Mechanisms of nerve growth factor mRNA regulation by interleukin‐1β in hippocampal cultures: Role of second messengers

Abstract

Cytokines such as interleukin-1, which are found in the brain after trauma, regulate expression of nerve growth factor (NGF) mRNA and protein in hippocampal cultures. We have investigated possible mechanisms by which Il-1 beta regulates NGF in hippocampal cells. The induction of NGF mRNA by Il-1 beta was blocked by a receptor antagonist indicating that this effect is receptor mediated. Il-1 beta elicited a dramatic induction of c-fos mRNA and a slight elevation of c-jun mRNA in a time dependent manner which may allow for a role in the induction of NGF mRNA expression. We examined whether specific second messenger pathways were involved in mediating the action of Il-1 beta in the hippocampus. Activation of cAMP with forskolin or treatment with 8-Br-cAMP had no effect on NGF mRNA levels. Moreover, exposure of hippocampal cultures to Il-1 beta evoked no change in cAMP levels, indicating that this second messenger system played little or no role in the regulation of NGF expression by Il-1 beta in these cells. Further, interleukin-1 elicited no change in membrane inositol phosphate turnover, nor did it affect intracellular calcium levels. Treatment of cell cultures with the phorbol ester PMA elicited an increase in NGF mRNA, suggesting that activation of protein kinase C (PKC) may mediate NGF mRNA expression. However, prolonged treatment of cultures with PMA to desensitize PKC did not eliminate the Il-1 beta induction of NGF mRNA. Il-1 beta, therefore, did not appear to activate NGF expression via cAMP, Ca2+, or a PKC isoform that is downregulated by prolonged PMA treatment. However, a phosphorylation event may be involved in the signal transduction mechanism, as treatment with okadaic acid to inhibit protein phosphatase 2a potentiated the induction of NGF mRNA by Il-1 beta. The results presented indicate that Il-1 beta acts via its receptor to induce a rise in NGF expression. Identification of the specific second messenger pathway has remained elusive; however, a phosphorylation event appears to be intermediary. Moreover, the induction of c-fos and c-jun may represent a final common path in activation of NGF gene expression by different signals such as Il-1 beta and PMA.

The immortalized astroglial cell line RC7 is a new model system for the study of nerve growth factor (NGF) regulation: Stimulation by interleukin‐1β and transforming growth factor‐β1 is additive and affected differently by dibutyryl cyclic AMP

“…In RC7 cells NGF synthesishecretion was dose-dependently upregulated by IL-16 as repeatedly described for astrocytes (Carman-Krzan et al, 1991;Friedman et al, 1992;Spranger et al, 1990;Vige et al, 1991;Yoshida and Gage, 1991) with a dose-effect range comparable to that determined by Carman-Krzan et al (1991) for NGF protein and by Vige et al (1991) for NGF mRNA levels in cortical primary astrocytes. In all three systems the half maximal effective concentration (EC,,) was about 1 U/ml and the maximal effective concentration (EC,,,) 3-10 U/ml.…”

Cell Type-Specific Interleukin-1β Signaling in the CNS

“…However, the pathway mediating IL-1␤ activation of CREB was undefined. Previous studies had shown no activation of cAMP by IL-1␤ in these cells (Friedman et al, 1992), suggesting that this cytokine may activate CREB by a distinct signaling pathway. Ca 2ϩ signaling also can lead to CREB activation; however, fura-2 analysis of IL-1␤-treated hippocampal neurons revealed no change in calcium signaling.…”

“…CREB can be activated by numerous second messengers, including Ca 2ϩ and cAMP, as well as by the ERK and p38 MAPK signaling pathways. In a previous study, we had shown that IL-1 did not elicit any change in cAMP levels in cultures of hippocampal neurons and glia (Friedman et al, 1992). To determine whether calcium signaling mediated CREB phosphorylation in hippocampal neurons, we monitored intracellular calcium by fura-2 imaging.…”

Regulation of Nerve Growth Factor mRNA by Interleukin-1 in Rat Hippocampal Astrocytes Is Mediated by NFκB

“…The signaling pathway does not appear to involve cAMP, protein kinase C, phosphatidylinositol turnover, or mobilization of intracellular calcium (16). Activation of c-fos (16), and the presence of an AP-1 site in the NGF promoter (17,18) suggested a potential role for this transcription factor in mediating Il-1 stimulation of NGF.…”

“…The signaling pathway does not appear to involve cAMP, protein kinase C, phosphatidylinositol turnover, or mobilization of intracellular calcium (16). Activation of c-fos (16), and the presence of an AP-1 site in the NGF promoter (17,18) suggested a potential role for this transcription factor in mediating Il-1 stimulation of NGF. Alternatively, the recent identification of the sphingomyelin pathway, leading ultimately to activation of NFB transcriptional activity by Il-1 and TNF (19,20), suggested that this pathway might mediate actions of Il-1 in astrocytes.…”