Expression of Sonic Hedgehog downstream genes is modified in rat embryos exposed in utero to a distal inhibitor of cholesterol biosynthesis

Therapeutic targeting of lipid synthesis metabolism for selective elimination of cancer stem cells

Cancer stem cells (CSCs) are believed to have an essential role in tumor resistance and metastasis; however, no therapeutic strategy for the selective elimination of CSCs has been established. Recently, several studies have shown that the metabolic regulation for ATP synthesis and biological building block generation in CSCs are different from that in bulk cancer cells and rather similar to that in normal tissue stem cells. To take advantage of this difference for CSC elimination therapy, many studies have tested the effect of blocking these metabolism. Two specific processes for lipid biosynthesis, i.e., fatty acid unsaturation and cholesterol biosynthesis, have been shown to be very effective and selective for CSC targets. In this review, lipid metabolism specific to CSCs are summarized. In addition, how monounsaturated fatty acid and cholesterol synthesis may contribute to CSC maintenance are discussed. Specifically, the molecular mechanism required for lipid synthesis and essential for stem cell biology is highlighted. The limit and preview of the lipid metabolism targeting for CSCs are also discussed.

Reduced cholesterol levels impair Smoothened activation in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a common autosomal-recessive disorder that results from mutations in the gene encoding the cholesterol biosynthetic enzyme 7-dehydrocholesterol reductase (DHCR7). Impaired DHCR7 function is associated with a spectrum of congenital malformations, intellectual impairment, epileptiform activity and autism spectrum disorder. Biochemically, there is a deficit in cholesterol and an accumulation of its metabolic precursor 7-dehydrocholesterol (7DHC) in developing tissues. Morphological abnormalities in SLOS resemble those seen in congenital Sonic Hedgehog (SHH)-deficient conditions, leading to the proposal that the pathogenesis of SLOS is mediated by aberrant SHH signalling. SHH signalling is transduced through the transmembrane protein Smoothened (SMO), which localizes to the primary cilium of a cell on activation and is both positively and negatively regulated by sterol molecules derived from cholesterol biosynthesis. One proposed mechanism of SLOS involves SMO dysregulation by altered sterol levels, but the salient sterol species has not been identified. Here, we clarify the relationship between disrupted cholesterol metabolism and reduced SHH signalling in SLOS by modelling the disorder in vitro. Our results indicate that a deficit in cholesterol, as opposed to an accumulation of 7DHC, impairs SMO activation and its localization to the primary cilium.

Involvement and alteration of the Sonic Hedgehog pathway is associated with decreased cholesterol level in trisomy 18 and SLO amniocytes

BACKGROUND

Trisomy 18 and Smith-Lemli-Opitz syndrome are two polymalformative conditions in which a cholesterol defect has been noted. When they occur prenatally, they are associated with a decreased maternal unconjugated estriol (uE(3)) level. Cholesterol plays an essential role in the Sonic Hedgehog pathway, allowing Shh protein maturation leading to its maximal activity. Many malformations in these two syndromes occur in Shh dependent tissues. We thus sought to assess whether a cholesterol defect could affect the Shh pathway and explain some of the observed malformations.

MATERIALS AND METHODS

We selected 14 cases of trisomy 18 and 3 cases of SLO in which the maternal uE(3) level was decreased and reported malformations were observed after fetopathological examination. We correlated the number of malformations with maternal uE(3) level. We then carried out cholesterol concentrations in separate culture media consisting of trisomy 18, SLO and control amniocytes. Finally, we analyzed the Shh pathway by testing the gene expression of several Shh components: GLI transcription factors, BMP2, BMP4, TGFβ1, COL1A1 and COL1A2.

RESULTS AND DISCUSSION

There was an inverse correlation between phenotypic severity and maternal uE(3) levels in SLO and trisomy 18. The cholesterol levels in the amniocyte culture media were correlated with maternal uE3 levels and were significantly lower in T18 and SLO amniocytes, reflecting cholesterol defects. There was an alteration in the Shh pathway since expression of several genes was decreased in T18 and SLO amniocytes. However, these cholesterol defects were not solely responsible for the altered Shh pathway and the malformations observed.

