Nonesterified cholesterol content of lysosomes modulates susceptibility to oxidant-induced permeabilization

Reactive oxygen species (ROS) can induce lysosomal membrane permeabilization (LMP). Photoirradiation of murine hepatoma 1c1c7 cultures preloaded with the photosensitizer NPe6 generates singlet oxygen within acidic organelles and causes LMP and the activation of procaspases. Treatment with the cationic amphiphilic drugs (CADs) U18666A, imipramine, and clozapine stimulated the accumulation of filipin-stainable nonesterified cholesterol/sterols in late endosomes/lysosomes, but not in mitochondria. Concentration-response studies demonstrated an inverse relationship between lysosomal nonesterified cholesterol/sterol contents and susceptibility to NPe6 photoirradiation-induced intracellular membrane oxidation, LMP, and activation of procaspase-9 and -3. Similarly, the kinetics of restoration of NPe6 photoirradiation-induced LMP paralleled the losses of lysosomal cholesterol that occurred upon replating U18666A-treated cultures in CAD-free medium. Consistent with the oxidation of lysosomal cholesterol, filipin staining in U18666A-treated cultures progressively decreased with increasing photoirradiating light dose. U18666A also suppressed the induction of LMP and procaspase activation by exogenously added hydrogen peroxide. However, neither U18666A nor imipramine suppressed the induction of apoptosis by agents that did not directly induce LMP. These studies indicate that lysosomal nonesterified cholesterol/sterol content modulates susceptibility to ROS-induced LMP and possibly does so by being an alternative target for oxidants and lowering the probability of damage to other lysosomal membrane lipids and/or proteins.

Effects of photodynamic therapy on the endocytic pathway

In this report, we describe an effect of photodynamic therapy (PDT) on membrane trafficking in murine 1c1c7 hepatoma cells. A brief exposure of 1c1c7 cells to a 20 nM concentration of the phosphatidylinositol kinase class-3 antagonist wortmannin led to the rapid appearance of cytoplasmic vacuoles. Fluorescence monitoring of plasma membrane-associated 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TDPH) over time demonstrated that the wortmannin-induced vacuoles were derived from endocytosed plasma membrane. Low-dose photodamage catalyzed by the lysosomal photosensitizer NPe6, prior to the addition of wortmannin, prevented formation of these vacuoles. NPe6 was found to suppress for several hours the normal trafficking of TDPH-labeled plasma membrane to the cytosol, and the formation of punctate TDPH-labeled cytoplasmic vesicles. The ability of NPe6-induced photodamage to suppress wortmannin-induced vacuolization occurred under conditions that did not disrupt lysosomes and were at or below the threshold of cytostatic/cytotoxic effects. Furthermore, the suppressive effects of NPe6-PDT were not prevented by inclusion of an agent that stabilized lysosomal membranes, or by E64d, an inhibitor of lysosomal cathepsin proteases. Mitochondrial photodamage was less effective at preventing wortmannin-induced vacuole formation and PDT directed against the ER had no effect. The role of photodamage to the endocytic pathway may be a hitherto unexplored effect on cells that selectively accumulate photosensitizing agents. These results indicate that photodamage directed against endosomes/lysosomes has effects independent of the release of lysosomal proteases.

On the use of fluorescence probes for detecting reactive oxygen and nitrogen species associated with photodynamic therapy

Fluorescent probes are frequently employed for the detection of different reactive oxygen and nitrogen species formed during the irradiation of photosensitized cells and tissues. Investigators often interpret the results in terms of information provided with the different probes without examining specificity or determinants of fluorogenic reactions. We examine five fluorescent probes in a cell-free system: reduced 2',7'-dichlorofluorescein, dihydroethidine, dihydrorhodamine, 3'-(p aminophenyl) fluorescein (APF), and 4',5'-diaminofluorescein. Of these, only APF demonstrates a high degree of specificity for a single reactive species. There is a substantial influence of peroxidase activity on all fluorogenic interactions. The fluorescence of the photosensitizing agent also must be taken into account in evaluating results.

