Receptors specific for thyroid hormones in nuclei of mammalian erythroid cells: involvement in erythroid cell proliferation.

Abstract

In search of evidence relating the effects of thyroid hormone on erythropoiesis to mechanisms at the chromatin level, we tested nucleated erythroid cells for the presence of specific receptors. Cells from spleens of hypoxic adult hamsters, 99% erythroid, were exposed for selected time periods to concentrations of 125I‐labelled triiodothyronine (T3) and thyroxine (T4) bracketing the physiologic ranges. Specific binding was assayed as the amount of labelled hormone displaced by subsequent addition of an excess of non‐labelled hormone. Saturable sites for both T3 and T4 were demonstrated, limited to nuclei. Binding to other sub‐cellular organelles and to plasma membrane was to low‐affinity sites only. Analysis of Scatchard plots revealed approximately 9200 binding sites per nucleus for either hormone, with a dissociation constant (Kd) of 2 · 7 nM for T3 and 20 · 5 nM for T4. The biological effect of 125I‐T3 on erythropoiesis was assayed in vitro. Erythroid colony growth was enhanced to the same extent by the radiolabelled and non‐labelled form of T3, showing that radioiodination had not altered this biological activity.