De novo sterologenesis in the intact rat

Cholesterol-mediated Degradation of 7-Dehydrocholesterol Reductase Switches the Balance from Cholesterol to Vitamin D Synthesis

Cholesterol is detrimental to human health in excess but is also essential for normal embryogenesis. Hence, enzymes involved in its synthesis possess many layers of regulation to achieve balanced cholesterol levels. 7-Dehydrocholesterol reductase (DHCR7) is the terminal enzyme of cholesterol synthesis in the Kandutsch-Russell pathway, converting 7-dehydrocholesterol (7DHC) to cholesterol. In the absence of functional DHCR7, accumulation of 7DHC and a lack of cholesterol production leads to the devastating developmental disorder, Smith-Lemli-Opitz syndrome. This study identifies that statin treatment can ameliorate the low DHCR7 expression seen with common Smith-Lemli-Opitz syndrome mutations. Furthermore, we show that wild-type DHCR7 is also relatively labile. In an example of end-product inhibition, cholesterol accelerates the proteasomal degradation of DHCR7, resulting in decreased protein levels and activity. The loss of enzymatic activity results in the accumulation of the substrate 7DHC, which leads to an increased production of vitamin D. Thus, these findings highlight DHCR7 as an important regulatory switch between cholesterol and vitamin D synthesis.

The role of abnormalities in the distal pathway of cholesterol synthesis in the Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects (CHILD) syndrome

CHILD syndrome (Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects) is a rare X-linked dominant ichthyotic disorder. CHILD syndrome results from loss of function mutations in the NSDHL gene, which leads to inhibition of cholesterol synthesis and accumulation of toxic metabolic intermediates in affected tissues. The CHILD syndrome skin is characterized by plaques topped by waxy scales and a variety of developmental defects in extracutaneous tissues, particularly limb hypoplasia or aplasia. Strikingly, these alterations are commonly segregated to either the right or left side of the body midline with little to no manifestations on the ipsilateral side. By understanding the underlying disease mechanism of CHILD syndrome, a pathogenesis-based therapy has been developed that successfully reverses the CHILD syndrome skin phenotype and has potential applications to the treatment of other ichthyoses. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Hair and skin sterols in normal mice and those with deficient dehydrosterol reductase (DHCR7), the enzyme associated with Smith-Lemli-Opitz syndrome

Our recent studies have focused on cholesterol synthesis in mouse models for 7-dehydrosterolreductase (DHCR7) deficiency, also known as Smith-Lemli-Opitz syndrome. Investigations of such mutants have relied on tissue and blood levels of the cholesterol precursor 7-dehydrocholesterol (7DHC) and its 8-dehydro isomer. In this investigation by gas chromatography/mass spectrometry (GC/MS) we have identified and quantified cholesterol and its precursors (7DHC, desmosterol, lathosterol, lanosterol and cholest-7,24-dien-3β-ol) in mouse hair. The components were characterized and their concentrations were compared to those found in mouse skin and serum. Hair appeared unique in that desmosterol was a major sterol component, almost matching in concentration cholesterol itself. In DHCR7 deficient mice, dehydrodesmosterol (DHD) was the dominant hair Δ(7) sterol. Mutant mouse hair had much higher concentrations of 7-dehydrosterols relative to cholesterol than did serum or tissue at all ages studied. The 7DHC/C ratio in hair was typically about sevenfold the value in serum or skin and the DHD/D ratio was 100× that of the serum 7DHC/C ratio. Mutant mice compensate for their DHCR7 deficiency with maturity, and the tissue and blood 7DHC/C become close to normal. That hair retains high relative concentrations of the dehydro precursors suggests that the apparent up-regulation of Dhcr7 seen in liver is slower to develop at the site of hair cholesterol synthesis.

Adaptation of cholesterol synthesis to fasting and TNF-alpha: profiling cholesterol intermediates in the liver, brain, and testis

Key players in pathogenesis of metabolic disorders are disturbed cholesterol balance and inflammation. In addition to cholesterol also sterol intermediates are biologically active, however, surprisingly little is known about their synthesis and roles. The aim of our study was to assess the interplay between the inflammatory cytokine TNF-alpha and cholesterol synthesis by measuring cholesterol and its intermediates in the liver, brain, and testis. Liquid chromatography-mass spectrometry has been applied to profile sterols of normally fed mice, during fasting and after TNF-alpha administration. In mice on normal chow diet, sterols other than cholesterol represent 0.5% in the liver, 1% in brain and 5% in testis. In the liver only 7-dehydrocholesterol, lanosterol and desmosterol were detected. Major sterol intermediates of the brain are desmosterol, testis meiosis activating sterol (T-MAS), and 7-dehydrocholesterol while in testis T-MAS predominates (4%), followed by desmosterol, lanosterol, 7-dehydrocholesterol and others. In 20h fasting there is no significant change in cholesterol of the three tissues, and no significant change in intermediates of the liver. In the brain sterol intermediates are lowered (significant for zymosterol) while in the testis the trend is opposite. TNF-alpha provokes a significant raise of some intermediates whereas the level of cholesterol is again unchanged. The proportion of sterols in the liver rises from 0.5% in controls to 1.2% in TNF-alpha-treated mice, which is in accordance with published expression profiling data. In conclusion, our data provide novel insights into the interaction between the inflammatory cytokine TNF-alpha and the tissue-specific cholesterol biosynthesis of the liver, brain and testis.

