Sequential operation of ceramide synthesis and ICE cascade in CPT-11-initiated apoptotic death signaling

Inhibition of Akt kinase by cell-permeable ceramide and its implications for ceramide-induced apoptosis

Ceramide is an important lipid messenger involved in mediating a variety of cell functions including apoptosis. However, mechanisms responsible for ceramide-induced apoptosis remain unclear. We investigated the possibility that ceramide may decrease antiapoptotic signaling in cells by inhibiting Akt kinase activity. Our data show that C2-ceramide induces apoptosis in HMN1 motor neuron cells and decreases both basal and insulin- or serum-stimulated Akt kinase activity 65-70%. These results are consistent with decreased Akt kinase activity being involved in the apoptotic effects of ceramide. This possibility is further supported by studies showing that constitutively active Akt kinase decreases C2-ceramide-induced death of HMN1 cells as well as COS-7 cells. Decreased Akt activity is not due to ceramide activating the ceramide-activated protein phosphatase or to a direct inhibition of Akt kinase by ceramide, suggesting that ceramide acts upstream of Akt kinase to decrease its activity. Treating cells with C2-ceramide does not affect phosphorylation of insulin receptor substrate-1, interactions between insulin receptor substrate-1 and p85, or insulin-stimulated phosphatidylinositol 3-kinase activity, suggesting that the effects of C2-ceramide on Akt kinase are not mediated through modulating phosphatidylinositol 3-kinase. In sum, our results suggest that inhibition of the key antiapoptotic kinase, Akt, may play an important role in ceramide-induced apoptosis.

Necessity of interleukin-1beta converting enzyme cascade in taxotere-initiated death signaling

Taxotere is a new type chemotherapeutic agent which targets tubulin. In the present study, we investigated the molecular machinery of taxotere-initiated death signaling. Taxotere induced cell death in mouse fibroblast L929 cells. Cell morphological analysis revealed that this effect showed characteristics of apoptotic and necrotic cell death. To further examine taxotere-induced cell death, we investigated the direct involvement of caspase. When cells were pretreated with the synthesized tetrapeptide inhibitor of caspase, YVAD-CHO (Ac-Tyr-Val-Ala-Asp-aldehyde: inhibitor of interleukin-1beta converting enzyme (ICE) subfamily) or DEVD-CHO (Ac-Asp-Glu-Val-Asp-aldehyde: inhibitor of CPP32 subfamily), taxotere-induced cell death was prevented. In addition, time course experiments demonstrated that activation of the ICE subfamily preceded activation of the CPP32 subfamily in taxotere-initiated death signaling, suggesting the direct involvement of the ICE cascade in taxotere-initiated death signaling. On the basis of these results, we suggest that taxotere causes the initiation of ICE cascade in its death signaling pathway and that the down-stream site of taxotere-initiated death signaling is the same as that of other chemotherapeutic agents.

Amyloid beta-protein induces necrotic cell death mediated by ICE cascade in PC12 cells

A major component of Alzheimer's disease plaque amyloid beta protein (betaAP) showed the cytolytic activity to rat pheochromocytoma PC 12 cells. Nuclear morphological study revealed that betaAP-induced cytolytic activity is due to necrotic cell death, rather than apoptotic cell death. To examine the molecular machinery of betaAP-induced necrotic cell death in detail, I investigated the direct involvement of caspase. When nerve growth factor-treated and -untreated PC12 cells were incubated with the synthesized tetrapeptide inhibitors of caspase, YVAD-CHO (Ac-Tyr-Val-Ala-Asp-CHO) or DEVD-CHO (Ac-Asp-Glu-Val-Asp-CHO), betaAP-induced necrotic cell death was prevented. In addition, the interleukin-1beta converting enzyme (ICE) subfamily activation preceded CPP32 subfamily activation during betaAP-induced necrotic cell death. On the basis of these findings, I suggest that betaAP induces necrotic cell death mediated by the ICE cascade and that the ICE cascade may possibly be involved in Alzheimer's disease.