Cytokine regulation of parathyroid hormone-related protein messenger ribonucleic acid levels in mouse spleen: paradoxical effects of interferon-gamma and interleukin-4

Under normal physiological conditions, PTH-related protein (PTHrP) is produced in a wide variety of tissues and is thought to act locally in an autocrine or paracrine fashion more analogous to cytokines than to classic hormones such as PTH. In addition, we have recently shown that, like cytokines, PTHrP is induced in the spleen during the response to sublethal doses of endotoxin [lipopolysaccharide (LPS)] an effect that is mediated by tumor necrosis factor (TNF). As complex cytokine cascades are induced in response to infectious or inflammatory stimuli, the effects of other prototypical inflammatory [interferon-gamma (IFN gamma)] or antiinflammatory [interleukin-4 (IL-4)] cytokines on PTHrP gene expression were studied. Paradoxically, IFN gamma (50 micrograms), a cytokine that usually synergizes with TNF, inhibited LPS induction of splenic PTHrP messenger RNA (mRNA) levels in LPS-sensitive C3H/OuJ (OuJ) and LPS-resistant C3H/HeJ (HeJ) mice. The stimulation of splenic PTHrP mRNA levels caused by the administration of TNF alpha or interleukin-1 beta was similarly inhibited by IFN gamma, a type II interferon. In contrast, IFN alpha (50 micrograms), a type I interferon, stimulated splenic levels of PTHrP mRNA. IL-4, a prototypical antiinflammatory cytokine, also had a paradoxical effect on LPS induction of splenic PTHrP mRNA levels. Instead of inhibiting LPS induction of splenic PTHrP mRNA levels in OuJ or HeJ mice, IL-4 (200 ng) actually stimulated PTHrP mRNA levels. These complex cytokine interactions suggest that the expression of PTHrP in response to infectious or inflammatory stimuli depends on the counterbalancing effects of the specific cytokine networks induced by each stimulus.

A role for ions in barrier recovery after acute perturbation

The epidermal cutaneous permeability barrier can be disrupted by treatment with topical solvents. Recent studies have shown that barrier recovery, measured by the recovery of transepidermal water loss towards normal, is inhibited by high extracellular Ca++ and K+, and accelerated by low extracellular concentrations of these ions. To examine the effects of Ca++ or K+ fluxes on barrier recovery, we tested the effects on transepidermal water loss recovery of agents that modify these fluxes. K+ channel agonists or blockers modified the inhibitory effects on barrier recovery induced by raised extracellular Ca++ and K+. In addition, Na+/K+ adenosine 5' triphosphatase inhibitors reversed the inhibitory effects of high extracellular Ca++ and K+. Our results suggest that barrier recovery requires both Ca++ and K+ fluxes and are consistent with the hypothesis that both verapamil or dihydropyridine-sensitive Ca++-permeable channels and Ca++-sensitive K+ channels participate in epidermal permeability barrier homeostasis.

Effects of TNF, IL-1, and the combination of both cytokines on cholesterol metabolism in Syrian hamsters

Infection and inflammation are associated with alterations in lipid metabolism that may be mediated by cytokines such as TNF and IL-1. This study determined the effects of TNF and IL-1 on certain aspects of cholesterol metabolism. TNF or IL-1 administration to Syrian hamsters increased serum cholesterol levels by 17 and 21%, respectively, and decreased HDL cholesterol levels by 20 and 15%, respectively. TNF + IL-1 increased serum cholesterol levels by 58% and decreased HDL cholesterol levels by 58%. TNF or IL-1 increased hepatic HMG CoA reductase mRNA levels by 3.5- and 3-fold, respectively. TNF + IL-1 increased HMG CoA reductase mRNA levels by 7-fold. IL-1 increased hepatic LDL receptor mRNA levels by 2-fold while TNF and a combination of TNF + IL-1 had minimal effects. TNF or IL-1 did not affect hepatic apo E or apo A-I mRNA levels while a combination of TNF + IL-1 decreased both mRNA levels by 50%. Our results demonstrate that TNF and IL-1 similarly affect the parameters of cholesterol metabolism studied. Furthermore, the combination of TNF + IL-1 was, in most cases, more effective than either cytokine alone, and reproduced many of the effects of LPS.

