Metabolism of very low density lipoproteins in rats with isotretinoin (13-cis retinoic acid)-induced hyperlipidemia

Acne Transcriptomics: Fundamentals of Acne Pathogenesis and Isotretinoin Treatment

This review on acne transcriptomics allows for deeper insights into the pathogenesis of acne and isotretinoin's mode of action. Puberty-induced insulin-like growth factor 1 (IGF-1), insulin and androgen signaling activate the kinase AKT and mechanistic target of rapamycin complex 1 (mTORC1). A Western diet (hyperglycemic carbohydrates and milk/dairy products) also co-stimulates AKT/mTORC1 signaling. The AKT-mediated phosphorylation of nuclear FoxO1 and FoxO3 results in their extrusion into the cytoplasm, a critical switch which enhances the transactivation of lipogenic and proinflammatory transcription factors, including androgen receptor (AR), sterol regulatory element-binding transcription factor 1 (SREBF1), peroxisome proliferator-activated receptor γ (PPARγ) and signal transducer and activator of transcription 3 (STAT3), but reduces the FoxO1-dependent expression of GATA binding protein 6 (GATA6), the key transcription factor for infundibular keratinocyte homeostasis. The AKT-mediated phosphorylation of the p53-binding protein MDM2 promotes the degradation of p53. In contrast, isotretinoin enhances the expression of p53, FoxO1 and FoxO3 in the sebaceous glands of acne patients. The overexpression of these proapoptotic transcription factors explains isotretinoin's desirable sebum-suppressive effect via the induction of sebocyte apoptosis and the depletion of BLIMP1(+) sebocyte progenitor cells; it also explains its adverse effects, including teratogenicity (neural crest cell apoptosis), a reduced ovarian reserve (granulosa cell apoptosis), the risk of depression (the apoptosis of hypothalamic neurons), VLDL hyperlipidemia, intracranial hypertension and dry skin.

13-cis-Retinoic Acid Affects Brain Perfusion and Function: In Vivo Study

Purpose. Study the effects of 13-cis-retinoic acid (13-RA), a synthetic analogue of a vitamin A used for the treatment of severe acne, on the blood flow in the rat brain using technetium-99m hexamethyl propylene amine oxime (99mTc-HMPAO) imaging. Methods. A total of 30 adult male Wistar rats were divided into the control (C), low-dose (L), and high-dose (H) groups. The L and H rats were exposed subcutaneously to 0.3 and 0.5 mg, respectively, of 13-RA per kg of body weight for seven days. Brain blood flow imaging was performed using a gamma camera. Then, a region of interest (ROI) around the brain (target, T), a whole-body region (WB), and a background region (BG) was selected and delimited. The net 99mTc-HMPAO brain counts were calculated as the net target counts, $\text{NTC}=\left(\text{T}-\text{BG}\right)/\left(\text{WB}-\text{BG}\right)$ in all groups. At the end of the 99mTc-HMPAO brain blood flow imaging, the brain, heart, kidney, lung, and liver were rapidly removed, and their uptake was determined. Brain histopathological analysis was performed using hematoxylin and eosin stains. In addition, the plasma fatty acids were studied using gas chromatography/mass spectrometry. Results. There were highly significant differences between L and H in comparison to C and across the groups. The 99mTc-HMPAO radioactivity in the brain showed increased uptake in a dose-dependent manner. There were also significant changes in the brain tissues and decreased free fatty acids among the groups compared to C. Conclusion. 13-RA increases 99mTcHMPAO brain perfusion, uptake, and function and reduces fatty acids.

Retinol-binding protein-4 in experimental and clinical metabolic disease

Retinol-binding protein-4 (RBP4), a 21-kDa protein synthesized in the liver and adipose tissue, has recently been described as a murine adipokine involved in the development of insulin resistance. The expression of the gene encoding RBP4 was increased in the adipose tissue, but not in the liver, of insulin-resistant adipose GLUT4(-/-) mice and five other mouse models of obesity and insulin resistance. In addition, intraperitoneal injection or transgenic overexpression of RBP4 in mice induced insulin resistance. While experimental clinical approaches (mostly applying clamp techniques) in humans confirmed correlations of RBP4 with insulin resistance, studies in larger groups out of clinical routine failed to demonstrate a connection with alternative measures of insulin sensitivity. Yet, significant associations of RBP4 with atherogenic lipids were found and a focus of future studies should be the influence on atherosclerosis and related complications. Based on current data, the function of RBP4 as an adipokine exerting metabolic effects in glucose metabolism in humans remains uncertain and might be restricted to rodent models.

