Subcellular distribution of [3H]-dexamethasone mesylate binding sites in Leydig cells using electron microscope radioautography

The present view is that glucocorticoid hormones bind to their cytoplasmic receptors before reaching their nuclear target sites, which include specific DNA sequences. Although it is believed that cytoplasmic sequestration of steroid receptors and other transcription factors (such as NFKB) may regulate the overall activity of these factors, there is little information on the exact subcellular sites of steroid receptors or even of any other transcription factors. Tritiated (3H)-dexamethasone 21-mesylate (DM) is an affinity label that binds covalently to the glucocorticoid receptor (GR), thereby allowing morphological localization of the receptor at the light and electron microscope levels as well as for quantitative radioautographic (RAG) analysis. After injection of 3H-DM into the testis, a specific radioautographic signal was observed in Leydig cells, which correlated with a high level of immunocytochemically demonstrable GR in these cells at the light-microscope level. To localize the 3H-DM binding sites at the electron microscope (EM) level, the testes of 5 experimental and 3 control adrenalectomized rats were injected directly with 20 microCi 3H-DM; control rats received simultaneously a 25-fold excess of unlabeled dexamethasone; 15 min later, rats were fixed with glutaraldehyde and the tissue was processed for EM RAG analysis combined with quantitative morphometry. The radioautographs showed that the cytosol, nucleus, smooth endoplasmic reticulum (sER), and mitochondria were labeled. Since the cytosol was always adjacent to tubules of the sER, the term sER-rich cytosol was used to represent label over sER networks, which may also represent cytosol labeling due to the limited resolution of the radioautographic technique. Labeling was highest in sER-rich cytosol and mitochondria, at 53% and 31% of the total, respectively