Functional aspects of polyisoprenoid protein substituents: roles in protein-protein interaction and trafficking

There are now numerous examples of post-translational modification with geranylgeranyl or farnesyl substituents. Once thought of as solely a mechanism for association of proteins with membranes, other functional aspects of protein prenylation have come to be appreciated. Although, in almost all instances, such proteins are membrane associated, they are often found to also engage in protein-protein interactions. In some instances, such interactions are critical aspects of prenylated protein trafficking. In this review, the role of prenylation in mediating protein-protein interactions will be considered. The hypothesis will be developed that such interactions occur through recognition of the prenyl group and a second domain, on the prenylated protein, by a heterodimeric protein partner.

Recent advances in the study of prenylated proteins

Post-translational modification of proteins with isoprenoids was first recognized as a general phenomenon in 1984. In recent years, our understanding, including mechanistic studies, of the enzymatic reactions associated with these modifications and their physiological functions has increased dramatically. Of particular functional interest is the role of prenylation in facilitating protein-protein interactions and membrane-associated protein trafficking. The loss of proper localization of Ras proteins when their farnesylation is inhibited has also permitted a new target for anti-malignancy pharmaceuticals. Recent advances in the enzymology and function of protein prenylation are reviewed in this article.

Subcellular localization and partial purification of prelamin A endoprotease: an enzyme which catalyzes the conversion of farnesylated prelamin A to mature lamin A

The nuclear lamina protein, lamin A is produced by proteolytic cleavage of a 74 kDa precursor protein, prelamin A. The conversion of this precursor to mature lamin A is mediated by a specific endoprotease, prelamin A endoprotease. Subnuclear fractionation indicates that the prelamin A endoprotease is localized at the nuclear membrane. The enzyme appears to be an integral membrane protein, as it can only be removed from the nuclear envelope with detergent. It is effectively solubilized by the detergent n-octyl-beta-D-glucopyranoside and can be partially-purified (approximately 1200-fold) by size exclusion and cation exchange (Mono S) chromatography. Prelamin A endoprotease from HeLa cells was eluted from Mono S with 0.3 M sodium chloride as a single peak of activity. SDS-PAGE analysis of this prelamin A endoprotease preparation shows that it contains one major polypeptide at 65 kDa and smaller amounts of a second 68 kDa polypeptide. Inhibition of the enzyme activity in this preparation by specific serine protease inhibitors is consistent with the enzyme being a serine protease.

Oleate potentiates oxysterol inhibition of transcription from sterol regulatory element-1-regulated promoters and maturation of sterol regulatory element-binding proteins

Activation of genes containing SRE-1 (sterol regulatory element 1) sequences is known to be under the regulation of sterols through modulation of the proteolytic maturation of SREBPs (SRE-1-binding proteins). Previous work has demonstrated SREBP-mediated transcriptional activation of genes encoding enzymes of sterol and fatty acid biosynthesis. Because synthesis of both sterols and C18 fatty acids are required for cell growth, in the absence of exogenous supplements of these lipids, we examined the hypothesis that fatty acid can also be regulatory in SREBP maturation. Our data indicate that C18 fatty acids can potentiate the biological activities of a typical, regulatory sterol: 25-hydroxycholesterol. Inhibition of C18 fatty acid synthesis in cells cultured in serum-free medium renders them resistant to killing by 25-hydroxycholesterol. Repression of expression of reporter constructs driven by promoters bearing SRE-1 element(s) by 25-hydroxycholesterol is increased by C18 fatty acid supplementation. C18 fatty acids also increase the inhibitory effect of 25-hydroxycholesterol on proteolytic maturation and nuclear localization of SREBPs. Furthermore, we also show that C18 fatty acid supplementation can enhance the inhibitory effect of 25-hydroxycholesterol on sterol and fatty acid biosynthesis. These results demonstrate that maximal down-regulation of SREBP maturation and the consequent repression of SRE-1 promoters occurs in response to both a regulatory sterol and fatty acid.

Evidence for a high affinity, saturable, prenylation-dependent p21Ha-ras binding site in plasma membranes

Oncogenic p21ras proteins can only exert their stimulation of cellular proliferation when plasma membrane-associated. This membrane association has an absolute requirement for post-translational modification with isoprenoids. The mechanism by which isoprenoids participate in the specific association of p21ras with plasma membranes is the subject of this report. We present in vitro evidence for a plasma membrane binding protein for p21(ras) that can recognize the isoprenoid substituent and, therefore, may facilitate the localization of p21ras.

Regulation of prelamin A endoprotease activity by prelamin A

The maturation of lamin A is completed by the endoproteolytic cleavage of its farnesylated precursor protein, prelamin A. In the absence of this cleavage, prelamin A can neither give rise to lamin A nor assemble into the nuclear lamina. We call the enzyme which catalyzes this endoproteolytic step the 'prelamin A endoprotease'. In this study, we begin characterization of the regulation of prelamin A endoprotease. In particular, we address the question as to whether prelamin A endoprotease activity is constitutive in cells or responds to expression of prelamin A. To do this, we compared the activity of this novel endoprotease in cells which express prelamin A with those that do not. Our data shows that the enzymatic activity of prelamin A endoprotease is enhanced by the expression of prelamin A.