Interactive effects of methyl tertiary-butyl ether (MTBE) and tertiary-amyl methyl ether (TAME), ethanol and some drugs: Triglyceridemia, liver toxicity and induction of CYP (2E1, 2B1) and phase II enzymes in female Wistar rats

The abilities of the gasoline additives methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) to cause liver damage following oral administration, dosed alone or in combination with model hepatotoxins, were investigated in the rat. Inducibility of liver drug-metabolizing enzyme activities was also studied. Exposure to these ethers (10-20mmol/kg) for 3 days resulted in hepatomegaly (13-30%) and induction of cytochrome P450 (CYP) activity towards N-nitrosodimethylamine (NDMAD), 7-pentoxyresorufin (PROD), and 7-ethoxyresorufin (EROD). Immunoinhibition assays with monoclonal antibodies showed that the ethers were equipotent as inducers of CYP2E1 activity (2-fold increase) but not of CYP2B1, which was elevated up to 260-fold in TAME-treated rats but only by 20-fold in MTBE rats. A slight or no modifying effect was observed on the NADPH:quinone oxidoreductase (NQO1), glutathione S-transferase (GST), and UDP-glucuronosyltransferase (UGT) activities. Alanine aminotransaminase (ALT) and aspartate aminotransaminase (AST) were elevated in blood plasma after administration of the ethers. No dramatic enhancement of liver damage could be detected by plasma enzyme analysis (ALT, AST, alkaline phosphatase, γ-glutamyltransferase) following ether administration (13.5mmol/kg) to rats pretreated with mildly hepatotoxic dosages of ethanol, pyrazole, phenobarbital, acetaminophen (paracetamol), or 13-cis-retinoic acid (13-cis-RA or isotretinoin). Plasma triglycerides increased in TAME-treated rats (1.7-fold) and in all 13-cis-RA-treated groups (2.1-2.8-fold). The findings that MTBE and TAME exhibited a clear but differential inducing effect on two ether-metabolizing CYP forms (2E1 and 2B1) with no marked effect on phase II activities may reflect the importance of these pathways in vivo. The observation that only TAME by itself induced hypertriglyceridemia while acetaminophen- and 13-cis-RA-induced hypertriglyceridemia were aggravated by both ethers, points to differences in their effects on lipid metabolism. TAME was clearly a more potent CNS depressant than MTBE. There was no marked potentiation of drug/chemical-induced acute liver damage either by MTBE or TAME.

Serum levels of albumin, triglycerides, total protein and glucose in rats are altered after oral treatment with low doses of 13-cis-retinoic acid or all-trans-retinoic acid

Currently used to treat severe acne, 13-cis-retinoic acid (13-cis-RA) is under investigation for its anticancer effects as is the isomer, all-trans-retinoic acid (all-trans-RA). Here, the effects of oral 13-cis-RA or all-trans-RA treatment on serum chemistry, leptin and adiponectin levels were evaluated. Adult Sprague-Dawley rats were gavaged once daily for 7 consecutive days with 13-cis-RA (7.5 or 15 mg kg(-1)), all-trans-RA (10 or 15 mg kg(-1)) (n=24/sex/dose), or soy oil (n=16/sex) and blood was sampled 30-480 min after the last gavage. The body weight was unaffected; however, the liver/body weight ratios were increased by both doses of all-trans-RA. Sex differences were noted for levels of cholesterol, creatine, triglycerides, albumin, alanine aminotransferase and total protein. Both doses of all-trans-RA reduced albumin levels to approximately 90% of the control and total protein levels to approximately 93% of the control while substantially elevating triglyceride levels to approximately 66%-99% above the control. Additionally, triglyceride levels of the 15 mg kg(-1) 13-cis RA group were approximately 62% higher than the controls and total protein levels were approximately 5% less. Glucose levels were affected by sex and RA treatment in that males treated with 15 mg kg(-1) of 13-cis-RA or 10 mg kg(-1) all-trans-RA had lower (13%-19%) levels than the same-sex controls; however, females were not similarly affected. Neither 13-cis-RA nor all-trans-RA treatment had significant effects on the levels of blood urea nitrogen, aspartate amino transferase, leptin or adiponectin. On a mg kg(-1) basis, all-trans-RA was more potent than 13-cis-RA. These results replicate previous findings of RA-induced increased triglyceride levels. Additionally, several new findings indicate there may be sex-specific effects of RA treatment. Finally, neither treatment appeared to alter the typical diurnal cycles of these endpoints.

