Photosensitization with protoporphyrin IX inhibits attachment of cancer cells to a substratum

Increasing cancer permeability by photodynamic priming: from microenvironment to mechanotransduction signaling

The dense cancer microenvironment is a significant barrier that limits the penetration of anticancer agents, thereby restraining the efficacy of molecular and nanoscale cancer therapeutics. Developing new strategies to enhance the permeability of cancer tissues is of major interest to overcome treatment resistance. Nonetheless, early strategies based on small molecule inhibitors or matrix-degrading enzymes have led to disappointing clinical outcomes by causing increased chemotherapy toxicity and promoting disease progression. In recent years, photodynamic therapy (PDT) has emerged as a novel approach to increase the permeability of cancer tissues. By producing excessive amounts of reactive oxygen species selectively in the cancer microenvironment, PDT increases the accumulation, penetration depth, and efficacy of chemotherapeutics. Importantly, the increased cancer permeability has not been associated to increased metastasis formation. In this review, we provide novel insights into the mechanisms by which this effect, called photodynamic priming, can increase cancer permeability without promoting cell migration and dissemination. This review demonstrates that PDT oxidizes and degrades extracellular matrix proteins, reduces the capacity of cancer cells to adhere to the altered matrix, and interferes with mechanotransduction pathways that promote cancer cell migration and differentiation. Significant knowledge gaps are identified regarding the involvement of critical signaling pathways, and to which extent these events are influenced by the complicated PDT dosimetry. Addressing these knowledge gaps will be vital to further develop PDT as an adjuvant approach to improve cancer permeability, demonstrate the safety and efficacy of this priming approach, and render more cancer patients eligible to receive life-extending treatments.

Her2 oncogene transformation enhances 5-aminolevulinic acid-mediated protoporphyrin IX production and photodynamic therapy response

Enhanced protoporphyrin IX (PpIX) production in tumors derived from the administration of 5-aminolevulinic acid (ALA) enables the use of ALA as a prodrug for photodynamic therapy (PDT) and fluorescence-guided tumor resection. Although ALA has been successfully used in the clinic, the mechanism underlying enhanced ALA-induced PpIX production in tumors is not well understood. Human epidermal growth receptor 2 (Her2, Neu, ErbB2) is a driver oncogene in human cancers, particularly breast cancers. Here we showed that, in addition to activating Her2/Neu cell signaling, inducing epithelial-mesenchymal transition and upregulating glycolytic enzymes, transfection of NeuT (a mutated Her2/Neu) oncogene in MCF10A human breast epithelial cells significantly enhanced ALA-induced PpIX fluorescence by elevating some enzymes involved in PpIX biosynthesis. Furthermore, NeuT-transformed and vector control cells exhibited drastic differences in the intracellular localization of PpIX, either produced endogenously from ALA or applied exogenously. In vector control cells, PpIX displayed a cell contact-dependent membrane localization at high cell densities and increased mitochondrial localization at low cell densities. In contrast, no predominant membrane localization of PpIX was observed in NeuT cells and ALA-induced PpIX showed a consistent mitochondrial localization regardless of cell density. PDT with ALA caused significantly more decrease in cell viability in NeuT cells than in vector cells. Our data demonstrate that NeuT oncogene transformation enhanced ALA-induced PpIX production and altered PpIX intracellular localization, rendering NeuT-transformed cells increased response to ALA-mediated PDT. These results support the use of ALA for imaging and photodynamic targeting Her2/Neu-positive tumors.

Glycolytic inhibitors 2-deoxyglucose and 3-bromopyruvate synergize with photodynamic therapy respectively to inhibit cell migration

Most cancer cells have the specially increased glycolytic phenotype, which makes this pathway become an attractive therapeutic target. Although glycolytic inhibitor 2-deoxyglucose (2-DG) has been demonstrated to potentiate the cytotoxicity of photodynamic therapy (PDT), the impacts on cell migration after the combined treatment has never been reported yet. The present study aimed to analyze the influence of glycolytic inhibitors 2-DG and 3-bromopyruvate (3-BP) combined with Ce6-PDT on cell motility of Triple Negative Breast Cancer MDA-MB-231 cells. As determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltertrazolium-bromide-Tetraz-olium (MTT) assay, more decreased cell viability was observed in 2-DG + PDT and 3-BP + PDT groups when compared with either monotherapy. Under optimal conditions, synergistic potentiation on cell membrane destruction and the decline of cell adhesion and cells migratory ability were observed in both 2-DG + PDT and 3-BP + PDT by electron microscope observation (SEM), wound healing and trans-well assays. Besides, serious microfilament network collapses as well as impairment of matrix metalloproteinases-9 (MMP-9) were notably improved after the combined treatments by immunofluorescent staining. These results suggest that 2-DG and 3-BP can both significantly potentiated Ce6-PDT efficacy of cell migration inhibition.

