3'-untranslated sequences mediate post-transcriptional regulation of 3-hydroxy-3-methylglutaryl-CoA reductase mRNA by 25-hydroxycholesterol

Cyclic fluctuations of 3-hydroxy-3-methylglutaryl-CoA reductase in aortic smooth muscle cell cultures

The cyclic fluctuations of HMG-CoA reductase activity and mRNA are reportedly related to feeding the cells in culture or to variations in food consumption by the animals over a 24-h cycle. In this work, we demonstrate cyclic increments in HMG-CoA reductase activity in smooth muscle cells (SMC) not associated with the culture feeding. Since reductase activity also shows a marked rise preceding the S phase, one of the major goals of the present work was to evaluate this dual role of reductase activity and mRNA fluctuations related to the cell cycle and to food intake in the SMC-C/SMC-Ch cultures derived from control-fed (SMC-C) and cholesterol-fed (SMC-Ch) chicks. The period and amplitude oscillations in HMG-CoA reductase activity varied depending on culture conditions: lipoprotein-deficient serum vs. FBS, young vs. senescent cells, or confluent vs. nonconfluent cultures. The HMG-CoA reductase mRNA concentration showed a marked rise after feeding not correlated to the fluctuation activity, suggesting posttranscriptional modulation. Reductase activity and mRNA were down-regulated in SMC-Ch. Since the nutritional culture conditions were the same in both cell lines, these findings indicate that consumption of a high-cholesterol diet by the animals prior to the establishment of the SMC cultures induced changes in the HMG-CoA reductase gene expression in-aortic SMC.

Proto oncogene/eukaryotic translation initiation factor (eIF) 4E attenuates mevalonate-mediated regulation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase synthesis

The rate-limiting enzyme for mevalonate synthesis in mammalian cells is 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Products of mevalonate synthesis are required for cell cycle progression as well as cell growth and survival. In tumor cells, HMG-CoA reductase is generally elevated because of attenuated sterol-mediated regulation of transcription. However, tumor cell HMG-CoA reductase remains sensitive to post-transcriptional regulation by mevalonate-derived isoprenoid intermediates of cholesterol synthesis. Isoprenoids suppress HMG-CoA reductase synthesis through a mechanism that reduces initiation of translation on HMG-CoA reductase mRNA. Because HMG-CoA reductase mRNA transcripts have 5'-untranslated regions (UTR) that are GC rich and contain stable secondary structure, we tested the hypothesis that overexpression of eIF4E would attenuate isoprenoid-mediated regulation of HMG-CoA reductase. eIF4E is elevated in many tumor cells and behaves as a proto-oncogene by aberrantly translating mRNAs whose translation is normally suppressed by 5-UTRs that are GC rich. A CHO cell line expressing high levels of eIF4E (rb4E) was developed by infecting cells with retroviruses containing a full-length mouse cDNA for eIF4E. Levels of reductase synthesis were elevated fivefold in rb4E cells compared to noninfected CHO cells; HMG-CoA reductase mRNA levels were not increased in rb4E cells compared to normal CHO cells. Total cellular protein synthesis was only increased by approximately 15% in rb4E cells compared to CHO cells. The mTOR inhibitor rapamycin lowered HMG-CoA reductase synthesis by 50 and 60% in rb4E and CHO cells, respectively; no equivalent effect was observed for HMG-CoA reductase mRNA levels with rapamycin treatment. These results indicate that HMG-CoA reductase mRNA is in a class of mRNAs with highly structured 5'-UTRs whose m(7)GpppX cap-dependent translation is closely linked to the rapamycin-sensitive mitogen activated pathway for protein synthesis.

