Regulated expression of sterol carrier protein 2 in the ovary: a key role for cyclic AMP

Role of STAR and SCP2/SCPx in the Transport of Cholesterol and Other Lipids

Cholesterol is a lipid molecule essential for several key cellular processes including steroidogenesis. As such, the trafficking and distribution of cholesterol is tightly regulated by various pathways that include vesicular and non-vesicular mechanisms. One non-vesicular mechanism is the binding of cholesterol to cholesterol transport proteins, which facilitate the movement of cholesterol between cellular membranes. Classic examples of cholesterol transport proteins are the steroidogenic acute regulatory protein (STAR; STARD1), which facilitates cholesterol transport for acute steroidogenesis in mitochondria, and sterol carrier protein 2/sterol carrier protein-x (SCP2/SCPx), which are non-specific lipid transfer proteins involved in the transport and metabolism of many lipids including cholesterol between several cellular compartments. This review discusses the roles of STAR and SCP2/SCPx in cholesterol transport as model cholesterol transport proteins, as well as more recent findings that support the role of these proteins in the transport and/or metabolism of other lipids.

Sterol carrier protein 2 in lipid metabolism and non-alcoholic fatty liver disease: Pathophysiology, molecular biology, and potential clinical implications

Non-alcoholic fatty liver disease (NAFLD) is considered as the most common chronic liver disease and has become a rapidly global public health problem. Sterol carrier protein 2 (SCP-2), also called non-specific lipid-transfer protein, is predominantly expressed by the liver. SCP-2 plays a key role in intracellular lipid transport and metabolism. SCP-2 has been closely implicated in the development of NAFLD-related metabolic disorders, such as obesity, atherosclerosis, Type 2 diabetes mellitus (T2DM), and gallstones. Recent studies indicate that SCP-2 plays a beneficial role in NAFLD by regulating cholesterol-, endocannabinoid-, and fatty acid-related aspects of lipid metabolism. Hence, in this paper, we summarize the latest findings about the roles of SCP-2 in hepatic steatosis and further describe its molecular function in the pathogenesis of NAFLD.

The Key Role of Peroxisomes in Follicular Growth, Oocyte Maturation, Ovulation, and Steroid Biosynthesis

The absence of peroxisomes can cause disease in the human reproductive system, including the ovaries. The available peroxisomal gene-knockout female mouse models, which exhibit pathological changes in the ovary and reduced fertility, are listed in this review. Our review article provides the first systematic presentation of peroxisomal regulation and its possible functions in the ovary. Our immunofluorescence results reveal that peroxisomes are present in all cell types in the ovary; however, peroxisomes exhibit different numerical abundances and strong heterogeneity in their protein composition among distinct ovarian cell types. The peroxisomal compartment is strongly altered during follicular development and during oocyte maturation, which suggests that peroxisomes play protective roles in oocytes against oxidative stress and lipotoxicity during ovulation and in the survival of oocytes before conception. In addition, the peroxisomal compartment is involved in steroid synthesis, and peroxisomal dysfunction leads to disorder in the sexual hormone production process. However, an understanding of the cellular and molecular mechanisms underlying these physiological and pathological processes is lacking. To date, no effective treatment for peroxisome-related disease has been developed, and only supportive methods are available. Thus, further investigation is needed to resolve peroxisome deficiency in the ovary and eventually promote female fertility.

Yellow fever mosquito sterol carrier protein-2 gene structure and transcriptional regulation

AeSCP-2, a sterol carrier protein, is involved in sterol trafficking in mosquitoes. The activity of the AeSCP-2 gene is important for mosquito development. An earlier study demonstrated that the transcription of this gene was upregulated by 20-hydroxyecdysone (20E) in cultured gut tissues. To investigate 20E-regulated transcription of the AeSCP-2 gene we truncated the upstream flanking region of AeSCP-2 gene and linked it to a reporter gene. The mosquito Aag-2 cell line was transfected with these promoter/reporter constructs and treated with 20E at various concentrations. Expression vectors of different transcription factors such as HR3 and beta FTZ-F1 were also co-transfected with the AeSCP-2 promoter/reporter constructs. The observed results demonstrated that varied combinations of transcription factors produce different promoter activities of the AeSCP-2 gene. This observation leads us to the conclusion that the partnership of transcription factors is crucial in regulating the transcriptional activity of the AeSCP-2 gene.

The differential transcriptome and ontology profiles of floating and cumulus granulosa cells in stimulated human antral follicles

Communication between various ovarian cell types is a prerequisite for folliculogenesis and ovulation. In antral follicles granulosa cells divide into two distinct populations of mural and cumulus granulosa cells (CGC), enveloping the antrum and surrounding the oocyte, respectively. Both cell types, with the mural compartment in excess, contribute to the floating granulosa cell (FGC) population in the follicular fluid. The aim of this study was to compare the transcriptomes of FGC and CGC in stimulated antral follicles obtained from 19 women undergoing IVF-ICSI procedure. FGC were obtained from follicular fluid during the follicle puncture procedure and CGC were acquired after oocyte denudation for micromanipulation. Gene expression analysis was conducted using the genome-wide Affymetrix transcriptome array. The expression profile of the two granulosa cell populations varied significantly. Out of 28 869 analysed transcripts 4480 were differentially expressed (q-value < 10(-4)) and 489 showed > or =2-fold difference in the expression level with 222 genes up-regulated in FGC and 267 in CGC. The transcriptome of FGC showed higher expression of genes involved in immune response, hematological system function and organismal injury, although CGC had genes involved in protein degradation and nervous system function up-regulated. Cell-to-cell signalling and interaction pathways were noted in both cell populations. Furthermore, numerous novel transcripts that have not been previously described in follicular physiology were identified. In conclusion, our results provide a solid basis for future studies in follicular biology that will help to identify molecular markers for oocyte and embryo viability in IVF.

