Sterol synergism in yeast.

Abstract

Sterol synergism as previously observed ) Biochemistry 19, 1462-1467 and defined as a greater-than-additive growth response to pairs of sterols by Mycoplasma capricolum [Dahl, J. S., Dahl, C. E. & Bloch, K. (1981) J. BioL Chem. 256, 87-91] is now demonstrated in the yeast mutant GL7, which is auxotrophic for sterol and unsaturated fatty acid. Mutant cells growing poorly when provided with cholesterol and oleic acid respond to ergosterol supplements (ergosterol-to-cholesterol ratio, 1:3) by a pronounced increase in growth rates and cell yields. Stigmasterol also elicits a significant synergistic effect, and 7-dehydrocholesterol, a smaller one. Evidence for a metabolic role of ergosterol in yeast membranes is presented. Cells raised on a 1:3 mixture of ergosterol to cholesterol up to midlogarithmic phase subsequently incorporate [1-'4C]oleic acid at significantly faster rates into phospholipids than do cells grown on cholesterol alone.In a series ofreports, we have described a phenomenon termed "sterol synergism" on the basis of the following observations. The sterol auxotroph Mycoplasma capricolum grows optimally on cholesterol but only poorly on lanosterol (1, 2). Bacterial cells supplied simultaneously with two sterols, cholesterol in limiting and lanosterol in nonlimiting quantities, grow much more rapidly and with higher yields than do cells receiving cholesterol or lanosterol alone (3). The response is synergistic, not additive. These findings were taken to indicate more than one function for sterols in membranes, and supporting evidence has been published (4). Here we describe a similar phenomenon for an eukaryotic cell, the yeast mutant GL7, chosen because it has an absolute sterol requirement for growth (5).In designing the present experiments, we took advantage of the facts that (i) GL7, a mutant deficient in both squalene epoxide cyclase and heme synthesis (5), grows optimally when supplied with ergosterol and one of a variety of mono-or polyunsaturated fatty acids, but (ii) cholesterol, a sterol foreign to yeast, replaces ergosterol effectively only when either linoleate or a mixture of palmitoleate and oleate serves as the external fatty acid source. When oleate is the only fatty acid supplied, growth on cholesterol is much slower than on ergosterol (6). We now find that under these latter conditions, ergosterol in amounts too small to elicit detectable growth by itself raises the growth rate of GL7 cells by a large factor.We also note that the sterol requirement for GL7 is remarkably small. Grown on a nearly optimal supply of ergosterol, these cells contain about 10% of the total sterol that wild-type yeast normally produces. Spheroplasts of GL7 cells grown on different sterol concentrations or "synergistic" sterol mixtures were fractionated, and subcellular membrane fractions were analyzed for their relative sterol and phospholipid content. Finally, we report that mutant cells raised on ergosterol/cholesterol show an enhanced capacity to incorporate external oleic acid into cellular phosph...

Effects of distal cholesterol biosynthesis inhibitors on cell proliferation and cell cycle progression

“…Similar results were obtained with the combination of other sterols in different organisms (29)(30)(31). The existing sterol synergism indicates that low concentrations of the natural sterol, which are insufficient to substantially sustain cell growth per se, elicit synergistic effects when added in combination with another sterol (27,32,33). These studies led to the postulation of a regulatory role for sterols during cell growth, which appears to be highly specific and fulfilled by very low levels of the natural sterol.…”

Dual roles for cholesterol in mammalian cells

“…These findings were taken to indicate that sterols play more than one role in membranes. Later, sterol synergism was demonstrated in unicellular eukaryotes including yeast (36) and Paramecium (37). Yeast mutants that require exogenous sterols can grow in a variety of sterols if trace amounts of ergosterol are present.…”

Sterol Synergism in Paramecium tetraurelia

“…In addition, we have demonstrated the phenomenon of sterol synergism or sparing in this simple eukaryote, i.e. the adequacy of non-growth-supporting sterols for the cell's bulk membrane structural requirement when a much lower concentration of an essential phytosterol is provided to meet the nutritional requirement (Pinto et al, 1983;Rampogal & Bloch, 1983;Whitaker & Nelson, 1987).…”