Microtubules and the regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase

CTP:cholinephosphate cytidylyltransferase in human and rat lung: association in vitro with cytoskeletal actin

CTP:cholinephosphate cytidylyltransferase activities were compared in saline homogenates of immature fetal (15-16 weeks gestation) and adult human lung. There were no differences in subcellular enzyme distribution, in Vmax activity, or in the phosphatidylglycerol-mediated stimulation of soluble enzyme activity. These results provide no support for a developmental translocation of cytidylyltransferase from a cytosolic to a microsomal location in human lung, such as that proposed to accompany the maturation of pulmonary surfactant phosphatidylcholine biosynthesis in rat. Soluble cytidylyltransferase activity from human but not rat lung was increased after manipulation in vitro. Resolution of human H form (greater than 10(3) kDa) and L form (200 kDa) enzyme by gel filtration led to an activity increase of 200%. Incubation at 37 degrees C for 2 h increased soluble enzyme recovery, although prior centrifugal removal of generated actin-rich aggregates was necessary in adult lung fractions. In contrast, 85% of soluble rat lung cytidylyltransferase was actin aggregate-associated after incubation. The apparent heteroassociation of rat and human lung enzyme with actin in the presence of poly(ethylene glycol) at 4 degrees C strongly suggested close in vitro and potential in vivo linkage. A partial co-purification of adult human lung cytidylyltransferase with actin was also consistent with this idea. We propose that some reported cytidylyltransferase translocation phenomena may be mediated by cytoskeletal interactions in vitro.

The effects of xenobiotics on hepatic lipid and lipo-protein metabolism

The liver occupies a central position in lipid and lipo-protein metabolism. Its function includes lipid and lipoprotein biosynthesis, assembly, packaging, transport, secretion, uptake and degradation of lipoproteins. In addition, enzymes synthesized and secreted by the liver into the blood stream or remaining bound to the endothelial cells in the capillaries, affect lipoprotein metabolism in the circulation. Xenobiotics may influence each of these steps. The mechanisms include more specific actions such as hormone or transmitter agonism and antagonism, membrane effects (stabilization or changes in trans-membrane gradients), influence on protein synthesis, influence on lipid metabolism by induction or inhibition of involved enzymes, or more general actions such as disturbances or damage of cellular membranes and cellular function. Some of these effects can easily be described as pharmacological actions, more or less independent of specific requirements in the chemical structure of the xenobiotics. Others are linked to specific chemical substituents such as carboxyl or alcoholic hydroxyl groups allowing the formation of lipid-xenobiotic-conjugates and/or the channeling of xenobiotics into lipid metabolism. This review will give a short overview of the mechanisms of xenobiotic-influenced hepatic lipid and lipoprotein metabolism.

Cyclic AMP-independent stimulation of steroidogenesis in Y-1 adrenal tumor cells by antimitotic agents

The stimulation of steroidogenesis by antimitotic drugs has been studied in wild-type (Y-1) and cAMP-dependent protein kinase-deficient (kin-8) mouse adrenal tumor cell lines. Unlike some other cells, Y-1 cells do not increase their cAMP output upon exposure to antimitotic drugs such as colchicine, vinblastine or podophyllotoxin, which readily increase steroidogenesis. Moreover, no increase in cAMP can be detected over an extended time span. Stabilization of tubulin polymers by taxol or high concentrations of vinblastine blocks ACTH-, cholera toxin- or colchicine-stimulated steroidogenesis without major effects on cAMP levels. Colchicine and podophyllotoxin stimulate steroidogenesis in the cAMP-dependent protein kinase-deficient mutant to the same degree as in the wild-type Y-1 cells, although absolute steroid yields are lower in the mutant cells. We suggest that the antimitotic agents stimulate adrenal steroidogenesis by a cAMP-independent pathway that may involve facilitation of cholesterol access to the mitochondrion.

The effect of colchicine on cholesterol processing by the progesterone-producing cells of the luteinized ovary

Previously, we indicated that luteal cells from colchicine-treated superovulatory (luteinized) rats show decreased capacity for progesterone production. The current study investigates the possibility that colchicine exerts this effect by interfering with the mechanism by which cholesterol is processed and/or synthesized by luteal cells. To this end, animals were treated with saline or colchicine after which the luteinized ovary or isolated luteal cells were assayed for their cholesterol content, their ability to synthesize cholesterol endogenously, or their ability to utilize lipoprotein-delivered cholesterol for the production of progesterone. The results show that animals treated with colchicine show a number of changes in luteal cell cholesterol metabolism: namely a 60% decline in stored cholesterol, a 3-fold rise in the activity of the cholesterol synthesizing enzyme HMG CoA reductase (although no change occurs in other cholesterol metabolizing enzymes), and a 3-fold rise in the capacity of the cells to incorporate precursor [14C]acetate into cholesterol. On the other hand, cells of animals treated with colchicine or cells treated with colchicine under in vitro circumstances are unable to fully utilize cholesterol provided by high density lipoproteins (HDL): this occurs despite the fact that the binding of HDL particles to luteal cells is quite normal after colchicine treatment. These findings are consistent with the view that a primary effect of colchicine on luteal cell progesterone production is in preventing the normal uptake of HDL-cholesterol.

