Evidence for modification of lamin B by a product of mevalonic acid

Protein prenylation in spinach chloroplasts

Protein prenylation in plants was studied by in vivo metabolic (3)H-mevalonate labeling in combination with a range of protein synthesis inhibitors. In spinach cotyledons, this posttranslational protein modification was found to be divided into two categories, one representing the conventional prenylation involving farnesyl and geranylgeranyl groups bound to cysteine residues via thioether linkages. This category revealed a similar pattern of prenylated proteins to that observed in mammalian cells and depends on nuclear gene expression. The other category was shown to represent a type of prenylation confined to chloroplasts. It depends on plastid gene expression and does not involve a thioether bond. The modifying isoprenoid could be released from the chloroplastic polypeptides by alkaline treatment and was identified as phytol upon GC-MS analysis. The phytol could readily be derived from all-trans-[(3)H]farnesol, which, like all-trans-[(3)H]geranylgeraniol, was taken up by the cotyledons, resulting in incorporation of radiolabel into proteins.

Subcellular distribution of farnesyl protein transferase in rat liver

Farnesyl protein transferase (FPT) activity was measured in rat liver subcellular fractions by using an unspecific acceptor for the farnesyl groups. The highest specific activity was found in mitochondria and it exceeded that of the microsomes three-fold. Considerably lower specific activities were found in the nuclei and cytosol. Further subfractionation revealed that the mitochondrial FPT activity is located in the matrix. The beta-subunit of the mitochondrial enzyme has an apparent molecular mass of 46 kDa, which is similar to its cytosolic counterpart. The results suggest that protein farnesylation can take place in a number of subcellular organelles.

Inhibition of cholesterol production but not of nonsterol isoprenoid products induces neuronal cell death

Deficiency of nonsterol isoprenoids, intermediate metabolites of the cholesterol biosynthetic pathway, has been known to cause an inhibition of DNA synthesis and cell growth, and to induce apoptosis in nonneuronal cells. To investigate whether this is also the case in neurons, we examined the effect of a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor on the viability of neuronal cultures prepared from fetal rat brains. Treatment with compactin, a competitive inhibitor of HMG-CoA reductase, induced neuronal death in a dose-dependent manner. Concurrent treatment with cholesterol, beta-migrating very low density lipoprotein, mevalonate, or squalene substantially inhibited the induction of neuronal death by compactin. Cell death was also induced by treatment with squalestatin, which specifically inhibits cholesterol biosynthesis at a site downstream from the generation of nonsterol metabolites. Furthermore, squalestatin-induced neuronal death was inhibited by concurrent incubation with squalene but not mevalonate. In contrast, cell growth of proliferating cells such as NIH 3T3 and PC12 cells was exclusively dependent on the level of nonsterol isoprenoid products and not that of cholesterol. The results of this study clearly indicate that the viability of neurons, different from that of nonneuronal cells, depends on the intracellular cholesterol content and not on the intermediate nonsterol isoprenoid products.

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.

In vivo prenylation of rat proteins: modification of proteins with penta- and hexaprenyl groups

In vivo protein prenylation was studied in newborn rats by repeated injections of [3H]mevalonate. The highest level of protein-bound mevalonate metabolites was found in the kidney, but incorporation was observed in all organs studied. After fluorography of SDS-polyacrylamide gel electrophoresis-separated polypeptides, labeling was found in the 21- to 28-kDa molecular mass region and, after prolonged exposure of the film, additional bands at both higher and lower molecular masses could be detected. Protein prenylation in the kidney increased during the first 5 days after birth, whereas that in the liver reached a maximum on the fourth day. After methyliodide treatment of the prenylated proteins, farnesol, geranylgeraniol, and two larger isoprenoids, pentaprenol and hexaprenol, were released. In the liver, the ratio of protein-bound geranylgeraniol to farnesol increased from 2 to 4.5 during the first 5 days after birth. Upon subfractionation of the kidney, the highest level of labeling was found in mitochondria and microsomes. When the mitochondria were subfractionated into outer membranes, intermembrane space and an inner membrane/matrix fraction, the labeling pattern of prenylated polypeptides differed in all fractions. The results demonstrate that in vivo labeling of rats can be performed to study the extent, type, and distribution of protein prenylation.

Nuclear lamins: their structure, assembly, and interactions

Nuclear lamins are intermediate filament-type proteins that are the major building blocks of the nuclear lamina, a fibrous proteinaceous meshwork underlying the inner nuclear membrane. Lamins can also be localized in the nuclear interior, in a diffuse or spotted pattern. Nuclei assembled in vitro in the absence of lamins are fragile, indicating that lamins mechanically stabilize the cell nucleus. Available evidence also indicates a role for lamins in DNA replication, chromatin organization, spatial arrangement of nuclear pore complexes, nuclear growth, and anchorage of nuclear envelope proteins. In this review we summarize the current state of knowledge on the structure, assembly, and possible functional roles of nuclear lamins, emphasizing the information concerning the ability of nuclear lamins to self-assemble into distinct oligomers and polymers.

Substrate specificity of mammalian prenyl protein-specific endoprotease activity

We have previously identified proteolytic activity in rat liver microsomes that cleaves an intact tripeptide, VIS, from S-farnesylated-CVIS tetrapeptide. This enzymatic activity, termed prenyl protein-specific endoprotease (PPEP) activity, has been solubilized in CHAPS and purified 5-fold. To probe the peptide recognition features of PPEP activity, 64 tripeptides [N-acetyl-C(S-farnesyl)a1a2] were prepared and tested as competitive inhibitors of PPEP activity-catalyzed hydrolysis of N-acetyl-C(S-farnesyl)VI[3H]S. It was found that PPEP activity prefers large hydrophobic residues in the a1 and a2 positions. A subset of N-acetyl-C(S-farnesyl)a1a2 peptides were prepared in radiolabeled form, and it was found that PPEP activity preferences for these substrates correlated well in most cases with the inhibition data. The exception is that R in the a1 position does not prevent binding of peptide to PPEP activity, but such peptides are poor substrates. The anionic residue D in the a2 position is not tolerated by PPEP activity. Five farnesylated radiolabeled tetrapeptides, Ac-C(F)FM[3H]L, Ac-C(F)LI[3H]L, Ac-C(F)LL[3H]L, Ac-C(F)LM[3H]L, and Ac-C(F)VI[3H]L were prepared, and PPEP activity kinetic studies revealed that they are good substrates and show comparable KM values (2.2-13.5 microM). Ac-C(F)RL[3H]S is a poor substrate. The reported peptide binding preferences of PPEP activity should be useful in designing compounds that block the C-terminal proteolysis of prenylated proteins. Nonprenylated peptides do not bind to PPEP activity, and replacement of the farnesyl group with ann-pentadecyl group modestly reduces binding. Peptide-membrane partitioning studies were used together with theoretical arguments to fully understand the substrate specificity of PPEP activity toward these compounds.

