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Differential regulation of IL-1 alpha and TNF alpha release from immortalized murine microglia (BV-2)
1Department of Pharmacology, CoCensys, Inc., Irvine, CA 92718.
Abstract:
Microglia, the resident macrophages of the brain, secrete a number of mediators involved in neural-immune function. The cytokines, IL-1 alpha and TNF alpha, are two such factors which are stored as inactive precursor molecules requiring post-translational proteolytic processing prior to release. From investigations of second messenger pathways involved in regulating the secretion of these cytokines, we have demonstrated that the PKC inhibitor, H-7, blocks the induction of TNF alpha secretion induced by LPS. In contrast, H-89 and HA-1077, inhibitors of cyclic nucleotide-dependent protein kinases (PKA and PKG), did not alter LPS-stimulation of TNF alpha release. Consistent with these observations, the weak PKC activator, mezerein, induced TNF alpha secretion in an H-7-reversible manner. In marked contrast, PKC activation did not induce IL-1 alpha secretion and H-7 potentiated IL-1 alpha release. In the case of the protein phosphatase inhibitor, okadaic acid, secretion of both cytokines was induced, indicating that protein phosphorylation is important for the induction of cytokine secretion but only in the case of TNF alpha is PKC involved. In the case of IL-1 alpha, a tonic inhibitory regulation involving PKC activation may be present. We therefore conclude that alterations in phosphorylation-dephosphorylation cycles may be important triggers in the switching of microglial cellular function from a resting to an activated state.
Insights
Protein kinase C (PKC) regulates tumor necrosis factor-alpha (TNF-α) secretion in microglia, but not interleukin-1 alpha (IL-1α). Altered phosphorylation cycles may activate these brain immune cells.
Area of Science:
- Neuroimmunology
- Cellular Signaling
- Neuroinflammation
Background:
- Microglia, the brain's resident macrophages, play a crucial role in neural-immune function by secreting various mediators.
- Key secreted cytokines, interleukin-1 alpha (IL-1α) and tumor necrosis factor-alpha (TNF-α), are stored as inactive precursors requiring post-translational processing for release.
Purpose of the Study:
- To investigate the role of second messenger pathways, specifically protein kinase C (PKC), in regulating microglial cytokine secretion.
- To elucidate the involvement of phosphorylation-dephosphorylation cycles in microglial activation.
Main Methods:
- Utilized PKC inhibitors (H-7) and activators (mezerein) to study TNF-α and IL-1α secretion.
- Examined the effects of cyclic nucleotide-dependent protein kinase inhibitors (H-89, HA-1077) and a protein phosphatase inhibitor (okadaic acid).
- Assessed cytokine release in response to lipopolysaccharide (LPS) stimulation.
Main Results:
- PKC inhibition (H-7) blocked LPS-induced TNF-α secretion, while PKC activation (mezerein) induced it.
- PKC modulation did not affect IL-1α secretion; H-7 potentiated IL-1α release, suggesting inhibitory regulation.
- Okadaic acid induced secretion of both cytokines, highlighting the importance of protein phosphorylation in cytokine release.
Conclusions:
- PKC is critically involved in TNF-α secretion but not IL-1α secretion in microglia.
- A tonic inhibitory regulation by PKC may exist for IL-1α.
- Changes in phosphorylation-dephosphorylation cycles are likely triggers for switching microglia from a resting to an activated state.