Maria Franca Mulas

Role of cholesterol ester pathway in the control of cell cycle in human aortic smooth muscle cells

Cholesterol esterification by acyl‐CoA:cholesterol acyltransferase (ACAT) and proliferation of vascular smooth muscle cells (VSMC) are key events in vascular proliferative diseases. Here we performed experiments to ascertain the role of cholesterol ester pathway in the control of human aortic VSMC cycle progression. Results showed that serum‐induced VSMC proliferation was preceded by an increased ability of the cells to esterify cholesterol as well as by an increased expression of ACAT and multidrug resistance (MDR1) mRNAs and extracellular related kinases 1/2 (ERK1/2), whereas caveolin‐1 levels were markedly decreased. Cell cycle analyses performed in the presence of two inhibitors of cholesterol esterification, directly inhibiting ACAT (Sandoz 58–035) or the transport of cholesterol substrate from plasma membrane to endoplasmic reticulum (progesterone), indicate that each inhibitor suppressed the serum‐induced DNA synthesis by accumulation of VSMCs in the G1 phase. The effect was associated with a rapid inhibition of ERK1/2 mitogenic signaling pathway; a down‐regulation of cyclin D1, ACAT, and MDR1 mRNA; and an up‐regulation of caveolin‐1. These data provide a plausible link between cholesterol esterification and control of cell cycle G1/S transition, supporting the hypothesis that cholesterol esterification may accelerate the progression of human vascular proliferative diseases by modulating the rate of the VSMC proliferation.

MDR1 gene expression in normal and atherosclerotic human arteries(1).

Recent studies have shown that a membrane p-glycoprotein, encoded by MDR1 gene, is involved in the transport of free cholesterol from the plasma membrane to endoplasmic reticulum, the site of cholesterol esterification by acyl-CoA:cholesterol acyltransferase (ACAT). Moreover, results deriving from our previous studies have shown that the rate of cell proliferation was positively correlated with cholesteryl ester levels as well as with ACAT and MDR1 gene expression. In this study, lipid content and the expression of the genes involved in cholesterol metabolism such as hydroxy-methylglutaryl coenzyme A reductase (HMGCoA-R), low-density lipoprotein receptor (LDL-R), ACAT and MDR1 have been investigated in control and atherosclerotic arteries. The results have shown that the levels of cholesteryl ester increase with the age of cadaveric donors in arteries prone to atherosclerosis (abdominal aorta, superficial femoral artery) and become predominant in advanced atherosclerotic lesions. The mRNA levels of ACAT and MDR1 showed the same age correlation, reaching the highest values in atherosclerotic specimens. These results suggest that MDR1 may be involved in the accumulation of intracellular cholesterol ester levels found in atherosclerotic lesions. Moreover, the levels of HMGCoA-R, LDL-R and ACAT gene expressions progressively increased with the age of cadaveric donors; conversely, in atherosclerotic specimens, the mRNA levels of HMGCoA-R and LDL-R drastically decreased while ACAT gene expression reached its maximum. These findings suggest a reactivation of normal homeostatic regulation of cholesterol in advanced and complicated lesions.

Comparative Effects of Insulin and Refeeding on DNA Synthesis, HMP Shunt and Cholesterogenesis in Diabetic and Fasted Rats

&NA; DNA synthesis, cholesterogenesis and the enzymes of the hexosemonophosphate (HMP) shunt pathway were investigated in liver of diabetic rats treated with insulin and in fasted/re‐fed rats. Both insulin and refeeding were found to induce liver cell proliferation, accompanied by a remarkable increase in cholesterogenesis. An enhancement of glucose‐6‐phosphate dehydrogenase (G6PD) and 6‐phosphogluconate dehydrogenase (G6PD) activities was also found in insulin‐treated diabetic rats and in re‐fed rats, supporting the concept that these two enzymes are involved in the proliferative process. Since insulin did not exert the same biochemical effects in a non replicating cell population, such as in insulin‐treated normal rats, these studies provide new evidence of a close correlation between DNA, cholesterol synthesis and HMP shunt enzymes during cell proliferation.

