Critical role of the plasma membrane for expression of mammalian mitochondrial side chain cleavage activity in yeast Catherine Duport1,*, Barbara Schoepp1,†, Elise Chatelain1, Roberto Sp
Trang 1Critical role of the plasma membrane for expression of mammalian mitochondrial side chain cleavage activity in yeast
Catherine Duport1,*, Barbara Schoepp1,†, Elise Chatelain1, Roberto Spagnoli2, Bruno Dumas3
and Denis Pompon1
1
Laboratoire d’Inge´nierie des Prote´ines Membranaires, CGM du CNRS, Gif sur Yvette, France;2Lead Discovery Technologies, Aventis Pharma, Romainville, France;3Functional Genomics, Aventis Pharma, 13 Quai Jules Guesde, F-94403 Vitry sur Seine, France
Engineered yeast cells efficiently convert ergosta-5-eneol to
pregnenolone and progesterone provided that endogenous
pregnenolone acetylase activity is disrupted and that
heterologous sterol D7-reductase, cytochrome P450 side
chain cleavage (CYP11A1) and 3b hydroxysteroid
dehydrogenase/isomerase (3b-HSD) activities are present
CYP11A1 activity requires the expression of the mammalian
NADPH-adrenodoxin reductase (Adrp) and adrenodoxin
(Adxp) proteins as electron carriers Several parameters
modulate this artificial metabolic pathway: the effects of
steroid products; the availability and delivery of the
ergosta-5-eneol substrate to cytochrome P450; electron flux and
protein localization CYP11A1, Adxp and Adrp are usually
located in contact with inner mitochondrial membranes
and are directed to the outside of the mitochondria by the
removal of their respective mitochondrial targeting sequen-ces CYP11A1 then localizes to the plasma membrane but Adrp and Adxp are detected in the endoplasmic reticulum and cytosol as expected The electron transfer chain that involves several subcellular compartments may control side chain cleavage activity in yeast Interestingly, Tgl1p, a potential ester hydrolase, was found to enhance steroid productivity, probably through both the availability and/or the trafficking of the CYP11A1 substrate Thus, the obser-vation that the highest cellular levels of free ergosta-5-eneol are found in the plasma membrane suggests that the sub-strate is freely available for pregnenolone synthesis Keywords: CYP11A1; plasma membrane; ergosta-5-eneol; Tgl1p
The large family of mammalian cytochrome P450 enzymes
includes drug metabolizing enzymes and enzymes that
mediate individual steps in the biosynthesis of biologically
active compounds Our interest is focused on the
cyto-chrome P450 enzymes that are involved in the synthesis of steroid hormones These steroids are critical for mammalian life and are involved in such distinct processes as stress response, immunosuppression, ion balance, general meta-bolite homeostasis and fetal, neonatal and gonadal devel-opment [1] Eukaryotic steroidogenic cells produce a large array of steroids using a limited set of cytochrome P450 enzymes [2] The biosynthesis of all hormonal steroids begins with the side chain cleavage (SCC) of cholesterol [3]
to form pregnenolone, the key precursor of biologically active steroids in all tissues [4,5] This reaction is catalysed
by cytochrome P450scc (also designated CYP11A1 [6]), a mitochondrial protein located on the matrix face of the inner membrane that requires electrons for activity These electrons are transferred from NADPH through a specific transport chain involving adrenodoxin reductase (Adrp) and adrenodoxin (Adxp) [7] Adxp is a small soluble iron– sulfur protein localized to the mitochondrial matrix, and Adrp is a larger flavodoxin protein bound to the inner mitochondrial membrane of steroid-producing cells [8] For many years, pregnenolone formation has been considered to
be the rate-limiting step in steroidogenesis [9] It has been shown that to initiate and sustain steroid production, a constant supply of cholesterol must be available in the cell Furthermore, there must be a mechanism to ensure the delivery of this substrate to the site where it is cleaved in the inner mitochondrial membrane, where CYP11A1 resides For example, substrate unavailability is a common cause of congenital lipoid adrenal hyperplasia, a disease character-ized by a dramatic decrease in steroid synthesis [10] In the
Correspondence to B Dumas, Functional Genomics, Aventis Pharma,
13 Quai Jules Guesde, F-94403 Vitry sur Seine, France.
Fax: + 33 1 5893 2625, Tel.: + 33 1 5893 2805,
E-mail: bruno.dumas@aventis.com
Abbreviations: ACAT, acyl coenzyme A cholesterol acyltransferase;
Adrp, adrenodoxin reductase protein; Adxp, adrenodoxin protein;
APAT, acetyl coenzyme A:pregnenolone acetyl transferase; ARE,
acyl coenzyme A:cholesteryl acyltransferase-related enzyme;
CYP2D6, cytochrome P450 2D6; CYP2E1, cytochrome P450 2E1;
CYP11A1, cytochrome P450 steroid side chain cleaving; CYP11B1,
cytochrome P450 steroid 11b hydroxylase; D7-Red, sterol D7
reduc-tase; ER, endoplasmic reticulum; 3b-HSD, 3b hydroxysteroid
dehydrogenase/isomerase; PM, plasma membrane; PGK1,
phospho-glycerate kinase; StAR, steroidogenic acute regulatory protein;
SCC, side chain cleavage; TEF1, transcription elongation factor.
Enzymes: ACAT, EC2.3 2.26; Adrp, EC1.18.1.2; CYP11A1,
EC1.14.15.6; CYP11B1, EC1.14.15.4; 3b-HSD, EC1.1.1.51.
*Present address: University of Paris VII and UMR A408,
INRA 84914 Avignon Cedex 09, France.
