Relationship among methylation, isoprenylation, and GTP binding in 21- to 23-kDa proteins of neuroblastoma

Simvastatin inhibits protein isoprenylation in the brain

Evidence suggests that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, or statins, may reduce the risk of Alzheimer's disease (AD). Statin action in patients with AD, as in those with heart disease, is likely to be at least partly independent of the effects of statins on cholesterol. Statins can alter cellular signaling and protein trafficking through inhibition of isoprenylation of Rho, Cdc42, and Rab family GTPases. The effects of statins on protein isoprenylation in vivo, particularly in the central nervous system, are poorly studied. We utilized two-dimensional gel electrophoresis approaches to directly monitor the levels of isoprenylated and non-isoprenylated forms of Rho and Rab family GTPases. We report that simvastatin significantly inhibits RhoA and Rab4, and Rab6 isoprenylation at doses as low as 50nM in vitro. We also provide the first in vivo evidence that statins inhibit the isoprenylation of RhoA in the brains of rats and RhoA, Cdc42, and H-Ras in the brains of mice treated with clinically relevant doses of simvastatin.

Characterization of the prenylated protein methyltransferase in human endometrial carcinoma

The processing of ras and of other GTP-binding proteins includes a final reversible step in which the carboxy terminal prenylated cysteine is methylated by the enzyme prenylated protein methyltransferase (PPMTase). The significance of this modification and of the role of PPMTase in human tumors has yet to be fully elucidated. Here we characterize the PPMTase of human endometrial carcinomas (tumors in which the frequency of ras gene mutations is relatively high) and compare it to the PPMTase of the normal endometrium. Our results show that in both types of tissues the enzyme is bound to the membranes. It can utilize synthetic substrates such as N-acetyl-S-farnesyl-L-cysteine (Km = 18-20 microM) and is blocked by the PPMTase inhibitor S-farnesylthioacetic acid (Ki = 2 microM). In vitro methylation assays and [alpha-32P]GTP blot-overlay assays showed that the major endogenous PPMTase substrates are small GTP-binding proteins. Methylation of these proteins in vitro is blocked by farnesylthioacetic acid. The kinetic properties of PPMTase from the carcinomas and the normal tissues are very similar. However, levels of PPMTase activity (but not of its endogenous substrates) are higher in the carcinomatous endometrium than in the normal one. The elevated enzyme activity is restricted to the crude mitochondrial fraction (8.0 +/- 0.4 vs. 5.4 +/- 0.1 pmol N-acetyl farnesylcysteine methyl ester formed/min/mg protein by the carcinoma and by the normal endometrial preparations, respectively). As this fraction is enriched in plasma membranes, it appears that the elevated enzyme activity could be related to ras protein methylation; if so, selective methylation blockers might inhibit the growth of endometrial carcinomas.

Lack of correlation between 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and lovastatin resistance in nerve growth factor treated PC-12 cells

1. The relationships among the mevalonic acid (MVA) forming enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (CoA) reductase, cell growth and differentiation, and the cytotoxic effects of the reductase inhibitor lovastatin were studied in PC-12 cells, exposed to growth factors. 2. When added individually, nerve growth factor (NGF), basic fibroblast growth factor, and epidermal growth factor induce an increase in HMG-CoA reductase activity in cells grown in serum-containing medium. In the presence of serum, the effect of NGF on HMG-CoA reductase is persistent. 3. Short-term serum starvation and long-term NGF treatment, in combination, have an additive effect, resulting in a high reductase activity. 4. Unlike serum and MVA, which downregulate levels of HMG-CoA reductase by accelerating its degradation, NGF upregulates reductase by slowing the rate of its degradation. This mechanism, however, appears to operate only in the presence of serum, as after prolonged growth with NGF in serum-free medium, cells have a low reductase activity. 5. PC-12 cells grown in the absence of NGF are highly sensitive to lovastatin (25 microM) and more than 70% of the cells die after 48 hr. NGF confers lovastatin resistance on cells grown in the presence or in the absence of serum (only 30-40% cell death after 48 hr with lovastatin). 6. NGF-induced resistance on lovastatin develops with time and is apparent only in the well-differentiated PC-12 cells whether or not the cells express a high reductase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Prenylated protein methyltransferase of rat cerebellum is developmentally co-expressed with its substrates

High levels of prenylated protein methyltransferase are expressed in the developing rat cerebellum and are responsible for methylation of endogenous G-proteins and 50-52 kDa synaptosomal proteins. Enzyme activity in cerebellar synaptosomes of 3 week postnatal rats is 2-fold higher than that found in adult rat cerebellum. A 10-fold rise in activity occurs at the end of the second and during the third postnatal weeks, followed by a subsequent decline. Expression of the enzymes' substrates follows the same pattern. The high methyltransferase activity in 3-week-old cerebellum coincides with the period of granule cell migration and synaptogenesis, suggesting a regulatory role for the enzyme and its substrates in cerebellar ontogenesis.

