Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.11.1 (protein kinase)
 81,284 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Several properties of the cytosolic cholesterol ester hydrolase from bovine adrenal cortex were investigated and those properties were compared directly with those of the well-characterised hormone-sensitive lipase, the rate-limiting enzyme in adipose tissue lipolysis. Properties examined included: (a) activity against different substrates; (b) susceptibility to inhibition by NaF, Hg2+ ions and diisopropyl fluorophosphonate; (c) subunit molecular weight as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate; (d) ability to serve as a substrate for cyclic AMP-dependent protein kinase; (e) effect of phosphorylation on enzyme activity; and (f) degradation pattern of polypeptides following limited proteolysis. In all respects the two enzymes exhibited essentially identical characteristics. It is therefore concluded that the same protein, or two very similar proteins, catalyses the hydrolysis of cholesterol esters in adrenal cortex and lipolysis in adipose tissue. The implication of this finding is discussed in relation to the hormonal control of steroidogenesis in adrenal cortex and of lipolysis in adipose tissue.
 ...
PMID:Direct evidence that cholesterol ester hydrolase from adrenal cortex is the same enzyme as hormone-sensitive lipase from adipose tissue. 710 23

Employing a perifusion technique, the activity of hormone-sensitive lipase (HSL) in human adipose tissue and isolated adipocytes was found to be significantly stimulated by 5 microM epinephrine (5-fold. P less than 0.001) or 0.5 mM theophylline (2-fold, P less than 0.03). The stimulatory effect of both, however, was not cumulative; instead, theophylline appeared to blunt the epinephrine effect. The two effectors together produced a 3-fold increase in activity over basal (P less than 0.01). Basal lipolysis was highly and significantly correlated with epinephrine and/or theophylline stimulated lipolysis in the tissue as well as in the isolated cells. The activity of the enzyme was examined in cultured human adipocytes using a cell-free system. The basal activity of HSL in the 20,000 X g supernatant (S20) fraction of cultured cells grown in fat-enriched medium was significantly higher than the value in the same fraction of cells grown in regular medium (6.06 +/- 1.49 versus 2.78 +/- 0.89 nmole glycerol/min/mg protein, mean +/- S.D., P less than 0.01), an was similar in the S20 fractions of cells grown in the enriched medium and the original tissue (6.06 +/- 1.49 versus 5.44 +/- 2.73 nmole glycerol/Min/mg protein, mean +/- S.D., P greater than 0.83). When the tissue and cells were stimulated in vitro before fractionation, the HSL activity in the S20 fraction of the original tissue increased 4-fold over basal (P less than 0.001), whereas that in the cultured cells increased 2-fold (P less than 0.01) regardless of the culture medium employed. In the S20 fractions derived from basally incubated samples, but not in those derived from the epinephrine-stimulated samples, the HSL enzyme was slightly but significantly (P less than 0.002) activated by exogenous addition of ATP, cyclic AMP, and protein kinase. The data suggest that the activity of HSL is retained in cultured human adipocytes and could be enhanced under conditions of culture which favor lipid accumulation and adipose conversion.
 ...
PMID:Hormone-sensitive lipase in human adipose tissue, isolated adipocytes, and cultured adipocytes. 715 75

The lipolytic action of theophylline was examined using both intact fat cells and a fat globule system. Theophylline had similar lipolytic actions in both systems. However theophylline did not activate hormone-sensitive lipase in the fat globule system as measured with added Ediol. Pretreatment of the fat globules with phospholipase C suppressed theophylline-induced lipolysis, but phospholipase D had no effect. A theophylline-sensitive system was reconstituted from endogenous fat and a lipase fraction. Inhibitors of theophylline-induced lipolysis such as quinine and propranolol inhibited theophylline binding to artificial lipid micelles. Purine nucleosides such as adenosine, inosine and guanosine inhibited theophylline-induced lipolysis in the fat globule system. These results suggest that theophylline has a lipolytic action similar to that of adrenaline. Both share a lipolytic mechanism additional to that involving the activation of hormone sensitive lipase through the cyclic-AMP dependent protein kinase. Phospholipids play an important role in this additional mechanism.
 ...
PMID:The mechanism of the lipolytic action of theophylline in fat cells. 724 46

