Uptake of dolichol into cultured cells

Metabolism of dolichol, dolichoic acid and nordolichoic acid in cultured cells

The uptake and metabolism of [1-(14)C]-labelled dolichol, dolichoic acid and nordolichoic acid were investigated in MDCK and HepG2 cells. Each of the three isoprenoids, bound to human serum albumin, was taken up effectively. None of the compounds was broken down in HepG2 cells, although these converted dolichol into fatty acid esters. In MDCK cells dolichoic acid gave rise to the formation of [14C]CO2 and radiolabelled formic acid, indicating that dolichoic acid can be broken down by alpha-oxidation. Dolichoic acid was also converted to a mixture of polar compounds, possibly polyols. MDCK cells generated radiolabelled CO2 from nordolichoic acid, presumably through beta-oxidation, although we could not find any labelled propionic acid. No oxidative breakdown of dolichol was found, apparently due to the lack of or very low conversion to dolichoic acid.

Mevalonate dependency of the early cell cycle mitogenic response to epidermal growth factor and prostaglandin F2 alpha in Swiss mouse 3T3 cells

Lovastatin (LOV), a hydroxy-methylglutaryl-coenzyme A (HMGCoA) reductase competitive inhibitor, blocks epidermal growth factor (EGF)- or prostaglandin F2 alpha (PGF2 alpha)-induced mitogenesis in confluent resting Swiss 3T3 cells. This inhibition occurs even in the presence of insulin, which potentiates the action of these mitogens in such cells. LOV exerts its effect in a 2-80 microM concentration range, with both mitogens attaining 50% inhibition at 7.5 microM. LOV exerted its effect within 0-8 h following mitogenic induction. Mevanolactone (10-80 microM) in the presence of LOV could reverse LOV inhibition within a similar time period. LOV-induced blockage of PGF2 alpha response is reflected in a decrease in the rate of cell entry into S phase. Neither cholesterol, ubiquinone, nor dolichols of various lengths could revert LOV blockage. In EGF- or PGF2 alpha-stimulated cells, LOV did not inhibit [3H]leucine or [3H]mannose incorporation into proteins, while tunicamycin, an inhibitor of N' glycosylation, prevented this last phenomenon. Thus, it appears that LOV exerts its action neither by inhibiting unspecific protein synthesis nor by impairing the N' glycosylation process. These findings strongly suggest that either EGF or PGF2 alpha stimulations generate early cell cycle signals which induce mevalonate formation, N' glycoprotein synthesis, and proliferation. The causal relationship of these events to various mechanisms controlling the onset of DNA synthesis is also discussed.

Uptake and transport of fluorescent derivatives of dolichol in human fibroblasts

We are using fluorescent derivatives to visualize the endocytic transport of dolichol intermediates from the cell surface to the lysosome, and to estimate their rate of turnover within the lysosome. Anthroyl dolichol and anthroyl [1-14C]dolichol were synthesized and purified by chromatography on silica and C18 Sep-Paks followed by high-performance liquid chromatography on C18. The successful synthesis of anthroyl polyisoprenoid alcohols was confirmed by the use of uv-visible spectrometry and by fluorescence spectrometry. The purified esters were taken up into Ham's media containing 10-30% fetal calf serum or alternatively reconstituted into phospholipid liposomes for delivery to human fibroblasts in culture. The uptake of fluorescent dolichol esters into the cells and into lysosomes was demonstrated using fluorescence microscopy. The localization of anthroyl dolichol in lysosomes was further documented by simultaneously labeling fibroblasts with anthroyl dolichol and FITC-dextran a recognized lysosomal marker. Fibroblasts generally showed several groupings (domains) of lysosomes, some were dually labeled while others were labeled exclusively with either anthroyl dolichol or FITC-dextran. Labeling with anthroyl dolichol was very slow relative to labeling of the same fibroblasts with FITC-dextran suggesting that anthroyl dolichol acts as a labeling agent for intracellular membranes, particularly those of the lysosome while the dextran fluorescence is presumably of lysosolic origin. Several types of experiments were done with anthroyl [1-14C]dolichol to establish that the fluorescence seen in lysosomes represents anthroyl dolichol. Anthroyl dolichol appears to enter fibroblasts intact, since we were unable to recover any free [1-14C]dolichol from total lipid extracts of (i) media used for the uptake of anthroyl dolichol or (ii) the media removed from cells labelled for 42 h. In addition, attempts to hydrolyze anthroyl [1-14C]dolichol in vitro using whole fibroblast homogenates at pH 4.0 and 7.5 were unsuccessful, even though the fibroblasts expressed acid lipase activity using 4-methylumbelliferyl palmitate as substrate.