Modulation of insulin secretion from normal rat islets by inhibitors of the post-translational modifications of GTP-binding proteins

Many GTP-binding proteins (GBPs) are modified by mevalonic acid (MVA)-dependent isoprenylation, carboxyl methylation or palmitoylation. The effects of inhibitors of these processes on insulin release were studied. Intact pancreatic islets were shown to synthesize and metabolize MVA and to prenylate several candidate proteins. Culture with lovastatin (to inhibit synthesis of endogenous MVA) caused the accumulation in the cytosol of low-M(r) GBPs (labelled by the [alpha-32P]GTP overlay technique), suggesting a disturbance of membrane association. Concomitantly, lovastatin pretreatment reduced glucose-induced insulin release by about 50%; co-provision of 100-200 microM MVA totally prevented this effect. Perillic acid, a purported inhibitor of the prenylation of small GBPs, also markedly reduced glucose-induced insulin secretion. Furthermore, both N-acetyl-S-trans,trans-farnesyl-L-cysteine (AFC), which inhibited the base-labile carboxyl methylation of GBPs in islets or in transformed beta-cells, and cerulenic acid, an inhibitor of protein palmitoylation, also reduced nutrient-induced secretion; an inactive analogue of AFC (which did not inhibit carboxyl methylation in islets) had no effect on secretion. In contrast with nutrients, the effects of agonists that induce secretion by directly activating distal components in signal transduction (such as a phorbol ester or mastoparan) were either unaffected or enhanced by lovastatin or AFC. These data are compatible with the hypothesis that post-translational modifications are required for one or more stimulatory GBPs to promote proximal step(s) in fuel-induced insulin secretion, whereas one or more inhibitory GBPs might reduce secretion at a more distal locus.

Protein methylation in cerebellar synaptosomes

Synaptosomes from five regions of adult rat brain were isolated, analyzed for methyl acceptor proteins, and probed for methyltransferases by photoaffinity labeling. Methylated proteins of 17 and 35 kDa were observed in all regions, but cerebellar synaptosomes were enriched in a 21-26-kDa family of methyl acceptor proteins and contained a unique major methylated protein of 52 kDa and a protein of 50 kDa, which was methylated only in the presence of EGTA. When cerebellar and liver subcellular fractions were compared, the cytosolic fractions of each tissue contained methylated proteins of 17 and 35 kDa; liver membrane fractions contained few methylated proteins, whereas cerebellar microsomes had robust methylation of the 21-26-kDa group. Differential centrifugation of lysed cerebellar synaptosomes localized the 17- and 35-kDa methyl acceptor proteins to the synaptoplasm, the 21-26-kDa family to the synaptic membranes, and the 52-kDa to synaptic vesicles. The 21-26-kDa family was identified as GTP-binding proteins by [alpha-32P]GTP overlay assay; these proteins contained a putative methylated carboxyl cysteine, based on the presence of volatile methyl esters and the inhibition of methylation by acetylfarnesylcysteine. The 52-kDa methylated protein also contained volatile methyl esters, but did not bind [alpha-32P]GTP. When synaptosomes were screened for putative methyltransferases by S-adenosyl-L-[methyl-3H]methionine photoaffinity labeling, a protein of 24 kDa was detected only in cerebellum, and this labeled protein was localized to synaptic membranes.

Nerve growth factor induces a succession of increases in isoprenylated methylated small GTP-binding proteins of PC-12 pheochromocytoma cells