Effects of endotoxin and cytokines on lipid metabolism

Endotoxin, via cytokines, induces marked changes in lipid metabolism: serum VLDL increases, whereas the effect on LDL levels varies among species. The increase in VLDL is caused by stimulation of hepatic VLDL secretion or inhibition of clearance, or both. These alterations can be deleterious or beneficial effects.

Influence of altered serum cholesterol levels and fasting on cutaneous cholesterol synthesis

Barrier perturbation stimulates epidermal cholesterol synthesis, which plays an important role in restoring barrier function. In the present study, we examined whether changes in serum cholesterol levels or nutrition regulate epidermal cholesterol synthesis in hairless mice. Serum cholesterol levels were lowered by 50% after injection with 4-aminopyrazolo (3,4-d) pyrimidine and were increased by 51% by feeding an atherogenic diet. In contrast to most other tissues, cholesterol synthesis in the epidermis and dermis was not inhibited by elevations or stimulated by decreases in serum cholesterol levels. Additionally, feeding a high-cholesterol diet did not decrease epidermal or dermal cholesterol synthesis. However, fasting significantly decreased both epidermal (38%) and dermal (34%) cholesterologenesis. Furthermore, barrier recovery after acetone disruption of the barrier was impaired in fasted animals. However, treatment with topical lipids did not restore barrier repair rate to normal, indicating that factors in addition to lipids are necessary to overcome the effects of fasting. These results demonstrate that cholesterol synthesis in the epidermis and dermis is regulated independently of changes in serum cholesterol levels.

Permeability barrier requirements regulate epidermal beta-glucocerebrosidase

The intercellular spaces of the outermost layers of the epidermis (stratum corneum, SC) of terrestrial mammals contain a mixture of lipids, enriched in ceramides that are critical for the epidermal permeability barrier. Whereas glucosylceramides (GlcCer) are synthesized in abundance in the epidermis, they disappear coincident with an increase of ceramides (Cer) in the SC. Hence, hydrolysis of GlcCer to Cer by beta-glucocerebrosidase (GlcCer'ase), may be required for permeability barrier homeostasis. We determined first whether modulations in epidermal GlcCer'ase activity and mRNA levels occur in response to barrier disruption; and second, how GlcCer'ase inhibitors influence barrier function and SC membrane ultrastructure. Barrier disruption significantly increased epidermal GlcCer'ase mRNA levels, with a 2.8-fold increase over untreated control levels at 8 h (P < 0.01). GlcCer'ase activity was increased in whole epidermis (34%; P < 0.02) 24 h after barrier disruption. Localization of GlcCer'ase activity showed an increase (33%; P < 0.05) in the outer epidermis (SC and stratum granulosum), without a change in lower epidermal activity (stratum spinosum and stratum basale). Furthermore, a single topical application of the GlcCer'ase inhibitor, bromoconduritol-B-epoxide (BrCBE), inhibited enzyme activity (98%) and significantly delayed permeability barrier recovery after acetone treatment. In addition, BrCBE treatment disrupted SC intercellular lamellar bilayers, without evidence of cellular toxicity. These results indicate that epidermal processing of GlcCer to Cer by GlcCer'ase is required for barrier homeostasis, and that this important enzymatic step is regulated by barrier requirements.

Selective obliteration of the epidermal calcium gradient leads to enhanced lamellar body secretion

The epidermal permeability barrier is formed by lipids delivered to the intercellular spaces through the secretion of lamellar bodies. Prior studies have shown that the rate of lamellar body secretion appears to be regulated by the extracellular calcium content of the upper epidermis, which is altered following permeability barrier disruption. To determine directly whether changes in extracellular calcium content in the upper epidermis versus disruption of the barrier regulate lamellar body secretion, we experimentally manipulated the Ca++ content of the upper epidermis by sonophoresis of aqueous solutions containing physiologic Ca++ (and K+) versus ion-free solutions across hairless mouse stratum corneum. Sonophoresis at 15 MHz did not alter barrier function, but in the absence of Ca++ the extracellular calcium content of the outer epidermis, as revealed by ion capture cytochemistry, was displaced downward toward the basal layer and dermis. In contrast, following sonophoresis of Ca(++)-containing solutions, the extracellular Ca++ gradient became obscured by excess Ca++ in the cytosol at all levels of the epidermis. These changes in the extracellular calcium content lead, in turn, to accelerated lamellar body secretion (with low Ca++), or basal rates of lamellar body secretion (with normal Ca++). These results demonstrate that the epidermal extracellular calcium content in the upper epidermis can be manipulated by sonophoresis without prior barrier disruption, and that changes in the Ca++ gradient induce lamellar body secretion, independent of barrier disruption.

