Showing papers on "Sterol published in 1987"

Intestinal cholesterol absorption efficiency in man is related to apoprotein E phenotype.

Abstract: Relationship between the efficiency of cholesterol absorption and apolipoprotein E (apoE) phenotype was studied in a random sample of middle-aged Finnish men. Subjects that were either heterozygous or homozygous for the allele epsilon 2 absorbed less and synthesized more cholesterol than those with the phenotype E4/3 and E4/4, the values for the individuals with the most frequent phenotype E3/3 (56% of the population sample) falling in between. Among the whole study group, the sum of the subscripts of apoE phenotype (e.g., E2/3 = 5) was correlated positively with the fractional absorption of cholesterol (r = 0.40; P less than 0.05) and negatively with the serum level of lathosterol, a cholesterol precursor sterol reflecting the activity of cholesterol synthesis (r = -0.48; P less than 0.01). Thus, apoE polymorphism appears to affect the efficiency of cholesterol absorption and may by this mechanism contribute to the variation in plasma total and low density lipoprotein cholesterol concentration.

Plasma Membrane Lipid Alterations Associated with Cold Acclimation of Winter Rye Seedlings (Secale cereale L. cv Puma)

Abstract: Highly enriched plasma membrane fractions were isolated from leaves of nonacclimated (NA) and acclimated (ACC) rye (Secale cereale L. cv Puma) seedlings. Collectively, free sterols, steryl glucosides, and acylated steryl glucosides constituted >50 mole% of the total lipid in both NA and ACC plasma membrane fractions. Glucocerebrosides containing hydroxy fatty acids constituted the major glycolipid class of the plasma membrane, accounting for 16 mole% of the total lipid. Phospholipids, primarily phosphatidylcholine and phosphatidylethanolamine with lesser amounts of phosphatidylglycerol, phosphatidic acid, phosphatidylserine, and phosphatidylinositol, comprised only 32 mole% of the total lipid in NA samples. Following cold acclimation, free sterols increased from 33 to 44 mole%, while steryl glucosides and acylated steryl glucosides decreased from 15 to 6 mole% and 4 to 1 mole%, respectively. Sterol analyses of these lipid classes demonstrated that free beta-sitosterol increased from 21 to 32 mole% (accounting for the increase in free sterols as a class) at the expense of sterol derivatives containing beta-sitosterol. Glucocerebrosides decreased from 16 to 7 mole% of the total lipid following cold acclimation. In addition, the relative proportions of associated hydroxy fatty acids, including 22:0 (h), 24:0 (h), 22:1 (h), and 24:1 (h), were altered. The phospholipid content of the plasma membrane fraction increased to 42 mole% of the total lipid following cold acclimation. Although the relative proportions of the individual phospholipids did not change appreciably after cold acclimation, there were substantial differences in the molecular species. Di-unsaturated molecular species (18:2/18:2, 18:2/18:3, 18:3/18:3) of phosphatidylcholine and phosphatidylethanolamine increased following acclimation. These results demonstrate that cold acclimation results in substantial changes in the lipid composition of the plasma membrane.

Total lipid production of the green alga nannochloropsis sp qii under different nitrogen regimes

Abstract: The green alga Nannochloropsis sp. QII was cultivated in media with sufficient and growth-limiting levels of nitrogen (nitrate). Nitrogen deficiency promoted lipid synthesis yielding cells with lipids comprising 55% of the biomass. The major lipids were triacylglycerols (79%), polar lipids (9%) and hydrocarbons (2.5%). The polar lipids consisted of a broad range of phospholipids, glycolipids and sulfolipids. Other lipids identified were pigments, free fatty acids, saponifiable and unsaponifiable sterol derivatives, various glycerides, a family of alkyl-1, 4-dioxane derivatives and a series of alkyl- and hydroxy-alkyl-dimethyl-acetals. Experiments in which /sup 14/CO/sub 2/ was provided at different times in the growth cycle demonstrated that enhanced lipid biosynthesis at low nitrogen levels resulted principally from de novo CO/sub 2/ fixation.

Group C Niemann-Pick disease: faulty regulation of low-density lipoprotein uptake and cholesterol storage in cultured fibroblasts.