The molecular control of corpus luteum formation, function, and regression

The corpus luteum (CL) is one of the few endocrine glands that forms from the remains of another organ and whose function and survival are limited in scope and time. The CL is the site of rapid remodeling, growth, differentiation, and death of cells originating from granulosa, theca, capillaries, and fibroblasts. The apparent raison d'etre of the CL is the production of progesterone, and all the structural and functional features of this gland are geared toward this end. Because of its unique importance for successful pregnancies, the mammals have evolved a complex series of checks and balances that maintains progesterone at appropriate levels throughout gestation. The formation, maintenance, regression, and steroidogenesis of the CL are among the most significant and closely regulated events in mammalian reproduction. During pregnancy, the fate of the CL depends on the interplay of ovarian, pituitary, and placental regulators. At the end of its life span, the CL undergoes a process of regression leading to its disappearance from the ovary and allowing the initiation of a new cycle. The generation of transgenic, knockout and knockin mice and the development of innovative technologies have revealed a novel role of several molecules in the reprogramming of granulosa cells into luteal cells and in the hormonal and molecular control of the function and demise of the CL. The current review highlights our knowledge on these key molecular events in rodents.

EGF-like factor epiregulin and amphiregulin expression is regulated by gonadotropins/cAMP in human ovarian follicular cells

Epiregulin and amphiregulin are growth factors involved in cancer development, but their potential role in signaling in the gonads is still obscure. We report here that basal expression of these growth factors is evident in human granulosa cells obtained from women treated for in vitro fertilization, when examined by RT-PCR using RNA isolated from primary cultures of ovarian granulosa cells. Expression of these factors was elevated concomitantly with elevation of progesterone production in these cells upon stimulation with luteinizing hormone (LH), and to a lesser extent with follicle stimulating hormone (FSH), both essential stimulants for ovulation and luteinization. Epiregulin and amphiregulin gene expression was dose- and time-dependent when measured subsequent to LH stimulation. Moreover, forskolin, which activates adenylate cyclase, was as efficient as LH in stimulating expression of these growth factors. It is suggested that upregulation of the epiregulin and amphiregulin expression is part of the signal transduction pathway which leads to ovulation and luteinization in the human ovary.

Peroxisome proliferator-activated receptor alpha induces rat sterol carrier protein x promoter activity through two peroxisome proliferator-response elements

Sterol carrier protein x (SCPx) plays a critical role in the peroxisomal oxidation of fatty acids. It has been previously demonstrated in streptozotocin-induced diabetic rats that SCPx expression is induced in association with an elevation in serum fatty acid and triglyceride levels. To elucidate the mechanisms underlying the expression of this gene during diabetes, the rat SCPx promoter was cloned and analyzed for regulatory motifs. Sequence analysis of this TATA-less promoter revealed two putative peroxisomal-proliferator-response element (PPRE) binding motifs at positions -134 and -869 relative to the translation start site. To examine peroxisomal-proliferator-activated receptor alpha (PPARalpha) effects on this gene, 935 bp of the SCPx promoter containing both PPRE motifs was cloned in front of the chloramphenicol acetyl-transferase gene or the luciferase gene and co-transfected into HTB-9 cells with vectors that encoded for PPARalpha and retinoid X receptor (RXR). The results indicate that PPARalpha was able to induce SCPx promoter activity in both cases, an effect that was enhanced by RXR and clofibrate. In addition, mutational analysis studies demonstrated that both PPREs contributed to the PPARalpha/RXRalpha-dependent activation of the SCPx promoter. Mobility shift assays and supershift analysis showed that nuclear extracts containing PPARalpha bound to the two PPRE motifs. This investigation indicates that similar to other genes involved in beta-oxidation, SCPx transcription may be controlled by fatty acid levels via PPARalpha.

Novel genes modulated by FSH in normal and immortalized FSH-responsive cells: new insights into the mechanism of FSH action