Malformation syndromes caused by disorders of cholesterol synthesis

Cholesterol homeostasis is critical for normal growth and development. In addition to being a major membrane lipid, cholesterol has multiple biological functions. These roles include being a precursor molecule for the synthesis of steroid hormones, neuroactive steroids, oxysterols, and bile acids. Cholesterol is also essential for the proper maturation and signaling of hedgehog proteins, and thus cholesterol is critical for embryonic development. After birth, most tissues can obtain cholesterol from either endogenous synthesis or exogenous dietary sources, but prior to birth, the human fetal tissues are dependent on endogenous synthesis. Due to the blood-brain barrier, brain tissue cannot utilize dietary or peripherally produced cholesterol. Generally, inborn errors of cholesterol synthesis lead to both a deficiency of cholesterol and increased levels of potentially bioactive or toxic precursor sterols. Over the past couple of decades, a number of human malformation syndromes have been shown to be due to inborn errors of cholesterol synthesis. Herein, we will review clinical and basic science aspects of Smith-Lemli-Opitz syndrome, desmosterolosis, lathosterolosis, HEM dysplasia, X-linked dominant chondrodysplasia punctata, Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects Syndrome, sterol-C-4 methyloxidase-like deficiency, and Antley-Bixler syndrome.

Identification of Lmo1 as part of a Hox-dependent regulatory network for hindbrain patterning

The embryonic functions of Hox proteins have been extensively investigated in several animal phyla. These transcription factors act as selectors of developmental programmes, to govern the morphogenesis of multiple structures and organs. However, despite the variety of morphogenetic processes Hox proteins are involved in, only a limited set of their target genes has been identified so far. To find additional targets, we used a strategy based upon the simultaneous overexpression of Hoxa2 and its cofactors Pbx1 and Prep in a cellular model. Among genes whose expression was upregulated, we identified LMO1, which codes for an intertwining LIM-only factor involved in protein-DNA oligomeric complexes. By analysing its expression in Hox knockout mice, we show that Lmo1 is differentially regulated by Hoxa2 and Hoxb2, in specific columns of hindbrain neuronal progenitors. These results suggest that Lmo1 takes part in a Hox paralogue 2-dependent network regulating anteroposterior and dorsoventral hindbrain patterning.

Partial rescue of neonatal lethality of Dhcr7 null mice by a nestin promoter-driven DHCR7 transgene expression

In humans, genetic disorders affecting post-squalene cholesterol biosynthesis result in a variety of dysmorphology syndromes. One key feature of all of these is the presence of mental retardation and another is the lack of a robust genotype-phenotype correlation. Knockout mice defective in the 3beta hydroxysterol Delta7 reductase (Dhcr7), a model for the most common of such disorders in humans, the Smith-Lemli-Opitz syndrome, all die within 24 h of birth. The cause of this postnatal mortality in these mice has not been fully established. In the present study, we tested the hypothesis that CNS dysfunction was a major cause of this lethality and investigated whether transgenic expression of normal human DHCR7 in neuronal tissues could rescue this neonatal lethality. Transgenic mice, expressing DHCR7 driven by murine nestin promoter, were bred onto Dhcr7 knock-out (Dhcr7(-1-)) background and resulted in a partial rescue of neonatal lethality in 11 of 91 (12%) of transgene-positive Dhcr7(-1-) pups. Despite biochemical analyses that showed continued profound cholesterol deficiency in brain, rescued animals survived between 3 and 17 days. Thus, one important conclusion to be drawn is that defects in CNS in Dhcr7 knockout mice may contribute to the early lethality. Another conclusion is that even small and subtle changes in the brain sterol metabolism were sufficient to enable rescue. These data also provide important clues as to the cause of the variable expressivity seen in SLOS.