Effects of growth factors on cell cycle arrest in dolichyl phosphate-depleted cultures

Previously we showed that CHO cell growth is arrested in the G1 or G0 phase within 24 h after the biosynthesis of mevalonic acid is blocked. The growth-limiting factor under these conditions appeared to be dolichyl phosphate or one of its glycosylated derivatives with consequent decrease in the synthesis of N-linked glycoproteins (Doyle, J.W., and A.A. Kandutsch, 1988, J. Cell Physiol. 137:133-140; Kabakoff, B., J.W. Doyle, and A.A. Kandutsch, 1990, Arch. Biochem. Biophys. 276:382-389). We show herein that cell surface glycoproteins are depleted in the inhibited cultures and that growth arrest is delayed when supraphysiological concentrations of insulin, insulin-like growth factor-1 (IGF-1) and bFGF are added to the culture medium. Apparently an elevated level of a growth factor increases the length of time during which a threshold level of occupied receptor is maintained as the number of glycosylated receptor molecules declines. The results support the idea that cellular levels of dolichyl phosphate and its derivatives may limit cell division by controlling the numbers of functional receptors for growth factors and of other glycoproteins on the cell surface.

Relationships among dolichyl phosphate, glycoprotein synthesis, and cell culture growth

Following treatment of Chinese hamster ovary cells with inhibitors of mevalonate biosynthesis in the presence of exogenous cholesterol, the cellular concentration of phosphorylated dolichol and the incorporation of [3H]mannose into dolichol-linked saccharides and N-linked glycoproteins declined coincident with a decline in DNA synthesis. Addition of mevalonate to the culture medium increased rates of mannose incorporation into lipid-linked saccharides and restored mannose incorporation into N-linked glycoproteins to control levels within 4 h. After an additional 4 h, synchronized DNA synthesis began. Inhibition of the synthesis of lipid-linked oligosaccharides and N-linked glycoproteins by tunicamycin prevented the induction of DNA synthesis by mevalonate, indicating that glycoprotein synthesis was required for cell division. The results suggest that the rate of cell culture growth may be influenced by the level of dolichyl phosphate acting to limit the synthesis of N-linked glycoproteins.

Requirement for mevalonate in cycling cells: quantitative and temporal aspects

In order to investigate a requirement for isoprenoid compounds in the cell cycle, DNA synthesis was examined in cultured Chinese hamster ovary cells in which mevalonate biosynthesis was blocked with mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Treatment of exponentially-growing cultures with mevinolin led to a decline in DNA synthesis and cell cycle arrest in G1. Synchronous DNA synthesis and cell division could be restored in the arrested cultures, in the absence of exogenous mevalonate, by removing the inhibitor from the culture thereby allowing expression of an induced level of HMG-CoA reductase. In order to quantitate the mevalonate requirement for entry into S phase, recovery of DNA synthesis was made dependent upon added mevalonate by preventing the induction of the enzyme using 25-hydroxycholesterol, a specific repressor of HMG-CoA reductase synthesis. When cultures were treated with both inhibitors, optimal recovery of DNA synthesis was obtained with 200 micrograms/ml mevalonate following an 8 h lag, whereas a progressively longer lag-time was found with lower concentrations of mevalonate. Exogenous dolichol, ubiquinone, or isopentenyladenine had no effect on the arrest or recovery of DNA synthesis. Cholesterol was required during the arrest incubation for cell viability, but was not sufficient for recovery in the absence of mevalonate. The recovery of DNA synthesis by 200 micrograms/ml mevalonate, which was maximal 14-16 h after the addition of mevalonate, only required that the mevalonate be present for the first 4 h, whereas more than an 8-h incubation was required for maximal recovery with 25 micrograms/ml mevalonate. Maximal recovery at either concentration of mevalonate was achieved after approximately 400 fmol mevalonate/micrograms protein was incorporated into non-saponifiable lipids. This quantity represents approximately 0.1% of the mevalonate required for the synthesis of total cellular isoprenoid compounds. The results indicate that production of a quantitatively minor product(s) of mevalonate metabolism is required during the first 4 h following release of the block before other cellular events necessary for entry into S phase can occur.