The role of cytoskeleton and adhesion proteins in the resistance to photodynamic therapy. Possible therapeutic interventions

It is known that Photodynamic Therapy (PDT) induces changes in the cytoskeleton, the cell shape, and the adhesion properties of tumour cells. In addition, these targets have also been demonstrated to be involved in the development of PDT resistance. The reversal of PDT resistance by manipulating the cell adhesion process to substrata has been out of reach. Even though the existence of cell adhesion-mediated PDT resistance has not been reported so far, it cannot be ruled out. In addition to its impact on the apoptotic response to photodamage, the cytoskeleton alterations are thought to be associated with the processes of metastasis and invasion after PDT. In this review, we will address the impact of photodamage on the microfilament and microtubule cytoskeleton components and its regulators on PDT-treated cells as well as on cell adhesion. We will also summarise the impact of PDT on the surviving and resistant cells and their metastatic potential. Possible strategies aimed at taking advantage of the changes induced by PDT on actin, tubulin and cell adhesion proteins by targeting these molecules will also be discussed.

Mechanisms in photodynamic therapy: part two-cellular signaling, cell metabolism and modes of cell death

Photodynamic therapy (PDT) has been known for over a hundred years, but is only now becoming widely used. Originally developed as a tumor therapy, some of its most successful applications are for non-malignant disease. In the second of a series of three reviews, we will discuss the mechanisms that operate in PDT on a cellular level. In Part I [Castano AP, Demidova TN, Hamblin MR. Mechanism in photodynamic therapy: part one-photosensitizers, photochemistry and cellular localization. Photodiagn Photodyn Ther 2004;1:279-93] it was shown that one of the most important factors governing the outcome of PDT, is how the photosensitizer (PS) interacts with cells in the target tissue or tumor, and the key aspect of this interaction is the subcellular localization of the PS. PS can localize in mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus and plasma membranes. An explosion of investigation and explorations in the field of cell biology have elucidated many of the pathways that mammalian cells undergo when PS are delivered in tissue culture and subsequently illuminated. There is an acute stress response leading to changes in calcium and lipid metabolism and production of cytokines and stress proteins. Enzymes particularly, protein kinases, are activated and transcription factors are expressed. Many of the cellular responses are centered on mitochondria. These effects frequently lead to induction of apoptosis either by the mitochondrial pathway involving caspases and release of cytochrome c, or by pathways involving ceramide or death receptors. However, under certain circumstances cells subjected to PDT die by necrosis. Although there have been many reports of DNA damage caused by PDT, this is not thought to be an important cell-death pathway. This mechanistic research is expected to lead to optimization of PDT as a tumor treatment, and to rational selection of combination therapies that include PDT as a component.

Anticancer efficacy of photodynamic therapy with hematoporphyrin-modified, doxorubicin-loaded nanoparticles in liver cancer

Photodynamic therapy (PDT) in combination with chemotherapy has great potential for cancer treatment. However, there have been very few attempts to developing cancer-targeted co-delivered systems of photosensitizers and anticancer drugs. We developed hematoporphyrin (HP)-modified doxorubicin (DOX)-loaded nanoparticles (HP-NPs) to improve the therapeutic effect of PDT in treating liver cancer. HP is not only a ligand for low density lipoprotein (LDL) receptors on the hepatoma cells but also a well-known photosensitizer for PDT. In vitro phototoxicity in HepG2 (human hepatocellular carcinoma) cells and in vivo anticancer efficacy in HepG2 tumor-bearing mice of free HP and HP-NPs were evaluated. The in vitro phototoxicity in HepG2 cells determined by MTT assay, annexin V-FITC staining and FACS analysis was enhanced in HP-NPs compared with free HP. Furthermore, compared with free HP-based PDT, in vivo anticancer efficacy in HepG2 tumor-bearing mice was markedly improved by HP-NPs-based PDT. Moreover, in both cases, the therapeutic effect was increased according to the irradiation time and number of PDT sessions. In conclusion, the HP-NPs prepared in this study represent a potentially effective co-delivery system of photosensitizer (HP) and anticancer drug (DOX) which improved the effects of PDT in liver cancer.