Differentiation of neuroblastoma cells induced by an inhibitor of mevalonate synthesis: relation of neurite outgrowth and acetylcholinesterase activity to changes in cell proliferation and blocked isoprenoid synthesis

Mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, stimulates neurite outgrowth and acetylcholinesterase (ACE) activity in C1300 (Neuro-2A) murine neuroblastoma cells. Sprouting of neurites began within 4-8 h, before changes in cell proliferation could be detected by [3H]thymidine incorporation or flow cytometry. In contrast, the increase in ACE activity was temporally correlated with suppression of DNA synthesis, which occurred after 8 h. The activity of the membrane marker enzyme phosphodiesterase I was not stimulated by mevinolin. Suppression of protein synthesis with cycloheximide blocked the induction of ACE activity but only partially inhibited neurite outgrowth in the mevinolin-treated cultures. When mevinolin was removed from the culture medium, most of the cells retracted their neurites within 2 h, but ACE activity did not decline until DNA synthesis began to return to control levels after 10 h. Similarly, retraction of neurites in differentiated cells exposed to colchicine was not accompanied by a decrease in ACE activity. DNA histograms suggested that mevinolin arrests neuroblastoma cells in both the G1 and G2/M compartments of the cell cycle. Other cytostatic drugs that arrest cells at different stages of the cell cycle did not cause Neuro-2A cells to form neurites such as those seen in the mevinolin-treated cultures. When incorporation of [3H]acetate into isoprenoid compounds was studied in cultures containing mevinolin in concentrations ranging from 0.25 microM to 25 microM, the labeling of cholesterol, dolichol, and ubiquinone was suppressed by 90% or more at all concentrations. However, significant growth arrest and cell differentiation were observed only at the highest concentrations of mevinolin. Supplementing the medium with 100 microM mevalonate prevented the cellular response to mevinolin, but additions of cholesterol, dolichol, ubiquinone, or isopentenyl adenine were generally ineffective. The cholesterol content of neuroblastoma cells incubated with 25 microM mevinolin for 24 h was not diminished, and protein glycosylation, measured by [3H]mannose incorporation, was decreased only after 24 h at high mevinolin concentration. These studies suggest that the stimulation of neurite outgrowth and the increase in ACE activity induced by mevinolin are independent phenomena. Whereas neurite outgrowth is not related directly to the effects of mevinolin on cell cycling, the induction of ACE is correlated with the inhibition of cell proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of antimicrotubule agents on phospholipid metabolism in rat hepatic subcellular membranes

Treatment of animals with antimicrotubule drugs has been shown to cause a perplexing variety of cellular changes which, theoretically, could be the result of changes in endomembrane biosynthesis, composition or flow. In the current study we have focused on this possibility by identifying antimicrotubule drug-induced changes in the phospholipid metabolism of hepatic subcellular membranes. Young adult rats were pretreated with radiolabeled [32 P]orthophosphate for 12 hr, and subsequently given saline, colchicine (2.5 mg/kg body wt) or vinblastine (20 mg/kg body wt) for 4 additional hr. Afterwards, the livers were homogenized, and separate microsomal and Golgi membrane fractions were prepared and subjected to phospholipid extraction and identification using two-dimensional thin-layer chromatography. The results show that colchicine and vinblastine given in vivo caused specific, rapid and in some cases, dramatic changes in phospholipid turnover in different membrane fractions of rat liver. The drugs specifically increased labeling of phosphatidylinositol-4-monophosphate and phosphatidylinositol-4,5-biphosphate and decreased the radioactivity associated with phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol in all fractions examined. In contrast, the antimicrotubule drugs produced a differential effect on the labeling pattern of sphingomyelin and lysophosphatidylcholine, i.e. they stimulated labeling of these phospholipids in microsomes, produced no changes in heavy Golgi fractions, and markedly increased their labeling in light Golgi fractions. These data suggest that antimicrotubule drugs restrict the incorporation of certain precursor phospholipids into forming membranes but do not affect the subsequent metabolism of these phospholipids. At the same time, the drugs appear to retard the flow of membranes from one cellular compartment to another.

