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Few Things in Life are "Free": Cellular Uptake of Steroid Hormones by an Active Transport Mechanism

Departments of Pharmaceutical Chemistry and Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94143-2280

SUMMARY

Conventional dogma holds that steroid hormones traverse cell membranes passively, owing to their lipophilic nature. The recently characterized protein megalin, however, functions as a transport protein on cell surfaces to carry steroids across the plasma membrane. Upon hydrolysis of steroid-associated binding globulins in lysosomes, free hormone is liberated and may exert its effects in the cell. Megalin-independent mechanisms of steroid uptake are likely important too, as the phenotypes of megalin-deficient mice do not completely mimic the phenotypes of androgen receptor– or estrogen receptor–null mice.