Association of dna strand breaks with accelerated terminal differentiation in mouse epidermal cells exposed to tumor promoters

We have studied the appearance of single strand breaks (SSB) in DNA of mouse keratinocytes exposed in vitro to various tumor promoters. Mouse basal keratinocytes were selectively cultured in low calcium medium, prelabeled with [¹⁴C]thymidine, exposed to test agents, and SSB quantified by alkaline elution. 12-0-Tetradecanoylphorbol-13-acetate (TPA) caused a dose-dependent (10^-9-10^-7 M) increase in SSB after 24 h but not after shorter exposures. DNA containing TPA-induced SSB was found only in cells which had detached from the culture plate as a consequence of TPA-induced terminal differentiation. Attached cells, resistant to the differentiation-inducing effects of TPA, had the low level of SSB found in DNA from vehicle-treated control cells. Attached cells were resistant to the formation of SSB and to induced differentiation when reexposed to TPA. Other tumor-promoting phorbol esters, mezerein and retinyl phorbol acetate, also produced SSB in detached cells, whereas phorbol or resin-iferatoxin caused neither SSB or cell detachment. Retinoic acid, which blocks the induction of differentiation by TPA, inhibited the production of SSB by TPA; however, fluocinolone acetonide, chymostatin, catalase, or superoxide dismutase blocked neither TPA-induced SSB nor terminal differentiation. Epidermal cell lines resistant to TPA-induced differentiation were also resistant to SSB production by TPA. Benzoyl peroxide (BP) (10^-4 M) induced SSB in basal keratinocytes within 1 h, and attached cells showed extensive SSB by 12 h. Retinoic acid had only a slight effect on BP-induced SSB, and 1 of 3 TPA-resistant cell lines developed SSB when exposed to BP. These results suggest that TPA-induced SSB in epidermal cells are an indirect consequence of the induction of terminal differentiation, whereas BP produces SSB by a more direct mechanism of DNA damage.