There is a growing interest in peroxisome proliferator-activated receptors (PPARs) as major players in the regulation of lipid and carbohydrate metabolism. Drugs targeting PPARs were in fact shown to have major relevance for the treatment of diseases associated with aging, such as arteriosclerosis and diabetes. However, a variety of toxic effects associated with PPAR ligand administration has been documented, including hepatocarcinogenesis, which may severely limit its therapeutic use. A better comprehension of the multiplicity of PPAR physiological functions is therefore mandatory for the development of novel, safer drugs. We here describe the generation of a novel transgenic mouse for the detection of the generalized activities of PPARs, the PPAR responsive element-Luc reporter mouse. In this model luciferase expression is under the control of a PPAR-inducible promoter in all target organs. By optical imaging and ex vivo analysis, we were able to demonstrate the remarkable gender specificity of the PPAR transcriptional activity in liver. In fact, in the liver of female PPAR responsive element-Luc, the PPAR reporter transgene is more than one order of magnitude less expressed, thus leading to the conclusion that the signaling in females is much less activated than in males. Diet or hormonal manipulations as demonstrated here by treatments with high-fat diet or gonad removal and hormone replacement do not influence this low activation. The extent of the gender difference in PPAR transcriptional activity and the ineffectiveness of hormone treatments or diet to significantly elevate liver PPAR activity in females led us to hypothesize that gender-specific epigenetic events occurring during development may affect PPAR signaling in the liver. This study sets the ground for understanding the differential susceptibility of the two genders to metabolic disorders; furthermore, the model generated provides a novel opportunity for the molecular characterization of PPAR activity in pathophysiological conditions.
Prostate cancer (PC) develops in response to an abnormal activation of androgen receptor induced by circulating androgens and, in its initial stages, is pharmacologically controlled by androgen blockade. However, androgen ablation therapy often allows androgen-independent PC development, generally characterized by increased invasiveness. We previously reported that 5a-androstane-3b,17b-diol (3b-Adiol) inhibits the migration of PC cell lines via the estrogen receptor b (ERb) activation. Here, by combining in vitro assays and in vivo imaging approaches, we analyzed the effects of 3b-Adiol on PC proliferation, migration, invasiveness, and metastasis in cultured cells and in xenografts using luciferase-labeled PC3 (PC3-Luc) cells. We found that 3b-Adiol not only inhibits PC3-Luc cell migratory properties, but also induces a broader anti-tumor phenotype by decreasing the proliferation rate, increasing cell adhesion, and reducing invasive capabilities in vitro. All these 3b-Adiol activities are mediated by ERb and cannot be reproduced by the physiological estrogen, 17b-estradiol, suggesting the existence of different pathways activated by the two ERb ligands in PC3-Luc cells. In vivo, continuous administration of 3b-Adiol reduces growth of established tumors and counteracts metastasis formation when PC3-Luc cells are engrafted s.c. in nude mice or are orthotopically injected into the prostate. Since 3b-Adiol has no androgenic activity, and cannot be converted to androgenic compounds, the effects here described entail a novel potential application of this agent against human PC.
Using a mouse model engineered to measure estrogen receptor (ER) transcriptional activity in living organisms, we investigated the effect of long-term (21 d) hormone replacement on ER signaling by whole-body in vivo imaging. Estrogens and selective ER modulators were administered daily at doses equivalent to those used in humans as calculated by the allometric approach. As controls, ER activity was measured also in cycling and ovariectomized mice. The study demonstrated that ER-dependent transcriptional activity oscillated in time, and the frequency and amplitude of the transcription pulses was strictly associated with the target tissue and the estrogenic compound administered. Our results indicate that the spatiotemporal activity of selective ER modulators is predictive of their structure, demonstrating that the analysis of the effect of estrogenic compounds on a single surrogate marker of ER transcriptional activity is sufficient to classify families of compounds structurally and functionally related. For more than one century, the measure of drug structure-activity relationships has been based on mathematical equations describing the interaction of the drug with its biological receptor. The understanding of the multiplicity of biological responses induced by the drug-receptor interaction demonstrated the limits of current approach and the necessity to develop novel concepts for the quantitative analysis of drug action. Here, a systematic study of spatiotemporal effects is proposed as a measure of drug efficacy for the classification of pharmacologically active compounds. The application of this methodology is expected to simplify the identification of families of molecules functionally correlated and to speed up the process of drug discovery.
By the use of in vivo imaging, we investigated the dynamics of estrogen receptor (ER) activity in intact, ovariectomized, and hormone-replaced estrogen response element-luciferase reporter mice. The study revealed the existence of a long-paced, noncircadian oscillation of ER transcriptional activity. Among the ER-expressing organs, this oscillation was asynchronous and its amplitude and period were tissue dependent. Ovariectomy affected the amplitude but did not suppress ER oscillations, suggesting the presence of tissue endogenous oscillators. Long-term administration of raloxifene, bazedoxifene, combined estrogens alone or with basedoxifene to ovariectomized estrogen response element-luciferase mice showed that each treatment induced a distinct spatiotemporal profile of ER activity, demonstrating that the phasing of ER activity among tissues may be regulated by the chemical nature and the concentration of circulating estrogen. This points to the possibility of a hierarchical organization of the tissue-specific pacemakers. Conceivably, the rhythm of ER transcriptional activity translates locally into the activation of specific gene networks enabling ER to significantly change its physiological activity according to circulating estrogens. In reproductive and nonreproductive organs this hierarchical regulation may provide ER with the signaling plasticity necessary to drive the complex metabolic changes occurring at each female reproductive status. We propose that the tissue-specific oscillatory activity here described is an important component of ER signaling necessary for the full hormone action including the beneficial effects reported for nonreproductive organs. Thus, this mechanism needs to be taken in due consideration to develop novel, more efficacious, and safer hormone replacement therapies.
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