Permeability barrier disruption alters the localization and expression of TNF alpha/protein in the epidermis

Previous studies have shown that (1) epidermal TNF alpha mRNA levels are increased following acute disruption of the cutaneous permeability barrier; (2) this increase is maximal at 1 h and decreases to control levels by 8 h; and (3) in essential fatty acid-deficient (EFAD) mice, a chronic model of barrier perturbation, TNF alpha mRNA levels are also elevated several-fold over controls. In the present study we determined, using immunocytochemical procedures, epidermal TNF alpha protein levels following either acute of chronic barrier disruption and the localization of any increase. Frozen, paraffin and Antibed sections of skin were incubated with polyclonal anti-mouse TNF alpha antisera and detection was accomplished by either immunoperoxidase or fluorescence procedures. We found that (1) TNF alpha-immunoreactive protein was present in normal mouse epidermis, and was primarily localized to the upper nucleated layers where it displayed a diffuse cytosolic pattern; (2) acute disruption of the barrier with acetone or tape-stripping resulted in TNF alpha staining that was more intense throughout all of the nucleated epidermal cell layers in comparison with normal epidermis; (3) the increase in TNF alpha staining occurred as early as 2 h after barrier disruption; and (4) increased TNF alpha staining was also observed in the stratum corneum of EFAD mice. These results indicate that epidermal TNF alpha protein levels increase after both acute and chronic barrier disruption, and are consistent with the hypothesis that TNF alpha may signal and/or coordinate portions of the cutaneous response to barrier disruption.

Barrier function coordinately regulates epidermal IL-1 and IL-1 receptor antagonist mRNA levels

Disruption of the cutaneous permeability barrier increases mRNA levels for TNF, GM-CSF, IL-1 alpha, and IL-1 beta in the epidermis. We have hypothesized that the cytokines mediate the changes in lipid and DNA synthesis which occur following barrier disruption. To further characterize the cytokine response to barrier abrogation, we examined the levels of epidermal IL-1ra mRNA in two acute models and one chronic model in the hairless mouse. IL-1ra mRNA levels increased shortly after acute disruption of the barrier with acetone, reached a peak at 3-4 h after treatment, and returned to control levels by 8h. These changes in mRNA levels parallel those which occur for IL-1 alpha and beta. Furthermore, IL-1ra mRNA levels were elevated 5-fold and 4-fold, at 2.5 h and 4 h, respectively, following tape-stripping, a second acute model of barrier disruption. Finally, IL-1ra mRNA levels were elevated 2.5-fold in the epidermis of EFAD mice, which have a chronic barrier defect. Thus, the cutaneous response to barrier disruption includes mechanisms which increase IL-1 and IL-1ra mRNA levels in a coordinate manner. The net result provides a regulatory mechanism for controlling the biological effects of increased IL-1 production.

Diabetes increases hepatic hydroxymethyl glutaryl coenzyme A reductase protein and mRNA levels in the small intestine

Previous studies have shown that both cholesterol synthesis and the activity of hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase, the rate-limiting enzyme in cholesterol synthesis, are increased in the small intestine of a wide variety of different animal models of diabetes. In the present study, we demonstrate that the mass of HMG CoA reductase protein is increased in the small intestine of both streptozocin-induced diabetic rats (2.5-fold) and streptozocin/alloxan-induced diabetic dogs (2.4-fold). These increases in HMG CoA reductase protein mass are of a magnitude similar to the previously observed increases in either HMG CoA reductase activity and/or cholesterol synthesis in the small intestine of diabetic animals. Furthermore, mRNA levels for HMG CoA reductase in the small intestine of diabetic rats and diabetic dogs are increased 2.1- and 1.7-fold, respectively. These results suggest that the increase in HMG CoA reductase protein levels in the small intestine of diabetic animals is due to an increase in mRNA levels. In contrast, mRNA levels for HMG CoA reductase in the liver of diabetic rats are not increased. Additionally, mRNA levels for the low-density lipoprotein (LDL) receptor are also increased in the small intestine of diabetic animals (rats, 43%; dogs, 59%). The increase in small-intestinal cholesterol synthesis has the potential for adversely affecting lipoprotein metabolism and increasing the risk of atherosclerosis in diabetes.

