Actinomycin D prevents nuclear processing of estrogen receptor

This communication describes a novel effect of actinomycin D (AcD) in inhibiting the nuclear processing or turnover of estrogen receptors in the human breast cancer cell line. MCF-7. In the absence of AcD, estradiol treatment results in rapid (5 min) hormone binding and translocation of unfilled cytoplasmic receptors (Rc) and binding of unfilled nuclear receptors (Rn). Thereafter, filled nuclear receptors (RnE) progressively deplete and, by 3 to 5 h, 70% are lost or processed. We now show that 1 to 2 micrometer AcD or chromomycin A3, both of which intercalate at G-C base-pairs on DNA, selectively and completely block RnE processing. In contrast, estrogen binding, translocation of receptor complex, and RnE accumulation in the nucleus are completely insensitive to AcD inhibition. At 1 to 2 micrometer, all other intercalators and inhibitors tested, including other inhibitors of transcription and replication, or inhibitors of translocation or of other functions, fail to prevent binding, translocation, or the nuclear processing step. AcD intranslocation, or the nuclear processing step. AcD inhibition of RnE processing is dependent on dose; at lower doses (100 nM decreasing to 1 nm), progressively greater RnE depletion occurs. AcD completely prevents RnE depletion if given together with or within 30 min after estradiol; at any time between 30 min and 5 h after estradiol, the processing of RnE is stopped instantly by addition of AcD. Because of the complexity of actinomycin action, several mechanisms can be proposed to explain its effect on nuclear ER levels.