Direct and indirect photodynamic therapy effects on the cellular and molecular components of the tumor microenvironment

Photodynamic therapy (PDT) is a novel cancer treatment. It involves the activation of a photosensitizer (PS) with light of specific wavelength, which interacts with molecular oxygen to generate singlet oxygen and other reactive oxygen species (ROS) that lead to tumor cell death. When a tumor is treated with PDT, in addition to affect cancer cells, the extracellular matrix and the other cellular components of the microenvironment are altered and finally this had effects on the tumor cells survival. Furthermore, the heterogeneity in the availability of nutrients and oxygen in the different regions of a tridimensional tumor has a strong impact on the sensitivity of cells to PDT. In this review, we summarize how PDT affects indirectly to the tumor cells, by the alterations on the extracellular matrix, the cell adhesion and the effects over the immune response. Also, we describe direct PDT effects on cancer cells, considering the intratumoral role that autophagy mediated by hypoxia-inducible factor 1 (HIF-1) has on the efficiency of the treatment.

Sodium butyrate increases the effect of the photodynamic therapy: a mechanism that involves modulation of gene expression and differentiation in astrocytoma cells

OBJECTIVES

In order to evaluate the improvement of the photodynamic therapy (PDT) due to sodium butyrate (NaBu), its effectiveness in U373-MG and D54-MG astrocytoma cell lines was evaluated.

METHODS

Cells were exposed to delta-aminolevulinic acid (δ-ALA) as a precursor to endogenous photosensitizer protoporphyrin IX (PpIX). In both astrocytoma cells, an important increase by ALA was observed in uroporphyrinogen synthetase gene expression: 1.8- and 52-fold for D54-MG and U373-MG cells, respectively. After irradiation, they showed 16.67 and 28.9% of mortality in U373-MG and D54-MG, respectively. These mortalities increased to 70.62 and 96.7% when U373-MG and D54-MG cells, respectively, were exposed 24 h to 8 mM NaBu, before to PpIX induction. NaBu induced expression of caspase-3, caspase-9, and Bcl-2 and increased Bax in U373-MG cells. ALA-induced morphological changes are compatible to differentiation.

CONCLUSIONS

Genes and differentiation induced mainly by NaBu improve cell death performed by PDT in astrocytoma cells. These facts prove the synergistic effect of NaBu on cytotoxic damage induced by PDT.

Stromal interactions as regulators of tumor growth and therapeutic response: A potential target for photodynamic therapy?

It has become increasingly widely recognized that the stroma plays several vital roles in tumor growth and development and that tumor-stroma interactions can in many cases account poor therapeutic response. Inspired by an emerging body of literature, we consider the potential role of photodynamic therapy (PDT) for targeting interactions with stromal fibroblasts and mechano-sensitive signaling with the extracellular matrix as a means to drive tumors toward a more therapeutically responsive state and synergize with other treatments. This concept is particularly relevant for cancer of the pancreas, which is characterized by tumors with a profoundly dense, rigid fibrous stroma. Here we introduce new in vitro systems to model interactions between pancreatic tumors and their mechanical microenvironment and restore signaling with stromal fibroblasts. Using one such model as a test bed it is shown here that PDT treatment is able to destroy fibroblasts in an in vitro 3D pancreatic tumor-fibroblast co-culture. These results and the literature suggest the further development of PDT as a potential modality for stromal depletion.

Preclinical photodynamic therapy research in Spain 4: Cytoskeleton and adhesion complexes of cultured tumor cells as targets of photosensitizers

Tumor cell death induced by photodynamic therapy (PDT) with different photosensitizers (PSs) is due to the selective damage of several membranous organelles including mitochondria, lysosomes and Golgi apparatus. Other cell structures such as the cytoskeleton (CSK) (microtubules, actin microfilaments and cytokeratin intermediate filaments) and the cell adhesion components (cadherins and integrins) are also implicated in cell death induced by PSs. CSK and adhesion components are responsible for many cellular functions such as the maintenance of morphology, motility, division and adhesion, all of them of fundamental importance for growth and dissemination of tumors. Therefore, they are considered very important targets for anticancer therapies, including PDT. In addition, similarly to the rest of the anticancer therapies, PDT often leaves a significant number of surviving tumor cells. The reorganization of CSK as well as the adhesion proteins in the PDT resistant cells affect their invasive migratory capabilities. Taking into account all these features, both CSK and adhesion proteins are crucial targets of PDT.

