Recombinagenic activity of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate in human lymphoblastoid cells

The potent mouse skin tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) was examined for its mutagenic and recombinagenic activity at the heterozygous thymidine kinase (tk +/-) locus and the hemizygous hypoxanthine phosphoribosyltransferase (hprt +/0) locus in the TK6 human lymphoblastoid cell line. TPA at concentrations of 0.01-1.0 micrograms/ml induced a low frequency of tk mutants showing the slow growth phenotype in a dose-dependent manner, but few normal growth tk mutants or hprt mutants. Concentrations of 1.0-10 micrograms/ml TPA induced all three types of mutants. The molecular structure of tk mutants arising spontaneously or induced by 1.0 and 10 micrograms/ml TPA was investigated by Southern hybridization with a human tk cDNA probe: 86% of all mutants arising after incubation with 10 micrograms/ml TPA lost the entire active tk allele, resulting in loss of heterozygosity (LOH), while 71% of spontaneously arising mutants showed LOH. Densitometric analysis indicated that the majority of LOH mutants induced by TPA were homozygous at the tk locus (retained two copies of the mutant allele), consistent with the occurrence of interchromosomal homologous recombination. These results support the hypothesis that tumor promoters such as TPA may increase the rate of chromosomal mitotic recombination and hence facilitate the segregation of recessive mutations. TPA may thus induce a type of genetic instability during the process of tumor promotion that involves enhanced recombinagenic activity.