Follicle-stimulating hormone (FSH) controls the development of follicle-enclosed oocytes in the mammalian ovary by interacting with specific receptors located exclusively on granulosa cells. Its biological activity involves stimulation of intercellular communication, intracellular signaling, and up-regulation of steroidogenesis; the entire spectrum of genes regulated by FSH is not yet fully characterized. We have established monoclonal rat FSH-responsive granulosa cell lines that express FSH receptors at 20-fold higher rates than with primary cells, and thus increased the probability of yielding a distinct spectrum of genes modulated by FSH. Using Affymetrix DNA microarrays, we discovered 11 genes not reported earlier to be up-regulated by FSH and 9 genes not reported earlier to be down-regulated by FSH. Modulation of signal transduction associated with G-protein signaling, phosphorylation of proteins, and intracellular-extracellular ion balance was suggested by up-regulation of decay accelerating factor GPI-form precursor (DAF), membrane interacting protein RGS16, protein tyrosine phosphatase (PTPase), oxidative stress-inducible protein tyrosine phosphatase (OSIPTPase), and down-regulation of rat prostatic acid phosphatase (rPAP), Na+, K+-ATPase, and protein phosphatase 1beta. Elevation in granzyme-like proteins 1 and 3, and natural killer (NK) cell protease 1 (NKP-1) along with reduction in carboxypeptidase E indicates possible FSH-mediated preparation of the cells for apoptosis. Up-regulation of vascular endothelial growth factors indicates the ability of FSH to produce angiogenic factors upon their maturation; whereas, reduction in insulin-like growth factor binding protein (IGFBP3) indicates its increased potential to promote p53-induced apoptosis. Striking similarities in FSH modulation of gene expression were found in primary cultures of human granulosa cells obtained from IVF patients although these cells expressed only 1% of FSH receptor compared with immortalized rat cells, as indicated by microarray technique, which probably is in the normal range of expression of this receptor in nontransformed cells. These findings should increase our understanding of the mechanism of FSH action in stimulating development of the ovarian follicular cells, of intracellular and intercellular communication, and of increasing the potential of ovarian follicular cells to undergo apoptosis during the process of selection of the dominant follicle.

Isolation and expression of a sterol carrier protein-2 gene from the yellow fever mosquito, Aedes aegypti

Trafficking of cholesterol in insects is a very important process due to the fact that insects depend on dietary cholesterol to fulfil their physiological needs. We identified a putative mosquito sterol carrier protein-2 (SCP-2) cDNA from fourth instar subtracted cDNA library. The AeSCP-2 protein has high degree homology in the sterol transfer domain to both rat and human SCP-2. Transcripts of AeSCP-2 in fourth instars were detected strongly in the midgut, and weakly in the head and hindgut. In the early pupae, AeSCP-2 transcription was observed in the thorax, head and body wall of abdomen, but not in the gut. The interaction of mosquito sterol carrier protein-2 (AeSCP-2) with cholesterol was examined. The Kd of purified recombinant AeSCP-2 to cholesterol was 5.6 +/- 0.6 x 10-9 m using radiolabelled cholesterol-binding assay. The results suggest that AeSCP-2 has high affinity to cholesterol and may function as a carrier protein in mosquitoes.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Mechanisms underlying the steroidogenic synergy of insulin and luteinizing hormone in porcine granulosa cells: joint amplification of pivotal sterol-regulatory genes encoding the low-density lipoprotein (LDL) receptor, steroidogenic acute regulatory (stAR) protein and cytochrome P450 side-chain cleavage (P450scc) enzyme

Growth of ovarian Graafian follicles and cytodifferentiation of granulosa and theca cells are regulated by gonadotropins, sex steroids and peptidyl growth factors. For example insulin and intraovarian insulin-like growth factor type I (IGF-I) may amplify the actions of both follicle stimulating hormone (FSH) and luteinizing hormone (LH) in promoting biochemical luteinization and enhancing steroidogenesis. To explore further the notion of interactions between insulinomimetic peptides and LH and to examine the associated mechanisms, we have established porcine granulosa cells in monolayer culture for 48 h in 3% serum with insulin (1 microg/ml), estradiol (0.5 microg/ml), and follicle stimulating hormone (FSH, 5 ng/ml) to allow cell anchorage, facilitate in vitro cytodifferentiation and confer LH responsiveness. To limit any carry-over effects of serum, granulosa cells were stabilized overnight in serum-free medium. Studies were then initiated to assess the impact of insulin on the dose-responsive actions of LH. A maximally effective concentration of insulin (1 microg/ml) synergistically augmented LH's dose-dependent ampilification of progesterone and cAMP accumulation; viz. by approximately twofold (progesterone) and approximately 2.5-fold (cAMP) above that observed in maximally LH-stimulated cultures (P < 0.001). Mechanistically, insulin significantly enhanced the sensitivity of granulosa cells to LH's drive of cAMP accumulation [ED50 for LH 61 +/- 14 ng/ml (control) vs. 10 +/- 1.0 ng/ml (insulin) (P < 0.01)]. Insulin also augmented the maximal stimulatory effect of LH; i.e. LH efficacy rose from 6.5 +/- 0.4 to 17 +/- 1.4 (pmole cAMP/microg DNA/48 h; P < 0.001). Insulin dose-response analysis showed that insulin alone minimally elevated basal, but significantly heightened LH's stimulation of progesterone and cAMP accumulation at (insulin) concentrations as low as 3-10 ng/ml. The molecular mechanisms underlying insulin and LH's synergy were assessed by RNase protection assays with (porcine) cRNA probes encoding the low density lipoprotein receptor (LDL-R), Steroidogenic Acute Regulatory Protein (StAR), P450 cholesterol sidechain cleavage enzyme (P450scc) and (as a possible negative control) Sterol Carrier Protein 2 (SCP-2) [data normalized to constitutive 18S rRNA]. Non linear least-squares analysis was applied to confirm or refute an hypothesis of interactive synergy between LH and insulin on gene expression. LH and insulin alone exerted no effect on StAR message accumulation, and LH alone minimally stimulated P450scc and LDL-R mRNA's accumulation at 48 h. In contrast, insulin in combination with LH augmented StAR mRNA concentrations by approximately 5-10-fold and stimulated LDL-R message levels by threefold above the respective maximally LH-driven values (P < 0.01). Maximal P450scc mRNA expression was enhanced twofold by cotreatment with LH and insulin compared with maximal LH-treated cultures. In contrast SCP-2 mRNA accumulation remained unaffected by any treatment. In summary, we have used a serum-free, in vitro differentiated porcine granulosa cell culture system to assess regulatory interactions between the disparate first messengers, LH and insulin. We observe marked LH-insulin steroidogenic synergy after 48 h of joint hormonal stimulation, and further clarify that the mechanism(s) of synergy include augmentation of cAMP production and increased steady-state concentrations of transcripts of key sterol-regulatory genes; namely, LDL-R, StAR, and P450scc, but not SCP-2. Since the encoded products of these genes variously control sterol substrate uptake, delivery to and utilization in mitochondrial steroidogenesis, we speculate that the concerted actions of insulin-like peptides and LH may contribute to steroidogenic differentiation during the later stages of follicular maturation and the granulosa-luteal cell transition.