Present address: Institut de Biologie Structurale et Microbiologie,
31 Chemin Joseph Aiguier, F-13402 Marseille, France.
(Received 20 November 2002, revised 27 January 2003,
accepted 11 February 2003)
Trang 2latter case, mutation(s) in the steroidogenic acute regulatory
protein (StAR) protein correlate(s) clearly with the absence
of pregnenolone synthesis The well-characterized StAR
protein is involved in the rapid transport of cholesterol to
the inner mitochondrial membrane [11] In contrast, the
factors and processes responsible for the intracellular supply
of cholesterol to the outer mitochondrial membrane are
poorly understood It is known, however, that cholesterol is
mobilized from cellular storage sites, such as lipid droplets,
in response to trophic hormones [12] This mobilization
requires the enzyme cholesteryl esterase, which mediates the
release of free cholesterol from cholesterol esters In
addition, the maintenance of cellular architecture requires
a stringent regulation of the concentration of free
choles-terol This is ensured by the enzyme, acyl coenzyme A
cholesterol acyltransferase (ACAT), which catalyzes its
esterification [13] The levels of expression of various
proteins (Adxp, Adrp, CYP11A1) involved in the reaction
can also affect the efficiency of cholesterol side chain
cleavage Indeed, it is apparent that the expression of these
three proteins is differentially modulated in
hormone-producing tissues For example, the corpus luteum and
adrenal cortex contain higher concentrations of Adxp and
Adrp than does the placenta ([14,15]) In the latter case, the
concentration of Adrp limits the production of
pregneno-lone [15] through a mechanism involving oxidized Adxp,
that is in excess in the human placenta [16] Whether ternary
complex formation is required for the optimal flow of
electrons from NADPH to CYP11A1 remains
controver-sial However, recent reports reinforce the idea that there is
a complex containing CYP11A1, cytochrome P450 11B1
(CYP11B1), Adxp and Adrp in the mitochondrial
mem-brane of steroid producing cells [17,18]
As reported previously [19], the simultaneous expression
of Arabidopsis thaliana sterol D7-reductase (D7-Red),
bovine CYP11A1, Adxp, Adrp and human
3b-hydroxy-steroid deshydrogenase/isomerase (3b-HSD) in modified
Saccharomyces cerevisiae cells allows the self-sufficient
biosynthesis of pregnenolone and progesterone (Fig 1),
thus reproducing the properties of steroidogenic tissues of
higher eukaryotes In these recombinant yeast strains, the
predominant sterol is ergosta-5-eneol, that replaces
ergos-terol in membranes and acts as a substrate for CYP11A1
Ergosta-5-eneol differs from ergosterol in that the C7–C8
doublebond is reduced and there is no doublebond at
position C22 It also differs from cholesterol in that it has a
methyl group at position C24 Ergosta-5-eneol is
synthe-sized [19] and esterified [20] in processes similar to those
that control cholesterol accumulation in mammalian cells
Ergosta-5-eneol and cholesterol act similarly as a substrate
for CYP11A1 and allow proper folding of CYP11A1 in
membrane microdomain The aim of this study was to
determine whether recombinant yeast can be used as a
model system to decipher the SCC reaction and its
poten-tial regulation during steroidogenesis To do so, we studied
the influence of ergosta-5-eneol availability and electron
carrier expression level on the production of pregnenolone
and progesterone, and we also determined the localization
of the components of the SCC reaction We found that
Adxp, Adrp and CYP11A1 appear to localize to three
compartments outside the mitochondrion, without
impair-ing the reaction This findimpair-ing has direct implications for the
potential formation of a complex containing CYP11A1, CYP11B1, Adxp and Adrp
Materials and methods
Culture conditions and genetic methods Yeast media, including SG (synthetic medium containing 2% glucose), SL (synthetic medium containing 2% galac-tose) and YP (complete medium without carbon source) are described [65] Low-density and high-density cultures were obtained as reported previously [19] Standard methods were used for transformation [21] and genetic manipulation
of S cerevisiae [22]
pUC-HIS3ADX is an integrative plasmid derived from pUC-HIS3 [23] that carries the TEF1prom::matADX:: PGK1term expression cassette This expression cassette contains the mature form of the ADX cDNA [24] under the control of the TEF1 promoter and PGK1 terminator [25] The matADX expression plasmid pTG10917 contains
an E coli replicon with an S cerevisiae replicon and a URA3marker The vector pUC18-HIS3 was linearized at the unique XhoI site in the intergenic region between the
S cerevisiae HIS3and DDE1 genes and blunt-ended with the Klenow enzyme A NotI linker was introduced into this linearized vector, giving pUC-HIS3N The 1235-bp NotI fragment carrying the expression cassette, TEF1prom:: matADX::PGK1term, was isolated from pTG10917 (see above) and subcloned into the NotI site of pUC-HIS3N to obtain pUC-HIS3ADX
pYeDP60 is a 2l replication origin-based expression plasmid that contains the URA3 and ADE2 selectable markers, a galactose inductible GAL10/CYC1 promoter, multiple cloning sites, and the PGK1 terminator [26] pCD69, a 2l-URA3-ADE2 plasmid expressing TGL1 under the control of the GAL10/CYC1 promoter was constructed as follows The TGL1 open reading frame was isolated from FY1679 genomic DNA by amplification using oligonucleotides lip1 (5¢-atagacacgcaaacacaaatacaca cactaaattaataatgaccggatcATGTACTTCCCCTTTTTAGG CAGAT-3¢) and lip2 (5¢-cagtagagacatgggagatcccccgcgg aattcgagctcggtacccgggTCATTCTTTATTTAGAGCATC CAGC-3¢)
