Compared intracellular localization of the glucocorticosteroid and progesterone receptors: an immunocytochemical study

The intracellular distribution of the glucocorticosteroid and progesterone receptors (GR and PR, respectively) was studied immunohistochemically. In control adrenalectomized (Adx) rat liver, immunostaining of paraffin sections revealed GR in cell nuclei, with a wide range of intensity between individuals. Following dexamethasone (Dex) treatment, the nuclear staining was uniformly high in all animals; the cytoplasmic staining was always weak and remained unchanged after Dex treatment. In frozen sections, the GR immunoreactivity in cell nuclei was weak in the absence and very strong in the presence of Dex, while no GR-specific cytoplasmic staining was observed. In frozen sections fixed in vapor of formaldehyde to avoid any artifactual redistribution of the receptor, some GR immunostaining was observed in the cytoplasm and the nucleus. In contrast, in paraffin as well as in frozen sections of chick oviduct, fixed by immersion or in vapor, PR was exclusively nuclear, including in the absence of progesterone, and the intensity of immunostaining was not modified by progesterone treatment. In order to verify if loss of nuclear receptors during tissue preparation could explain the differences in nuclear immunostaining observed between hormone-free and hormone-occupied GR, and between GR and PR, frozen sections of Adx rat liver and chick oviduct were preincubated at 4 degrees C in buffer solutions before the fixation procedure. It was found that hormone-free GR diffused out of the nucleus faster than hormone-occupied GR nuclei, and that nuclear GR diffused faster than nuclear PR. Based on these results, we propose that, during the fixation procedure, the fraction of nuclear GR which diffuses out of the nucleus is much smaller in the presence than in the absence of Dex. This lesser loss of nuclear GR after Dex treatment results in an increase of immunostaining after hormonal administration, which might have been erroneously interpreted as a sign of translocation from cytoplasm to nucleus. That the nuclear PR detection is not modified by progesterone treatment may be explained by its reduced diffusibility as compared to nuclear GR. This hypothesis does not rule out the existence of some cytoplasmic GR, whose significance remains unclear, but it offers a unified mechanism of action for all steroid hormone receptors. In the case of glucocorticosteroids, as already proposed for estradiol and progesterone, no step of cytoplasm to nucleus translocation would be required for hormone action, and transformation-activation would occur in the nucleus, resulting in tighter binding of the hormone receptor complexes.(ABSTRACT TRUNCATED AT 250 WORDS)