Unveiling the mechanism of action and regulation of the steroidogenic acute regulatory protein

Stimulation of steroid-producing cells of the gonads and adrenals with trophic hormone (LH, and ACTH, respectively) produces a marked increase in steroid hormone synthesis within minutes. The rate-limiting step in this acute steroidogenic response is the transport of cholesterol from the outer to the inner mitochondrial membrane, where the first committed step in steroid synthesis is performed by the side-chain cleavage enzyme system (P450scc), resulting in the production of pregnenolone. This process of cholesterol translocation is blocked by inhibitors of protein synthesis (i.e. cycloheximide) indicating that the effect of trophic hormones, acting through the intermediacy of cAMP, most likely involves the de novo synthesis of a protein that is rapidly inactivated. The recently identified steroidogenic acute regulatory protein (StAR) appears to be the most likely candidate for the labile protein: (1) StAR is synthesized in response to cAMP and the StAR preprotein disappears rapidly in the presence of inhibitors of protein synthesis; (2) StAR has an N-terminal targeting sequence that directs the protein to the mitochondria; and (3) StAR protein is expressed almost exclusively in steroid-producing cells, its presence is correlated with steroid hormone production, and lack of functional StAR causes the autosomal recessive disease congenital lipoid adrenal hyperplasia (lipoid CAH), characterized by markedly impaired gonadal and adrenal steroid hormone synthesis. We have demonstrated that StAR is a target for serine phosphorylation mediated by protein kinase A (PKA), a process that is essential to maximizing StAR activity. StAR import by mitochondria is not essential to its steroidogenesis enhancing activity, and more likely, represents a means of rapidly inactivating StAR. Truncation mutations and site-directed mutations in StAR demonstrated that the C-terminus of the protein contains the functionally important domains. Further, we have demonstrated potent steroidogenic activity of recombinant StAR protein on isolated mitochondria from bovine corpus luteum using protein that lacks the mitochondrial targeting sequence. These observations confirm that StAR import is not essential for its steroidogenic activity and suggest that StAR acts directly on the outer mitochondrial membrane in the absence of intermediary cytosolic factors. More recently, we have found that StAR functions as a cholesterol transfer protein that does not require a protein receptor or co-factor, suggesting that StAR acts directly on lipids of the outer mitochondrial membrane to promote cholesterol translocation.

Defective peroxisomal catabolism of branched fatty acyl coenzyme A in mice lacking the sterol carrier protein-2/sterol carrier protein-x gene function

Gene targeting in mice was used to investigate the unknown function of Scp2, encoding sterol carrier protein-2 (SCP2; a peroxisomal lipid carrier) and sterol carrier protein-x (SCPx; a fusion protein between SCP2 and a peroxisomal thiolase). Complete deficiency of SCP2 and SCPx was associated with marked alterations in gene expression, peroxisome proliferation, hypolipidemia, impaired body weight control, and neuropathy. Along with these abnormalities, catabolism of methyl-branched fatty acyl CoAs was impaired. The defect became evident from up to 10-fold accumulation of the tetramethyl-branched fatty acid phytanic acid in Scp2(-/-) mice. Further characterization supported that the gene disruption led to inefficient import of phytanoyl-CoA into peroxisomes and to defective thiolytic cleavage of 3-ketopristanoyl-CoA. These results corresponded to high-affinity binding of phytanoyl-CoA to the recombinant rat SCP2 protein, as well as high 3-ketopristanoyl-CoA thiolase activity of the recombinant rat SCPx protein.

Peroxisomes in adrenal steroidogenesis

Peroxisomes, cytoplasmic organelles limited by a single membrane and with a matrix of moderate electron density, are present in a great number of cells, namely in adrenal cortex and other steroid-secreting organs. Presently peroxisomes are considered to be involved in important metabolic processes. They intervene in: (1) the production and degradation of H2O2; (2) biosynthesis of ether-phospholipids, cholesterol, dolichol, and bile acids; (3) oxidation of very long chain fatty acids, purines, polyamines, and prostaglandins; (4) catabolism of pipecolic, phythanic and glyoxylic acids; and (5) gluconeogenesis. Recent studies demonstrated that the experimental alterations in the normal steroidogenesis, produce significant morphological and biochemical changes in peroxisomes. Besides this, the presence of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (the key enzyme in the de novo cholesterol synthesis from acetate) and of sterol carrier protein-2 (SCP2), which is involved in the cholesterol metabolism and steroid metabolic pathways, are located in peroxisomes of steroid-secreting cells. In addition, patients with peroxisome diseases present deficiency in steroidogenesis, as well as reduced levels of SCP2. These data pointed out the important role of peroxisomes in steroid biosynthesis.

