Tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced chromosome aberrations in mouse keratinocyte cell lines: a possible genetic mechanism of tumor promotion

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Abstract
The effect of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) on the induction of numerical and structural chromosomal aberrations, double minute chromosomes (DM) and homogeneously staining regions (HSR) was investigated in two mouse keratinocyte cell lines (HEL-30 and HEL-37) using G- and C-banded chromosomal preparations. The frequency of polyploid metaphases and of structural aberrations increased several fold during the first two cell cycles. The increase of gaps, chromatid and isochromatid breaks was accompanied by inter- and intrachromosomal exchanges, which were virtually absent in control cultures. The induction of chromosomal aberrations was similarly expressed in both cell lines at different passage levels and was largely independent of the TPA dose applied (10-9-10-6 M). The non-promoting phorbolester 4-O-methyl-TPA (10-6 M) did not produce any chromosomal aberrations above the control level. Effects were already visible within one cell cycle (24 h) after treatment and increased with longer and multiple exposure. Moreover, the TPA-induced chromosomal aberrations persisted for several days after elimination of the promoter and further cultivation of cells in fresh medium. Structural alterations were not randomly distributed but chromosomes 1 and 2 were preferentially involved in breaks, while chromosomes 1-3 and 9-13 were mainly involved in chromosomal exchanges. Chromosomes 7 and 14, which are numerically underrepresented in HEL cells, were least involved in breaks. Most interesting, the frequency of metaphases carrying DMs increased 2- to 3-fold after TPA treatment, and the alterations were partially reversible after removal of TPA. These early effects of TPA at the chromosomal level could play an essential role in the mechanism of tumor promotion and may be responsible for the observed persistence of alterations induced during the fist stage of tumor promotion.

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