Correlation among agonist dose, rate of import, and transcriptional activity of liganded progesterone receptor B isoform in living cells

Abstract

Purpose: To determine the correlation among progesterone dose, rate of import of progesterone-occupied progesterone receptor (PR) complexes into the nucleus of cells, and transcriptional activity of progesterone-PR complexes.

Methods: Live cell imaging and time-lapse microscopy of green fluorescent protein-tagged PR were performed to measure the rate of import into the nucleus of progesterone-PR complexes. To measure transcriptional activity, a progesterone-PR-sensitive luciferase reporter gene assay was used.

Results: For low doses of progesterone, there was a correlation among dose, import into the nucleus, and transcriptional activity. At higher doses of progesterone (beyond 12.5 nM), transcriptional activity increased, but there was no further increase in the rate of import, indicating a saturation of the import machinery. In both cases, a simple one-compartment model was sufficient to describe the import data.

Conclusions: At low doses, progesterone dose correlates well with rate of import and transcriptional activity. At high doses, more progesterone can get into the nucleus and can activate unoccupied receptors already in the nucleus, leading to higher transactivation than would be expected from rates of import of progesterone-PR complexes into the nucleus.