Statins are a class of medications that lower lipid levels and are administered to reduce illness and mortality in people who are at high risk of cardiovascular disease. Atorvastatin (trade name: Lipitor) is a well-tolerated orally-administered synthetic statin that reduces levels of total cholesterol, low-density lipoprotein (LDL)-cholesterol, triglyceride, and very-low-density lipoprotein (VLDL)-cholesterol. It also increases levels of high-density lipoprotein (HDL)-cholesterol. Atorvastatin's efficacy is greater than other statins in reducing total cholesterol and LDL-cholesterol levels. This is theorized to be the result of a prolonged duration of HMG-CoA reductase inhibition. Reported side effects of atorvastatin include constipation, flatulence, dyspepsia (indigestion), abdominal pain, headache, and myalgia (muscle pain). The primary therapeutic mechanism of action of statins is the inhibition of the rate-limiting enzyme 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase in hepatocytes. HMG-CoA reductase catalyzes the conversion of HMG-CoA into mevalonic acid, a precursor for cholesterol biosynthesis. Statins bind reversibly to the active site of HMG-CoA reductase and the subsequent structural change in the enzyme effectively disables it. Due to the resulting decrease in intracellular sterol levels, the ER membrane protein INSIG no longer binds to SREBP cleavage-activating protein (SCAP) which is, itself, bound to the transcription factor sterol regulatory element-binding protein (SREBP). Freed from INSIG, SCAP escorts SREBP to the Golgi apparatus from the ER as cargo in COPII vesicles. At the Golgi membrane, two proteases, S1P and S2P, sequentially cleave the SCAP-SREBP complex, releasing the mature form of SREBP into the cytoplasm. SREBP then translocates to the nucleus where it is actively transported into the nucleoplasm by binding directly to importin beta in the absence of importin alpha. SREBP binds to the sterol regulatory element (SRE) present in the promoter region of genes involved in cholesterol uptake and cholesterol synthesis, including the gene encoding low-density lipoprotein (LDL) receptor (LDL-R). As a result, LDL-R gene transcription increases which then leads to an increased synthesis of the LDL-R protein. LDL-R localizes to the endoplasmic reticulum for transport and exocytosis to the cell surface. The elevated amount of LDL-R results in the binding of more circulating free LDL cholesterol and subsequent internalization via endocytosis. Lysosomal degradation of the internalized LDL cholesterol elevates cellular cholesterol levels to maintain homeostasis.

Atorvastatin Pathway References