The effect of molybdate on the intracellular distribution of estrogen receptor in mammary tumors.

The addition of sodium molybdate (20 mM) to human breast tumor homogenates results in an increased yield of estrogen receptor sites in the cytosol, but reduces the number of sites detectable in salt extracts of purified nuclei. Only about 25% of the increase in cytosol receptor can be accounted for by the loss of nuclear sites. Addition of molybdate to isolated nuclear salt extracts has no inhibitory effect on the estrogen receptor assay, but addition of the oxyanion to the cytosol after separation of the initial nuclear pellet still gives a significant increase in estradiol binding capacity. Furthermore, addition of molybdate to the suspension from the first nuclear pellet still results in loss of binding sites in the nuclear salt extract. This rules out the possibility that the negative effect of molybdate on nuclear receptor recovery is by its inhibition of cytosol receptor activation and translocation during tissue preparation. The number of nuclei isolated from estrogen receptor-containing tumors does not differ significantly in the presence or absence of molybdate. Estrogen receptor-negative tumors however yield fewer nuclei, particularly when they are processed in the presence of molybdate. We conclude that when homogenization of tumors is carried out in the presence of molybdate, a significant fraction of estrogen receptor which would normally be present in purified nuclei is lost, perhaps by leakage through the nuclear membrane.