<p>CYP27A1 and CYP7B1 are widely expressed in various human tissues and are two key enzymes involved in the pathways for conversion of cholesterol to bile acids. Also, CYP27A1 is involved in bioactivation of vitamin D3 and CYP7B1 plays a role in 7alpha-hydroxylation of dehydroepiandrosterone and other steroids. Both enzymes have been reported to be relevant to prostate cell proliferation. The current study examines the hormonal regulation of CYP27A1 and CYP7B1.</p><p>CYP7B1 was shown to be regulated by estrogens and androgens in human embryonic kidney HEK293 and prostate cancer LNCaP cells. Quantitation of CYP7B1 mRNA in adult and fetal human tissues showed markedly higher CYP7B1 mRNA levels in fetal tissues compared with the corresponding adult ones, except in the liver. This indicates a tissue-specific, developmental regulation of CYP7B1 and suggests an important function for this enzyme in fetal life. CYP7B1 regulation by estrogens may be of importance in fetal development and in other processes where CYP7B1 is involved, including cholesterol homeostasis, cellular proliferation, and CNS function. The regulation of CYP7B1 by sex hormones also suggests an important role for CYP7B1 in balancing prostate hormone levels in human cells. </p><p>Results show that CYP27A1 can be regulated by dexamethasone, growth hormone, IGF-1, PMA, estrogens and androgens in liver-derived HepG2 cells. Dexamethasone, growth hormone and IGF-1 stimulated the promoter and endogenous activity of CYP27A1, whereas thyroid hormones and PMA inhibited CYP27A1. The regulatory effects of estrogens and androgens are different depending on the cell types. Thus, the results imply that human CYP27A1 gene is a target for estrogens and androgens, and the expression of CYP27A1 may be affected by endogenous sex hormones and pharmacological compounds with estrogenic or androgenic effects. </p><p>The mechanism for the dexamethasone-induced effect on the human CYP27A1 promoter was examined. A GRE was identified important for GR-mediated regulation of CYP27A1 transcriptional activity. </p>

CYP27A1 and CYP7B1 are widely expressed in various human tissues and are two key enzymes involved in the pathways for conversion of cholesterol to bile acids. Also, CYP27A1 is involved in bioactivation of vitamin D3 and CYP7B1 plays a role in 7alpha-hydroxylation of dehydroepiandrosterone and other steroids. Both enzymes have been reported to be relevant to prostate cell proliferation. The current study examines the hormonal regulation of CYP27A1 and CYP7B1. CYP7B1 was shown to be regulated by estrogens and androgens in human embryonic kidney HEK293 and prostate cancer LNCaP cells. Quantitation of CYP7B1 mRNA in adult and fetal human tissues showed markedly higher CYP7B1 mRNA levels in fetal tissues compared with the corresponding adult ones, except in the liver. This indicates a tissue-specific, developmental regulation of CYP7B1 and suggests an important function for this enzyme in fetal life. CYP7B1 regulation by estrogens may be of importance in fetal development and in other processes where CYP7B1 is involved, including cholesterol homeostasis, cellular proliferation, and CNS function. The regulation of CYP7B1 by sex hormones also suggests an important role for CYP7B1 in balancing prostate hormone levels in human cells. Results show that CYP27A1 can be regulated by dexamethasone, growth hormone, IGF-1, PMA, estrogens and androgens in liver-derived HepG2 cells. Dexamethasone, growth hormone and IGF-1 stimulated the promoter and endogenous activity of CYP27A1, whereas thyroid hormones and PMA inhibited CYP27A1. The regulatory effects of estrogens and androgens are different depending on the cell types. Thus, the results imply that human CYP27A1 gene is a target for estrogens and androgens, and the expression of CYP27A1 may be affected by endogenous sex hormones and pharmacological compounds with estrogenic or androgenic effects. The mechanism for the dexamethasone-induced effect on the human CYP27A1 promoter was examined. A GRE was identified important for GR-mediated regulation of CYP27A1 transcriptional activity.