Transepidermal water loss in developing rats: role of aquaporins in the immature skin

In the extremely preterm infant, high transepidermal water loss (TEWL) can result in severe dehydration. TEWL has been attributed to the structural properties of the epidermis but might also be influenced by mechanisms that facilitate water transport. To investigate whether aquaporins (AQP) may be involved in the extreme losses of water through immature skin, we examined the presence and cellular distributions of AQP-1 and AQP-3 in embryonic and adult rat skin by immunohistochemistry. The expression of AQP mRNA in skin was analyzed with the use of semiquantitative reverse transcription-PCR. In rat pups of different embryonic (E) and postnatal (P) ages (days), TEWL and skin hydration were measured. AQP-1 was detected in dermal capillaries, and AQP-3 was abundant in basal epidermal layers. Both AQP displayed several times higher expression in embryonic than in adult skin. TEWL was highest at embryonic day 18 (E18) (133 +/- 18 g/m2h) and lower at E20 (25 +/- 1 g/m2h) and P4 (9 +/- 2 g/m2h). Skin hydration measured as skin electrical capacitance paralleled TEWL, being highest in fetal skin (794 +/- 15 pF at E18) and decreasing to 109 +/- 11 pF at E20 and to 0 +/- 0 pF at P4. We conclude that, as in infants, water loss through the skin of rats decreases markedly with maturation during the perinatal period. The expression and cellular localization of the AQP are such that they might influence skin hydration and water transport and contribute to the high losses of water through the immature skin.

The activity of HMG-CoA reductase and acetyl-CoA carboxylase in human apocrine sweat glands, sebaceous glands, and hair follicles is regulated by phosphorylation and by exogenous cholesterol

Human apocrine and sebaceous glands function to secrete lipids, predominantly triglycerides, fatty acids, cholesterol and its esters, and, in the sebaceous gland, squalene. The enzymes that catalyze the important regulatory steps in cholesterol and fatty acid biosyntheses, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acetyl-CoA carboxylase, respectively, were therefore studied in isolated human skin appendages, and their relevant kinetic parameters determined. The enzyme activities that were observed can account for previously described rates of incorporation of radiolabeled substrates into the appropriate lipids by glands in vitro. Reduced enzyme activities following homogenization in the presence of fluoride indicated that both of these enzymes in skin appendages are inactivated by phosphorylation. The activity of the enzyme known to catalyze this phosphorylation, the AMP-activated protein kinase, was also measured. Compactin was shown to inhibit HMG-CoA reductase in homogenates of these appendages. Conversely, incubation of whole sebaceous glands with compactin resulted in the stimulation of enzyme activity, which suggests that these appendages can respond to diminishing cholesterol levels. The effect of exogenous low density lipoprotein and 25-hydroxycholesterol on HMG-CoA reductase activity from skin appendages was investigated. HMG-CoA reductase activity in both apocrine and sebaceous glands was reduced following incubation with either low density lipoprotein or 25-hydroxycholesterol. Low density lipoprotein receptor and lipoprotein lipase mRNA expression was also detected in skin appendages. These results indicate that apocrine and sebaceous glands have the capacity to sequester dietary cholesterol and fatty acids that may have important implications for the understanding of both acne and axillary odor.

Cutaneous lipid synthesis during late fetal development in the rat

Lipid synthesis in fetal skin may be important both for the development of a mature epidermal permeability barrier and for growth. In these studies, we measured cutaneous cholesterol, sphingolipid and fatty acid synthesis during the critical period of epidermal barrier development in fetal rats to determine whether barrier function influences synthetic rates. In addition, the activities of HMG CoA reductase, serine palmitoyl transferase and acetyl coenzyme A carboxylase were evaluated. In whole skin, synthesis of cholesterol, ceramide, sphingomyelin and fatty acid decreased from day 17 to day 21 of gestation, as did the activity of HMG CoA reductase, serine palmitoyl transferase and acetyl coenzyme A carboxylase. In both the epidermis and dermis, a decrease in cholesterol, ceramide, sphingomyelin and fatty acid synthesis was measured over days 19-21 of gestation. Epidermal HMG CoA reductase activity also decreased over this same time period. In summary, epidermal and dermal synthetic rates and enzyme activity were highest early in gestation when the barrier was least competent and decreased as competence was achieved. Since other studies with mature animals have revealed that epidermal synthetic rates and enzyme activity are highest when barrier disruption is maximal, enhanced epidermal lipid synthesis precedes the establishment of a competent barrier in both fetal and mature rodents.

Barrier function regulates epidermal lipid and DNA synthesis

The stratum corneum, the permeability barrier between the internal milieu and the environment, is composed of fibrous protein-enriched corneocytes and a lipid-enriched intercellular matrix. The lipids are a mixture of sphingolipids, cholesterol and free fatty acids, which form intercellular membrane bilayers. Lipid synthesis occurs in the keratinocytes in all nucleated layers of the epidermis, and the newly synthesized lipids are delivered by lamellar bodies to the interstices of the stratum corneum during epidermal differentiation. Disruption of barrier function by topical acetone treatment results in an increase in the synthesis of free fatty acids, sphingolipids and cholesterol in the living layers of the epidermis, leading to barrier repair. Cholesterol and sphingolipid synthesis are regulated by the rate-limiting enzymes HMG CoA reductase and serine palmitoyl transferase (SPT), respectively. Acute barrier disruption leads to an increase in both enzymes, but with a different time curve: increase in HMG CoA reductase activity begins at 1.5 h, whereas the increase in SPT activity occurs 6 h after barrier impairment. Topical application of HMG CoA reductase or SPT inhibitors after acetone treatment delays barrier repair, providing further evidence for a role of cholesterol and sphingolipids in epidermal barrier function. Repeated application of lovastatin to untreated skin results in disturbed barrier function accompanied by increased DNA synthesis and epidermal hyperplasia. Therefore, we have examined the specific relationship between barrier function and epidermal DNA synthesis. After acute and chronic disturbances not only lipid, but also DNA synthesis, is stimulated. Thus, stimulation of DNA synthesis leading to epidermal hyperplasia may be a second mechanism by which the epidermis repairs defects in barrier function. The link between barrier function and both lipid and DNA synthesis is supported further by occlusion studies. Artificial barrier repair by latex occlusion prevents an increase in both lipid and DNA synthesis. In addition, increased epidermal lipid and DNA synthesis in essential fatty-acid deficiency can be reversed by topical applications of the n-6 unsaturated fatty acids, linoleic or columbinic acid. These studies may be of relevance in understanding the pathogenesis of hyperproliferative skin diseases, such as ichthyosis, psoriasis, atopic dermatitis, and irritant contact dermatitis.

