Internalization of Sex Hormone-Binding Globulin into Neurons and Brain Cells in vitro and in vivo

Abstract

Background: Sex hormone-binding globulin (SHBG) is a 94-kDa homodimer that binds steroids and is made in the hypothalamus. We have demonstrated that infusions of SHBG into the hypothalami of rats increase their female sexual receptivity except when SHBG is coupled to dihydrotestosterone (DHT) suggesting that SHBG has an active function in behavioral neuroendocrinology. Methods: This study examines the possibility that SHBG is internalized by neuronal and/or non-neuronal brain cells as one possible mode of action using in vitro and in vivo techniques. Results: First, analysis of the uptake of radiolabeled SHBG (¹²⁵I-SHBG) found ¹²⁵I-SHBG uptake in HT22 hippocampal cells stably transfected with cDNA for ERβ (HT22-ERβ). The addition of DHT to ¹²⁵I-SHBG significantly inhibited ¹²⁵I-SHBG uptake in HT22-ERβ cells but not in HT22-ERα or HT22 wild-type cells. SHBG internalization was specific as it did not occur in either the human neuroblastoma cell line SK-N-SH or the glioma cell line C6. Second, SHBG was labeled with a fluor (Alexa-555^TM), and infused into the lateral cerebroventricles of ovariectomized rats. Optimal SHBG uptake was seen 10 min after these infusions. SHBG uptake was seen in specific parts of the choroid plexus and periventricular cells as well as into cells in the paraventricular nucleus, the medial forebrain bundle, and the habenula. Conclusions: These studies suggest that SHBG is internalized by brain cells, which may be affected by the presence of ERβ. The gonadal steroids have numerous effects in brain and the discovery that the steroid-binding protein SHBG is taken up into neurons and brain cells may demand a change in thinking about how steroids are delivered to brain cells to affect neurophysiology.

Cellular uptake of steroid carrier proteins—Mechanisms and implications

“…Last but not least, surface binding sites have been proposed to mediate the cellular uptake of carrier-bound steroid hormones. For example, uptake of SHBG has been documented in epithelial cells of the epididymis and endometrium (Feldman et al, 1981;Gerard et al, 1988;Noe et al, 1992), in MCF-7 cells (Porto et al, 1991), and in neurons in the brain (Caldwell et al, 2007). A biological process, where uptake of SHBG is suggested to play a physiological role is the delivery of testosterone to principal cells in the epididymis (Fig.…”

Identification of sex hormone binding globulin-interacting proteins in the brain using phage display screening

“…It is possible that one function of SHBG in the brain is due to actions at the level of myelin in brain fiber tracts. SHBG internalization was often associated with such tracts in the brain (15). It is also interesting to note that MBP is present in various different cell types including lymphoid cells and all types of myeloid lineage cells (i.e., macrophages, dendritic cells, granulocytes, megakaryocytes, and erythroblasts) (56).…”

“…SHBG internalization has been demonstrated in multiple tissues (7)(8)(9)(11)(12)(13)54) including the brain (15) suggesting that SHBG along with associated steroids are selectively brought into cells, where cytosolic proteins may bind the SHBG. This work may have identified proteins that are candidates for an intracellular SHBGbinding complex.…”

“…Other studies (5,6) also suggested that there are membrane-associated receptors for SHBG in the uterus. These membrane-associated receptors may be important for internalization of SHBG, which was observed in many tissues (7)(8)(9)(10)(11)(12)(13)(14) including the brain (15). The Nykjaer and Willnow laboratory demonstrated that a membrane-associated protein called megalin, a 600-kDa member of the LDL receptor gene family, is capable of binding to and facilitating the internalization of both SHBG and its bound steroid (16).…”

“…After binding to membraneassociated receptors, it is possible that SHBG, its associated steroid, and the SHBG receptor are all internalized into brain cells. Caldwell et al (15) demonstrated uptake of SHBG in vivo in the choroids plexus, paraventricular and supraoptic nuclei, and in specific periventricular cells. Despite observations that suggest the presence of both central and peripheral SHBG membrane receptors, the identity of brain SHBG receptors has not been elucidated.…”