Circadian rhythm of hepatic cytosolic and nuclear estrogen and androgen receptors

Abstract

Mammalian liver is a sex steroid-responsive tissue. The effects of these hormones presumably are mediated by hepatic estrogen receptors (ER) and androgen receptors (AR). Serum levels of sex hormones display circadian rhythms. Further, estrogens and androgens are commonly administered; administration of these agents is associated frequently with liver disease. Therefore, we investigated whether the cytosolic and nuclear sex steroid receptors also display a similar circadian rhythm, and whether variations occurred in the distribution of receptors between cytosolic and nuclear compartments. Animals were killed every 4 h from midnight till the following midnight; cytosolic and nuclear levels of both ER and AR were measured. Cytosolic ER reached a maximum level at 4 AM, and a minimum at 8 PM and midnight of both days. Nuclear ER was highest at 8 AM and lowest at 4 PM and 8 PM, a pattern which parallels variations in serum estradiol levels. Cytosolic AR was highest at 8 PM and lowest at midnight and 4 AM. Nuclear AR was highest at 4 AM and lowest at 4 PM and 8 PM. The highest level of nuclear AR does not correspond to the maximum serum testosterone level, which occurred at 4 PM. The total hepatic content of both ER and AR was not constant over the 24-h period, but varied considerably with time of day. These studies suggest that both ER and AR show a distinct circadian rhythm in subcellular compartmentalization, and that total hepatic content of ER and AR varies significantly during a 24-h period.

Figure 1

Specific binding of [³H]E₂ in hepatic cytosol of the male rat. Aliquots (200 μl)of cytosol (5 mg protein/ml) precipitated with protamine sulfate were incubated with six different concentrations of [³H]E₂ (0.15–3.0 nM) for 18 h at 0°C in the absence (total binding) and presence (nonspecific binding) of 100-fold excess of unlabeled E₂. Specific binding was calculated by subtracting nonspecific binding from total binding. Each point is the mean ± SEM of triplicate determinations. The insert shows the Scatchard analysis of [³H]E₂ specific binding.

Figure 2

Specific binding of [³H]R1881 in cytosol of the normal male rat. Aliquots (200 μl)of cytosol were incubated with several different concentrations of [³H]R1881 (0.2–4.0 nM) for 18 h at 4°C in the absence (total binding) and presence (nonspecific binding) of 100-fold excess unlabeled R1881. Specific binding was calculated by subtracting nonspecific binding from total binding. Each point is the mean ± SEM of triplicate determinations. In the insert, the Scatchard plot of specific binding is indicated.

Figure 3

Variation in specific [³H]E₂ binding in male rat liver during a 24-h period. Specific [³H]E₂ binding is expressed as picomoles per gram of liver, for total liver content (panel A). cytosolic (panel B) and nuclear (panel C) binding. Serum estradiol levels are shown in panel D.

Figure 4

Variation in specific [³H]-R1881 binding in male rat liver during a 24-h period. Specific [³H]-R1881 binding is expressed as femtomoles per gram of liver, for total liver content (panel A). cytosolic (panel B), and nuclear (panel C) binding. Serum testosterone levels are shown in panel D.