradation by the metalloprotease insulin-degrading enzyme.11-12 Insulin-like peptides may also regulate carrier proteins that transport A between the CNS and the periphery.¹³

Intervening factors may modify insulin’s proinflammatory effects. In the brain, insulin regulates levels of norepinephrine,¹⁴ an endogenous, anti-inflammatory neuromodulator that attenuates A42-provoked increases in IL-1 levels in rats and potentially modulates the effects of IL-1 on A processing.¹⁵ Thus, noradrenergic depletion in AD may increase vulnerability to A-provoked inflammation. Additionally, insulin regulates apolipoprotein E (apoE) levels, in part though interactions with low-density lipoprotein receptor–related protein.¹⁶ The apoE protein down-regulates the inflammatory cascade, lowering IL-6 and TNF- levels in animals following inflammatory stimulation.¹⁷

In the present study, we tested the hypothesis that peripheral insulin administration would modulate CSF inflammatory markers. We raised plasma insulin levels (while maintaining euglycemia) in 16 healthy older adults, achieving moderately high physiological elevations typical of postprandial levels in patients with insulin resistance. We then measured changes in plasma and CSF levels of inflammatory markers (IL-1, IL-1, IL-6, TNF-, and F₂-isoprostane), modulators (transthyretin, apoE, and norepinephrine), and A.

METHODS

This study was approved by the University of Washington institutional review board. All of the subjects gave written informed consent. Participants included 16 cognitively normal adults ranging in age from 55 to 81 years (Table) in good health who received extensive medical and cognitive screening as previously described.¹⁸ None were receiving medications with known CNS or glucoregulatory effects.

View this table: [in this window] [in a new window] [as a PowerPoint slide]   Table. Subject Characteristics

PROCEDURE

On separate mornings at least 1 week apart, fasting participants received 2 randomized infusions: (1) saline (baseline) and (2) insulin (1.0 mU · kg^-1 · min^-1), yielding corresponding plasma insulin levels of approximately 10 µU/mL and 85 µU/mL, with variable dextrose levels (20%) to maintain plasma glucose levels of approximately 100 mg/dL. Intravenous catheters were inserted for infusions and blood sampling. Following a 30-minute habituation period, infusions were begun; target plasma levels were attained in approximately 90 minutes. Afterward, subjects completed a 15-minute cognitive protocol and then underwent lumbar puncture to collect CSF (both procedures are described in a separate article¹⁸). Blood samples were obtained prior to beginning infusions and prior to CSF acquisition.

ASSAYS

Insulin and norepinephrine levels were determined by radioenzymatic assay or radioimmunoassay.¹⁹ The F₂-isoprostane levels were quantified using gas chromatography with negative-ion chemical-ionization mass spectrometry and selective ion monitoring.⁵ The IL-1, IL-1, IL-6, TNF-, and transthyretin levels (plasma and CSF) and A40 and A42 levels (plasma) were determined using enzyme-linked immunosorbent assays.^20-21 The CSF A42 levels were determined with enzyme-linked immunosorbent assays (Athena Diagnostics, Worcester, Mass).

STATISTICAL ANALYSIS

Biomarker values were subjected to repeated-measures analysis of covariance with infusion condition (saline or insulin) as the within-subjects factor. Age and body mass index (BMI), which are associated with insulin resistance, were used as covariates. When they did not contribute significantly to analyses, they were deleted from the model. Relationships among biomarker changes due to induced hyperinsulinemia were examined by correlating difference scores (value in the insulin condition minus the corresponding value in the saline condition). Higher scores reflected greater increases due to insulin infusion. The relationship between changes in inflammatory markers and A levels in response to insulin was examined using stepwise multiple regression analysis. Dependent variables were difference scores for plasma and CSF A40 and A42 levels (separate analyses). Predictors were age, BMI, and plasma or CSF inflammatory reactants.

RESULTS

Intravenous insulin administration produced reliable elevations in CSF insulin levels, which is consistent with animal models showing insulin transport into the brain and subsequent egress into CSF22 (mean [SEM] saline and insulin infusions were 1.44 [0.20] µU/mL and 2.22 [0.35] µU/mL, respectively; P = .02).¹⁸

We then examined changes in cytokine and F₂-isoprostane levels during hyperinsulinemia. Insulin increased CSF levels of all 4 cytokines (Figure 1A-D; IL-1 [P<.001], IL-1 [P<.001], IL-6 [P = .007], and TNF- [P = .002]) and F₂-isoprostane (Figure 1E; P = .01). Adults with greater BMIs tended to have higher CSF TNF- levels in response to insulin (r = 0.49, P = .06). In contrast, plasma cytokine levels did not change reliably in response to insulin. Plasma and CSF cytokine levels were uncorrelated, as were insulin-induced changes. Insulin did not affect CSF protein, suggesting that changes in inflammatory reactants were not due to nonspecific effects on CSF turnover (P = .33).