Plant-derived monoterpenes suppress hamster kidney cell 3-hydroxy-3-methylglutaryl coenzyme a reductase synthesis at the post-transcriptional level

The rate-limiting enzyme for mevalonate and cholesterol synthesis in mammalian cells is 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Control occurs through both transcriptional and post-transcriptional actions signaled by the end product, cholesterol, and by isoprenoid intermediates. End products of plant mevalonate metabolism, i.e., plant-derived isoprenoids, also suppress mammalian HMG-CoA reductase. Previous studies reported that isoprenoids suppress reductase synthesis at a post-transcriptional level. We tested the hypothesis that plant-derived isoprenoids also regulate mammalian HMG-CoA reductase synthesis at a post-transcriptional level by incubating lovastatin-treated C100 cells with mevalonate or a plant-derived isoprenoid (the monoterpenes, limonene, perillyl alcohol or geraniol) either alone or combined with the oxysterol, 25-hydroxycholesterol (25-OH C). Mevalonate decreased HMG-CoA reductase synthesis and mRNA levels by 65 and 66%, respectively (P < 0.05). The cyclic monoterpenes, limonene and perillyl alcohol, lowered HMG-CoA reductase synthesis by 70 and 89%, respectively (P < 0.05); although neither reduced HMG-CoA reductase mRNA levels (P = 0.88). Geraniol, an acyclic monoterpene, suppressed HMG-CoA reductase synthesis by 98% and lowered mRNA levels by 66% (P < 0.05). A combination of 25-OH C and either mevalonate or any three monoterpenes reduced HMG-CoA reductase mRNA levels (P < 0.05) compared with lovastatin-only treated cells. However, the dual combination of 25-OH C and either mevalonate or a monoterpene resulted in a greater decrease in HMG-CoA reductase synthesis than in mRNA levels. The difference between changes in HMG-CoA reductase synthesis and mRNA levels reflects a specific effect of isoprenoids on HMG-CoA reductase synthesis at the translational level. Mevalonate enhanced HMG-CoA reductase degradation, but no such effect was observed for the monoterpenes. These results indicate that the three plant-derived isoprenoids primarily suppress HMG-CoA reductase synthesis at a post-transcriptional level by attenuating HMG-CoA reductase mRNA translational efficiency.

Alterations in 3-hydroxy-3-methylglutaryl-CoA reductase mRNA concentration in cultured chick aortic smooth muscle cells

We observed and compared alterations in 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase at the transcriptional level in unsynchronized, three-passage cultures of smooth-muscle cells from the aorta of chicks fed on a control diet (C-SMC) and those of chicks fed on a similar diet plus cholesterol (Ch-SMC). Alterations in reductase mRNA concentrations in senescent cultures were much lower. We used a modification of the competitive (c) reverse transcription polymerase chain reaction method, using a Thermus thermophilus DNA polymerase (Tth pol) to quantify the very scarce species of HMG-CoA reductase mRNA in samples of cytoplasmic SMC mRNA. We cloned and sequenced a 199 bp cDNA fragment of chicken HMG-CoA reductase, which encoded a region of 66 amino acids belonging to the catalytic domain of the enzyme. HMG-CoA reductase mRNA concentrations from young C-SMC cultures rose 3.89-fold 4 h after the change of medium and returned to base levels between 8 to 12 h afterward. Concentrations in Ch-SMC cultures increased less (2.36-fold) 8 h after the change to fresh medium. Increases in reductase mRNA in senescent cultures of Ch-SMC and C-SMC measured under similar conditions were only 1.28- and 1.39-fold, respectively.

The regulatory effects of thyroid hormone on the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase

The effects of 3, 3', 5-triiodothyronine (T3) on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were evaluated in the C100 baby hamster kidney cell line. Cells cultured in Minimal Essential Medium (MEM) were supplemented with 10% thyroid hormone-depleted fetal bovine serum (THDS-MEM) and had a 70.1% lower level of HMG-CoA reductase activity than the cells grown in a medium supplemented with fetal bovine serum (FBS). When T3 was added to THDS-MEM, the reduction of the reductase activity was blocked in a dose-dependent manner. In the cells grown in THDS-MEM for 48 hours, T3 (10(-6) M) treatment rapidly increased HMG-CoA reductase activity, achieving the control level six hours after treatment. Such effects of T3 were blocked by actinomycin D (5 microg/ml) or cycloheximide (10 microg/ml). The transcriptional rate of the HMG-CoA reductase gene did not change significantly regardless of the presence of T3, while T3 inhibited the 25-hydroxycholesterol-mediated decay of the reductase mRNA significantly. Our results show that T3-dependent regulation of HMG-CoA reductase activity, via the de novo synthesis of the reductase enzyme, seems to be mediated at least partially by the stabilization of HMG-CoA reductase mRNA.

