Sterol regulatory element binding protein 2 (SREBP-2) regulates cholesterol synthesis by activating the transcription of genes for HMG-CoA reductase and other enzymes of the cholesterol synthetic pathway. When cellular sterol levels are high, SREBP is bound by SCAP and Insig to ER membranes as a glycosylated precursor protein. Upon cholesterol depletion, the protein is cleaved to its active form and translocated into the nucleus to stimulate transcription of genes involved in the uptake and synthesis of cholesterol. Betulin, the precursor of betulinic acid, is a pentacyclic triterpene found in the bark of birch trees. Betulin inhibits the SREBP-driven pathway of cholesterol and fatty acid biosynthesis by promoting SCAP–Insig binding which prevents the activation and release of SREBP-2 from the ER. At 15-30 mg/kg/day, betulin has been shown to decrease lipid levels and increase insulin sensitivity in mice fed a western-type diet. In an atherosclerosis disease model, 30 mg/kg/day betulin can reduce the size and improve the stability of atherosclerotic plaques in LDLR-knockout mice. At 2.5-5 μg/ml betulin, in combination with cholesterol, demonstrates anticancer effects by inducing apoptosis in Jurkat cells, A549 lung carcinoma cells, and HeLa cervical carcinoma cells.