The sequences in lower-case are complementary to the end of the GAL10/CYC1 promoter (lip1) and to the beginning of the PGK1 terminator (lip2) The 1647 bp PCR fragment was transformed into yeast along with BamHI– EcoRI-linearized pYeDP60, permitting cloning by homo-logous recombination between the plasmid and the PCR fragment and giving pCD69
The pDP10037 TRP1), pCD63 (2l-URA3-TRP1) and pV13SCC (2l-URA3) plasmids were con-structed as reported previously [19] pDP10037 carries the GAL10/CYC1prom::matADR::PGK1term and GAL10/ CYC1prom::matADX::PGK1termexpression cassettes separ-ated by the URA3 marker pCD63 was obtained from pDP10037 by replacing sequences coding for the mature Adrp (preceded by a methionine codon) by sequences coding for the mature form of cytochrome CYP11A1 (preceded by a methionine codon) pV13SCC expresses the mature form of CYP11A1 (preceded by a methionine codon) driven by the GAL10/CYC1 promoter
Trang 3Yeast strains used in this study are listed in Table 1 The
structure of the CA10 D7 reductase expression locus has
been verified by PCR, Southern and direct sequencing
analysis of the promoter region, showing that it contains the
GAL10/CYC1 promoter instead of the expected PGK1
promoter (Table 1) The APAT-deficient strain, CA14, was
generated by disrupting the ATF2 gene of CA10 with the
KanMX4 cassette, which confers G418 resistance Primers
5¢ATF2-Kan 5¢-AGACTTTCAAACGAATAATAACTT
CAGCAATAAAAATTGTCCAGGTTAATtccagcgacatg
gaggccc-3¢ and 3¢ATF2-Kan: 5¢-TTGTACGAGCTCGG
CCGAGCTATACGAAGGCCCGCTACGGCAGTATC
GCAcattcacatacgattgacgc-3¢ (nucleotides in lower case are
specific to the KanMX4 module) were used for PCR with
pFA6-MX4 as a template [27] to produce the KanMX
cassette flanked by ATF2 sequences [23]
The strain, CA19 was obtained by introducing the
GAL10/CYC1prom::3bHSD::PGK1term cassette into the
region between the HIS3 and DDE1 genes of CA14, as
previously described [19]
CDR07 (an FY-1679–18B derivative that contains only
the GAL10/CYC1prom::D7 reductase::PGK1term expression
cassette) was obtained by sporulation of the diploid resulting from a cross between CA10 (MATa) and FY1679–18B (MATa)
TGY120.2 (MATa) was obtained by transformation of FY1679–28C(MATa) with XbaI-linearized pTG10925 This plasmid contains an S cerevisiae genomic DNA fragment covering the LEU2 and SPL1 locus derived from pFL26CD [19] A bovine mature Adrp expression cassette (TEF1prom:: matADR::PGK1term) [24] was introduced into the unique NotI site of pFL26CD, that is in the noncoding region between LEU2 and SPL1 Transformed colonies were grown
in selective medium, and the expression of mature Adrp was verified by Western blot analysis as described [24] One clone, TGY120.2 MATa, was selected for further studies The yeast strain, CA15 was isolated by mating CDR06 (MATa) to TGY120.2 (MATa), that contains the TEF1prom::matADR::PGK1term cassette in the intergenic region between LEU2 and SPL1
The strain, CA17 was generated by integrating a TEF1prom::matADX::PGK1term cassette into the intergenic region between the HIS3 and DDE1 of CA15 with the pUC-HIS3ADX integrative plasmid
The are1::KanMX4 are2::HIS3 double mutant strain CA23 was constructed by crossing CA10 (MATa ARE 1
Table 1 Yeast strains and expression plasmids.
Strain or plasmid Relevant genotype or encoded protein (promoter) Source
S cerevisiae strains
FY1679 MATa/arho + , GAL2, ura3–52, trp1-D63, his3-D200, leu2-D1 [64] CDR07 MATa, rho+,GAL2, ura3–52, trp1-D63, his3-D200 leu2-D1,
ade2::GAL10/CYC1::D7Reductase::PGK1.
This study CDS04 MATa, rho + , GAL2, ura3–52, trp1-D63, his3-D200, leu2-D1, are1::G418 R , are2::HIS3 [28] CA10 MATa, rho +
, GAL2, ura3–52, trp1-D6, his3-D200, erg5::HYGROR, ade2::GAL10/CYC1::D7Reductase::PGK1,
LEU2::GAL10/CYC1::matADR::PGK1.
[19]
CA15 MATa, rho + , GAL2, ura3–52, trp1-D63, his3-D200, erg5::HYGRO R ,
ade2::GAL10/CYC1::D7Reductase::PGK1, LEU2::TEF1::matADR::PGK1
This study
CA17 MATa, rho + , GAL2, ura3–52, trp1-D63, erg5::HYGRO R ,
ade2::GAL10/CYC1::D7Reductase::PGK1, LEU2::
TEF1::matADR::PGK1, HIS3::TEF1::matADX::PGK1.
This study
CA14 MATa, rho + , GAL2, ura3–52, trp1-D63, his3-D200, erg5::HYGRO R atf2:: G418 R ,
ade2::GAL10/CYC1::D7Reductase::PGK1, LEU2::GAL10/CYC1::matADR::PGK1.
This study
CA19 MATa, rho + , GAL2, ura3–52, trp1-D6, his3-D200, erg5::HYGRO R , atf2:: G418 R ,
ade2:: GAL10/CYC1::D7Reductase::PGK1, LEU2::GAL10/CYC1::matADR::PGK1, HIS3::GAL10/CYC1::3b-HSD::PGK1.
This study
CA23 MATa, rho + , GAL2, ura3–52, trp1-D63, his3-D200,
erg5:: HYGROR, are1::G418R, are2::HIS3, ade2::
GAL10/CYC1::D7Reductase::PGK1, LEU2::GAL10/CYC1::matADR::PGK1
This study
TGY 120.2 MATa, rho+, GAL2, ura3–52, trp1-D63,
his3-D200, LEU2::TEF1::matADR::PGK1
This study
Plasmids (2micron replicon and URA3 selection marker and GAL10/CYC1 promoter for all the above cDNAs and gene)
Trang 4ARE2) [19] with CDS04 (MATa are1D are2D) [28] and
searching for the appropriate haploid segregants
Subcellular fractionation and Western blot analysis
In all subcellular fractionation experiments, recombinant
yeast cells were grown in synthetic SL medium to a density
of 107cells per mL
Lipid particles and mitochondrial and endoplasmic
reticulum (ER) membranes were prepared according to a
published protocol [29] The plasma membrane (PM)
fraction was obtained using electrostatic attachment of
spheroplasts on cationic silica beads as described [30]
For differential fractionation and sucrose density
centri-fugation, cell-free extracts were prepared in a manner
similar to that reported above Spheroplasts were disrupted
with a Dounce homogenizer and loaded onto a sucrose
gradient after centrifugation at 500 g to eliminate cellular
debris The gradient was continuous from 0.7–1.6Msucrose
in 10 mM Tris/HCl, pH 7.6, 10 mM EDTA and 1 mM
dithiothreitol and centrifuged for 4 h at 100 000 g and
4Cin a swinging bucket rotor 1.5 mL fractions were
collected from the bottom of the gradient The protein
contents were determined using a protein assay kit (Pierce
Chemical Co.)