Association of Ad4BP/SF-1 transcription factor with steroidogenic activity in oncogene-transformed granulosa cells

Adrenal binding protein 4 (Ad4BP) known also as steroidogenic factor 1 (SF-1) is a cell specific transcription factor regulating all steroidogenic P450 genes and is present exclusively in steroidogenic tissues. In this study, we examined whether Ad4BP expression is affected by oncogene-induced cell transformation. Using a gel shift assay we report here that nuclear extracts of steroidogenic granulosa cell lines, transformed by SV40 DNA and the Ha-ras oncogene show specific binding activity towards an Ad4 recognition sequence oligonucleotide. In contrast, nuclear extracts obtained from granulosa cells transformed with SV40 alone, which lost their steroidogenic activity, did not exhibit any binding to the Ad4 oligonucleotide. Using a specific antibody to Ad4BP, it was demonstrated that only the steroidogenic cell lines, i.e. transfected with SV40 + Ha-ras, expressed significant amount of the protein. No binding activity to the Ad4 oligonucleotide was evident in fibroblasts transformed with the same oncogenes (SV40 + Ha-ras). Steroidogenic activity in SV40 + Ha-ras transformed granulosa cells was markedly elevated following forskolin or follice stimulating factor (FSH) and further augmented by incubation of the cells with dexamethasone. However, no change in Ad4BP expression and binding activity was observed following such stimulations. It is suggested that Ha-ras expression in SV40 transformed granulosa cells can play an important role in restoring Ad4BP expression and activity, which are required for their steroidogenic function. Thus, expression of Ad4BP is essential for steroidogenesis both in primary and in oncogene transformed granulosa cells.

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.

Cross-talk between cAMP and p53-generated signals in induction of differentiation and apoptosis in steroidogenic granulosa cells

In each menstrual cycle only very few follicles in the mammalian ovary undergo maturation and ovulation while most of the follicles degenerate in the process of atresia. Moreover, in the absence of pregnancy, the newly formed corpora lutea will degenerate and disappear in the process of luteolysis. Recent studies suggest that ovarian follicular atresia is associated with DNA fragmentation and degeneration of follicular cells, characteristics of programmed cell death (apoptosis). Apoptosis can be induced in vitro, in primary granulosa cell culture, by serum deprivation and by induction of a high intracellular level of cAMP. This induction of apoptosis can be blocked by fibroblast growth factor, suggesting that receptor-medicated activation of a tyrosine kinase can serve as a survival signal. Apoptosis can also be induced in immortalized steroidogenic granulosa cells, transformed by SV40 DNA and Ha-ras oncogene, by overexpression of the wild-type p53 tumor suppressor gene in cAMP-stimulated cells. Omitting the cAMP stimulus prevents the p53-induced apoptosis in these cells, suggesting cross-talk between p53 and c-AMP-generated signals in the induction of apoptosis. Steroidogenic activity in these cells, as well as in nontransformed granulosa cells, does not decline during apoptosis but is rather significantly elevated before total cell collapse occurs. Cytochemical studies using confocal laser microscopy, electron microscopy, and three-dimensional reconstruction reveal a specific reorganization pattern of proteasomes, the most abundant nonlysosomal protease, and of the steroidogenic organelles, such as mitochondria and lipid droplets, in the apoptotic cell. Our results suggest that compartmentalization of intracellular organelles during apoptosis permits proteolysis without interfering with steroidogenesis, characteristic of the differentiated phenotype of the granulosa cell. Moreover, cytoskeletal rearrangement may serve as a barrier between these cellular activities.

Role of the steroidogenic acute regulatory protein (StAR) in steroidogenesis

The rate-limiting, hormone-regulated, enzymatic step in steroidogenesis is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme system (CSCC), which is located on the matrix side of the inner mitochondrial membrane. However, it has long been observed that hydrophilic cholesterol-like substrates capable of traversing the mitochondrial membranes are cleaved to pregnenolone by the CSCC in the absence of any hormone stimulation. Therefore, the true regulated step in the acute response of steroidogenic cells to hormone stimulation is the delivery of cholesterol to the inner mitochondrial membrane and the CSCC. It has been known for greater than three decades that transfer of cholesterol requires de novo protein synthesis; however, prior to this time the regulatory protein(s) had yet to be identified conclusively. It is the purpose of this commentary to briefly review a number of the candidates that have been proposed as the acute regulatory protein. As such, we have summarized the available information that describes the roles of transcription, translation, and phosphorylation in this regulation, and have also reviewed the supporting cases that have been made for several of the proteins put forth as the acute regulator. We close with a comprehensive description of the Steroidogenic Acute Regulatory protein (StAR) that we and others have identified and characterized as a family of proteins that are synthesized and imported into the mitochondria in response to hormone stimulation, and for which strong evidence exists indicating that it is the long sought acute regulatory protein.

