Transcriptional regulation of prolactin receptor gene expression by sodium butyrate in MCF-7 human breast cancer cells.

The prolactin receptor (PRLR) mediates the diverse effects of prolactin, which in the mammary gland include the development of lobuloalveolar structures and increased tumor cell proliferation. Treatment of mammary carcinoma cells with the differentiating agent sodium butyrate (NaB) is known to reduce PRLR binding activity and PRLR gene expression, however the mechanism which mediates these changes is unknown, prompting this investigation. Using MCF-7 human breast cancer cells, assay of the rate of PRLR gene transcription by the nuclear run-on technique indicated that 3 mM NaB reduced PRLR gene transcription by 50% after 3 h of treatment and that this effect was maintained for at least 24 h. The protein synthesis inhibitor cycloheximide failed to abrogate this effect, which indicated that NaB did not require continuing protein synthesis to reduce the rate of PRLR transcription. Measurement of PRLR mRNA stability, using Northern blot analysis at various times after the inhibition of transcription with actinomycin D, showed that NaB treatment did not alter PRLR mRNA half-life. These results indicate that NaB inhibits PRLR gene expression by a transcriptional mechanism that does not require continuing protein synthesis.