Abstract: Incubation of mutant Niemann-Pick C fibroblasts with low-density lipoprotein (LDL) resulted in excessive internalization of lipoprotein and extensive cellular over-accumulation of unesterified cholesterol. The uptake of LDL by the mutant cells appeared to occur through the classic LDL receptor pathway and internalized lipoprotein was processed in lysosomes. Lipoprotein uptake into mutant cells was associated with delays in the initiation of established cellular cholesterol homeostatic responses. Subcellular fractionation of mutant Niemann-Pick C fibroblasts accumulating LDL-cholesterol showed excess unesterified sterol to be localized in the light lysosome-light membrane region of a Percoll gradient, and revealed that cholesterol storage was associated with a specific alteration in the normal profiles of lysosomal marker enzymes.

[58] Plant membrane sterols: Isolation, identification, and biosynthesis

Abstract: Publisher Summary This chapter presents detailed analysis of sterols and sterol derivatives of well-characterized (by a rich array of both morphological and biochemical markers) membrane fractions from etiolated maize coleoptiles, with a special focus on a plasma membrane (PM)-rich fraction. Then data about the different cellular sites of plant sterol biosynthesis in the same material are presented. It is presented in the chapter that it is possible to modify the sterol profile of higher plant membranes using sterol biosynthesis inhibitors. Results that have obtained with maize roots treated by fenpropimorph are given. If both free sterols and sterol conjugates of membrane fractions are to be analyzed, then the total lipid must be extracted three times with 6 vol of dichloromethane-methanol (2:1, v/v). To obtain sterols from steryl esters (SE), the corresponding fraction can be hydrolyzed by heating for one hour with 5 ml of 6% (w/v) KOH in methanol and pyrogallol.

Fungal resistance to sterol biosynthesis inihitors: a new challenge

Abstract: Yield losses caused by plant pathogens have threatened the security and efficiency of crop production since agriculture became the main source of the human food supply. Fortunately, agriculture has made tremendous progress during the last century, and part of this progress has been the development of modern means of plant disease control. In particular, the introduction of chemical disease control agents has contributed to a substantial increase in c r o p p roduc t ion , t o a smoothing of annual undulations in crop yields, and, ultimately, to today's high level of food security. T h e f i rs t mi les tone in fungic ide development was the introduction of ino rgan ic fungic ides such a s sul fur , copper, or mercury compounds, followed by the development of organic fungicides such as dithiocarbamates (e.g., maneb) and phthalimides (e.g., captan). These two classes of protective compounds have been used extensively for decades without development of field resistance. D u r i n g the s a m e pe r iod , o rgan ic insect ic ides had a l r eady encoun te red cumbersome drawbacks. Gordon (16) introduced his 196 1 review on insecticide resistance with a clear statement: \"The number of insect species or populations resistant t o one or more of the synthetic organic insecticides has increased every year since 1947, and there is yet no indication that this trend can be halted or reversed.\" The conclusion on fungicide res is tance d r a w n 6 years l a t e r by Georgopoulos and Zaracovitis (14) was clearly different: \"Tolerance to organic fungicides used in the control of fungal diseases of plants or storage rot has created practical difficulties in only a few instances.\"The future prospects, however, sounded less optimistic, and the authors must have seen the dawn of a major change: \"If future fungicides must be selective, interfering with the metabolism

A protein kinase antigenically related to pp60v-src possibly involved in yeast cell cycle control: positive in vivo regulation by sterol

Abstract: The effects of ergosterol, yeast's natural sterol, on cell cycling and a protein kinase antigenically related to pp60v-src were examined in a sterol auxotroph of Saccharomyces cerevisiae. Sterol-depleted cells accumulate in an unbudded, G1 state. Cell budding and proliferation are reinitiated upon addition of nonlimiting ergosterol or cholesterol with trace ergosterol, whereas cholesterol or trace ergosterol alone is less effective. Stimulation of a protein kinase associated with immune complexes of yeast protein and anti-pp60v-src shows a positive correlation with exit from the G1 phase following ergosterol addition. Ergosterol-stimulated cells also demonstrate an increase in phosphatidylinositol kinase activity. The data suggest that hormonal levels of ergosterol (effective concentration, approximately equal to 1 nM) participate in a signaling process associated with a protein kinase possibly involved in yeast cell cycle control.

Partial deletion of membrane-bound domain of 3-hydroxy-3-methylglutaryl coenzyme A reductase eliminates sterol-enhanced degradation and prevents formation of crystalloid endoplasmic reticulum.

