Photosensitizers as Novel Compounds in Apoptosis Research

Death is an intricate process in all living cellular organisms. Apoptosis, also known as programmed cell death, is a normal physiological process occurring in all eukaryotic cells. This normal cell death process has been known for over half a century. Mechanisms leading to apoptotic cell death have also been studied extensively. Induction of tumour cell apoptosis becomes a new research direction for the treatment of malignant disease. In the past decade, photodynamic therapy (PDT) has been developed as a tumour therapy. The treatment relies on the accumulation of photosensitizers in the tumour mass. Tumour cell death, triggered by irradiating the tumour mass with visible light, is one of the major mechanisms leading to tumour eradication. Generation of singlet oxygen and induction of oxidative stress are the major intracellular events after PDT. In view of the short half-life of reactive oxygen species, the localization of photosensitizers in various subcellular compartments, such as the plasma membrane, mitochondria, lysosomes, Golgi body, and endoplasmic reticulum has been implicated in the induction of various modes of cell death, Photosensitizers targeting mitochondria received particular interest as mitochondria are the key organelles in the mitochondria-mediated apoptotic pathway. Other cellular events, such as activation of transcription factors and intracellular stress signaling pathways and changes of Ca²⁺ and lipid metabolism are also seen in PDT treated cells. This review focuses on the understanding of PDT-induced tumour cell apoptosis.