View larger version (20K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 1. Cerebrospinal fluid cytokines and F2-isoprostane (mean ± SEM) for study participants (n = 15). Hyperinsulinemia increased cerebrospinal fluid levels of interleukin (IL) 1 (F1,14 = 61.46, P<.001) (A), IL-1 (F1,14 = 33.14, P<.001) (B), IL-6 (F1,14 = 9.90, P = .007) (C), tumor necrosis factor (TNF-) (F1,14 = 14.15, P = .002) (D), and F2-isoprostane (F1,14 = 8.07, P = .01) (E). Shaded bars indicate saline condition; solid black bars, insulin condition; asterisks, significant change with insulin infusion relative to saline infusion.

INSULIN AND A

Plasma A42 increased with insulin, an effect that was associated with BMI (Figure 2A; P = .046). Adults with greater BMIs showed greater plasma A42 elevations with insulin (r = 0.49, P = .047) (Figure 2B). Consistent with the observation that TNF- modulates A transport between the CNS and the periphery,¹³ insulin-induced changes in CSF TNF- levels predicted changes in plasma A42 levels (R² = 0.44, P = .007); subjects with higher TNF- levels during insulin infusion had greater increases in plasma A42 levels (r = 0.64, P = .01). Higher plasma A42 levels were also associated with increased CSF transthyretin levels (Figure 2C; r = 0.63, P = .02), which binds A and facilitates its transport from the brain to the periphery. Interestingly, insulin infusion did not affect plasma A40 levels (mean [SEM] plasma A40 level was 224.7 [26.2] pg/mL for saline conditions and 221.6 [26.4] pg/mL for insulin conditions). Insulin-induced changes in plasma A40 or A42 levels were unrelated to changes in plasma inflammatory markers.

View larger version (49K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 2. A, Hyperinsulinemia increased plasma -amyloid 42 (A42) levels (F1,13 = 4.86, P = .046). Open bar indicates saline condition; filled bar, insulin condition. B, Participants with greater body mass index (BMI) showed greater increases in plasma A42 levels with hyperinsulinemia. C, Insulin-induced elevations of cerebrospinal fluid (CSF) transthyretin (TTR) levels were associated with higher plasma A42 levels. D, Insulin-induced elevations of CSF TTR levels were associated with lower CSF A42 levels. Mean ± SEM CSF TTR levels were 4.75 ± 0.48 mg/L and 4.91 ± 0.52 mg/L during saline and insulin infusions, respectively. Mean ± SEM CSF A42 levels were 1113.45 ± 117.95 pg/mL and 1159.65 ± 115.14 pg/mL during saline and insulin infusions, respectively.

We previously reported that insulin provoked an age-dependent increase in CSF A42 levels for this group of normal adults.¹⁸ We have now determined that transthyretin and inflammatory marker levels strongly predict insulin-induced changes in CSF A42 levels (omnibus F_4,9 = 11.14, P = .002). The best predictors were age (P = .003) and difference scores for IL-6 (P = .003), F₂-isoprostane (P = .002), and transthyretin (P = .01). Older age and greater increases in IL-6 and F₂-isoprostane levels were associated with greater increases in CSF A42 levels following insulin infusion. In contrast, increased transthyretin levels predicted lowering of CSF A42 levels, which is consistent with enhanced transport from the CNS to the periphery (Figure 2D; r = –0.59, P = .03).

CSF NOREPINEPHRINE, IL-1, AND A42

Since norepinephrine attenuates A42-provoked increases in IL-1 levels in rodents,¹⁵ we examined whether insulin-induced increases in CSF norepinephrine levels attenuate increases in CSF A42 and IL-1 levels. Subjects with higher CSF norepinephrine levels during insulin infusion had lower levels of A42 (Figure 3A; r = –0.51, P = .04) and IL-1 (Figure 3B; r = –0.60, P = .02).

View larger version (26K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 3. Inverse relationship between cerebrospinal fluid (CSF) norepinephrine (NE) and insulin-induced changes in CSF -amyloid 42 (A42) (A) and interleukin 1 (IL-1) (B).

CSF APOE AND CYTOKINES

Insulin regulates apoE levels,¹⁶ and apoE moderates the inflammatory cascade.¹⁷ Hyperinsulinemia provoked age-related changes in CSF apoE levels (P = .04). Insulin raised apoE levels for most subjects, which was an effect that increased with age (Figure 4B; r = 0.46, P = .08). Higher CSF apoE levels with insulin infusion were associated with smaller increases in CSF IL-6 (r = –0.54, P = .04) and TNF- (r = –0.42, P = .12) levels, and they were associated with greater CSF IL-1 levels (r = 0.60, P = .02). Plasma and CSF apoE levels were uncorrelated.