Immunological analysis of each subcellular fraction was
carried out after separation of proteins by 8–12% SDS/
PAGE and transfer onto nitrocellulose membranes
(Hybond-C; Amersham Pharmacia Biotech) by standard
procedures as described [18, 24] Filters were probed with
antibodies to porin (an outer mitochondrial membrane
marker [31]), the 40-kDa microsomal protein [32], 3-PGK (a
cytosol marker from Molecular Probes Inc [33]), and
Pma1p (an integral membrane-bound H+-ATPase of the
PM [34]), to characterize yeast organelles For detection of
recombinant CYP11A1, Adxp, Adrp and 3b-HSD, rabbit
polyclonal antibodies obtained from Oxygene (Dallas, Tx,
USA) were used Anti-peptide D7-Red was generated using
a synthetic peptide containing amino acids 311–324 of
D7-reductase (H2
N-Tyr-Asp-Arg-Gln-Arg-Gln-Glu-Phe-Arg-Arg-Thr-Asn-Gly-Lys-COOH) coupled to keyhole
limpet hemocyanin by
N-maleimidobenzoyl-N-hydrosuc-cinimide ester cross-linking The resulting peptide/keyhole
limpet hemocyanin conjugate was injected subcutaneously
into female New Zealand White Rabbits (Neosystem
Laboratoire, Strasbourg, France) Immune complexes were
visualized using HRP-conjugated secondary antibodies
(Amersham Biosciences, Little Chalfont, UK), followed
by chemiluminescence (SuperSignal, Pierce Chemical Co.,
Rockford, IL USA)
Fluorescence and confocal microscopy
Yeast cells were fixed in 2% paraformaldehyde, converted
to spheroplasts, attached to polyL-lysine coated coverslips
and permeabilized as described [31] Samples were
incuba-ted with 1/20 dilutions of CYP11A1, Adrp,
anti-Adxp, anti-PGK, anti-Porin, anti-Gpa1p [35] (Plasma
Membrane marker, Santa Cruz Biotech Inc) Igs and a
1/5 dilution of anti-Dpm1p (dolichol phosphate mannose
synthase, Molecular Probes Inc) [36] in NaCl/Picontaining
1% BSA and 0.1% Tween 20 overnight at 4C They were
washed four times in 1· NaCl/Piand stained with a 1/150 dilution of CY2TM-conjugated anti-rabbit IgG (Interchim Inc.) and a 1/150 dilution of FITC-conjugated goat anti-mouse IgG (Santa Cruz Biotech Inc) for 30 min Samples were washed and mounted in 95% glycerol containing 0.1% p-phenylenediamine Observations were made with
a confocal microscope (model MRC-1000; Bio-Rad House, Hertfordshire, England; 1-lm optical serial sections) attached to a camera (model Optiphot; Nikon Inc.) equipped with a 60· plan apochromat objective (NA 1.3; Carl Zeiss Inc.) Images were collected using Bio-Rad image capture software, and projections were generated using confocal assistant software LASER SHARP 2000 (Bio-Rad)
Enzymatic activity assays Previously, steryl ester hydrolase assays were performed
as reported [37] except that cholesteryl[4-14C]oleate (100 mCiÆmL)1 in toluene, NEN Life Science Products Inc.) was used instead of cholesteryl[1-14C]oleate Side chain cleavage activity was measured as described [18]
Steroid and sterol analyses Steroids and sterols were extracted from yeast cells and analyzed by gas chromatography as described previously [19] Total concentration was measured after 100 h galac-tose induction in three independent assays Error bars on histograms indicate the standard errors of the means Statistical significance by paired t-tests was performed using theSTATVIEWprogram Statistical significance was assumed when P < 0.05
Results
The cellular pool of ergosta-5-eneol is not limiting for CYP11A1 activity
In the strain, CA10/pCD63, the reconstituted SCC system catalyzes the conversion of ergosta-5-eneol to pregnenolone, that accumulates also as the 3-acetyl ester form ([19], and Fig 1) Pregnenolone esterification was found to compete with progesterone production when mammalian 3b-HSD activity was introduced into this recombinant strain [19] Cauet and coworkers [38] further showed that in S cere-visiae, pregnenolone is acetylated by acetyl-coenzyme A: pregnenolone acetyl-transferase (APAT) activity encoded
by the ATF2 gene Therefore, both free pregnenolone production and the efficient coupling of the SCC and the 3b-HSD activities in yeast require the use of strains lacking this acetylating activity For this reason, the CA10 ATF2 gene was disrupted, yielding the strain, CA14 The strain, CA19 was constructed by integrating a human 3b-HSD expression cassette into the CA14 genome (see Materials and methods and Table 1)
As expected, pregnenolone acetate accumulation was not observed in the atf2D strains, CA14 and CA19 transformed with pCD63 compared to the ATF2 control strain CA10/ pCD63 (Fig 2A) In addition, pregnenolone was almost completely converted into progesterone when 3b-HSD activity was expressed (compare the CA19/pCD63 and
Trang 5CA14/pCD63 gas chromatography profiles) However, in
these two strains, the accumulation of ergosta-5-eneol,
which is both a CYP11A1 substrate and the main sterol of
CA10/pCD63 membranes, was greatly reduced, while at
least four other sterols accumulated (Fig 2B) They were
identified (by mass spectrometry and relative retention time
to cholesterol [39]) as intermediates of the ergosterol
biosynthesis pathway Namely, in CA14/pCD63
mem-branes, desmosterol (cholesta-5,24-dieneol) becomes the
major sterol while ergosta-5-enol, zymosterol
(cholesta-8,24-dieneol), fecosterol [8,24(28)-dieneol],
ergosta-5,7-dieneol and another sterol (Mr¼ 384, putatively
cholesta-7,24-dieneol) accumulate (Figs 1 and 2B) To a
lesser extent, the same phenomenon is observed in CA19/
pCD63 membranes; but in this strain, ergosta-5-eneol
remains the main sterol (Fig 2B) Thus, the production of
both pregnenolone and progesterone correlates with the