Dynamics of signaling, cytoskeleton and cell cycle regulation proteins in glioblastoma cells after sub-lethal photodynamic treatment: antibody microarray study

BACKGROUND

Photodynamic therapy (PDT) that induces oxidative stress and cell death is used for tumor destruction in oncology. To characterize early molecular events in photosensitized glioblastoma cells, we studied expression of 224 proteins after sublethal PDT that doesn't kill but wounds cells.

METHODS

Cultured glioblastoma D54Mg cells were photosensitized with 5-aminolevulinic acid so that cell survival was 95-100%. At following 0.5-5.5h protein expression and phosphorylation was assayed using proteomic antibody microarrays.

RESULTS

Within the first post-treatment hour we observed phosphorylation of protein kinase Raf, adhesion-related kinases FAK and Pyk2, and microtubule-associated protein tau. Protein kinase Cγ and microtubule-associated protein MAP-1B were overexpressed. Dystrophin, calponin, and vinculin, components of the actin cytoskeleton scaffold, microtubule-associated proteins MAP2 and CNP, cytokeratins 4 and 7 were down-regulated that indicated changes in adhesion and cell shape. Down-regulation of cyclins A, D1 and D3, c-Myc, checkpoint proteins chk1/2 and up-regulation of Smad4 could arrest the cell cycle. Overexpression of Bcl-xL and down-regulation of caspase 9 demonstrated anti-apoptotic response. At 2h post-treatment protein expression changed lesser but at 5.5h levels of PKCγ and β-synuclein and phosphorylation of Raf, FAK, Pyk2, and tau increased again.

CONCLUSIONS

Sub-lethal PDT induces complex response of glioblastoma cells including changes in activity and expression of proteins involved in adhesion-mediated signaling, signal transduction, cytoskeleton remodeling, cell cycle regulation and anti-apoptotic processes.

GENERAL SIGNIFICANCE

Multiple reactions of various cellular subsystems including adhesion, cytoskeleton, signal transduction, cell cycle, and apoptosis are integrated into the general cell response to a sublethal impact.

Targets and mechanisms of photodynamic therapy in lung cancer cells: a brief overview

Lung cancer remains one of the most common cancer-related causes of death. This type of cancer typically develops over a period of many years, and if detected at an early enough stage can be eliminated by a variety of treatments including photodynamic therapy (PDT). A critical discussion on the clinical applications of PDT in lung cancer is well outside the scope of the present report, which, in turn focuses on mechanistic and other aspects of the photodynamic action at a molecular and cellular level. The knowledge of these issues at pre-clinical levels is necessary to develop, check and adopt appropriate clinical protocols in the future. This report, besides providing general information, includes a brief overview of present experimental PDT and provides some non-exhaustive information on current strategies aimed at further improving the efficacy, especially in regard to lung cancer cells.

PDT-induced inflammatory and host responses

Photodynamic therapy (PDT) is used in the management of neoplastic and nonmalignant diseases. Its unique mechanisms of action include direct cytotoxic effects exerted towards tumor cells, destruction of tumor and peritumoral vasculature and induction of local acute inflammatory reaction. The latter develops in response to (1) damage to tumor and stromal cells that leads to the release of cell death-associated molecular patterns (CDAMs) or damage associated molecular patterns (DAMPs), (2) early vascular changes that include increased vascular permeability, vascular occlusion, and release of vasoactive and proinflammatory mediators, (3) activation of alternative pathway of complement leading to generation of potent chemotactic factors, and (4) induction of signaling cascades and transcription factors that trigger secretion of cytokines, matrix metalloproteinases, or adhesion molecules. The majority of studies indicate that induction of local inflammatory response contributes to the antitumor effects of PDT and facilitates development of systemic immunity. However, the degree of PDT-induced inflammation and its subsequent contribution to its antitumor efficacy depend on multiple parameters, such as chemical nature, concentration and subcellular localization of the photosensitizers, the spectral characteristics of the light source, light fluence and fluence rate, oxygenation level, and tumor type. Identification of detailed molecular mechanisms and development of therapeutic approaches modulating PDT-induced inflammation will be necessary to tailor this treatment to particular clinical conditions.