The P-407–Induced Murine Model of Dose-Controlled Hyperlipidemia and Atherosclerosis

We are continuing to both elucidate underlying mechanisms and identify clinical applications for a chemically induced murine model of dose-controlled hyperlipidemia and atherosclerosis. This murine model neither utilizes genetically modified mice nor a high-fat, cholate-containing diet, although simultaneous ingestion of a high-fat, cholate-enriched diet potentiates the hyperlipidemic response and the number and size of aortic atherosclerotic lesions formed in C57BL/6 mice. The chemical agent used to induce hyperlipidemia is poloxamer 407 (P-407), a nonionic surface-active-agent. To date, we have investigated the effect of P-407 on the biologic activity of a variety of key enzymes involved with lipid metabolism and transport. These enzymes include 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, lipoprotein lipase (LPL), cholesterol 7alpha-hydoxylase (C7alphaH), sterol 27-hydroxylase (S27H), lecithin cholesterol acyltransferase (LCAT), cholesteryl-ester-transfer-protein (CETP), hepatic lipase (HL), and endothelial lipase (EL). P-407 directly inhibits the heparin-releasable fraction of LPL and HL and indirectly increases the biologic activity of CETP and LCAT. Long-term (> 4 months) administration of P-407 to C57BL/6 mice appears to have no effect on the biologic activity of S27H and HMG-CoA reductase, but decreases the activity of C7alphaH. This would suggest that hypertriglyceridemia and hypercholesterolemia result from inhibition of LPL and C7alphaH, respectively, while the biologic activity of CETP and LCAT are indirectly increased to compensate for the increased cholesterol burden. The above model has proven useful for predicting the therapeutic efficacy of existing and possibly newer statin drugs, as well as evaluating the potential of one statin drug (atorvastatin calcium) to cause the regression of P-407-induced atherosclerotic lesions in mice. The P-407-induced murine model of atherogenesis represents an alternative to the use of either genetically modified or diet-induced models and may also prove beneficial for the evaluation of newer classes of antihyperlipidemic agents such as antioxidants, CETP inhibitors, and liver X receptor (LXR) agonists.

A comparison of the effectiveness of soy protein isolate and fish oil for reducing the severity of retinoid-induced hypertriglyceridemia

The effectiveness of soy protein isolate (SPI) to reduce the severity of retinoid-induced hypertriglyceridemia has been demonstrated in the rat, but not in human subjects. Because fish oil has been demonstrated to be effective at lowering serum triglyceride concentration in human subjects undergoing retinoid therapy, a study was conducted to compare the ability of SPI with that of fish oil to reduce the severity of retinoid-induced hypertriglyceridemia in the rat. Male Fischer 344 rats, n=8/group, were fed one of four isonitrogenous, isoenergetic diets, consisting of a control diet containing 24% casein +20% corn oil (C), and three 13-cis retinoic acid (13cRA)-supplemented diets containing 24% casein +20% corn oil (R), 24% SPI +20% corn oil (SR), and 24% casein +15% fish oil and 5% corn oil (FR). There was no effect of diet on food intake or final body weight. Serum triglyceride concentration for group R was higher (P<0.001) than for groups C, SR, and FR (7.20 vs. 2.50, 2.84, and 2.02 mmol/L, respectively); values for groups SR and FR did not differ for this parameter. The serum concentration of 13cRA for group R did not differ from that for groups SR and FR. Thus, SPI was as effective as fish oil in reducing the severity of retinoid-induced hypertriglyceridemia in an animal model, suggesting that it may be effective for this purpose in human subjects.

Short-term administration of isotretinoin elevates plasma triglyceride concentrations without affecting insulin sensitivity in healthy humans