Abstract: 3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase is anchored to the endoplasmic reticulum (ER) membrane by a hydrophobic NH2-terminal domain that contains seven apparent membrane-spanning regions and a single N-linked carbohydrate chain. The catalytic domain, which includes the COOH-terminal two-thirds of the protein, extends into the cytoplasm. The enzyme is normally degraded with a rapid half-life (2 h), but when cells are depleted of cholesterol, its half-life is prolonged to 11 h. Addition of sterols accelerates degradation by fivefold. To explore the requirements for regulated degradation, we prepared expressible reductase cDNAs from which we either deleted two contiguous membrane-spanning regions (numbers 4 and 5) or abolished the single site for N-linked glycosylation. When expressed in hamster cells after transfection, both enzymes retained catalytic activity. The deletion-bearing enzyme continued to be degraded with a rapid half-life in the presence of sterols, but it no longer was stabilized when sterols were depleted. The glycosylation-minus enzyme was degraded at a normal rate and was stabilized normally by sterol deprivation. When cells were induced to overexpress the deletion-bearing enzyme, they did not incorporate it into neatly arranged crystalloid ER tubules, as occurred with the normal and carbohydrate-minus enzymes. Rather, the deletion-bearing enzyme was incorporated into hypertrophied but disordered sheets of ER membrane. We conclude that the carbohydrate component of HMG CoA reductase is not required for proper subcellular localization or regulated degradation. In contrast, the native structure of the transmembrane component is required to form a normal crystalloid ER and to allow the enzyme to undergo regulated degradation by sterols.

Sterol Carrier and Lipid Transfer Proteins

Abstract: Publisher Summary The discovery of the lipid exchange, transfer, or carrier proteins is a result of the findings that cells contained cytosolic factors that were required for microsomal synthesis of cholesterol or could accelerate the transfer or exchange of phospholipids between membrane preparations. Most of the lipid exchange transfer or carrier activities have been characterized as cytosolic factors. These activities might be concentrated in cell particulate fractions, such as microsomes, and might be released to varying extents by tissue homogenization and/or treatment of the “cytosolic” fraction. The phospholipid exchange and transfer activities have been determined by the exchange or transfer of labeled phospholipids between microsomes and mitochondria or liposomes. These more direct assays involving aspects of cholesterol metabolism suggest the possibility that there is greater biological specificity for sterol carrier protein 2 (SCP 2) than is implied by the term non-specific lipid transfer protein. Another protein with apparent biological specificity is fatty acid-binding protein (FABP). Three approaches have been employed to determine fatty acid-binding protein (FABP) activities: Sephadex G-50 chromatography of liver cytosol following addition of [14 C] oleate; use of hydrophobic Lipidex 1000 to separate FABP-bound from unbound fatty acids; and immunoprecipitation technique.

The lipid composition and permeability to azole of an azole- and polyene-resistant mutant of Candida albicans

Abstract: Candida albicans 6.4, which is resistant to both polyene and azole groups of antifungal antibiotics, has a larger lipid content and lower polar lipid to neutral lipid ratio compared with other strains that are sensitive or resistant only to azoles. C. albicans 6.4 contains a relatively greater proportion of triacylglycerol in its neutral lipid in the exponential phase of batch culture compared with other strains, but, unlike them, does not accumulate triacylglycerols or any other stored lipid in the stationary phase. Like other strains, in C. albicans 6.4 the major phospholipids are phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, but sphingomyelin is absent; the major fatty acids are palmitic, palmitoleic, oleic and linoleic acids. In common with other C. albicans strains, strain 6.4 contains non-specific (lyso)phospholipase activity. The main distinctive feature of the lipid composition of C. albicans 6.4 is the absence of ergosterol, which is replaced by methylated sterol; mainly lanosterol, 24-methylene-24,25-dihydrolanosterol and 4-methylergostadiene-3-ol. It is suggested that the altered membrane sterol pattern provides a common basis for the double resistance by preventing polyene binding and reducing azole permeability.

Isomers of sterol synthesis inhibitors: Fungicidal effects and plant growth regulator activities

Abstract: The chemical diversity of fungicides and antimycotics interfering with the C-14 demethylation of fungal sterols indicates a rather high structural flexibility of the sterol binding site at the specific cytochrome P-450 mixed-function oxygenase. The structural flexibility, however, is opposed by a remarkable stereochemical selectivity. A similar, though not identical, stereochemical discrimination is observed with a group of plant growth regulators chemically related to the fungicidal demethylation inhibitors. The relationship between chemical structure and stereochemical requirements is discussed for both biological activities.

