David W. C. Hunt

Rapid cytochrome c release, activation of caspases 3, 6, 7 and 8 followed by Bap31 cleavage in HeLa cells treated with photodynamic therapy

Photodynamic therapy (PDT) is a clinical approach that utilizes light‐activated drugs for the treatment of a variety of pathologic conditions. The initiating events of PDT‐induced apoptosis are poorly defined. It has been shown for other pro‐apoptotic stimuli that the integral endoplasmic reticulum protein Bap31 is cleaved by caspases 1 and 8, but not by caspase‐3. Further, a 20 kDa Bap31 cleavage fragment is generated which can induce apoptosis. In the current report, we sought to determine whether Bap31 cleavage and generation of p20 is an early event in PDT‐induced apoptosis. The mitochondrial release of cytochrome c, involvement of caspases 1, 2, 3, 4, 6, 7, 8, and 10 and the status of several known caspase substrates, including Bap31, were evaluated in PDT‐treated HeLa cells. Cytochrome c appeared in the cytosol immediately following light activation of the photosensitizer benzoporphyrin derivative monoacid ring A. Activation of caspases 3, 6, 7, and 8 was evident within 1–2 h post PDT. Processing of caspases 1, 2, 4, and 10 was not observed. Cleavage of Bap31 was observed at 2–3 h post PDT. The caspase‐3 inhibitor DEVD‐fmk blocked caspase‐8 and Bap31 cleavage suggesting that caspase‐8 and Bap31 processing occur downstream of caspase‐3 activation in PDT‐induced apoptosis. These results demonstrate that release of mitochondrial cytochrome c into the cytoplasm is a primary event following PDT, preceding caspase activation and cleavage of Bap31. To our knowledge, this is the first example of a chemotherapeutic agent inducing caspase‐8 activation and demonstrates that caspase‐8 activation can occur after cytochrome c release.

Mechanism of Colon Cancer Cell Apoptosis Mediated by Pyropheophorbide-a Methylester Photosensitization

Pyropheophorbide-a methylester (PPME) is a second generation of photosensitizers used in photodynamic therapy (PDT). We demonstrated that PPME photosensitization triggered apoptosis of colon cancer cells as measured by using several classical parameters such as DNA laddering, PARP cleavage, caspase activation and mitochondrial release of cytochrome c. Preincubation of cells with N-acetyl cysteine (NAC) or pyrolidine dithiocarbamate (PDTC) protected against apoptosis mediated by PPME photosensitization showing that reactive oxygen species (ROS) are involved as second messengers. On the other hand, photosensitization carried out in the presence of deuterium oxide (D2O) which enhances singlet oxygen (1O2) lifetime only increases necrosis without affecting apoptosis. Since PPME was localized in the endoplasmic reticulum (ER)/Golgi system and lysosomes, other messengers than ROS were tested such as calcium, Bid, Bap31, phosphorylated Bcl-2 and caspase-12 but none was clearly identified as being involved in triggering cytochrome c release from mitochondria. On the other hand, we demonstrated that the transduction pathways leading to NF-κB activation and apoptosis were clearly independent although NF-κB was shown to counteract apoptosis mediated by PPME photosensitization.

Release of Cytochrome c, Bax Migration, Bid Cleavage, and Activation of Caspases 2, 3, 6, 7, 8, and 9 during Endothelial Cell Apoptosis

Although the executioner phase of apoptosis has been well defined in many cell types, the subcellular events leading to apoptosis in endothelial cells remain undefined. In the current study, apoptosis was induced in primary human umbilical venous endothelial cells by the photosensitizer verteporfin and light. Release of mitochondrial cytochrome c into the cytosol was detectable immediately and accumulated over 2 hours after treatment while cytosolic levels of the proapoptotic Bcl-2 family member, Bax, decreased reciprocally over the same time period. Cleavage of another proapoptotic Bcl-2 family member, Bid, was observed by 2 hours after treatment. Although Bid cleavage has been shown to occur as an upstream event responsible for inducing cytochrome c release, we demonstrate that Bid cleavage can also occur after cytochrome c release. Activation of caspases 2, 3, 6, 7, 8, and 9 occurred following the release of cytochrome c, and cleavage of downstream substrates was observed. In summary, endothelial cell death involves the cellular redistribution of Bax and cytochrome c, followed by the activation of multiple caspases which manifest the apoptotic phenotype.