The enzymes CYP27A1 and CYP7B1 are widely expressed in various human tissues and perform catalytic reactions in cholesterol homeostasis and endocrine signaling. We have investigated the metabolism of a synthetic oxysterol. In this study, we show that CYP27A1 is the enzyme responsible for a 28-hydroxylation of this oxysterol and that the rate of CYP27A1-mediated metabolism is relatively slow. This may give an explanation for the prolonged inhibitory effects on cholesterol biosynthesis that have been shown for this oxysterol. The current study contributes to the knowledge of synthetically produced oxysterols and their potential use as cholesterol lowering drugs. In two studies we investigated CYP7B1-mediated metabolism of different sex hormones. Our data indicate that CYP7B1 may carry out a previously unknown catalytic reaction involving an androgen. Taken together the data suggest that varying steroid concentrations in cells and tissues may be important for CYP7B1-dependent metabolism of sex hormones and sex hormone precursors. CYP7B1-mediated hydroxylation of sex hormones may influence the cellular levels of these steroids and may be a potential pathway for elimination of the steroids from the cell. Some known CYP7B1 substrates are agonists for ERα and ERβ but the reported role(s) of CYP7B1 for ER action are not fully understood. In the last study we investigated the role(s) of CYP7B1-mediated metabolism for ER-mediated action. Our data indicate that CYP7B1-mediated conversion of steroids that affect ER-mediated response into their 7α-hydroxymetabolites will result in loss of action. This indicates that CYP7B1 may have an important role for regulation of ER-mediated processes in the body. In summary, results from this thesis contribute to the knowledge on the metabolism of synthetic oxysterols of potential use as cholesterol lowering drugs and the role(s) of CYP7B1-mediated metabolism for processes related to the functions of sex hormones. / Disputationsordförande;Professor Eva Brittebo, Inst. för Biovetenskap, Avd. för Toxikologi, Uppsala Universitet, UppsalaBetygsnämndens ledamöten; Docent Lena Ekström, Inst. för Laboratoriemedicin, Avd. för Klinisk Kemi, Karolinska Universitetssjukhuset, HuddingeDocent Ulf Diczfaluzy, Inst. för Laboratoriemedicin, Avd. för Klinisk Kemi, Karolinska Universitetssjukhuset, HuddingeProfessor Agneta Oskarsson, Inst. BVF, Avd. för farmakologi och toxikologi, SLU, Uppsala

Enzyme-catalyzed reactions are important to regulate steroidogenesis and nuclear receptor activation. The present investigation examines the role of steroid metabolism catalyzed by CYP7B1 for regulation of hormone receptor activation and the effects of vitamin D on enzymatic regulation of steroidogenesis. The study reports data indicating that CYP7B1 can regulate estrogenic signaling by converting estrogens into inactive or less active metabolites. Similar results were obtained for CYP7B1-mediated metabolism of some androgen receptor ligands, indicating that CYP7B1 can be involved also in the regulation of androgenic signaling. CYP7B1 substrates and metabolites were found to exert androgenic effects in a cell line-specific manner. Furthermore, cell line differences were observed in the expression pattern for androgen receptor comodulators. This thesis reports that 1α,25-dihydroxyvitamin D3 alters the gene expression and enzyme activity of CYP21A2 and CYP17A1 leading to suppressed production of aldosterone, dehydroepiandrosterone and androstenedione in adrenocortical cells. These are novel findings on vitamin D action. A mechanism is reported for the vitamin D-mediated regulation of the CYP21A2 gene. Data indicate that vitamin D receptor interacting repressor (VDIR) and Williams syndrome transcription factor (WSTF) are key comodulators in this novel vitamin D receptor (VDR)-mediated mechanism. Furthermore, the results indicate that altered expression levels of VDIR and WSTF can shift the suppressing effect of vitamin D to a stimulatory effect. Also, epigenetic components were found to be involved in the effects of vitamin D on CYP21A2 transcriptional rate. In addition, a functional vitamin D response element was identified in the CYP21A2 promoter. This study also reports that 1α,25-dihydroxyvitamin D3 affects sex hormone production in a tissue-specific way. Gene expression and enzyme activity of aromatase were found to be downregulated in cells derived from breast, but not in cells derived from prostate and adrenal cortex. The production of estradiol and dihydrotestosterone was altered in a tissue-selective manner following vitamin D treatment. These findings are of importance for the discussion on vitamin D as a potential anti-breast cancer agent.