Apoptotic Signaling Proteins Research Articles

Expression of apoptotic signaling proteins in leukoplakia and oral lichen planus: quantitative and topographical studies.

Although the pathogenesis of leukoplakia has been unclear, carcinogenic transformation is postulated to result from alterations of apoptotic signal transduction proteins in epithelial cells. The pathogenesis of oral lichen planus (OLP) has also been unclear, but apoptotic changes of the epithelial cells in OLP have been reported. In the present study, we used a histochemical approach to describe human keratinocyte-expression of several apoptotic signaling proteins in leukoplakia, in OLP, and in normal oral mucosa as a control. Mucosal biopsies from patients with leukoplakia (n=13), OLP (n=10), and normal oral mucosa (n=9) were frozen, sectioned and immunostained with monoclonal antibodies to wild-type (wt) tumor suppressive protein p53, cyclin-dependent kinase inhibitor p21WAF1/CIP1 and the oncoproteins MDM2, and Bcl-2. Apoptosis was assessed in all cases by the TUNEL method. MDM2 and Bcl-2 expression in keratinocytes were quantitatively greater in leukoplakia than in OLP. Wt-p53 and p21WAF1/CIP1 expression was quantitatively greater in keratinocytes in OLP than in leukoplakia. Keratinocyte maturation appeared histologically normal in OLP, even though wt-p53 and p21WAF1/CIP1 were expressed in these cells. Altered keratinocyte maturation was seen in leukoplakia lesions expressing MDM2 and Bcl-2. No significant difference for the number of apoptotic epithelial cells was observed between leukoplakia and OLP, in spite of the divergent outcomes of the apoptotic signaling proteins.

Mitochondria in Ca2+ signaling and apoptosis.

Cellular Ca2+ signals are crucial in the control of most physiological processes, cell injury and programmed cell death; mitochondria play a pivotal role in the regulation of such cytosolic Ca2+ ([Ca2+]c) signals. Mitochondria are endowed with multiple Ca2+ transport mechanisms by which they take up and release Ca2+ across their inner membrane. These transport processes function to regulate local and global [Ca2+]c, thereby regulating a number of Ca2+-sensitive cellular mechanisms. The permeability transition pore (PTP) forms the major Ca2+ efflux pathway from mitochondria. In addition, Ca2+ efflux from the mitochondrial matrix occurs by the reversal of the uniporter and through the inner membrane Na+/Ca2+ exchanger. During cellular Ca2+ overload, mitochondria take up [Ca2+]c, which, in turn, induces opening of PTP, disruption of mitochondrial membrane potential (delta(psi)m) and cell death. In apoptosis signaling, collapse of delta(psi)m and cytochrome c release from mitochondria occur followed by activation of caspases, DNA fragmentation, and cell death. Translocation of Bax, an apoptotic signaling protein from the cytosol to the mitochondrial membrane, is another step during this apoptosis-signaling pathway. The role of permeability transition in the context of cell death in relation to Bcl-2 family of proteins is discussed.

The modular nature of apoptotic signaling proteins.

Apoptosis, initiated by a variety of stimuli, is a physiological process that engages a well-ordered signaling cascade, eventually leading to the controlled death of the cell. The most extensively studied apoptotic stimulus is the binding of death receptors related to CD95 (Fas/Apo1) by their respective ligands. During the last years, a considerable number of proteins have been identified which act together in the receptor-proximal part of the signaling pathway. Based on localized regions of sequence similarity, it has been predicted that these proteins consist of several independently folding domains. In several cases these predictions have been confirmed by structural studies; in other cases they are at least supported by experimental data. This review focuses on the three most widespread domain families found in the apoptotic signaling proteins: the death domain, the death effector domain and the caspase recruitment domain. The recently discovered analogies between these domains, both in structure and in function, have shed some light on the overall architecture of the pathway leading from death receptor ligation to the activation of caspases and eventually to the apoptotic phenotype.

Tumor Necrosis Factor Induces Ceramide Oscillations and Negatively Controls Sphingolipid Synthases by Caspases in Apoptotic Kym-1 Cells

The role, origin, and mode of action of the lipid messenger ceramide in programmed cell death and its linkage to receptor-associated apoptotic signal proteins is still unresolved. We show here in Kym-1 rhabdomyosarcoma cells that tumor necrosis factor (TNF)-induced apoptosis is preceded by a multiphasic increase in intracellular ceramide levels. Distinct enzymes were found to contribute to three waves of ceramide, neutral sphingomyelinase, ceramide synthase, and acid sphingomyelinase, with peak activities at 1-2, 40, and around 200 min, respectively, the latter coinciding with progression to irreversible damage. In parallel with ceramide generation, TNF-mediated inhibition of glucosylceramide and sphingomyelin (SM) synthase prevents the immediate metabolization of this lipid mediator. In the presence of benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z-VAD-fmk) or benzyloxycarbonyl-Asp-Glu-Val-Asp-chloromethyl ketone (Z-DEVD-cmk), a broad spectrum and a caspase 3-selective inhibitor, respectively, glucosylceramide and SM synthase activity remains unaffected by TNF, and intracellular ceramide accumulation is not observed. Our results show that several lipid enzymes contribute to generation of ceramide in response to TNF and identify glucosylceramide and SM synthase as important regulators of the kinetics and magnitude of intracellular ceramide accumulation. As glucosylceramide and SM synthase activity is caspase-sensitive, our data suggest a novel functional link between caspase(s) and ceramide during apoptotic processes.