Internalization of Sex Hormone Binding Globulin into Fibroblast 3T3 Cells

Introduction

A recent volume of this journal compiled articles suggesting there are “Emerging Roles of Steroid Binding Globulins” (Hormone and Metabolic Research, Volume 38, Issue 4, April, 2006). One such binding globulin of interest is sex hormone binding globulin (SHBG), which has been demonstrated to be internalized in many tissues (see [1]). Hryb et al. [2] solubilized a membrane-associated receptor for SHBG and discovered that when SHBG was bound to dihydrotestosterone (DHT) it did not bind to this receptor, suggesting that SHBG-DHT acts as an antagonist at the SHBG receptor. Nakhla, Khan, and Rosner [3] developed an assay for SHBG uptake whereby surface-bound SHBG was washed off with a protease (Pronase E), so that any labeled SHBG remaining in cells after the pronase E treatment was determined to have been internalized by those cells. Nakhla, Khan, and Rosner [3] did not find SHBG uptake into differentiated prostate cell lines. However, non-tumorigenic 3T3 fibroblast cells, that do not have either androgen receptors or estradiol receptor-α (ERα) [4] [5], still are stimulated to become tumorigenic by steroids. If such cells internalize SHBG, it may be a novel mechanism for steroid-induced tumorigenesis. We have suggested that the neuropeptide oxytocin, which is co-localized in the same synaptic vesicles with SHBG [6], interacts with a membrane-associated SHBG receptor. Here we utilize a modification of the Pronase E method to examine SHBG internalization with or without DHT, estradiol, or oxytocin in the fibroblast cell line NIH3T3 versus the pheochromocytoma adrenal medullary cell line PC-12. We have utilized this method to demonstrate SHBG uptake into mouse hippocampal cells stably transfected with cDNA for ERβ[7].

References

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