Cholesterol ester hydrolase activity in mammary tissue of the lactating rat

Expression, activity, and localization of hormone-sensitive lipase in rat mammary gland during pregnancy and lactation

We examined the presence of hormone-sensitive lipase (HSL) in mammary glands of virgin, pregnant (12, 20, and 21 days), and lactating (1 and 4 days postpartum) rats. Immunohistochemistry with antibody against rat HSL revealed positive HSL in the cytoplasm of both alveolar epithelial cells and adipocytes. In virgin rats, immunoreactive HSL was observed in mammary adipocytes, whereas diffuse staining was found in the epithelial cells. Positive staining for HSL was seen in the two types of cells in pregnant and lactating rats. However, as pregnancy advanced, the staining intensity of immunoreactive HSL increased in the epithelial cells parallel to their proliferation, attaining the maximum during lactation. An immunoreactive protein of 84 kDa and a HSL mRNA of 3.3. kb were found in the rat mammary gland as in white adipose tissue. Both HSL protein and activity were lower in mammary glands from 20 and 21 day pregnant rats than from those of virgin rats, although they returned to virgin values on days 1 and 4 of lactation. Mammary gland HSL activity correlated negatively to plasma insulin levels. Immunoreactive HSL and HSL activity were found in lactating rats' milk. The observed changes indicate an active role of HSL in mammary gland lipid metabolism.

The esterification of cholesterol in the yolk sac membrane of the chick embryo

The uptake of lipid from the yolk by the yolk sac membrane of the chick embryo is accompanied by the rapid esterification of a large proportion of the yolk cholesterol. This could arise from enhanced acyl-CoA:cholesterol acyltransferase (ACAT) activity and/or inhibition of cholesteryl ester hydrolase (CEH) activity. The activity of ACAT was therefore measured in microsomes obtained from yolk sac membranes at various stages of development. A high level of activity (up to 929 pmol of cholesteryl oleate formed per min per mg protein) was found during the second half of this period. Supplementation with exogenous cholesterol stimulated ACAT activity in microsomes obtained from the tissue at the earlier, but not at the later, stages of development suggesting that the enzyme became saturated with microsomal cholesterol as development proceeded. Correlating with this, the concentration of cholesterol in the microsomes increased 4-fold between 9 and 20 d of development. The activity of CEH was very low in the microsomes and could not be detected in the cytosolic fraction. The activity of a protein, which has been shown to function as an inhibitor of CEH, was found to be present at all stages of development. The high activity of ACAT, together with the low activity of CEH and an active CEH inhibitor protein is a combination well suited to promote an essentially unidirectional conversion of cholesterol to cholesteryl ester. This process may be a major determinant of the rate of lipid transfer from the yolk to the embryo.

Characterization of a cytosolic protein in rat liver inhibiting neutral cholesteryl ester hydrolase

Neutral cholesteryl ester hydrolase activity (EC 3.1.1.13) present in microsomes isolated from lactating rat mammary glands was found to be inhibited by a factor (or factors) occurring in the cytosolic fraction of male rat liver. The inhibitor was heat-labile, non-dialyzable, destroyed by proteolysis, and was stable following preparation of an acetone/diethyl ether powder of the cytosolic fraction. The protein also inhibited the activity of hormone-sensitive lipase (HSL) (from bovine adipose tissue) and esterase from Candida cylindracea, but seemed to be more active against the neutral hydrolase found in rat liver microsomes. For the mammary gland microsomal cholesteryl ester hydrolase, the extent of the inhibitory effect was dependent on the concentration of the cytosolic protein, 50% inhibition being achieved by about 100 micrograms of cytosolic protein, and on the method of initiating the enzyme assay. Kinetic analysis indicated that, under circumstances where the reaction was initiated by the addition of substrate, the inhibition was characterized as "uncompetitive." When an inhibitor/substrate complex was allowed to form in the absence of enzyme, an element of "competitive" inhibition was introduced into the reaction. Food withdrawal reduced the activity of the inhibitor in liver by 56%, but activity was fully restored by short-term re-feeding. In contrast, feeding a diet high in fat led to a 34% increase in activity. The present findings suggest that the inhibitory factor(s) may be involved in the regulation of the hydrolysis of cholesteryl esters in the liver and also in other cell types.

The inhibition of neutral cholesteryl ester hydrolase by a cytosolic protein factor in female rat liver: the influence of varying hormonal and nutritional conditions on the inhibitory activity

A cytosolic protein, that is inhibitory to neutral cholesteryl ester hydrolase, has been investigated in the livers of female rats using microsomes isolated from the mammary gland of lactating rats as an enzyme source. To facilitate comparisons, inhibitory activity is expressed in terms of the amount (micrograms) of cytosolic protein required to reduce esterase activity by 50% and is compared to the hepatic content of both cholesterol and cholesteryl esters. The experiments revealed a sexual difference in the level of inhibitory activity, with the livers of both suckling and mature male animals containing less of the material than the corresponding females. Alterations in the physiological status of the females, such as pregnancy and lactation, led to a decrease in the activity of the protein. This was reversed by blocking lactation with a combination of an antiserum to rat growth hormone and the anti-prolactin drug, bromocriptine, but not by premature weaning of the animals. Food withdrawal for 24 hr also had the effect of increasing inhibitory activity. In general the cholesteryl ester content of the livers correlated with the level of inhibitory activity. Thus the activity of the cytosolic inhibitor of neutral cholesteryl ester hydrolase responded to changes in both the hormonal and the nutritional status of the female animal. It is suggested that the presence of the greater cholesteryl ester hydrolase inhibitory activity in the female liver may help to explain the lower risk of coronary heart disease in premenopausal females by facilitating increased hepatic storage of the sterol in the form of the ester.

Cholesterol ester hydrolysis and hormone-sensitive lipase in lactating rat mammary tissue

Neutral cholesterol esterase activity is expressed in extracts of mammary epithelial cells. The identity of the enzyme catalyzing this hydrolysis was investigated. Anti-hormone-sensitive lipase immunoglobulin elicited the total inhibition of this activity and also immunoprecipitated a single phosphoprotein of Mr 84 kDa from mammary cell extracts previously phosphorylated in vitro with [gamma-32P]ATP and cyclic AMP-dependent protein kinase. It is concluded that mammary cell cholesterol esterase activity results from the presence of hormone-sensitive lipase.