Non-monotonic changes in clonogenic cell survival induced by disulphonated aluminum phthalocyanine photodynamic treatment in a human glioma cell line

BACKGROUND

Photodynamic therapy (PDT) involves excitation of sensitizer molecules by visible light in the presence of molecular oxygen, thereby generating reactive oxygen species (ROS) through electron/energy transfer processes. The ROS, thus produced can cause damage to both the structure and the function of the cellular constituents resulting in cell death. Our preliminary investigations of dose-response relationships in a human glioma cell line (BMG-1) showed that disulphonated aluminum phthalocyanine (AlPcS2) photodynamically induced loss of cell survival in a concentration dependent manner up to 1 microM, further increases in AlPcS2concentration (>1 microM) were, however, observed to decrease the photodynamic toxicity. Considering the fact that for most photosensitizers only monotonic dose-response (survival) relationships have been reported, this result was unexpected. The present studies were, therefore, undertaken to further investigate the concentration dependent photodynamic effects of AlPcS2.

METHODS

Concentration-dependent cellular uptake, sub-cellular localization, proliferation and photodynamic effects of AlPcS2 were investigated in BMG-1 cells by absorbance and fluorescence measurements, image analysis, cell counting and colony forming assays, flow cytometry and micronuclei formation respectively.

RESULTS

The cellular uptake as a function of extra-cellular AlPcS2 concentrations was observed to be biphasic. AlPcS2 was distributed throughout the cytoplasm with intense fluorescence in the perinuclear regions at a concentration of 1 microM, while a weak diffuse fluorescence was observed at higher concentrations. A concentration-dependent decrease in cell proliferation with accumulation of cells in G2+M phase was observed after PDT. The response of clonogenic survival after AlPcS2-PDT was non-monotonic with respect to AlPcS2 concentration.

CONCLUSIONS

Based on the results we conclude that concentration-dependent changes in physico-chemical properties of sensitizer such as aggregation may influence intracellular transport and localization of photosensitizer. Consequent modifications in the photodynamic induction of lesions and their repair leading to different modes of cell death may contribute to the observed non-linear effects.

Size and Surface Charge Effect of 5-Aminolevulinic Acid-Containing Liposomes on Photodynamic Therapy for Cultivated Cancer Cells

5-Aminolevulinic acid (ALA)-containing liposomes having various average diameters and/or positive surface charges were prepared, and their photodynamic therapy (PDT) efficacy for murine thymic lymphoma cells, EL-4 cells, cultivated in vitro was investigated. The PDT efficacy for EL-4 cells and the accumulation of ALA-induced protoporphyrin IX (PpIX) in the cells increased with a decrease in the average diameter of liposomes. In particular, the ALA-containing liposomes smaller than 63.5 nm in diameter promoted the PDT efficacy in comparison with that of ALA alone. We also found no significant changes in PDT efficacy and PpIX accumulation with increasing positive surface charges of liposomes.

Decreased metastatic phenotype in cells resistant to aminolevulinic acid-photodynamic therapy

Photodynamic therapy (PDT) is a novel cancer treatment utilising a photosensitiser, visible light and oxygen. PDT often leaves a significant number of surviving tumour cells. In a previous work, we isolated and studied two PDT resistant clones derived from the mammary adenocarcinoma LM3 line (Int. J. Oncol. 29 (2006) 397-405). The isolated Clon 4 and Clon 8 exhibited a more fibroblastic, dendritic pattern and were larger than the parentals. In the present work we studied the metastatic potential of the two clones in comparison with LM3. We found that 100% of LM3 invaded Matrigel, whereas only 19+/-6% and 24+/-7% of Clon 4 and Clon 8 cells invaded. In addition, 100% of LM3 cells migrated towards a chemotactic stimulus whereas 38+/-8% and 73+/-10% of Clones 4 and 8, respectively, were able to migrate. In vivo, 100% of the LM3 injected mice developed spontaneous lung metastasis, whereas none of the Clon 8 did, and only one of the mice injected with Clon 4 did. No differences were found in the proteolytic enzyme profiles among the cells. Anchorage-dependent adhesion was also impaired in vivo in the resistant clones, evidenced by the lower tumour take, latency time and growth rates, although both clones showed in vitro higher binding to collagen I without overexpression of beta1 integrin. This is the first work where the metastatic potential of cells surviving to PDT has been studied. PDT strongly affects the invasive phenotype of these cells, probably related to a higher binding to collagen. These findings may be crucial for the outcome of ALA-PDT of metastatic tumours, although further studies are needed to extrapolate the results to the clinic employing another photosensitisers and cell types.

