Radio-sensitive murine thymoma cell line 3SB: characterization of its apoptosis-resistant variants induced by repeated X-irradiation

Regulation of radiation-induced protein kinase Cdelta activation in radiation-induced apoptosis differs between radiosensitive and radioresistant mouse thymic lymphoma cell lines

Protein kinase Cdelta (PKCdelta) has an important role in radiation-induced apoptosis. The expression and function of PKCdelta in radiation-induced apoptosis were assessed in a radiation-sensitive mouse thymic lymphoma cell line, 3SBH5, and its radioresistant variant, XR223. Rottlerin, a PKCdelta-specific inhibitor, completely abolished radiation-induced apoptosis in 3SBH5. Radiation-induced PKCdelta activation correlated with the degradation of PKCdelta, indicating that PKCdelta activation through degradation is involved in radiation-induced apoptosis in radiosensitive 3SBH5. In radioresistant XR223, radiation-induced PKCdelta activation was lower than that in radiosensitive 3SBH5. Cytosol PKCdelta levels in 3SBH5 decreased markedly after irradiation, while those in XR223 did not. There was no apparent change after irradiation in the membrane fractions of either cell type. In addition, basal cytosol PKCdelta levels in XR223 were higher than those in 3SBH5. These results suggest that the radioresistance in XR223 to radiation-induced apoptosis is due to a difference in the regulation of radiation-induced PKCdelta activation compared to that of 3SBH5. On the other hand, Atm(-/-) mouse thymic lymphoma cells were more radioresistant to radiation-induced apoptosis than wild-type mouse thymic lymphoma cells. Irradiated wild-type cells, but not Atm(-/-) cells, had decreased PKCdelta levels, indicating that the Atm protein is involved in radiation-induced apoptosis through the induction of PKCdelta degradation. The decreased Atm protein levels induced by treatment with Atm small interfering RNA had no effect on radiation-induced apoptosis in 3SBH5 cells. These results suggest that the regulation of radiation-induced PKCdelta activation, which is distinct from the Atm-mediated cascade, determines radiation sensitivity in radiosensitive 3SBH5 cells.

Involvement of Protein Kinase C-Related Anti-apoptosis Signaling in Radiation-Induced Apoptosis in Murine Thymic Lymphoma(3SBH5) Cells

Protein kinase C (PKC; also known as PRKC) is known to be an important participant in radiation-induced apoptosis. However, its role is not fully clarified. Using 3SBH5 cells, which are radiation-sensitive thymic lymphoma cells, the involvement and functions of PKC were assessed in radiation- induced apoptosis. PMA (phorbol 12-myristate 13-acetate), a PKC activator, inhibited the radiation-induced apoptosis in 3SBH5 cells. On the other hand, chelerythrine, a PKC inhibitor, potentiated apoptosis. In addition, Gö6976, a classical PKC (cPKC) inhibitor, which specifically inhibits PKC (alpha and betaI), also promoted apoptosis. Interestingly, post-treatment (20 min after irradiation) with Gö6976 had no effect on the radiation-induced apoptosis. These results suggest that cPKC is activated early after irradiation for anti-apoptosis signaling and contributes to the balance between cell survival and death. Indeed, an increase of cPKC activity involving PKC (alpha, betaI and betaII) was observed in the cytosolic fraction 3 min after irradiation with 0.5 Gy. However, no translocation of cPKC was observed in the cells after irradiation. Our findings indicate that activation of cPKC (alpha or beta) soon after irradiation is critical to the understanding of the regulation of radiation-induced apoptosis in radiation-sensitive cells.

Phosphatidylserine induces apoptosis in CHO cells without mitochondrial dysfunction in a manner dependent on caspases other than caspases-1, -3, -8 and -9

Treatment of Chinese hamster ovary K1 cells with phosphatidylserine (PS) caused typical apoptosis with distinct morphological and biochemical features in a dose- and time-dependent manner. However, unlike camptothecin-induced apoptosis, changes in mitochondrial transmembrane potential were not observed. In addition, cytochrome c release did not occur in PS-induced apoptosis. A pan caspase inhibitor, Z-VAD, significantly inhibited the apoptosis, but inhibitors of caspase-1, -3, -8 and -9 did not. Activities of caspase-1, -3, -8 and -9 were increased by treatment of the cells with camptothecin, but not with PS. These results suggest that PS-induced apoptosis occurs without the collapse of mitochondrial transmembrane potential and without the release of cytochrome c, in a manner independent of caspase-1, -3, -8 and -9.

