Decrease of Hormone Binding Capacity of Estrogen Receptor by Calcium |
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Abstract: | AbstractWe have adressed the question as to whether calcium may modify the 3H]estradiol (3H]E2) binding properties of the estrogen receptor (ER). A human recombinant full length ER (yER) expressed in yeast was used to limit the potential interference of ER-associated proteins and proteases present in the target tissues.Ca++ (0.1–10 mM) always produced an important loss of 3H]E2 binding capacity without any effect on the hormone binding affinity of residual receptors. This loss was reflected in a decrease of immunoreactivity for monoclonal antibodies raised against the hormone binding domain. An ER recombinant expressing solely this domain confirmed that the ion operated at this level. Binding of 125I]Z-17 α-(2-iodovinyl)-11β-chloromethyl estradiol-17β (an compound with very high selectivity for ER) as well as 125I]tamoxifen aziridine were similarly affected. Size-exclusion chromatography failed to reveal the emergence of any ER isoforms of low molecular weight rejecting the hypothesis of a Ca++- induced proteolysis. In agreement with this conclusion, EDTA reversed the loss of 3H]E2 binding capacity. Phosphoamino acids (PY, PT and PS) partly antagonized the effect of Ca++ suggesting its interaction with phosphoamino acid residues. Worthy of note, the effect of Ca++ appeared more marked when assessed by DCC than HAP assay. The phosphocalcic nature of the HAP matrix may explain this phenomenon which was observed with cytosolic ER from various origins. |
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