Role for monokines in the metabolic effects of endotoxin. Interferon-gamma restores responsiveness of C3H/HeJ mice in vivo

To examine the role of cytokines in mediating the lipogenic effects of endotoxin (LPS), we studied the effects of LPS and cytokines on hepatic fatty acid synthesis in LPS-sensitive C3H/OuJ mice and in LPS-resistant C3H/HeJ mice, whose macrophages are defective in the ability to produce tumor necrosis factor (TNF) and IL-1 in response to LPS. HeJ mice were 16-fold less sensitive than OuJ mice to the lipogenic effect of LPS. In OuJ mice, 10 micrograms of LPS caused a maximal increase in hepatic lipogenesis (3.86 +/- 0.41-fold), whereas in HeJ mice the maximal increase was only 1.79 +/- 0.32-fold after 100 micrograms of LPS. This lipogenic response paralleled the decreased ability of LPS to increase hepatic and splenic levels of mRNAs for TNF and IL-1 and serum levels of TNF in HeJ mice. In contrast, the maximal effect of TNF on lipogenesis was greater and the sensitivity to TNF was increased 2.4-fold in HeJ mice compared to OuJ mice. Administration of IFN-gamma before LPS in HeJ mice had no effect on IL-1 mRNA, but partially restored the LPS-induced increase in hepatic and splenic mRNA for TNF and serum TNF levels, which may account for the partial restoration of sensitivity to the lipogenic effect of LPS after IFN-gamma treatment. These results indicate that cytokines produced by mononuclear leukocytes mediate the lipogenic effects of LPS.

Structural basis for the barrier abnormality following inhibition of HMG CoA reductase in murine epidermis

Recent studies have shown that increased epidermal 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG CoA) reductase activity is crucial for the barrier recovery response that follows solvent-induced barrier perturbation. Upregulation of this enzyme leads to increased cholesterologenesis, formation and secretion of cholesterol-enriched lamellar bodies, and barrier repair. Topical lovastatin-induced inhibition of HMG CoA reductase activity both delays the acute barrier-repair response, as well as leading to a chronic barrier abnormality when applied repeatedly to intact skin. Presently, we assessed the effects of repeated topical applications of two different specific inhibitors of HMG CoA reductase on barrier function, the lamellar body-secretory system, and stratum corneum intercellular domains, with functional and morphologic parameters. Once-daily applications of lovastatin or fluindostatin (XU62-320; Sandoz) for 4-8 d to intact hairless mouse epidermis produced a progressive abnormality in barrier function (transepidermal water loss greater than 2.0-5.0 in treated versus less than 0.25 mg/cm2/h for weakly active analogues or vehicle controls). The barrier defect was preceded by alterations in lamellar body internal structure and a partial failure of lamellar body secretion into the stratum corneum interstices, further confirmed by enzyme cytochemistry. Moreover, the deposition of abnormal lamellar body contents resulted in the formation of clefts in the intercellular spaces at the stratum granulosum-stratum corneum interface, resulting in increased permeability through these domains shown by lanthanum perfusion. Applications of irritants, even when producing a barrier abnormality, did not alter the lamellar body secretory system. Co-applications of cholesterol with the inhibitors reversed both the barrier abnormality and the abnormalities in the lamellar body secretory system that occur with the inhibitor alone. Finally, membrane bilayer structures in the mid-to-outer stratum corneum of inhibitor-treated specimens appeared normal, but the intercellular domains displayed enormously expanded lacunae. However, because similar dilatations also occurred in vehicle-treated samples, they can be attributed to the vehicle alone. These studies provide further evidence that the inhibitor-induced defect in barrier function a) is initiated by inhibition of HMG CoA reductase; b) can be attributed to defects in both lamellar body structure and deposition with resultant abnormalities in intercellular membrane domains in the lower stratum corneum; and c) is further enhanced by permissive effects of the vehicle on the permeability of the outer stratum corneum.

Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase induce reductase accumulation and altered lamellar bodies in rat forestomach keratinocytes

Lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and a potent hypocholesterolemic agent, induces a hyperplastic thickening of the rat forestomach mucosa after oral administration of its active form, a hydroxyacid. We studied the effects of lovastatin on the intracellular accumulation of HMG-CoA reductase immunostaining and the accompanying morphological changes in rat forestomach keratinocytes by immunofluorescence microscopy and transmission electron microscopy (TEM). Administration of lovastatin hydroxyacid induced increases in HMG-CoA reductase levels within forestomach keratinocytes that were dose and time dependent and reversible. The adjacent glandular stomach epithelium did not exhibit induction of reductase. A pharmacologically inactive epimer of lovastatin hydroxyacid did not increase keratinocyte reductase accumulation, and lovastatin lactone induced minimal forestomach reductase. TEM of forestomachs from rats given lovastatin hydroxyacid demonstrated profound alterations in epidermal lamellar bodies (organelles that transport lipids and steroids to the intercellular spaces of the stratum corneum). Treated cells lacked internal lipid lamellae and failed to secrete sheets of lipid material into the intercellular spaces of the stratum corneum. We hypothesize that sustained inhibition of HMG-CoA reductase in rat forestomach keratinocytes induces accumulation of HMG-CoA reductase and hyperplasia by inhibiting sterol synthesis, assembly of lamellar bodies, and formation of intercellular lipid sheets.

