Squalene synthase: a critical enzyme in the cholesterol biosynthesis pathway.

High levels of plasma low-density lipoprotein cholesterol (LDL-C) are a significant risk factor for heart disease. Statins (3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors) have been extensively used to treat high-plasma LDL-C levels and are effective in preventing heart disease. However, statins can be associated with adverse side effects in some patients and do not work effectively in others. As an alternative to statins, the development of cholesterol-lowering agents that directly inhibit squalene synthase have shown promise. Clinical studies have shown that squalene synthase inhibitors are effective in lowering plasma levels of total cholesterol and LDL-C. Squalene synthase plays an important role in the cholesterol biosynthesis pathway as it is responsible for the flow of metabolites into either the sterol or the non-sterol branches of the pathway. In addition, variants of the squalene synthase gene appear to modulate plasma cholesterol levels in human populations and therefore may be linked to cardiovascular disease. In this review, we examine squalene synthase and the gene that codes for it (farnesyldiphosphate farnesyltransferase 1). In particular, we investigate their role in the regulation of cellular and plasma cholesterol levels, including data that suggest that squalene synthase may be involved in the etiology of hypercholesterolemia.