ICAM-1 is a transmembrane glycoprotein of the Ig superfamily involved in cell adhesion. ICAM-1 is aberrantly expressed by astrocytes in CNS pathologies such as multiple sclerosis, experimental allergic encephalomyelitis, and Alzheimer’s disease, suggesting a possible role for ICAM-1 in these disorders. ICAM-1 has been shown to be important for leukocyte diapedesis through brain microvessels and subsequent binding to astrocytes. However, other functional roles for ICAM-1 expression on astrocytes have not been well elucidated. Therefore, we investigated the intracellular signals generated upon ICAM-1 engagement on astrocytes. ICAM-1 ligation by a mAb to rat ICAM-1 induced mRNA expression of proinflammatory cytokines such as IL-1α, IL-1β, IL-6, and TNF-α. Examination of cytokine protein production revealed that ICAM-1 ligation results in IL-6 secretion by astrocytes, whereas IL-1β and IL-1α protein is expressed intracellularly in astrocytes. The involvement of mitogen-activated protein kinases (MAPKs) in ICAM-1-mediated cytokine expression in astrocytes was tested, as the MAPK extracellular signal-regulated kinase (ERK) was previously shown to be activated upon ICAM-1 engagement. Our results indicate that ERK1/ERK2, as well as p38 MAPK, are activated upon ligation of ICAM-1. Studies using pharmacological inhibitors demonstrate that both p38 MAPK and ERK1/2 are involved in ICAM-1-induced IL-6 expression, whereas only ERK1/2 is important for IL-1α and IL-1β expression. Our data support the role of ICAM-1 on astrocytes as an inflammatory mediator in the CNS and also uncover a novel signal transduction pathway through p38 MAPK upon ICAM-1 ligation.

In the human astroglioma cell line CH235-MG, interleukin-1 beta (IL-1 beta) induces transcriptional activation of the tumor necrosis factor-alpha (TNF-alpha) gene, resulting in expression of TNF-alpha mRNA and biologically active TNF-alpha protein. This study was undertaken to elucidate intracellular signaling pathways involved in IL-1 beta induction of the TNF-alpha gene. We demonstrated that the protein kinase C (PKC) activator 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) in concert with Ca++ ionophore A23187 induced expression of TNF-alpha mRNA and protein, whereas an inactive PMA analogue (alpha PMA) had no effect. Various cyclic nucleotide activators such as 8-Bromo cAMP, cholera toxin, and forskolin had no effect on TNF-alpha production. Two PKC inhibitors, H7 and staurosporine (SS), abrogated IL-1 beta induced TNF-alpha expression in a dose-dependent fashion. Treatment of CH235-MG cells with a high concentration of PMA (1 microM) for an extended period of time (48 h) caused a greater than 90% reduction in total PKC activity. Further strengthening a role for PKC in this cytokine response is the fact that IL-1 beta was no longer able to induce TNF-alpha expression in these PKC depleted cells. Last, IL-1 beta treatment produced an increase of total PKC activity in CH235-MG cells. Taken together, these data demonstrate that IL-1 beta induces TNF-alpha gene expression in CH235-MG cells in a PKC-dependent manner.

The molecular mechanism(s) by which three cytokines (IFN-gamma, TNF-alpha, TGF-beta) affect class II MHC gene expression in primary rat microglia was examined. IFN-gamma is a potent inducer of the class II gene, and this induction is unaffected by treatment with either TNF-alpha or TGF-beta. Transient transfection of primary rat microglia with an HLA-DRA promoter linked to the chloramphenicol acetyltransferase reporter gene (DRA-CAT) demonstrated that IFN-gamma acts at the transcriptional level to induce class II MHC gene expression, and that TNF-alpha and TGF-beta have no influence on IFN-gamma-induced promoter activity. Experiments using a series of DRA substitution mutants that individually affect the W, X1, X2, or Y elements, as well as a double mutation in both X1 and X2, indicate that all four of these elements are required for responsiveness of the DRA promoter to IFN-gamma. The effect of IFN-gamma and TNF-alpha on DNA binding proteins by microglia was examined. A constitutive complex with specificity for the X2 box was detected in extracts from unstimulated microglia. IFN-gamma treatment changed this complex to migrate with slower mobility, and TNF-alpha had no effect on either the constitutive or IFN-gamma-induced complexes. These studies provide information on the molecular regulation of the class II MHC gene in microglia, a cell type critically involved in immune regulation within the central nervous system.

Class II MHC Ags are critical in the regulation of immune responses by presenting Ag to T lymphocytes, resulting in their activation and differentiation. Class II expression is rare in the normal central nervous system, but elevated expression on glial cells has been observed in several neurologic diseases. We have previously demonstrated that IFN-gamma-induced class II expression in glial cells involves activation of both tyrosine kinase and protein kinase C. IFN-gamma induces tyrosine phosphorylation of the tyrosine kinases Jak1 and Jak2 and of Stat1 alpha. In addition, IFN-gamma enhances expression of Stat1 alpha mRNA and protein. We utilized antisense oligonucleotides against Stat1 alpha to determine directly whether IFN-gamma-induced activation and/or enhancement of Stat1 alpha is involved in class II expression. Antisense oligonucleotides complementary to Stat1 alpha mRNA were introduced in CH235-MG astroglioma cells by transient transfection; such treatment inhibited both constitutive and IFN-gamma-enhanced expression of Stat1 alpha. IFN-gamma-induced class II MHC expression was also inhibited in cells exposed to Stat1 alpha antisense oligonucleotides. The fact that the class II promoter does not contain IFN-gamma-activated sequences for binding Stat1 alpha suggests that Stat1 alpha must activate another protein that is directly involved in class II expression. A likely candidate is the class II MHC transactivator (CIITA). IFN-gamma induction of CIITA mRNA was also inhibited in cells treated with antisense oligonucleotides against Stat1 alpha. These findings demonstrate that Stat1 alpha is involved in IFN-gamma induction of CIITA expression, resulting in class II MHC expression.

We previously showed the presence of receptors for granulocyte- macrophage colony-stimulating factor (GM-CSF) on tumor tissues and tumor cell lines that are derived from the neural crest. To determine whether normal neural cells express functional GM-CSF receptors, we isolated and analyzed primary rat brain cells, including microglia, astrocytes, and oligodendrocytes. Scatchard analysis of equilibrium binding of 125I-GM-CSF to primary rat oligodendrocytes showed an average of 1,110 GM-CSF binding sites per cell, with a kd of 20 pmol/L. In six separate experiments, no specific binding was detectable on the astrocyte population. Microglia were used in competitive binding experiments with oligodendrocytes, and addition of microglia did not increase the specific binding of labeled ligand to oligodendrocytes. In dose-response assays, we measured 3H-thymidine uptake in rat oligodendrocytes, microglia and control murine 32D cells stimulated with various concentrations of GM-CSF. Over concentration ranges of 0.025 to 1000 pmol/L, cell proliferation and peak 3H-thymidine incorporation was observed at approximately 30 pmol/L for both the control cells and the oligodendrocytes. However, the microglial cells did not proliferate in response to GM-CSF. These data indicate the presence of a functional receptor for GM-CSF on primary rat oligodendrocytes, and suggest that hematopoietic growth factors such as GM-CSF may play a role in nerve cell development, function, or response to injury.