Integrity of the permeability barrier is crucial for maintenance of the epidermal calcium gradient

Prior studies have demonstrated a Ca2+ gradient within the epidermis, with the highest concentration in the outer nucleated layers, disappearance of the Ca2+ gradient when the permeability barrier is acutely disrupted, and reappearance of the Ca2+ gradient in parallel with barrier repair, and disruption of the gradient in psoriasis. These observations suggest that integrity of the permeability barrier may maintain the epidermal Ca2+ gradient. To determine further whether a functional barrier is crucial for maintaining the Ca2+ gradient, we examined Ca2+ distribution by ion-capture cytochemistry in essential-fatty-acid-deficient (EFAD) and topical-lovastatin-treated mice, which display a chronic barrier abnormality. In both models, loss of the Ca2+ gradient occurred due to increased cytosolic Ca2+ in the lower epidermis, which normally displays a paucity of Ca2+. Moreover, artificial barrier restoration for 48 h with a water vapour-impermeable wrap normalized the Ca2+ distribution pattern. Acute barrier disruption also leads to the loss of the Ca2+ gradient, but in contrast with the chronic models, loss of the gradient was due to decreased Ca2+ in the upper epidermis. Occlusion with a vapour-impermeable wrap blocked restoration of the Ca2+ gradient after acute barrier disruption. These results demonstrate that chronic barrier disruption increases Ca2+ in the epidermis, and blockade of water flux normalizes Ca2+ distribution, whereas acute barrier disruption leads to loss of Ca2+, and blockade of water flux prevents the return of Ca2+. We conclude: (i) that the epidermal Ca2+ reservoir is derived from the movement of fluids and Ca2+ across the basement membrane, and (ii) that the integrity of the permeability barrier maintains the epidermal Ca2+ gradient.

Fatty acid synthesis in obese insulin resistant diabetic mice

Diabetes is associated with hypertriglyceridemia and it has been suggested that the intestine contributes to this elevation. Recent studies have shown that in animals with IDDM, fatty acid (FA) synthesis is decreased in the liver and increased in the small intestine (SI). The purpose of the present study was to measure FA synthesis in the liver and SI of animals with NIDDM. In both db/db and ob/ob animals the incorporation of 3H2O into FA was increased in the SI (10 fold in db/db and 2.2 fold in ob/ob). FA synthesis was also increased in the liver of NIDDM (9 fold in db/db and 6 fold in ob/ob). These results provide further evidence that diabetes leads to changes in lipid metabolism in the SI.

Role of exogenous oxygen in cutaneous barrier repair

Occlusion of the skin with a water-vapor-impermeable membrane following disruption of the permeability barrier prevents the epidermal changes which lead to the restoration of barrier function, suggesting that water transit could be an important regulatory signal for barrier repair. However, occlusion with a water-vapor-impermeable membrane also prevents the movement of gases, which could also potentially influence permeability barrier homeostasis. Since O2 is known to have an effect on epidermal cell function, we have determined the effect of gases containing different levels of O2 on barrier repair 6 h following topical treatment of hairless mice with acetone. The disrupted barrier of air-exposed animals (O2 approximately 20%) recovered by 50.8 +/- 3.4% (mean +/- SEM) after 6 h. Under flowing air (O2 approximately 20%), O2/CO2 95/5% and argon (O2 = 0%) the barrier recovered by 43.9 +/- 28, 36.2 +/- 8.5 and 39.2 +/- 4.6%, respectively. These values were not statistically different from each other. The slightly lower levels of recovery at 6 h with the flowing gases in comparison to exposure to static air probably can be attributed to a slight cooling of the skin caused by the flowing gases. These results suggest that exogenous O2 is neither required for barrier repair nor a signal for barrier repair.