Effect of interleukin-1 on lipid metabolism in the rat. Similarities to and differences from tumor necrosis factor

Infection and inflammation are associated with hypertriglyceridemia, which is thought to be mediated by cytokines. Previous studies at our laboratory and others have shown that tumor necrosis factor acutely increases serum triglyceride levels primarily by stimulating hepatic lipid synthesis and secretion. The role of interleukin-1 (IL-1), a cytokine that is also secreted by stimulated macrophages and that has many actions that overlap those of tumor necrosis factor, has not been studied in depth. The present study demonstrates that IL-1, at doses similar to those that cause fever and anorexia and that stimulate adrenocorticotropic hormone secretion, rapidly increases serum triglyceride levels; this elevation persists for at least 17 hours. Serum cholesterol levels are not altered by IL-1. Neither is the clearance of triglyceride-rich lipoproteins affected by IL-1. However, hepatic triglyceride secretion, measured by the Triton WR-1339 technique, is increased in IL-1-treated animals. Accompanying this stimulation in hepatic lipid secretion is an increase in de novo fatty acid synthesis in the liver. IL-1 does not increase serum free fatty acid and glycerol levels, suggesting that IL-1 does not stimulate lipolysis in vivo. Additionally, inhibition of lipolysis does not prevent the increase in serum triglyceride levels, providing further evidence that lipolysis does not play a crucial role in the increased hepatic lipid synthesis and secretion induced by IL-1. In contrast, tumor necrosis factor increases lipolysis, which contributes to the increase in serum triglycerides. That multiple cytokines rapidly elevate plasma triglyceride levels suggest that these changes in lipid metabolism may play an important role in the organism's response to infection and inflammation.

Localization and regulation of epidermal 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity by barrier requirements

Recent studies have shown that epidermal cholesterol synthesis is regulated by HMG CoA reductase activity and that this activity is modulated by changes in the cutaneous permeability barrier. Here, we quantitated HMG CoA reductase activity after acute and chronic barrier disruption in the upper and lower layers of murine epidermis. In unperturbed epidermis, 13 and 87% of enzyme activity localized to the upper and lower epidermis, respectively, with the majority of activity in the stratum basale. Acute barrier disruption with either acetone or sodium dodecylsulfate provoked an increase in HMG CoA reductase activity (54% and 30%) in the lower layers, but only a small change in the upper layers. However, the activation state of the enzyme was increased 50% in the upper epidermis. Correction of barrier function by occlusion with an impermeable Latex wrap prevented the increase both in enzyme activity and activation state. After chronic barrier disruption; i.e., essential fatty acid deficient (EFAD) diet, HMG CoA reductase activity was increased in the upper epidermis (161%); a change prevented by occlusion. These results show: (1) that HMG CoA reductase activity is present in both the upper and lower cell layers; (2) that acute insults to barrier integrity stimulate enzyme activity in both the upper and lower epidermis; and (3) that chronic insults provoke an increase in enzyme activity in the upper layers. These studies provide further insights into the linkage of the permeability barrier with epidermal cholesterol metabolism.

The regulation and role of epidermal lipid synthesis

One of the key functions of the epidermis is to form a barrier between the organism and the outside world. As shown in Fig. 3, disruptions of the barrier result in a cascade of events that ultimately leads to barrier repair. The initial signal that initiates this repair response is unknown. The exocytosis of preformed lipid-enriched lamellar bodies is the first step in this response, which is followed by an increase in lipid synthesis in the epidermis. Our studies demonstrate that this increase in epidermal lipid synthesis is required for the synthesis of new lamellar bodies and repair of the barrier. Inhibition of epidermal lipid synthesis by artificial membranes or drugs impairs barrier recovery by preventing the reformation of lamellar bodies and the continued secretion of lipid. Whether the stimulation of lipid synthesis is primarily regulated by disturbances in barrier function or secondarily by decreases in the lipid content of the cells due to the utilization of lipid for the formation of lamellar bodies is unknown. Additionally, the precise mechanisms by which lipid synthesis is increased (enzyme activation, transcriptional regulation, etc.) remain to be elucidated. The secretion of lipid-containing lamellar bodies results in the reaccumulation of lipid in the intercellular spaces of the stratum corneum and the recovery of normal barrier function. Epidermal lipid synthesis also is probably required to provide lipid for new cell membrane formation to allow for the increase in epidermal cell proliferation, which is stimulated following barrier disruption. Additionally, epidermal lipid synthesis may provide regulatory molecules or crucial substrates that are required for DNA synthesis. Thus, epidermal lipid synthesis plays a key role in the major biological functions of the epidermis, the cutaneous permeability barrier, and cell proliferation.

Cholesterol synthesis is required for cutaneous barrier function in mice

Previous studies have shown that topical acetone treatment results in the removal of stratum corneum lipids and disruption of the permeability barrier. This disruption stimulates epidermal lipid synthesis which is associated with the rapid restoration of stratum corneum lipids and barrier function. The aim of this study was to determine the role of cutaneous cholesterol synthesis in the barrier recovery. Here we show that topical lovastatin, a competitive inhibitor of HMG CoA reductase, inhibits cholesterol synthesis. After acetone disruption of the barrier, the normal rapid return of cholesterol to the stratum corneum and recovery of barrier function is impaired in animals treated topically with lovastatin. When lovastatin animals are simultaneously treated topically with either mevalonate, the immediate product of HMG CoA reductase, or cholesterol, the final end product of the pathway, the recovery of the barrier is normalized. Lovastatin resulted in the delayed secretion and abnormal appearance of lamellar bodies. These results provide the first evidence demonstrating that cholesterol synthesis is required for the maintenance of barrier structure and function and suggests a crucial role for cholesterol synthesis in allowing for terrestrial existence.