Human sterol carrier protein x/sterol carrier protein 2 gene has two promoters

The human sterol carrier protein x (SCPx)/sterol carrier protein 2 (SCP2) gene gives rise to two mRNAs: a 2.8 kb mRNA encoding SCPx, a peroxisome-associated thiolase, and a 1.5 kb mRNA encoding SCP2, which is thought to be an intracellular lipid transfer protein. The SCPx/SCP2 gene is highly expressed in organs involved in lipid metabolism, but the relative abundance of SCPx and SCP2 mRNAs varies. Here we report that the two transcripts are produced under the direction of two independent promoters. We determined the DNA sequence of 3.4 kb of the proximal promoter governing the transcription of SCPx sequences. The promoter governing the transcription of SCP2 sequences was identified 45 kb downstream from the SCPx promoter in intron XI. This promoter initiates transcription within exon XII. Both the SCPx and SCP2 promoters lack TATA boxes and initiate transcription at multiple sites. They share features that are found in the promoters of genes encoding other peroxisomal proteins. The basal activities of the two promoters were tested as fusion gene constructs in selected host cells, including BeWo choriocarcinoma cells, HepG2 hepatoblastoma cells, murine Y1 adrenocortical tumor cells, and Balb 3T3 fibroblasts. Cell host-specific patterns of promoter activity were observed. In addition, 8-Br-cAMP and phorbol myristate acetate were found to increase SCPx promoter activity in a host cell-specific manner. The SCP2 promoter was not significantly influenced by these agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced hepatic sterol carrier protein-2 expression in the streptozotocin treated diabetic rat

While a strong relationship between the hypercholesterolemia of diabetes and premature atherosclerosis is established, the etiology for the elevation in serum cholesterol in this disease is unknown. To determine whether diabetic hypercholesterolemia may be related to alterations in hepatic cholesterol transport capacity, sterol carrier protein-2 (SCP2) expression was examined in rats treated with streptozotocin (SZT). Furthermore, this study examined whether 17β-estradiol and insulin confer a protective effect on liver cholesterol homeostasis by maintaining hepatic SCP2 levels. SCP2 protein and mRNA expression were examined 13 days following SZT-induced diabetes onset and in diabetic rats treated with estradiol (1 cm silastic implant) or insulin (12 units/day). Data indicate that SCP2 protein levels were significantly reduced in the diabetic animals and that SCP2 protein expression in the liver was inversely related to the level of serum cholesterol in the diabetic animals. In contrast, SCP2 mRNA levels examined by slot blot, ribonuclease protection assay, and Northern blot analysis were significantly elevated. Both insulin and estradiol were able to enhance the expression of SCP2 protein in the liver following SZT treatment. The results of this investigation clearly indicate that hepatic SCP2 protein levels are significantly altered in the diabetic state suggesting that cholesterol transport capacity is reduced in the SZT-treated diabetic rat. The inverse relationship between serum cholesterol and hepatic SCP2 protein content suggests that the reduction in this protein may be a contributing factor in diabetic hypercholesterolemia.

Inhibition of gonadotropin-stimulated ovarian steroid production by polyunsaturated fatty acids in teleost fish

The effects of the polyunsaturated fatty acids (PUFAs)--eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (AA)--on in vitro steroid production by full-grown prematurational ovarian follicles from goldfish and rainbow trout were investigated. EPA and DHA inhibited gonadotropin-stimulated testosterone production in a dose-related manner, but AA was inhibitory only at the highest dose tested (400 microM). AA alone stimulated testosterone production by increasing cAMP production, but the effects of other PUFAs alone were marginal. The inhibitory actions by PUFAs were not restricted to long-chain PUFAs, as linoleic and linolenic acids had similar actions in the goldfish. The inhibitory action of EPA on testosterone production was reversible upon removal of the PUFA from medium. Testosterone production stimulated by the addition of the cAMP analogues, dibutyryl cAMP, and 8-bromo cAMP, was attenuated by PUFAs, suggesting that they act at a site distal to cAMP formation. A post-cAMP site regulating cholesterol availability may be involved as testosterone production induced by addition of 25OH-cholesterol was not affected by the PUFAs in either fish species. Together, these findings underscore the importance of lipids in ovarian physiology and suggest that PUFAs may participate in the regulation of ovarian steroidogenesis in teleost fish.

Immunohistochemical localization of two types of fatty acid-binding proteins in rat ovaries during postnatal development and in immature rat ovaries treated with gonadotropins

BACKGROUND

The ovary of adult rats expresses two types of cytoplasmic fatty acid binding proteins (FABP), i.e., heart FABP (H-FABP) and intestinal 15 kDa protein (I-15P). We studied immunohistochemically the cellular localizations of these FABPs in the ovaries of rats at various postnatal ages and in the ovaries of immature (3-week-old) rats treated with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (HCG).

METHODS

The cryosections of ovaries were incubated with polyclonal antibodies against H-FABP and I-15P, and the immunoreactions were visualized at both light and electron microscopic levels.

RESULTS

The immunoreactivity for H-FABP occurred temporarily in the follicular epithelial (granulosa) cells from 3 days to 2 weeks post partum, and then was localized exclusively to the theca/interstitial gland cells from 2 weeks to adulthood. In contrast, the immunoreactivity for I-15P appeared temporarily in a small subset of theca/interstitial gland cells from 2 to 3 weeks, disappeared at 4 weeks, and was localized exclusively to the corpus luteum cells after the onset of ovulation in the animal around 5 weeks. In the immature rat ovaries induced to ovulate by treatment with gonadotropins, I-15P-immunoreactive cells were first recognized in the luteinized granulosa layer of large preovulatory follicles, and increased in number progressively in the developing corpora lutea after the ovulation.

