Steroid hormone receptors

Retinoic acid receptors at 35 years

For almost a century, vitamin A has been known as a nutrient critical for normal development, differentiation, and homeostasis; accordingly, there has been much interest in understanding its mechanism of action. This review is about the discovery of specific receptors for the vitamin A derivative, retinoic acid (RA), which launched extensive molecular, genetic, and structural investigations into these new members of the nuclear receptor superfamily of transcriptional regulators. These included two families of receptors, the RAR isotypes (α, β, and γ) along with three RXR isotypes (α, β, and γ), which bind as RXR/RAR heterodimers to cis-acting response elements of RA target genes to generate a high degree of complexity. Such studies have provided deep molecular insight into how the widespread pleiotropic effects of RA can be generated.

Cracking the estrogen receptor's posttranslational code in breast tumors

Estrogen signaling pathways, because of their central role in regulating the growth and survival of breast tumor cells, have been identified as suitable and efficient targets for cancer therapies. Agents blocking estrogen activity are already widely used clinically, and many new molecules have entered clinical trials, but intrinsic or acquired resistance to treatment limits their efficacy. The basic molecular studies underlying estrogen signaling have defined the critical role of estrogen receptors (ER) in many aspects of breast tumorigenesis. However, important knowledge gaps remain about the role of posttranslational modifications (PTM) of ER in initiation and progression of breast carcinogenesis. Whereas major attention has been focused on the phosphorylation of ER, many other PTM (such as acetylation, ubiquitination, sumoylation, methylation, and palmitoylation) have been identified as events modifying ER expression and stability, subcellular localization, and sensitivity to hormonal response. This article will provide an overview of the current and emerging knowledge on ER PTM, with a particular focus on their deregulation in breast cancer. We also discuss their clinical relevance and the functional relationship between PTM. A thorough understanding of the complete picture of these modifications in ER carcinogenesis might not only open new avenues for identifying new markers for prognosis or prediction of response to endocrine therapy but also could promote the development of novel therapeutic strategies.

Mediation of estrogen mitogenic effect in human breast cancer MCF-7 cells by PC-cell-derived growth factor (PCDGF/granulin precursor)

PC-cell-derived growth factor (PCDGF) is an 88-kDa glycoprotein corresponding to the granulin precursor. We have reported that PCDGF was expressed in human breast cancer cells. In estrogen-receptor positive cells, 17-beta-estradiol (E(2)) transcriptionally stimulated PCDGF expression in a dose- and time-dependent fashion. We demonstrate here that PCDGF mediates the mitogenic effect of E(2) in MCF-7 cells. PCDGF substituted for E(2) to stimulate DNA synthesis. The E(2) mitogenic effect was inhibited in a dose-dependent fashion by anti-PCDGF neutralizing antibody. Inhibition of PCDGF expression by antisense transfection also inhibited the E(2) mitogenic effect. In contrast, overexpression of PCDGF in MCF-7 cells resulted in cells that were able to proliferate in the absence of estrogen and were tamoxifen resistant. The PCDGF signaling pathway was examined. Like E(2), PCDGF stimulated mitogen-activated protein kinase activity. PCDGF could substitute for E(2) in stimulating cyclin D1 expression. The cyclin D1 stimulation by E(2) was 50% inhibited by anti-PCDGF antibody. In contrast, PCDGF did not stimulate c-myc expression, another molecular target of E(2). We conclude that autocrine PCDGF mediates the E(2) mitogenic effect via stimulation of cyclin D1. These studies provide information on estrogen action and identify an autocrine molecular target in human breast cancer cells.

Mediation of estrogen mitogenic effect in human breast cancer MCF-7 cells by PC-cell-derived growth factor (PCDGF/granulin precursor)

PC-cell-derived growth factor (PCDGF) is an 88-kDa glycoprotein corresponding to the granulin precursor. We have reported that PCDGF was expressed in human breast cancer cells. In estrogen-receptor positive cells, 17-beta-estradiol (E(2)) transcriptionally stimulated PCDGF expression in a dose- and time-dependent fashion. We demonstrate here that PCDGF mediates the mitogenic effect of E(2) in MCF-7 cells. PCDGF substituted for E(2) to stimulate DNA synthesis. The E(2) mitogenic effect was inhibited in a dose-dependent fashion by anti-PCDGF neutralizing antibody. Inhibition of PCDGF expression by antisense transfection also inhibited the E(2) mitogenic effect. In contrast, overexpression of PCDGF in MCF-7 cells resulted in cells that were able to proliferate in the absence of estrogen and were tamoxifen resistant. The PCDGF signaling pathway was examined. Like E(2), PCDGF stimulated mitogen-activated protein kinase activity. PCDGF could substitute for E(2) in stimulating cyclin D1 expression. The cyclin D1 stimulation by E(2) was 50% inhibited by anti-PCDGF antibody. In contrast, PCDGF did not stimulate c-myc expression, another molecular target of E(2). We conclude that autocrine PCDGF mediates the E(2) mitogenic effect via stimulation of cyclin D1. These studies provide information on estrogen action and identify an autocrine molecular target in human breast cancer cells.

