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 micromol ar ETO, 100 mu M DEX and UV irradiation for 90 min reduced the cell nu mber to 20% of that of the control. The cytotoxic effects of ETO and D EX were dose-dependent, while those of UV irradiation were time-depend ent. Endonuclease cleavage of DNA into internucleosomal fragments was not recognized on DOS C14 osteosarcoma treated with 50 mu M ETO, 100 m u M DEX or UV irradiation. Aurintricarboxylic acid (ATA) also failed t o inhibit the reduction in the number of viable cells. However, DNA fr om DOS C14 osteosarcoma cells exposed to 1 h UV irradiation showed sme aring DNA fragments after treatment with S1 endonuclease, while such s ingle strand modification was not detected in DNA extracted from cells exposed to ETO or DEX. On the other hand, pulse field gel electrophor esis revealed that cleavage of DNA into high molecular weight fragment s 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, wh ile UV-induced cytotoxicity was not reduced by okadaic acid. Furthermo re, okadaic acid inhibited the formation of high molecular weight DNA fragments in a dose-dependent manner. These results suggest that two t ypes of DNA degradation exist in osteosarcoma death; one is random bre akage of DNA, and the other is large DNA fragmentation, which may be p roduced by an activation of putative ATA-insensitive and okadaic acid- sensitive endonuclease.