Evidence for the contribution of a sterol 14-reductase to the 14 alpha-demethylation of lanosterol by yeast

Resumption of oocyte meiosis in mammals: on models, meiosis activating sterols, steroids and EGF-like factors

De novo synthesis of meiosis activating sterols (MAS) was stimulated by LH- and AY-9944 in rat cultured follicles and cumulus oocyte complexes (COCs), but could not be measured in denuded oocytes. Thus, MAS synthesized by the somatic compartment of the follicle could serve as a signal for the resumption of meiosis. Nevertheless, the delay in germinal vesicle breakdown (GVB) after MAS or AY-9944 stimulation as compared with gonadotropins, obtained by several groups, remains the strongest evidence against the suggested role of MAS as an essential mediator of LH in meiosis resumption. Recently several studies using mammalian COCs in culture have implied that steroids, like in fish and amphibians, serve as signals in mediating the LH/hCG stimulation of meiosis. However, in these studies there was no clear distinction between the requirement for steroids for the acquisition of meiotic competence, oocyte and follicle wellbeing or as a signal for meiotic resumption. Further, some of the authors overlooked earlier studies showing that blocking ovarian or follicular steroidogenesis does not affect GVB, the first step of meiosis resumption. Finally, in vivo and in vitro studies in the rat confirm and extend recent studies showing that locally produced and released EGF-like factors, such as epiregulin, seem to mediate at least part of the LH/hCG actions on oocyte maturation and release of ova at ovulation.

Meiosis-Activating Sterol Synthesis in Rat Preovulatory Follicle: Is It Involved in Resumption of Meiosis?1

Meiosis-activating sterol (MAS) was shown to overcome the inhibitory effect of hypoxanthine on spontaneous maturation of mouse oocytes and was suggested to mediate the stimulation of meiosis by gonadotropins. Follicular fluid (FF)-MAS is synthesized by cytochrome P450 lanosterol 14alpha-demethylase (LDM). Follicular LDM was preferentially localized in oocytes by immunohistochemistry. Using [3H]acetate or R-[5-3H]mevalonate as precursors as well as high-performance liquid chromatographic and thin-layer chromatographic separation, we have measured the concentrations of de novo-synthesized lanosterol, FF-MAS, and cholesterol in rat graafian follicles, cumulus-oocyte complexes (COCs), and denuded oocytes (DOs) treated with LH, AY-9944 (an inhibitor of Delta14-reductase, which was anticipated to increase FF-MAS levels by inhibiting its metabolism), or both after 8 h of culture. In follicles, both LH and AY-9944 increased the accumulation of FF-MAS as compared to controls. In COCs, AY-9944 caused a marked increase in FF-MAS, but we were unable to detect accumulation of FF-MAS in DOs. Neither the endogenous increases in FF-MAS accumulation nor the addition of FF-MAS to the culture medium could overcome the inhibition on resumption of meiosis by phosphodiesterase inhibitors. Compared to LH-induced resumption of meiosis in follicles, that induced by AY-9944 was much delayed. These results call into question any role of FF-MAS as an obligatory mediator of LH activity on germinal vesicle breakdown. The discrepancy between the positive staining for LDM in oocytes and our inability to detect de novo synthesized FF-MAS in DOs may relate to the sensitivity of the methodology employed and either the number of oocytes used or a deficiency in LDM synthetic activity in such oocytes. Further studies are required to confirm any of these alternatives.

STUDIES ON THE EXPRESSION LEVELS OF STEROL-METABOLIZING ENZYMES IN THE OBESE MODEL SHR/NDmcr-cp RATS

1. Expression levels of four key enzymes of cholesterol metabolism, namely 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, lanosterol 14-demethylase (CYP51), cholesterol 7alpha-hydroxylase (CYP7A1) and sterol 12alpha-hydroxylase (CYP8B1), in metabolic syndrome model rats (SHR/NDmcr-cp) were examined. 2. Decreased expression of CYP51, which may be linked to the development of obesity, was found in the rats. 3. Expression of CYP8B1 was significantly higher in young rats. 4. No substantial change was observed in the mRNA levels of the dominant rate-limiting enzymes of sterol metabolism, namely HMG-CoA reductase and CYP7A1, in the rats. 5. These findings suggest that the expression levels of two key enzymes managing the downstream parts of the cholesterol-metabolizing pathways are altered in the rats, although little change was observed in the expression levels of the dominant rate-limiting enzymes of cholesterol metabolism.

