Molecular effect of phytosterols in the regulation of cholesterol metabolism

Intracellular cholesterol concentration regulates both cholesterol synthesis and uptake from blood. Sterol regulatory element binding proteins (SREBP) are master regulators. Nuclear SREBP concentration decreases when intracellular cholesterol is high, so reducing the transcription of genes related to cholesterol metabolism, as those encoding for HMG-CoA reductase and LDL receptor. Cholesterol can regulate its own metabolism in the cell due to the presence of a sterol sensor domain (SSD) in SREBP proteins. It is not clear if this domain can sense free cholesterol only, or other sterol such as phytosterols (PS) can contribute to the regulation. Since PS chemical structure is very similar to cholesterol, they could mimic its action within the cell, and their increased concentration could reduce nuclear SREBP concentration. To verify this hypothesis, selected PS (sitosterol, campesterol, and brassicasterol) were added - separately or in a mixture - to cardiac cells in culture. PS supplemented cells were compared to unsupplemented ones (control) and to cells supplemented with cholesterol or treated with a cholesterol-lowering drug, mevastatin. All PS were incorporated into cells, and this was accomplished by a decrease in cholesterol content similar to that obtained with mevastatin treatment. Notwithstanding, the expression of genes encoding for SREBP-1, SREBP-2, HMG-CoA reductase, and LDL receptor was not modified by PS supplementation. The absence of modification in gene expression could be due to the insensitivity of the SSD to PS, or to a too low PS intracellular concentration. Another possibility is that PS concentrate in the membrane and not in the cytosol, where the SSD-mediated regulation takes place. Preliminary data confirmed this hypothesis, indicating no increase in PS cytosolic concentration. On the contrary, PS appeared incorporated into the membranes at the expenses of cholesterol. This cholesterol displacement from membranes,particularly mitochondrial ones, could explain the reduction of metabolic activity (measured as MTT conversion) observed in PS supplemented cardiomyocytes, in agreement with our previous data [1]. Further studies are needed for the successful understanding of PS molecular effects. This is important also in the light of the suggested long-term relevant PS intake suggested for cholesterol-lowering.