Diet affects the mechanisms by which TNF stimulates hepatic triglyceride production

Tumor necrosis factor (TNF) induces hyperlipidemia in rodents by increasing hepatic triglyceride production. We now explore the mechanism of this increase. TNF does not increase phosphatidate phosphohydrolase, glycerolphosphate acyltransferase, or diacylglycerol acyltransferase, which are enzymes of triglyceride synthesis. Rather, TNF increases triglyceride production by providing increased fatty acids (FA) as substrate. In chow-fed rats, TNF increases plasma free fatty acids (FFA). The antilipolytic drug, phenylisopropyl adenosine (PIA), prevents the TNF-induced increase in plasma FFA and, most importantly, inhibits the TNF-induced increase in plasma triglycerides. Thus increased lipolysis with delivery of FA to liver contributes to TNF-induced hyperlipidemia in chow-fed animals. In contrast, in rats fed a high-sucrose diet, TNF causes hyperlipidemia without increasing plasma FFA, and PIA has no effect on TNF-induced increases in plasma triglycerides. However, in sucrose-fed rats, TNF markedly stimulates hepatic de novo FA synthesis, which provides FA. This diet determines the mechanism by which TNF stimulates hepatic triglyceride production. The use of multiple mechanisms to increase plasma triglycerides suggests that this TNF action plays an important role in the response to infection or inflammation.

The effect of diet on tumor necrosis factor stimulation of hepatic lipogenesis

Previous studies have demonstrated that tumor necrosis factor (TNF) acutely increases serum triglyceride levels and stimulates hepatic lipid synthesis. In this study, we determined the effects of TNF on serum lipid levels and hepatic lipid synthesis in animals whose diets and feeding conditions were varied to induce changes in baseline serum lipid levels and/or rates of hepatic lipid synthesis. In animals studied at both the nadir and peak of the diurnal cycle of hepatic lipid synthesis, TNF acutely increases serum triglyceride levels, stimulates hepatic fatty acid synthesis, and increases the quantity of newly synthesized fatty acids found in the serum. Similarly, in animals ingesting either high-sucrose or cholesterol-enriched diets, TNF induces the characteristic rapid increase in serum triglyceride levels, hepatic fatty acid synthesis, and quantity of labeled fatty acids in the serum. In animals fed a diet high in triglycerides, using either corn oil or lard, TNF stimulates hepatic fatty acid synthesis and increases the quantity of newly synthesized fatty acids in the serum, but serum triglyceride levels do not change. However, TNF inhibits gastric emptying, which results in a marked decrease in fat absorption in TNF-treated animals. It is likely that a decrease in the dietary contribution to serum triglyceride levels during high-triglyceride feeding counterbalances the increased hepatic contribution induced by TNF treatment. In animals fasted before TNF administration there was no acute change in either serum lipid levels, hepatic fatty acid synthesis, or the quantity of labeled fatty acids in the serum. Thus, TNF stimulates hepatic fatty acid synthesis and increases serum triglyceride levels under many diverse dietary conditions, suggesting that there is a strong linkage between the immune system and lipid metabolism that is independent of most dietary manipulations and may be of fundamental importance in the body's response to infection.

Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in murine epidermis. Modulation of enzyme content and activation state by barrier requirements

Epidermal cholesterol biosynthesis is regulated by barrier function. We quantitated the amount and activation state (phosphorylation-dephosphorylation) of the rate-limiting enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, in epidermis before and after barrier disruption. In murine epidermis we found high enzyme activity (1.75 +/- 0.02 nmol/min per mg protein). After acute barrier disruption, enzyme activity began to increase after 1.5 h, reaching a maximum increase by 2.5 h, and returned to normal by 15 h. Chronic barrier disruption increased total enzyme activity by 83%. In normal epidermis, measurement of HMG CoA reductase activity in microsomes isolated in NaF- vs. NaCl-containing buffers demonstrated that 46 +/- 2% of the enzyme was in the active form. After acute or chronic barrier disruption, a marked increase in the percentage of HMG CoA reductase in the active form was observed. Acute disruption increased enzyme activation state as early as 15 min, reaching a maximum after 2.5 h, with an increase still present at 15 h, indicating that changes in activation state had a close temporal relationship with barrier function. Increases in total HMG CoA reductase activity occurred only after profound barrier disruption, whereas changes in activation state occur with lesser degrees of barrier disruption. Artificial correction of barrier function prevented the increase in total HMG CoA reductase activity, and partially prevented the increase in enzyme activation. These results show that barrier requirements regulate epidermal cholesterol synthesis by modulating both the HMG CoA reductase amount and activation state.

