Estrogen Receptor and Progesterone Receptor Synthesis and Degradation in Breast Cancer Cells (Turnover, Precursor, Density Shift, Dense Amino Acids)
Mullick, Alaka
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https://hdl.handle.net/2142/71442
Description
Title
Estrogen Receptor and Progesterone Receptor Synthesis and Degradation in Breast Cancer Cells (Turnover, Precursor, Density Shift, Dense Amino Acids)
Author(s)
Mullick, Alaka
Issue Date
1986
Department of Study
Physiology and Biophysics
Discipline
Physiology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Animal Physiology
Abstract
This study was aimed at determining synthesis and degradation rates of the estrogen and progesterone receptors in a human breast cancer cell line and investigating the effect of ligand (both agonist and antagonist) binding on these rates.
To perform these studies, a powerful technique, viz., density labelling of proteins, has been used. Cells are exposed to dense (('14)N,('12)C,('2)H) amino acids so that newly synthesized proteins are of a higher density than the preexistent (('14)N,('12)C,('1)H-normal density amino acid) pool of proteins and by virture of differential migration on sucrose gradients the "old-normal" proteins can be distinguished from the "new-dense" proteins.
Using dense amino acid labelling we have determined that in the MCF-7 human breast cancer cells, the estrogen receptor is a rapidly turning over protein with a half-life of ca. 4 h. Ligand binding (both agonist and antiagonist) does not markedly influence the rates of synthesis and degradation. A comparison of the time course of density labelling of the nuclear 5 S estrogen receptor complex with that of the 4 S monomeric form of the receptor indicates that the 5 S complex is composed of subunits that turnover with similar half lives (ca. 4 h). The data are consistent with a homodimeric model for the 5 S complex.
In the same cell line, the progesterone receptor whose levels are stimulated by estrogen treatment, is a somewhat more long lived protein. The dense amino acid labelling of progesterone receptor does not follow linear kinetics and suggests the existence of a biosynthetic precursor that lacks the ability to bind hormone. In the unoccupied state the progesterone receptor levels are reduced to half in ca. 17 h. However, the R5020-receptor complexes are degraded more rapidly such that half initial levels are attained by ca. 12 h. Because of the nonlinear kinetics, 17 and 12 h do not represent accurate half-lives. A kinetic model has been used to determine more accurate valves and to gain information about precursor pool size and activation rate.
Data are also presented to show that binding of antihormones seem to confer on the receptor a higher propensity to aggregate than their agonist counterparts. (Abstract shortened with permission of author.)
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