Immunocytochemical localization of the sex steroid-binding protein of plasma in tissues of the adult monkey Macaca nemestrina.

The sex steroid-binding protein (SBP) present in the serum of the monkey Macaca nemestrina is shown to exist in cells of tissue involved in reproduction. The localization was demonstrated by immunofluorescence with monospecific antibodies raised against homogeneous human SBP. These antibodies were previously shown to crossreact with monkey SBP. The protein appears to be located in the cytoplasm of epithelial cells lining the prostate alveoli, the ducti of the epididymis, and the seminiferous tubula of the testes of the monkey. The protein is also present in the cytoplasm of parenchymal cells of the liver, where SBP is believed to be synthesized, and in cells of the adrenal cortex, where steroids are known to be synthesized. Controls appear dark and illustrate specificity of the immunofluorescence, ruling out both tissue autofluorescence and other nonspecific interactions. In all cases, the relative intensity of fluorescence appears minimal in the nuclei of cells. Experiments performed with cultured MCF-7 cells indicate that SBP can across the plasma membrane and enter into the cytoplasm but not into the nucleus. Additional studies indicate that the monospecific antibodies do not crossreact with the monkey prostate androgen receptor, as shown by ultracentrifugation in sucrose densty gradients. The physiological significance of these observations is not known; however, the existence of this protein in cells of target tissues for sex steroids introduces a new dimension in our thinking about the role of this protein in androgen action.