View larger version (27K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 4. Insulin infusion caused greater increases in plasma apolipoprotein E (apoE) levels for younger subjects (<70 years) than for older subjects (70 years) (A), but greater increases in cerebrospinal fluid (CSF) apoE levels for older subjects than for younger subjects (B). Mean ± SEM plasma apoE levels with saline and insulin infusions were 3.68 ± 0.37 pg/mL and 4.00 ± 0.36 pg/mL, respectively, for younger subjects, and 4.49 ± 0.37 pg/mL and 4.22 ± 0.36 pg/mL, respectively, for older subjects. Mean ± SEM CSF apoE levels with saline and insulin infusions were 0.46 ± 0.05 pg/mL and 0.47 ± 0.05 pg/mL, respectively, for younger subjects, and 0.53 ± 0.05 pg/mL and 0.54 ± 0.06 pg/mL, respectively, for older subjects.

COMMENT

Insulin may also have indirectly affected CSF cytokine levels through modulation of CSF and plasma A42 levels. Our data provide, to our knowledge, the first demonstration of acute manipulation of peripheral A42 in vivo in humans. The role of plasma A42 in AD pathogenesis is uncertain; however, elevations have been documented in patients with AD and in adults who later develop AD.²⁴ Notably, insulin’s effect on plasma A42 levels was enhanced in subjects with greater BMIs, a characteristic associated with both insulin resistance and AD risk.²⁵ Thus, the interactive effects of hyperinsulinemia and BMI on plasma A42 levels may contribute to this increased risk. It has been hypothesized that prolonged elevations of plasma A levels obstruct a peripheral sink through which CNS A is cleared, leading to increased accumulation in the brain.²⁶ High insulin levels may inhibit peripheral clearance of A42 by insulin-degrading enzyme in the liver or other tissues. The selective effects of insulin on A42 levels but not on A40 levels are puzzling. Such effects may reflect the increased tendency of A42 to oligomerize, rendering it impervious to degradation by insulin-degrading enzyme, or insulin-induced changes in lipids that differentially bind and enhance clearance of A species.

Alternatively, insulin may have increased A42 efflux from the brain to the plasma. Levels of transthyretin, a protein that can bind A and facilitate transport from the CNS to the periphery, are reduced in patients with AD.¹³ We found that insulin-induced elevations of CSF transthyretin levels were associated with increased plasma A42 levels and decreased CSF A42 levels. This inverse relationship suggests that insulin-induced transthyretin changes facilitated A clearance from the CNS to the periphery for some participants. Transthyretin is synthesized in the liver and the choroid plexus, sites rich with insulin receptors, and its synthesis is increased by insulin-like growth factor I, a peptide closely related to insulin. An insulin-responsive element has recently been identified in the promoter region of the transthyretin gene (D.G., unpublished data, 2004). Transthyretin is also regulated by IL-6 and TNF-.^27-28 Thus, insulin-induced increases of cytokine levels may have reduced transthyretin levels for some participants.

The A42 peptide interacts with inflammatory agents in a cyclically reinforcing manner, such that elevations in A levels increase proinflammatory cytokine levels.²⁹ In vitro, soluble A oligomers rapidly increase IL-1 and TNF- levels.³⁰ Conversely, several cytokines affect A production or clearance. Both IL-6 and IL-1 can regulate processing of the amyloid precursor protein from which A is derived and can increase production of A42.^31-32 The mutually reinforcing effects of A, TNF-, IL-1, and IL-6 may, therefore, create a "cytokine cycle."²⁹ Aspects of our results support this model. Changes in CSF A42 levels were predicted by increases in the levels of these 3 cytokines, but these changes were unrelated to changes in the levels of IL-1. Also, levels of CSF F₂-isoprostane, a lipid peroxidation marker produced by neurons and glia, increased with insulin infusion, and the magnitude of this effect was directly related to elevations of CSF A42 levels. In contrast, elevations of plasma A42 levels following insulin infusion were associated solely with increased CSF TNF- levels. This pattern contradicts a rodent study¹³ showing that TNF- inhibits A42 clearance from the brain, although effects of TNF- only on CSF A and not on plasma A were reported. It is possible that in humans, the insulin-induced rise in plasma A42 levels is multifactorial, reflecting A transport from the CNS, effects on peripheral clearance, or A release from peripheral sources such as platelets.