Protein prenylation: from discovery to prospects for cancer treatment

A specific set of proteins in eukaryotic cells contain covalently attached carboxy-terminal prenyl groups (15-carbon farnesyl and 20-carbon geranylgeranyl). Many of them are signaling proteins including Ras, heterotrimeric G proteins and Rab proteins. The protein prenyltransferases which attach prenyl groups to proteins have been well characterized, and an X-ray structure is available for protein farnesyltransferase. Inhibitors of protein farnesyltransferase are showing sufficient promise in preclinical trials as anti-cancer drugs to warrant widespread interest in the pharmaceutical industry.

Expression of nuclear lamins in human tissues and cancer cell lines and transcription from the promoters of the lamin A/C and B1 genes

We have examined the expression of lamins A, B1, and C in human tissues and cancer cell lines and the function of the lamin A/C and B1 gene promoters in transfected cells. Northern analysis and immunoblotting demonstrated that lamin A/C mRNA and protein were not detectable in some human cell lines whereas lamin B1 was always present. Sequencing of approximately 2.6 kb of the lamin A/C and 1.6 kb of the lamin B1 genes 5' to the translation initiation sites showed that they did not contain typical TATA boxes near the transcription start sites. The lamin B1 and A/C proximal promoter regions were transcribed in transfected HeLa, Raji, and NT2/D1 cell lines even if the cells did not contain detectable endogenous lamin A/C mRNA or protein. These results show that, similar to most cytoplasmic intermediate filament genes, transcriptional regulatory elements in the promoters of the human nuclear lamin A/C and B1 genes do not control their cell type-specific expression in culture lines.

Genomic organization and molecular characterization of a gene encoding HsPXF, a human peroxisomal farnesylated protein

A protein modification essential for the cellular sorting of many biologically relevant proteins is the covalent attachment of prenyl lipids by specific transferases. Isoprenylation is known to render protein domains hydrophobic, thereby facilitating the interaction with lipid bilayers and/or membrane proteins. The target for the modification with farnesyl groups is the COOH-terminal sequence CaaX. Among the variety of farnesylated proteins the only one reported so far to be located to peroxisomes is the 37-kDa peroxisomal farnesylated hamster protein PxF. Recently we published data on the cDNA of the human gene HK33 (A. Braun et al., 1994, Gene 146: 291-295), which was revealed to be the human ortholog of PxF and was consequently renamed HsPXF. The genomic structure, molecular characterization, and evolutionary conservation of HsPXF are described herein. The exact location of the gene was defined as chromosome 1q22. The gene spans a region of approximately 9 kb, containing eight exons and seven introns. The 5' upstream region showed two potential Sp1-binding sites and an Alu repetitive sequence. Luciferase reporter activating capacity confirmed the presumed promoter activity of this region. On the transcriptional level, we detected four splice variants originating either from exon skipping or from alternative splicing events. For the HsPXF protein, a carboxyterminal farnesylation at cysteine residues was demonstrated. Through the use of HsPXF-specific antibodies, the protein was shown to be attached to the outer surface of peroxisomes. This localization together with the similarity to a peroxisomal assembly protein from Saccharomyces cerevisiae suggests HsPXF is involved in the process of peroxisomal biogenesis or assembly.

GST-lamin fusion proteins act as dominant negative mutants in Xenopus egg extract and reveal the function of the lamina in DNA replication

A cDNA encoding Xlamin B1 was cloned from a whole ovary mRNA by RT-PCR. GST-lamin fusion constructs were generated from this cDNA by first creating convenient restriction sites within the Xlamin B1 coding sequence, using PCR directed mutagenesis, and then sub-cloning relevant sequences into pGEX-4T-3. Two expression constructs were made, the first, termed delta 2+ lacked sequences encoding the amino-terminal 'head domain' of lamin B1 but included sequences encoding the nuclear localization signal sequence (NLS). The second expression construct, termed delta 2-, lacked sequences encoding the amino-terminal 'head domain' as well as sequences encoding the NLS. Purified fusion proteins expressed from these constructs, when added to egg extracts prior to sperm pronuclear assembly, formed hetero-oligomers with the endogenous lamin B3. The delta 2+ fusion protein prevented nuclear lamina assembly but not nuclear membrane assembly. The resulting nuclei were small (approximately 10 microns in diameter), did not assemble replication centers and failed to initiate DNA replication. When the delta 2- fusion protein was added to egg extracts prior to sperm pronuclear assembly, lamina assembly was delayed but not prevented. The resulting nuclei although small (approximately 12 microns), did form replication centers and initiated DNA replication. When added to egg extracts after sperm pronuclear assembly was completed delta 2+, but not delta 2-, entered the pre-formed nuclei causing lamina disassembly. However, the disassembly of the lamina by delta 2+ did not result in the disruption of replication centers and indeed these centres remained functional. These results are consistent with the hypothesis that lamina assembly precedes and is required for the formation of replication centers but does not support those centers directly.

Chloroplastic prenylated proteins

By in vivo [3H]mevalonate labelling of spinach combined with biochemical analysis, evidence is provided for the existence of protein prenylation in chloroplasts. Approximately 20 prenylated polypeptides were resolved by SDS-PAGE followed by autoradiography. Thermolysin treatment of intact chloroplasts revealed that about 40% of the prenylated polypeptides were associated with the cytoplasmic surface of the outer envelope membrane. The remaining portion was present in thylakoids and/or the inner envelope membrane. The majority of the prenylated polypeptides were associated with larger membrane protein complexes. A farnesyl protein transferase activity was found to be associated with the thylakoid membrane.

HMG-CoA reductase inhibitors suppress macrophage growth induced by oxidized low density lipoprotein

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors ameliorate atherosclerotic diseases in several models of vascular disease. This is largely due to their ability to reduce plasma cholesterol levels in vivo. Proliferation of cellular components is one of the major events in the development and progression of atherosclerotic lesions. We recently demonstrated that oxidized low density lipoprotein (Ox-LDL), a likely atherogenic lipoprotein present in vivo, is capable of inducing macrophage growth in vitro. In the present study, we investigated the effect of HMG-CoA reductase inhibitors, simvastatin and pravastatin, on Ox-LDL-induced macrophage growth. Our results demonstrated that these inhibitors effectively suppressed Ox-LDL-induced macrophage growth with concentrations required for 50% inhibition by simvastatin and pravastatin being 0.1 and 80 microM, respectively, and that this inhibitory effect was reversed by mevalonate but not by squalene. Under these conditions, simvastatin did not affect the endocytic degradation of Ox-LDL, nor subsequent accumulation of intracellular cholesteryl esters. Our results suggest that a non-cholesterol metabolites(s) of mevalonate pathway may play an important role in Ox-LDL-induced macrophage growth. Since it is well known that macrophage-derived foam cells are the key cellular element in the early stage of atherosclerosis, a significant inhibition of Ox-LDL-induced macrophage growth by HMG-CoA reductase inhibitors in vitro, particularly simvastatin, may also explain, at least in part, their anti-atherogenic action in vivo.

Protein prenylation in spinach--tissue specificity and greening-induced changes

Etiolated spinach seedlings, as well as petioles and blades of leaves of green seedlings, were labeled with [3H]mevalonate to study protein prenylation in several plant developmental stages. The polypeptide prenylation pattern of the leaf petiole and the leaf blade differed considerably, although some prenylated proteins were present in both tissues. During greening several prenylated polypeptides in the 30- to 46-kDa molecular mass region and two at 15 kDa became more abundant, while others in the 21.5- to 30-kDa region and one at 62 kDa showed a relative decrease. However, the relative amount of several of the prenylated polypeptides did not appear to be altered during the greening process. In etiolated seedlings, more thioether-linked farnesol than geranylgeraniol was found, whereas in seedlings grown under normal light conditions the converse situation prevailed.

Nuclear lamina and nuclear matrix organization in sperm pronuclei assembled in Xenopus egg extract

Nuclear lamina and matrices were prepared from sperm pronuclei assembled in Xenopus egg extracts using a fractionation and extraction procedure. Indirect immunofluorescence revealed that while chromatin was efficiently removed from nuclei during the extraction procedure, the distribution of lamins was unaffected. Consistent with this data, the amount of lamin B3, determined by immunoblotting, was not affected through the extraction procedure. Nuclear matrices were visualised in DGD sections by TEM. Within these sections filaments were observed both at the boundary of the nucleus (the lamina) and within the body of the nucleus (internal nuclear matrix filaments). To improve resolution, nuclear matrices were also prepared as whole mounts and viewed using field emission in lens scanning electron microscopy (FEISEM). This technique revealed two distinct networks of filaments. Filaments lying at the surface of nuclear matrices interconnected nuclear pores. These filaments were readily labelled with monoclonal anti-lamin B3 antibodies. Filaments lying within the body of the nuclear matrix were highly branched but were not readily labelled with antilamin B3 antibodies. Nuclear matrices were also prepared from sperm pronuclei assembled in lamin B3 depleted extracts. Using FEISEM, filaments were also detected in these preparations. However, these filaments were poorly organised and often appeared to aggregate. To confirm these results nuclear matrices were also observed as whole mounts using TEM. Nuclear matrices prepared from control nuclei contained a dense array of interconnected filaments. Many (but not all) of these filaments were labelled with anti-lamin B3 antibodies. In contrast, nuclear matrices prepared from "lamin depleted nuclei' contained poorly organised or aggregated filaments which were not specifically labelled with anti-lamin B3 antibodies.

Effects of monoterpenes and mevinolin on murine colon tumor CT-26 in vitro and its hepatic "metastases" in vivo

Tumors derived from the colonic epithelium exhibit cholesterol metabolism which is clearly different from that in fibroblasts, hepatocytes, adrenals, and ovaries. In hepatocytes and fibroblasts MEV inhibition of the rate limiting step in cholesterol synthesis HMG Co A reductase can be overcome by the uptake of LDL. Colon cancer cells however do not overcome MEV inhibition by LDL uptake but rather exhibit further growth suppression Mevinolin (Mevacor), a drug used to lower serum cholesterol levels has the advantage of accumulating in the liver to approximately 95% with the first pass. A small but variable percentage of non-sterol precursors may escape inhibition and be utilized for other pathways in the isoprenylation of certain proteins, among them members of the ras family. Mutated ras, an oncogene, is found in 40-50% of colon tumors and the expression of a functional gene product is dependent on isoprenylation for anchorage to the tumor cell membrane. d-Limonene, a relatively non-toxic monoterpene found in orange skin oil, selectively inhibits isoprenylation and also accumulates to some extent in the liver. It was hypothesized that the differences in mevalonate metabolism between hepatocytes and colon tumor cells could provide a chemotherapeutic advantage in which MEV and/or d-limonene could effectively inhibit cholesterol synthesis and post-translational modification of proteins with non-sterol cholesterol precursors in colon tumor derived hepatic metastases and thus inhibit their growth. Since each drug affects aspects of mevalonate synthesis at different points, the effects of the combination of their agents on inhibiting tumor metastases was investigated to ascertain if these could be additive. In tissue culture, MEV and d-limonene significantly inhibited the growth of CT-26, a murine transplantable colon tumor. Cholesterol synthesis assessed in these cells indicated that in lipid deficient media the following additions-25-hydroxycholesterol, and LDL significantly reduced cholesterol synthesis. Conversely, perillyl alcohol increased cholesterol synthesis 2.5 fold. In cells cultured in FBS based medium, which have an FBS control, MEV treatment reduced cholesterol synthesis to 65% of control. Perillyl alcohol increased synthesis 1.4 fold and when given in conjunction with MEV, it abolished the effects of this inhibitor. In isoprenylation studies of 14C-mevalonate incorporation into proteins, MEV impaired isoprenylation by restricting synthesis of mevalonate derived intermediates. Results of CT-26 treatment with perillyl alcohol are inconsistent with its putative role as a protein isoprenylation inhibitor. The combination of these agents indicates an additive action which requires additional investigation to elucidate their mechanism(s). Dietary MEV and d-limonene were evaluated alone and in combination for their chemotherapeutic potential in a hepatic "metastasis" model. Using splenic colonization in which CT-26 was implanted into the spleen and ultimately seeded the liver, each of these compounds were found to inhibit the growth of resultant tumors both alone and in combination by approximately 80% versus controls at 35 days post-implantation. Assessment of HMGCoA reductase in liver and tumor indicated that these agents were effective in reaching these target sites. The findings to date indicate that while d-limonene and MEV may differentially affect the same pathway, and their individual actions may appear antagonistic in vitro, their overall action individually or together, appears promising as a chemotherapeutic modality for the possible management of hepatic metastases from colon cancer.

Xenopus lamin B3 has a direct role in the assembly of a replication competent nucleus: evidence from cell-free egg extracts

Xenopus egg extracts which assemble replication competent nuclei in vitro were depleted of lamin B3 using monoclonal antibody L6 5D5 linked to paramagnetic beads. After depletion, the extracts were still capable of assembling nuclei around demembranated sperm heads. Using field emission in lens scanning electron microscopy (FEISEM) we show that most nuclei assembled in lamin B3-depleted extracts have continuous nuclear envelopes and well formed nuclear pores. However, several consistent differences were observed. Most nuclei were small and only attained diameters which were half the size of controls. In a small number of nuclei, nuclear pore baskets, normally present on the inner aspect of the nuclear envelope, appeared on its outer surface. Finally, the assembly of nuclear pores was slower in lamin B3-depleted extracts, indicating a slower overall rate of nuclear envelope assembly. The results of FEISEM were confirmed using conventional TEM thin sections, where again the majority of nuclei assembled in lamin B3-depleted extracts had well formed double unit membranes containing a high density of nuclear pores. Since nuclear envelope assembly was mostly normal but slow in these nuclei, the lamin content of 'depleted' extracts was investigated. While lamin B3 was recovered efficiently from cytosolic and membrane fractions by our procedure, a second minor lamin isoform, which has characteristics similar to those of the somatic lamin B2, remained in the extract. Thus it is likely that this lamin is necessary for nuclear envelope assembly. However, while lamin B2 did not co-precipitate with lamin B3 during immunodepletion experiments, several protein species did specifically associate with lamin B3 on paramagnetic immunobeads. The major protein species associated with lamin B3 migrated with molecular masses of 102 kDa and 57 kDa, respectively, on one-dimensional polyacrylamide gels. On two-dimensional O'Farrell gels the mobility of the 102 kDa protein was identical to the mobility of a major nuclear matrix protein, indicating a specific association between lamin B3 and other nuclear matrix proteins. Nuclei assembled in lamin B3-depleted extracts did not assemble a lamina, judged by indirect immunofluorescence, and failed to initiate semi-conservative DNA replication. However, by reinoculating depleted extracts with purified lamin B3, nuclear lamina assembly and DNA replication could both be rescued. Thus it seems likely that the inability of lamin-depleted extracts to assemble a replication competent nucleus is a direct consequence of a failure to assemble a lamina.

Specific detection of the precursor of ras p21 with a mouse monoclonal anti-C-terminal peptide antibody, SARA-K1

In an attempt to clarify the post-translational modifications of ras oncogene product p21, we have established a mouse monoclonal antibody specific for the precursor of p21. The C-terminal peptide (156-188) of K(4A)-ras oncogene product p21 (p21K(4A), termed K(4A)-peptide, was used as the immunogen. In Western blotting, monoclonal antibodies were examined for their differential reactivity between two types of p21K(4A) expressed in Escherichia coli (esh-p21K(4A)) and mammalian cell (mam-p21K(4A)). One monoclonal antibody, designated SARA-K1, reacted selectively with esh-p21K(4A). The epitope for SARA-K1 was defined on tryptic peptide (177-184), containing Cys180, of the K(4A)-peptide. Pulse-chase experiments of mam-p21K(4A) synthesis at 24 degrees C revealed that SARA-K1 precipitated a 21 kDa protein within a 7 min chase but not after a 10 min chase, indicating that SARA-K1 recognizes the precursor of mam-p21K(4A). Furthermore, in Triton X-114 partitioning experiments using mammalian cells pre-treated with Mevalotin, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitor, SARA-K1 precipitated [35S]methionine-labeled, [3H]mevalonic acid-unlabeled mam-p21K(4A) in the aqueous phase, but did not precipitate [3H]mevalonic acid-labeled mam-p21K(4A) in either aqueous or detergent phase. The data presented clearly show that the SARA-K1 specifically recognizes the primary translational product pro-p21K(4A).

Effects of pravastatin, a hydroxymethylglutaryl-CoA reductase inhibitor, on two human tumour cell lines

Competitive inhibitors of 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase are currently used to treat patients with hypercholesterolaemia. These inhibitors affect not only cholesterol biosynthesis, but also the production of non-steroidal mevalonate derivatives, that are involved in a number of growth-regulatory processes. As a consequence, their potential use as anticancer drugs has been suggested. In order to examine long-term effects of this potential therapeutic approach, we cultivated the gastric carcinoma cell line, EPG85-257, and the breast tumour cell line, MDA-MB231, in the presence of increasing concentrations of the HMG-CoA reductase inhibitor, pravastatin. For both cell lines, this procedure led to the selection of resistant variants able to proliferate in more than 1000 microM inhibitor. By competitive reverse transcriptase/polymerase chain reaction assay (cRT-PCR), the expression of the mRNA for two key proteins of cellular cholesterol metabolism, HMG-CoA reductase and low-density lipoprotein (LDL) receptor, were analysed in sensitive and resistant cells. Despite similar growth rates, MDA-MB231 cells expressed approximately four times more HMG-CoA reductase mRNA than EPG85-257 cells and over 30 times more LDL receptor mRNA. Both mRNA species were coordinately regulated in the parental and in the pravastatin-resistant variant cells. Expression was highly stimulated (3- to 4-fold for the HMG-CoA reductase and 2- to 3-fold for the LDL receptor) in the resistant variants when cultured in lipoprotein-deficient medium in the presence of 1000 microM pravastatin. Immunocytological analysis of the expression of the HMG-CoA reductase and LDL receptor protein were in accordance with the data on specific mRNA expression obtained by cRT-PCR. Southern blot analysis revealed a 1.5-fold amplification of the HMG-CoA reductase gene in resistant MDA-MB231 cells, but not in the resistant EPG85-257 variant. Our data provide evidence for resistance mechanisms to pravastatin that are independent of the amplification of the HMG-CoA reductase gene. By analogy to the cell-culture models employed in this study, it is conceivable that similar mechanisms might occur in human tumour cells in vivo during long-term treatment with HMG-CoA reductase inhibitors. This might limit their application as chemotherapeutic anticancer agents.

Effect of lovastatin alone and as an adjuvant chemotherapeutic agent on hepatoma tissue culture-4 cell growth

BACKGROUND

Cholesterol is essential for cell viability and growth. Interference with the cholesterol biosynthetic pathway with a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (e.g., lovastatin) may preferentially slow malignant cell growth and offer a new approach to cancer chemotherapy. To test this hypothesis, we evaluated the effect of lovastatin alone, and as an adjuvant chemotherapeutic agent, on the growth and function of hepatoma tissue culture-4 (HTC-4) cells.

METHODS

HTC-4 cells were treated with lovastatin at concentrations of 1, 3, 5, and 10 microM, with mitomycin-C at concentrations of 10, 25, 50, and 100 nM, or with combinations of the two drugs. Cell growth was evaluated by daily cell counts and substrate adhesion to fibronectin.

RESULTS

Lovastatin alone slowed HTC-4 cell growth at concentrations as low as 1 microM (p < 0.01). Mitomycin-C alone slowed HTC-4 cell growth at concentrations of 25 nM and above (p < 0.01). Lovastatin added to mitomycin-C-treated cells resulted in a significant adjuvant effect, with cell growth slowed by an additional 20-30% by 1 microM lovastatin and by an additional 43-63% by 5 microM lovastatin, compared to mitomycin-C alone (p < 0.01). Lovastatin-treated cells also exhibited decreased adherence to substrate (p < 0.05).

CONCLUSIONS

Lovastatin is effective alone and as an adjuvant to mitomycin-C in slowing the growth of HTC-4 cells. These in vitro results support further investigation of lovastatin as an adjuvant chemotherapeutic agent in animal models.

Isoprenylation of plant proteins in vivo. Isoprenylated proteins are abundant in the mitochondria and nuclei of spinach

Protein isoprenylation in vivo is demonstrated using spinach seedlings labeled with [3H]mevalonate. This report provides evidence for the occurrence of a large number of isoprenylated proteins in plants. Seedlings, without roots, were labeled quantitatively through the cut stem. Mevinolin treatment of the seedlings resulted in increased incorporation of radiolabel into proteins. Approximately 30 labeled bands could be detected after autoradiography of SDS-polyacrylamide gel electrophoresis-separated polypeptides, ranging in molecular mass from 6 to 200 kDa. Methyl iodide hydrolysis resulted in the release of covalently bound farnesol, geranylgeraniol, phytol, and some unidentified isoprenoid compounds from mevalonate-labeled proteins. It was found that all cellular fractions contained some isoprenylated proteins, although most were located in the mitochondria and nuclei. Subfractionation of the nucleus revealed that the majority of isoprenylated proteins in this compartment were components of the nuclear matrix. The results demonstrate that in vivo labeling of a complex organism can be performed using a plant system in order to study protein isoprenylation and distribution of modified proteins in different cellular compartments.

Nutritional factors and colon cancer

During the last 2 decades, substantial progress has been made in understanding the relationship between dietary constituents and the development of colon cancer in man. Unlike studies of cancer among smokers and nonsmokers, nutritional epidemiologic studies are confronted with the inherent difficulty of assessing reasonably precise exposures. The lack of consistency between international correlation studies and case-control studies does not necessarily negate a dietary etiology of colon cancer because these inconsistencies may have arisen, at least in part, from methodological limitations. Some of these deficiencies in epidemiological studies of diet and cancer have been corrected; recent case-control studies demonstrated that high dietary fat is a risk factor for colon cancer development and that an overall increase in intake of foods high in fiber might decrease the risk for colon cancer. The results of epidemiologic studies may be assumed to present conservative estimates of the true risk for cancer associated with diet. The populations with high incidences of colon cancer are characterized by high consumption of dietary fat, which may be a risk factor in the absence of factors that are protective, such as whole-grain cereals and of other high fiber. Laboratory-animal model studies have shown that certain dietary lipids and fibers influence tumorigenesis in the colon. The data of metabolic epidemiological and laboratory-animal model studies are sufficiently convincing with respect to the enhancement of colon cancer by type of fat and protection by certain dietary fibers.

Weaving a pattern from disparate threads: lamin function in nuclear assembly and DNA replication

The major residual structure that remains associated with the nuclear envelope following extraction of isolated nuclei or oocyte germinal vesicles with non-ionic detergents, nucleases and high salt is the lamina (Fawcett, 1966; Aaronson and Blobel, 1975; Dwyer and Blobel, 1976). The nuclear lamina is composed of intermediate filament proteins, termed lamins (Gerace and Blobel, 1980; Shelton et al., 1980), which polymerise to form a basket-weave lattice of fibrils, which covers the entire inner surface of the nuclear envelope and interlinks nuclear pores (Aebi et al., 1986; Stewart and Whytock, 1988; Goldberg and Allen, 1992). At mitosis, the nuclear envelope and the lamina both break down to allow chromosome segregation. As a consequence, each structure has to be rebuilt during anaphase and telophase, allowing cells an opportunity to reposition chromosomes (Heslop-Harrison and Bennett, 1990) and to reorganise looped chromatin domains (Franke, 1974; Franke et al., 1981; Hochstrasser et al., 1986), which may in turn control the use of subsets of genes. Because of the position that it occupies, its dynamics during mitosis and the fact that it is an essential component of proliferating cells, the lamina has been assigned a number of putative roles both in nuclear metabolism and in nuclear envelope assembly (Burke and Gerace, 1986; Nigg, 1989). However, to date there is little clear cut evidence that satisfactorily explains the function of the lamina in relation to its structure. In this Commentary we will describe some of the recent work that addresses this problem and attempt to provide a unified model for the role of lamins in nuclear envelope assembly and for the lamina in the initiation of DNA replication.

Chemoprevention of colon cancer by dietary fatty acids

During the last two decades, substantial progress has been made in the understanding of the relationship between the dietary constituents and development of colon cancer in man. Unlike studies of cancer among smokers and nonsmokers, nutritional epidemiologic studies are confronted with the inherent difficulty of assessing reasonably precise exposures. The lack of consistency between international correlation studies and case-control studies does not necessarily negate a dietary etiology of colon cancer because these inconsistencies may have arisen, at least in part, from methodological limitations. Some of these deficiencies in epidemiological studies of diet and cancer have been corrected; recent case-control studies demonstrated that high dietary fat is a risk factor for colon cancer development and that an overall increase in intake of foods high in fiber might decrease the risk for colon cancer. The results of epidemiologic studies may be assumed to present conservative estimates of the true risk for cancer associated with diet. The populations with high incidence of colon cancer are characterized by high consumption of dietary fat, which may be a risk factor in the absence of factors that are protective, such as whole-grain cereals and of other high-fiber. Laboratory animal model studies have shown that certain dietary lipids and fibers influence tumorigenesis in the colon. The data of metabolic epidemiological and laboratory animal model studies are sufficiently convincing with respect to enhancement of colon cancer by type of fat and protection by certain dietary fibers.

Inhibition of human vascular smooth muscle cell proliferation by lovastatin: the role of isoprenoid intermediates of cholesterol synthesis

Restenosis remains the largest single obstacle to the long-term success of invasive vascular interventions. Lovastatin, an HMG-CoA reductase inhibitor, has been shown to reduce myointimal hyperplasia in animal models of restenosis and in one clinical coronary restenosis trial. We have assessed the effect of lovastatin on the growth of cultured human vascular smooth muscle cells derived from saphenous vein and vascular graft stenoses. Lovastatin (2 microM) inhibited proliferation over 14 days in saphenous vein (and graft stenoses) derived vascular smooth muscle cells by 42% and 32% respectively: this was not significantly different. Lovastatin (10 microM) reduced [methyl 3H]-thymidine uptake by 51% in saphenous vein-derived cells. These concentrations were significantly higher than those achieved in plasma during therapeutic dosage. Lovastatin-induced inhibition of vascular smooth muscle cell proliferation and [methyl 3H]-thymidine uptake was completely reversed by adding mevalonate (100 microM) but cholesterol (10-40 micrograms ml-1) had no effect. Isopentenyl adenine (25-50 microM) did not affect the inhibition of [methyl 3H]-thymidine uptake by lovastatin (10 microM), but farnesol (20 microM), another isoprenoid precursor of cholesterol synthesis, reversed the antiproliferative effect.

Molecular cloning and characterization of an isoprenylated 67 kDa protein

The cDNA coding for a 67 kDa protein (p67) was isolated from a rat Schwann cell library. A recombinant form of p67 expressed in bacteria was used to produce polyclonal anti-p67 antibodies. By immunoblot analysis p67 was found to be expressed in most tissues and cell lines examined. Inspection of the deduced amino acid sequence revealed a COOH-terminal consensus sequence for isoprenylation. Consistent with this finding, p67 was a substrate for isoprenylation in vitro by geranylgeranylpyrophosphate. p67 was associated predominantly with the particulate fraction of rat smooth muscle cells. The rat p67 sequence was highly homologous to a family of recently described human and mouse gamma-interferon inducible, guanine nucleotide binding proteins.

Isoprenylated proteins in myelin

Incubation of rat brainstem slices with [3H]-mevalonate ([3H]MVA) in the presence of lovastatin resulted in the incorporation of label into three groups of myelin-associated proteins with molecular masses of 47, 21-27, and 8 kDa, as revealed on sodium dodecyl sulfate-polyacrylamide rod gel electrophoresis. Although the gel patterns of [3H]MVA-derived prenylated proteins were similar, the relative level of 3H incorporated into each protein species differed between myelin and the brainstem homogenate. Immunoprecipitation studies identified the 47-kDa prenylated protein as a 2'-3'-cyclic nucleotide phosphodiesterase, whereas the 8-kDa protein proved to be the gamma subunit of membrane-associated guanine nucleotide regulatory protein. The 3H-labeled 21-27-kDa group in myelin corresponds to the molecular mass of the extensive Ras-like family of monomeric GTP-binding proteins known to be prenylated in other tissues. Increase in lovastatin concentration resulted in reduced levels of [3H]MVA-labeled species in myelin and concomitantly increased levels in the cytosol. A cold MVA chase restored to normality the appearance of [3H]MVA-labeled proteins in myelin. Furthermore, a high lovastatin concentration in the brainstem slice incubation mixture altered the appearance of newly synthesized nonprenylated myelin proteins, including proteolipid protein and the 17-kDa subspecies of myelin basic protein. Because other myelin proteins were unaffected by the high lovastatin concentration, restricting the availability of MVA in myelin-forming cells may selectively alter processes required for myelinogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Prenylated proteins and lymphocyte proliferation: inhibition by d-limonene related monoterpenes

The aim of the present study was to explore the role of post-translational isoprenoid modification of cellular proteins in the proliferation of human lymphocytes. We here report that treatment of phytohemagglutinin-stimulated peripheral blood mononuclear cells with monoterpenes including d-limonene, perillic acid and perillyl alcohol (0.5-5 mM) which selectively inhibit the isoprenylation of 21-26-kDa proteins resulted in a dose-dependent inhibition of DNA synthesis. Cell cycle analysis revealed that perillic acid arrested cells in G1 and prevented cells from entering S phase in a manner similar to that induced by the specific 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor, compactin. However, unlike compactin, the perillic acid-induced effects on lymphocyte proliferation were not prevented by addition of mevalonate. We also examined the incorporation of [3H]mevalonate into proteins in resting and phytohemagglutinin-stimulated lymphocytes during the first 30 h of culture. While in unstimulated lymphocytes radioactivity was predominantly incorporated into a cluster of 21-26-kDa proteins, mitogenic stimulation was associated with a striking increase in [3H]mevalonate incorporation into a protein (approximately 68 kDa) with migration characteristics similar to that of nuclear lamin B. Treatment of phytohemagglutinin-stimulated lymphocytes with 5 mM d-limonene, 2.5 mM perillic acid or 1.25 mM perillyl alcohol strongly suppressed [3H]mevalonate-labeling of proteins to a degree that correlated with the level of DNA synthesis inhibition. These findings suggest that those mevalonate-derived products required for lymphocyte proliferation may include one or more isoprenylated proteins and that the isoprenylation of these proteins is required for cell cycle progression.

Protein prenylation in eukaryotic microorganisms: genetics, biology and biochemistry

Modification of proteins at C-terminal cysteine residue(s) by the isoprenoids farnesyl (C15) and geranylgeranyl (C20) is essential for the biological function of a number of eukaryotic proteins including fungal mating factors and the small, GTP-binding proteins of the Ras superfamily. Three distinct enzymes, conserved between yeast and mammals, have been identified that prenylate proteins: farnesyl protein transferase, geranylgeranyl protein transferase type I and geranylgeranyl protein transferase type II. Each prenyl protein transferase has its own protein substrate specificity. Much has been learned about the biology, genetics and biochemistry of protein prenylation and prenyl protein transferases through studies of eukaryotic microorganisms, particularly Saccharomyces cerevisiae. The functional importance of protein prenylation was first demonstrated with fungal mating factors. The initial genetic analysis of prenyl protein transferases was in S. cerevisiae with the isolation and subsequent characterization of mutations in the RAM1, RAM2, CDC43 and BET2 genes, each of which encodes a prenyl protein transferase subunit. We review here these and other studies on protein prenylation in eukaryotic microbes and how they relate to and have contributed to our knowledge about protein prenylation in all eukaryotic cells.

Endoproteolysis of non-CAAX-containing isoprenylated peptides

A microsomal endoprotease specifically cleaves isoprenylated peptides of the CAAX motif, such as N-acetyl-S-all-trans-farnesyl-L-cysteine (AFC-VIM), at the isoprenylated cysteine residue. It is shown here that endoproteolysis will also occur with peptides which are not of the CAAX type. Peptide substrates modeled after the Delta virus large antigen carboxyl-terminus (CRPQ) are endoproteolytically hydrolyzed by liver microsomes. AFC-RPQ is hydrolyzed with a KM = 12.4 microM and a Vmax = 0.27 nmol/min/mg, and AGGC-RPQ is hydrolyzed with a KM = 7.9 microM and a Vmax = 0.042 nmol/min/mg. Moreover, a series of potent inhibitors of the endoproteolysis of AFC-AAX-containing peptides are ineffective at inhibiting the hydrolysis of AFC-RPQ and AGGC-RPQ, suggesting the existence of isoforms of the endoprotease.

Modification of eukaryotic signaling proteins by C-terminal methylation reactions

Eukaryotic polypeptides that are initially synthesized with the C-terminal sequence -Cys-Xaa-Xaa-Xaa, including a variety of signal-transducing proteins, such as small G-proteins, large G-proteins and cGMP phosphodiesterases, can be targeted for a series of sequential post-translational modifications. This processing pathway includes the isoprenylation of the cysteine residue with a farnesyl or geranylgeranyl moiety, followed by proteolysis of the three terminal residues and alpha-carboxyl methyl esterification of the cysteine residue. The potential reversibility of the last step suggests that it may be involved in modulating the function of these proteins. Firstly, methylation may play a role in the activation of cellular peptides or proteins. Secondly, this modification may aid in the membrane attachment of cytosolic precursor proteins. Thirdly, methylation may protect the polypeptide from C-terminal proteolytic degradation once the three terminal amino acid residues are removed. Finally, reversible methylation may directly regulate the function of its target proteins. Therapeutically, inhibitors of C-terminal isoprenylcysteine methylation or demethylation reactions may prove to be useful pharmacological tools as anti-cancer and anti-inflammatory agents.

The differential expression of lamin epitopes during mouse spermatogenesis

The presence of lamin proteins in mouse spermatogenic cells has been examined by using an anti-lamin AC and an anti-lamin B antisera which recognize somatic lamins A and C, and somatic lamin B, respectively. Anti-lamin B binds to the nuclear periphery of all cell types examined, including Sertoli cells, primitive type A spermatogonia, preleptotene, leptotene, zygotene and pachytene spermatocytes, and round spermatids. In sperm nuclei, the antigenic determinants are localized to a narrow domain of the nucleus. However, after removing the perinuclear theca, anti-lamin B localizes to the entire nuclear periphery in a punctate pattern, suggesting that it is binding to determinants previously covered by the theca constituents. On immunoblots anti-lamin B reacts with a approximately 68 kD polypeptide in all germ cells and, to a lesser extent, with four additional polypeptides present only in meiotic and post-meiotic nuclear matrices. Anti-lamin AC also reacts with the perinuclear region of the somatic cells in the testes, in particular, those of the interstitium and also the Sertoli cells of the seminiferous epithelium. In contrast to anti-lamin B, anti-lamin AC does not bind to the germ cells at any stage of spermatogenesis. In addition, nuclear matrix proteins from isolated spermatogenic cells do not bind anti-lamin AC on immunoblots, suggesting the lack of reactivity is not due to the masking of any antigenic sites. These data demonstrate that germ cells contain lamin B throughout spermatogenesis, even during meiosis and spermiogenesis when the nuclear periphery lacks a distinct fibrous lamina.

Mapping of multiple mouse loci related to the farnesyl pyrophosphate synthetase gene

The prenyltransferases are a class of enzymes involved in the synthesis of sterol and nonsterol isoprene compounds. We report here the chromosomal mapping of nine loci in the mouse that hybridize to the cDNA for the enzyme farnesyl pyrophosphate synthetase (FPS), a prenyltransferase that catalyzes the synthesis of an intermediate common to both the sterol and nonsterol branches of the isoprene biosynthetic pathway. Mapping was performed with genomic DNA from a mouse-hamster somatic cell hybrid panel, and by linkage analysis with recombinant inbred strains and the progeny of an interspecific backcross. The mapped loci have been designated farnesyl pyrophosphate synthetase-like-1 (Fpsl-1) on mouse Chromosome (Chr) 3; Fpsl-2 on Chr 4; Fpsl-3, Fpsl-4, and Fpsl-5, dispersed on Chr 10; Fpsl-6 on Chr 12; Fpsl-7 on Chr 13; Fpsl-8 on Chr 17; and Fpsl-9 on Chr X. It is presently unclear which of these loci encode active prenyltransferases and which may correspond to pseudogenes. The strongly hybridizing loci provide convenient genetic markers for seven mouse chromosomes.

Reversal of ras-induced inhibition of gap-junctional intercellular communication, transformation, and tumorigenesis by lovastatin

The plasma-membrane association and transforming activity of the ras oncoprotein p21 are dependent upon posttranslational farnesylation. Farnesyl synthesis and p21 ras farnesylation are inhibited by hydroxymethylglutaryl-CoA reductase inhibitors such as lovastatin. In this study, we examined whether lovastatin could reverse the transformed phenotype of a v-Ha-ras-transformed rat liver epithelial cell line (WB-ras cells) and if changes were associated with the enhancement of gap-junctional intercellular communication (GJIC). WB-ras cells grow in soft agar, have reduced GJIC, and are highly tumorigenic. Membrane association of p21 ras in these cells was inhibited after in vitro treatment with lovastatin (0.1-0.5 microM) for 48 h. Concomitantly, the cells displayed a more normal morphology, decreased growth in soft agar, and enhanced GJIC. These changes were prevented by cotreatment with mevalonic acid. The morphology and GJIC of rat liver epithelial cells transformed with other oncogenes (src, neu, and raf/myc) were not affected by lovastatin. Intrahepatic WB-ras tumors were induced in male rats by intraportal-vein injection of WB-ras cells. The size and DNA labeling index of these tumors were decreased approximately 75% by administration of lovastatin (5 mg/kg orally twice daily for 2 wk). These results suggest that lovastatin reversed the transformed phenotype of WB-ras cells by inhibiting p21 ras plasma membrane association. Furthermore, the concomitant enhancement of GJIC in lovastatin-treated cells suggests a role for reduced GJIC in the expression of the transformed phenotype.

Evidence for up-regulated low density lipoprotein receptor in human lung adenocarcinoma cell line A549

Human lung adenocarcinoma cell line A549 was studied with respect to the metabolism of human low density lipoprotein (LDL) and 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGR) activity. After incubation in medium containing lipoprotein-deficient serum (LPDS) for 24 h, the A549 cell line expresses a single class of high affinity LDL binding sites (KD at 37 degrees C of 15.1 +/- 0.7 nM and capacity of 118 +/- 2.8 ng/mg cell protein) and an HMGR activity of 111.4 +/- 7 pmol/min/mg cell protein. After binding, the LDLs were internalized and degraded by a common saturable process. The HMGR activity was higher in A549 cells than in fibroblasts but LDL affinity and binding capacity were similar in both cell types. However, in the presence of lipoproteins, A549 cells showed a two-fold higher binding capacity than fibroblasts. When the cells were deprived of cholesterol, the amount of LDLR sites increased but the extent of stimulation was lower in A549 than in fibroblast cells (2.5-fold versus six-fold respectively). This increase was accompanied by a similar increase in the specific LDLR mRNA cellular levels (two-fold versus six-fold respectively). When cells were deprived of exogenous and endogenous cholesterol (biosynthesis blocked by compactin), the binding capacity and the LDLR mRNA levels were yet again increased in A549 cells but not in fibroblasts. Taken together these results suggest that the level of expression of the LDLR is up-regulated in A549 cells compared to fibroblasts.

Pharmacology of smooth muscle cell replication

The suggestion that smooth muscle cell proliferation contributes to hypertension, atherosclerosis, and restenosis after angioplasty has led to a growing interest in the use of drugs to inhibit this process. This review summarizes pharmacological studies of smooth muscle cell proliferation in vitro and in vivo and identifies specific mediators of proliferation that are implicated by drugs binding with high affinity to enzymes or receptors.

Effect of pravastatin, a potent 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, on survival of AH130 hepatoma-bearing rats

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor is known to have an inhibitory effect on cell growth in addition to a cholesterol-lowering effect. This study examined the effect of pravastatin, a potent inhibitor of HMG-CoA reductase, on the survival of AH130 hepatoma-bearing rats. Pravastatin (1, 2, or 8 mg/kg body weight) was intraperitoneally injected once a day into tumor-bearing rats. The difference in the survival curves was significant between the controls and the rats treated with 8 mg/kg of pravastatin (P < 0.019 by logrank test) but not between the controls and the rats treated with 1 or 2 mg/kg of the inhibitor. The tumor volume was significantly decreased in the rats treated with 8 mg/kg of pravastatin (P < 0.05). These observations showed that intraperitoneal injection of pravastatin could improve the survival of AH130 hepatoma-bearing rats and had an inhibitory effect on the growth of the ascites form tumor.

Assembly and cell cycle dynamics of the nuclear lamina

The nuclear lamina is a karyoskeletal structure composed of intermediate filament type proteins. It underlies the inner nuclear membrane and confers mechanical stability to the nuclear envelope. In addition, it interacts with chromatin and may thereby participate in determining the three-dimensional organization of the interphase nucleus. During mitosis, the nuclear lamina is transiently disassembled, most probably through hyperphosphorylation of lamin proteins by the protein kinase p34cdc2, a key regulator of the eukaryotic cell cycle. Mitotic disassembly of the lamina is necessary but not sufficient for nuclear envelope breakdown. Electron microscopic analyses have begun to provide insights into the principles that govern lamina assembly in vitro, and sequence motifs required for targeting newly synthesized lamins to the nuclear envelope have been identified. Of particular interest, lamins were shown to undergo a type of hydrophobic modification known as isoprenylation. Finally, recent studies addressing the nature of lamin-chromatin interactions may provide the basis for elucidating the role of lamins in organizing the distribution of interphase chromatin.