Steroidogenic cells store cholesteryl esters, precursors for steroid hormone synthesis, in intracellular lipid droplets. Cholesteryl ester hydrolysis is activated by protein kinase A and catalyzed by cholesteryl esterase. The esterase is similar, if not identical, to hormone-sensitive lipase in adipocytes where an analogous lipolytic mechanism occurs. Perilipins, proteins located exclusively at lipid droplet surfaces in adipocytes, are polyphosphorylated by protein kinase A in response to lipolytic stimuli, suggesting a role for these proteins in mediating lipid metabolism. The present study reveals that perilipins are associated with cholesteryl ester droplets in two steroidogenic cell lines: Y-1 adrenal cortical cells and MA-10 Leydig cells. The relative abundance of perilipin mRNAs and protein is much less in steroidogenic cells than in adipocytes. Like adipocytes, steroidogenic cells express perilipin A; additionally, the latter cells contain relatively abundant amounts of perilipin C, a protein that is not detectable in adipocytes by Western analysis. The data suggest a strong link between perilipins and lipid hydrolysis that is mediated by the hormone-sensitive lipase/cholesteryl esterase class of enzymes.
 ...
PMID:Perilipins are associated with cholesteryl ester droplets in steroidogenic adrenal cortical and Leydig cells. 762 16

Enhancement of cAMP degradation by increased cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) activity is thought to be an important component of the mechanism whereby insulin counteracts catecholamine-induced lipolysis in adipocytes. In this study the selective cGI-PDE inhibitor OPC3911 was used to evaluate this role of cGI-PDE activation in intact rat adipocytes with special reference to changes in cAMP levels measured as cAMP-dependent protein kinase (cAMP-PK) activity ratios. OPC3911 completely blocked (IC50 = 0.3 microM) the maximal inhibitory effect of insulin on noradrenaline-induced lipolysis and the net dephosphorylation of hormone-sensitive lipase and other intracellular target proteins for insulin action, whereas insulin-induced lipogenesis was not changed. The effect of OPC3911 on cAMP-PK activity ratios at different levels of lipolysis achieved by noradrenaline stimulation revealed that the reduction of cAMP-PK caused by 1 nM insulin was completely blocked by 3 microM OPC3911. The effect of OPC3911 was not due to an excessive increase in cellular cAMP resulting in 'supramaximal' lipolysis unresponsive to insulin. These data demonstrate that reduction in cAMP levels by the activation of cGI-PDE may be sufficient to account for the antilipolytic action of insulin.
 ...
PMID:Evidence for the key role of the adipocyte cGMP-inhibited cAMP phosphodiesterase in the antilipolytic action of insulin. 771 14

The pathophysiology of familial combined hyperlipidemia (FCHL) is unknown, but altered lipid turnover in peripheral tissues as well as hepatic overproduction of apolipoprotein B have been suggested as possible causes. In the present study, we explored whether a change in triglyceride breakdown by lipolysis in fat cells is present in FCHL. Lipolysis activation by catecholamines was examined in isolated subcutaneous adipocytes from 10 patients with FCHL and 22 healthy control subjects. Lipolysis rate was linear for at least 3 h in both groups. However, a marked (approximately 65%) decrease in the lipolytic response to noradrenaline was found in FCHL. This was also true when lipolysis was maximally stimulated at the receptor level with isoprenaline (nonselective beta-adrenergic agonist), at the adenylyl cyclase level with forskolin, or at the level of the protein kinase hormone-sensitive lipase complex with dibutyryl cAMP. The maximum enzymatic activity of hormone-sensitive lipase was decreased by approximately 40% in FCHL. On the other hand, the lipolytic sensitivity of alpha 2-, beta 1-, and beta 2-adrenoceptors was normal in this condition, as was the number and affinity of beta 1- and beta 2-adrenoceptors. Variations in the maximum lipolysis rate correlated significantly with the variations in hormone-sensitive lipase activity in the whole material, and with the serum values for triglycerides, HDL cholesterol and apoB lipoprotein within the control group, but the serum triglyceride values in FCHL were higher than this correlation predicted. In conclusion, the data demonstrate a marked resistance to the lipolytic effect of catecholamines in fat cells from patients with FCHL, in spite of normal adrenoceptor function. The lipolytic defect appears predominantly to be due to a defect in hormone-sensitive lipase, and may be of importance in the pathophysiology of FCHL.
 ...
PMID:Impaired activation of adipocyte lipolysis in familial combined hyperlipidemia. 773 84