Bcl-2 and Bcl-xL overexpression inhibits cytochrome c release, activation of multiple caspases, and virus release following coxsackievirus B3 infection

Coxsackievirus B3, a cytopathic virus in the family Picornaviridae, induces degenerative changes in host cell morphology. Here we demonstrate cytochrome c release and caspases-2, -3, -6, -7, -8, and -9 processing. Enforced Bcl-2 and Bcl-xL expression markedly reduced release of cytochrome c, presentation of the mitochondrial epitope 7A6, and depressed caspase activation following infection. In comparison, cell death using TRAIL ligand caused caspase-8 processing prior to cytochrome c release and executioner caspases and cell death was only partially rescued by Bcl-2 and Bcl-xL overexpression. Disruption of the mitochondrial inner membrane potential following CVB3 infection was not inhibited by zVAD.fmk treatment. Bcl-2 or Bcl-xL overexpression or zVAD.fmk treatment delayed the loss of host cell viability and decreased progeny virus release following infection. Our data suggest that mitochondrial release of cytochrome c may be an important early event in caspase activation in CVB3 infection, and, as such, may contribute to the loss of host-cell viability and progeny virus release.

Photodynamic therapy induces caspase-3 activation in HL-60 cells.

Caspases have been shown to play a crucial role in apoptosis induced by various deleterious and physiologic stimuli. In this study, we show for the first time that photodynamic therapy (PDT), using benzoporphyrin derivative monoacid ring A (BPD-MA, verteporfin) as the photosensitizer, induces the complete cleavage and subsequent activation of caspase-3 (CPP32/Yama/Apopain) but not caspase-1 (ICE) in human promyelocytic leukemia HL-60 cells. Poly(ADP-ribose) polymerase (PARP) and the catalytic subunit of DNA dependent protein kinase (DNA PKCS) were cleaved within 60 min of light activation of BPD-MA. The general caspase inhibitor Z-Asp-2,6 dichlorobenzoyloxymethylketone (Z-Asp-DCB) blocked PARP cleavage while the serine protease inhibitors 3,4-dichloroisocoumarin (DCI) and N-tosyl-lysyl chloromethyl ketone (TLCK) blocked the cleavage of caspase-3 suggesting that they act upstream of caspase-3 activation. All three inhibitors were able to block DNA fragmentation that was induced by treatment with BPD-MA followed by light application. These studies demonstrate that protease activity, particularly that of caspase-3, is triggered in HL-60 cells treated with lethal levels of BPD-MA and visible light.

Mitochondrial Release of Apoptosis-Inducing Factor and Cytochrome c During Smooth Muscle Cell Apoptosis

Photodynamic therapy (PDT) is under investigation for the treatment of intimal hyperplastia in conditions such as atherosclerosis and restenosis. Although smooth muscle cells (SMCs) may be a key target for treatment, the effects of PDT on these cells are poorly characterized. In the present study, apoptosis was induced in primary human aortic SMCs by the combination of the photosensitizer verteporfin and visible light. After PDT, an increase in mitochondrial cytochrome c (cyt c) and apoptosis-inducing factor (AIF) levels were detected in the cytosol immediately and their levels increased steadily up to 2 hours. Cytosolic levels of the pro-apoptotic Bcl-2 family member Bax decreased reciprocally throughout this period, but this change did not occur before cyt c release. Confocal microscopy revealed a diffuse staining pattern of cyt c within apoptotic cells as compared to a distinct mitochondrial staining in normal cells. AIF translocated from mitochondria to the nucleus during the progression of apoptosis. After cyt c release, caspase-9 and caspase-3 processing was visible by 1 hour and caspase-6, -7, and -8 processing was apparent by 2 hours after PDT. In summary, these results demonstrate for the first time the cellular redistribution of mitochondrial AIF during SMC apoptosis, as well as the early release of cyt c and the subsequent activation of multiple caspases during PDT-induced SMC apoptosis.