Cholesterol esterification during differentiation of mouse erythroleukemia (Friend) cells.

Cholesterol is an essential constituent of all mammalian cell membranes and its availability is therefore a prerequisite for cellular growth and other functions. Several lines of evidence are now indicating an association between alterations of cholesterol homeostasis and cell cycle progression. However, the role of cholesterol in cell differentiation is still largely unknown. To begin to address this issue, in this study we examined changes in cholesterol metabolism and in the mRNA levels of proteins involved in cholesterol import and esterification (multi-drug resistance, MDR-3) and acylCoA: cholesterol acyltransferase (ACAT) and cholesterol export (caveolin-1) in Friend virus-induced erythroleukemia cells (MELC), in the absence or in the presence of the chemical inducer of differentiation, hexamethylene bisacetamide (HMBA). FBS-stimulated growth of MELC was accompanied by an immediate elevation of cholesterol synthesis and cholesterol esterification, and by an increase in the levels of MDR-3 and ACAT mRNAs. A decrease in caveolin-1 expression was also observed. However, when MELC were treated with HMBA, the inhibition of DNA synthesis caused by HMBA treatment, was associated with a decrease in cholesterol esterification and in ACAT and MDR-3 mRNA levels and an increase in caveolin-1 mRNA. Detection of cytoplasmic neutral lipids by staining MELC with oil red O, a dye able to evidence CE but not FC, revealed that HMBA-treatment also reduced growth-stimulated accumulation of cholesterol ester to approximately the same extent as the ACAT inhibitor, SaH. Overall, these results indicate for the first time a role of cholesterol esterification and of some related genes in differentiation of erythroid cells.

Cholesterol esterification as a mediator of proliferation of vascular smooth muscle cells and peripheral blood mononuclear cells during atherogenesis

Background/Aims: We determined growth rates, cholesterol esterification and mRNA levels for caveolin-1 (Cav-1), neutral cholesterol esters hydrolase (n-CEH) and ATP-binding cassette transporter (ABCA-1), in quiescent and growth-stimulated peripheral blood mononuclear cells (PBMCs) and intimal vascular smooth muscle cells (VSMCs) from blood and primary atherosclerotic plaques, respectively. These cells were cultured in the presence or absence of the mTOR inhibitor 40-O-(2-hydroxyethyl) rapamycin (RAD). Methods: The rate of cell proliferation was determined by 3H-thymidine incorporation into DNA and that of lipid metabolism by utilizing 14C-acetate and 14C-oleate as precursors. Lipid deposit in the vascular cells was evaluated by Oil Red O staining and lipid mass by thin layer chromatography-linked enzymatic assay. Results: Growth stimulation of PBMCs and VSMCs caused a rapid increase in intracellular cholesterol esterification and an accumulation of cholesterol esters (CEs) accompanied by a reduction of free cholesterol (FC) and Cav-1, ABCA-1 and n-CEH mRNAs. RAD reduced intracellular lipid accumulation in growth-stimulated cells and also increased expression of Cav-1, n-CEH and ABCA-1 genes. Conclusion: Collectively, these data provide evidence that the determination of CEs in PBMCs may be an easy prescreening test to identify subjects at risk for vascular proliferative disease and that FC, CE, Cav-1, n-CEH and ABCA-1 may be suitable targets for antiproliferative therapies.

Modifying Influence of Fasting on DNA Synthesis, Cholesterol Metabolism and HMP Shunt Enzymes in Liver Hyperplasia Induced by Lead Nitrate

Previous studies by Columbano et al.1 have shown that lead nitrate when injected intravenously as a single dose to rats, induces an hyperplastic response in the liver as indicated by an increased DNA synthesis and by an enhanced mitotic index. Liver hyperplasia was accompanied by a stimulation of cholesterol synthesis and hexose-monophosphate (HMP) shunt enzymes2.