Steroid Hormone Agonists and Antagonists in the Treatment of Cancer

Definition of the critical cellular components which distinguish between hormone and antihormone activated progesterone receptor

The steroid hormone progesterone is a key modulator of the cellular processes associated with the maintenance and development of female reproductive function. The biological activity of this hormone is mediated by specific nuclear receptors located in target cell nuclei which upon activation are capable of modulating the transcriptional activity of promoters containing progesterone response elements. Abnormalities in the progesterone receptor (PR) signal transduction pathway are implicated in pathological states such as breast cancer, endometriosis, and uterine fibroids. As a result of the medical need to modulate PR transcriptional activity, antiprogestins, compounds which oppose the actions of progesterone and novel progesterone receptor agonists, have been developed. This review outlines our current understanding of the critical cellular components which define the pharmacology of progesterone receptor agonists and antagonists, and how this information will impact the discovery and development of additional therapeutics.

Progesterone receptor and the mechanism of action of progesterone antagonists

Currently available progesterone antagonists have been suggested to fall into two categories based on differences in how they interact with and inactivate the progesterone receptor (PR). The anti-progestin ZK98299 (Type I) impairs PR association with DNA, while Type II compounds (RU486, ZK112993, ZK98734) promote PR binding to DNA. Type II agents, therefore, appear to inhibit receptor activity at a step downstream of DNA binding, presumably failing to induce conformational changes in PR structure requird for enhancement of transcription. This paper discusses both published and unpublished data supporting the concept of two types of progestin antagonists. Using PR-mediated induction of reporter genes in breast cancer cells as an assay for biological response, both types of anti-progestins, after correction for difference in steroid binding affinity, inhibit progestin induction substoichiometrically. However, Type II anti-progestins are more potent, inhibiting at lower ratios of antagonist to agonist than ZK98299. This suggests that in addition to behaving by classical competitive mechanisms these compounds (in particular Type II) may exhibit additional activity as transrepressors of PR in the same cell bound to hormone agonist. Transrepression may occur by the combined mechanisms of heterodimerization and competition for binding to DNA. In support of this, mixed ligand dimers form readily in solution between a PR subunit bound to agonist and another bound to either type of anti-progestin, whereas these mixed ligand dimers bind poorly, if at all, to specific progesterone response elements (PREs) in vitro. Additionally, when added as a single ligand, Type II agents increase PR dimerization in solution and PR affinity for PREs as compared with single ligand dimers formed by progestin agonist. This contrasts with ZK98299, when given as a single ligand, which reduces PR affinity for PREs without disrupting solution dimerization. Thus the higher affinity of PR for PREs may account for the greater biological potency of Type II compounds as compared with ZK98299. As a further distinction between types of antiprogestins, ZK98299 minimally stimulates phosphorylation of PR whereas RU486 increases site-specific phosphorylation of PR in a manner indistinguishable from that of hormone agonist. Additionally, ZK98299 is not susceptible in vivo to functional switching to a partial agonist by cross talk with cAMP signal transduction pathways, as occurs with Type II compounds. Thus, ZK98299 under certain conditions may be a more pure antagonist than Type II compounds.

Signal-transduction therapy. A novel approach to disease management

In the past decade it has become apparent that many diseases result from aberrations in signaling pathways. These include proliferative diseases such as cancers, atherosclerosis and psoriasis and inflammatory conditions such as sepsis, rheumatoid arthritis and tissue rejection. These findings refocused the research of the medical community to seek new modalities for disease management which essentially consist of designing drugs which intercept cell signaling. In this review, the emerging success in using tyrosine kinase blockers and other signal interceptors, such as protein kinase C blockers, Ras blockers, Ca2+ signaling inhibitors and estrogen antagonists which inhibit growth of cancer cells in vitro and in vivo, will be discussed. These signal interceptors, especially tyrosine-kinase blockers, are also able to block inflammatory responses and the proliferation of vascular smooth muscle cells and psoriatic keratinocytes. The utility of signal interceptors in analyzing signal-transduction pathways is also discussed.

The mechanism of action of steroid hormones: a new twist to an old tale

Steroid hormones, vitamins, and thyroid hormone are potent chemical messengers that exert dramatic effects on cell differentiation, homeostasis, and morphogenesis. These molecules, though diverse in structure, share a mechanistically similar mode of action. The effector molecules diffuse across cellular membranes and bind to specific high affinity receptors in the target cell nuclei. This interaction results in the conversion of an inactive receptor to one that can interact with the regulatory regions of target genes and modulate the rate of transcription of specific gene sets. The recent cloning and characterization of the functional receptors for these hormones has been enlightening as to the individual steps involved in steroid signal transduction. In addition, emerging evidence suggests that receptor function can be influenced by cell and promoter context indicating that it may be possible to develop tissue specific or tissue-restricted drugs. The concept that a single receptor can modulate gene transcription in a cell-specific manner is of great medical and pharmaceutical importance. The focus of this review is to highlight the recent developments in the steroid receptor field and to illustrate the novel approaches been undertaken to identify novel pharmaceuticals.

Nuclear hormone receptors as targets for new drug discovery

There are two basic types of receptor transducing systems: those which utilize membrane bound receptors and are activated at the cell surface by the appropriate hormone and transmit their signal to the internae of the cell via a second messenger (i.e. cAMP), and those that utilize internal, cytoplasmic or nuclear receptors (intracellular receptors) which upon activation by hormones interact directly with DNA and alter the genetic program of a cell. This review focuses on the mechanism of action of these intracellular receptors and discusses how such an understanding is expected to facilitate the discovery of new therapeutic agents.