Sterol biosynthesis via cycloartenol and other biochemical features related to photosynthetic phyla in the amoebae Naegleria lovaniensis and Naegleria gruberi

Abstract: The sterols and sterol precursors of two amoebae of the genus Naegleria, Naegleria lovaniensis and Naegleria gruberi were investigated. Cycloartenol, the sterol precursor in photosynthetic organisms, is present in both amoebae. In N. lovaniesis, it is accompanied by lanosterol and parkeol, as well as by the 24,25-dihydro derivatives of these triterpenes. One of the most striking features of these amoebae is the accumulation of 4 alpha-methylsterols which are present in similar amounts as those of 4,4-desmethylsterols (3-5 mg/g, dry weight). 4 alpha-Methylergosta-7,22-dienol was identified as a new compound. Ergosterol was the major 4,4-desmethylsterol, accompanied by small amounts of C27 and other C28 sterols. Treatment of N. lovaniensis with fenpropimorph modified the sterol pattern of this amoeba and inhibited its growth. This fungicide, known to inhibit steps of sterol biosynthesis in fungi and plants, induced the disappearance of 4 alpha-methyl-delta 7-sterols and the appearance of the unusual delta 6,8,22-ergostatrienol as in A. polyphaga. These results might be explained by a partial inhibition of the delta 8----delta 7 isomerase, the small amounts of delta 7-sterols formed being converted into ergosterol which is still present in fenpropimorph-exposed cells. De novo sterol biosynthesis in N. lovaniensis was shown by incorporation of [1-14C]acetate into sterols and sterol precursors, especially cycloartenol. Lanosterol and parkeol were not significantly labelled. Furthermore, [3-3H]squalene epoxide was efficiently cyclized by a cell-free system of this amoeba into cycloartenol, and again no significant radioactivity was detected in lanosterol and parkeol. This shows that cycloartenol, the sterol precursor in plants and algae, is also the sterol precursor in Naegleria species, and that these amoebae, like A. polyphaga, are related by some biosynthetic pathways to photosynthetic phyla. Lanosterol, the sterol precursor in non-photosynthetic phyla (animal and fungi) and parkeol are more likely dead-ends of this biosynthetic pathway. The peculiar phylogenetic position of these protozoa was further emphasized by the action of indole acetic acid and other auxine-like compounds on their growth. Indeed amoebic growth was enhanced in the presence of these higher plant growth hormones. The differences in the sterol composition of the protozoa we have hitherto examined is related to their sensitivity toward polyene macrolide antibiotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Fecal steroids and colorectal cancer

Abstract: The fecal steroid profiles of healthy subjects were compared with those of colorectal cancer (CRC) patients. The multicomponent profiles did not differ qualitatively in that CRC patients, like control subjects, had similar fecal steroids. The major bile acids detected in fecal extracts were lithocholic acid (LCA) and deoxycholic acid (DCA). The major sterol of animal origin was cholesterol and its bacterial metabolite coprostanol, whereas the major plant sterols were beta-sitosterol, stigmasterol, campesterol, and their corresponding bacterial metabolites. CRC patients excreted higher amounts of total major bile acids (LCA and DCA) than did the control group, but this difference was not significant. However, the LCA-to-DCA ratio was much higher in the CRC group [(1.43, p less than 0.01) compared with the control group (0.72)]. The control group excreted significantly higher amounts of total neutral sterols (p less than 0.001), sterols of animal origin (p less than 0.001), and plant sterols (p less than 0.001) compared with the CRC group; the plant sterols represented a much lower proportion of excreted total neutral sterols in the CRC group (p greater than 0.001) compared with the control group. We propose the following hypotheses. The LCA-to-DCA ratio may be an important discriminant market for CRC susceptibility. The fecal LCA-to-DCA ratio may depend on the differential hepatic synthesis of their respective precursors chenodeoxycholic acid (CDCA) and cholic acid. Hepatic synthesis of CDCA may be increased by more efficient conservation of dietary cholesterol because it has been shown that cholesterol of exogenous origin is the main precursor of this bile acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of aging on cholesterol content and cholesterol-metabolizing enzymes in the rat adrenal gland.