In vitro assay and characterization of the farnesylation-dependent prelamin A endoprotease

The 72-kDa nuclear lamina protein lamin A is synthesized as a 74-kDa farnesylated precursor. Conversion of this precursor to mature lamin A appears to be mediated by a specific endoprotease. Prior studies of overexpressed wild-type and mutant lamin A proteins in cultured cells have indicated that the precursor possesses the typical carboxyl-terminal S-farnesylated, cysteine methyl ester and that farnesylation is required for endoproteolysis to occur. In this report, we describe the synthesis of an S-farnesyl, cysteinyl methyl ester peptide corresponding to the carboxyl-terminal 18 amino acid residues of human prelamin A. This peptide acts as a substrate for the prelamin A endoprotease in vitro, with cleavage of the synthetic peptide at the expected site between Tyr657 and Leu658. Endoproteolytic cleavage requires the S-prenylated cysteine methyl ester and, in agreement with transfection studies, is more active with the farnesylated than geranylgeranylated cysteinyl substrate. N-Acetyl farnesyl methyl cysteine is shown to be a noncompetitive inhibitor of the enzyme. Taken together, these observations suggest that there is a specific farnesyl binding site on the enzyme which is not at the active site.

Strongyloides stercoralis: identification of antigens in natural human infections from endemic areas of the United States

Using Western-blot analysis, we identified eight immunodominant antigens (apparent molecular weights 96, 86, 75, 56, 41, 32, 28, and 26 kDa) of Strongyloides stercoralis in natural human infections. For this study, 78 individual serum samples were obtained from S. stercoralis-infected patients residing in endemic areas of the United States. Poly A+ RNA was translated in vitro in the rabbit-reticulocyte lysate system. The newly synthesized translation products were immuno-precipitated with S. stercoralis human infection sera. All eight of the identified antigens were detected in the immunoprecipitates. The potential of these antigens as targets for immunodiagnosis is also discussed.

The farnesyl protein transferase inhibitor BZA-5B blocks farnesylation of nuclear lamins and p21ras but does not affect their function or localization

BZA-5B is a peptidomimetic inhibitor of protein farnesylation in mammalian cells. We have examined the specificity of this compound toward inhibition of farnesylation of p21ras and the nuclear lamin proteins, prelamin A and lamin B. We have also used the Raney nickel cleavage technique in conjunction with radio-gas liquid chromatography to assess the ability of this compound to block total protein farnesylation. These studies show that BZA-5B blocks farnesylation of the lamin proteins with an IC50 comparable to that seen for p21ras. At a concentration in excess of 25 microM, BZA-5B inhibits all protein farnesylation in CHO-K1 cells below the limits of detection. Furthermore, we found that after a 2-day exposure to high concentrations of BZA-5B, CHO-K1 cell lines exhibit no loss in sensitivity to inhibition of prenylation by this compound. Yet, despite the potent and general inhibition of protein farnesylation, BZA-5B does not interfere with a variety of cellular functions expected to be farnesylation dependent, including cell growth and viability, assembly of the nuclear lamina, membrane association of p21ras, and p21ras-dependent differentiation of PC-12 cells in response to treatment with nerve growth factor. The maintenance of farnesylation-dependent events in the presence of BZA-5B stands in marked contrast to the inhibition of the oncogenic ras-mediated transformed phenotype that has been observed with this compound and other farnesyl protein transferase inhibitors. This specificity for inhibition of ras transformation by BZA-5B is quite encouraging to its eventual development as an antimalignancy pharmaceutical.

Expression of prelamin A but not mature lamin A confers sensitivity of DNA biosynthesis to lovastatin on F9 teratocarcinoma cells

The role of inhibition of prelamin A processing in the inhibition of DNA synthesis by lovastatin was examined by expressing prelamin A in F9 teratocarcinoma cells. These cells, normally lacking expression of the A/C lamins, were transfected with constructs expressing either prelamin A or mature lamin A and the effect of lovastatin on DNA biosynthesis was assessed. It was found that expression of prelamin A specifically conferred sensitivity to inhibition of DNA biosynthesis by lovastatin on F9 cells.

An antibody which specifically recognizes prelamin A but not mature lamin A: application to detection of blocks in farnesylation-dependent protein processing

A polyclonal antibody [anti-prelamin A antibody (alpha-PA)] has been obtained against the peptide LLGNSSPRTQSPQN which is proteolytically removed during the farnesylation-dependent processing of prelamin A to mature lamin A. We tested the ability of this antibody to detect inhibition of farnesylation-dependent protein processing of prelamin A. The alpha-PA antibody was shown to immunoprecipitate prelamin A from lovastatin-treated HeLa cells but not mature lamin A from untreated cells. Further studies were performed after antigen-affinity chromatographic purification of the antibody. Western blotting of lovastatin-treated HeLa cell extract demonstrated that the purified alpha-PA antibody recognizes prelamin A. Furthermore, this signal could be competed away by incubation with the peptide. Indirect immunofluorescence helped detect nuclear accumulation of the antigen in response to treatment of HeLa cells with lovastatin or in Chinese hamster ovary K1 cells transiently transfected with a prelamin A mutant blocked in farnesylation. This antibody should be useful for screening compounds that may block any of the three common steps in the farnesylation-dependent processing of proteins (farnesylation, endoproteolysis, and carboxymethylation) since it appears that prelamin A undergoes all of these reactions prior to removal of the antigenic peptide. Inhibitors of these reactions have been proposed as potential anticancer drugs, since they would be expected to block the biological activity of oncogenic p21ras proteins. Since such screening would be performed most efficiently by enzyme-linked immunosorbent assays, we can detect the accumulation of prelamin A after treatment with lovastatin by performing this procedure as well. Application of alpha-PA in an enzyme-linked immunosorbent assay, which demonstrates the activity of a peptidomimetic farnesyltransferase inhibitor, supports the use of this antibody in large scale screening for inhibitors of farnesylation-dependent protein processing.

