Nuclear chromatin texture and sensitivity to DNase I in human leukaemic CEM cells incubated with nanomolar okadaic acid

It is now known that the analysis of chromatin texture can be used in oncology as a sensitive detection method, either to define diagnostic classifications or to locate a lesion along a defined trend curve. However, the functional significance of these variations in textural features remains sometimes unclear. Several drugs have been shown to be able to modulate chromatin structure. Among them, the phosphatase inhibitor okadaic acid at low concentration can increase accessibility to DNA in chromatin of carcinoma cells. This paper demonstrates that short exposures (0–3h) to a 10‐nM dose of okadaic acid induced an increased sensitivity to DNase I digestion in human CEM leukaemic cell nuclei and that this sensitization was associated to variations of nuclear texture characteristics, as evaluated by image cytometry. CEM cells treated with okadaic acid for 0–3h displayed changes in chromatin supraorganization with a more homogeneous and fine chromatin texture, as compared to control cells. This suggests that the appearance of an open configuration of chromatin structure as evaluated by biochemical methods corresponds to a more decondensed texture of nuclei measured by image cytometry. Longer exposures (6–24h) of CEM cells to 10nM okadaic acid lead to apoptosis. As reported previously for camptothecin‐treated HL60 cells, okadaic acid‐treated CEM cells display biphasic nuclear chromatin texture changes, i.e. a decondensation phase followed by the appearance of typical apoptotic cells with a smaller nuclear area and a highly condensed chromatin. Finally, using the multidrug‐resistant CEM‐VLB cell line, it was confirmed that these multidrug‐resistant cells also display cross‐resistance to okadaic acid, as this compound was unable to induce either increased DNase I sensitivity, apoptosis, or altered nuclear texture in this particular cell line.