Abstract: Previous studies from this laboratory have documented a progressive decline in basal and ACTH-stimulated corticosterone production in isolated adrenocortical cells as rats age. In the current study we examined the possibility that the aging process exerts this effect by interfering with the mechanism(s) by which cholesterol is processed and/or synthesized by the adrenal gland. Freshly excised adrenals from 2-, 5-, 12-, and 18-month-old rats were used for the measurement of cholesteryl ester, free cholesterol, cholesteryl esterases, and acyl co-enzyme A (CoA)-cholesterol acyltransferase activities as well as key enzymes involved in cholesterol biosynthesis. The results showed that cholesteryl ester content increased in a linear manner with advancing age, while neutral cholesteryl esterase activity decreased steadily until at 18 months of age it reached 40% that of 2-month-old control rats. In contrast, lysosomal acid cholesteryl esterase did not change with age, and acyl CoA: Cholesterol acyltransferase showed only a 33% decrease at 12 months of age. The activity of 3-hydroxy-3-methylglutaryl CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis, decreased steadily with advancing age, and at 18 months of age, activity was only half of that in 2-month-old control rats. In contrast, the activities of other enzymes involved in the de novo synthesis of cholesterol, namely acetoacetyl CoA thiolase and HMG CoA synthase, were similar in 2- and 12-month-old rats, while mevalonate kinase activity was significantly lower in the 12-month-old rats. After depletion of plasma lipoprotein cholesterol by 4-aminopyrazolo-[3,4-d]pyrimidine, the intraadrenal cholesteryl ester content in young and aged animals fell significantly. Furthermore, such treatment enhanced the activities of all of the cholesterol de novo synthetic enzymes examined. In addition, HMG CoA synthase and HMG CoA reductase activities rose to much greater levels in both young and old rats compared to acetoacetyl CoA thiolase and mevalonate kinase. Finally, markedly higher activities of HMG CoA reductase were observed in 12- and 18-month-old rats after 4-aminopyrazolo-[3,4-d]pyrimidine treatment. Similar results were seen using 17 alpha-ethinyl estradiol to deplete cholesterol and adrenal sterol ester stores. The metabolism of endogenous cholesterol and exogenous hydroxysterols (which bypass the cAMP-dependent transport of endogenous cholesterol to mitochondrial side-chain cleavage enzyme complex) to corticosterone by collagenase-dispersed adrenocortical cells isolated from rats of various ages were also studied.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid composition of plasma membranes from barley leaves and roots, spinach leaves and cauliflower inflorescences

Abstract: Highly purified plasma membranes (PM) were obtained from barley (Hordeum vulgare L. cv. Kristina) leaves and roots, spinach (Spinacia oleracea L. cv. Viking II) leaves, and cauliflower (Brassica oleracea) inflorescences by partitioning in an aqueous polymer two-phase system. The sterol and polar lipid composition of the PM, including the fatty acid composition of the glycerolipids, was determined. Dominating lipids were free sterols, glucocerebroside, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), although large variations in content were observed between the PM of the different species and organs. Thus, the spinach leaf PM contained only 7% (mol %) free sterol compared to over 30% free sterol in the other PM analysed, with the barley root PM as the other extreme (57% free sterol). On the other hand, sterol derivatives were more abundant in the spinach leaf PM, containing 13% acylated sterol glycosides. Cerebroside constituted 16% of the lipids in the barley leaf PM but only 3% in cauliflower. The phospholipids PC and PE ranged from 25 and 24%, respectively, in the spinach leaf PM to 8 and 7%, respectively, in the barley root PM. As a result of the large variations in sterol and phospholipid content, the ratio of free sterol to phospholipid varied from 2.2 in the barley root PM to only 0.1 in the spinach leaf PM. Sitosterol, campesterol and stigmasterol were the completely dominating sterols in the barley and cauliflower PM, whereas the unique sterol composition of spinach was dominated by spinasterol. Palmitic (16:0), linoleic (18:2) and linolenic (18:3) acid were the major glycerolipid fatty acids. The fatty acid composition of the barley root PM was the most saturated (44% 16:0, 13% 18:3), whereas that of the cauliflower PM was the most unsaturated (21% 16:0,42% 18:3). Thus, very large variations were observed in both total lipid and fatty acid composition of the PM investigated, which represent both mono— and dicotyledons, as well as both photosynthetic and non-photosynthetic tissue. The consequences of this large diversity in composition of the lipid bilayer for the function of integral PM proteins are discussed.