Effect of tumor necrosis factor (TNF) on lipid metabolism in the diabetic rat. Evidence that inhibition of adipose tissue lipoprotein lipase activity is not required for TNF-induced hyperlipidemia

Tumor necrosis factor (TNF) administration produces an increase in plasma triglycerides that may be due to inhibition of adipose lipoprotein lipase activity and/or a stimulation of hepatic lipogenesis. We now report that TNF administration to insulinopenic diabetic rats increases serum triglycerides (2 h, 2.4-fold; 17 h, 4.3-fold). Adipose tissue lipoprotein lipase activity was markedly decreased in diabetic animals compared with controls and was not further inhibited by TNF. Incorporation of tritiated water into fatty acids in the liver was increased 45% 1-2 h after TNF and 87% at 16-17 h. These results indicate that the TNF-induced increase in circulating lipid levels can occur in the absence of a TNF-induced inhibition of adipose tissue lipoprotein lipase activity. Moreover, the clearance from the circulation of triglycerides in chylomicrons was similar in control and TNF-treated animals; these results provide further evidence that the removal of triglyceride-rich lipoproteins is not altered in the TNF-treated animals. Our data suggest that the TNF-induced stimulation of hepatic lipid synthesis may play an important role in the increase in serum triglycerides. In addition, TNF administration to diabetic animals leads to an elevation in serum glucose levels (73% at 17 h) without a change in serum insulin levels. Thus, TNF stimulation of hepatic lipogenesis is independent of changes in insulin.

Cholesterol synthesis in bypassed segments of the small intestine in hyperphagic rats

Previous studies have demonstrated that a variety of conditions that result in an increase in food intake lead to an increase in small-intestinal cholesterol synthesis. In the present study, it was determined whether hyperphagia induces an increase in cholesterol synthesis in segments of the small intestine excluded from contact with the food stream and whether this increase would occur in bypassed segments of the proximal or mid-small intestine. In hyperphagic diabetic rats, cholesterol synthesis is increased 91% in the proximal portion of the small intestine excluded from contact with nutrients. In lactating rats, another model of hyperphagia, cholesterol synthesis is increased 2.4-fold in midintestinal segments excluded from contact with the food stream and 2.9-fold in segments of the proximal intestine that have been bypassed. These observations demonstrate that the hyperphagia-induced increase in small-intestinal cholesterol synthesis will occur in portions of the small intestine, even if contact with the food stream is prevented. In addition, this data demonstrated that the mass of the bypassed portion of the small intestine is increased in hyperphagic animals. In diabetic animals, the weight of the bypassed proximal intestine is increased 2.1-fold, whereas in lactating animals the mass is increased 50% in the bypassed midintestine and 74% in the bypassed proximal small intestine. In conclusion, the present study suggests that circulating or neurologic factors, or both, play a role in stimulating intestinal cholesterol synthesis in hyperphagic animals. These findings also suggest that indirect factors play a role in the increase in intestinal mass associated with hyperphagia.

Lipid-related barriers and gradients in the epidermis

The stratum corneum, once regarded as a degenerate, inconsequential tissue, is now respected as a structurally heterogeneous and metabolically active tissue. The segregation of lipids into intercellular domains, and the shift in composition from a mixture of polar lipids and neutral lipids to sphingolipids and neutral lipids have important implications for both barrier function and desquamation. Metabolic studies demonstrate the capacity of epidermis to synthesize lipids, the relative autonomy of such synthesis from extracutaneous influences, and the regulation of epidermal lipogenesis by local barrier requirements. Abundant lipid biosynthesis appears to occur in the stratum granulosum, consistent with the rapid recovery of barrier function following solvent treatment. Whereas nonpolar lipids, including sterol esters and hydrocarbons, provide a superficial barrier, sphingolipids and free sterols provide a more profound barrier. In parallel with the synthesis of lipids for barrier function, hydrolysis of phospholipids, glycolipids, and cholesterol sulfate occurs in the outer epidermis leading to a more hydrophobic lipid mixture with the evolution of broad membrane bilayers that may regulate both transcutaneous water loss and desquamation.

Regulation of cholesterol metabolism in a slow-growing hepatoma in vivo

Cholesterol metabolism and its regulation are altered in hepatomas as compared to normal liver. We investigated parameters of cholesterol metabolism and their regulation in rats bearing the well-differentiated Morris hepatoma 9108. The numbers of membrane associated receptors recognizing chylomicron remnants, the lipoproteins that deliver dietary lipid to the liver, were substantially decreased in the 9108 tumor relative to the host liver. Cholesterol synthetic rates were 2-3-fold higher in the tumor, while the activity of 3-hydroxy-3-methylglutarylcoenzyme A reductase (EC 1.1.1.88), a rate-limiting enzyme for sterol synthesis, was elevated 6-14-fold. Although tumor free and esterified cholesterol contents were elevated, the activity of acylcoenzyme A:cholesterol acyltransferase (EC 2.3.1.26), the enzyme responsible for intracellular sterol esterification, was unchanged. Similar to the host liver, cholesterol synthesis and 3-hydroxy-3-methylglutarylcoenzyme A reductase were inhibited in the tumor when rats were fed a diet containing cholesterol, cholate and lard, and there was no effect on the numbers of chylomicron remnant receptors. Administering an intravenous bolus of very low density lipoproteins obtained from hypercholesterolemic rats caused an inhibition of tumor reductase activity, but had little effect on cholesterol content or cholesterol esterification. Thus, hepatoma 9108 expressed quantitative differences in cellular parameters involved in the uptake, metabolism, and synthesis of cholesterol and their susceptibility to regulation when compared with the host liver. These differences are best explained by changes in the hepatoma of multiple factors involved in the regulation of normal hepatic cholesterol metabolism.