The protective effect of serum lipoproteins against bacterial lipopolysaccharide

Lipoproteins bind and inactivate bacterial endotoxin, both in vitro and in vivo. Both cholesterol ester-rich and TG-rich lipoproteins, and TG-rich lipid emulsions can prevent death in mice when pre-incubated with a lethal dose of endotoxin before intraperitoneal administration. Chylomicrons can also prevent death when given intravenously after endotoxin in rats. The metabolic fate of lipoprotein-bound endotoxin appears to be directed by the lipoprotein particle. When administered with chylomicrons, the plasma clearance and hepatic uptake of endotoxin are enhanced. Endotoxin is shunted preferentially to hepatocytes and away from hepatic macrophages, thereby increasing endotoxin excretion [corrected] in bile. The survival benefit and alterations in metabolism afforded by chylomicrons correlate with a reduction in peak serum levels of tumour necrosis factor (TNF), providing a possible mechanism by which lipoproteins protect against endotoxin-induced death. These findings suggest a possible role for lipoproteins or lipid emulsions in the body's defence against endotoxaemia.

Effect of endotoxin on cholesterol biosynthesis and distribution in serum lipoproteins in Syrian hamsters

Infection and inflammation increase serum triglyceride and cholesterol levels in rodents and rabbits. Endotoxin (LPS) has been used as a model of infection and its effects on triglyceride metabolism have been previously characterized. In the present study we demonstrate that both low (100 ng/100 g body weight) and high dose (100 micrograms/100 g body weight) LPS increase serum cholesterol levels in hamsters. The increase in serum cholesterol is first observed 16 h after LPS and persists for at least 24 h. This increase is primarily due to an increase in low density lipoprotein (LDL) cholesterol. High density lipoprotein (HDL) cholesterol levels decrease after LPS treatment. Both low and high dose LPS increase hepatic cholesterol synthesis (low dose 85%, high dose 205%) and total HMG-CoA reductase activity (low dose 2.97-fold, high dose 9.96-fold). However, the proportion of HMG-CoA reductase in the active form is reduced by LPS treatment. Additionally, the mass of HMG-CoA reductase protein in the liver, measured by Western blotting, is increased after LPS. Moreover, LPS increases hepatic HMG-CoA reductase mRNA levels (low dose 3.1-fold, high dose 14.2-fold). The increase in hepatic HMG-CoA reductase mRNA levels is first seen 4 h after LPS and persists for at least 24 h. In contrast, LPS had only minimal effects on hepatic LDL receptor protein and mRNA levels. These results suggest that LPS increases serum cholesterol levels by increasing hepatic cholesterol synthesis. LPS administration decreases apoE mRNA levels in the liver while having no effect on apoA-I mRNA levels. These results suggest that HMG-CoA reductase is a member of a group of hepatic proteins that are positively regulated by inflammatory stimuli (acute phase proteins) while apoE can be considered a negative acute phase protein in hamsters. It is possible that increases in hepatic HMG-CoA reductase provide cholesterol that allows for the increased production of lipoproteins and elevations in serum lipid levels that may be beneficial to the body's host defense.

Endotoxin increases parathyroid hormone-related protein mRNA levels in mouse spleen. Mediation by tumor necrosis factor

Parathyroid hormone-related protein (PTHrP) causes hypercalcemia in malignancy. However, the role and regulation of PTHrP in normal physiology is just beginning to be explored. PTHrP is found in the spleen and has several other features common to cytokines. Since endotoxin (LPS) causes many of its effects indirectly by inducing cytokines, studies were undertaken to determine whether LPS might also induce splenic PTHrP expression. LPS (100 ng/mouse) increased splenic PTHrP mRNA levels 3.6-fold in C3H/OuJ mice. This effect was maximal at 2 h and returned to baseline by 4 h. PTHrP peptide levels also increased 3.3-fold in splenic extracts in response to LPS (1 microgram/mouse). Murine TNF-alpha and human IL-1 beta, cytokines that mediate many of the effects of LPS, also increased splenic PTHrP mRNA levels. LPS-resistant C3H/HeJ mice, which produce minimal amounts of TNF and IL-1 in response to LPS, were resistant to LPS induction of splenic PTHrP mRNA, while TNF-alpha and IL-1 beta readily increased PTHrP mRNA levels in C3H/HeJ mice. Anti-TNF antibody blocked LPS induction of splenic PTHrP mRNA in C3H/OuJ mice by 68%, indicating that TNF is a mediator of the LPS induction of PTHrP levels. In contrast, an IL-1 receptor antagonist (IL-1ra) was ineffective. The increase in PTHrP in the spleen during the immune response suggests that PTHrP may play an important role in immune modulation, perhaps by mediating changes in lymphocyte proliferation and/or function.

