Progesterone-Binding Proteins: In Vitro Binding and Biological Activity of Different Steroidal Ligands

Abstract

Progesterone-binding proteins from human, rabbit, sheep and guinea pig myometrial cytosol, all induced with oestradiol, as well as from pregnant guinea pig myometrium and plasma were investigated. The physico-chemical properties of the oestradiol-induced binding proteins were very similar in all the species studied. In all, 63 steroids were tested for their ability to compete with tritiated progesterone for the binding sites on these six proteins and their relative affinities were determined. The studies reveal that the ligand specificities of oestrogen-induced myometrial binding proteins from human, rabbit and sheep are rather similar, whereas that from guinea pig myometrium has different binding characteristics. The properties of the binding proteins from pregnant guinea pig uterus and plasma differ substantially from all of the induced proteins. It is clear from the different physico-chemical characteristics and binding specificities that the oestrogen-induced myometrial protein of the guinea pig is not the same as that which appears in the myometrium and plasma during pregnancy. The binding energies of the well bound progestational compounds were of the order of −12 Kcal/mole, half of which stems from the interaction of the steroid nucleus with the protein. The specific interaction of the protein with the two functional groups, the 3-keto-4-ene system and the acetyl side chain each contributed −3 Kcal/mole. In the case of the rabbit, sheep and human proteins a 17α-ethynyl-17β-hydroxyl function could replace the acetyl side chain. For a large number of steroids reasonable agreement existed between the degree of binding to the rabbit myometrial protein and in vivo biological activity (Clauberg-McPhail test) in the same species. The data suggest that as far as the binding aspect is concerned, the rabbit is an appropriate model for assessing the biological activity of compounds under development for human application. The in vitro binding system is also a useful tool to assess whether steroids need to be bio-activated before eliciting a biological response.

Estrogen-induced loss of progesterone receptor expression in normal and malignant ovarian surface epithelial cells

“…Yet, it acted as a mitogen in MCF-7 cells. It has been widely established that estrogens induce PR expression not only in female reproductive tissues but also in normal and malignant mammary epithelial cells (Kontula et al, 1975;Nardulli et al, 1988;Wei et al, 1988). In sharp contrast, the present study revealed significantly decreased PR-A and PR-B expression in HOSE or OVCA cell lines following exposure to an estrogen.…”

Antibodies to rabbit progesterone receptor: crossreaction with human receptor.

“…If this hypothesis were true, then human progesterone receptor would have more such determinants in common with rabbit receptor than the guinea pig and the rat receptor. Similarity in ligand specificity between human and rabbit progesterone receptors has been reported (8). These crossreactions with the corresponding receptor from various species are similar to observations made for estrogen receptors (5); however, a difference resides in the fact that antirabbit progesterone antibodies do not interact with a nonmammalian receptor (from chicken oviduct) whereas the anti-calf estrogen receptor antibodies do interact with the chicken oviduct estrogen receptor (3).…”

Progesterone Receptor of Adult Rabbit Lung

“…The [3H]R5020 specific binding was sensitive to proteases and sulfhydryl re agents but not to nucleases, indicating ste roid binding stability depended on protein and sulfhydryl group integrity. Furthermore, the general pattern of efficacy shown in the competition studies parallels the patterns observed for progesterone receptors of both rabbit and human uterus [Kontula et al, 1975;Laumas and Kasid, 1980]. The most potent competitors (R5020, norgestrel.…”