ABSTRACT
Tumor necrosis factor-alpha (TNFalpha) released in the brain by HIV-activated macrophages/microglia is suspected to compromise neuronal survival. Previously, we have demonstrated that activated receptor for insulin-like growth factor I (IGF-IR) protects neurons from TNFalpha-induced neuronal damage (Wang et al. [ 2006] J. Neurosci. Res. 83:7-18). Because TNFalpha triggers phosphorylation of insulin receptor substrate 1 (IRS-1) on serine residues (pS-IRS-1; Rui et al. [ 2001] J. Clin. Invest. 107:181-189), and pS-IRS-1 binds integrins (Reiss et al. [ 2001] Oncogene 20:490-500), we asked how these events affect neuronal processes. We show that beta1-integrin and pS-IRS-1 colocalize in PC12 cells and in primary cortical neurons. TNFalpha treatment elevated membrane-associated pS-IRS-1, enhanced pS-IRS-1 interaction with beta1-integrin, and attenuated cell attachment to collagen IV. In contrast, IGF-I inhibited pS-IRS-1-beta1-integrin complexes and improved cell attachment. The domain of IRS-1 involved in beta1-integrin binding mapped between amino acids 426 and 740, and the expression of 426-740/IRS-1 mutant attenuated neuronal outgrowth. Our results indicate that TNFalpha facilitates the interaction of pS-IRS-1 and beta1-integrin and destabilizes neuronal processes. IGF-I counteracts TNFalpha-mediated accumulation of pS-IRS-1-beta1-integrin complexes supporting the stability of neuronal processes.
Subject(s)
Integrin beta1/metabolism , Neurons/metabolism , Phosphoproteins/metabolism , Serine/metabolism , Animals , Blotting, Western , Cell Adhesion/physiology , Cells, Cultured , Cerebral Cortex/metabolism , Fluorescent Antibody Technique , Immunoprecipitation , Insulin Receptor Substrate Proteins , Insulin-Like Growth Factor I/metabolism , Membrane Microdomains/metabolism , Phosphoproteins/chemistry , Phosphorylation , Protein Binding , Rats , Tumor Necrosis Factor-alpha/metabolismABSTRACT
Insulin-like growth factor 1 receptor (IGF-1R) supports neuronal survival against a wide variety of insults. This includes tumor necrosis factor-alpha (TNFalpha)-mediated neuronal damage, which represents one of the factors suspected to play a role in HIV-associated dementia (HAD). PC12 neurons engineered to express human IGF-1R (PC12/IGF-1R) maintain neuronal processes on collagen IV for several weeks. However, prolonged treatment with TNFalpha caused degeneration of neuronal processes, with no apparent signs of apoptosis. In this process, TNFalpha did not affect IGF-1-mediated phosphorylation of IRS-1, IRS-2, Akt, or Erks. In addition, PC12/IGF-1R cells were found to express predominantly alpha1beta1 integrin, which has high affinity to collagen IV. The treatment of PC12/IGF-1R neurons with a specific alpha1beta1 integrin inhibitor, obtustatin, also caused loss of neuronal processes, accompanied by a quick cell detachment and extensive apoptosis. In the presence of IGF-1, both TNFalpha-induced and obtustatin-induced degeneration of neuronal processes were effectively inhibited. Furthermore, TNFalpha-mediated neuronal degeneration correlated with decreased attachment of PC12/IGF-1R cells to collagen IV and with a reduced level of alpha1beta1 integrin, consistent with a role for this surface protein in the maintenance of neuronal processes. Thus the neuroprotective effects of IGF-1 are not restricted to its antiapoptotic properties but also involve an additional neuroprotective mechanism, by which IGF-1 counteracts the negative effect of TNFalpha on alpha1beta1 integrin-mediated attachment to collagen IV.