Impaired regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase degradation in lovastatin-resistant cells

L-90 cells were selected to grow in the presence of serum lipoproteins and 90 microM lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). L-90 cells massively accumulate HMGR, a result of >10-fold amplification of the gene and 40-fold rise in mRNA, and also overexpress other enzymes of the mevalonate pathway. Western blot and promoter-luciferase analyses indicate that transcriptional regulation of sterol-responsive genes by 25-hydroxycholesterol or mevalonate is normal. Yet, none of these genes is regulated by lipoproteins, a result of severe impairment in the low density lipoprotein receptor pathway. Moreover, L-90 cells do not accelerate the degradation of HMGR or transfected HMGal chimera in response to 25-hydroxycholesterol or mevalonate. This aberrant phenotype persists when cells are grown without lovastatin for up to 37 days. The inability to regulate HMGR degradation is not due to its overproduction since in LP-90 cells, which were selected for lovastatin resistance in lipoprotein-deficient serum, HMGR is overexpressed, yet its turnover is regulated normally. Also, the rapid degradation of transfected alpha subunit of T cell receptor is markedly retarded in L-90 cells. These results show that in addition to gene amplification and overexpression of cholesterogenic enzymes, statin resistance can follow loss of regulated HMGR degradation.

Regulatory signals in messenger RNA: determinants of nutrient–gene interaction and metabolic compartmentation

Nutrition has marked influences on gene expression and an understanding of the interaction between nutrients and gene expression is important in order to provide a basis for determining the nutritional requirements on an individual basis. The effects of nutrition can be exerted at many stages between transcription of the genetic sequence and production of a functional protein. This review focuses on the role of post-transcriptional control, particularly mRNA stability, translation and localization, in the interactions of nutrients with gene expression. The effects of both macronutrients and micronutrients on regulation of gene expression by post-transcriptional mechanisms are presented and the post-transcriptional regulation of specific genes of nutritional relevance (glucose transporters, transferrin, selenoenzymes, metallothionein, lipoproteins) is described in detail. The function of the regulatory signals in the untranslated regions of the mRNA is highlighted in relation to control of mRNA stability, translation and localization and the importance of these mRNA regions to regulation by nutrients is illustrated by reference to specific examples. The localization of mRNA by signals in the untranslated regions and its function in the spatial organization of protein synthesis is described; the potential of such mechanisms to play a key part in nutrient channelling and metabolic compartmentation is discussed. It is concluded that nutrients can influence gene expression through control of the regulatory signals in these untranslated regions and that the post-transcriptional regulation of gene expression by these mechanisms may influence nutritional requirements. It is emphasized that in studies of nutritional control of gene expression it is important not to focus only on regulation through gene promoters but also to consider the possibility of post-transcriptional control.

Role of fibronectin and sulfated proteoglycans in endothelial cell migration on a cultured smooth muscle layer

Endothelial cell (EC) migration is essential for the healing of denudation injuries to the vessel wall. This event occurs in vivo in a pericellular environment and should be regulated in part by juxtacrine and paracrine interactions with underlying smooth muscle cells (SMCs). To investigate the EC migration behavior over SMCs under direct cell-cell contact conditions, we have utilized human umbilical vein endothelial cells (HUVECs) grown to confluence on microcarrier beads and radiolabeled with chromium-51. The EC carrying beads were spread over a confluent human aortic SMC monolayer and cocultured for 48 hr, allowing the EC to migrate from beads onto the underlying SMC monolayer in the presence of sodium chlorate, cycloheximide (CHM), or anti-fibronectin monoclonal antibody (anti-FN mAb). The level of EC migration was quantified by counting the amount of radioactivity on the SMC layer after the removal of the beads. The presence of a SMC layer enhanced EC migration more than threefold (P < 0.05). Furthermore, EC migration was inhibited from 30 to 80% (P < 0.05) by sodium chlorate, CHM, and anti-FN mAb in a dose-dependent fashion. This model has shown that smooth muscle cells augment endothelial motility. Both fibronectin and sulfated proteoglycans released by ECs and SMCs likely play an important role in the regulation of EC migration behavior over SMCs. The method described of using radiolabeled ECs on microcarrier beads should prove to be a useful tool in the study of cell migration in a heterotypic cellular environment.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase degradation by the nonsterol mevalonate metabolite farnesol in vivo