depletion of ergosta-5-eneol and the accumulation of
ergosta-5-eneol precursors in atf2D strains Moreover, the
addition of pregnenolone, or to a lesser extent of
progester-one, into the culture medium of CA14 (in the absence or
presence of CYP11A1) similarly induces the accumulation
of ergosterol biosynthesis intermediates (data not shown) The levels of free ergosta-5-eneol final (stationary phase) were reduced 10- and fivefold for CA14/pCD63 and CA19/ pCD63, respectively when compared to CA10/pCD63 (Fig 3B) However, the extent of steroid formation did not reflect this dramatic change in CYP11A1 substrate availability and was comparable for the strains CA10/ pCD63, CA14/pCD63, CA19/pCD63, which produce preg-nenolone acetate, pregpreg-nenolone and progesterone, respect-ively (Fig 3A) An almost complete conversion of pregnenolone into progesterone in CA19/pCD63 was associated with a more limited decrease in ergosta-5-eneol content than in the pregnenolone accumulating strain CA14/pCD63 This result is consistent with observation of the absence of inhibition by pregnenolone in the pregneno-lone acetylation-competent strain CA10 (Fig 2)
In conclusion, we showed that in atf2D strains the SCC reaction is readily coupled with 3b-HSD activity, permitting the efficient biosynthesis of progesterone A disruption of pregnenolone acetylase activity causes in turn a dramatic decrease in the cellular production of free ergosta-5-eneol, the CYP11A1 substrate, but has only limited effects on the
Fig 1 Schematic representation of the connected sterol and steroid pathway in yeast C, cytosol; ER, endoplamic reticulum; LP, lipid particles; mat, mature form of the proteins; PM, plasma membrane Steroids are shown in green Ncp1p, NADPH P450 reductase; Adxp, adrenodoxin; Adrp, adrenodoxin reductase; Are1p, Are2p, Atf2p, alcohol O-acetyltransferase (acetyl pregnenolone acetyl transferase); CYP11A1, P450 side chain cleaving; Erg2p, sterol C8-C7 isomerase; Erg3p, C-5 sterol desaturase; Erg5p, D 22(23) sterol desaturase; Erg6p, S-adenosyl methionine D-24-sterol-C-methyl-transferase; 3b-HSD, 3b-hydroxy steroid dehydrogenase.
Trang 6activity of CYP11A1 This suggests that the availability of
the CYP11A1 substrate is not limiting in the experimental
conditions used, but it might be limiting for higher levels of
CYP11A1 activity or expression
Tgl1p, a putative ester hydrolase, regulates
SCC activity
In wild-type yeast, more than 90% of the predominant
sterol is stored as esters [37] The balance between the levels
of free and esterified sterols is regulated by esterification and
hydrolysis and is modified in strains disrupted for the
ester-synthase genes ARE 1 and ARE 2 ([20,28]), or in which
steryl ester hydrolase activity is altered [40] To further
investigate the potential limiting effect of sterol availability
on CYP11A1 activity, we cloned and expressed the TGL1
gene, that is predicted to code for a 46-kDa protein with a
potential steryl ester hydrolase catalytic domain [41] Tgl1p
function was first evaluated in vitro by monitoring the
cholesteryl hydrolase activities of cell-free extracts prepared
from the wild-type strain, FY1679/pCD69 and an isogenic
are1D are2D double mutant, CDS04/pCD69 Surprisingly,
ester hydrolase activity in extracts of the control strain,
FY1679/pYeDP60 was higher than in extracts of FY1679/
pCD69, which overexpresses Tgl1p (Fig 4A) In contrast,
although control CDS04/pYeDP60 extracts exhibited the
same activity as FY1679/pYeDP60 extracts, CDS04/
pCD69 cell-free extracts exhibited a twofold increase in
cholesteryl ester hydrolase activity These apparently
con-tradictory results can be rationalized if cholesteryl esterase
activity can be monitored in vitro only in cellular extracts
devoid of endogenous sterol esters, that likely compete with
the labeled cholesteryl oleate used as a substrate Inhibition
in FY1679/pCD69 extracts also suggested a strong
prefer-ence of Tgl1p for yeast sterol esters compared to cholesteryl
oleate The hydrolytic activity of Tgl1p was further analyzed
in vivo(Fig 4B,C) Similar twofold, increases in the ratios of
free vs total ergosterol and ergosta-5-eneol were observed in
Tgl1p-overexpressing FY-1679 and CA10 cells, when
com-pared to their respective control strains (Fig 4B) This effect
was not observed in the corresponding are1D are2D double
mutant strains CDS04 and CA23, consistent with the
disruption of ester synthase genes (data not shown) As
established for ergosterol in wild-type strains [40], the
highest free ergosta-5-eneol to total protein ratio was
Fig 2 The efficient production of pregnenolone and progesterone (A) in
the atf2D recombinant strains CA14/pCD63 (C) (Derg5, expressing
CYP11A1) and CA19/pCD63 (Derg5, expressing CYP11A1 and
3bHSD) is accompanied by an accumulation of ergosta-5-eneol
precur-sors (B) Gas chromatography (GC) profiles were obtained from
cel-lular lysates prepared from cultures harvested after 100 h of induction
with galactose The sterol extraction procedure allows free sterol to be
detected (B) The ATF2 strain CA10/pCD63 was used as a control.
Relative retention times to cholesterol under our conditions are shown
between brackets Steroids are: P, pregnenolone (0.598); Pr,
pro-gesterone (0.685), PA, pregnenolone acetate (0.714) Atypical sterols
detected in CA14/pCD63 and CA19/pCD63 cells are the following: D,
desmosterol (1.04); E5, ergosta-5-enel (1.13); E5,7, ergosta-5,7-dieneol
(1.16); F, fecosterol (1.11); U, unknown sterol with a MW of 384 which
might be cholesta-7,24-dieneol (1.17); Z, zymosterol (1.06).
