Differential inhibition of long-chain acyl-CoA hydrolases by hypolipidemic drugs in vitro

Effect of carbon chain length in acyl coenzyme A on the efficiency of enzymatic transformation of okadaic acid to 7-O-acyl okadaic acid

Okadaic acid (OA), a product of dinoflagellate Prorocentrum spp., is transformed into 7-O-acyl OA in various bivalve species. The structural transformation proceeds enzymatically in vitro in the presence of the microsomal fraction from the digestive gland of bivalves. We have been using LC-MS/MS to identify OA-transforming enzymes by detecting 7-O-acyl OA, also known as dinophysistoxin 3 (DTX3). However, an alternative assay for DTX3 is required because the OA-transforming enzyme is a membrane protein, and surfactants for solubilizing membrane proteins decrease the sensitivity of LC-MS/MS. The present study examined saturated fatty acyl CoAs with a carbon chain length of 10 (decanoyl), 12 (dodecanoyl), 14 (tetradecanoyl), 16 (hexadecanoyl) and 18 (octadecanoyl) as the substrate for the in vitro acylation reaction. Saturated fatty acyl CoAs with a carbon chain length of 14, 16 and 18 exhibited higher yields than those with a carbon chain length of 10 or 12. Acyl CoAs with carbon chain lengths from 14 to 18 and containing either a diene unit, an alkyne unit, or an azide unit in the carbon chain were synthesized and shown to provide the corresponding DTX3 with a yield comparable to that of hexadecanoyl CoA. The three functional units can be conjugated with fluorescent reagents and are applicable to the development of a novel assay for DTX3.

AMP-activated protein kinase and nitric oxide regulate the glucose sensitivity of ventromedial hypothalamic glucose-inhibited neurons

The mechanisms by which glucose regulates the activity of glucose-inhibited (GI) neurons in the ventromedial hypothalamus (VMH) are largely unknown. We have previously shown that AMP-activated protein kinase (AMPK) increases nitric oxide (NO) production in VMH GI neurons. We hypothesized that AMPK-mediated NO signaling is required for depolarization of VMH GI neurons in response to decreased glucose. In support of our hypothesis, inhibition of neuronal nitric oxide synthase (nNOS) or the NO receptor soluble guanylyl cyclase (sGC) blocked depolarization of GI neurons to decreased glucose from 2.5 to 0.7 mM or to AMPK activation. Conversely, activation of sGC or the cell-permeable analog of cGMP, 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), enhanced the response of GI neurons to decreased glucose, suggesting that stimulation of NO-sGC-cGMP signaling by AMPK is required for glucose sensing in GI neurons. Interestingly, the AMPK inhibitor compound C completely blocked the effect of sGC activation or 8-Br-cGMP, and 8-Br-cGMP increased VMH AMPKalpha2 phosphorylation. These data suggest that NO, in turn, amplifies AMPK activation in GI neurons. Finally, inhibition of the cystic fibrosis transmembrane regulator (CFTR) Cl(-) conductance blocked depolarization of GI neurons to decreased glucose or AMPK activation, whereas decreased glucose, AMPK activation, and 8-Br-cGMP increased VMH CFTR phosphorylation. We conclude that decreased glucose triggers the following sequence of events leading to depolarization in VMH GI neurons: AMPK activation, nNOS phosphorylation, NO production, and stimulation of sGC-cGMP signaling, which amplifies AMPK activation and leads to closure of the CFTR.

Characterization of glucosensing neuron subpopulations in the arcuate nucleus: integration in neuropeptide Y and pro-opio melanocortin networks?

Four types of responses to glucose changes have been described in the arcuate nucleus (ARC): excitation or inhibition by low glucose concentrations <5 mmol/l (glucose-excited and -inhibited neurons) and by high glucose concentrations >5 mmol/l (high glucose-excited and -inhibited neurons). However, the ability of the same ARC neuron to detect low and high glucose concentrations has never been investigated. Moreover, the mechanism involved in mediating glucose sensitivity in glucose-inhibited neurons and the neurotransmitter identity (neuropeptide Y [NPY] or pro-opio melanocortin [POMC]) of glucosensing neurons has remained controversial. Using patch-clamp recordings on acute mouse brain slices, successive extracellular glucose changes greater than and less than 5 mmol/l show that glucose-excited, high glucose-excited, glucose-inhibited, and high glucose-inhibited neurons are different glucosensing cell subpopulations. Glucose-inhibited neurons directly detect decreased glucose via closure of a chloride channel. Using transgenic NPY-green fluorescent protein (GFP) and POMC-GFP mice, we show that 40% of NPY neurons are glucose-inhibited neurons. In contrast, <5% of POMC neurons responded to changes in extracellular glucose >5 mmol/l. In vivo results confirm the lack of glucose sensitivity of POMC neurons. Taken together, hypo- and hyperglycemia are detected by distinct populations of glucosensing neurons, and POMC and NPY neurons are not solely responsible for ARC glucosensing.

Effect of clofibrate on the chiral inversion of ibuprofen in healthy volunteers

OBJECTIVES

To determine the influence of the hypolipidemic drug clofibrate on the stereoselective metabolism of ibuprofen in humans.

METHODS

Healthy male subjects (n = 12) ingested a dose of 400 mg pseudoracemic ibuprofen (200 mg R-ibuprofen, 160 mg S-ibuprofen, and 40 mg 13C-S-ibuprofen) on two occasions after either pretreatment with clofibrate (2 gm/day over 1 week) or no pretreatment in a randomized order.

RESULTS

When subjects were pretreated with clofibrate, clearances of R-ibuprofen and 13C-S-ibuprofen increased significantly from 55.0 and 66.4 ml/min to 186.2 and 106.7 ml/min (p < 0.01), respectively. This increase was similarly reflected in the clearance by inversion of R-ibuprofen (control, 36.0 ml/min; treated, 118.8 ml/min; p < 0.01), as well as in the clearance by noninversion (control, 19.0 ml/min; treated, 67.4 ml/min; p < 0.01). Unbound clearance values significantly increased for R-ibuprofen (control, 19.5 L/min; treated, 38.7 L/min) but not for 13C-S-ibuprofen (11.8 versus 10.6 L/min, respectively). The fractional inversion of ibuprofen calculated from the urinary metabolite data was increased after clofibrate pretreatment (clofibrate group, 66.4%; control, 53.5%; p < 0.01). However, this was not evident when fractional inversion was calculated from the plasma concentration-time data for the unmetabolized drug.

CONCLUSIONS

Clofibrate altered the stereoselective disposition of ibuprofen in healthy volunteers by increased formation of R-ibuprofenoyl-coenzyme A rather than by an effect on oxidative metabolism of ibuprofen. This interaction has potential therapeutic implications.

Distinct responses of mouse hepatic CYP enzymes to corn oil and peroxisome proliferators

We studied the response of male DBA/2N mouse liver monooxygenases to acute (one-day) and subacute (7-day) exposure to clofibrate, gemfibrozil, and corn oil. The day following a single treatment with clofibrate (200 mg/kg), coumarin 7-hydroxylase (COH) activity decreased significantly (by 70%) with a concomitant decrease in the CYP2A4/5 protein and mRNA levels. The 7-day treatment schedule also decreased COH activity by only by 30%, though the levels of CYP2A4/5 protein and mRNA were still low. Treatment 1 and 7-day with clofibrate decreased 7-pentoxyresorufin O-dealkylase (PROD) activity by 40%. No changes were seen in testosterone 15 alpha-hydroxylase (T15 alpha OH) activity after 1 day of treatment with clofibrate but, after 7 days, it was decreased by 50%. Clofibrate treatment had no significant effects on testosterone 7 alpha-hydroxylase (T7 alpha OH), 7-ethoxyresorufin O-deethylase (EROD), or benzphetamine N-demethylase (BZDM) activities. Gemfibrozil (200 mg/kg) did not alter COH activity or CYP2A4/5 protein content after a single treatment, but a slight decrease was seen in the mRNA level. Treatment for 7 days significantly increased (2.5-fold) the activity and mRNA content but the amount of protein remained unchanged. Gemfibrozil enhanced (2-2.7-fold PROD and EROD (2-2.5-fold) activities by both treatments, whereas T15 alpha OH, T7 alpha OH, or BZDM activities were not significantly affected. Treatment with corn oil for 7 days significantly decreased (65%) COH activity and CYP2A4/5 protein and mRNA levels. PROD (55%) and T15 alpha OH (65%) activities were significantly decreased even after a single dose although injection for 7 days had no effect. Neither of the corn oil schedules had any marked effect on T7 alpha OH, EROD, or BZDM activities. These results demonstrate: 1. a decrease in the expression of CYP2A4/5 gene by clofibrate and corn oil; 2. substantial differences within the CYP2A subfamily in their responses to corn oil, clofibrate, and gemfibrozil; and 3. distinct responses of other xenobiotic metabolizing CYP subfamily enzymes to clofibrate and gemfibrozil.

Different effects of fibrates on the microsomal fatty acid chain elongation and the acyl composition of phospholipids in guinea-pigs

1. The effects in vitro and in vivo of three fibric acid derivatives, clofibrate (CFB), bezafibrate (BFB) and gemfibrozil (GFB) on some enzyme activities related to fatty acid biosynthesis, namely palmitoyl-CoA synthetase and hydrolases (microsomal and cytosolic), NADH and NADPH cytochrome c reductases and acyl-CoA elongases were investigated in guinea-pigs. 2. The three fibrates inhibited acyl-CoA elongation in vitro, irrespective of the substrate of elongation used (saturated, monounsaturated, polyunsaturated) and with an order of potency GFB > BFB > CFB. In the case of GFB, inhibition occurred at concentrations that can be reached in vivo. 3. Microsomal palmitoyl-CoA hydrolase and synthetase were also inhibited in vitro (GFB > or = BFB > CFB), whereas NADH cytochrome c reductase activity was increased by GFB. Nevertheless, the magnitude of changes were lower than those observed in elongation activities. 4. Treatment with fibrates did not produce peroxisomal proliferation in guinea-pigs, as measured by peroxisomal beta-oxidation activity and liver weight/body weight ratio. Nevertheless, fibrates provoked a reduction in plasma cholesterol and triglycerides, at least in GFB- and BFB-treated animals. 5. Fatty acid elongation was significantly modified by GFB treatment in vivo. The remaining enzyme activities studied were only slightly changed by fibrate treatment. 6. Treatment with BFB and to a lesser extent with CFB, increased the relative proportion of MUFA (palmitoleic and oleic acids) in microsomal phospholipids, whereas PUFA (mainly linoleic acid) decreased. GFB behaved differently, increasing palmitic and linoleic acids and decreasing stearic and oleic acids. The latter changes are attributable to an inhibition of elongation activity by GFB. 7. The changes observed after fibrate treatment in both rats and guinea-pigs, as they are not directly related to peroxisome proliferation, could be more reliably extrapolated to man than those observed only in rats.

Differential effects of fibrates on the acyl composition of microsomal phospholipids in rats

1. The time-course and comparative effects of treatment with clofibrate (CFB), bezafibrate (BFB), and gemfibrozil (GFB) on the acyl composition of the main microsomal phospholipids, i.e. phosphatidylcholine and phosphatidylethanolamine, have been studied in male Sprague-Dawley rats. 2. The administration of the three fibrates caused a strong peroxisomal induction and a hypolipidaemic effect. Concerning the changes in acyl composition, CFB and BFB behaved in a similar way, with differences which could be attributed to their different potency as peroxisome inducers, whereas GFB showed a somewhat distinct profile. 3. The three drugs increased the relative content of palmitic, palmitoleic and oleic acids, whereas the levels of stearic acid and also those of long chain, highly unsaturated fatty acids docosatetraenoic, docosapentaenoic and docosahexaenoic acids were reduced. In general, these effects appeared from the first day of treatment and were highly correlated with peroxisomal proliferation. In addition, they were more evident in the phosphatidylcholine than in the phosphatidylethanolamine fraction. 4. Fibrates increased total monounsaturated fatty acids, whereas a decrease in total polyunsaturated fatty acids in the phosphatidylcholine fraction was observed in CFB- and BFB-, but not in GFB-treated rats. Clear differences appeared between CFB and BFB on the one hand, and GFB on the other when the influence of fibrate treatment on the molar percentages of linoleic, eicosatrienoic, arachidonic and mead acids was analyzed. 5. GFB increased linoleic acid content in phosphatidylethanolamine, whereas CFB and BFB decreased its level in both phospholipid fractions. In contrast, CFB and BFB enhanced eicosatrienoic and mead acids in both fractions and arachidonic acid in phosphatidylethanolamine, whereas GFB had practically no effect.

Selective modification of rat hepatic microsomal fatty acid chain elongation and desaturation by fibrates: relationship with peroxisome proliferation

1. The time-course of the effect of clofibrate (CFB), bezafibrate (BFB) and gemfibrozil (GFB) on lipid plasma levels and palmitoyl-, palmitoleoyl- and gamma-linolenoyl-CoA elongase, delta-9, delta-6 and delta-5 desaturase activities, and microsomal electron transport chains, as well as the correlation with the peroxisomal proliferation phenomenon have been studied in male Sprague-Dawley rats. 2. As reported in our previous work, the three drugs behave as peroxisomal proliferators (the order of potency was BFB > CFB > or = GFB) and induced a clear reduction in both plasma cholesterol and triglyceride levels. 3. Palmitoyl-CoA elongation activity was increased by the three drugs (BFB = GFB > CFB), whereas palmitoleoyl-CoA elongation activity was only enhanced by GFB. Elongation activity was not modified by fibrates when gamma-linolenoyl-CoA was used as substrate. These results are in accordance with the existence of three different elongation systems for saturated, mono- and polyunsaturated fatty acids. 4. delta-9, delta-6 and delta-5 desaturase activities were increased by the three fibrates, with an order of potency BFB > CFB = GFB for delta-9 and delta-5, and GFB > BFB = CFB for delta-6. 5. Of the enzyme activities integrated in the microsomal electron transport chains, NADH cytochrome b5 reductase was not affected by fibrate treatment, NADPH cytochrome c reductase activity was enhanced (BFB = GFB > CFB), whereas NADH cytochrome c reductase activity was reduced by CFB and BFB. 6. The increase in Delta-9 and Delta-5 desaturase activities was highly dependent on the peroxisomal proliferation phenomena, whereas the increase in Delta-6 desaturase activity and the decrease in NADH cytochromec reductase was mainly independent. The modifications of palmitoyl-CoA elongase and NADPH cytochrome c reductase activities, as well as plasma lipid levels, were partially correlated with peroxisomal beta-oxidation, but the r2 values obtained point to the existence of additional independent mechanisms.7. As man is assumed to be a species refractory to peroxisomal proliferation, only those fibrate effectsnot absolutely related to this phenomenon are expected to appear after fibrate therapy.

Relationship between plasma lipids and palmitoyl-CoA hydrolase and synthetase activities with peroxisomal proliferation in rats treated with fibrates

1. The time-course of the effect of clofibrate (CFB), bezafibrate (BFB) and gemfibrozil (GFB) on lipid plasma levels and palmitoyl-CoA hydrolase and synthetase activities, as well as the correlations with the peroxisomal proliferation phenomenon have been studied in male Sprague-Dawley rats. 2. The administration of the three drugs caused a significant reduction in body weight gain, accompanied with a paradoxical increase in food intake in groups treated with BFB and GFB. 3. Drug treatment produced gross hepatomegaly and increase in peroxisomal beta-oxidation, and these parameters were strongly correlated. The order of potency was BFB > CFB > or = GFB. 4. Both plasma cholesterol (BFB approximately CFB > GFB) and triglyceride (BFB approximately GFB > CFB) levels were reduced in treated animals. There was an inverse correlation between these parameters and peroxisomal beta-oxidation, although the peroxisomal proliferation seemed to explain only a small part of the hypolipidemic effect observed. 5. Cytosolic and microsomal (but not mitochondrial) palmitoyl-CoA hydrolase activities were increased by the three drugs (BFB > CFB > GFB), probably by inducing the hydrolase I isoform, which is insensitive to inhibition by fibrates in vitro. The increased hydrolase activities were directly and strongly correlated with peroxisomal beta-oxidation. 6. Palmitoyl-CoA synthetase activity was also increased by the treatment with fibrates (BFB > CFB > GFB), probably as a consequence of the enhancement of hydrolase activities. 7. Some of the effects of fibrate treatment can be explained, at least in part, in terms of peroxisomal induction and caution should be exercised in the extrapolation of these results to species, such as man,that are insensitive to peroxisomal proliferation.

Fibrates modify rat hepatic fatty acid chain elongation and desaturation in vitro

Three fibric acid derivatives, clofibric acid (CFB), bezafibrate (BFB), and gemfibrozil (GFB), mainly used in the treatment of hypertriglyceridaemic or mixed hyperlipidaemic states, have been tested for their ability to modify fatty acid chain elongation and desaturation in vitro. Both endogenous and exogenous (saturated, monounsaturated and polyunsaturated) fatty acid elongations were inhibited by fibrates at concentrations well within the physiological range (IC50 values for GFB were between 0.1 and 0.3 mM). The potency order was GFB > BFB > CFB. Inhibition was not due to an impairment of the activation step from free fatty acids to acyl-CoAs, as palmitoyl-CoA synthetase was only slightly inhibited (IC50 value for GFB = 2.8 mM). Fibrates (GFB) appeared to behave as mixed non-competitive inhibitors with respect to malonyl-CoA when the rate limiting step of elongation, the condensing enzyme, is assayed. Further, delta 6 and delta 5 desaturates were inhibited by the three drugs (GFB > BFB > CFB), although not to the same extent as the elongation system. In contrast, delta 9 desaturase activity was not affected by fibrates.

Gemfibrozil modifies acyl composition of liver microsomal phospholipids from guinea-pigs without promoting peroxisomal proliferation

Treatment with gemfibrozil modifies acyl composition of hepatic microsomal phosphatidylcholine and phosphatidylethanolamine in guinea-pigs. Palmitic (16:0) and palmitoleic (16:1) fatty acids are increased, and stearic (18:0) and oleic (18:1) are decreased; further, while linoleic acid [18:2 (n-6)] is increased by gemfibrozil treatment, the other constituents of the n-6 fatty acids family, including arachidonic acid [20:4 (n-6)], are decreased. As gemfibrozil is a potent inhibitor of fatty acid elongation in vitro (Sánchez et al., FEBS Lett 300: 89-92, 1992), the inhibition of this enzyme system by gemfibrozil treatment could be responsible for the observed results in vivo. These changes in fatty acid composition are accompanied by a decrease in serum lipids and, more important, are independent of peroxisomal proliferation.

Cytosolic lipogenic enzymes: effect of fibric acid derivatives in vitro

The effect of fibric acid derivatives, clofibric acid (CFB), bezafibrate (BFB), and gemfibrozil (GFB) on hepatic cytosolic enzymatic activities involved in saturated fatty acid synthesis has been estudied in vitro. From all the activities tested (fatty acid synthetase, acetyl-CoA carboxylase, ATP-citrate lyase, malic enzyme, malic dehydrogenase, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase), only acetyl-CoA carboxylase and glucose-6-phosphate dehydrogenase were significantly inhibited by fibrates, with the following order of potency: GFB > BFB > > CFB. The characteristics of the inhibition phenomena (IC50, kinetic analysis, time and protein dependence, etc) and their transcendence to the effects of fibric acid derivatives in vivo are discussed.