A photodynamic pathway to apoptosis and necrosis induced by dimethyl tetrahydroxyhelianthrone and hypericin in leukaemic cells: possible relevance tophotodynamic therapy

The mechanism of cell death induction by dimethyl tetrahydroxyhelianthrone (DTHe), a new second-generation photodynamic sensitizer, is analysed in hum an leukaemic cell lines in comparison with the structurally related hyperic in. DTHe has a broad range of light spectrum absorption that enables effect ive utilization of polychromatic light. Photosensitization of HL-60 cells w ith low doses of DTHe (0.65 mu M DTHe and 7.2 J cm(-2) light energy) induce d rapid apoptosis of greater than or equal to 90% of the cells. At doses gr eater than or equal to 2 mu M, dying cells assumed morphological necrosis w ith perinucleolar condensation of chromatin in HL-60 and K-562 cell lines. Although nuclear fragmentation that is characteristic to apoptosis was prev ented, DNA digestion to oligonucleosomes proceeded unhindered. Such incompl ete apoptosis was more prevalent with the related analogue hypericin throug hout most doses of photosensitization. Despite hypericin being a stronger p hotosensitizer, DTHe exhibited advantageous phototoxic properties to tumour cells, initiating; apoptosis at concentrations about threefold lower than hypericin. Photosensitization of the cells induced dissociation of the nucl ear envelope, releasing lamins into the cytosol. DTHe also differed from hy pericin in effects exerted on the nuclear lamina, causing release of an 86- kDa lamin protein into the cytosol that was unique to DTHe. Within the nucl eus, nuclear envelope lamin B underwent covalent polymerization, which did not affect apoptotic nuclear fragmentation at low doses of DTHe. At higher doses, polymerization may have been extensive enough to prevent nuclear col lapse. Hut-78, CD4(+) cells were resistant to the photodynamically activate d apoptotic pathway. Beyond the tolerated levels of photodynamic damage, th ese cells died exclusively via necrosis. Hut-78 cells overexpress Bcl-X-L, as well as a truncated Bcl-X-L(tr) isoform that could contribute to the obs erved resistance to apoptosis.