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 various sterols (cholesterol, lathosterol, 7‐dehydrocholesterol, allocholesterol and dihydrocholesterol) regulate synthesis de novo of phosphatidylcholine, cholesterol and cholesteryl (or steryl) esters in human skin fibroblasts. When human skin fibroblasts were incubated for 1 h with 130 µm cholesterol/CyD complexes, the mass of cellular free cholesterol increased by 100 nmol·mg−1 protein (from 90 nmol·mg−1 to 190 nmol·mg−1 protein). A similar exposure of cells to different sterol/CyD complexes increased the cell sterol content between 38 and 181 nmol sterol per mg cell protein. In cholesterol‐enriched cells, the rate of phosphatidylcholine synthesis was doubled compared to control cells, irrespective of the type of precursor used ([3H]choline, [3H]palmitic acid, or [14C]glycerol). Enrichment of fibroblasts with 7‐dehydrocholesterol, allocholesterol, or dihydrocholesterol also upregulated phosphatidylcholine synthesis, whereas cells enriched with lathosterol failed to upregulate their phosphatidylcholine synthesis. The activity of membrane‐bound CTP:phosphocholine cytidylyltransferase, the rate‐limiting enzyme, was increased by 47 ±â€ƒ4% in cholesterol‐enriched cells whereas its activity was unchanged in lathosterol‐enriched cells. Sterol enrichment with all tested sterols (including lathosterol) down‐regulated acetate‐incorporation into cholesterol, and upregulated sterol esterification in the sterol‐enriched fibroblasts. Using 31P‐NMR to measure the lamellar‐to‐hexagonal (Lα–HII) phase transition in multilamellar lipid dispersions, lathosterol‐containing membranes underwent their transition at significantly higher temperatures compared to membranes containing any of the other sterols. In a system with 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphoethanolamine and either cholesterol or lathosterol (70 : 30 mol/mol), differential scanning calorimetry also revealed that the Lα–HII‐transition occurred at a higher temperature with lathosterol compared to either cholesterol, allocholesterol, or dihydrocholesterol. These findings together suggest that there may exist a correlation between the propensity of a sterol to stabilize the Lα–HII‐transition and its capacity to upregulate the activity of CTP:phosphocholine cytidylyltransferase in cells.