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Phosphatidylinositol 3-kinase/Akt activity regulates c-FLIP expression in tumor cells
1Department of Hematology and Oncology, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02215, USA.
Abstract:
The caspase-8 homologue FLICE-inhibitory protein (FLIP) functions as a caspase-8 dominant negative, blocking apoptosis induced by the oligomerization of the adapter protein FADD/MORT-1. FLIP expression correlates with resistance to apoptosis induced by various members of the tumor necrosis factor family such as TRAIL. Furthermore, forced expression of FLIP renders cells resistant to Fas-mediated apoptosis. Although FLIP expression is regulated primarily by MEK1 activity in activated T cells, the oncogenic signaling pathways that regulate FLIP expression in tumor cells are largely unknown. In this report, we examined the roles of the MAP kinase and phosphatidylinositol (PI) 3-kinase signaling pathways in the regulation of FLIP expression in tumor cells. We observed that the MEK1 inhibitor PD98059 reduced FLIP levels in only 2 of 11 tumor cell lines tested. In contrast, disruption of the PI 3-kinase pathway with the specific inhibitor LY294002 reduced Akt (protein kinase B) phosphorylation and the levels of FLIP protein and mRNA in all cell lines evaluated. The introduction of a dominant negative Akt adenoviral construct also consistently reduced FLIP expression as well as the phosphorylation of the Akt target glycogen synthase kinase-3. In addition, infection of the same cell lines with a constitutively active Akt adenovirus increased FLIP expression and the phosphorylation of GSK-3. These data add FLIP to the growing list of apoptosis inhibitors in which expression or function is regulated by the PI 3-kinase-Akt pathway.
Insights
The phosphatidylinositol 3-kinase (PI3K)-Akt pathway, not MEK1, regulates FLICE-inhibitory protein (FLIP) in tumor cells. This pathway controls FLIP expression, impacting apoptosis resistance and offering potential therapeutic targets.
Area of Science:
- Oncology
- Molecular Biology
- Cell Signaling
Background:
- FLICE-inhibitory protein (FLIP) inhibits apoptosis by blocking caspase-8 activation.
- FLIP expression is linked to resistance against tumor necrosis factor (TNF) family-induced apoptosis and Fas-mediated cell death.
- While MEK1 regulates FLIP in T cells, the pathways controlling FLIP in tumor cells remain unclear.
Purpose of the Study:
- To investigate the roles of MAP kinase and phosphatidylinositol 3-kinase (PI3K) signaling pathways in regulating FLIP expression in tumor cells.
Main Methods:
- Treatment of tumor cell lines with MEK1 inhibitor PD98059 and PI3K inhibitor LY294002.
- Assessment of FLIP protein and mRNA levels, and Akt phosphorylation.
- Utilized dominant-negative and constitutively active Akt adenoviral constructs to modulate Akt activity.
Main Results:
- MEK1 inhibition only affected FLIP levels in a minority of tumor cell lines.
- PI3K pathway disruption with LY294002 consistently reduced FLIP protein/mRNA and Akt phosphorylation across all tested cell lines.
- Modulation of Akt activity via adenoviral constructs directly altered FLIP expression and Akt target phosphorylation (GSK-3).
Conclusions:
- The PI3K-Akt pathway, rather than MEK1, is a key regulator of FLIP expression in tumor cells.
- These findings identify FLIP as another apoptosis inhibitor regulated by the PI3K-Akt pathway, suggesting potential therapeutic implications.