The mechanism underlying hypertriglyceridemia-associated insulin resistance in humans remains poorly understood. It has been proposed that hypertriglyceridemia only produces insulin resistance when associated with an increased lipid delivery to muscle. Accordingly, hypertriglyceridemia secondary to a decreased clearance of triglyceride-rich particles should not cause insulin resistance. To verify this hypothesis, we assessed whole body and adipose tissue insulin sensitivity in 15 healthy male volunteers before and after a 5-day administration of isotretinoin (1 mg/kg/d), a vitamin A derivative that decreases the clearance of triglyceride-rich particles. Whole body insulin-mediated glucose disposal (6,6 (2)H(2)glucose), glucose oxidation (indirect calorimetry), lipolysis ((2)H(5) glycerol), and subcutaneous adipose lipolysis (microdialysis) were evaluated during a 3-step hyperinsulinemic euglycemic clamp. Isotretinoin increased plasma triglyceride from 0.97 +/- 0.15 to 1.30 +/- 0.22 mmol/L (P <.02), but did not change whole body insulin-mediated glucose disposal and lipolysis. These observations are consistent with an isotretinoin-induced inhibition of very-low-density lipoprotein (VLDL)-triglyceride clearance. The suppression of endogenous glucose production and the reduction in subcutaneous adipose glycerol concentrations by insulin remained equally unaffected after isotretinoin administration. We conclude that the impaired clearance of triglyceride-rich particles secondary to a 5-day isotretinoin administration does not impair insulin-mediated antilipolysis or glucose disposal. The data support the concept that hypertriglyceridemia-associated insulin resistance develops primarily when triglyceride production is increased.

Retinoid X receptor agonist elevation of serum triglycerides in rats by potentiation of retinoic acid receptor agonist induction or by action as single agents

Hypertriglyceridemia is a major side-effect of retinoid therapy in humans. We previously reported that agonists for the retinoic acid receptors (RARs), but not the retinoid X receptors (RXRs), elevate serum triglycerides in male Fischer rats, and that, surprisingly, the RAR/RXR pan-agonists 9-cis-retinoic acid and AGN 191659 [(E)-5-[2-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphthyl)propen-1-yl]-2-thiophenecarboxylic acid] induce 2- to 3-fold higher levels of serum triglycerides than the RAR-selective agonists alone. We have now demonstrated that hypertriglyceridemia induced by an RAR agonist, AGN 190121 [4-[4-(2',6',6'-trimethylcyclohex-1-enyl)-but-1-yne-3-enyl]benzoic acid], is substantially potentiated by the RXR-selective agonists AGN 191701 [(E) 2-[2-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphthyl)propen-1-yl]-4-thiophene-carboxylic acid] and AGN 192849 [(3,5,5,8,8,-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl) (5 carboxypyrid-2-yl)sulfide] in a dose-dependent manner. RXR-specific retinoids, as previously reported, had no independent effect on serum triglycerides when tested at 24 hr after final dosing, but did elicit a reversible hypertriglyceridemia at 2.5 and 5 hr. This induction of serum triglycerides could not be blocked by the potent RAR-specific antagonist AGN 193109 [4-[(5,6-dihydro-5,5-dimethyl-8-(4-methylphenyl)-2-naphthalenyl)-ethynyl] benzoic acid]. The RXR ligand-induced hypertriglyceridemia was independent of the effect of feeding or fasting. The relative potencies of RXR-specific retinoids for acute triglyceride elevation (AGN 194204 [3,7-dimethyl-6S,7S-methano-7-[1,1,4,4-tetramethyl-1,2,3,4-tetrahydronaphth-7-yl] 2(E),4(E) heptadienoic acid] > AGN 192849 approximately AGN 191701) approximately correlated with potencies in the activation of the RXR receptors. The RAR/RXR pan-agonist effect included >50% inhibition of total heparin-releasable lipase activity in serum, consistent with inhibition of lipase-mediated triglyceride disposal. These data also indicate that RAR and RXR ligands can act synergistically to induce hypertriglyceridemia through distinct mechanisms of action.

Microwave assisted synthesis of N-arylphthalamic acids with hyperlipidemic activity

A series of substituted N-arylphthalamic acids 3a-i has been synthesized by the reaction of phthalic anhydride 1 and aryl- or heterocyclic amines 2a-i, in the absence of solvents, in a domestic microwave oven. The formation of nine N-arylphthalamic acids was accomplished in 1-3 min giving excellent yields for compounds 3a-g, but moderate yield of compounds 3h and 3i, respectively. Compounds 3h and 3i are new. Interestingly, N-arylphthalamic acids 3a-i induced hyperlipidemia in Swiss white mice and also increased animals' body weight.