Mechanistic and epidemiologic considerations in the evaluation of adverse birth outcomes following gestational exposure to statins

The cholesterol-lowering "statin" drugs are contraindicated in pregnancy, but few data exist on their safety in human gestation. We reviewed case reports for patterns suggesting drug-related effects on prenatal development and considered a variety of mechanisms by which such effects, if confirmed, might occur. This uncontrolled case series included all FDA reports of statin exposures during gestation, as well as others from the literature and from manufacturers. Exposures and outcomes were reviewed and were tabulated by individual drug. Age-specific rates of exposure to each drug among women of child-bearing age were estimated. Of 214 ascertained pregnancy exposures, 70 evaluable reports remained after excluding uninformative cases. Among 31 adverse outcomes were 22 cases with structural defects, 4 cases of intrauterine growth restriction, and 5 cases of fetal demise. There were two principal categories of recurrent structural defects: cerivastatin and lovastatin were associated with four reports of severe midline CNS defects; simvastatin, lovastatin, and atorvastatin were all associated with reports of limb deficiencies, including two similar complex lower limb defects reported following simvastatin exposure. There were also two cases of VACTERL association among the limb deficiency cases. All adverse outcomes were reported following exposure to cerivastatin, simvastatin, lovastatin, or atorvastatin, which are lipophilic and equilibrate between maternal and embryonic compartments. None were reported following exposure to pravastatin, which is minimally present in the embryo. Statins reaching the embryo may down-regulate biosynthesis of cholesterol as well as many important metabolic intermediates, and may have secondary effects on sterol-dependent morphogens such as Sonic Hedgehog. The reported cases display patterns consistent with dysfunction of cholesterol biosynthesis and Sonic Hedgehog activity. Controlled studies are needed to investigate the teratogenicity of individual drugs in this class.

Sterol profiling of amniotic fluid: a routine method for the detection of distal cholesterol synthesis deficit

OBJECTIVES

Smith Lemli Opitz syndrome (SLOS) caused by a deficit of 3beta-hydroxysterol-Delta7 reductase was the first sterol deficit described with multiple malformations. The lack of specificity of many morphological abnormalities detected by ultrasound and their frequency have justified routine screening of amniotic fluid (AF) for sterols by GC-MS. The examination contributes to an improved knowledge of the sterol status in the fluid.

METHODS

A series of sterol profiles is collated here. Accumulation of 7- and 8-dehydrocholesterol are diagnostic for SLOS. However, a number of other sterols have also been detected by GC-MS in control AF and their presence may be confusing.

RESULTS AND CONCLUSIONS

In addition to cholesterol, the level of which varies as function of the gestational age, lathosterol is present together with trace amounts of 7- and 8-dehydrocholesterol and other precursors such as desmosterol, lanosterol, and dimethylsterol. Phytosterols are also present in 70% of AF samples that have been tested. Besides SLOS, GC-MS examination of amniotic fluid can detect various sterol deficits associated with malformations (lathosterolosis, desmosterolosis, X-linked chondrodysplasia, and particular Antley-Bixler syndrome). Practical conclusions support GC-MS as a routine method to investigate skeletal and central nervous system malformations.

Expression and role of cubilin in the internalization of nutrients during the peri-implantation development of the rodent embryo

Histiotrophic nutrition is essential during the peri-implantation development in rodents, but little is known about receptors involved in protein and lipid endocytosis derived from the endometrium and the uterine glands. Previous studies suggested that cubilin, a multiligand receptor for vitamin, iron, and protein uptake in the adult, might be important in this process, but the onset of its expression and function is not known. In this study, we analyzed the expression of cubilin in the pre- and early post-implantation rodent embryo and tested its potential function in protein and cholesterol uptake. Using morphological and Western blot analysis, we showed that cubilin first appeared at the eight-cell stage. It was expressed by the maternal-fetal interfaces, trophectoderm and visceral endoderm, but also by the future neuroepithelial cells and the developing neural tube. At all these sites, cubilin was localized at the apical pole of the cells exposed to the maternal environment or to the amniotic and neural tube cavities, and had a very similar distribution to megalin, a member of the LDLR gene family and a coreceptor for cubilin in adult tissues. To analyze cubilin function, we followed endocytosis of apolipoprotein A-I and HDL cholesterol, nutrients normally present in the uterine glands and essential for embryonic growth. We showed that internalization of both ligands was cubilin dependent during the early rodent gestation. In conclusion, the early cubilin expression and its function in protein and cholesterol uptake suggest an important role for cubilin in the development of the peri-implantation embryo.