The effect of vitamin D status on cutaneous sterologenesis in vivo and in vitro

Recent studies have shown that cutaneous sterologenesis is autonomous from the influence of circulating sterols, and that the epidermis is an important site of sterologenesis which is regulated by permeability barrier requirements. In addition to barrier function, an additional, important function of the epidermis is to synthesize sterol precursors of vitamin D3. The present study was designed, first, to determine whether vitamin D status and/or circulating levels of 1,25-dihydroxyvitamin D3 might play a role in regulating cutaneous sterol synthesis in vivo and, second, whether 1,25-dihydroxyvitamin D3 modulates sterologenesis in cultured human keratinocytes. Hairless mice were maintained on a vitamin D-deficient diet in the dark and supplemented with various doses of vitamin D3/day. Despite demonstrating serum 25-hydroxyvitamin D3 levels ranging from less than 10 to 343 ng/ml, the incorporation of tritiated water into cholesterol and total nonsaponifiable lipids in both the epidermis and dermis was similar in the four groups of animals. Likewise, administration of various doses of 1,25-dihydroxyvitamin D3 to vitamin D-deficient mice resulted in serum levels of 1,25-dihydroxyvitamin D3 ranging from less than 10 to 85 pg/ml; yet, cholesterol and total nonsaponifiable lipid synthesis was similar in both the dermis and epidermis in all groups of animals. Moreover, administration of 0.6 micrograms/kg per day of 1,25-dihydroxyvitamin D3 to 'normal' vitamin D-replete mice also had no effect on cutaneous sterol synthesis. Furthermore, conversion of 7-dehydrocholesterol to cholesterol in vitamin D-deficient vs. supplemented animals did not differ. Finally, addition of 1,25-dihydroxyvitamin D3 to cultured keratinocytes over a concentration range of 10(-12)-10(-7) M did not affect sterologenesis, except at supraphysiologic doses (10(-7) M). Together, these results suggest that vitamin D status does not influence sterol synthesis in the skin.

Tumor necrosis factor-alpha stimulates hepatic lipogenesis in the rat in vivo

The hyperlipidemia accompanying infection has been attributed to production of tumor necrosis factor. This cytokine inhibits adipose tissue lipoprotein lipase, which could decrease clearance of lipoproteins. Infections also increase hepatic lipogenesis. We now have demonstrated that tumor necrosis factor-alpha stimulates lipid synthesis in vivo. 2 h after administration of tumor necrosis factor (25 micrograms/200 g), plasma triglycerides increase 2.2-fold and remain elevated for 17 h. Plasma cholesterol also increases, but this effect appears after 7 h. Tumor necrosis factor rapidly stimulates incorporation of tritiated water into fatty acids in the liver (1-2 h), which persists for 17 h. Also, tumor necrosis factor stimulates hepatic sterol synthesis. Of note, tumor necrosis factor treatment does not stimulate lipid synthesis in other tissues, including adipose tissue. Labeled fatty acids rapidly increase in the plasma, raising the possibility that stimulation of hepatic lipogenesis by tumor necrosis factor contributes to the hyperlipidemia of infection.

Effect of essential fatty acid deficiency on cutaneous sterol synthesis

The fact that the skin is a major site of total body sterologenesis, coupled both with the apparent absence of low density lipoprotein receptors on keratinocytes and with the lack of influence of serum cholesterol on epidermal sterologenesis, has created the impression that epidermal lipid synthesis might be autonomous, i.e., nonregulatable. Recent studies have shown, however, that disruption of cutaneous barrier function with acetone or detergents stimulates epidermal sterologenesis (J Lipid Res 26:418-427, 1985). To correlate further sterologenesis with barrier function, we measured de novo synthesis of cholesterol and total nonsaponifiable lipids in essential fatty acid-deficient (EFAD) hairless mice. Animals with defective barrier function, manifested by abnormal transepidermal water loss, demonstrated a 2-fold increase in epidermal cholesterol and total nonsaponifiable lipid synthesis over controls while synthesis in the dermis was unchanged. Epidermal sterologenesis in EFAD animals, repleted with linoleic acid either systematically or topically, returned toward normal as barrier function improved. Moreover, plastic occlusion of EFAD mouse skin normalized epidermal sterologenesis at 1 and 3 days. These results provide further evidence that epidermal sterologenesis is not entirely autonomous, and can be regulated by water barrier requirements.

Evidence for different isotopic enrichments of acetyl-CoA used for cholesterol synthesis in the liver and intestine: a study in the rat by mass fragmentography after intravenous infusion of [13C]acetate

Wistar rats were killed 4 h after an intravenous infusion of [1,2-13C]- and [1-14C]acetic acid sodium salt (39 mg, 12.5 microCi/ml, constant rate: 1.2 ml/h). At this time, labeled free cholesterol movements between the organs are still weak and cholesterol labeling in each tissue mainly originates from the in situ incorporation of the exogenous substrate. In male rats, the specific radioactivity of free cholesterol was found to be higher in the intestine (mucosa and wall) than in the liver and plasma. In female and in cholestyramine-fed male rats, cholesterol 14C labeling was close to that of male rats in the intestine, and was markedly higher in the liver. The same variations of 13C excess, calculated by mass fragmentography, indicated that there was no isotopic effect between 13C and 14C precursors. The advantage of this method consisted in obtaining the proportions of labeled molecules according to their molecular weight (M + 1-M + 11) for each sample. Then the distribution of 13C atoms in newly synthesized cholesterol was assessed in each sterogenesis site. In the intestine, about 3/4 of the 13C atoms were found in molecules of weight of at least M + 4 (after incorporation of at least two labeled acetate units). This proportion was only 1/3 in hepatic and plasma free cholesterol. These distinct 13C-labeling patterns clearly indicate that local variations occurred in the isotopic enrichment of acetyl-CoA used for cholesterol formation. Whatever the experimental conditions of this study, cholesterol was synthesized from an acetyl-CoA more 13C enriched in the intestine than in the liver. Such variations probably result from the different dilutions of exogenous acetyl-CoA by the endogenous pool in the liver and intestine. Consequently, the 14C or 13C incorporations measured in the liver and intestinal sterols do not account for absolute rates of cholesterol production by these organs. This study also indicated that after a few hours of infusion, free cholesterol labeling in the plasma originated mainly from cholesterol newly formed in the liver, even when acetate incorporation into cholesterol was higher in the intestine than in the liver.

