Sterol-induced upregulation of phosphatidylcholine synthesis in cultured fibroblasts is affected by the double-bond position in the sterol tetracyclic ring structure

We have examined how a specific enrichment of cultured fibroblasts with var ious sterols (cholesterol, lathosterol, 7-dehydrocholesterol, allocholester ol and dihydrocholesterol) regulate synthesis de novo of phosphatidylcholin e, cholesterol and cholesteryl (or steryl) esters in human skin fibroblasts . When human skin fibroblasts were incubated for 1 h with 130 mu m choleste rol/CyD complexes, the mass of cellular free cholesterol increased by 100 n mol.mg(-1) protein (from 90 nmol.mg(-1) to 190 nmol.mg(-1) protein). A simi lar exposure of cells to different sterol/CyD complexes increased the cell sterol content between 38 and 181 nmol sterol per mg cell protein. In chole sterol-enriched cells, the rate of phosphatidylcholine synthesis was double d compared to control cells, irrespective of the type of precursor used ([H -3]choline, [H-3]palmitic acid, or [C-14]glycerol). Enrichment of fibroblas ts with 7-dehydrocholesterol, allocholesterol, or dihydrocholesterol also u pregulated phosphatidylcholine synthesis, whereas cells enriched with latho sterol failed to upregulate their phosphatidylcholine synthesis. The activi ty of membrane-bound CTP:phosphocholine cytidylyltransferase, the rate-limi ting enzyme, was increased by 47 +/- 4% in cholesterol-enriched cells where as its activity was unchanged in lathosterol-enriched cells. Sterol enrichm ent with all tested sterols (including lathosterol) down-regulated acetate- incorporation into cholesterol, and upregulated sterol esterification in th e sterol-enriched fibroblasts. Using P-31-NMR to measure the lamellar-to-he xagonal (L-alpha-H-II) phase transition in multilamellar lipid dispersions, lathosterol-containing membranes underwent their transition at significant ly higher temperatures compared to membranes containing any of the other st erols. In a system with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolami ne and either cholesterol or lathosterol (70 : 30 mol/mol), differential sc anning calorimetry also revealed that the L-alpha-H-II-transition occurred at a higher temperature with lathosterol compared to either cholesterol, al locholesterol, or dihydrocholesterol. These findings together suggest that there may exist a correlation between the propensity of a sterol to stabili ze the L-alpha-H-II-transition and its capacity to upregulate the activity of CTP:phosphocholine cytidylyltransferase in cells.