Interaction between protein tyrosine phosphatase and protein tyrosine kinase is involved in androgen-promoted growth of human prostate cancer cells

Steroid hormones play key roles in regulating cell proliferation and differentiation in targeting tissues. However, in advanced cancers, the steroid hormone regulation is frequently attenuated through a yet unknown mechanism even in the presence of functional steroid hormone receptors. We investigate the functional role of tyrosine phosphorylation signaling in the hormone-refractory growth of human prostate tumors. Initial studies demonstrate that the androgen-responsive phenotype of human prostate cancer cells associates with a low phosphotyrosine (p-Tyr) level of ErbB-2, which is regulated by cellular prostatic acid phosphatase (PAcP), a protein tyrosine phosphatase. In prostate cancer cells, the p-Tyr level, but not the protein level, of ErbB-2 inversely correlates with the androgen-responsiveness of cell proliferation. Androgen-stimulated cell growth concurs with a down-regulation of cellular PAcP, an elevated p-Tyr level of ErbB-2, and the activation of mitogen-activated protein kinases. Furthermore, only the ErbB-2 inhibitor AG 879, but not the EGFR inhibitor AG 1478, abolishes androgen-induced cell proliferation. Forced expression of ErbB-2 can also attenuate androgen promotion of cell growth. Data taken collectively conclude that in human prostate cancer cells, the tyrosine phosphorylation of ErbB-2 regulated by cellular PAcP plays a key role in regulating androgen-mediated proliferation signaling. Oncogene (2000).

The important prognostic value of Ki-67 expression as determined by image analysis in breast cancer

A series of 191 female breast carcinomas (with long-term follow-up) were analysed immunohistochemically (with a monoclonal MIB1 antibody) for Ki-67 (a proliferation marker) expression with special reference to well-established prognostic factors and patient survival. Expression of Ki-67 was directly related to the S-phase fraction (P < 0.0001), the volume-corrected mitotic index (P < 0.0001), histological grade (P < 0.0001), the apoptotic index (P < 0.0001), oestrogen and progesterose receptor content (P < 0.0001 for both) and p53 accumulation (P = 0.001). No correlation was found between Ki-67 expression and lymph node status (P = 0.25), metastasis at operation (n = 0.81) or tumour size (n = 0.38). The proliferation rate, as measured by image analysis of Ki-67 expression, predicted survival in the entire cohort (P = 0.001) and in axillary-lymph-node-negative (ANN) patients (P = 0.003). The difference in recurrence-free survival between the high- and low-expression groups was greatest in ANN tumours, 40% (P = 0.008). In axillary-lymph-node-positive tumours, the Ki-67 expression was not significantly related to recurrence-free survival (P = 0.723). The results of multivariate survival analysis showed that tumour size, axillary lymph node status, and mitotic index were independent prognostic factors in the entire series whereas, in ANN cases, tumour size and Ki-67 labelling were independent prognostic factors. These findings imply that Ki-67 expression could be an important prognostic determinant in breast cancer. Because of the evident loss of the predictive power of tumour size in the 1990s, the prognostic value of Ki-67 expression may even be accentuated in the currently diagnosed small breast carcinomas.

Biotin-labelled and photoactivatable aldosterone and progesterone derivatives as ligands for affinity chromatography, fluorescence immunoassays and photoaffinity labelling

New derivatives of progesterone and aldosterone were synthesized and functionally tested with commercially available antibodies. The covalent labelling of antibodies specific for aldosterone and progesterone was detected by SDS/PAGE analysis and subsequent autoradiography after using 3-(O-carboxymethyl)-oximino-(3-[125I]iodo-4-azidosalicylamidobu tylamine) derivatives of aldosterone and progesterone, respectively, as photoactivatable radioligands. Labelling was not observed in the presence of an excess of the unlabelled steroid. Aldosterone was labelled with biotin and used as a tracer in a time-resolved fluorescence immunoassay. The nonradioactive tracer is highly selective for its antibody-binding site, with almost no detectable cross-reactivity for other steroids. Biotin-labelled progesterone was immobilized by avidin-agarose and used for affinity chromatography. This yielded a more than 20-fold enrichment of an anti-progesterone polyclonal antibody. These results demonstrate that derivatives of steroids are particularly useful for the development of nonradioactive assays for the determination of natural steroids and may be also useful for the detection of specific binding sites in biological material such as plasma membranes.

Screening and characterization of estrogenic activity from a hydroxystilbene library

BACKGROUND

Compounds that either inhibit or induce an estrogen response in vivo are important as potential drugs and biochemical tools. Non-steroidal stilbene analogs such as tamoxifen are known to function as both estrogen agonists and antagonists depending upon the analog structure. This family of compounds is amenable to parallel-manifold synthesis because stilbene analogs are easily synthesized using a single-step olefination reaction.

RESULTS

We have prepared a small 23-component hydroxystilbene library using a solid phase synthesis approach. The library was screened for estrogenic and antiestrogenic activity using a cell-based bioassay that measures estrogen receptor-mediated transcription of a reporter gene. Three of the analogs proved to have dose-dependent estrogenic activity with EC50 values between 5 microM and 15 microM. Further characterization of the hydroxystilbene-mediated estrogenic activity suggests that the agonist activity results from direct binding to the steroid site on the estrogen receptor with IC50 values of 1-10 microM.

CONCLUSIONS

The results of this study show that classic olefination chemistry can be adapted to a solid-phase format for parallel synthesis of analog libraries. Although yields varied for the individual analogs, sufficient quantity of pure material was obtained directly from the resin for structural characterization and biological evaluation. This study further validates solid-phase organic synthesis as a useful approach for rapid parallel-manifold library synthesis to augment both lead compound discovery and optimization.

Reproducibility of subjective immunohistochemical estrogen- and progesterone receptor determination in human endometrium

To obtain a reliable scoring system for semi-quantitation of estrogen- (ER) and progesterone receptors (PgR) in human endometrial tissue, we investigated the reproducibility of subjective immunohistochemical ER and PgR determination. Specimens of frozen endometrial tissue were stained once (n = 129) or twice (n = 22) using the ER-ICA and PgR-ICA kits from Abbott. The semi-quantitative approach we used included subjective estimates of the overall staining intensity (I) and of the fraction of stained cells (%). Scoring was performed twice by the same observer and once by another observer (n = 87). Intra- and inter-observer agreement were evaluated using Kappa statistics. We found that more comprehensive scorings of I and % could not be agreed upon by the observers. Only simplified estimates of the fraction of cells stained and overall staining intensity were reproducible. Subjective estimates obtained by this method agreed with estimates obtained by counting (n = 38). Simplified H-scores, which were obtained by multiplication of the simplified estimates of % and I, were reproducible, too. In addition, semi-quantitation of ER and PgR by immunohistology was significantly correlated to quantitation by enzyme-immunoassay (n = 39). Thus, it was possible to reproducibly semi-quantitate ER and PgR only by employing a very simple immunohistochemical scoring of ER and PgR.

Memory-enhancing effects in male mice of pregnenolone and steroids metabolically derived from it

Immediate post-training intracerebroventricular administration to male mice of pregnenolone (P), pregnenolone sulfate (PS), dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEAS), androstenedione, testosterone, dihydrotestosterone, or aldosterone caused improvement of retention for footshock active avoidance training, while estrone, estradiol, progesterone, or 16 beta-bromoepiandrosterone did not. Dose-response curves were obtained for P, PS, DHEA, and testosterone. P and PS were the most potent, PS showing significant effects at 3.5 fmol per mouse. The active steroids did not show discernible structural features or known membrane or biochemical effects that correlated with their memory-enhancing capacity. The above, together with the findings that DHEA acted even when given at 1 hr after training and that P, PS, and DHEA improved retention over a much wider dose range than do excitatory memory enhancers, led to the suggestion that the effects of the active steroids converge at the facilitation of transcription of immediate-early genes. P and PS, for which receptors have not yet been demonstrated, may exert their effects by serving as precursors for the formation of a panoply of different steroids, ensuring near-optimal modulation of transcription of immediate-early genes required for achieving the plastic changes of memory processes. Low serum levels of P in aging and the increases of cancer and behavioral disorders in individuals receiving drugs that block synthesis of cholesterol, the immediate precursor of P, suggest possible clinical utility for P.