On the binding of dolichol by human serum

The presence of a dolichol binding system is demonstrated in human serum. The dolichol binding exhibits normal saturation kinetics with an apparent affinity constant Kd of 6.9 X 10(-6) M. Optimal binding is obtained at pH 7.4 and 5 degrees C. After binding the [3H]dolichol cannot be chased by unlabelled dolichol. The selectivity is examined by competition studies showing that only dolichyl derivatives equally compete for binding sites. From buoyant density centrifugation and gel filtration it is deduced that the dolichol binding is due to a serum protein fraction, displaying the characteristics of VLDL.

Effects of 25-hydroxycholesterol, cholesterol, and isoprenoid derivatives on the G1 progression in Swiss 3T3 cells

The effect of inhibition of 3-Hydroxy-3-methylglutaryl Coenzyme A reductase (HMG CoA reductase) on cell cycle progression in proliferating 3T3 cells was studied. It was found that short transient exposures to the HMG CoA reductase inhibitor 25-hydroxycholesterol temporarily blocked the cell cycle traverse in the postmitotic half of G1 (G1pm), whereas cells in the subsequent cell cycle phases were unaffected. The kinetics of the cell cycle delay, induced by 25-hydroxycholesterol, resembled the kinetics of the delay induced by serum depletion, which also inhibited the activity of HMG CoA reductase. In contrast to the case of serum depletion, platelet derived growth factor (PDGF), which efficiently prevented the decrease of HMG CoA reductase in serum-free medium, was not capable of preventing the growth inhibitory effect following treatment by 25-hydroxycholesterol. However, cholesterol and two isoprenoids, dolichol and coenzyme Q, were effective in this respect. In addition, dolichol counteracted the cell cycle delay following short periods of serum starvation.

Distribution, metabolism and function of dolichol and polyprenols

Polyisoprenoid alcohols consisting of 9 or more isoprene units are present in all living cells. They can be fully unsaturated (polyprenols) or alpha-saturated (dolichol). Dolichol forms may have additional saturation at or near the omega-end. Some species contain ony dolichol or only polyprenols while others have nearly equal amounts of both types. Some polyisoprenoid alcohols consist entirely of trans isoprene units but most, including dolichol, contain both trans and cis units. Considerable advances in lipid methodology have occurred since the first review of polyisoprenoid alcohols by Hemming in 1974. For example, direct analysis of both dolichol and Dol-P by HPLC has replaced earlier methods which were often both insensitive and inaccurate. The availability of radiolabeled dolichol and polyprenols has facilitated studies concerning the metabolism and distribution of these compounds. Those studies suggest that only a small portion of the dolichol present in cells is likely to be involved in glycosylation. Polyisoprenoid alcohols are usually present at a family of homologues where each differs in size by one isoprene unit. Little or no size related specificity has been observed for any reaction involving dolichol or polyisoprenol intermediates. The overall length of polyisoprenoid alcohols may, however, affect the manner in which these compounds influence the physical and biochemical properties of membranes. Studies on the biosynthetic pathway leading from cis, trans Pol-PP by phosphatase action. The formation of the dolichol backbone from a polyprenol requires the action of an additional enzyme, an alpha-saturase. This enzyme does not always act at the level of a single common substrate, since Pol-PP, Pol-P, and polyprenol all appear to be utilized as substrates. The major product of the de novo pathway differs among different species. Dol-P would appear to be the most energy efficient end-product since it can participate directly in glycoprotein formation. Most often, however, Dol-P is not the major product of metabolic labeling experiments. In some cases, dolichol is formed so that rephosphorylation is required to provide Dol-P for participation in glycoprotein formation. The kinase responsible for this phosphorylation appears to bypass the considerable stores of dolichol present in tissues (i.e. sea urchin eggs) in favor of dolichol derived directly from de novo synthesis. Although HMGR is a major regulatory component of the pathway leading to polyisoprenoid alcohols and cholesterol, control is most often not co-ordinated.(ABSTRACT TRUNCATED AT 400 WORDS)

Dolichyl phosphate supplementation increases N-linked protein glycosylation in rat parotid acinar cells without increasing glycoprotein secretion

N-linked protein glycosylation was increased three- to five-fold, in dispersed rat parotid acinar cells in vitro, by supplementation with exogenous dolichylphosphate. Despite this increase, glycoprotein secretion from both control and dolichylphosphate-supplemented cells was comparable.