The AMP-activated protein kinase (AMPK) is believed to protect cells against environmental stress (e.g. heat shock) by switching off biosynthetic pathways, the key signal being elevation of AMP. Identification of novel targets for the kinase cascade would be facilitated by development of a specific agent for activating the kinase in intact cells. Incubation of rat hepatocytes with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) results in accumulation of the monophosphorylated derivative (5-aminoimidazole-4-carboxamide ribonucleoside; ZMP) within the cell. ZMP mimics both activating effects of AMP on AMPK, i.e. direct allosteric activation and promotion of phosphorylation by AMPK kinase. Unlike existing methods for activating AMPK in intact cells (e.g. fructose, heat shock), AICAR does not perturb the cellular contents of ATP, ADP or AMP. Incubation of hepatocytes with AICAR activates AMPK due to increased phosphorylation, causes phosphorylation and inactivation of a known target for AMPK (3-hydroxy-3-methylglutaryl-CoA reductase), and almost total cessation of two of the known target pathways, i.e. fatty acid and sterol synthesis. Incubation of isolated adipocytes with AICAR antagonizes isoprenaline-induced lipolysis. This provides direct evidence that the inhibition by AMPK of activation of hormone-sensitive lipase by cyclic-AMP-dependent protein kinase, previously demonstrated in cell-free assays, also operates in intact cells. AICAR should be a useful tool for identifying new target pathways and processes regulated by the protein kinase cascade.
 ...
PMID:5-aminoimidazole-4-carboxamide ribonucleoside. A specific method for activating AMP-activated protein kinase in intact cells? 774 80

Recent developments in understanding the biochemical and molecular nature of the CE hydrolases and their impact on cellular cholesterol trafficking have further defined the enzyme's mechanism of action with reasonable clarity. The availability of the cDNA probe for the human lysosomal acid lipase/CE hydrolase and the hormone-sensitive lipase now makes it possible to study CE hydrolase gene regulation and expression in human tissue; and it can now be stated with more assurance that the cytoplasmic CE hydrolase (NCEH) is most likely activated through phosphorylation by the cyclic AMP-dependent protein kinase. Evidence also shows that the NCEH is most likely identical to the hormone-sensitive lipase and that it plays an important role in cholesterol efflux properties of the cell. Recent advances in the discovery of the role of the eicosanoid/cytokine network in the regulation of CE hydrolysis, highlighted in Figure 10, further emphasize the interesting but complex nature of the cholesterol trafficking processes in cells, particularly under pathophysiological conditions such as cell injury, repair, and inflammation. It can be speculated that in several years, when the crystal structure of the CE hydrolase is known, the structure-function properties of this enzyme's catalytic domain, as it relates to the physical state of the CE substrates, should further clarify the precise role of this enzyme in intracellular cholesterol mobilization and trafficking under a variety of cellular conditions.
 ...
PMID:Regulation of cholesteryl ester hydrolases. 781 70

The AMP-activated protein kinase is responsible for the regulation of fatty acid synthesis by phosphorylation of acetyl-CoA carboxylase. It may also regulate cholesterol synthesis via phosphorylation and inactivation of hormone-sensitive lipase and hydroxymethylglutaryl-CoA reductase. We have purified the AMP-activated protein kinase 14,000-fold from porcine liver. The 63-kDa catalytic subunit co-purifies with two proteins of 40 and 38 kDa that may function as subunits. Partial amino acid sequence of the 63-kDa subunit revealed a striking homology with the catalytic domain of the yeast protein kinase transcriptional regulator Snf1 and its plant homologs. The Snf1 (72 kDa) and Snf4 (36 kDa) complex was also purified and found to phosphorylate the AMP-activated protein kinase peptide substrate, HMRSAMSGLHLVKRR-amide, but was not activated by AMP. Both Snf1/4 and the AMP-activated protein kinase phosphorylate and inactivate yeast acetyl-CoA carboxylase in vitro. These results indicate that during evolution the catalytic domain sequences of the Snf1 protein kinase subfamily have been exploited in the control of mammalian lipid metabolism and raise the possibilities that the AMP-activated protein kinase may have other substrates involved in regulating gene expression pathways, as well as Snf1 homologs participating in the control of lipid metabolism in many eukaryotic organisms.
 ...
PMID:Mammalian AMP-activated protein kinase shares structural and functional homology with the catalytic domain of yeast Snf1 protein kinase. 790 77

In vivo, hormone-sensitive lipase (HSL) is known to be phosphorylated on two sites termed the regulatory and basal sites. However, the intracellular role of the basal site or the identity of the protein kinase phosphorylating this site has not been established. We show that 5-amino-4-imidazolecarboxamide ribonucleoside (AICAR) markedly activates cellular AMP-activated protein kinase (AMPK) in a time- and dose-dependent manner. As expected for an agent that activates AMPK intracellularly, AICAR had no effect on the basal activity of HSL. However, preincubation of adipocytes with AICAR led to a reduced response of these cells to the lipolytic agent isoprenaline. AICAR was also shown to profoundly inhibit lipogenesis through increased phosphorylation of acetyl-CoA carboxylase (ACC). Thus it appears that in addition to regulating lipogenesis, AMPK also plays an important antilipolytic role by regulating HSL in rat adipocytes.
 ...
PMID:Inhibition of lipolysis and lipogenesis in isolated rat adipocytes with AICAR, a cell-permeable activator of AMP-activated protein kinase. 792 17


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>