Trang 7observed in the PM of steroid-producing cells (Fig 4C) In
contrast, internal membranes (ER, lipid particles and
mitochondrial membranes) exhibited only residual levels
of ergosta-5-eneol Tgl1p overexpression did not modify this
subcellular distribution, and the influence of Tgl1p was
observed only in the PM fraction In summary, we showed
that Tgl1p containing extracts have a steryl ester hydrolytic
activity and that these extracts could mediate the release of
free ergosta-5-eneol from esterified forms in
steroid-produ-cing cells with the same efficiency as observed for ergosterol
in wild-type yeast cells In addition, Tgl1p overproduction
leads to an increase of free ergosta-5-eneol in tested
organelles, especially in the PM, which contains the highest
concentration of substrate
To determine whether the Tgl1p-dependent increase in
free ergosta-5-eneol content could affect the SCC reaction,
the concentrations of accumulated pregnenolone were
measured in both the wild-type strain CA10/pCD63 and
in the are1D are2D double mutant CA23/pCD63, in the
absence or presence of the Tgl1p overexpression construct
(Fig 5) As expected, CA23/pCD63, that is devoid of ester
synthase activity [42], contained higher amounts of free
ergosta-5-eneol than CA10/pCD63 (data not shown) This
phenomenon could explain the limited enhancement of
CYP11A1 activity observed in the are1D are2D mutant
(Fig 5) When CA10 and CA23 were cotransformed with
pCD69, only the free ergosta-5-eneol content of the former
strain was increased (Fig 4B and data not shown), whereas pregnenolone production was enhanced in both strains, with similar final concentrations of the steroid (Fig 5) Therefore, these results reveal two levels of complexity of the CYP11A1 reaction in yeast On one hand, the content
of free ergosta-5-eneol poorly correlates with the extent of pregnenolone production, suggesting that SCC activity is only partially limited by substrate concentration (Fig 1)
On the other, an artificial increase in the level of free ergosta-5-eneol improves the yield of steroid (Fig 5) This latter effect may involve steryl hydrolase activity potentially encoded by TGL1 Moreover, the effects of Tgl1p on CYP11A1 activity is observed both in the presence and absence of ester synthase activity
In recombinant yeast, the concentration of Adxp, but not of Adrp, controls SCC activity
To determine whether electron transfer from NADPH to CYP11A1 via Adrp and Adxp could regulate the extent of synthesis, we built two CA10 derivatives, CA15 and CA17 Like CA10, CA15 carries a unique ADR expression cassette integrated in the LEU2-SPL1 intergenic region but in CA15 the mature ADR ORF is under the control of the constitu-tive TEF1 promoter instead of the inducible GAL10/CYC1 promoter CA17 also carries a cassette with the mature ver-sion of ADX integrated in the HIS3-DED1 intergenic region (Table 1) The accumulation of steroids in the strains CA15/pCD63 (that carries a multicopy plasmid bearing CYP11A1 and ADX) and CA17/pV13sccm (that carries a multicopy plasmid for CYP11A1 and has a single integrated copy of ADX) was compared to that in CA10/pCD63 The level of expression of mature Adrp was found to be lower in CA15/pCD63 as compared to CA10/pCD63, as judged by Western blot analysis (data not shown and [43]) In contrast, CA17 exhibited the same level of Adrp as CA15 but a lower level of Adxp as Adxp was expressed from a single genomic copy (data not shown and Table 1) Similar concentra-tions of pregnenolone (2.9 ± 0.5 mgÆL)1A600units) were observed for CA10/pCD63 and CA15/pCD63, whereas a 36-fold decrease was observed for CA17/pCD63 These results suggest that CYP11A1 activity depends strongly on the levels of expression of Adxp but not Adrp and therefore that the concentration of Adxp is the major factor controlling pregnenolone synthesis in recombinant yeast
In conclusion, the SCC reaction appears to be regulated similarly in yeast and in adrenal cells; in the latter, Adxp and not Adrp limits the activity of CYP11A1 and hence controls the extent of pregnenolone production [14]
Protein partners involved in the yeast SCC system localize to three distinct subcellular compartments
To gain a deeper insight into how the artificial steroid path-way is coordinated with the endogenous ergosta-5-eneol pathway, we determined the subcellular localizations of Adrp, Adxp, D7-Red and 3b-HSD A total cell extract from the progesterone-producing strain CA19/pCD63 was subjected to sucrose gradient fractionation (Fig 6) Char-acterization of each fraction with Pma1p (PM), anti-3-PGKp (cytosol), anti-40 kDa protein (ER) and anti-porin (mitochondrial membranes) antisera revealed that Adrp,
Fig 3 The final levels of accumulated steroids are independent of Datf2
genetic background (A) and do not correlate with the content of free
ergosta-5-eneol (B) Steroid-producing cells were grown as described in
Material and methods Accumulated steroid contents are the sum of
pregnenolone and pregnenolone acetate (CA10/pCD63) (Derg5,
expressing CYP11A1), pregnenolone (CA14/pCD63) (Fig 2),
preg-nenolone and progesterone (CA19/pCD63) (Fig 2) Statistical
signi-ficance by paired t-tests was performed using the STATVIEW program.
