Molecular studies of the insulin-like growth factor system in the brain : focus on growth, tumorigenesis and trauma

Abstract

Insulin-like growth factor (IGF) 1 and -2 are synthesized by most tissues of the body and function as autocrine or paracrine agents in a tissue-specific manner. mRNA for IGF-1 and -2, as well as the IGF-binding proteins, are synthesized locally in various regions of the brain. Specific IGF receptors (type I and type II) are expressed throughout the brain, suggesting that the IGFs play an important role in the brain.

This thesis is focused mainly on the expression and regulation of the IGFs in the brain, in particular their role in growth, tumorigenesis and trauma. IGF-1 and -2 mRNA were identified in normal human brain during development, with higher levels of both molecules in fetuses. Higher levels of IGF-1 than IGF-2 mRNA were observed at both stages. These findings suggest that the IGFs are important during brain development. The cDNA sequence corresponding to human fetal brain IGF-1, was characterized by RT-PCR and sequencing for two precursor protein forms, Ea and Eb IGF-1. The sequences showed no differences to previously reported sequences from-human liver indicating that the truncated IGF-1 molecules found in the brain are produced by posttranslational modifications.

In brain tumors, elevated levels of IGF mRNA and peptide were identified. Northern blots detected enhanced levels of expression of both IGF-1 and -2 in gliomas, with most of the tumors expressing higher levels of IGF-1 than IGF-2. The cDNA sequences of the two precursor forms of IGF-1 found in the human fetal brain, was not changed in one anaplastic astrocytoma analyzed. A high IGF-2/IGFBP-2 mRNA ratio was found in anaplastic or atypical meningiomas, whereas meningiomas of lower histopathological grade showed a low IGF-2/IGFBP-2 mRNA ratio. In situ hybridization on paraffin embedded tumor tissue identified a non-uniform labelling pattern of IGF-2 with specific foci showing strong labelling in many of the tumors, whereas the IGFBP-2 mRNA was expressed in a more uniform pattern. A high IGF-2/IGFBP-2 mRNA ratio may thus serve as a molecular marker for the detection of malignant meningiomas. Some of the molecules in the IGF cascade respond to traumatic injury with a transient but striking increase in mRNA synthesis.

We used an experimental traumatic brain injury model in rat, making a cerebral cortical contusion. Marked increases in gene expression of IGF-1, IGFBP-2 and -4 were found two-three days following the impact. This increase in mRNA levels was inhibited by glutamate receptor blockade, a treatment used to reduce excitotoxic neuronal damage. Injections of an NMDA (MK-801) or an AMPA/kainate (CNQX) receptor antagonist, before and after the injury blocked the induction of IGF-1 mRNA at the site of contusion. The neuronal cortical increase of IGFBP-2 and -4 mRNA levels were blocked by MK-801 but not by CNQX. Previous studies have shown that IGF-I treatment may increase neuronal survival. A combination of glutamate receptor blockade and IGF- 1 treatment may thus increase neuronal survival. The IGFs are developmentally regulated and display regional specificity in the brain which suggest a specific physiological role. Increased levels of mRNA coding for molecules within the IGF-system were observed during development, following trauma and in certain brain tumors. An increased understanding of the role of the IGF system in the brain following trauma and in tumors, may be significant for developing novel strategies in regeneration and to prevent tumor progression.