Relationship of apolipoprotein E phenotypes to hypocholesterolemia

PURPOSE

Persons with total cholesterol (TC) levels less than 130 mg/dL (less than 3.26 mmol/L) make up less than 1% of a healthy population. Causes of hypocholesterolemia include a diet very low in cholesterol and saturated fat, disease, genetic factors (including low apolipoprotein B-100 [apo B-100] and the apo E allele), and drug therapy. The purpose of this study was to determine the causes of hypocholesterolemia in a healthy Kaiser Foundation Health Plan (KFHP) population.

PATIENTS AND METHODS

We conducted a dietary and health survey of 201 healthy hypocholesterolemic adults (range: 2.04 to 3.88 mmol/L [79 to 150 mg/dL]) and 200 matched control subjects with TC levels in the middle quintile of the population (range: 5.0 to 5.61 mmol/L [194 to 217 mg/dL]) who had routine health screening from 1983 through 1985. We did apo E phenotyping studies and lipid and apo A-1 and B-100 measurements in a subgroup of 45 hypocholesterolemic subjects (mean TC level: 3.26 mmol/L [126 mg/dL]) and in a comparison group of 49 unmatched volunteers (mean TC level: 5.04 +/- 0.75 mmol/L [195 +/- 29 mg/dL]).

RESULTS

We found no differences in dietary intake or clinically significant medical illness between hypocholesterolemic and control subjects. In the hypocholesterolemic subgroup, we found an increased frequency of the apo E2 allele (epsilon 2) and a decreased frequency of the apo E4 allele (epsilon 4); the frequencies of the epsilon 2, epsilon 3, and epsilon 4 alleles were 33.3%, 63.3%, and 3.3%, respectively. The corresponding apo E allele frequencies in the comparison subgroup were 8.2%, 73.5%, and 18.4%, similar to those previously reported for the general population and significantly different from those found in the hypocholesterolemic subgroup (p < 0.0001). One hypocholesterolemic subject (a 46th patient) had a mutation in the apo B gene that resulted in the synthesis of a truncated species of apo B (apo B-46).

CONCLUSION

Our study indicates that hypocholesterolemia in our KFHP urban population is usually not caused by diet or disease. Biochemical factors, including the increased frequency of the apo E-2 phenotype and the decreased frequency of the apo E-4 phenotype, are more important.

Indices of thyroid function and weight loss in human immunodeficiency virus infection and the acquired immunodeficiency syndrome

Abnormalities of thyroid hormone levels have been reported in the acquired immunodeficiency syndrome (AIDS), but there has been debate as to whether they are appropriate for the clinical status of the patients. Inappropriate maintenance of circulating 3,3',5-triiodothyronine (T3) levels could contribute to weight loss. Although many patients with AIDS have a history of wasting, recent data indicate that prolonged periods of stable weight occur in AIDS and that short-term weight loss is present in a subset of patients with anorexia, many of whom have active secondary infection (AIDS-SI). Therefore we analyzed thyroid hormone levels in a cohort of subjects that have been characterized in terms of recent weight loss and caloric intake. Asymptomatic patients with human immunodeficiency virus infection (HIV+) had short-term stable weights, normal caloric intake, and normal serum T3 levels. In AIDS, average short-term weight was stable, caloric intake was normal, and T3 levels were decreased by 19%. In AIDS-SI, both short-term weight loss and anorexia were significant, and this group showed a 45% decrease in T3 levels. The free T3 (FT3) index was decreased by 30% in AIDS and by 50% in AIDS-SI. Free thyroxine (FT4) levels were decreased while thyroxine-binding globulin (TBG) capacity was increased in HIV+ and AIDS; TBG sialylation was unchanged. Thyrotropin (TSH) levels were slightly increased in AIDS, although levels remained within the normal range. 3,3',5'-triiodothyronine (rT3) levels were decreased in HIV+, AIDS, and AIDS-SI. Thus asymptomatic patients with HIV infection whose weight is stable maintain normal T3 levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of cholesterol and sphingolipid synthesis causes paradoxical effects on permeability barrier homeostasis

