Perinatal development of hepatic cholesterol synthesis in the rat

Hepatic Gene Expression During the Perinatal Transition in the Rat

During the immediate postnatal (PN) period, the liver, with its role in energy metabolism and macromolecule synthesis, plays a central role in the perinatal transition. Using RNA microarrays and several complementary computational analyses, we characterized changes in hepatic gene expression in the rat across a developmental period starting with the late gestation fetus (embryonic day 21), and including 30 min PN, 4 h PN, 12 h PN, 1 day PN, and 1 week after birth. Following subtle changes in gene expression at the earliest PN time point, there were marked changes that occurred between 4 and 12 h after birth. These reflected changes in multiple metabolic pathways, with expression of enzymes involved in glycolysis and cholesterol synthesis showing the greatest change. Over 50% of nuclear-encoded mitochondrial genes changed in the first 7 days of PN life, with 25% changing within the first 24 h. We also observed changes coinciding with a transient period of synchronous hepatocyte proliferation that we had observed previously, which occurs during the first PN week. Analysis for upstream regulators of gene expression indicated multiple initiating factors, including cell stress, hormones, and cytokines. Also implicated were multiple canonical transcription factor networks. We conclude that changes in gene expression during the early phases of the perinatal transition involve a complex, choreographed network of signaling pathways that respond to a variety of environmental stimuli. This transcriptomic response during the immediate PN period reflects a complex metabolic adaptive response that incorporates a panoply of signaling pathways and transcriptional regulators.

The liver plays a key role in whole body sterol accretion of the neonatal Golden Syrian hamster

Neonates have a significant requirement for cholesterol. From -1 to 25 days of age, the liver accrues 6.9 mg cholesterol and the extra-hepatic tissues accrue 107.7 mg cholesterol in the hamster. It is currently unknown if each of these body compartments synthesizes their own cholesterol or if they have alternative source(s) of sterol. Using (3)H(2)O, in vivo hepatic sterol synthesis rates (per g liver per animal) increased between -1 and 5 days of age, decreased by 10 days of age, and increased again by 15 days of age. HMG-CoA reductase (HMGR) expression levels paralleled in vivo synthesis rates. Extra-hepatic sterol synthesis rates followed the same pattern as sterol synthesis rates in the liver. When sterol synthesis rates were converted to the mass of sterol synthesized per day, the liver synthesized 38.9 and the extra-hepatic tissues synthesized 63.9 mg cholesterol in the 26-day neonatal period. Comparing the amount of cholesterol accrued to that synthesized, one can conclude that the liver is a major source of sterol for the whole body during the neonatal period of the hamster. These results may help elucidate the cause(s) of reduced growth rates in neonates with liver disease or in neonates with compromised sterol synthesis rates.

Regulation of fatty acid and cholesterol synthesis: co-operation or competition?

Fatty acids and sterols originally evolved symbiotically as structural components of cell membranes. In some respects, control of their biosynthetic pathways reflects their mutual interdependence in defining changes in the physicochemical properties of the membranes in response to the changing internal and external cellular environments. In some tissues of higher animals, however, cholesterol and fatty acids have multifunctional roles. In particular, the liver synthesizes these lipids for export as multimolecular complexes in the form of micellar bile components and lipoproteins. Intrahepatic fatty acid and cholesterol synthesis is dependent upon the balance between hepatic output of these complexes and dietary input of fat and cholesterol. Thus physiological control of these synthetic processes is often co-ordinated at both the transcriptional and post-translational levels. On the other hand, changes in flux through major metabolic pathways, particularly during physiological transitions and as a result of genetic manipulation, affects substrate availability for these pathways. Under these circumstances, regulation reflects a compensatory response to ensure that flux through the lipid pathways remains unchanged. These regulatory changes can best be interpreted in terms of a Metabolic Control Analysis approach. In summary, flux through the fatty acid and cholesterol pathways reflects (a) cellular demand for these lipids, (b) a variable availability of substrates, (c) a combination of (a) and (b).

Charting the fate of the "good cholesterol": identification and characterization of the high-density lipoprotein receptor SR-BI

Risk for cardiovascular disease due to atherosclerosis increases with increasing concentrations of low-density lipoprotein (LDL) cholesterol and is inversely proportional to the levels of high-density lipoprotein (HDL) cholesterol. The receptor-mediated control of plasma LDL levels has been well understood for over two decades and has been a focus for the pharmacologic treatment of hypercholesterolemia. In contrast, the first identification and characterization of a receptor that mediates cellular metabolism of HDL was only recently reported. This receptor, called scavenger receptor class B type I (SR-BI), is a fatty acylated glycoprotein that can cluster in caveolae-like domains on the surfaces of cultured cells. SR-BI mediates selective lipid uptake from HDL to cells. The mechanism of selective lipid uptake is fundamentally different from that of classic receptor-mediated endocytic uptake via coated pits and vesicles (e.g. the LDL receptor pathway) in that it involves efficient receptor-mediated transfer of the lipids, but not the outer shell proteins, from HDL to cells. In mice, SR-BI plays a key role in determining the levels of plasma HDL cholesterol and in mediating the regulated, selective delivery of HDL-cholesterol to steroidogenic tissues and the liver. Significant alterations in SR-BI expression can result in cardiovascular and reproductive disorders. SR-BI may play a similar role in humans; thus, modulation of its activity may provide the basis of future approaches to the treatment and prevention of atherosclerotic disease.

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.