Plasma mevalonate as a measure of cholesterol synthesis in man

Measurement of mevalonic acid (MVA) concentrations in plasma or 24-h urine samples is shown to be useful in studies of the regulation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and cholesterol synthesis. Plasma MVA concentrations, measured either at 7-9 a.m. after an overnight fast, or throughout the 24-h cycle, were compared with cholesterol synthesis rates that were measured by the sterol balance method: plasma MVA concentrations were directly related to the rate of whole body cholesterol synthesis (r = 0.972; p less than 0.001; n = 18) over a tenfold range of cholesterol synthesis rates. Moreover, hourly examination of MVA concentrations throughout the day demonstrated that interventions such as fasting or cholesterol feeding cause suppression of the postmidnight diurnal rise in plasma MVA concentrations, with little change in the base-line of the rhythm. Thus, the daily rise and fall of plasma MVA appears to reflect changes in tissues and organs, such as the liver and intestine, that are known to be most sensitive to regulation by fasting or by dietary cholesterol. The hypothesis that short-term regulation of HMG-CoA reductase in tissues is quickly reflected by corresponding variations in plasma MVA was tested by using a specific inhibitor of HMG-CoA reductase, mevinolin, to block MVA synthesis. Mevinolin caused a dose-dependent lowering of plasma MVA after a single dose; and in patients who received the drug twice a day for 4 wk, it decreased 24-h urinary MVA output. Significant lowering of plasma cholesterol was achieved through administration of mevinolin at doses that only moderately limit MVA production.

Effect of dietary cholesterol on mevalonate metabolism by sterol and nonsterol pathways

Results in the present communication demonstrate for the first time that the shunt pathway of mevalonate not leading to sterols is regulated by cholesterol feeding in a reverse fashion to the sterol pathway. Mevalonate incorporation into nonsaponifiable lipids by liver slices was inhibited by cholesterol feeding while the shunt pathway was clearly enhanced. Moreover, inhibition of renal sterologenesis by dietary cholesterol is also reported. These changes in the mevalonate metabolism are closely correlated with the increase observed in the esterified cholesterol content in neonatal chick liver and kidneys after 10 days of 2% cholesterol supplementation of the diet.

Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and cholesterol synthesis and esterification during the first cell cycle of liver regeneration

The regenerating rat liver provides a unique in vivo synchronized system for study of the interrelationships between mevalonate and sterol metabolism during the cell cycle. The regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, cholesterol synthesis and acyl coenzyme A: cholesterol acyltransferase during the first cell cycle was investigated. At 8 h postoperative and prior to onset of DNA synthesis or S phase, cholesterol synthesis was depressed in the regenerating liver relative to that in sham-operated controls. This suppression was observed whether assayed in vitro with liver homogenates utilizing radiolabeled acetate, mevalonate or water or in vivo with tritium water. In contrast, at this time point, 3-hydroxy-3-methylglutaryl-CoA reductase activity was increased in microsomes prepared both in the presence and absence of NaF. By 24 h, well into S phase and approaching mitosis, reductase activity and cholesterol synthesis both approached levels observed in the sham-operated control animals. There were no detectable changes in acyl-CoA: cholesterol acyltransferase activity at any time point. Thus, at the 8 h time point, the regulation of the three processes appeared uncoupled. The increased levels of in vitro expressed 3-hydroxy-3-methylglutaryl-CoA reductase activity compared with the decrease in the rate of both cholesterol and squalene biosynthesis suggested diversion of mevalonate into products other than squalene or sterols. We propose that this may reflect the needs of the cell for a nonsterol metabolite of mevalonate necessary for entry of cells into S phase.

Localization of de novo sterologenesis in mammalian skin

Previous studies have demonstrated that the skin is an important site of de novo sterol synthesis and that there is a sex difference in cutaneous sterologenesis with male animals synthesizing more sterols than females. The aim of the present study was to localize the major sites of sterol synthesis within the skin and to determine which of these sites accounted for the sex differences in sterologenesis. In male and female rats whose dermal and epidermal layers are separated by dithiothreitol treatment, the dermis is the major site of skin sterologenesis (males 86% of total, females 82% of total). Moreover, the sex difference in skin sterol synthesis is quantitatively localized primarily within the dermal layer (approximately 2.5-fold greater in the dermis of males). Sterol synthesis is also increased in the epidermis of males. To rule out the possibility that sebaceous gland production accounted for our observations, we treated animals with isotretinoin (13-cis-retinoic acid), a drug that suppresses sebaceous gland sebum production. Sterol synthesis in the skin of both male and female rats is not significantly altered by isotretinoin administration and the sex difference in skin sterologenesis is unaffected. To further localize the sites of sterol synthesis within the skin, studies of hairless mice whose skin was split by DTT were initiated. In hairless mice, DTT separates the epidermis into upper (stratum corneum and granulosum) and lower (basal and spinous cells) strata. The basal layer was separated from the dermis by gentle scraping. As in rats, the dermis is the chief site of sterol synthesis in the skin. In addition, the lower layer of the epidermis (basal and spinous cells) is also a very active site of sterologenesis, accounting for 20% of total skin nonsaponifiable lipid synthesis. The upper epidermis accounted for only a small portion of total skin synthesis. It is highly likely that the bulk of cutaneous sterol synthesis occurs in the pilosebaceous epithelium.