Effect of Sex, Ovariectomy and Female Sex Hormones on Neutral Cholesterol Ester Hydrolase in Rat Liver

Regulation of hepatic neutral cholesteryl ester hydrolase by hormones and changes in cholesterol flux

To understand molecular events in regulation of hepatic neutral cholesteryl ester hydrolase (EC3.1.1.13; CEH), catalytic activity, protein mass, and mRNA levels were measured in rats with various perturbations of hepatic cholesterol metabolism. Cholesterol feeding decreased activity (56 +/- 2%), mass (44 +/- 2%), and mRNA (14 +/- 3%). The cholesterol precursor mevalonate also decreased activity (42 +/- 6%), mass (76 +/- 3%), and mRNA (23 +/- 16%). Inhibition of cholesterol biosynthesis by lovastatin increased activity (65 +/- 12%) and mRNA (31 +/- 24%). Stimulation of cholesterol efflux by chronic biliary diversion increased activity (138 +/- 34%), mass (29 +/- 7%), and mRNA (146 +/- 28%). Chenodeoxycholate feeding decreased activity (46 +/- 6%) and mRNA (26 +/- 12%). These data suggest rational regulation of CEH in response to changes in cholesterol flux through the liver. In primary hepatocytes, steady-state mRNA markedly decreased during 72-h cultures and addition of L-thyroxine and dexamethasone synergistically maintained mRNA levels near control values. Lovastatin increased mRNA levels by 103 +/- 15%. Taurocholate and phorbol 12-myristate 13-acetate suppressed mRNA (61 +/- 4% and 49 +/- 13%, respectively), suggesting that protein kinase C mediated effects of bile acids on CEH mRNA levels. These data suggest regulation of CEH by hormones and signal transduction in addition to changes in cholesterol flux.

Age-related changes in catalytic activity, enzyme mass, mRNA, and subcellular distribution of hepatic neutral cholesterol ester hydrolase in female rats

Activity and protein mass of hepatic neutral cholesteryl ester hydrolase (CEH) were measured in liver cytosol and washed microsomes of female Sprague-Dawley rats aged 3, 4, 7, 9, 13, and 16 wk. CEH mRNA was also measured. The microsomal component varied with age and contributed a greater fraction of total activity in females than previously reported in males. Nevertheless, the cytosolic component accounted for 62-80% of activity and 77-94% of immunoreactive protein in postmitochondrial fractions. Cytosolic and microsomal CEH specific activities, relative to total protein, decreased 94 and 83%, respectively, from 3 to 4 wk, prior to onset of puberty at 5 wk, and increased 360 and 137%, respectively, from 12 to 16 wk. These results contrast with an earlier study, in which cytosolic CEH activity of males increased with puberty and declined after 12 wk. Although cytosolic CEH was activated by protein kinase A and inhibited by alkaline phosphatase treatment at all ages, protein kinase activation peaked at 4 wk, coinciding with the initial decrease in specific activity. Specific activity in cytosol and microsomes correlated with CEH mass at all ages, suggesting that this CEH accounts for most variation in cellular activity. In contrast, CEH mRNA varied little from 3-16 wk, indicating that transcriptional regulation does not make a major contribution to the variation in CEH activity and mass in females, although it may make an important contribution to male-female differences in CEH expression. Specific activities of cytosolic and microsomal CEH, relative to immunoreactive CEH protein mass, exhibited changes consistent with posttranslational regulation. These results indicate gender-specific multivalent regulation of hepatic CEH by posttranslational mechanisms during development of female rats.

Age-related changes in mRNA, protein and catalytic activity of hepatic neutral cholesterol ester hydrolase in male rats: evidence for transcriptional regulation

Messenger RNA, protein mass and catalytic activity of hepatic neutral cholesteryl ester hydrolase (CEH) were measured in male Sprague-Dawley rats, aged 6, 8, 9.5, 12 and 24 weeks (wks). CEH mRNA increased 101% from 6 to 9.5 wks, corresponding to onset of puberty, and declined by 52% from 12 to 24 wks. CEH mass was highly correlated with mRNA levels at all ages, increasing 170% from 6 to 9.5 wks and declining 61% from 12 to 24 wks. CEH activity was highly correlated with mass and mRNA from 8-24 wks, but was greater at 6 wks than the activity predicted by the measured mass. In all age groups, activity was consistently increased by activation of endogenous protein kinase A and consistently inhibited by alkaline phosphatase, suggesting that age-related differences in catalytic activity were not due to differences in the level of enzyme phosphorylation. These data suggest transcriptional regulation and indicate an important role for CEH in cholesterol homeostasis in the developing rat.

