The
sex steroid hormone progesterone modulates the developmental and lactogenic activity of
prolactin in the mammary gland. Regulation of the level of
prolactin receptor (PRLR) provides one possible mechanism by which this may occur, prompting this investigation of the molecular mechanisms involved in
progestin regulation of
prolactin receptor levels. Treatment of T-47D and MCF-7 human
breast cancer cells with 10 nM of the
synthetic progestin ORG 2058 for 24 hr resulted in an increase in all four PRLR
mRNA transcripts detected. The effect of
ORG 2058 was shown in T-47D cells to be time- and concentration-dependent and resulted in an approximate two-fold increase in PRLR
mRNA after 24 hr of treatment with 10 nM or 100 nM
ORG 2058. Nuclear run-on assays indicated that
ORG 2058 increased the rate of T-47D PRLR gene transcription at all times between 1 hr and 28 hr of treatment. The
protein synthesis inhibitors cycloheximide and
puromycin abrogated the induction of PRLR gene transcription at 1 hr and 2 hr, which demonstrated that on-going
protein synthesis was required for the
ORG 2058 effect and suggested that
progestins may exert some transcriptional effects via the induction of an intermediary
protein. These experiments demonstrated that
progestin induced a transcriptionally based increase in PRLR gene expression and provided a mechanism by which
progesterone may modulate the mitogenic activity of
prolactin during mammary gland development.