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Recent evidence suggests that the level of interleukin-6 (IL-6) is elevated in Alzheimer's disease (AD) brains. IL-6 is produced by reactive glial cells and could potentially affect neuronal survival. Understanding the biochemical mechanism that regulates the production and release of IL-6 by astrocytic cells may help to identify potential targets for therapeutic intervention in AD. In the present study, glial fibrillary acidic protein-positive human U373MG astrocytoma cells were used as a model of reactive astrocytes. Production of IL-6 in response to drug treatment was monitored with an ELISA assay. Histamine (1-100 microM), substance P (SP; 1-100 nM), and human interleukin-1 beta (IL-1 beta; 1-30 pM) stimulated the release of IL-6 in a time- and concentration-dependent manner, with EC50 values of 4.5 microM, 8 nM, and 4.5 pM, respectively. The respective effects of histamine, SP, and IL-1 beta were effectively blocked by the histamine H1, SP, and IL-1 receptor antagonists, supporting a receptor-mediated event for these agents. Both histamine and SP enhanced the formation of inositol phosphates and increase intracellular calcium levels, suggesting that the phosphatidylinositol bisphosphate/protein kinase C pathway may be involved in the IL-6 release process. Indeed, phorbol 12-myristate 13-acetate, a protein kinase C activator, also evoked IL-6 release from the U373MG cells. On the other hand, IL-1 beta, which produces a much more robust release of IL-6 than histamine or SP, has no effect on inositol phosphate formation or intracellular calcium levels.(ABSTRACT TRUNCATED AT 250 WORDS)

We have previously shown that activation of the phosphatidyl-inositol/phospholipase C pathway could induce interleukin 6 (IL-6) release from U373MG human astrocytomes cells. We also found that, although interleukin 1 beta (IL-1 beta) did not activate phosphatidy-linositol turnover, it induced, a robust release of IL-6. In the present study, we examined the role of adenylate cyclase/cyclic 3',5'-adenosine monophosphate (cAMP) pathway in IL-6 release. Agents which mimicked (dibutyryl cAMP) or stimulated (isoproterenol and forskolin) cAMP formation were found to induce IL-6 release and their effects could be potentiated by 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor. On the other hand, in spite of its robust action on IL-6 release, IL-1 beta did not stimulate cAMP formation. Other possible signal transduction mechanisms involved in IL-1 beta-induced IL-6 release are discussed.

Activation of the classical complement cascade by beta-amyloid peptides has been hypothesized to underlie the neurodegeneration observed in Alzheimer's diseased brains. In this study, various lots of synthetic beta-amyloid peptides, A beta(1-40), A beta(1-42), and A beta(25-35), were tested for their ability to activate both early complement cascade events and formation of the membrane attack complex through terminal pathway activation. Unlike recent reports which did not assess activation of complement terminal pathway, we found that concentrations of beta-amyloid which activated early cascade events, to an extent comparable to aggregated IgG, failed to elicit formation of comparable levels of membrane attack complex.

To investigate the consequences of complement activation on neuronal viability, the effects of serum treatment on neuron-rich and mixed neuronal/glial cultures were evaluated. The neurotoxicity observed following treatment with either human or rat serum was variable and did not appear to be mediated through a complement-mediated mechanism. Serum lots lacking CH50 activity induced neurotoxicity, and heat treatment of toxic lots of either human or rat sera did not abolish toxicity. In cases where serum treatment did not induce cell death, treatment with PIPLC to remove endogenous membrane-bound complement inhibitors prior to serum exposure, did not result in cell death.