Effect of photodynamic therapy on the extracellular matrix and associated components

In many countries, photodynamic therapy (PDT) has been recognized as a standard treatment for malignant conditions (for example, esophageal and lung cancers) and non-malignant ones such as age-related macular degeneration and actinic keratoses. The administration of a non-toxic photosensitizer, its selective retention in highly proliferating cells and the later activation of this molecule by light to form reactive oxygen species that cause cell death is the principle of PDT. Three important mechanisms are responsible for the PDT effectiveness: a) direct tumor cell kill; b) damage of the tumor vasculature; c) post-treatment immunological response associated with the leukocyte stimulation and release of many inflammatory mediators like cytokines, growth factors, components of the complement system, acute phase proteins, and other immunoregulators. Due to the potential applications of this therapy, many studies have been reported regarding the effect of the treatment on cell survival/death, cell proliferation, matrix assembly, proteases and inhibitors, among others. Studies have demonstrated that PDT alters the extracellular matrix profoundly. For example, PDT induces collagen matrix changes, including cross-linking. The extracellular matrix is vital for tissue organization in multicellular organisms. In cooperation with growth factors and cytokines, it provides cells with key signals in a variety of physiological and pathological processes, for example, adhesion/migration and cell proliferation/differentiation/death. Thus, the focus of the present paper is related to the effects of PDT observed on the extracellular matrix and on the molecules associated with it, such as, adhesion molecules, matrix metalloproteinases, growth factors, and immunological mediators.

Milestones in the development of photodynamic therapy and fluorescence diagnosis

Many reviews on PDT have been published. This field is now so large, and embraces so many sub-specialties, from laser technology and optical penetration through diffusing media to a number of medical fields including dermatology, gastroenterology, ophthalmology, blood sterilization and treatment of microbial-viral diseases, that it is impossible to cover all aspects in a single review. Here, we will concentrate on a few basic aspects, all important for the route of development leading PDT to its present state: early work on hematoporphyrin and hematoporphyrin derivative, second and third generation photosensitizers, 5-aminolevulinic acid and its derivatives, oxygen and singlet oxygen, PDT effects on cell organelles, mutagenic potential, the basis for tumour selectivity, cell cooperativity, photochemical internalization, light penetration into tissue and the significance of oxygen depletion, photobleaching of photosensitizers, optimal light sources, effects on the immune system, and, finally, future trends.

The effect of sub-lethal ALA-PDT on the cytoskeleton and adhesion of cultured human cancer cells

5-Aminolevulinic acid (ALA), a precursor of the endogenous photosensitizer protoporphyrin IX, is used in the photodynamic therapy (PDT) of cancer. Sub-lethal ALA-PDT (1-min irradiation with 370-450 nm blue light, 0.6 mW/cm(2) after 2-h incubation with 1 mM ALA) has been earlier shown to change cell morphology and to inhibit both trypsin-induced detachment of cultured cancer cells from the plastic substrata and cell attachment to the bottom of the plastic well plates. In the present study, we found that such treatment of human adenocarcinoma WiDr cells grown in dense colonies stimulated the formation of actin cortex between cells in the colonies and increased the number of actin stress fibres in some, but not in all, cells. However, ALA-PDT did not change the microtubular cytoskeleton in these cells. A similar treatment of glioblastoma D54Mg cells, which grow separately and communicate by protrusions, caused loss of fibrillar actin structures in growth cones, retraction of protrusions, and surface blebbing in some cells. The application of the cytoskeleton inhibitors cytochalasin D, colchicine or taxol showed that the inhibition of trypsin-induced detachment of photosensitized WiDr cells was related to ALA-PDT-induced changes in actin and microtubular cytoskeleton. Some signal transduction processes are suggested to be involved in ALA-PDT-induced changes in cytoskeleton, cell shape, and adhesion.