Progesterone receptor synthesis and degradation in MCF-7 human breast cancer cells as studied by dense amino acid incorporation. Evidence for a non-hormone binding receptor precursor

We have used the technique of density labeling of proteins by biosynthetic incorporation of ²H, ¹³C, ¹⁵N (dense) amino acids to study the synthesis and degradation rates of the progesterone receptor in MCF-7 human breast cancer cells. In cells grown in the absence of progestin, sucrose gradient shift analyses reveal that it takes 17 h for the normal density progesterone receptor levels to be reduced to half the initial value, whereas in the presence of 10 nM of the synthetic progestin, [³H]R5020, the receptor turns over more rapidly, such that the normal density R5020-occupied progesterone receptor complexes are reduced to half in 12 h. The accelerated progesterone receptor turnover in the presence of [³H]R5020 reflects increased turnover rates of both the A (M(r)-85,000) and B (M(r)-115,000) subunits, as determined by sodium dodecyl sulfate gel analyses of dense and light receptors photoaffinity labeled with [³H]R5020. In both control and progestin-exposed cells, the time course of progesterone receptor turnover shows a lag of approximately 6 h after dense (¹⁵N, ¹³C, ²H) amino acid exposure, before dense hormone binding receptor species are seen and before normal density progestin binding activity starts decreasing. Since our evaluations of progesterone receptor depend upon its binding of radiolabeled ligand ([³H]R5020), this lag in the density shift kinetics would be consistent with the presence of a non-hormone binding biosynthetic precursor, from which the hormone-binding form of progesterone receptor is derived. A kinetic model is used to analyze the lag-decay profiles and to determine the rate constants for progesterone receptor synthesis, activation to the hormone-binding form, and degradation.