The processing pathway of prelamin A

The conversion of mammalian prelamin A to mature lamin A proceeds through the removal of 18 amino acids from the carboxyl terminus. The initial step in this processing is the isoprenylation of a CAAX box cysteine. This proteolytic event is distinctive for prelamin A among the known prenylated mammalian proteins. Since the carboxyl terminus of prelamin A is removed during maturation, it is not obvious that this protein would undergo the two reactions subsequent to prenylation observed in other CAAX box proteins--the endoproteolytic removal of the carboxyl-terminal 3 amino acids and the subsequent methylation of the now carboxyl-terminal cysteine. To characterize the maturation of prelamin A further, we have developed a CHO-K1 cell line that possesses a dexamethasone-inducible human prelamin A against a genetic background of high mevalonate uptake. Utilizing this cell line in association with antibodies specific to the transgenic prelamin A, we have been able to demonstrate directly in vivo that prelamin A undergoes farnesylation and carboxymethylation prior to conversion to lamin A, as is the case for other prenylated proteins. We have demonstrated previously that in the absence of isoprenylation, conversion of prelamin A to lamin A is blocked, but that unprocessed prelamin A is transported to the nucleus where it can still undergo maturation. Consistent with the implications of these prior studies, we now demonstrate the presence of both subunits of farnesyl-protein transferase in the nucleus.

Ras (CXXX) and Rab (CC/CXC) prenylation signal sequences are unique and functionally distinct

Rab proteins typically lack the consensus carboxyl-terminal CXXX motif that signals isoprenoid modification of Ras and other isoprenylated proteins and, instead, terminate in either CC or CXC sequences (C = cysteine, X = any amino acid). To compare the functional relationship between the Ras CXXX and the Rab CC/CXC motifs, we have generated chimeric Ras proteins terminating in Rab carboxyl-terminal CC or CXC sequences. These mutant Ras proteins were not isoprenylated in vitro or in vivo, demonstrating that the CC and CXC sequences alone are not sufficient to replace a CXXX sequence to signal Ras isoprenoid modification. Surprisingly, chimeric Ras/Rab proteins terminating in significant lengths of carboxyl-terminal sequences from Rab1b (7-139 residues), Rab2 (5-151 residues), or Rab3a (12 residues) were also not isoprenylated. These results demonstrate that the sequence requirements for isoprenoid modification of Rab proteins are more complex than the simple tetrapeptide CXXX sequence for isoprenoid modification of Ras proteins and suggest that the Rab geranylgeranyl transferase(s) requires recognition of protein conformation to signal the addition of geranylgeranyl groups. Finally, competition studies demonstrate that a common geranylgeranyl transferase activity is responsible for the modification of Rab proteins terminating in CC or CXC motifs.

Post-transcriptional regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by 24(S),25-oxidolanosterol

We have examined the mechanism of regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by 24(S),25-oxidolanosterol, a C30-sterol naturally occurring in mammalian tissues. In the absence of enzymatic demethylation to the C27-sterol, 24(S),25-epoxycholesterol, oxidolanosterol is shown to be a post-transcriptional regulator of reductase synthesis in both primary rat hepatocytes and Chinese hamster ovary cells. Under these conditions, oxidolanosterol also increases the rate of degradation of reductase protein in these cells. When demethylation is not inhibited, oxidolanosterol treatment produces transcriptional regulation of sterol-sensitive genes in Chinese hamster ovary cells. In contrast to previous findings with the oxygenated C27-sterol, 25-hydroxycholesterol, oxidolanosterol can act as a post-transcriptional regulator in cells starved for mevalonate. These findings are consistent with the hypothesis that oxidolanosterol down-regulates sterol synthesis in a fashion mechanistically distinct from that of C27-sterols.

Feedback inhibition of polyisoprenyl pyrophosphate synthesis from mevalonate in vitro. Implications for protein prenylation

The prenylation of proteins utilizes the polyisoprenyl pyrophosphates (FPP) and geranylgeranyl pyrophosphate (GGPP) as prenyl donors. These polyisoprenoids are also precursors to ubiquinone and dolichol synthesis. We have previously described the geranylgeranylation of rab 1b from labeled mevalonate in rabbit reticulocyte lysates (Khosravi-Far, R., Lutz, R. J., Cox, A. D., Conroy, L., Bourne, J. R., Sinensky, M., Balch, W. E., Buss, J. C., and Der, C. J. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 6264-6268). We now directly demonstrate the incorporation of mevalonate into FPP and GGPP in rabbit reticulocyte cytosol. High pressure liquid chromatography analysis reveals that only all-trans-E,E,E-GGPP, the prenyl donor for in vivo protein geranylgeranylation, is synthesized. Incubations with recombinant H-ras and rab1b result in an increased synthesis of farnesyl and geranylgeranyl derivatives, respectively. The increase is wholly accounted for by protein-incorporated polyisoprenoids with no change in the polyisoprenyl pyrophosphate pools. Further, GGPP inhibits its own synthesis, without affecting FPP synthesis, with half-maximal inhibition at approximately 3 microM GGPP. Inhibition of FPP synthesis by the inhibition of isopentenyl isomerase causes a dramatic increase in isopentenyl pyrophosphate synthesis. FPP also inhibits conversion of mevalonate into FPP. These findings indicate that these polyisoprenyl pyrophosphates can down-regulate their own synthesis in vitro, and this regulation may control the levels of these polyisoprenoids in vivo.