Pheochromocytoma (PC-12) cells exposed to nerve growth factor (NGF) acquire a sympathetic neuron-like phenotype. This NGF-response is blocked by methylation inhibitors and can be mimicked by the farnesylated methylated small GTP-binding protein p21ras. The implicated involvement of prenylation, methylation and a small GTP-binding protein in the NGF-response has been studied by directly measuring 3H-mevalonic acid (MVA)-metabolites incorporated into proteins, protein carboxy [methyl-3H]ester formation and levels of [alpha-32P]GTP-binding proteins in NGF-induced PC-12 cells. We demonstrate that NGF induces a 2-3-fold increase in 21-24 kDa methylated membrane proteins that incorporate 3H-MVA-metabolites, and bind GTP. Levels of [alpha-32P]GTP-binding in these proteins were increased by 2-3-fold. Methylation and membrane association of the small GTP-binding proteins were blocked by lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, which also enhanced their labeling by 3H-MVA-metabolites. Cycloheximide reduced the levels of [methyl-3H] labeled 21-24 kDa proteins and of the overlapping [alpha-32P]GTP binding-proteins. About 70% of the [methyl-3H]-groups found in these proteins were recovered from two dimensional gel blots in nine distinct spots of [alpha-32P]GTP-binding proteins. Taken together these results strongly suggest that in PC-12 cells, NGF induces an increase in the synthesis of prenylated methylated small GTP-binding proteins. The efficacy of lovastatin blockage of protein methylation and enhancement of 3H-MVA-metabolites incorporation into GTP-binding proteins was lower in NGF-induced cells than in controls. This suggests that NGF also induces an increase in HMG-CoA reductase activity. At the early phase of the NGF response in PC-12 cells (15 min-1 h), the levels of two small GTP-binding proteins (molecular mass of 21-22 kDa and 23-24 kDa) were increased. Thus, at least two proteins, of which one but not the other may be p21ras, appear to be involved in the early response. After a lag period of 24 h with NGF, a second more robust phase of increase in methylated small GTP-binding proteins was apparent. This relatively late response, which was almost completed within 24 h, may reflect involvement of small GTP-binding proteins in neurite-outgrowth and in the functional activity of the differentiated cells. Many small GTP-binding proteins were increased during the second phase, precluding electrophoretic separation of all of them. 3 proteins, however, were well separated (one 23-24 kDa protein and two 21-22 kDa proteins).(ABSTRACT TRUNCATED AT 400 WORDS)

Isoprenylation and carboxylmethylation in small GTP-binding proteins of pheochromocytoma (PC-12) cells

1. A group of 21 to 24-kDa proteins of pheochromocytoma (PC-12) cells was found in blot overlay assays to bind specifically [alpha-32P]GTP. Binding was inhibited by GTP analogues but not by ATP. Such small GTP-binding proteins were found in the cytosolic and in the particulate fraction of the cells, but they were unevenly distributed: about 75% of the small GTP-binding proteins were localized within the particulate fraction of the cells. Separation of these proteins by two-dimensional gel electrophoresis revealed the existence of seven distinct [alpha-32P]GTP-binding proteins. 2. Targeting of the small GTP-binding proteins to the particulate fraction of PC-12 cells requires modification by isoprenoids, since depleting the cells of the isoprenoid precursor mevalonic acid (MVA) by the use of lovastatin resulted in a 50% decrease in membrane-bound small GTP-binding proteins, with a proportionate increase in the cytosolic form. This blocking effect of lovastatin was reversed by exogenously added MVA. 3. In addition, metabolic labeling of PC-12 cells with [3H]MVA revealed incorporation of [3H]MVA metabolites into the cluster of 21 to 24-kDa proteins in a form typical of isoprenoids; the label was not removed from the proteins by hydroxylamine, and labeling was enhanced in cells incubated with lovastatin. The latter effect reflects a decrease in the isotopic dilution of the exogenously added [3H]MVA, as the addition of exogenous MVA reversed the effect of lovastatin on [3H]MVA-metabolite incorporation into the 21 to 24-kDa proteins. 4. Additional experiments demonstrated that isoprenylation is required not only for membrane association of small GTP-binding proteins, but also for their further modification by a methylation enzyme. This was evident in experiments in which the cells were metabolically labeled with [methyl-3H]methionine, a methylation precursor. The group of 21 to 24-kDa proteins was labeled with a methyl-3H group in a form typical of C-terminal-cysteinyl carboxylmethyl esters. Their methylation was blocked by the methylation inhibitors methylthioadenosine (MTA), 3-deazadenosine and homocysteine thiolactone as well as by lovastatin. MVA reversed the lovastatin block of methylation. 5. Two-dimensional gel analysis of the [3H]methylated proteins detected seven methylated small GTP-binding proteins that correspond to the isoprenylated proteins. Levels of the small GTP-binding proteins as well as isoprenylation and methylation were reduced by cycloheximide. 6. Distribution of the methylated proteins between particulate and cytosolic fractions was found to be similar to that of the small GTP-binding proteins (i.e., a 4:1 ratio).(ABSTRACT TRUNCATED AT 400 WORDS)