Neonatal dendritic cells

The capacity of lymphoid dendritic cells from human cord blood or adult peripheral blood to support a mixed leukocyte reaction in cord blood and adult T cells has been compared. Cord blood dendritic cells have a limited ability to induce either adult or cord blood T cells to proliferate in response to typical concentration of phytohemagglutinin or concanavalin A. Adult blood dendritic cells, on the other hand, induce equivalent mitogen responses in cord blood and adult blood T cells. This relative deficiency can be overcome by increasing the concentration of mitogen or the numbers of dendritic cells in the culture. Neonatal primary immune responses may, in part, reflect the reduced function of dendritic cells.

Overexpression of Bcl‐XL prevents caspase‐3‐mediated activation of DNA fragmentation factor (DFF) produced by treatment with the photochemotherapeutic agent BPD‐MA

Photodynamic therapy (PDT) is a clinically effective cancer treatment. For human promyelocytic leukemia HL‐60 cells, cleavage of pro‐caspase‐3 (CPP32/Yama/apopain) into its proteolytically active subunits rapidly follows the photodynamic treatment of these cells with cytotoxic levels of the photosensitizer benzoporphyrin derivative monoacid ring A and visible light. Cleavage of a recently identified cytosolic 45 kDa protein, DNA fragmentation factor (DFF), is required for endonuclease activation leading to DNA fragmentation. In the present study, DFF was rapidly processed following PDT. Overexpression of the anti‐apoptotic Bcl‐XL gene in HL‐60 cells prevented PDT‐induced caspase activation, DFF cleavage and DNA fragmentation. These results demonstrate for the first time an example of chemotherapeutic drug‐induced activation of DFF and its regulation by Bcl‐XL.

Consequences of the photodynamic treatment of resting and activated peripheral T lymphocytes

The impact of the immunomodulatory photosensitizer benzoporphyrin derivative monoacid ring A (BPD-MA, verteporfin) and visible light on the survival and surface receptor pattern of resting and activated murine T cells was evaluated. T cells treated for 48 h with immobilized anti-CD3 monoclonal antibody upregulated expression of the interleukin-2 receptor alpha-chain (CD25), transferrin receptor (CD71), the apoptosis-regulating Fas receptor (CD95), contained a greater level of the anti-apoptotic protein Bcl-2 and accumulated significantly more BPD-MA than their unactivated counterparts. Activated T cells displayed a modestly greater susceptibility to the photodynamic induction of DNA fragmentation than resting T cells. Resting T cells treated with sub-lethal levels of BPD-MA and light did not exhibit changes in surface levels of CD3, CD4, CD8, CD28, CD45 or T cell receptor (TCR) beta-chain structures. However, levels of major histocompatibility complex (MHC) class I antigens were decreased while the density of Thy-1.2 (CD90) increased on these cells. Photodynamically treated T cells failed to express optimal CD25 levels when exposed to the mitogenic anti-CD3 antibody. Activated T cells treated with sub-lethal levels of BPD-MA and light exhibited lower CD25 levels, a temporary block in cell cycle transition, but unaltered expression of MHC Class I, CD3, CD4, CD8, CD45, CD54, CD71, CD122 (IL-2R beta-chain) or TCR beta-chain antigens 24 h afterward. Resting and activated T lymphocytes differ in susceptibility to PDT-mediated apoptosis but both types are sensitive to anti-proliferative effects the treatment exerts at sub-lethal photosensitizer levels. The marked sensitivity of activated T cells to photodynamic inactivation likely contributes to the immunomodulatory action of BPD-MA.

Immunomodulatory aspects of photodynamic therapy.

In its conventional form, photodynamic therapy (PDT) is a clinically effective technique with which to treat tumours accessible to visible light. PDT utilises light absorbing compounds which catalyse the generation of toxic oxygen species, to produce localised antitumour effects. It has become apparent over the past decade that PDT also exhibits immunomodulatory attributes. Experimental animals may possess heightened antitumour immunity after tumour ablation with PDT. In contrast, at sub-phototoxic levels of photosensitiser, in combination with whole body light irradiation, PDT lessened disease severity when applied in different models of autoimmunity. Although the behaviour of lymphocytes may be affected by treatment, the ability of PDT to down-regulate autoimmune processes appears to be related to its capacity to influence the immunostimulatory attributes of antigen presenting cells.