Progesterone altered amino acid accumulation by human endometrium in vitro

When human endometrium is exposed to progesterone, the rate of amino acid incorporation into its protein pool is altered. Accumulation into specific proteins has been evaluated by one-dimensional polyacrylamide gel electrophoresis. Comparison of steroid-exposed and nonexposed tissue has revealed a change in the rate of radiolabeled amino acid uptake into a few stained protein bands. To further characterize the effect of progesterone on endometrial protein metabolism, we have examined the one-dimensional electrophoretic band that undergoes the greatest change in leucine accumulation using two-dimensional gel and monoclonal antibody technologies. The band of interest results from altered amino acid uptake into a single protein spot on two-dimensional gels. This particular protein has an approximate size of 50 kDa and pI of 5.8. It exists in a monomeric form under nondenaturing conditions. During organ culture of proliferative endometrium exposed to 0.1 micrograms/ml progesterone, there is a sixfold increase in the rate of methionine uptake into this protein. The rate of amino acid accumulation is maximal after 24 h of culture in medium containing progesterone. Partial purification provided a product enriched 120-fold for this specific protein and facilitated development of a monoclonal antibody that was used to identify the protein in several human tissues besides endometrium. Antibody-antigen complex could not be detected in medium from secretory endometrium, suggesting that this protein resides in the cytosol without being secreted by the tissue.