S-phase arrest and apoptosis induced in normal mammary epithelial cells bya novel retinoid

The addition of all-trans-retinoic acid has been found to mediate a G(1) ce ll cycle phase arrest but not apoptosis in normal mammary epithelial cells. We have now found that addition of the novel retinoid 6-[3-(1-adamantyl)]- 4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437), which appears to fu nction through a pathway independent of retinoic acid nuclear receptors, re sults in an S-phase arrest that is preceded by a 4-fold elevation in the le vels of the cyclin-cyclin dependent kinase (cdk) inhibitor p21(WAF1/CIP1). Failure to inhibit E2F-1 activation of genes through its phosphorylation by the cyclin cdk2 kinase has been shown to result in S-phase arrest and apop tosis in a number of cell types. Although exposure of the normal mammary ce lls to CD437 does not result in modulation of cyclin A or cdk2 levels, an i ncrease in E2F-1 levels and a marked inhibition of cyclin A/cdk2 kinase act ivity are observed. Exposure to CD437 results in enhanced E2F-1 binding to its DNA consensus sequences and transcriptional activity during S phase. We hypothesize that this enhanced E2F-1 transcriptional activity results in S -phase arrest and subsequent apoptosis that has been observed in other syst ems.