Abnormal sterol metabolism in a patient with Antley-Bixler syndrome and ambiguous genitalia

Antley-Bixler syndrome (ABS) is a rare multiple anomaly syndrome comprising radiohumeral synostosis, bowed femora, fractures of the long bones, premature fusion of the calvarial sutures, severe midface hypoplasia, proptosis, choanal atresia, and, in some, ambiguous genitalia. Of fewer than 40 patients described to date, most have been sporadic, although reports of parental consanguinity and affected sibs of both sexes suggests autosomal recessive inheritance in some families. Known genetic causes among sporadic cases of ABS or ABS-like syndromes are missense mutations in the IgII and IgIII regions of FGFR2, although the assignment of the diagnosis of ABS to such children has been disputed. A third cause of an ABS-like phenotype is early in utero exposure to fluconazole, an inhibitor of lanosterol 14-alpha-demethylase. The fourth proposed cause of ABS is digenic inheritance combining heterozygosity or homozygosity for steroid 21-hydroxylase deficiency with effects from a second gene at an unknown locus. Because fluconazole is a strong inhibitor of lanosterol 14-alpha-demethylase (CYP51), we evaluated sterol metabolism in lymphoblast cell lines from an ABS patient without a known FGFR2 mutation and from a patient with an FGFR2 mutation and ABS-like manifestations. When grown in the absence of cholesterol to stimulate cholesterol biosynthesis, the cells from the ABS patient with ambiguous genitalia but without an FGFR2 mutation accumulated markedly increased levels of lanosterol and dihydrolanosterol. Although the abnormal sterol profile suggested a deficiency of lanosterol 14-alpha-demethylase, mutational analysis of its gene, CYP51, disclosed no obvious pathogenic mutation in any of its 10 exons or exon-intron boundaries. Sterol metabolism in lymphoblasts from the phenotypically unaffected mother was normal. Our results suggest that ABS can occur in a patient with an intrinsic defect of cholesterol biosynthesis at the level of lanosterol 14-alpha-demethylase, although the genetic nature of the deficiency remains to be determined.

Is meiosis activating sterol (MAS) an obligatory mediator of meiotic resumption in mammals

In-vitro studies of mouse oocytes have provided evidence that two closely related sterols, subsequently named meiosis-activating sterols (MAS), can overcome the inhibitory effect of hypoxanthine on resumption of meiosis. These sterols are synthesized by cytochrome P450 (CYP) lanosterol 14alpha-demethylase (LDM), a key enzyme in cholesterol biosynthesis. Our studies in the rat with specific inhibitors and molecular approaches did not support the hypothesis that MAS is an obligatory step in the stimulation of the resumption of meiosis. (i) Specific inhibitors of MAS synthesizing enzymes did not prevent spontaneous or LH-stimulated meiosis at doses that have previously been shown to effectively suppress LDM activity. At higher doses, they caused degeneration of oocytes. (ii) The timing of LDM expression in the ovary was incompatible with a role for MAS in meiosis. (iii) The preferential localization of LDM protein in the oocytes suggests MAS production in oocytes, rather than its transport from the somatic compartment as expected by the suggested role of MAS in the regulation of meiosis as a putative cumulus-oocyte signal molecule. (iv) AY-9944, which supposedly increases MAS levels by inhibiting its metabolism, induced the maturation of follicle-enclosed oocytes that was much delayed as compared with gonadotropic stimulation. Thus, the resumption of meiosis induced by added MAS [Biol. Reprod. 61 (1999) 1362, Biol. Reprod. 64 (2001) 418] or presumed endogenous MAS accumulation by AY-9944, resulted in oocyte maturation with remarkably slower kinetics than observed with LH stimulation. This delay in meiosis after MAS stimulation, the studies with LDM inhibitors and its spatial and temporal expression, cast serious doubts whether MAS is indeed mediating the meiosis inducing action of the gonadotropins, as suggested.

