The mechanism of cell death induction by
dimethyl tetrahydroxyhelianthrone (DTHe), a new second-generation photodynamic sensitizer, is analysed in human leukaemic cell lines in comparison with the structurally related
hypericin. DTHe has a broad range of light spectrum absorption that enables effective utilization of polychromatic light.
Photosensitization of HL-60 cells with low doses of DTHe (0.65 microM DTHe and 7.2 J cm(-2) light energy) induced rapid apoptosis of > or =90% of the cells. At doses > or =2 microM, dying cells assumed morphological
necrosis with perinucleolar condensation of
chromatin in HL-60 and K-562 cell lines. Although nuclear fragmentation that is characteristic to apoptosis was prevented,
DNA digestion to oligonucleosomes proceeded unhindered. Such incomplete apoptosis was more prevalent with the related analogue
hypericin throughout most doses of
photosensitization. Despite
hypericin being a stronger
photosensitizer, 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 nuclear envelope, releasing
lamins into the cytosol. DTHe also differed from
hypericin 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 nucleus, 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 collapse. Hut-78, CD4+ cells were resistant to the photodynamically activated apoptotic pathway. Beyond the tolerated levels of photodynamic damage, these 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 observed resistance to apoptosis.