Published online before print August 10, 2006, doi: 10.1161/01.STR.0000236496.30106.4b
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(Stroke. 2006;37:2368.)
© 2006 American Heart Association, Inc.
Short Communication |
Intravenous Tissue Plasminogen Activator in Patients With Stroke Increases the Bioavailability of Insulin-Like Growth Factor-1
From the Departments of Neurology (N.W., J.W.E., D.S., J.D.) and Laboratory Medicine (I.P.K.), University Medical Center Groningen, Groningen, The Netherlands.
Correspondence to Professor Jacques De Keyser, Department of Neurology, University Medical Center Groningen, Hanzeplein 1, 9713 GZ Groningen, The Netherlands. E-mail j.h.a.de.keyser{at}neuro.umcg.nl
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Abstract |
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Background and Purpose— Insulin-like growth factor (IGF)-1 has potent neuroprotective properties. We investigated the effects of intravenous administration of tissue plasminogen activator (tPA) on serum levels of IGF-1 and IGF-binding protein (IGFBP)-3 in patients with acute ischemic stroke.
Methods— Serum levels of total IGF-1, free IGF-1, and IGFBP-3 were measured by radioimmunoassay in 10 patients with ischemic stroke treated with intravenous tPA (0.9 mg/kg body weight) and 10 untreated controls.
Results— During tPA treatment, total IGF-1 and IGFBP-3 serum levels did not change, but there was an &70% increase in free IGF-1 serum levels from 0.98±0.25 at baseline to 1.69±0.18 nmol/L at the end of the 1-hour infusion (P=0.01).
Conclusions— Intravenous therapy with tPA enhances the bioavailability of IGF-1.
Key Words: acute stroke • insulin-like growth factor 1 • neuroprotection • thrombolysis • tissue plasminogen activator
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Introduction |
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Insulin-like growth factor I (IGF-1) is a survival factor for neurons and glial cells and prevents apoptosis by activating several signal transduction pathways.1,2 IGF-1 protects neurons against oxidative stress, nitric oxide and excitotoxicity, which are key elements of the biochemical cascade causing cell death in cerebral ischemia. IGF-1 administered within a few hours after brain injury is protective in both gray and white matter.1,2 The bioavailability of IGF-1 is tightly regulated by IGF-binding proteins (IGFBPs).1 Proteolytic cleavage of IGFBPs is an important mechanism for the release of IGF-1, which is then able to bind to IGF-1 receptors on target cells. In serum, IGF-1 circulates predominantly as a 150-kDa ternary complex consisting of IGF-1, IGFBP-3, and the acid-labile subunit. This ternary complex constitutes both a reservoir and a carrier system for IGF-1.3
Tissue plasminogen activator (tPA) initiates the process of clot degradation by converting plasminogen to plasmin and is used for the treatment of acute ischemic stroke with the intention to rescue ischemic brain tissue by restoring blood flow. Both tPA and plasmin are serine proteases, and in vitro studies have shown that tPA and plasmin induce limited proteolysis of IGFBP-3.4,5 Limited proteolysis by tPA or plasmin lowers the affinity of IGFBP-3 in the ternary complex for IGF-1 and constitutes a mechanism for enhancing IGF-1 bioavailability. Our study aimed at investigating whether intravenous administration of tPA elevates free serum levels of IGF-1 in patients with acute ischemic stroke.
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Patients and Methods |
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Patients
The study was approved by the local ethics committee and all patients gave informed consent before the inclusion in the study. The study population consisted of 20 patients with partial or total anterior circulation ischemic stroke, aged 18 years or older, and known stroke onset time. A computed tomography scan of the brain was performed in all patients at baseline. Ten patients were treated with intravenous 0.9 mg/kg body weight tPA (no more than 90 mg total) given as a 10% intravenous bolus over 2 minutes through a dedicated line followed immediately by a 60-minute infusion of the remaining dose. Ten patients who were not eligible for tPA treatment served as controls. Reasons for not giving tPA were: rapidly improving symptoms (2), use of oral anticoagulants and elevated prothrombin time (4), mild neurological deficit (1), sustained systolic blood pressure >185 mm Hg (2), and age (1).
Assays
Venous blood samples were taken at baseline within 5 hours after stroke onset. In patients receiving tPA, the first sample was taken just before the administration of the bolus. Further samples were taken at 1 hour (for the tPA-treated patients at the end of the infusion), 2 hours, and 4 hours after baseline. Blood samples were immediately centrifuged and serum was stored at –80°C until assayed. Total IGF-1, free IGF-1, and IGFBP-3 levels were measured by radioimmunoassay kits according to the manufacturer’s instructions (Nichols Institute Diagnostics). The samples from each individual were measured in duplicate in the same assay.
