Different types of DNA cleavage involved in osteosarcoma cell death

To induce cell death in osteosarcoma, a murine osteosarcoma clone, DOS C14 was exposed to: i) topoisomerase II-reactive epipodophyllotoxin, etoposide (ETO); ii) glucocorticoid analogue, dexamethasone (DEX); and iii) ultraviolet light (UV) irradiation. In MTT assay, fifty micromolar ETO, 100 mu M DEX and UV irradiation for 90 min reduced the cell number to 20% of that of the control. The cytotoxic effects of ETO and DEX were dose-dependent, while those of UV irradiation were time-dependent. Endonuclease cleavage of DNA into internucleosomal fragments was not recognized on DOS C14 osteosarcoma treated with 50 mu M ETO, 100 mu M DEX or UV irradiation. Aurintricarboxylic acid (ATA) also failed to inhibit the reduction in the number of viable cells. However, DNA from DOS C14 osteosarcoma cells exposed to 1 h UV irradiation showed smearing DNA fragments after treatment with S1 endonuclease, while such single strand modification was not detected in DNA extracted from cells exposed to ETO or DEX. On the other hand, pulse field gel electrophoresis revealed that cleavage of DNA into high molecular weight fragments estimated as 50-150 kilobase pairs (kbp) with a peak of 100 kbp was found in DOS C14 osteosarcoma cells exposed to 50 mu M ETO and 100 mu M DEX. The cytotoxicity of ETO and DEX was reduced by okadaic acid, while UV-induced cytotoxicity was not reduced by okadaic acid. Furthermore, okadaic acid inhibited the formation of high molecular weight DNA fragments in a dose-dependent manner. These results suggest that two types of DNA degradation exist in osteosarcoma death; one is random breakage of DNA, and the other is large DNA fragmentation, which may be produced by an activation of putative ATA-insensitive and okadaic acid-sensitive endonuclease.