Use of arginine to reduce the severity of retinoid-induced hypertriglyceridemia

Previous research with an animal model of retinoid-induced hypertriglyceridemia, rats fed a 13-cis retinoic acid-containing diet in which casein was the dietary protein, has demonstrated that replacement of dietary casein with soy protein isolate can reduce the severity of this condition. A depressant action of soy protein vs. that of casein on serum triglyceride concentration has also been demonstrated in rats fed purified diets without supplemental retinoid. Because this action of soy protein appears to be due to its having a higher arginine-to-lysine ratio than casein, a study was done to determine how feeding a casein-containing diet supplemented with sufficient arginine, to give a dietary arginine-to-lysine ratio equivalent to that of soy protein, would affect the development of retinoid-induced hypertriglyceridemia. Groups of five-week-old male Fischer 344 rats (n = 7/group) were fed a control diet containing casein or one of three 13-cis retinoic acid-containing diets in which dietary nitrogen was provided as casein, casein + arginine, or soy protein. Incorporation of dietary 13-cis retinoic acid resulted in hypertriglyceridemia, with serum triglyceride concentrations of 2.00 and 7.23 mmol/l, or 177 and 640 mg/dl, for groups fed the control and casein + 13-cis retinoic acid diets, respectively. For animals fed the 13-cis retinoic acid-containing diets, serum triglyceride levels for the casein + arginine and soy protein diet groups (4.75 and 2.92 mmol/l, or 421 and 258 mg/dl, respectively) were significantly lower than for the casein group (p < 0.05); however, the value was significantly lower for the group fed the soy protein diet than for the group fed the casein + arginine diet. Serum and dietary arginine-to-lysine ratios were highly correlated (r = 0.93, p < 0.0001). Thus, supplementing dietary casein with arginine reduced the severity of retinoid-induced hypertriglyceridemia, but not as effectively as replacing casein with soy protein.

The use of soy protein isolate to reduce the severity of 13-cis retinoic acid-induced hypertriglyceridemia

The ability of soy protein isolate to reduce the severity of 13-cis retinoic acid (13cRA)-induced hypertriglyceridemia was assessed in an animal model of this condition (rats fed a 13cRA-containing, casein-based diet). Male Fischer 344 rats (N = 7/group) were fed purified diets containing either casein or soy protein isolate and having either no 13cRA or retinoid at a level of 100 mg/kg diet-designated as Groups C, S, C + R, and S + R, respectively. Diet had no effect on either food intake or final body weight, and the serum levels of 13cRA were not significantly different for Groups C + R and S + R. Groups fed diets having 13cRA had significantly (p < 0.05) higher serum triglyceride levels than ones fed the corresponding retinoid-free diets, but the serum triglyceride level for Group S + R was significantly lower than for Group C + R, being 2.99 vs. 6.97 mM (265 vs. 617 mg/dl). Thus, soy protein isolate can reduce the severity of 13cRA-induced hypertriglyceridemia without affecting serum retinoid level.

Poloxamer 407-induced atherogenesis in the C57BL/6 mouse

Poloxamer 407 (P-407) induces hyperlipidemia in the rat. It was the purpose of this investigation to determine if chronic P-407 administration would produce atherogenic arterial lesions in the C57BL/6 mouse, a strain reported to be susceptible to hyperlipidemia-induced atherosclerotic plaque formation. One injection (i.p.) of P-407 (0.5g/kg) produced hypercholesterolemia in the mouse that peaked at 24 h and returned to control levels by 96 h following treatment. Four groups of mice were maintained: (1) saline injected (C); (2) P-407-injected (0.5g/kg every 3rd day) (P); (3) P-407 injected plus cholic acid in the diet (PC); and (4) mice fed a high cholesterol (CHOL) diet containing cholic acid (HF). Mice from each group were sacrificed following 90, 145, 200, or 300 days of treatment. Plasma lipid concentrations, hepatic CHOL concentrations (145 and 300 day), and aortic atherogenic lesion areas were measured. Plasma CHOL and triglyceride remained at control levels throughout the 300 days in the C group. CHOL of the HF animals plateaued at approximately 225 mg/dl. P-407 produced CHOL concentrations of 600 mg/dl in P mice and 1000-1500 mg/dl in PC animals. There was no lesion formation in C mice. However, by 90 days lesions were present in the three other groups. Size of the lesions progressed through day 300 with the largest lesions (184.33 + 27.99 mu2 x 10(-3)) being present in the PC mice. HF and P animals had lesions of 70.50 + 11.35 and 43.33 + 7.88 mu2 x 10(-3), respectively. This study provides an animal model where atherogenesis has been produced with hyperlipidemia induced using a chemical agent.

The poloxamer 407-induced hyperlipidemic atherogenic animal model

We are attempting to develop a chemically-induced murine model for the study of atherosclerosis. Injection of poloxamer-407 (P-407) into rats and mice causes significant dose-dependent hypercholesterolemia and hypertriglyglyceridemia. The elevated triglycerides (TG) seem to result primarily from the compound's inhibition of lipoprotein lipase. P-407 also indirectly stimulates the activity of the rate limiting enzyme in cholesterol (CHOL) biosynthesis, HMG CoA reductase. In addition, P-407 promotes changes in the concentration of hepatic CHOL content. These date indicate that the hyper CHOL could be the result of increased CHOL synthesis, as well as a clearing of CHOL from the liver. Chronic injection into mice of P-407 for 145 d produced atherogenic lesions in the aortas of C57BL/6 mice. The response was equivalent to that seen in animals eating a high CHOL diet for 145 d. Cholic acid potentiated the P-407-induced atherogenesis. These data suggest that P-407 could be used as an agent for the study of hyperlipidemia-induced atherogenesis.

Retinoids stimulate ApoA-I synthesis by induction of gene transcription in primary hepatocyte cultures from cynomolgus monkey (Macaca fascicularis)

The influence of different retinoids on apolipoprotein A-I (apoA-I) synthesis and secretion was investigated in primary monolayer cultures of hepatocytes from cynomolgus monkeys. Addition of retinol (vitamin A) and retinoic acid to the culture medium resulted in a time- and dose-dependent increase in the secretion of apoA-I. No effect was observed during the first 24-hour incubation period; however, apoA-I secretion was enhanced 1.5-fold in the following 24-hour period in the presence of 10 mumol/L retinoic acid. Maximal stimulation (2.7-fold) was obtained at 10 mumol/L retinoic acid during a third 24-hour incubation. In these experiments apoB-100 secretion was unaffected. When [35S]methionine incorporation studies were performed de novo synthesis of apoA-I was increased, whereas total protein synthesis remained constant. These observations indicated that the induction of apoA-I synthesis is not part of a general effect of retinoic acid on hepatic protein synthesis. Among different natural and synthetic retinoids, retinoic acid and its 9-cis and 13-cis isomers were equally active and were the most potent inducers of apoA-I synthesis, whereas the maximal stimulation induced by retinol was lower (1.6-fold). ApoA-I mRNA abundance was increased threefold in hepatocytes exposed for 72 hours to 10 mumol/L retinoic acid, which was associated with a twofold increase in the transcriptional rate of the apoA-I gene. In contrast, no changes were found in the apoB-100 mRNA level and transcriptional activity of the apoB-100 gene. We conclude that retinoids enhance apoA-I synthesis in simian hepatocytes by transcriptional regulation.

13-cis-retinoic acid and hepatic steatosis in rats

The effect of administration of 13-cis-retinoic acid (100 mg/kg diet) on lipid metabolism was examined in male rats fed either a 20% casein + 0.3% methionine diet, a 20% casein diet, a 10% casein + 0.3% methionine diet, or a 10% casein + 0.6% methionine diet for 10 days. Hepatic triglyceride concentrations of rats fed either 10% casein diet were 3-fold greater than animals receiving diets containing 20% casein. The addition of 13-cis-retinoic acid to the diet further increased the total hepatic lipid (43-56%) and triglyceride (approximately 2-fold) concentrations in rats fed the 10% casein diets. 13-cis-Retinoic acid supplementation did not alter the total liver lipid or triglyceride concentrations in rats fed either of the 20% casein diets. Thus, under specific dietary conditions, the administration of 13-cis-retinoic acid resulted in a marked accumulation of hepatic lipids which did not appear to be related to the total methionine content of the diet nor to the hepatic concentrations of S-adenosylmethionine and glutathione. In addition, all four groups of 13-cis-retinoic acid-fed rats exhibited elevations in the concentration of serum triglycerides, and 10-20% reductions in serum cholesterol concentrations.

Long-term safety of retinoid therapy

The concern about long-term toxicity of oral synthetic retinoids has developed because many patients, especially those with genodermatoses, require lifelong therapy. Several organ systems are at risk, especially the hepatic, skeletal, and cardiovascular systems. Although acute hepatotoxicity is a rare side effect of etretinate and acitretin therapy, prospective studies have not demonstrated chronic liver toxicity. The frequency of bone changes induced by retinoids is difficult to estimate, because this adverse effect is usually asymptomatic and requires x-ray or scintigraphic examination for detection. Atherosclerosis develops in many patients who receive long-term retinoid therapy, but the extent to which the process is aggravated by drug-induced hyperlipidemia is not known. Many patients have now been treated with either etretinate or isotretinoin continuously for as many as 15 years and have not developed any signs of severe chronic toxicity. However, continued intense surveillance is recommended for patients expected to require lifelong therapy.