A role for hydrogen peroxide in the pro-apoptotic effects of photodynamic therapy

Although the first reactive oxygen species (ROS) formed during irradiation of photosensitized cells is almost invariably singlet molecular oxygen (1O2), other ROS have been implicated in the phototoxic effects of photodynamic therapy (PDT). Among these are superoxide anion radical (*O2(-)), hydrogen peroxide (H2O2) and hydroxyl radical (*OH). In this study, we investigated the role of H2O2 in the pro-apoptotic response to PDT in murine leukemia P388 cells. A primary route for detoxification of cellular H2O2 involves the peroxisomal enzyme catalase. Inhibition of catalase activity by 3-amino-1,2,4-triazole led to an increased apoptotic response. PDT-induced apoptosis was impaired by addition of an exogenous recombinant catalase analog (CAT-skl) that was specifically designed to enter cells and more efficiently localize in peroxisomes. A similar effect was observed upon addition of 2,2'-bipyridine, a reagent that can chelate Fe+2, a co-factor in the Fenton reaction that results in the conversion of H2O2 to *OH. These results provide evidence that formation of H2O2 during irradiation of photosensitized cells contributes to PDT efficacy.

Assessing autophagy in the context of photodynamic therapy

Photodynamic therapy (PDT) is a procedure that has applications in the selective eradication of neoplasia where sites of malignant lesions are clearly delineated. It is a two-step process whereby cells are first sensitized to light and then photoirradiated. This results in the formation of singlet molecular oxygen and other reactive oxygen species that can cause photodamage at sites where the photosensitizing agent has localized. Photosensitizers found to be clinically useful show affinity for the endoplasmic reticulum (ER), mitochondria, lysosomes, or combinations of these sites. The induction of apoptosis and/or autophagy in photosensitized cells is a common outcome of PDT. This report explores the following issues: (1) Does the induction of autophagy in PDT protocols occur independent of, or in association with, apoptosis? (2) Does the resulting autophagy play a prosurvival or prodeath role? (3) Do photosensitizers damage/inactivate specific proteins that are components of, or that modulate the autophagic process? (4) Can an autophagic response be mounted in cells in which lysosomes are specifically photodamaged? In brief, autophagy can occur independently of apoptosis in PDT protocols, and appears to play a prosurvival role in apoptosis competent cells, and a prodeath role in apoptosis incompetent cells. Mitochondrial and ER-localized sensitizers cause selective photodamage to some (i.e., Bcl-2, Bcl-x(L), mTOR) proteins involved in the apoptotic/autophagic process. Finally, an aborted autophagic response occurs in cells with photodamaged lysosomes. Whereas autophagosomes form, digestion of their cargo is compromised because of the absence of functional lysosomes.

Preoperative imaging of left portal vein at the Rex recess for Rex shunt formation using wedged hepatic vein carbon dioxide portography

BACKGROUND

In children with extrahepatic portal vein obstruction (EHPVO), formation of a mesentericoportal bypass (Rex shunt) restores hepatopetal flow, relieves portal hypertension, and reduces variceal bleeding and hypersplenism. The Rex shunt is created by inserting a vein graft between the superior mesenteric vein and the umbilical segment (Rex) of the left portal vein within the Rex recess of the liver. The preoperative evaluation of a patient with EHPVO includes an accurate assessment of the venous inflow and outflow. The inflow portal vein is readily assessed by ultrasound and magnetic resonance imaging. The outflow intrahepatic portal vein is harder to assess. We report our experience of patients evaluated with wedged hepatic vein carbon dioxide portography (WHVCP).

METHOD

All children referred for venography from October 2001 to October 2007 were prospectively identified, and clinical and radiologic data were reviewed retrospectively. The imaging findings were correlated to findings at surgery.

RESULTS

Eleven children (range, 3-14 years, median, 6 years) were referred for preoperative wedged hepatic venography. The left portal vein at the Rex recess was clearly identified in 9 patients (82%). In the other 2 patients (18%), the Rex segment was not identified despite opacification of left and right intrahepatic portal veins; this was taken to indicate an occluded segment. Wedged venography was performed with carbon dioxide in 10 patients (91%). Carbon dioxide was contraindicated in the final patient because of the presence of a ventricular septal defect.

