BIOCHEMISTRY ond MOLECULAR BIOLOGY INTERNATIONAL

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1 Vol. 44, No. 3, March 1998 BIOCHEMISTRY ond MOLECULAR BIOLOGY INTERNATIONAL Pages SHORT-TERM EFFECT OF DEXAMETHASONE ON FATTY ACID AND CHOLESTEROL SYNTHESIS IN ISOLATED RAT t lepatocytes. Anna Maria Giudetti and Gabriele V. Gnoni* Laboratory of Biochemistry, Dept. of Biology, University of Lecce, Lecce (Italy) Received October 23, 1997 SUMMARY The short-term effect of dexamethasone, a synthetic glucocorticoid, on both fatty acid and cholesterol synthesis has been investigated in rat hepatocyte cultures. Within 4h following hormone addition to the cultures, a noticeable stimulation of labelled acetate incorporation into fatty acids was observed. Similar behaviour was noticed when [:H]H20 was used as an independent index of the lipogenic activity. In the same cultures, however, cholesterol synthesis from both [l*c]acetate or [~H]H20 was significantly reduced by dexamethasone addition. In these conditions, no significative variation of cholesterol synthesis starting from labelled tnevalonate was observed. KEY WORDS Early effect; dexamethasone; fatty acid synthesis; cholesterol synthesis; hepatocytes. INTRODUCTION Traditionally, the glucocorticoids have been viewed as catabolic hormones. I lowever a stimulation of the synthesis of anabolic pathway enzymes by glucocorticoids has also been shown (1). Increased concentrations of glucocorticoids arc associated with enhanced rates of l ipogenesis in the liver of maternal, fetal and newborn rats (2,3), in fetal rat hmg (4), in adult rat testis (5), as ~,~ell as with increased plasma lipids (6). The interactions of glucocorticoids with other hormones, such as triiodothymnme (7) and insulin (8) in stimulating fatty acid and glycerolipid synthesis have been well documented In contrast, widely divergent results on the effect of dexamethasonc on cholesterol synthesis and on the activity of ~3-hydroxy-13-methylglutaryl-Coenzyme A reductase, the key enzyme of this pathway, have been reported (9-12), It must be pointed out that in all the above-mentioned studies, the long-term effects of m v/vo dexamethasonc administration on fatty acid (2-8) or cholesterol synthesis (9-12) ha~e been investigated. Long-term treatment could point to a secondary' or unspecific response * To whom the correspondence should be addressed /98/ /0 Copyrigh t by A cademic Press A ustralia. All rights r reproduction in any forn~ reserved

2 BIOCHEMISTRYond MOLECULAR BIOLOGY INTERNATIONAL of these metabolic pathways to hormone administration. On the other hand, short-term control events may precede the onset of long-term induction. A short term regulation of lipogenesis by triiodothyronine, another long-term acting hormone, has been demonstrated (13). Therefore, we decided to investigate whether acute effects of dexamethasone on hepatic lipogenesis and cholesterologenesis occur. Moreover, since in vivo direct effects of the horrnone have been difficult to differentiate from indirect ones, we utilized isolated hepatocytes as experimental tool. This study shows for the first time that, m adult rat hepatocytes, an acute and direct effect of dexamethasone on lipid metabolism occur. In particular, while lipogenesis is noticeabl> stimulated, a reduction of cholesterol synthesis is observed. METffODS Male Wistar rats weighing g were used throughout in this study. Rat liver parenchimal cells were isolated by perfusing the liver with collagenase (0.05%) as previously described (13). 20 mm glucose was added to all buffers employed in the isolation procedure to minimize glycogenolysis. Cell viability, determined by' trypan blue exclusion, was greater than 90%. The standard culture medium was flares F-12 containing 10% fetal calf serum, 1% fatty acid free bovine serum albumin, 100 units/ml penicillin and 100 mg/ml streptomycin and buffered (ph 7.4) with 14.5 mm sodium bicarbonate and 12.5 mm each of 2(N-morpholino) ethanesulfonic acid (MES) and N-tris (hydroxymethyl) methyl-2-aminoethanesulfonic acid (TES). Incubation conditions were essentially as described (14). Briefly, 4 ml of cell suspensions (5 x 105 cells per ml, corresponding to 1.2 mg protein), were initially placed m 60 mm plastic Petri dishes tbr plating and kept in a humidified incubator at 37~ with a continuous flow of 95% air - 5% CO2. In 2 h, the majority of cells became firmly attached to the bottom of the dishes so that medium could be changed without substantial loss of hepatocytes. After this 2 h plating period, 4 ml fresh medium without fetal calf serum was added, where indicated, together with the hormone. Incubations lasted for a total of 4 h, during which no detachment of cells was observed. Fatty acid and cholesterol synthesis were monitored by measuring the rate of [l-hc]acetate (1.56 mci/lnrnole) or [3H]H20 (1 mci/ml) incorporation into these lipid fractions (13). Fresh medium, with or without the hormone, as well as labelled substrate were added I h before ending the incubations. At the end of the experiment, the radioactive medium was drawn off, monolayers were carefully washed three times with 0.9% NaC1 and the reactions were stopped with 1.5 lnl of 0.5 N KOH. Cells were scraped off with a rubber policeman. A small amount (50 ~d) was reserved for protein determination by the Lowry method (15). To the rest of the cell suspension, 2 ml of ethanol were added and the mixture was placed in a 90~ water bath for 2 h. Nonsaponifiable lipids were extracted with petroleum ether, and cholesterol, inthe ether extract was precipitated with digitonin according to (16). Digitonin-precipitable cholesterol and fatty acids were extracted and counted as in (17). Results are expressed as nmoles [l-14c]acetate or as nmoles [3H]H20 incorporated x h ~1 protein

3 BIOCHEMISTRYond MOLECULAR BIOLOGY INTERNATIONAL RESULTS AND DISCUSSION Primary cultures of hepatocytes were used in this study because they offer the opportunity of examining dexamethasone action free from secondary effects, tissue interactions and blood-borne factors. Over 80% of freshly isolated hepatocytes became firmly attached to the surface of the plastic dishes as a monolayer within a period of 2 h. Thereafter, fresh medium supplemented with dexamethasone was added where indicated, and the cultures were incubated for an additional 4 h. The capacity of the cells to incorporate labelled acetate into fatty acids and cholesterol was measured in the last hour of the experiment. Table 1 shows that fatty acid synthesis from labelled acetate is noticeably stimulated by dexamethasone addition to the hepatocyte cultures. This stimulation is already evident at 1.25 ~,tm hormone concentration, which compares favorably with m vivo levels of glucocorticoids (18). At 1.25 x 10 5 M hormone concentration, the incorporation of labelled acetate into fatty acids was increased by almost 50% (specific activity nmoles vs 1.4l 0.13 mnoles of the control). Our results are in agreement with those of Diamant and Shafrir (19), who indicated that glucocorticoid administration increases in rat liver cytoplasm the activities of de novo fatty acid synthesis enzymes, i.e. acetyl-coa carboxylase and fatty acid synthetase. However, they are in contrast with those obtained in cultures of adult hepatocytes where dexamethasone had no effect on basal lipogenesis (20). It must be underlined that in adult rat hepatocytes only long-term effects of glucocorticoids have been reported so far. In fact, in these experiments, the failure of dexamethasone to stimulate lipogenesis was observed after 20 h of incubation of cells with the hon-none (20). In the cell, acetate is precursor of both fatty acid and cholesterol synthesis. Table 1 also shows that, in contrast to lipogenesis, cholesterol synthesis is significantly inhibited by dexamethasone addition to the hepatocyte cultures. At 1.25 x 10-5 M honaaone concentration, a reduction of about 30% in tile incorporation of labelled acetate into cholesterol was observed. From these data we can not indicate which enzymatic step(s) of cholesterol synthesis is influenced by the hormone. To note, however, that we observed (data not reported) that dexamethasone addition to isolated hepatocytes showed almost no effect on cholesterol synthesis from mevalonate, which is the product of 13-hydroxy-13- methyl-glutaryl-coa reductase, the well known regulatory enzyme of cholesterol 517

4 BIOCHEMISTRYand MOLECULAR BIOLOGY INTERNATIONAL Table 1. Effect of dexamethasone on the rate of fatty acid and cholestctol synthesis fiom [1-~L~C]acetate in isolated rat hepatocytes. Dexamethasone (M) ( nmoles [1 -I~C]acetate incorporated x h-~ protein -l) Fatty acid synthesis Cholesterol synthesis (100) (100) (104) 1.52 _+0.12 (86) 1.25 ~ 10 -c' 1.92 ~ 0.15 (136) 1.33 _ (76) 1.25 x (149) 1.19+_0.09(68) Plated cells were in contact with the indicated dexamethasone concentration for a total period of 4 h. At 3 h, fresh medium containing the appropriate hormone concentration together with labelled acetate was added. Reactions were stopped after incubation for an additional hour and li D& extracted. Values are the mean + S.D. of 6 experiments. In each experiment the assays were carried out in triplicate. synthesis. This suggests that the hormone could influence cholesterol synthesis by inhibiting some enzymatic step(s) preceding mevalonate formation. In Figure 1 a plot of the rate of fatty acid (panel A) and cholesterol (panel 13) synthesis as a function of time in the absence and presence of dexamethasone is reported. Within 2 la following addition of the hormone, a noticeable stimulation of fatty acid synthesis is observed. During the same 211 time period, however, cholesterol synthesis is significantly reduced. Maximal effect of the hormone on both these pathways was observed at 4 h. The important point here is that the response to dexamethasone is rather rapid, indicating that short-term control mechanisms may make a contribution to the overall effects of dexamethasone on lipogenesis in rat liver cells. It has been proposed that the tritium incorporation method provides a more accurate measurement of fatty acid and cholesterol synthesis than acetate incorporation because the specific radioactivity of [3H]H=O is less likely to be influenced by fluctuations in endogenous substrate concentrations (21, 22). Therelbre, studies were initiated using [3H]H20 incorporation into lipids as an independent index of tipogenic activity. Data 518

5 Voi. 44, No. 3, 1998 BIOCHEMISTRY and MOLECULAR BIOLOGY INTERNATIONAL A 1,5.m ~ 0 l B 1,5 0, Hormone incubation (h) Fig. I. Changes in the rate of fatty acid and cholesterol synthesis in response to dexamethasone. After monolayer formation, medium was changed and hormone added. Labelled acetate was added at 0,1,2 or 3 h and one hour later the reaction was stopped, fatty acids and cholesterol were extracted and radioactivity counted. Panel A : fatty acid synthesis; Panel B : cholesterol synthesis 4, Control x 10.5 M Dexamethasone Specific activity = nmoles [ 1-14C]acetate incorporated h -~ proteiff I. Each point is the mean _+ S.D. (verticle bars) of 4 experiments. reported in Table 2 clearly indicate that fatty acid and cholesterol synthesis from [3tt]H20 arc inlquenced by dexamethasone addition to isolated hepatocytes in a manner similar to that measured by the incorporation of [1-14C]acetate. It has been previously reported that t04 M gtucocorticoid addition to isolated rat hcpatocytes produces a clear stimulation of some enzymatic activities of neutral lipid synthesis, e.g. glycerophosphate acyltransfcrase and phosphatidate phosphatase, accompanied by a strong increase in the capacity to 519

6 BIOCHEMISTRY(/nd MOLECULAR BIOLOGY INTERNATIONAL Table 2. Influence of dexamethasone on fatty acid and cholesterol synthesis from [3H]H20 in hepatocyte cultures. Dexamethasone (M) (nmoles [3H]H20 incorporated h -I mg protein -~) Fatty acid synthesis Cholesterol synthesis _ 0.29 (100) (100) 1.25 x 10 -v (111) (92) 1.25 x (126) (74) (147) (61) Experimental conditions were as in Table 1. Data are the mean + SD. of 6 experiments, with each determination carried out in triplicate. synthesize triacylglycerol from glycerol phosphate and palmitate (23, 8). In addition, administration of dexamethasone increased triacylglycerol content in both neonatal and adult rat liver (24). Our data add further support to these findings. The dexamethasoneinduced stimulation of fatty acid synthesis we observed could determine a greater substrate availability for the neutral lipid synthesis enzyme activities. This study represents the first report on the early and direct effect of dexamethasone on fatty acid and on cholesterol synthesis. Studies are in progress in our laboratory to determine the molecular mechanism of the hormone action and the enzymatic step(s) of these pathways influenced by dexamethasone addition to the hepatocyte cultures. ACKNOWLEDGEMENTS This research was supported by a grant (40%) from the Ministero dell'universit/t e della R.S.T. Dr. A. M. Giudetti is a recipient ofa "Progetto C.A.R.S.O." fellowship. REFERENCES 1. Berdanier, C. D. (1989) FASEB J. 3, Ben M., Lorenzo, M., and Medina, J. M. (1982) Biochem J, 204, Ben M., Lorenzo, M., and Cald6s, T. (1984) Biochem. J. 224, Xu, Z. X., and Rooney, S. A. (1989) Biochim. Biophys. Acta 1005,

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