Norepinephrine may also mediate insulin’s effects on A and inflammatory reactants. Insulin can regulate CNS norepinephrine,¹⁴ an endogenous, anti-inflammatory neuromodulator that blocks IL-1 expression.¹⁵ Increased A plaque load in AD has been linked to neuronal loss in the locus coeruleus, the primary source of brain norepinephrine.³³ Thus, decreased norepinephrine activity in AD may potentiate the deleterious inflammatory effects of A. Consistent with this notion, higher CSF norepinephrine levels with insulin infusion were associated with selective attenuation in elevated IL-1 levels and reduced CSF A42 levels.

Insulin produced age-dependent effects on CSF levels of apoE, a lipoprotein that plays a critical role in cholesterol metabolism and injury repair and that down-regulates TNF- and IL-6 production in animal models.¹⁷ In the periphery, insulin reduces hepatic production of apoE and regulates its uptake by low-density lipoprotein receptor–related protein.¹⁶ We found that insulin reduced plasma apoE levels, an effect that increased with age. In contrast, insulin increased CSF apoE concentrations for older subjects. Increased brain apoE levels have been reported in AD in association with polymorphisms in the promoter region of the APOE gene that influence protein expression.³⁴ Insulin may influence CNS apoE expression through interactions with these polymorphisms or through other factors, such as low-density lipoprotein receptor–related protein. We observed that insulin-induced elevations of CSF apoE levels were associated with attenuated increases in levels of proinflammatory cytokines IL-6 and TNF- and with higher levels of IL-1, an anti-inflammatory cytokine. This selective pattern suggests multiple insulin effects that modulate the role of apoE in response to inflammation.

Our results can be integrated into a model describing the role of peripheral insulin resistance and hyperinsulinemia in AD pathogenesis. During early pathogenesis, high plasma insulin levels raise plasma A42 levels by promoting A release and inhibiting its clearance by insulin-degrading enzyme. As a result, more A42 may be transported from the periphery into the brain, or the transport of A42 from the brain to the periphery may be obstructed. Failure of insulin to appropriately regulate transthyretin may also interfere with clearance of A42 from the brain. Concomitantly, peripheral hyperinsulinemia increases CNS levels of IL-1, IL-6, TNF-, and F₂-isoprostane, agents that interact synergistically to promote A synthesis (IL-6 and IL-1) and reduce its clearance (TNF-). The resulting elevations of A levels provoke a correspondingly greater inflammatory response. Prolonged inflammation also likely exerts deleterious effects independent of A that contribute to AD pathogenesis. For example, noradrenergic dysfunction that characterizes patients with insulin resistance may reduce norepinephrine’s anti-inflammatory influence. Reduced availability or efficacy of apoE may affect its ability to inhibit IL-1 expression and thereby to modulate the inflammatory response.

Although this model has obvious relevance for diabetes mellitus, hyperinsulinemia and insulin resistance are widespread conditions that affect many nondiabetic adults with obesity, impaired glucose tolerance, cardiovascular disease, and hypertension. Our results provide a cautionary note for the current epidemic of such conditions, which, in the context of an aging population, may provoke a dramatic increase in the prevalence of AD. More encouragingly, greater understanding of insulin’s role in AD pathogenesis may lead to novel and more effective strategies for treating, delaying, or even preventing this challenging disease.

AUTHOR INFORMATION

Accepted for Publication: June 21, 2005.

Author Contributions: Study concept and design: Watson, Cook, Peskind, Baker, Asthana, and Craft. Acquisition of data: Fishel, Watson, Montine, Wang, Green, Kulstad, Goldgaber, Nie, Plymate, and Craft. Analysis and interpretation of data: Watson, Montine, Baker, Goldgaber, Nie, Schwartz, and Craft. Drafting of the manuscript: Watson, Wang, Cook, and Craft. Critical revision of the manuscript for important intellectual content: Fishel, Watson, Montine, Green, Kulstad, Cook, Peskind, Baker, Goldgaber, Nie, Asthana, Plymate, Schwartz, and Craft. Statistical analysis: Watson, Baker, and Craft. Obtained funding: Montine, Baker, and Craft. Administrative, technical, and material support: Fishel, Watson, Montine, Green, Cook, Peskind, Baker, Goldgaber, Nie, Plymate, and Craft. Study supervision: Watson, Cook, Peskind, Baker, Asthana, Plymate, and Craft.

Funding/Support: This work was supported by the Department of Veterans Affairs, Washington, DC, by grants RO1-AG-10880 (S.C.), RO1-AG-16835, and RO1-AG-24011 from the National Institute on Aging, Bethesda, Md, and by the Alvord Endowment (T.J.M), University of Washington, Seattle.