Effects of perturbations in hepatic free and esterified cholesterol pools on bile acid synthesis, cholesterol 7 alpha-hydroxylase, HMG-CoA reductase, acyl-CoA:cholesterol acyltransferase and cytosolic cholesteryl ester hydrolase

Effects of expansion of the hepatic free cholesterol pool on bile acid and cholesterol metabolism and homeostasis were examined in rats fed cholesterol in high-fat diets or treated with oleyl-p-(n-decyl)-benzenesulfonate (ODS) or progesterone. Cholesterol feeding for 10-16 days, which increased free (33%) and esterified (6-fold) cholesterol, had no effect on cholate synthesis, total bile acid synthesis, or cholate turnover, whereas these activities were increased 60-80% by ODS and progesterone, which produced only small increases (19%) in free cholesterol. Cholesterol feeding reduced beta-hydroxy-beta-methylglutaryl (HMG)-CoA reductase (72%) and cholesteryl ester hydrolase (48%) and increased acyl-CoA:cholesterol acyltransferase (184%), whereas ODS and progesterone reversed these compensatory responses in cholesterol-fed rats. Cholesterol 7 alpha-hydroxylase was changed no more than 22% by any treatment. A bolus of ODS elevated biliary cholesterol output 41% and shifted biliary bile acid synthesis and composition toward 12-deoxy bile acids. These effects were not seen in ODS-fed or progesterone-treated rats, in which cholesteryl ester stores were depleted. It is concluded that effects of free cholesterol on bile acid synthesis and biliary cholesterol are probably mediated by specific precursor or regulatory pools which can be independently regulated and which represent a relatively small fraction of hepatic free cholesterol.

Kinetic properties and solubilization of microsomal cholesterol ester hydrolase from rat liver

Some kinetic properties of the microsomal cholesterol ester hydrolase (CEH) have been examined in rat liver. The reaction was linear with time up to 60 min and with enzyme concentration up to 0.3 mg/mL, and a pH optimum of 6.7 for enzyme activity was observed. Cholesterol esterase exhibited the following apparent kinetic constants: Km, 68.88 microM and Vmax, 33 Units/mg protein. Cholesteryl palmitate was hydrolyzed to a much greater extent than cholesteryl oleate by the enzyme. Product inhibition with cholesterol and palmitic acid was not apparent; however, oleic acid added to the system reduced markedly microsomal CEH activity. The present paper also reports the solubilization of cholesteryl palmitate hydrolase from the microsomal fraction by pretreating it with Triton X-100, sodium deoxycholate, and sodium dodecylsulfate. All ionic and non-ionic detergents tested are capable of making the enzyme soluble, and maximal effects were found at higher concentrations of detergents although the esterase activity was strongly inhibited. Triton X-100 was found to be more effective than sodium deoxycholate and sodium dodecylsulfate in enzyme and protein solubilization. When the direct effects of detergents on CEH activity were studied, progressive concentration-dependent inhibitions were observed.

Cholesteryl ester hydrolysis in rat liver lysosomes: different response to female sex hormones

The regulation of the hydrolysis of cholesteryl oleate by female sex hormones was studied in the lysosomal fraction of rat liver. Cholesterol ester hydrolase activity was determined at pH 5.0 with an acetone-dissolved cholesteryl [1-14C]oleate substrate preparation. The administration of a single dose of progesterone decreased the enzyme activity during a 3- to 24-hr period following hormone injection. This effect was not correlated to changes in the lysosomal protein synthesis rate. The lysosomal hydrolysis of cholesteryl esters was also inhibited in a noncompetitive manner by the addition of progesterone at concentrations higher than 100 microM. The esterase failed to respond to the estradiol in vivo as well as in vitro. The findings of the present paper suggest that the lysosomal breakdown of cholesteryl esters in rat liver may be under selective hormonal regulation and that the inhibitory effect of progesterone on the enzyme activity might be, at least in part, responsible for the liver cholesterol ester accumulus produced by the administration of the hormone.