Tetraploidization is a frequent intermediate of aneuploidy characterized by genomic instability, a common feature of most cancer cells. Our study describes the induction of tetraploidy in diploid keratinocytes and compares a tumorigenic potential of the tetraploid cells to their diploid precursor. Diploid skin keratinocytes isolated from newborn p53 knock-out mice were converted into tetraploid cells by an inhibitor of the mitotic spindle formation, nocodazole. This treatment induced a brief mitotic cell cycle arrest followed by mitotic slippage and replication of the tetraploid nucleus in a majority of cells (>95%). The tetraploid cells and the parental diploid cells had a similar doubling time (14-15h) and a similar sensitivity to a single dose of UVB-irradiation (1-5mJ/cm2). However, the tetraploid cells were 4-fold more resistant to chronic UVB-irradiation (four consecutive 1.5mJ/cm2 doses at 24-hour intervals) than the diploid cells as measured by growth inhibition. The tetraploid cells also formed small colonies in soft agar following twice a week UVB-irradiation with a 1.5mJ/cm2 dose for three weeks, whereas similarly treated parental diploid cells and untreated controls did not. These data indicate that UVB-irradiation preferentially induces an anchorage-independent cellular growth, a hallmark of tumorigenicity, in tetraploid cells. A mouse xenograft model was used to examine the growth of diploid and tetraploid keratinocytes in vivo and it is currently ongoing. Results of our studies suggest that tetraploid keratinocytes may have higher resistance to growth inhibition induced by chronic UVB-irradiation and a higher tumorigenic potential than their parental diploid cells, which is in accord with the hypothesis that tetraploidy generates genomic instability and leads to carcinogenesis.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA