Antiproliferative effects of GnRH agonists: prospects and problems for cancer therapy

Gonadotropin-releasing hormone (GnRH) receptor activation has been demonstrated to inhibit cell proliferation in vitro and in vivo. These effects are dependent on the degree of receptor expression and the intracellular signaling protein milieu. The physiological and pathophysiological relevance is largely undefined, and its potential for exploitation in the treatment of specific malignancies is the subject of ongoing investigations. GnRH receptors are expressed in embryonic, juvenile and adult tissues, including brain, pituitary, gonads, accessory reproductive organs and placenta. The levels of receptor expression vary, from high in pituitary gonadotropes to low in peripheral tissues, although quantification of functional receptor protein has been determined in relatively few cell types. Roles for GnRH receptor signaling at different stages of animal development and its influence on reproductive health remain largely unexplored, except in cases of hereditary hypogonadal infertility. In addition to regulating hormone secretion, GnRH is postulated to act as a chemokine or a growth- and differentiation-inducing factor. Hence, receptor activation may influence the function of neuronal networks in the brain and the maturation of reproductive tissue epithelia. GnRH may also potentially influence the biology of cancerous cells in reproductive tissue since receptor activation may signal terminal differentiation, cell cycle arrest or apoptosis. In this context, the cell surface expression of GnRH receptor is important since it influences the intensity of intracellular signaling, and correlates with the ability to inhibit proliferation in transformed cells in vitro. Here, we review data on the effects of GnRH agonists on cell proliferation and apoptosis, and put forward hypotheses for investigation to determine whether the GnRH receptor acts as a tumor suppressor in neuroendocrine or epithelial cells.

Potentiation of ICI182,780 (Fulvestrant)-induced estrogen receptor-alpha degradation by the estrogen receptor-related receptor-alpha inverse agonist XCT790

ICI182,780 (Fulvestrant) is a pure anti-estrogen used in adjuvant therapies of breast cancer. This compound not only inhibits the transcriptional activities of the estrogen receptor-alpha (ER alpha) but also induces its proteasome-dependent degradation. The latter activity is believed to be required for the antiproliferative effects of ICI182,780. Estrogen receptor-related receptor-alpha (ERR alpha) is an orphan member of the nuclear receptor superfamily that is expressed in a wide range of tissues including breast tumors, in which its high expression correlates with poor prognosis. Although not regulated by any natural ligand, ERR alpha can be deactivated by the synthetic molecule XCT790. Here we demonstrate that this compound also induces a proteasome degradation of ERR alpha. We also show that although it does not act directly on the steady-state level of ER alpha, XCT790 potentiates the ICI182,780-induced ER alpha degradation. We suggest that treatment with XCT790 could thus enhance the efficacy of ICI182,780 in ER alpha-dependent pathologies such as breast cancer.

Activation Function-1 Domain of Estrogen Receptor Regulates the Agonistic and Antagonistic Actions of Tamoxifen

The antiestrogen tamoxifen has been widely used for decades as selective estrogen receptor (ER) modulator for ERalpha-positive breast tumors. Tamoxifen significantly reduces tumor recurrence by binding to the activation function-2 (AF-2) domain of the ER. Acquired resistance to tamoxifen in breast cancer patients is a serious therapeutic problem. Antiestrogen-resistant breast cancer often shows increased expression of the epidermal growth factor receptor (EGFR) family members, EGFR and ErbB2. In this report we now show that overexpression of EGFR or activated AKT-2 in MCF-7 cells leads to phosphorylation of Ser167 in the AF-1 domain of ERalpha, enhanced ER-amplified in breast cancer 1 (ER:AIB1) interaction in the presence of tamoxifen, and resistance to tamoxifen. In contrast, transfection of activated MAPK kinase, an immediate upstream activator of MAPK (ERK 1 and 2) into MCF-7 cells leads to phosphorylation of Ser118 in the AF-1 domain of ERalpha, inhibition of ER-amplified in breast cancer 1 (ER:AIB1) interaction in the presence of Tam, and maintenance of sensitivity to tamoxifen. Inhibition of AKT by short inhibitory RNA blocked Ser167 phosphorylation in ER and restored tamoxifen sensitivity. However, maximum sensitivity to tamoxifen was observed when both AKT and MAPK were inhibited. Taken together, these data demonstrate that different phosphorylation sites in the AF-1 domain of ERalpha regulate the agonistic and antagonistic actions of tamoxifen in human breast cancer cells.

Effect of a new leuprorelin formulation on testosterone levels in patients with advanced prostate cancer

BACKGROUND AND SCOPE

Leuprorelin is a well known luteinising hormone releasing hormone (LHRH) agonist. The drug is effective in the treatment of advanced prostate cancer and is well tolerated. This article reviews published literature (based on a search of PubMed, EMBASE and Biosis databases to the end of 2005) and other sources of data on a new formulation of leuprorelin acetate (Eligard) for use in the treatment of hormone-dependent advanced prostate cancer. This product takes advantage of a novel delivery system (Atrigel) which forms an implant in situ that is capable of delivering double doses of leuprorelin consistently to provide better, more sustained testosterone suppression compared with a microsphere leuprolide acetate formulation. Two formulations, 7.5 mg and 22.5 mg, are currently available with duration of action of 1 and 3 months, respectively. The 2-week stability at room temperature prior to mixing facilitates its use and reduces the potential for waste.

FINDINGS

In clinical studies of the new leuprorelin acetate formulation reviewed here, all patients achieved testosterone levels < or = 50 ng/dL and up to 98% of patients showed levels comparable to those resulting from surgical bilateral orchidectomy (< or = 20 ng/dL). Both formulations showed minimal breakthroughs, defined as a rise in testosterone levels after reaching levels of 50 ng/dL. The safety profile is typical of LHRH agonists, with mild to moderately severe 'hot flushes' being the most common adverse event. The higher dose of 22.5 mg, with a volume of 0.375 mL is administered subcutaneously via a small 20G needle, causing little local discomfort.

CONCLUSION

Prostate cancer remains a major cause of morbidity and mortality in older men. In the majority of cases, suppression of serum testosterone levels is very effective. The level of testosterone suppression is currently under debate, with ideal suppression levels ranging from 20 to 50 ng/dL. Not all LHRH agonist therapy achieves the same degree of testosterone suppression as bilateral orchidectomy. The new leuprorelin acetate (Eligard) appears to achieve a testosterone suppression of 20 ng/dL in 98% of patients, while maintaining a side effect profile comparable to other products in its class.

Molecular changes associated with the agonist activity of hydroxy-tamoxifen and the hyper-response to estradiol in hydroxy-tamoxifen-resistant breast cancer cell lines

The aim of this study was to explore the pharmacological response to 4-hydroxy-tamoxifen (OH-Tam) and to estradiol (E2) in three cell lines: MVLN, a human breast carcinoma cell line derived from MCF-7, and two MVLN-derived OH-Tam-resistant (OTR) cell lines, called CL6.8 and CL6.32. The OH-Tam response in the OTR cells was associated with the development of both an agonist activity of the drug on cell proliferation and the resistance of the cells to OH-Tam-induced apoptosis. The OTR cells also developed an increased sensitivity to the E2 growth-stimulating activity. To delineate the genes that determine such responses, we combined a mini-array-based gene-selection approach and an extensive real-time quantitative PCR exploration in the MVLN and OTR cell lines exposed to three pharmacological conditions: a 4-day treatment with E2, OH-Tam or both E2 and OH-Tam. Compiled data revealed a hyper-response to E2 and a modification of the OH-Tam pharmacological response (loss of antagonist action and agonist activity) at the gene-expression level. The proteins encoded by the genes selected in this study have been reported to be involved in the regulation of cell proliferation, cell transformation, DNA repair and apoptosis, or belong to the ErbB/epidermal growth factor receptor-driven pathway. Our data also provide evidence of changes in transcriptional co-regulator expression, elevated mitogen-activated protein kinase activity and increase in the phosphorylation status of estrogen receptor alpha on serine residue 118 in the OTR cell lines, suggesting the possible involvement of such mechanisms in the agonist activity of OH-Tam and/or the hyper-response of cells to E2. Taken together, our study should enhance our knowledge of the multifactorial events associated with the development of Tam resistance in two independent cell lines issued from the same selection process and should help in the identification of potential molecular targets for diagnosis or therapy.

Inhibition of the membrane translocation and activation of protein kinase C, and potentiation of doxorubicin-induced apoptosis of hepatocellular carcinoma cells by tamoxifen

Hepatocellular carcinoma (HCC) is characterized by high drug resistance to currently available chemotherapeutic agents. In a prospective clinical study, we have demonstrated that high-dose tamoxifen significantly enhanced the therapeutic efficacy of doxorubicin in patients with far-advanced HCC. In a search for a possible mechanism, we found that tamoxifen at a clinically achievable concentration (2.5 microM) significantly enhanced doxorubicin-induced cytotoxicity and apoptosis of Hep-3B cells, a multidrug resistance (MDR)-1 expressing HCC cell line. This synergistic cytotoxic effect of tamoxifen, at this concentration, however, was not mediated by MDR inhibition. Instead, as evidenced by both western blot and immunofluorescence studies, tamoxifen inhibited the cytoplasmic-membrane translocation of protein kinase C (PKC)-alpha. 12-O-Tetradecanoylphorbol-13-acetate (TPA) restored the membrane translocation of PKC-alpha and abrogated the synergistic cytotoxicity of tamoxifen. We also showed that tamoxifen, at this concentration, did not directly affect the enzyme activity of PKC. Further, membrane translocation of other membrane-bound proteins, such as Ras protein, was similarly inhibited by tamoxifen, but could not be restored by the addition of TPA. Together, these data suggested that tamoxifen may act on the cytoplasmic membrane, and thereby inhibit PKC-alpha translocation to the membrane where it is activated. We hypothesize that high-dose tamoxifen may be an effective modulator of doxorubicin in the treatment of HCC, and suggest that biochemical modulation of PKC as a measure to improve systemic chemotherapy for HCC deserves further investigation.