Colchicine inhibition of insulin induction of stearoyl-CoA desaturase and fatty acid synthetase in cultured avian liver explants

Chicken embryo liver explants cultured in chemically defined medium in the absence of serum provide a unique system to probe into the mechanism of insulin induction of lipogenic enzymes. Colchicine at concentrations of 0.2 and 1 microM in the culture medium caused inhibition of insulin induction of stearoyl-CoA desaturase and fatty acid synthetase by 50 and 90%, respectively. As measured by immunochemical techniques, the inhibition of the induction of these two enzyme systems resulted from the decreased content of the delta 9-terminal desaturase component of the stearoyl-CoA desaturase and the fatty acid synthetase. Colchicine, however, had no effect on the general protein synthesis, nor did it affect the malic enzyme, which is induced by triiodothyronine but not by insulin. Also, colchicine had no influence on the binding of 125I-insulin to isolated plasma membrane. Pretreatment of liver explants with insulin for 0.5-1 h and subsequent incubation in insulin-free media for 48 h resulted in induction of the desaturase and fatty acid synthetase. However, inclusion of colchicine in the media for 3 h subsequent to the treatment with insulin completely abolished the inductive effect of insulin, suggesting that colchicine affects events occurring subsequent to insulin binding to the cell surface membranes. Since lumicolchicine, an inactive isomer of colchicine, had no effect on insulin action, it is suggested that the inhibition of insulin induction of the desaturase and synthetase is related to the depolymerizing action of colchicine. Therefore, in eliciting long-term responses to insulin, microtubular integrity of the cell may be required for the transfer of a putative from cell surface insulin receptor to intracellular sites.

Cytoskeletal structures and oligodendroglial differentiation in C-6 glial cells

The relationship of the cytoskeleton to a biochemical expression of oligodendroglial differentiation was studied in cultured C-6 glial cells. Specifically, we investigated the effect of the cytoskeletal perturbants, colchicine and cytochalasin D, on the induction of the oligodendroglial marker enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), caused by removal of serum from the culture medium. Each drug inhibited CNP induction in a concentration-dependent manner, and essentially complete inhibition of induction was observed with 0.25 microM colchicine or 2.0 microM cytochalasin D. Detailed study of the effect of colchicine was carried out. This antimicrotubular agent not only totally prevented induction if added at the onset of serum removal, but also prevented further induction when added at various times after serum removal. That the effect of colchicine related to the drug's effect on microtubules was supported by the demonstration that lumicolchicine, a colchicine isomer which has no effect on microtubules, had no effect on the CNP induction. Moreover, colchicine, but not lumicolchicine, prevented the morphological signs of differentiation provoked by serum removal. The effect of colchicine was reversible and relatively specific. Thus, no concomitant effect of colchicine on the activity of another plasma membrane enzyme of C-6 cells, i.e., (Na+ + K+)-activated ATPase, or on the rate of incorporation of [3H]leucine into total protein of intact cells could be discerned. The possibility that the site of the effect of colchicine is on intracellular events was suggested by the observation that the drug inhibited the induction of CNP by dibutyryl cyclic AMP. The data suggest that the cytoskeleton is involved in oligodendroglial differentiation.

Induction of differentiation in murine neuroblastoma cells by mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase

Mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, stimulated outgrowth of neurites and increased acetylcholinesterase activity in C1300-N2A murine neuroblastoma cells cultured in medium containing 10% fetal calf serum. Changes in cell morphology and enzyme activity were concentration-dependent in the range of 0.25-25 microM mevinolin, and were accompanied by decreased incorporation of [3H]thymidine into DNA. The expression of differentiated characteristics induced by 25 microM mevinolin was blocked by simultaneous addition of 100 microM mevalonate to the culture medium. The data suggest that changes in intracellular levels of mevalonate or one of its isoprenoid derivatives may play a role in the regulation of cell differentiation.