Lipids and sterols of Pinus sylvestris L. sapwood and heartwood.

Abstract: The lipid and sterol content and composition of three lipid fractions (free fatty acids/ sterols, triacylglycerols and sterol/triterpenoid esters) extracted from three stem discs of Pinus sylvestris were assessed to investigate metabolic changes related to heartwood formation. The wood was separated into (1) cambial zone, (2) outer sapwood, (3) inner sapwood, (4) transition zone, (5) outer heartwood and 6) inner heart-wood. The fractions were separated by thin-layer chromatography (TLC) and analysed by gas-liquid chromatography (GLC). The amount of fatty acids of sapwood triacylglycerols was about 1.5% (dry wt.) but a large reduction occurred in the transition zone. In contrast, noticeable amounts of free fatty acids were present only in the heart-wood. The most important fatty acids in the sapwood fractions were 16:0, 18:0, 18:1, 18:2 (the dominant fatty acid in all fractions), 18:3 and 20:3. Together 18:1 and 18:2 formed about 70% of the total triacylglycerol fatty acids. Of the sterol/ triterpenoid esters, 18:2 and 18:3 were predominant. The fatty acid composition of all fractions changed in the transition zone. The sterols found were sitosterol, stigmastanol, campesterol and campestanol. The amount of sterol esters increased towards the heartwood, and the amount of free sterols was lowest in the inner sapwood. Sitosterol was the dominant sterol in both free sterols and sterol esters.

Ecdysteroid biosynthesis and embryonic development are disturbed in insects (Locusta migratoria) reared on plant diet (Triticum sativum) with a selectively modified sterol profile.

Abstract: Wheat seedlings germinating in the presence of the systemic fungicide fenpropimorph accumulate 9β,19-cyclopropylsterols (95% of total sterols) in place of Δ5-sterols, which are normally produced in these plants. Adult females of the phytophagous insect Locusta migratoria show a dramatic decrease in their cholesterol content when reared on fenpropimorph-treated wheat. These females lay eggs with the ecdysteroid concentration reduced by up to 80% as compared to controls. Injection of fenpropimorph to the insects or feeding them on wheat coated with the fungicide (normal sterol composition) does not affect their sterol or ecdysteroid profiles; addition of cholesterol to fenpropimorph-treated wheat prior to feeding restores normal ecdysteroid titers in the insects. The severe reduction of the ecdysteroid content in eggs laid by females reared on fenpropimorph-treated wheat is associated with a series of developmental arrests and/or abnormalities. The results show that the dietary 9β,19-cyclopropylsterols cannot be used by Locusta in place of Δ5-sterols for ecdysteroid biosynthesis. They suggest that the selective inhibition of specific enzymes in the sterol biosynthetic pathway of the plants can be used as a strategy to control insect development.

The insulin-like growth factor, somatomedin-C, modulates low density lipoprotein metabolism by swine granulosa cells.

Abstract: Insulin-like growth factor I (IGF-I) synergistically amplified the stimulatory effect of low density lipoprotein (LDL) on progesterone biosynthesis by primary cultures of swine ovarian cells. The mechanisms subserving this facilitative interaction included the following. IGF-I's synergism with LDL was associated with a decrease in the mean half-maximally stimulatory concentration of LDL from 20-3.5 micrograms/ml. IGF-I increased by 3- to 6-fold the number of specific high affinity LDL receptors on ovarian cells, with no change in apparent binding affinity. IGF-I augmented by 3- and 18-fold the maximal rates of [125I]iodo-LDL internalization and degradation, respectively, without altering half-maximally effective concentrations of LDL supporting these processes. IGF-I increased by 2- to 2.5-fold the total mass of free and esterified cholesterol contained in granulosa cells. IGF-I stimulated the intracellular accumulation of free [3H]cholesterol and [3H]cholesteryl ester from exogenous [3H]cholesteryl linoleate-labeled LDL, and amplified [3H]progesterone secretion by granulosa cells exposed to this source of lipoprotein-borne sterol. These actions of IGF-I were demonstrated at 30- to 100-fold lower concentrations of IGF-I than insulin. We conclude that IGF-I and LDL synergistically enhance progesterone biosynthesis by ovarian cells. This synergism occurs in part via mechanisms that regulate the effectual delivery of lipoprotein-borne cholesterol substrate into cellular sterol pools that participate in steroid hormone biosynthesis.