**P < 0.05 when a strain is compared to the two others (B).
Trang 8D7-Red and 3b-HSD clearly localize to the ER whereas
Adxp is a soluble protein, as shown previously [19] In
contrast to the other enzymes, recombinant CYP11A1 was
not restricted to a single subcellular compartment but
instead was found distributed broadly throughout the
gradient Either this experiment reflects a broad intracellular
distribution of the protein, or a cell surface transport of the
protein as described [44,45] To distinguish between these
two hypotheses, indirect immunofluorescence studies were
performed with polyclonal Igs raised against markers for
each of the different compartments (Fig 7 A,D,G, red
fluorescence, CY-2TMconjugated secondary Ig) The PM
(Fig 7B), ER (Fig 7E) and mitochondrial membranes
(Fig 7H) were simultaneously visualized in the same cells
with Igs to Gpa1p, Dpm1p and porin, respectively (green
fluorescence, FITC-conjugated secondary antibodies)
Fluorescence was not detected in control experiments
performed in the absence of the primary anti-CYP11A1
and anti-marker Igs (data not shown) Confocal microscopy
was used to evaluate the degree of CYP11A1 colocalization
with each of these organelle markers, as seen in merged
images (Fig 7 C,F,I) CYP11A1 was found to colocalize
with the PM marker Gpa1p (Fig 7C; yellow areas
corres-pond to regions where the red and green signals are
superimposed), suggesting that most of the CYP11A1
antigen resides in the PM There was no overlap between the
red and green signals corresponding to the CYP11A1 and
porin mitochondrial markers, respectively, but some yellow
signals could be seen in cells labeled with both the CYP11A1 and the ER marker Ig (Fig 7) However, the best corres-pondence was observed for the CYP11A1-derived signal and the PM-derived signal In conclusion, the CYP11A1 antigen appears to be excluded from mitochondria in vivo, and most of the antigen is detected in the plasma membrane, with a minor fraction localizing to the ER
To determine whether the SCC reaction occurs in the
PM, where most CYP11A1 is found, or in the ER, we performed an analysis of free ergosta 5-eneol distribution in the steroid-producing strain, CA19/pCD63 and in the control strain, CA19/pDP10037 Figure 8 shows that the level of free sterol in the PM is significantly depleted upon the expression of SCC activity, whereas no decrease is
Fig 4 Tgl1p has a steryl ester hydrolase activity Tgl1p activity was
illustrated by comparing the steryl esterase cell free extract activities
of Tgl1p-overexpressing cells (strains transformed with pCD69,
stripped bars) vs the controls (strains transformed with the vector
pYeDP60, gray bars) The CDS04 strain is an Dare1, Dare2 isogenic
derivative of FY1679-28 Cand was generated by disruption of both
ARE 1 and ARE 2 genes, that encode two sterol ester-transferases
that catalyze the synthesis of steryl ester in yeast (A) The cholesteryl
esterase activity of Tgl1p is detected only in the sterol esterification
deficient strain, CDS04 (Dare1, Dare2) Experiments used
choleste-ryl[4-14C]oleate as a substrate [37] Specific activities were measured
in whole cell extracts prepared from lysed spheroplasts of FY1679/
pYeDP60, FY1679/pCD69 (expressing Tgl1p), CDS04/pCD69 (see
above), and CDS04/pYeDP60 cells Data are mean values ± SEM
from three independent experiments with a maximum deviation of
5% (B) The free ergosterol and ergosta-5-eneol contents are
increased in the Tgl1p-overproducing strains FY1679, CA10 (Derg5)
and CA10/pCD63 (Derg5 expressing CYP11A1), respectively Sterols
were extracted from cellular lysates and analyzed by gas
chroma-tography with or without preliminary saponification for detection of
total sterols and free sterols, respectively Data are expressed as ratio
of free vs total sterol Data are mean values ± SEM obtained from
three independent experiments (C) The subcellular partitioning of
free ergosta-5-eneol is not changed by TGL1 overexpression Whole
cell lysates were subjected to fractionation to isolate subcellular
organelles, mitochondria, lipid particles and ER as described [29]
and PM as described [30] Data are expressed as the sterol : protein
ratio and are mean values from three independent experiments with
a maximum deviation of 5% The ratio obtained in lipid particles
were 6.69 · 10)4 and 6.78 · 10)4 mgÆmg protein)1 in CA10/
pYeDP60 and CA10/pCD69, respectively.
Trang 9observed in the ER or mitochondrial fractions These data
are consistent with ergosta-5-eneol bioconversion at the
PM level In conclusion, the mature form of recombinant
CYP11A1 (e.g., the protein without the mitochondrial
targeting sequence and with an extra methionine at the
NH2 terminus) is primarily in the PM in yeast, and the SCC system depends on electron transfer from ER-localized Adrp to cytosolic Adxp and finally to CYP11A1
in the PM
Discussion
The reconstruction in S cerevisiae of the steroidogenic pathway allows the self-sufficient production of pregneno-lone and progesterone, as reported previously [19] (Fig 1) Like mammals, S cerevisiae possesses a system that efficiently protects against the toxic accumulation of preg-nenolone In mammals, pregnenolone is present as a biologically active sulfo-conjugate [46], whereas in yeast, APAT activity converts pregnenolone into the correspond-ing acetate ester [38] Acetylation or sulfatation at position 3
of pregnenolone prevents further metabolism by 3b-HSD Yeast strains devoid of APAT activity allow high-yield biosynthesis of free pregnenolone or progesterone but accumulate ergosta-5-eneol biosynthesis intermediates, such
as desmosterol, fecosterol and zymosterol (Fig 1) This phenomenon likely results from the inhibition of S-adenosyl sterol 24-methyl transferase (Erg6p [47]), and to a lesser extent of sterol C8–C7 isomerase (Erg2p [48]) Erg6p permits the transformation of cholesta-derivatives into ergosta-derivatives Thus, inhibition of Erg6p allows the accumulation of zymosterol that is sequentially transformed