CONCLUSION

Our series demonstrates the use of WHVCP as a diagnostic tool in preoperative assessment of the Rex segment of left portal vein in children with extrahepatic portal vein obstruction.

Monitoring singlet oxygen and hydroxyl radical formation with fluorescent probes during photodynamic therapy

Singlet oxygen (1O2) is the primary oxidant generated in photodynamic therapy (PDT) protocols involving sensitizers resulting in type II reactions. 1O2 can give rise to additional reactive oxygen species (ROS) such as the hydroxyl radical (*OH). The current study was designed to assess 3'-p-(aminophenyl) fluorescein (APF) and 3'-p-(hydroxyphenyl) fluorescein (HPF) as probes for the detection of 1O2 and *OH under conditions relevant to PDT. Cell-free studies indicated that both APF and HPF were converted to fluorescent products following exposure to 1O2 generated by irradiation of a water-soluble photosensitizing agent (TPPS) and that APF was 35-fold more sensitive than HPF. Using the 1O2 probe singlet oxygen sensor green (SOSG) we confirmed that 1 mm NaN3 quenched 1O2-induced APF/HPF fluorescence, while 1% DMSO had no effect. APF and HPF also yielded a fluorescent product upon interacting with *OH generated from H2O2 via the Fenton reaction in a cell-free system. DMSO quenched the fluorogenic interaction between APF/HPF and *OH at doses as low as 0.02%. Although NaN3 was expected to quench *OH-induced APF/HPF fluorescence, co-incubating NaN3 with APF or HPF in the presence of *OH markedly enhanced fluorescence. Cultured L1210 cells that had been photosensitized with benzoporphyhrin derivative exhibited APF fluorescence immediately following irradiation. Approximately 50% of the cellular fluorescence could be suppressed by inclusion of either DMSO or the iron-chelator desferroxamine. Combining the latter two agents did not enhance suppression. We conclude that APF can be used to monitor the formation of both 1O2 and *OH in cells subjected to PDT if studies are performed in the presence and absence of DMSO, respectively. That portion of the fluorescence quenched by DMSO will represent the contribution of *OH. This procedure could represent a useful means for evaluating formation of both ROS in the context of PDT.

Initiation of autophagy by photodynamic therapy

Photodynamic therapy (PDT) involves the irradiation of photosensitized cells with light. Depending on localization of the photosensitizing agent, the process can induce photodamage to the endoplasmic reticulum (ER), mitochondria, plasma membrane, and/or lysosomes. When ER or mitochondria are targeted, antiapoptotic proteins of the Bcl-2 family are especially sensitive to photodamage. Both apoptosis and autophagy can occur after PDT, autophagy being associated with enhanced survival at low levels of photodamage to some cells. Autophagy can become a cell-death pathway if apoptosis is inhibited or when cells attempt to recycle damaged constituents beyond their capacity for recovery. While techniques associated with characterization of autophagy are generally applicable, PDT introduces additional factors related to unknown sites of photodamage that may alter autophagic pathways. This chapter discusses issues that may arise in assessing autophagy after cellular photodamage.

A physics based method for combining multiple anatomy models with application to medical simulation

We present a physics based approach to the construction of anatomy models by combining components from different sources; different image modalities, protocols, and patients. Given an initial anatomy, a mass-spring model is generated which mimics the physical properties of the solid anatomy components. This helps maintain valid spatial relationships between the components, as well as the validity of their shapes. Combination can be either replacing/modifying an existing component, or inserting a new component. The external forces that deform the model components to fit the new shape are estimated from Gradient Vector Flow and Distance Transform maps. We demonstrate the applicability and validity of the described approach in the area of medical simulation, by showing the processes of non-rigid surface alignment, component replacement, and component insertion.