Statistical Analysis
Results are expressed as the mean±SE of mean. Categorical data were compared between groups with Fisher exact test and continuous data with the Mann Whitney U test. Repeated-measures analysis of variance used to detect differences in the biochemical variables. A probability value <0.05 was considered significant. The data were analyzed using Instat 3 for Macintosh.
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Results |
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The clinical characteristics of the 2 patient groups are listed in the Table.
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At baseline, total IGF-1, free IGF-1, and IGFBP-3 serum levels were not different between the 2 groups. In the control group, all 3 biochemical variables remained unchanged after 1, 2, and 4 hours (Figure). In the group receiving tPA, free serum IGF-1 levels were significantly increased from 0.98±0.25 nmol/L at baseline to 1.67±0.18 nmol/L at the end of the infusion (P=0.01). Total IGF-1 and IGFBP-3 serum levels were not affected by tPA (Figure). The tPA-induced increase in bioavailable IGF-1 was no longer present 1 hour after the infusion was stopped.
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Discussion |
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There has been concern that tPA might enhance neuronal damage in focal cerebral ischemia because of reports that this compound increased neuronal cell death mediated by glutamate overstimulation of N-methyl-D-aspartate receptors.6 However, these effects were observed in cell culture experiments with doses of tPA in the micromolar range. It is unlikely that these effects are relevant for patients with stroke treated with tPA, because concentrations of tPA that extravasate in the ischemic brain after the intravenous administration of 10 mg/kg tPA, leading to reperfusion in a thromboembolic model of focal cerebral ischemia in rats, were in the nanomolar range.7
Contradictory data have been collected from tPA knockout mice, in which transient focal cerebral ischemia produced infarcts that, compared with wild-type mice, were smaller in one study8 yet larger in another.9 Although the exact reason for these variable results is not known, differences in genetic background of the mice used could play a significant role. Klein and coworkers found no increase in either hippocampal neuronal injury or infarct volume in rat models of global and focal brain ischemia, even at doses 10 times that used in patients with stroke.10 Other studies found that tPA administration after transient focal cerebral ischemia in rats and mice showed a tendency to reduce the degree of injury.11,12
Our study for the first time demonstrates that intravenous administration of tPA to patients with acute ischemic stroke transiently increases free IGF-1 serum levels. The mean delay between stroke onset and baseline sample was 60 minutes longer in the control group than the tPA-treated group. However, this difference was not significant. If a phenomenon related to the stroke itself was responsible for the rise in free IGF-1 occurring 180 minutes after stroke onset and having disappeared 60 minutes later, then we would have expected to observe a significant difference between baseline and 1-hour blood samples in the controls.
The increase in free serum IGF-1 levels may be explained by tPA or plasmin-induced limited proteolysis of IGFBP-3, which has been demonstrated in vitro.4,5 IGFBP-3 serum levels as measured by radioimmunoassay were not affected by tPA, which is consistent with the observations that limited proteolysis does not result in a disruption of the ternary complex, but only decreases the affinity of IGF-1 for IGFBP-3 &10-fold.13 Free IGF-1 can enter the ischemic brain parenchyma because the blood-brain barrier is disrupted, which is further enhanced by tPA itself.14 Because tPA mainly activates plasminogen associated with fibrin, which accumulates not only in the clot, but also in the ischemic cerebral microvasculature,15 free IGF-1 levels may be more pronounced at the site of the ischemic injury than those that we measured in the systemic circulation.
A short increase in bioactive IGF-1 levels may be sufficient to offer neuroprotection. A single dose of IGF-1 given over 20 minutes into the lateral ventricle 2 hours after focal brain ischemia in rats significantly reduced cortical infarction and neuronal cell death and improved somatosensory function when examined 20 days later.16 In a model of hypoxic-ischemic injury in fetal sheep, caused by transient bilateral carotid artery occlusion, a continuous infusion of IGF-1 for 24 hours offered no better neuroprotection than a single dose of IGF-1 given over 1 hour.17
It remains unclear why intravenous tPA improves outcome in patients with lacunar stroke.18,19 Another unresolved issue is why a substantial number of patients with ischemic stroke do not experience immediate clinical improvement after intravenous tPA administration, but rather experience a slower recovery of brain functions leading to a good outcome at 3 months.19,20 Both effects might be explained by mechanisms mediated by tPA that are independent of its thrombolytic action. IGF-1 strongly reduces white matter injury, which could explain a beneficial effect in lacunar ischemia,2 and its role in preventing programmed cell death would be consistent with a slower functional recovery of stroke patients after tPA treatment. Our findings suggest that intravenous therapy with tPA might have neuroprotective properties in addition to its thrombolytic action.
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Acknowledgments |
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We thank A. Teelken and M. Schaaf for excellent technical assistance.
Disclosures
None.
Received April 4, 2006; revision received May 20, 2006; accepted June 19, 2006.
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