Phenolphthalein induces
tumors in rodents but because it is negative in assays for mutation in Salmonella and in mammalian cells, for
DNA adducts and for
DNA strand breaks, its primary mechanism does not seem to be DNA damage.
Chromosome aberration (Ab) induction by
phenolphthalein in vitro is associated with marked cytotoxicity. At very high doses,
phenolphthalein induces weak increases in micronuclei (MN) in mouse bone marrow; a larger response is seen with chronic treatment. All this suggests genotoxicity is a secondary effect that may not occur at lower doses. In heterozygous TSG-p53((R)) mice,
phenolphthalein induces
lymphomas and also MN, many with kinetochores (K), implying chromosome loss. Induction of
aneuploidy would be compatible with the loss of the normal p53 gene seen in the
lymphomas. Here we address some of the postulated mechanisms of genotoxicity in vitro, including metabolic activation, inhibition of
thymidylate synthetase, cytotoxicity, oxidative stress, DNA damage and
aneuploidy. We show clearly that
phenolphthalein does not require metabolic activation by S9 to induce Abs. Inhibition of
thymidylate synthetase is an unlikely mechanism, since
thymidine did not prevent Ab induction by
phenolphthalein.
Phenolphthalein dramatically inhibited
DNA synthesis, in common with many non-
DNA reactive chemicals that induce Abs at cytotoxic doses.
Phenolphthalein strongly enhances levels of intracellular
oxygen radicals (ROS). The radical scavenger
DMSO suppresses
phenolphthalein-induced toxicity and Abs whereas H(2)O(2) potentiates them, suggesting a role for peroxidative activation.
Phenolphthalein did not produce
DNA strand breaks in rat hepatocytes or
DNA adducts in Chinese hamster ovary (CHO) cells. All the evidence points to an indirect mechanism for Abs that is unlikely to operate at low doses of
phenolphthalein. We also found that
phenolphthalein induces mitotic abnormalities and MN with kinetochores in vitro. These are also enhanced by H(2)O(2) and suppressed by
DMSO. Our findings suggest that induction of Abs in vitro is a high-dose effect in oxidatively stressed cells and may thus have a threshold. There may be more than one mechanism operating in vitro and in vivo, possibly indirect genotoxicity at high doses and also chromosome loss, both of which would likely have a threshold.