Nucleoplasmic localization of prelamin A: implications for prenylation-dependent lamin A assembly into the nuclear lamina

The synthesis of the nuclear lamina protein lamin A requires the prenylation-dependent processing of its precursor protein, prelamin A. Unlike p21ras, which undergoes similar initial posttranslational modifications, maturation of lamin A results in the proteolytic removal of the prenylated portion of the molecule. We have used an in vitro prenylation system to demonstrate the nature of the prenyl substituent on prelamin A to be a farnesyl group. Further, the in vitro farnesylation of prelamin A requires an intact cysteine-aliphatic-aliphatic-other (CAAX) amino acid sequence motif at its carboxyl terminus. The effect of blocking the prenylation of prelamin A on its localization and assembly into the nuclear lamina was investigated by indirect immunofluorescence. Expression of wild-type prelamin A in lovastatin-treated cells showed that nonprenylated prelamin A accumulated as nucleoplasmic particles. Upon addition of mevalonate to lovastatin-treated cells, the wild-type lamin A was incorporated into the lamina within 3 hr. Expression of a mutant lamin A in which the carboxyl-terminal 21 amino acids were deleted resulted in a lamin molecule that was directly assembled into the lamina. These results indicate that the carboxyl-terminal peptide of prelamin A blocks its proper assembly into the nuclear lamina and that the prenylation-initiated removal of this peptide can occur in the nucleus.

p23rab2, a ras-like GTPase with a -GGGCC C-terminus, is isoprenylated but not detectably carboxymethylated in NIH3T3 cells

With the development of a specific anti-rab2 antiserum, p23rab2, a ras-like GTPase with a -GGGCC C-terminus, has been localized mainly to the particulate (P100) fraction in NIH3T3 cells, although a small amount of this protein also appears in the soluble fraction. The endogenous p23rab2 is isoprenylated in intact cells, and recombinant murine p23rab2 is also isoprenylated in an in vitro system using reticulocyte lysate. Both the cytosolic and membrane-bound forms of p23rab2 are isoprenylated in intact cells. Recombinant p23rab2 is specifically geranylgeranylated at one or both of the C-terminal cysteine residues in the in vitro system. Blocking isoprenoid synthesis with lovastatin results in an accumulation of a totally cytosolic unisoprenylated form, indicating that isoprenylation is a prerequisite for membrane association of p23rab2. Surprisingly, unlike p21ras and p25rab3A, no carboxymethylation of p23rab2 is detectable in either the soluble or particulate fractions.

The prenylation of proteins

The prenylated proteins represent a newly discovered class of post-translationally modified proteins. The known prenylated proteins include the oncogene product p21ras and other low molecular weight GTP-binding proteins, the nuclear lamins, and the gamma subunit of the heterotrimeric G proteins. The modification involves the covalent attachment of a 15-carbon (farnesyl) or 20-carbon (geranylgeranyl) isoprenoid moiety in a thioether linkage to carboxyl terminal cysteine. The nature of the attached substituent is dependent on specific sequence information in the carboxyl terminus of the protein. In addition, prenylation entrains other posttranslational modifications forming a reaction pathway. In this article, we review our current understanding of the biochemical reactions involved in prenylation and discuss the possible role of this modification in the control of cellular functions such as protein maturation and cell growth.

Isoprenoid modification of rab proteins terminating in CC or CXC motifs

Mevalonate starvation of hamster fibroblasts resulted in a shift of rab1b from the membrane to the cytosolic fraction, suggesting that rab1b depends upon an isoprenoid modification for its membrane localization. rab1b and rab3a proteins expressed in insect cells incorporated a product of [3H]mevalonate, and gas chromatography analysis of material released by Raney nickel cleavage demonstrated that rab1b and rab3a are modified by geranylgeranyl groups. Additionally, in vitro prenylation analysis demonstrated farnesyl modification of H-ras but geranylgeranyl modification of five rab proteins (1a, 1b, 2, 3a, and 6). Together, these results suggest that the carboxyl-terminal CC/CXC motifs (X = any amino acid) specifically signal for addition of geranylgeranyl, but not farnesyl, groups. A rab1b mutant protein lacking the two carboxyl-terminal cysteine residues was not prenylated in vitro. However, since a mutant H-ras protein that terminates with tandem cysteine residues was also not modified, the CC motif may be essential, but not sufficient, to signal prenylation of rab1b. Finally, rab1b and rab3a proteins were not efficient substrates for either farnesyl- or geranylgeranyltransferase activities that modify CAAX-containing proteins (A = any aliphatic amino acid). Therefore, rab proteins may be modified by a prenyltransferase(s) distinct from the prenyltransferases that modify carboxyl-terminal CAAX proteins.

Differential inhibitory effects of lovastatin on protein isoprenylation and sterol synthesis

It has been reported that when 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors are utilized for treatment of hypercholesterolemia, as much as 50% inhibition of whole body cholesterol biosynthesis is observed. As general inhibitors of isoprenoid biosynthesis, these compounds can also inhibit the synthesis of the substituents of isoprenylated proteins. For two mammalian proteins (p21ras and lamin A), it has been demonstrated that such inhibition of biosynthesis of the isoprenoid substituent blocks proteolytic maturation of these proteins. It has been argued that advantage may be taken of this phenomenon to block the synthesis of p21ras in malignancies. It is also possible that treatment of hypercholesterolemia with lovastatin might produce problematic inhibition of protein processing dependent upon isoprenylation. In this report, we compare the concentration dependence of inhibition of isoprenylation dependent protein processing and sterol biosynthesis. Effects of partial inhibition of isoprenylated protein processing on whole cells can be sensitively assessed by visualization of lamina structure through indirect immunofluorescence. Our results indicate that the degree of inhibition of p21ras and prelamin A maturation by lovastatin is identical. Thus, 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors are unlikely to be useful as anti-malignancy drugs. However, the conditions of lovastatin treatment which produce 50% inhibition of sterol biosynthesis analogous to pharmacological conditions, produce no observable effects on isoprenylated protein maturation.

Defective elongation of fatty acids in a recessive 25-hydroxycholesterol-resistant mutant cell line

The Chinese hamster ovary recessive mutant, crB, has been selected for its resistance to the cytotoxic effects of 25-hydroxycholesterol in sterol-free media (Sinensky, M., Logel, J., and Torget, R. (1982) J. Cell. Physiol. 113, 314-319). Growth of crB in a chemically defined lipid-poor medium is very slow and is enhanced by a mixture of saturated and unsaturated fatty acids. Incorporation of [3H]acetate into total fatty acids is 4-fold lower in crB compared to that in parental Chinese hamster ovary K1 and in contrast to the wild-type cells, crB cells are unable to synthesize either stearate or oleate. In addition, crB cells can not elongate exogenous palmitate, while they are capable of desaturating exogenous stearate. The mutant cells are also pleiotropically defective in the regulation of mRNA levels for the enzymes of cholesterol biosynthesis. 25-Hydroxycholesterol is a poor regulator of the synthesis and degradation of the rate-limiting enzyme, 3-hydroxy-3-methylglutaryl-coenzyme A reductase in crB in comparison to the wild-type Chinese hamster ovary K1 cells. The defect in the elongation of fatty acids is reversed in revertants of crB selected for their ability to grow in lipid-poor medium. Such revertants exhibit normal regulation of 3-hydroxy-3-methylglutaryl-CoA reductase activity by 25-hydroxycholesterol. Regulation of reductase activity in crB cells can also be restored by supplementing the culture medium with a mixture of fatty acids that restores normal growth rate. The defective regulation of reductase in crB does not appear to be due to nonspecific adverse effects of fatty acid starvation nor is it due to any gross change in the fatty acid composition of cellular phospholipids. These results strongly suggest a direct relationship between the fatty acid auxotrophy of crB and defective regulation of the enzymes of cholesterol biosynthesis.

Isoprenylation is required for the processing of the lamin A precursor

The nuclear lamina proteins, prelamin A, lamin B, and a 70-kD lamina-associated protein, are posttranslationally modified by a metabolite derived from mevalonate. This modification can be inhibited by treatment with (3-R,S)-3-fluoromevalonate, demonstrating that it is isoprenoid in nature. We have examined the association between isoprenoid metabolism and processing of the lamin A precursor in human and hamster cells. Inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase by mevinolin (lovastatin) specifically depletes endogenous isoprenoid pools and inhibits the conversion of prelamin A to lamin A. Prelamin A processing is also blocked by mevalonate starvation of Mev-1, a CHO cell line auxotrophic for mevalonate. Moreover, inhibition of prelamin A processing by mevinolin treatment is rapidly reversed by the addition of exogenous mevalonate. Processing of prelamin A is, therefore, dependent on isoprenoid metabolism. Analysis of the conversion of prelamin A to lamin A by two independent methods, immunoprecipitation and two-dimensional nonequilibrium pH gel electrophoresis, demonstrates that a precursor-product relationship exists between prelamin A and lamin A. Analysis of R,S-[5-3H(N)]mevalonate-labeled cells shows that the rate of turnover of the isoprenoid group from prelamin A is comparable to the rate of conversion of prelamin A to lamin A. These results suggest that during the proteolytic maturation of prelamin A, the isoprenylated moiety is lost. A significant difference between prelamin A processing, and that of p21ras and the B-type lamins that undergo isoprenylation-dependent proteolytic maturation, is that the mature form of lamin A is no longer isoprenylated.

Inhibition of isoprenoid biosynthesis and the post-translational modification of pro-p21

It has recently been reported that a precursor of p21ras (pro-p21ras) becomes modified by a metabolite of mevalonic acid prior to conversion to mature p21ras. We have examined the effect of blocking isoprenoid biosynthesis on this process. Fluoromevalonate, which inhibits the conversion of pyrophosphomevalonate to isopentenyl pyrophosphate, blocks the incorporation of radioactive mevalonate into pro-p21ras, demonstrating the mevalonate must be converted to an isoprenoid prior to such incorporation. Starvation of CHO-K1 cells for mevalonic acid by treatment with mevinolin, an inhibitor of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, or mevalonate deprivation in a mevalonate auxotroph defective in HMG-CoA synthase activity results in the accumulation of pro-p21ras. The precursor, accumulated due to either of these treatments, is converted through an intermediate form to the mature p21ras by incubation of cells with mevalonate. Incubation of cells with 25-hydroxycholesterol, the pleiotropic transcriptional down-regulator of cholesterol biosynthesis does not, however, result in the accumulation of pro-p21ras. This result indicates that in contrast to the regulation of cholesterol biosynthesis in mammalian cells, important regulatory control other than at the level of HMG-CoA reductase is involved in the isoprenoid biosynthesis required for protein isoprenylation.

Sterol-independent regulation of 3-hydroxy-3-methylglutaryl-CoA reductase by mevalonate in Chinese hamster ovary cells. Magnitude and specificity

In this paper, we assess the relative degree of regulation of the rate-limiting enzyme of isoprenoid biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, by sterol and nonsterol products of mevalonate by utilizing cultured Chinese hamster ovary cells blocked in sterol synthesis. We also examine the two other enzymes of mevalonate biosynthesis, acetoacetyl-CoA thiolase and HMG-CoA synthase, for regulation by mevalonate supplements. These studies indicate that in proliferating fibroblasts, treatment with mevalonic acid can produce a suppression of HMG-CoA reductase activity similar to magnitude to that caused by oxygenated sterols. In contrast, HMG-CoA synthase and acetoacetyl-CoA thiolase are only weakly regulated by mevalonate when compared with 25-hydroxycholesterol. Furthermore, neither HMG-CoA synthase nor acetoacetyl-CoA thiolase exhibits the multivalent control response by sterol and mevalonate supplements in the absence of endogenous mevalonate synthesis which is characteristic of nonsterol regulation of HMG-CoA reductase. These observations suggest that nonsterol regulation of HMG-CoA reductase is specific to that enzyme in contrast to the pleiotropic regulation of enzymes of sterol biosynthesis observed with oxygenated sterols. In Chinese hamster ovary cells supplemented with mevalonate at concentrations that are inhibitory to reductase activity, at least 80% of the inhibition appears to be mediated by nonsterol products of mevalonate. In addition, feed-back regulation of HMG-CoA reductase by endogenously synthesized nonsterol isoprenoids in the absence of exogenous sterol or mevalonate supplements also produces a 70% inhibition of the enzyme activity.

Treatment of CHO-K1 cells with 25-hydroxycholesterol produces a more rapid loss of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity than can be accounted for by enzyme turnover

A key enzyme in the regulation of mammalian cellular cholesterol biosynthesis is 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). It is well established that treatment with the compound 25-hydroxycholesterol lowers HMG-CoA reductase activity in cultured Chinese hamster ovary (CHO-K1) cells. After brief incubation (0-4 h) with 25-hydroxycholesterol (0.5 microgram/ml), cellular HMG-CoA reductase activity is decreased to 40% of its original level. This also occurs in the presence of exogenous mevinolin, a competitive inhibitor of HMG-CoA reductase which has previously been shown to inhibit its degradation. The inhibition of HMG-CoA reductase activity by 25-hydroxycholesterol is complete after 2 h. Radio-immune precipitation analysis of the native enzyme under these conditions shows a degradation half-life which is considerably longer than that of the observed inhibition. Studies with sodium fluoride, phosphatase 2A, bacterial alkaline phosphatase and calf alkaline phosphatase indicate that the observed loss of activity is not due to phosphorylation. These data are not consistent with described mechanisms of HMG-CoA reductase activity regulation by phosphorylation or degradation but are consistent with a novel mechanism that regulates the catalytic efficiency of this enzyme.

Further characterization of a somatic cell mutant defective in regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase

Two enzymes of mammalian cellular mevalonate biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase and HMG-CoA reductase, have been shown to be regulated by exogenous sterols. It has been demonstrated that these enzymes are regulated, at least in part, by transcriptional control of their synthesis. We have previously described a somatic cell mutant (CR1) of the CHO-K1 cell line that is defective in regulation of the activity of these enzymes in response to exogenous sterols. In this report, we demonstrate that this mutant is defective in regulation of the mRNA levels for HMG-CoA reductase and HMG-CoA synthase by 25-hydroxycholesterol and mevinolin. In the case of HMG-CoA reductase, this loss of apparent transcriptional control is not accompanied by a comparable loss in regulation of synthesis of this enzyme. This observation is consistent with prior studies suggesting that HMG-CoA reductase can be regulated translationally. We also show that CR1 cells exhibit a constitutively rapid rate of degradation of HMG-CoA reductase.

Incorporation of a product of mevalonic acid metabolism into proteins of Chinese hamster ovary cell nuclei

We have examined the nuclear localization of isoprenylated proteins in CHO-K1 cells labeled with [14C]mevalonate. Nuclear proteins of 68, 70, and 74 kD, posttranslationally modified by an isoprenoid, are also components of a nuclear matrix-intermediate filament preparation from CHO cells. Furthermore, the 68-, 70-, and 74-kD isoprenylated polypeptides are immunoprecipitated from cell extracts with two different anti-lamin antisera. Based on exact two-dimensional comigration with lamin B, both from rat liver lamin and CHO nuclear matrix-intermediate filament preparations, and its immunoprecipitation with anti-lamin antisera, we conclude that the 68-kD isoprenylated protein found in nuclei from [14C]mevalonate-labeled CHO cells is lamin B. The more basic 74-kD isoprenylated nuclear protein is similar in molecular mass and isoelectric pH variants to the lamin A precursor polypeptide reported by others. Starving cells for mevalonate results in a dramatic accumulation of a polypeptide that comigrates on two-dimensional, non-equilibrium pH gradient electrophoresis (NEPHGE) gels with the 74-kD isoprenylated protein. The 70-kD isoprenylated protein, which is resolved on NEPHGE gels as being higher in molecular mass and slightly more basic than lamin B, has not yet been identified.

Somatic cell genetics and the study of cholesterol metabolism

The regulation of cholesterol biosynthesis by extracellular cholesterol occurs both in whole animal tissue and in permanent somatic cell lines in culture. Permanent mammalian cells lines, under optimized growth conditions, are easily manipulated both biochemically and genetically. The Chinese hamster ovary cell line (CHO-K1) is the most widely used cell line for genetic studies. CHO-K1 is a pseudo-diploid mammalian cell exhibiting a short doubling time and a relatively high plating efficiency. Somatic cell mutants can be generated through mutagenesis and also by drug adaptation. Following mutagenesis, auxotrophs may be isolated either by selection or by screening. Most selection procedures for mutants of cholesterol metabolism must be done in serum depleted of cholesterol which requires the endogenous biosynthetic pathway to be intact. Mutants failing to produce cholesterol do not replicate their DNA and exhibit reduced concentrations of cholesterol in their membranes. BUdR and polyene antibiotics have both been used to select against the wild-type cells which incorporate these compounds and are killed, allowing the survival of the mutant cells. Both mevalonate and cholesterol auxotrophs have been isolated with the BUdR technique and have proven useful for elucidation of the early steps in cholesterol biosynthesis, particularly for the ratelimiting enzyme HMG-CoA reductase. Somatic cell fusion of a mutant and wild-type cell followed by chromosomal segregation, routinely used to map human genes, has also been used to map the human gene for HMG-CoA synthase. Such hybrids also provide valuable information on the dominance or recessivity of a specific lesion. DNA-mediated gene transfer into somatic cell mutants allows the selection of DNA sequences which complement the mutation, and is also useful for analysis of regions of regulatory significance. Mutants, resistant to the regulatory effects of oxygenated sterols, can be isolated following mutagenesis. Mutants of this type vary the lipid content of their membranes in response to cholesterol concentration in the medium. All such mutants tested exhibit a pleiotropic regulatory effect on more than one enzyme in the cholesterol biosynthetic pathway. Adaptation to drugs such as compactin and mevinolin, which inhibit HMG-CoA reductase, have been used to produce mutants which overexpress enzymes in the pathway. These amplified cells are useful sources of specific mRNAs for construction of cDNA libraries and gene isolation. Structure-function relationships of membrane sterols can be studied in cholesterol auxotrophs where changes in acyl-chain ordering can be manipulated by exogenous sterols in the medium.

Endogenous sterol synthesis is not required for regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by low density lipoprotein

It has been proposed that an endogenously synthesized oxysterol mediates the regulation of cholesterol biosynthesis by low density lipoprotein in cultured mammalian cells. Studies in this report demonstrate that under conditions in which squalene conversion to sterols is blocked either by inhibition of squalene cyclization or lanosterol demethylation, or both, low density lipoprotein regulates 3-hydroxy-3-methylglutaryl coenzyme A reductase normally. These observations rule out the hypotheses that either an endogenously synthesized oxygenated cholesterol biosynthetic intermediate or epoxysterol is required to mediate the inhibition of this enzyme by low density lipoprotein.

Characterization of HMG CoA synthase activity of rat liver and CHO-K1 cells

This report describes the characterization and partial purification of rat liver 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase activity. A preliminary characterization of Chinese hamster ovary (CHO) cell HMG CoA synthase activity is also presented. Ion-exchange chromatography of ammonium sulfate precipitates of rat liver cytosol indicate the existence of two isoenzymes of HMG CoA synthase. These isoenzymes are physically, catalytically, and immunologically distinct. One of these isoenzymes, peak 1, resembles mitochondrial HMG-CoA synthase activity as evidenced by similarities in elution upon ion-exchange chromatography, inhibition by MgCl2, and cross reactivity with an antibody prepared against the mitochondrial enzyme. As peak 1 activity is unstable, further purification studies were performed on peak 2 activity. Peak 2 can be further resolved into two activities (peaks 2A and 2B) by gel filtration. In contrast, CHO-K1 cells (a permanent fibroblast line) possess only peak 2 type HMG CoA synthase activity.

Requirement for 24(S),25-epoxycholesterol for the viability of cultured fibroblasts

Previous studies on a somatic cell mutant auxotrophic for mevalonate (Mev-1) have shown that these cells rapidly lose viability when deprived of mevalonic acid in culture medium supplemented with serum cholesterol. Testing of all known end products of mevalonate metabolism in cultured mammalian cells has been conducted to determine the basis for this mevalonate requirement. It has been found that the recently discovered mevalonate metabolite 24(S),25-epoxycholesterol produces a partial restoration of viability of Mev-1 cells starved for mevalonate, whereas other structurally similar oxysterols do not. It appears that 24(S),25-epoxycholesterol has a specific, vital cellular function in CHO-K1 cells.

Localization of the gene encoding 3-hydroxy-3-methylglutaryl-coenzyme A synthase to human chromosome 5

A series of hybrids between primary human cells and a Chinese hamster somatic cell mutant (Mev-1), defective in expression of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase [(S)-3-hydroxy-3-methylglutaryl-CoA acetoacetyl-CoA-lyase (CoA-acetylating, EC 4.1.3.5], has been prepared that complements the mutant defect. A technique based on differential sensitivity of this enzyme activity to inhibition by magnesium ion is described that allows the discrimination of expression of human and hamster HMG-CoA synthase in these hybrids. The results indicate a structural gene defect in expression of HMG-CoA synthase activity in Mev-1 cells. Segregation of human chromosomes that do not possess the complementing marker have allowed the assignment of human HMG-CoA synthase activity to chromosome 5. This is the second demonstrably transcriptionally regulated enzyme of cholesterologenesis to be assigned to chromosome 5, the other being HMG-CoA reductase.

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase synthesis by a non-sterol mevalonate-derived product in Mev-1 cells. Apparent translational control

A somatic cell mutant (Mev-1) auxotrophic for mevalonate by virtue of a complete lack of detectable 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase activity has been shown to demonstrate a requirement for a non-sterol mevalonate-derived product for regulation of synthesis of HMG-CoA reductase. A comparison of the effects of 25-hydroxycholesterol and the combination of 25-hydroxycholesterol and mevalonate on HMG-CoA reductase activity, synthesis, and mRNA levels in Mev-1 is presented in this report. The results show a close correlation between activity, rate of synthesis, and mRNA levels for Mev-1 cells treated with 25-hydroxycholesterol alone. Under the conditions of these experiments these effects are relatively small (approximately a 4-fold decrease). A much larger inhibition of HMG-CoA reductase activity and rate of synthesis (approximately 50-fold) is observed upon treatment of Mev-1 cells with a combination of 25-hydroxycholesterol and mevalonate. Yet, under these conditions mRNA levels are still reduced by only a factor of 4. These results are interpreted to suggest that the non-sterol mevalonate-derived regulatory product of HMG-CoA reductase acts by a translational control mechanism.

A somatic cell mutant with a kinetically altered 3-hydroxy-3-methylglutaryl coenzyme A reductase

A somatic cell mutant has been isolated which is resistant to killing and growth inhibition by mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. The resistance phenotype is dominant with respect to the wild-type cell and can largely be ascribed to a 6-7-fold lowering of the KM for HMG-CoA. We thus conclude that mevinolin resistance can be utilized to obtain a genetic marker for the structural gene encoding HMH-CoA reductase.

Defective macromolecule biosynthesis and cell-cycle progression in a mammalian cell starved for mevalonate

The isolation of a somatic cell mutant (Mev-1) with a block in one of the mevalonate-biosynthesizing enzymes (3-hydroxy-3-methylglutaryl-coenzyme A synthase, EC 4.1.3.5) has afforded us the opportunity to test and to extend the hypothesis that a product of mevalonate biosynthesis other than cholesterol is required for cellular proliferation. We present evidence here that both DNA synthesis and protein synthesis are inhibited in this mutant by mevalonate starvation, although RNA synthesis appears to be unaffected. The loss of DNA synthesis and the loss of protein synthesis in this mutant appear to be due to independent processes. DNA synthesis is reversibly inhibited by mevalonate starvation at a unique point in the cell cycle. Resumption of DNA synthesis after readdition of mevalonate exhibits a long lag; the peak of S-phase DNA synthesis occurs approximately 17 hr after mevalonate readdition, suggesting that mevalonate starvation puts cells into a quiescent (G0) state owing to their failure to transit a restriction point. The loss of DNA biosynthesis in the Mev-1 cell is well correlated with the rate of turnover of mevalonate label of certain terpenylated polypeptides.

Analysis of the coordinate expression of 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase activities in Chinese hamster ovary fibroblasts

Decreased activities of both 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase and HMG CoA reductase are observed in the presence of sterol in the Chinese hamster ovary (CHO) fibroblast. In three different genotypes of CHO cell mutants resistant to 25-hydroxycholesterol both enzyme activities exhibit a decreased response to 25-hydroxycholesterol compared to wild-type cells. Permanently repressed levels of both HMG CoA synthase and HMG CoA reductase activities are observed in another CHO mutant, phenotypically a mevalonate auxotroph. Mevinolin, a competitive inhibitor of HMG CoA reductase, has no effect on HMG CoA synthase activity measured in vitro. Incubation of CHO cells with sublethal concentrations of mevinolin produces an inhibition of the conversion of [14C]acetate to cholesterol and results in elevated levels of both HMG CoA synthase and HMG CoA reductase activities. Studies of CHO cells in sterol-free medium supplemented with cycloheximide indicate that continuous protein synthesis is not required for the maximal expression of HMG CoA synthase activity and provide an explanation for the lack of temporal similarity between HMG CoA synthase and reductase activities after derepression. These results support the hypothesis of a common mode of regulation for HMG CoA synthase and HMG CoA reductase activities in CHO fibroblasts.

Inhibition of degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by mevinolin

Mevinolin is demonstrated to inhibit the degradation of 3-hydroxy-3-methylglutaryl-CoA reductase in Chinese hamster ovary cells. Evidence is presented that this effect of mevinolin is not general to degradation of total cellular proteins and is not due to the block to mevalonate synthesis produced by this compound.

Complementary recessive 25-hydroxycholesterol-resistant somatic cell mutants--assay of 25-hydroxycholesterol binding activity

Complementation analysis of recessive 25-hydroxycholesterol-resistant mutants of the CHO-Kl cell shows the existence of at least two complementation groups, one of which is missing a binding activity for 25-hydroxycholesterol. Both complementation groups are shown to be refractory to inhibition of cellular HMG-CoA reductase activity and in the inhibition of biosynthesis of this enzyme by 25-hydroxycholesterol.

Analysis of regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in a somatic cell mutant auxotrophic for mevalonate

Regulation of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activity and synthesis by 25-hydroxycholesterol is aberrant in a somatic cell mutant of the Chinese hamster ovary K1 cell auxotrophic for mevalonate by virtue of a defect in HMG-CoA synthase activity. Normal regulation of HMG-CoA reductase activity and synthesis in this mutant by 25-hydroxycholesterol can be restored by simultaneous incubation with a small (0.4 mM) mevalonate supplement. Normal regulation of HMG-CoA reductase is also observed in a revertant of the mutant cell with normal HMG-CoA synthase activity.