Differential accessibility to DNA in tumor cell chromatin is important to growth, differentiation apoptosis, and the targeting of DNA modifying drugs. We now show that endonuclease accessibility to DNA in the nuclei of A431 human carcinoma cells is increased within 90 min by nontoxic nanomolar levels of okadaic acid, known to inhibit protein phosphatase 2A. This genomic hypersensitivity was partly enhanced by joint treatment with epidermal growth factor and okadaic acid but did not appear without the latter. Nuclei with greater DNA susceptibility showed a decrease in Mr 80,000 DNA binding protein double specific for dAT-rich sequences concurrent with the “apparent” hyperphosphorylation of a Mr 70,000 nuclear matrix protein.

We propose that some of the tumor-promoting effects of okadaic acid may be partly associated with its ability to promote genomic susceptibility.

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Supported by the Andean Development Corporation (CAF).

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