Cholesterol, fatty acid, and sphingolipid synthesis are required for barrier homeostasis, as demonstrated by studies where synthesis of these species is stimulated in parallel with barrier repair. Moreover, blockade of synthesis of these lipids with inhibitors of two of the rate-limiting enzymes, HMGCoA reductase (lovastatin, fluvastatin) and serine palmitoyl transferase (beta-chloroalanine), alters the kinetics of barrier repair. Whereas these studies demonstrated a requirement for these lipids individually, we asked here whether these lipids are required in either an additive or cooperative fashion. We applied each class of inhibitor alone or the two classes of inhibitors together to acetone-treated skin, or each class separately to essential fatty acid deficient murine skin. When fluvastatin or beta-chloroalanine was applied individually to acetone-treated skin, each caused a delay in the early or late stages of barrier recovery, respectively (assessed as transepidermal water loss). However, when applied together they caused no further worsening at the early time point and a paradoxical improvement at the later time points. This improvement correlated with an accelerated return of sphingolipids, which was perhaps due to a global stimulation of lipid synthesis induced by HMGCoA reductase inhibitors. In essential fatty acid deficient animals, inhibition of HMGCoA reductase caused drastic worsening of both clinical appearance and barrier function, but beta-chloroalanine caused a paradoxical improvement, which correlated with a significant reduction in epidermal sphingolipids. These results are consistent with a requirement for both cholesterol and sphingolipids for barrier homeostasis, and also with the suggestion that both of these lipids must be present (with free fatty acids) for optimal barrier function.

The effect of oxidized lipids in the diet on serum lipoprotein peroxides in control and diabetic rats

The levels of oxidized serum lipoproteins are increased in humans and animals with diabetes. We have examined the contribution of dietary oxidized lipids on the levels of oxidized lipoproteins. In both control and streptozocin induced diabetic rats, the oxidized lipid content of mesenteric lymph chylomicrons (CM) increased when increasing quantities of oxidized lipids were administered intragastrically. However, at all levels of administered oxidized lipids, the quantity of oxidized lipids in CM was greater in the diabetic animals. These results indicate that oxidized lipids are absorbed and packaged into CM and suggest that there is increased absorption of oxidized lipids in diabetic animals. In nondiabetic rats fed a fat-free diet, the levels of oxidized lipids in their serum lipoproteins were very low. When oxidized lipids were added to the diet, the quantity of peroxides in serum lipoproteins increased about fivefold. In diabetic animals fed a fat-free diet, there were also very low levels of oxidized lipids in their serum lipoproteins, and there was no difference between control and diabetic rats. However, when diabetic animals were fed a diet containing oxidized lipids, the quantity of oxidized lipids in their serum lipoproteins increased 16-fold and were significantly greater than in controls. Thus, in both control and diabetic rats the quantity of oxidized lipids in the diet largely determines the levels of oxidized lipids in circulating lipoproteins. However, in diabetic animals the effect of diet is more pronounced. Together with the CM studies, these results demonstrate that dietary oxidized lipids make a major contribution to the levels of oxidized lipids in circulating lipoproteins and indicate that increased absorption of oxidized lipids in diabetic animals may play a role in the elevation of oxidized lipoproteins observed in this disorder.