Vincristine and vinblastine lower plasma cholesterol concentrations in rhesus monkeys

3-4 days after a single clinical dose of vincristine or vinblastine in rhesus monkeys there was a marked decrease in plasma low-density lipoprotein cholesterol concentrations. There was also a concomitant increase in plasma triacylglycerol concentrations. Plasma lipid levels returned to normal concentrations within 7-10 days after injection. Plasma high-density lipoprotein cholesterol concentrations were unaltered by the drugs. Electron micrographs of the hepatocytes from monkeys treated with vincristine or vinblastine showed an accumulation of glycogen particles and proliferation of smooth endoplasmic reticulum, which was accompanied by an increase in the number of lipoprotein-containing vesicles. These results indicate that vincristine and vinblastine alter plasma cholesterol and triacylglycerol concentrations in part by interfering with hepatic lipid and lipoprotein metabolism. These studies further suggest the possibility that other less cytotoxic alkaloids from Catharanthus species with clinically useful hypocholesterolemic activity may be discovered.

The synthesis of the brain specific S100 protein in colcemid resistant mutants of rat glial cells

We have isolated two colcemid-resistant mutant sublines, CMR (7A) and CMR (7B), from rat glial cells, C6, using multiple consecutive selections with increasing concentrations of colcemid. The mutant sublines show a decreased uptake of [3H]colchicine but have no apparent defect in the cytoplasmic binding of the drug. The synthesis of the brain-specific S100 protein is less sensitive to colcemid inhibition in the mutant cell lines than in parental C6 cells, suggesting that colcemid must enter the cell to inhibit S100 protein synthesis.

Interrelationships of ubiquinone and sterol syntheses in cultured cells of neural origin

Ubiquinone synthesis has been studied in cultured C-6 glial and neuroblastoma cells by utilizing an inhibitor, 3-beta-(2-diethylaminoethoxy) androst-5-en-17-one hydrochloride (U18666A), of cholesterol biosynthesis. Exposure of C-6 glial cells to nanomolar quantities of U18666A caused a marked inhibition of total sterol synthesis from [14C]acetate or [3H]mevalonate within minutes. A 95% inhibition was apparent after a 3-h exposure to 200 ng/ml of U18666A. These observations, together with studies of the incorporation of radioactivity from the two precursors into cholesterol, desmosterol, lanosterol, and squalene, indicated that although the most sensitive site to inhibition by U18666A is desmosterol reduction to cholesterol, a major site of inhibition is demonstrable at a more proximal site, perhaps squalene synthetase. As a consequence of the latter inhibition, exposure of C-6 glial cells to U18666A caused a marked stimulation of incorporation of [14C]acetate or [3H]mevalonate into ubiquinone. Over a wide range of U18666A concentrations, the increase in ubiquinone synthesis was accompanied by an approximately similar decrease in total sterol synthesis. Whereas in the absence of U18666A only approximately 7% of the radioactivity incorporated from [3H]mevalonate into isoprenoid compounds was found in ubiquinone, in the presence of the drug approximately 90% of incorporated radioactivity was found in ubiquinone. The reciprocal effects of U18666A on ubiquinone and sterol syntheses were apparent also in the neuronal cells. THe data thus demonstrate a tight relationship between ubiquinone and sterol biosyntheses in cultured cells of neural origin. In such cells ubiquinone synthesis is exquisitely sensitive to the availability of isoprenoid precursors derived from the cholesterol biosynthetic pathway.

Reverse transformation of vole cells transformed by avian sarcoma virus containing the src gene

Vole cells transformed by avian sarcoma virus carrying the src gene lose their fibroblastic morphology, the organized cytoskeletal system of the normal fibroblastic cell, the typical fibronectin deposit around the cell membrane, and the ability to shut off multiplication when suspended in liquid medium. All of these transformation characteristics are reversed by treatment with cAMP derivatives. Moreover, the cAMP treatment does not cause loss of activity of the src gene product. These data imply that cAMP exerts its effect at or after the point in the metabolic pathway affected by the src gene product, pp60src. Presumably, the decision to adopt the transformed or the normal state is determined by the degree to which the src gene or cAMP-mediated kinase activities respectively predominante in the cell. The development of all four transformation characteristics as a result of introduction of the src gene, and their coordinate reversal by cAMP derivatives, supports the previous thesis that in the normal vole or CHO fibroblast all four properties are part of a common regulatory system.

Selective action of colchicine on protein synthesis and release in a clonal line of rat glial cells

The effect of colchicine on protein synthesis and secretion in stationary cultures of clonal rat glial cells C6 was examined. Colchicine inhibited the synthesis of the brain specific S100 protein in intact cells but not in a cell-free protein synthesizing system derived from these cells. There was no demonstrable effect of the drug on the synthesis of any of the several hundred proteins resolved by a two-dimensional electrophoretic analysis. However, colchicine specifically enhanced the secretion of several proteins of molecular weighs of 30,000 and of 200-300,000 into the medium. Two of the high molecular weight proteins were apparently membrane proteins whose release into the medium was stimulated by the drug.