by Erg2p and Erg3p (the C-5 desaturase [49]), into cholesta-7,24-dieneol and cholesta-5,7,24-trieneol, respectively (Fig 1) In the presence of D7-reductase, the latter is modified into cholesta-5,24-dieneol (desmosterol) that is detected in the membranes of CA14/pCD63 (Fig 2B) Accumulation of zymosterol indicates that Erg2p might also be inhibited A similar effect was observed in mamma-lian cells, in that progesterone [50] and pregnenolone [51] inhibit cholesterol biosynthesis, resulting in the accumula-tion of cholesterol precursors Finally, in yeast strains devoid of APAT activity, there is almost no accumulation of pregnenolone when 3b-HSD activity is present, and the rate
of progesterone biosynthesis is only marginally reduced compared to the rate observed for pregnenolone alone Therefore, an efficient coupling of the two first steps of steroidogenesis is possible, and as reported for mammalian cells [9], the SCC reaction is the rate-determining step in progesterone synthesis in yeast
The yeast SCC system offers the possibility of increasing
or decreasing the availability of the endogenous substrate Ergosterol or related yeast sterols exist in the free form or as esters conjugated to fatty acids Conversion between free sterols and steryl esters is thus a critical homeostatic determinant for membrane function in yeast as in all eukaryotic cells ([52,53]) The PM is the major subcellular location of free sterol [40] Steryl esters are synthesized in the
ER by the ACAT-related ARE 1 and ARE 2 gene products [20,42], stored in lipid droplets and mobilized by a process involving steryl ester hydrolases in the PM [37]
Our work shows that Tgllp over-expressing cells and extracts exhibit steryl ester hydrolase activities in vivo and
in vitro, respectively As it has been shown previously that Tgl1p had significant homologies to triglyceride lipase, it is rather likely that Tgl1p is a steryl ester hydrolase [41] In the steroid-producing yeast strain, CA10/pCD63 free
Fig 6 Subcellular localization of heterologous CYP11A1, Adrp, Adxp,
D7-Red, 3b-HSD and organelle membrane after sucrose density
frac-tionation in recombinant yeast The heterologous Adrp, D7-Red and
3b-HSD proteins cofractionate with the ER marker (40 kDa protein)
and Adxp cofractionates with the cytosol marker 3-PGK on a sucrose
continuous gradient No clear subcellular localization in the PM, ER
or mitochondrial membranes was observed for CYP11A1 CA19/
pCD63 (Datf2, Derg5, expressing CYP11A1 and 3bHSD) cells were
grown as described in Materials and methods, converted to
sphero-plasts, lysed and fractionated on a continuous sucrose gradient (0.7–
1.6 M ) from the bottom (fraction 1) to the top (fraction 25) Fractions
were subjected to Western blot analysis with Igs against the following
organelle marker proteins: 3-PGK for cytosol, Pma1p for the PM, the
40-kDa protein for the ER and porin for mitochondria The
distri-bution of heterologous proteins was similarly detected with Igs against
CYP11A1, Adrp, Adxp, 3b-HSD and D7-Red.
Fig 5 Pregnenolone biosynthesis is increased in recombinant yeast cells
overproducing Tgl1p Pregnenolone was extracted from cellular lysates
prepared from cultures harvested after 100 h of induction by galactose
[19] The Tgl1p-overproducing effect was analyzed in ARE 1 ARE 2
strains (CA10/pCD63 + pCD69) (Derg5, expressing CYP11A1
and Tgl1p) and (CA10/pCD63 + pYeDP60) (Derg5, expressing
CYP11A1) and in Dare 1 Dare 2 strains (CA23/pCD63 + pCD69)
(Derg5, expressing CYP11A1 and Tgl1p) and CA23/pCD63 +
pYeDP60) (Derg5, expressing CYP11A1).
Trang 10ergosta-5-eneol represents, as expected, only a minor
fraction of the cellular pool of sterols, and this fraction
increases when Tgl1p is overexpressed or when both the
ARE 1and ARE 2 genes are deleted Thus, the two ester synthases, Arelp and Are2p and the probable steryl ester hydrolase Tgllp, play complementary roles in maintaining free ergosta-5-enol at appropriate levels, reminiscent of the mechanism reported for cholesterol in adrenal cells ([40,54])
In addition to normally cycling between the PM and other cellular organelles, free ergosta-5-eneol also must be avail-able in the yeast cell to serve as a substrate for CYP11A1 While SCC driven formation of pregnenolone likely occurs
at the PM, ergosta-5-enol biosynthesis occurs mainly in the
ER, consistent with D7-Red localization (Fig 6) Are1p and Are2p are localized in the ER [42] while Tgl1p could be localized in lipid particles, and activated with the supply of sterol from lipid particles to PM Therefore, de novo synthesis and transport processes are critical for continued activity
In theory, the sterol pool used for steroidogenesis must be constantly supplied from cellular sterol stores while the membrane structural pool is more static, at least for cells in stationary phase, in which CYP11A1-dependent conversion
is active but cell growth has ceased Thus, free and esterified sterol pools are exchanged rapidly Indeed, Tgllp overpro-duction results in an increased level of free sterol (Fig 4B), and it would also be expected to increase the cycling of sterols by esterification and hydrolysis, resulting in an increase in intracellular sterol trafficking It is unclear, then, whether the corresponding increase in pregnenolone pro-duction results from an increase in free sterol concentration
or from an enhancement of sterol trafficking
Fig 7 CYP11A1 colocalizes with the
endo-genous Gpa1p plasma membrane protein.
Spheroplasts from CA19/pCD63 (Datf2,
Derg5, expressing CYP11A1 and 3b )HSD)
were fixed, permeabilized and then incubated
with primary polyclonal CYP11A1 Igs A
CY2TMconjugated secondary Ig (red
fluores-cence in A, D, G) was used to visualize the
CYP11A1 protein PM (B), ER (E) and
mitochondrial (H) membranes were detected
in the same cells with anti-Gpa1p (guanine
nucleotide-binding protein alpha subunit),
Dpm1p (dolichol-phosphate
mannosyltrans-ferase) and porin monoclonal Igs coupled to
FITCsecondary Igs (green fluorescence) The
merged CY2TMand FITCimages are shown
in C, F and I.
Fig 8 Distribution of the CYP11A1 substrate Whole cell lysates were
fractionated to isolate the ER and mitochondrial membranes as
des-cribed [29] and PM as desdes-cribed [30] Data are expressed as percent of
the total activity measured for the whole cellular extract Relative to
CA10/pDP10037 (gray bars, Derg5), the pregnenolone-producing
strain CA10/pCD63 (stripped bars, Derg5 expressing CYP11A1)
shows a significant depletion of the levels of free ergosta 5-eneol in the
PM fraction and increased overall levels in the ER and mitochondrial
fractions.