Fatty acids are required for epidermal permeability barrier function

The permeability barrier is mediated by a mixture of ceramides, sterols, and free fatty acids arranged as extracellular lamellar bilayers in the stratum corneum. Whereas prior studies have shown that cholesterol and ceramides are required for normal barrier function, definitive evidence for the importance of nonessential fatty acids is not available. To determine whether epidermal fatty acid synthesis also is required for barrier homeostasis, we applied 5-(tetradecyloxy)-2-furancarboxylic acid (TOFA), an inhibitor of acetyl CoA carboxylase, after disruption of the barrier by acetone or tape stripping. TOFA inhibits epidermal fatty acid by approximately 50% and significantly delays barrier recovery. Moreover, coadministration of palmitate with TOFA normalizes barrier recovery, indicating that the delay is due to a deficiency in bulk fatty acids. Furthermore, TOFA treatment also delays the return of lipids to the stratum corneum and results in abnormalities in the structure of lamellar bodies, the organelle which delivers lipid to the stratum corneum. In addition, the organization of secreted lamellar body material into lamellar bilayers within the stratum corneum interstices is disrupted by TOFA treatment. Finally, these abnormalities in lamellar body and stratum corneum membrane structure are corrected by coapplication of palmitate with TOFA. These results demonstrate a requirement for bulk fatty acids in barrier homeostasis. Thus, inhibiting the epidermal synthesis of any of the three key lipids that form the extracellular, lipid-enriched membranes of the stratum corneum results in an impairment in barrier homeostasis.

Chylomicrons enhance endotoxin excretion in bile

Chylomicrons prevent endotoxin toxicity and increase endotoxin uptake by hepatocytes. As a consequence, less endotoxin is available to activate macrophages, thereby reducing tumor necrosis factor secretion. To determine whether the chylomicron-mediated increase in hepatocellular uptake of endotoxin results in increased endotoxin excretion into bile, we examined bile after endotoxin administration. A sublethal dose (7 micrograms/kg) of 125I-endotoxin was incubated with either rat mesenteric lymph containing nascent chylomicrons (500 mg of chylomicron triglyceride per kg of body weight) or an equal volume of normal saline (controls) for 3 h and then infused into male Sprague-Dawley rats. Bile samples were collected via a common bile duct catheter for 24 h. Infusion of endotoxin incubated with chylomicrons increased biliary excretion of endotoxin by 67% at 3 h (P < or = 0.006) and by 20% at 24 h (P < or = 0.01) compared with infusion of endotoxin incubated in saline. Endotoxin activity, as measured by the Limulus assay, was not detected in the bile of test animals. However, endotoxin activity was detected after hot phenol-water extraction of bile, demonstrating that endotoxin is inactive in the presence of bile but retains bioactivity after hepatic processing. Since the majority of an intravenous endotoxin load has been shown to be cleared by the liver, acceleration of hepatocyte clearance and biliary excretion of endotoxin may represent a component of the mechanism by which chylomicrons protect against endotoxin-induced lethality.

Extrahepatic lipogenesis contributes to hyperlipidemia in the analbuminemic rat

Hepatic lipid and apolipoprotein synthesis is increased in the nephrotic syndrome. Catabolism of triglyceride-rich lipoproteins is impaired in nephrotic syndrome but not in rats with hereditary analbuminemia (NA), suggesting that lipid synthesis should be increased by analbuminemia in the absence of proteinuria. In this study the rate of cholesterol and fatty acid synthesis in liver and extrahepatic tissue was measured in female NA and control Sprague-Dawley (SD) rats to determine whether lipid synthesis was indeed increased in isolated analbuminemia and to identify the site(s) of increased lipogenesis. We also measured the concentrations of apolipoproteins (apo) AI, B, and E in plasma, as well as the levels of the respective mRNAs in liver. Plasma cholesterol, triglycerides, and apo AI, B, and E were all increased severalfold in the NA rat (P < 0.001). Although liver apolipoprotein mRNA content was significantly increased (P < 0.001) for apo AI (643%), B (273%), and E (299%), 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in liver microsomes and hepatic cholesterol synthesis were not significantly increased in the NA rats. Hepatic fatty acid synthesis and intestinal cholesterol synthesis were not increased in the NA rats. Surprisingly, intestinal fatty acid synthesis was elevated by 60% (P < 0.01). The NA rats demonstrated approximately fourfold increases in the incorporation of 3H2O into circulating cholesterol and fatty acids (P < 0.001). A 56% increase in the synthesis of total nonsaponifiable lipid was found in the extravisceral carcass (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)