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Nitric oxide inhibits tumor necrosis factor-alpha-induced apoptosis by reducing the generation of ceramide
C De Nadai1, P Sestili, O Cantoni
1Department of Neuroscience, DIBIT-H San Raffaele Institute, and Consiglio Nazionale delle Ricerche, Centre of Cellular and Molecular Pharmacology, 20132 Milan, Italy.
Abstract:
Apoptosis triggered by death receptors proceeds after defined signal-transduction pathways. Whether signaling at the receptor level is regulated by intracellular messengers is still unknown. We have investigated the role of two messengers, ceramide and nitric oxide (NO), on the apoptotic pathway activated in human monocytic U937 cells by tumor necrosis factor-alpha (TNF-alpha) working at its p55 receptor. Two transduction events, the receptor recruitment of the adapter protein, TRADD, and the activation of the initiator caspase, caspase 8, were investigated. When administered alone, neither of the messengers had any effect on these events. In combination with TNF-alpha, however, ceramide potentiated, whereas NO inhibited, TNF-alpha-induced TRADD recruitment and caspase 8 activity. The effect of NO, which was cGMP-dependent, was due to inhibition of the TNF-alpha-induced generation of ceramide. Our results identify a mechanism of regulation of a signal-transduction pathway activated by death receptors.
Insights
This study reveals how ceramide enhances and nitric oxide (NO) inhibits tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis signaling. NO achieves this by blocking ceramide generation, uncovering a novel regulatory mechanism for death receptor pathways.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Immunology
Background:
- Apoptosis, or programmed cell death, is initiated by death receptors through defined signaling pathways.
- The regulation of these pathways by intracellular messengers at the receptor level remains largely unexplored.
- Tumor necrosis factor-alpha (TNF-alpha) is a key cytokine that triggers apoptosis via its p55 receptor.
Purpose of the Study:
- To investigate the role of ceramide and nitric oxide (NO) as intracellular messengers in TNF-alpha-induced apoptosis.
- To determine how these messengers affect key signal transduction events: TRADD recruitment and caspase 8 activation.
Main Methods:
- Utilized human monocytic U937 cells treated with TNF-alpha.
- Assessed the impact of ceramide and NO, alone and in combination with TNF-alpha, on TRADD recruitment and caspase 8 activation.
- Investigated the mechanism of NO's action, including its cGMP dependence and effect on ceramide generation.
Main Results:
- Ceramide potentiated TNF-alpha-induced TRADD recruitment and caspase 8 activity.
- Nitric oxide (NO) inhibited TNF-alpha-induced TRADD recruitment and caspase 8 activity.
- NO's inhibitory effect was cGMP-dependent and mediated by blocking TNF-alpha-induced ceramide generation.
Conclusions:
- Ceramide and NO act as critical regulators of the TNF-alpha-mediated apoptotic signaling pathway.
- NO inhibits apoptosis by suppressing the generation of ceramide, a pro-apoptotic messenger.
- These findings elucidate a novel regulatory mechanism controlling signal transduction initiated by death receptors.