Adventures in photodynamic therapy: 1976-2008

While the concept of photodynamic therapy dates from 1900, and there have been periodic re-discoveries, the clinical era really began with the studies by Dougherty and associates in the early 1970s. This report relates my encounter with the field of PDT, along with experimental approaches to the elucidation of pertinent phototoxic mechanisms.

The Bcl-2 antagonist HA14-1 forms a fluorescent albumin complex that can be mistaken for several oxidized ROS probes

The proapoptotic effects of the Bcl-2 antagonist HA14-1 are believed to derive from its affinity for the hydrophobic groove on Bcl-2 and Bcl-x(L), thereby displacing proapoptotic factors, e.g. Bax and Bak. We have reported that HA14-1 promotes the efficacy of low-dose photodynamic therapy (PDT). A recent report proposed that the proapoptotic activity of HA14-1 reflects its ability to generate reactive oxygen species (ROS) when incubated in an aqueous environment. This later study, like several other HA14-1 investigations, relied on the use of fluorescent probes for ROS detection. We found that HA14-1 reacts with the albumin in serum to yield a fluorescent product. After correcting for this effect, the putative formation of ROS by HA14-1 could not be demonstrated with the fluorescent probes H(2)DCFDA, dihydroethidium or dihydrorhodamine. Indeed, the fluorescence excitation/emission spectra of HA14-1 encompassed the excitation/emission wavelengths used to detect these ROS probes. Cells cultured in a medium supplemented with ovalbumin, instead of serum, underwent apoptosis following HA14-1 addition, but did not exhibit fluorescence. Hence, HA14-1 fluorescence was unrelated to its proapoptotic activity. We conclude that the enhancement of PDT by HA14-1 reflects a pharmacologic effect, rather than its direct contribution of ROS.

Promotion of PDT efficacy by a Bcl-2 antagonist

Photodynamic therapy (PDT) directed against the endoplasmic reticulum (ER) is also known to target antiapoptotic Bcl-2 family proteins. This effect is associated with the initiation of both apoptosis, a cell death pathway, and autophagy, an organelle recycling system that can lead to survival or cell death. In this study, we examined the ability of the Bcl-2 antagonist HA14-1 to promote the photodynamic efficacy of PDT directed at the ER. At concentrations that independently caused only a small loss of viability, HA14-1 markedly enhanced the proapoptotic and phototoxic effects of ER photodamage. These results provide additional evidence that the antiapoptotic properties of Bcl-2 constitute an important determinant of photokilling, and demonstrate that synergistic effects can result when PDT is coupled with pharmacologic suppression of Bcl-2 function.

Apoptosis and autophagy after mitochondrial or endoplasmic reticulum photodamage

Photodynamic therapy (PDT) can cause lethal photodamage by both direct and indirect mechanisms. Direct modes of cell death relate to nonspecific necrosis and the initiation of signaling pathways that elicit apoptosis, autophagy or both. In this report, effects of low-dose and high-dose PDT are explored, comparing sensitizers that localize in the endoplasmic reticulum (the porphycene termed CPO) or mitochondria (mesochlorin). To explore the role of autophagy, two cell lines were examined--the murine L1210 leukemia and an Atg7 knockdown derivative of L1210. The Atg7 gene is central to the process of autophagy. High-dose PDT with either sensitizer resulted in a substantial loss of the Bcl-2 protein. As Bcl-2 regulates both apoptosis and autophagy, loss of this protein can lead to initiation of either or both processes. Low-dose PDT with either sensitizer resulted in the initiation of apoptosis in the L1210/Atg7- cell line and a 20% loss of viability. In contrast, the same PDT dose led to the rapid appearance of autophagic cells in the L1210 line, less apoptosis and only a 5% loss of viability. These results are consistent with autophagy serving as a pro-survival response via the recycling of damaged organelles. At a higher PDT dose more apoptosis was again seen in the L1210/Atg7- line, but both cell lines exhibited comparable cytotoxicity in colony formation assays. We conclude that autophagy offers protection from the phototoxic effects of low-dose PDT, but can serve as an alternate death mode when the PDT dose is increased.