CONCLUSIONS

Two types of FABPs are expressed in distinct steroid-producing cell types of rat ovary, and their expressions seem to be regulated in coincidence with the expressions of respective steroid hormones. These results suggest that FABPs play specific roles in the ovarian hormone synthesis.

Regulation of sterol carrier protein 2 (SCP2) gene expression in rat peritoneal macrophages during foam cell formation. A key role for free cholesterol content

Sterol carrier protein 2 (SCP2) has been shown to be involved in intracellular transport and metabolism of cholesterol. However, there have been no reports concerning SCP2 in macrophages, the major source of atheromatous foam cells. We investigated whether SCP2 is present in rat peritoneal macrophages and determined the changes of SCP2 and its mRNA levels in macrophages during form cell formation induced by acetylated LDL (AcLDL). Immunoblot analysis and Northern blot analysis demonstrated that both SCP2 and its mRNA are expressed in rat peritoneal macrophages. Incubations with AcLDL caused a dose- and time-dependent increase of cellular esterified cholesterol, SCP2 and its mRNA in rat peritoneal macrophages. The inhibitor of acyl-CoA:cholesterol acyltransferase further enhanced AcLDL-induced increase of SCP2 protein and its mRNA. Incubations with 25-hydroxy cholesterol also caused a dose-dependent stimulation of SCP2 gene expression in macrophages, while incubation with maleylated BSA had no effect. These results suggest that the increment of cellular-free cholesterol is responsible for enhanced SCP2 gene expression in macrophages. The enhancement of SCP2 gene expression by AcLDL suggests that SCP2 may play an important role during foam cell formation induced by AcLDL which may be most important step for the atherosclerosis.

Phospholipid-transfer proteins and their mRNAs in developing rat lung and in alveolar type-II cells

Gene expression of non-specific lipid-transfer protein (nsL-TP; identical with sterol carrier protein 2) and phosphatidylinositol-transfer protein (PI-TP) was investigated in developing rat lung. During the late prenatal period (between days 17 and 22) there is a 7-fold increase in the level of nsL-TP and a 2-fold rise in that of PI-TP. The prenatal increases in the levels of nsL-TP and PI-TP are accompanied by parallel increases in the levels of their mRNAs, indicating pretranslational regulation. Compared with whole lung, isolated alveolar type-II cells are enriched in nsL-TP and its mRNA, but not in PI-TP and its mRNA. The observation that the levels of nsL-TP and its mRNA in rat lung show a pronounced increase in the period of accelerated surfactant formation, together with the observation that the surfactant-producing type-II cells are enriched in nsL-TP and its mRNA, suggest that nsL-TP plays a role in the metabolism of pulmonary surfactant.

Plasticity of cell organization during differentiation of normal and oncogene transformed granulosa cells

Granulosa cells, which nurse the oocyte and serve as a major source for estradiol and progesterone production, undergo major morphological changes which correlate very well with modulation of their steroidogenic capacity. These include changes in intercellular contacts and communication, in cell membrane receptors, and in the development and organization of organelles associated with steroidogenesis (i.e., mitochondria, smooth endoplasmic reticulum, lipid droplets, and lysosomes). These biochemical and morphological changes can also be obtained in primary cultures as well as in oncogene transformed granulosa cell lines established recently in our laboratory. A growing body of evidence suggests that plasticity of the cytoskeleton plays a major role in the biochemical and morphological differentiation of granulosa cells as well as in other steroidogenic cells.

Sterol carrier protein 2: a role in steroid hormone synthesis?

The intracellular movement of cholesterol is an important regulated step in the process of steroidogenesis. However, the molecular mechanisms by which cholesterol is translocated to key organelles, including the mitochondria, remains poorly understood. Lipid transfer proteins may have an important function in this process. One candidate lipid transfer protein is sterol carrier protein 2 (SCP2). This 13.2 kDa protein enhances the movement of cholesterol between vesicles and isolated mitochondria. It also stimulates mitochondrial pregnenolone synthesis. When introduced into intact cells, anti-SCP2 antibodies reduce steroid secretion. Moreover, expression of SCP2 in COS cells engineered to produce progestins increases steroid formation. SCP2 is abundant in steroidogenic glands and the pattern of SCP2 gene expression is consistent with a role for the protein in hormone synthesis: SCP2 transcripts are more prominent in the most steroidogenic compartments of the ovary and tropic hormones that stimulate steroidogenesis increase SCP2 gene expression. Other evidence that suggests that SCP2 plays important roles in cellular function includes a remarkable conservation of primary structure across species. The mechanisms by which SCP2 promotes intracellular sterol movement have not been elucidated. The protein appears to bind sterols and is synthesized with a 20 amino acid N-terminal "pro-" sequence that may serve to target SCP2 to mitochondria. In addition, the C-terminus of SCP2 contains a peroxisome-targeting sequence. SCP2 is derived from a large gene that encodes transcripts that are translated into larger proteins of 30 and 58 kDa. The 58 kDa protein, which has some structural homologies with thiolases, seems to be specifically targeted to peroxisomes whereas SCP2 has a broader subcellular distribution. The significance of the peroxisome association of SCP2 and steroidogenesis has not been disclosed. However, diseases of peroxisome function, including adrenoleukodystrophy and Zellweger syndrome, have notable deficits in steroid and bile acid metabolism, thus linking peroxisomes and steroidogenesis. SCP2 is deficient in fibroblasts of patients with these diseases.