We have previously reported that degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the rate-limiting enzyme in the isoprenoid pathway leading to cholesterol production, can be accelerated in cultured cells by the addition of farnesyl compounds, which are thought to mimic a natural, nonsterol mevalonate metabolite(s). In this paper we report accelerated reductase degradation by the addition of farnesol, a natural product of mevalonate metabolism, to intact cells. We demonstrate that this regulation is physiologically meaningful, shown by its blockage by several inhibitory conditions that are known to block the degradation induced by mevalonate addition. We further show that intracellular farnesol levels increase significantly after mevalonate addition. Based on these results, we conclude that farnesol is a nonsterol, mevalonate-derived product that plays a role in accelerated reductase degradation. Our conclusion is in agreement with a previous report (Correll, C. C., Ng, L., and Edwards, P. A. (1994) J. Biol. Chem. 269, 17390-17393), in which an in vitro system was used to study the effect of farnesol on reductase degradation. However, the apparent stimulation of degradation in vitro appears to be due to nonphysiological processes. Our findings demonstrate that in vitro, farnesol causes reductase to become detergent insoluble and thus lost from immunoprecipitation experiments, yielding apparent degradation. We further show that another resident endoplasmic reticulum protein, calnexin, similarly gives the appearance of protein degradation after farnesol addition in vitro. However, after the addition of farnesol to cells in vivo, calnexin remains stable, whereas reductase is degraded, providing further evidence that the in vivo effects of farnesol are physiologically meaningful and specific for reductase, whereas the in vitro effects are not.

Mevalonate regulates polysome distribution and blocks translation-dependent suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA: relationship to translational control

We reported previously that 3-hydroxy-3-methylglutaryl coenzyme A reductase synthesis is regulated at the translational level by mevalonate. To determine at what stage mevalonate affects reductase synthesis, we examined the distribution of reductase mRNA in polysomes from cells treated with lovastatin alone; lovastatin and 25-hydroxycholesterol; or lovastatin, 25-hydroxycholesterol, and mevalonate. In lovastatin-treated cells, reductase mRNA was primarily associated with heavy polysome fractions. When 25-hydroxycholesterol was added to lovastatin-treated cells, reductase mRNA levels were reduced approximately fourfold in all polysome fractions, with no accompanying redistribution of reductase mRNA into lighter polysome fractions. However, addition of both 25-hydroxycholesterol and mevalonate to lovastatin-treated cells shifted reductase mRNA from heavier to lighter polysome fractions. No change in the distribution of control beta-actin or ribosomal protein S17 mRNA occurred with any of the treatments. These results suggest that mevalonate suppresses reductase synthesis at the level of initiation. When the translation inhibitor cycloheximide was added to all three regimens, reductase mRNA shifted into heavy polysome fractions. Treatment with either lovastatin alone or lovastatin plus 25-hydroxycholesterol resulted in a 50% greater loss of reductase mRNA from the heavy polysome fractions compared to the same fractions from noncycloheximide-treated cells. No loss of reductase mRNA occurred when cycloheximide was added to cells treated with both 25-hydroxycholesterol and mevalonate. beta-Actin mRNA levels and polysome distribution were not significantly changed by cycloheximide under any of these conditions. Translationally mediated suppression of reductase mRNA did not occur when protein synthesis was inhibited with puromycin. Our results indicate that regulation of reductase mRNA levels is translation-dependent and is linked to the rate of elongation.