Late gestational lung hypoplasia in a mouse model of the Smith-Lemli-Opitz syndrome

Background

Normal post-squalene cholesterol biosynthesis is important for mammalian embryonic development. Neonatal mice lacking functional dehydrocholesterol Δ7-reductase (Dhcr7), a model for the human disease of Smith-Lemli-Opitz syndrome, die within 24 hours of birth. Although they have a number of biochemical and structural abnormalities, one cause of death is from apparent respiratory failure due to developmental pulmonary abnormalities.

Results

In this study, we characterized further the role of cholesterol deficiency in lung development of these mice. Significant growth retardation, beginning at E14.5~E16.5, was observed in Dhcr7^-/- embryos. Normal lobation but smaller lungs with a significant decrease in lung-to-body weight ratio was noted in Dhcr7^-/- embryos, compared to controls. Lung branching morphogenesis was comparable between Dhcr7^-/- and controls at early stages, but delayed saccular development was visible in all Dhcr7^-/- embryos from E17.5 onwards. Impaired pre-alveolar development of varying severity, inhibited cell proliferation, delayed differentiation of type I alveolar epithelial cells (AECs) and delayed vascular development were all evident in knockout lungs. Differentiation of type II AECs was apparently normal as judged by surfactant protein (SP) mRNAs and SP-C immunostaining. A significant amount of cholesterol was detectable in knockout lungs, implicating some maternal transfer of cholesterol. No significant differences of the spatial-temporal localization of sonic hedgehog (Shh) or its downstream targets by immunohistochemistry were detected between knockout and wild-type lungs and Shh autoprocessing occurred normally in tissues from Dhcr7^-/- embryos.

Conclusion

Our data indicated that cholesterol deficiency caused by Dhcr7 null was associated with a distinct lung saccular hypoplasia, characterized by failure to terminally differentiate alveolar sacs, a delayed differentiation of type I AECs and an immature vascular network at late gestational stages. The molecular mechanism of impaired lung development associated with sterol deficiency by Dhcr7 loss is still unknown, but these results do not support the involvement of dysregulated Shh-Patched-Gli pathway in causing this defect.

Role of cholesterol in the regulation of growth plate chondrogenesis and longitudinal bone growth

Inborn errors of cholesterol synthesis are associated with multiple systemic abnormalities, including skeletal malformations. The regulatory role of cholesterol during embryogenesis appears to be mediated by Shh, a signaling molecule in which activity depends on molecular events involving cholesterol. Based on this evidence, we hypothesized that cholesterol, by modifying the activity of Ihh (another of the Hedgehog family proteins) in the growth plate, regulates longitudinal bone growth. To test this hypothesis, we treated rats with AY 9944, an inhibitor of the final reaction of cholesterol synthesis. After 3 weeks, AY 9944 reduced the cumulative growth, tibial growth, and the tibial growth plate height of the rats. To determine whether cholesterol deficiency affects bone growth directly at the growth plate, we then cultured fetal rat metatarsal bones in the presence of AY 9944. After 4 days, AY 9944 suppressed metatarsal growth and growth plate chondrocyte proliferation and hypertrophy. The inhibitory effect on chondrocyte hypertrophy was confirmed by the AY 9944-mediated decreased expression of collagen X. Lastly, AY 9944 decreased the expression of Ihh in the metatarsal growth plate. We conclude that reduced cholesterol synthesis in the growth plate, possibly by altering the normal activity of Ihh, results in suppressed longitudinal bone growth and growth plate chondrogenesis.