The non-specific lipid-transfer protein (sterol carrier protein 2) and its relationship to peroxisomes

The non-specific lipid-transfer protein (nsL-TP), also known as sterol carrier protein 2 (SCP2), is a small (M(r) 13,000) basic protein which catalyzes in vitro the transfer of a great variety of lipids, including cholesterol, between membranes. Inherent to this transfer activity, the protein stimulates in vitro various aspects of cholesterol metabolism. nsL-TP is synthesized as a precursor (pre-nsL-TP) with a leader sequence of 20 amino acid residues. It appears that the peroxisomes play an important role in the conversion of pre-nsL-TP into the mature form. In fact, nsL-TP appears to be mainly present in peroxisomes as shown by immunogold labeling of rat liver, adrenals and testes using the anti-nsL-TP antibody. However, interpretation of the data is complicated by the fact that the antibody raised against nsL-TP also reacts with a protein with a M(r) of 58,000. From cDNA analysis it became apparent that the cross-reactive 58-kDa protein contains the complete sequence of pre-nsL-TP at its C-terminus. However, pre-nsL-TP and the 58-kDa protein are synthesized from different mRNAs. Interestingly, the N-terminal part of the 58-kDa protein was found to have significant sequence similarity with 3-oxoacyl-CoA thiolase. Both pre-nsL-TP and the 58-kDa protein contain the C-terminal peroxisomal targeting tripeptide Ala-Lys-Leu. However, as shown by subcellular fractionation studies the 58-kDa protein is exclusively localized in the peroxisomes whilst nsL-TP is not only detected in the peroxisomes but also in other subcellular fractions. Moreover, a membrane-bound form of nsL-TP was detected. This membrane-bound form is present at the cytosolic side of the membranes. The physiological function of nsL-TP is still unclear; some recent developments are discussed briefly in the last part of this review.

Changes in intracellular localization of proteasomes in immortalized ovarian granulosa cells during mitosis associated with a role in cell cycle control

We describe the isolation and characterization of proteasomes from recently established immortalized ovarian granulosa cell lines and their intracellular distribution during mitosis and during cAMP-induced differentiation, as revealed by immunofluorescence microscopy. In interphase, proteasomes were localized in small clusters throughout the cytoplasm and the nuclear matrix. In prophase, a substantial increase in proteasomal staining was observed in the perichromosomal area. A dramatic increase occurred in metaphase and in early anaphase; the chromosomes remained unstained. In late anaphase, intensive staining remained associated mainly with the spindle fibers. In telophase and early interphase of the daughter cells, intensive staining of proteasomes persisted in the nuclei. In contrast, in cells stimulated to differentiate by forskolin, which substantially elevates intracellular cAMP in these cell lines, only a weak staining of proteasomes was revealed in both the nucleus and the cytoplasm. Double staining of nondividing cells with antibodies to proteasomes and to tubulin did not show colocalization of proteasomes and microtubules. In contrast, dividing cells show a preferential concentration of proteasomes around spindle microtubules during metaphase and anaphase. The observed spatial and temporal distribution pattern of proteasomes during mitosis is highly reminiscent of the behavior of cyclins [Pines, J. & Hunter, T. (1991) J. Cell Biol. 115, 1-17]. Since proteasome accumulation appears to coincide with disappearance of cyclins A and B1 from the spindle apparatus, it is suggested that proteasomes may play a role in termination of mitosis by degrading the cyclins, which act as regulatory elements.

Oncogene-transformed granulosa cells as a model system for the study of steroidogenic processes

Highly steroidogenic granulosa cell lines were established by transfection of primary granulosa cells from preovulatory follicles with SV40 DNA and Ha-ras oncogene. Progesterone production in these cells was enhanced to levels comparable to normal steroidogenic cells, by prolonged (> 12 h) stimulation with 8-Br-cAMP, forskolin and cholera toxin, which elevate intracellular cAMP. The steroidogenic capacity of individual lines correlated with the expression of the ras oncogene product (p21) and the morphology of the cells. Formation of the steroid hormones was associated with de novo synthesis of the mitochondrial cytochrome P450scc system proteins. Since cholesterol import into mitochondria is essential for steroidogenesis, the expression of the peripheral benzodiazepine receptor (PBR) and the sterol carrier protein 2 was characterized in these cells. The induction of the expression of the genes coding for both proteins appeared to be mediated, at least in part, by cAMP. Stimulation of the PBR by specific agonists enhanced progesterone production in these cells. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) dramatically suppressed the cAMP-induced steroidogenesis, in spite of enhanced intracellular cAMP levels, suggesting that TPA can modify the effects of cAMP. cAMP stimulation suppressed growth of transformed cells concomitantly with induction of steroidogenesis. The transformed cells lacked receptors for the native stimulants, the gonadotropic hormones. After transfection of the cells with a lutropin (LH) receptor expression plasmid, the LH and hCG response was reconstituted. In these newly established cell lines gonadotropins were able to stimulate the formation of cAMP and progesterone in a dose-dependent manner with an ED₅₀ characteristic of the native receptor. High doses caused desensitization to gonadotropins as observed in normal cells. These newly established oncogene-transformed granulosa cell lines can serve as a useful model to study inducible steroidogenesis and the effect of oncogene expression on this process.