Role of meiosis-activating sterols in rat oocyte maturation: effects of specific inhibitors and changes in the expression of lanosterol 14alpha-demethylase during the preovulatory period

In vitro studies on mouse oocytes have shown that two closely related sterols, subsequently named meiosis-activating sterols (MAS), can overcome the inhibitory effect of hypoxanthine on the resumption of meiosis. These sterols are synthesized by cytochrome P(450) lanosterol 14alpha-demethylase (LDM), a key enzyme in cholesterol biosynthesis. We have used specific inhibitors of LDM, azalanstat (RS-21607) and RS-21745, to test whether MAS is an obligatory mediator in the resumption of meiosis in the rat. Addition of azalanstat and RS-21745 (1-200 microM) to culture medium of rat isolated cumulus-enclosed oocyte and preovulatory follicle-enclosed oocyte stimulated by LH/hCG did not allow separation between their inhibition of the resumption of meiosis and the degeneration of oocytes. In both models, doses of the drug that inhibited oocyte maturation also increased oocyte degeneration. The inhibitors only partially suppressed follicular progesterone production. We have examined by reverse transcriptase-polymerase chain reaction, Western blotting, and immunocytochemistry the ovarian expression of LDM mRNA and protein during the preovulatory period. We did not find evidence for the stimulation of this enzyme by LH/hCG. The strongest staining by LDM antiserum was obtained in primordial and primary oocytes, and the staining was reduced with oocyte growth. In addition, strong LDM staining could be observed in some of the granulosa cells, especially of the corona radiata localized in close proximity to the oocyte. In conclusion, our results with specific inhibitors and molecular approaches do not reveal evidence to support the hypothesis that MAS is an obligatory step in the stimulation of the resumption of meiosis. Specific inhibitors of MAS synthesis did not prevent spontaneous or LH-stimulated meiosis at doses that have previously been shown to effectively suppress LDM activity. Much higher concentrations of the inhibitors, which affected meiosis, were detrimental to oocytes, leading to their degeneration. The timing of LDM expression in the ovary was incompatible with a role for MAS in meiosis. Finally, the preferential localization of LDM protein to the oocytes suggests MAS production in oocytes rather than its transport from the somatic compartment as implied by the proposed role of MAS as a cumulus-oocyte signal molecule.

Cholesterol biosynthesis from lanosterol: regulation and purification of rat hepatic sterol 14-reductase

We have previously characterized the membrane-bound sterol 14-reductase (14-reductase) that catalyzes anaerobically NADPH-dependent reduction of the 14-double bond of delta 8,14-diene or delta 7,14-diene sterols that are sterol intermediates in cholesterol biosynthesis in mammals (Paik et al. (1984) J. Biol. Chem. 259, 13413-13423). To elucidate the regulatory mechanism as well as molecular characteristics of the 14-reductase, we extended our investigation on the consequences of alteration of the enzymic activity under various physiological conditions. The enzymic activity of rat hepatic sterol 14-reductase was induced more than 11-fold by feeding 5% cholestyramine plus 0.1% lovastatin (the CL-diet) for 7 days but was severely suppressed by feeding 5% cholesterol or 0.01% AY-9944 (an inhibitor of 14-reductase) for the same period. The increase or decrease in the 14-reductase activity also parallels the same change in the cholesterol synthetic rate in hepatocytes from rats that had been fed either the CL-diet or 0.01% AY-9944. In vitro inhibition studies revealed that AY-9944 acts as a competitive inhibitor of the 14-reductase (Ki = 0.26 microM). A diurnal variation was observed for the 14-reductase with peak activity near the middle of the dark cycle (10 p.m.), which was abolished by administration of cycloheximide. With induced enzyme conditions 14-reductase has been further purified with chromatographic procedures to near homogeneity. Purified 14-reductase appears to be a M(r) = 70,000 protein that is composed of two equally-sized subunits having a M(r) = 38,000. All properties of the purified 14-reductase suggest that the solubilized enzyme is the principal 14-reductase of microsomes. Taken together, our results provide the first evidence in support of a previously unknown regulatory role for the 14-reductase in the overall cholesterol synthetic pathway.