The interplay of genetic and environmental factors in craniofacial morphogenesis: holoprosencephaly and the role of cholesterol

Cyclopia, the paradigmatic "face [that] predicts the brain" in severe holoprosencephaly (HPE) (DeMyer et al., 1964), has been recognized since ancient times. Descriptive embryologists and pathologists have noted the continuum of defective separation of the forebrain and loss of central nervous system (CNS) midline structures for more than a century. It has been recognized more recently that inhibitors of cholesterol biosynthesis, whether consumed in native plants by range sheep, or experimentally applied to early embryos, could phenocopy the natural malformation, as could a variety of other teratogens (maternal diabetes, alcohol). Yet it has been less than a decade that the genomic knowledge base and powerful analytic methods have brought the sciences of descriptive, molecular, and genetic embryology within range of each other. In this review, we discuss the clinical presentations and pathogenesis of HPE. We will outline various genetic and teratogenic mechanisms leading to HPE. Lastly, we will attempt to examine the pivotal role of cholesterol and the Sonic Hedgehog (Shh) pathway in this disorder and in normal embryonic forebrain development.

Sonic hedgehog rescues cranial neural crest from cell death induced by ethanol exposure

Alcohol is a teratogen that induces a variety of abnormalities including brain and facial defects [Jones, K. & Smith, D. (1973) Lancet 2, 999-1001], with the exact nature of the deficit depending on the time and magnitude of the dose of ethanol to which developing fetuses are exposed. In addition to abnormal facial structures, ethanol-treated embryos exhibit a highly characteristic pattern of cell death. Dying cells are observed in the premigratory and migratory neural crest cells that normally populate most facial structures. The observation that blocking Sonic hedgehog (Shh) signaling results in similar craniofacial abnormalities prompted us to examine whether there was a link between this aspect of fetal alcohol syndrome and loss of Shh. We demonstrate that administration of ethanol to chick embryos results in a dramatic loss of Shh, as well as a loss of transcripts involved in Shh signaling pathways. In contrast, other signaling molecules examined do not demonstrate such dramatic changes. Furthermore, we demonstrate that both the ethanol-induced cranial neural crest cell death and the associated craniofacial growth defect can be rescued by application of Shh. These data suggest that craniofacial anomalies resulting from fetal alcohol exposure are caused at least partially by loss of Shh and subsequent neural crest cell death.

7-Dehydrocholesterol-dependent proteolysis of HMG-CoA reductase suppresses sterol biosynthesis in a mouse model of Smith-Lemli-Opitz/RSH syndrome

Smith-Lemli-Opitz/RSH syndrome (SLOS), a relatively common birth-defect mental-retardation syndrome, is caused by mutations in DHCR7, whose product catalyzes an obligate step in cholesterol biosynthesis, the conversion of 7-dehydrocholesterol to cholesterol. A null mutation in the murine Dhcr7 causes an identical biochemical defect to that seen in SLOS, including markedly reduced tissue cholesterol and total sterol levels, and 30- to 40-fold elevated concentrations of 7-dehydrocholesterol. Prenatal lethality was not noted, but newborn homozygotes breathed with difficulty, did not suckle, and died soon after birth with immature lungs, enlarged bladders, and, frequently, cleft palates. Despite reduced sterol concentrations in Dhcr7(-/-) mice, mRNA levels for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme for sterol biosynthesis, the LDL receptor, and SREBP-2 appeared neither elevated nor repressed. In contrast to mRNA, protein levels and activities of HMG-CoA reductase were markedly reduced. Consistent with this finding, 7-dehydrocholesterol accelerates proteolysis of HMG-CoA reductase while sparing other key proteins. These results demonstrate that in mice without Dhcr7 activity, accumulated 7-dehydrocholesterol suppresses sterol biosynthesis posttranslationally. This effect might exacerbate abnormal development in SLOS by increasing the fetal cholesterol deficiency.