Chapter 26: Cholesterol Synthesis, Transport, & Excretion

Cholesterol Synthesis, Transport, & Excretion overview explores the metabolic pathways of cholesterol, a vital amphipathic lipid that serves as a fundamental structural component of cell membranes and the primary precursor for all other steroids, including sex hormones, vitamin D, and bile acids. The de novo synthesis of cholesterol occurs in the cytosol and endoplasmic reticulum of almost all nucleated cells, proceeding through five distinct stages: the synthesis of mevalonate from acetyl-CoA, the formation of isoprenoid units, the condensation of these units into squalene, the cyclization of squalene into lanosterol, and the final conversion into cholesterol. Central to this process is the enzyme HMG-CoA reductase, which acts as the principal regulatory checkpoint; its activity is modulated by hormonal signals like insulin and glucagon, and it is the primary target for cholesterol-lowering statin medications. The chapter details the transport of cholesterol between tissues via plasma lipoproteins, identifying LDL as the primary vehicle for delivery to peripheral tissues and HDL as the mediator for reverse cholesterol transport back to the liver. Cellular cholesterol balance is tightly maintained through the regulation of LDL receptor expression and the SREBP transcription factor pathway. Furthermore, the text examines the elimination of cholesterol through its conversion into primary bile acids in the liver—a process controlled by the rate-limiting enzyme CYP7A1—and its subsequent excretion or recycling via the enterohepatic circulation. Clinical discussions highlight the link between elevated serum cholesterol levels and the development of atherosclerosis and coronary heart disease. Finally, the summary addresses inherited disorders of lipoprotein metabolism, such as familial hypercholesterolemia, and the impact of diet, lifestyle, and pharmacological interventions like PCSK9 inhibitors in managing cardiovascular risk.