Fas-independent apoptosis induced by UVC in p53-mutated human epithelial tumor A431 cells through activation of caspase-8 and JNK/SAPK

A431 cells/UVC-induced apoptosis/Caspase 8/Fas/JNK/PAPK. We previously observed that p53-mutated human epithelial tumor A431 cells underwent apoptosis after ultraviolet C (UVC) irradiation through the caspases-8 and -3 pathway. Fas/FasL is known to initiate apoptosis in several cell lines via caspase-8 activation. Then, to determine if Fas/FasL mediates apoptosis in A431. we investigated Fas expression and modulation in UVC-irradiated A431 cells. A431 constitutively expressed Fas, which gradually decreased after UVC-irradiation. Pretreatment with a neutralizing anti-Fas antibody, ZB4, did not abrogate the UVC-induced apoptosis. An agonistic anti-Fas antibody, CH11, very slowly induced apoptosis in A431. suggesting that the constitutively expressed Fas had a low functional potential. Hence, UVC-induced apoptosis in A431 seems to occur independent of the Fas signal. Interestingly, however, a pretreatment with CH11 remarkably potentiated UVC-induced apoptosis. An inhibitor of caspase-8, Ac-IETD-CHO, partially inhibited UVC-induced apoptosis. JNK was phosphorylated immediately after exposure to UVC. prior to apoptotic chromatin condensation. Our data suggest that the activation of caspase-8 occurs independent of Fas upregulation, and that JNK/ SAPK contributes to UVC-induced apoptosis in human epithelial A431 cells.

Targeting ceramide metabolism--a strategy for overcoming drug resistance

Inherent or acquired drug resistance, which frequently characterizes cancer cells, is caused by multiple mechanisms, including dysfunctional metabolism of the lipid second messenger ceramide. Ceramide, the basic structural unit of the sphingolipids, plays a role in activating cell death signals initiated by cytokines, chemotherapeutic agents, and ionizing radiation. Recent discoveries about the metabolism of ceramide suggest that this agent may have an important influence on the effectiveness of various cancer therapeutics. In particular, the cytotoxic effect of chemotherapy is decreased when generation of ceramide is impaired but is increased when the degradation of ceramide is blocked. Herein, we review the mechanisms of resistance to chemotherapeutic agents in terms of ceramide metabolism.

UVC-induced apoptosis in human epithelial tumor A431 cells: sequence of apoptotic changes and involvement of caspase (-8 and -3) cascade

Human epidermoid tumor A431 cells underwent apoptosis following exposure to ultraviolet C (UVC). The apoptosis was of the interphase death type, and mostly occurred within one cell cycle, independent of the cell-cycle phases. We further examined the detailed sequential order of apoptotic changes in cells after UVC exposure and the involvement of caspases using six caspase inhibitors. The loss of mitochondrial transmembrane potential (delta psi m) appeared in the earliest phase; subsequently, the chromatin condensation and DNA-fragmentation occurred. Cell shrinkage and loss of the plasma-membrane integrity, judged by propidium iodide (PI) staining, were observed in the later phase. A broad-spectrum caspase inhibitor, z-VAD-fmk, completely prevented all apoptotic changes, except for the depletion of delta psi m. Both Ac-DEVD-CHO and Ac-IETD-CHO, inhibitors of caspase -3 and -8, respectively, effectively inhibited typical chromatin condensation to almost the same extent. However, the nuclei still showed partial condensation. A caspase -9 inhibitor, Ac-LEHD-CHO, did not prevent chromatin condensation, though it partially inhibited cell-size reduction and PI-stainability. None of the caspase inhibitors could inhibit the delta psi m reduction. These results strongly suggest that the collapse of delta psi m is not a part of the central apoptotic machinery, and that caspase cascade(s), especially caspase-8 to -3, play an important role in UVC-induced apoptosis in A431.