Estradiol Binds to a Receptor-like Cytosol Binding Protein and Initiates a Biological Response in Paracoccidioides Brasiliensis

Paracoccidioidomycosis, a disease caused by Paracoccidioides brasiliensis, which is endemic to Latin America, is much more common in men than women, suggesting a role for hormonal factors. We recently showed that two other yeasts possess steroid binding proteins and postulated that these receptor-like molecules represented a mechanism by which the hormonal milieu of the host might influence an infecting pathogen. Therefore, we examined P. brasiliensis for a sex steroid binding protein. Because tritiated steroids rapidly dissociated from the other fungal binding proteins, we developed a fast binding method with Sephadex G-50 microcolumns speeded by centrifugation. This method detected specific binding of [3H]estradiol in P. brasiliensis cytosol. Other tritiated steroid hormones, including testosterone and corticosterone, failed to exhibit specific binding. Scatchard analysis of [3H]estradiol binding showed an apparent dissociation constant (Kd) of 1.7 X 10(-8) M and a maximal binding capacity (Nmax) of 235 fmol/mg of protein. Susceptibility to trypsin indicated the binding site was protein in nature. The protein had a Stokes radius of approximately equal to 32 A by HPLC exclusion column and a sedimentation coefficient of 4.4 S by sucrose gradient, consistent with an apparent Mr of approximately equal to 60,000. Competition experiments revealed that estrone, estriol, and progesterone had 25% of the affinity of estradiol, whereas diethylstilbestrol, androgens, and corticosteroids had low affinity. Investigation of steroid hormone actions in P. brasiliensis indicated that estradiol inhibited the fungal transformation from mycelial form to yeast form, the initial step of infection. This suppressive effect was dose-dependent and not found with testosterone. We hypothesize that endogenous estrogens in the host, acting through the cytosol binding protein in the fungus, inhibit mycelial-to-yeast transformation, thus explaining the resistance of women to paracoccidioidomycosis.