Chemical structure of sterols that activate oocyte meiosis

Gonadotrophins and various growth factors, but not sex steroids, can induce resumption of meiosis in vitro, but only in oocytes enclosed by cumulus-granulosa cells. Follicular purines prevent resumption of meiosis. This process can be overcome, in vitro, by a transient elevation of cyclic AMP resulting in the production of a diffusible meiosis-inducing substance secreted by the cumulus cells. A meiosis-inducing activity has been detected in gonads of different species, for example, in preovulatory follicular fluid of women and in mouse testes. We report here the isolation and characterization of meiosis-activating sterols from human follicular fluid and bull testes and the synthesis of two closely related C29 sterols. All these sterols induce a resumption of meiosis in cultured cumulus-enclosed and naked mouse oocytes indicating their nonspecificity across species and sex. This family of sterols is for the first time considered crucial to meiosis.

Induction and substrate specificity of the lanosterol 14 alpha-demethylase from Saccharomyces cerevisiae Y222

The potential inducibility of the lanosterol 14 alpha-demethylase (P-45014DM) from Saccharomyces cerevisiae Y222 by xenobiotics was investigated. This enzyme and NADPH-cytochrome P-450 reductase were unaffected by a number of compounds known to induce mammalian and some yeast cytochrome P-450 monooxygenases. Furthermore, dibutyryl cyclic AMP did not affect P-45014DM or P-450 reductase levels, while growth at 37 degrees C resulted in a slight decrease. P-45014DM was found to be specific for lanosterol and did not metabolize a number of P-450 substrates including benzo[a]pyrene.

Microsomal delta 8,14-sterol delta 14-reductase in higher plants. Characterization and inhibition by analogues of a presumptive carbocationic intermediate of the reduction reaction

An enzymatic assay for delta 8,14-sterol delta 14-reductase, an enzyme involved in sterol biosynthesis, has been developed for the first time in higher plants. The properties of the microsomal enzyme have been established with respect to cofactor requirements, kinetics and substrate specificity. This enzymatic double-bound reduction is thought to proceed through an electrophilic addition mechanism, involving a C14 putative carbonium ion high-energy intermediate. Using this in vitro assay, ammonium and iminium analogues of this cationic intermediate were shown to be potent inhibitors of the reduction reaction. Thus, compounds of the N-alkyl-8-aza-4 alpha,10-dimethyl-trans-decal-3 beta-ol series strongly inhibited sterol reductase (I50 = 0.07 - 4 microM) (I50/Km = 10(-4) - 10(-3), as did the antimycotic agent 15-azasterol (I50 = 0.03 microM); all of these compounds act as reaction-intermediate analogues of the proposed C14 carbonium ion intermediate. Moreover, the in vitro inhibition of the plant sterol reductase by a series of ammonium-ion-containing fungicides was demonstrated. The relative specificity of these different series of inhibitors toward cycloeucalenol-obtusifoliol isomerase, delta 8----delta 7-sterol isomerase and delta 8,14-sterol delta 14-reductase, was directly studied.

Characterization of a cytochrome P450 deficient mutant of Candida albicans

A previously described Candida albicans nystatin resistant mutant blocked in 14 alpha-demethylation of lanosterol was shown to also lack all traces of cytochrome P450 as determined by carbon monoxide difference spectra. This strain does not require ergosterol for growth and reverted to an ergosterol producing, cytochrome P450 containing strain indicating no other lesions. Cytochrome P450 mutants described in Saccharomyces cerevisiae are auxotrophic for ergosterol or contain a second mutation in 5,6 desaturation of the sterol B ring. These results suggest that a cytochrome P450 lesion in these yeasts have different phenotypes and may reflect different sterol requirements for the two organisms.

The 14 alpha-demethylation of obtusifoliol by a cytochrome P-450 monooxygenase from higher plants' microsomes

Microsomes isolated from corn embryos (Zea mays) can demethylate the 14 alpha-methyl group of obtusifoliol 2. An enzymatic assay has been developed for obtusifoliol 14 alpha-methyl-demethylase in higher plants. The enzymatic reaction was shown to occur sequentially, converting obtusifoliol 2 to 4 alpha-methyl-5 alpha-ergosta-8,24(28)-dien-3 beta-ol 4 via the trienol 4 alpha-methyl-5 alpha-ergosta-8,14,24(28)-trien-3 beta-ol 3 which was thoroughly identified. This enzymatic reaction is dependent of NADPH and molecular oxygen. It is inhibited by CO, menadione and specific inhibitors of cytochrome P-450, the CO inhibition being partially reversed by light. It is concluded that in Zea mays microsomes, obtusifoliol is demethylated at C-14 by a cytochrome P-450 containing monooxygenase system.