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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.
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Betulin may be used in the preparation of betulinic acid, which shows anti-HIV, antimalarial and anti-inflammatory activities.
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ChEBI: A pentacyclic triterpenoid that is lupane having a double bond at position 20(29) as well as 3beta-hydroxy and 28-hydroxymethyl substituents.
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A cell-permeable pentacyclic triterpenoid from birch bark extract that interacts with SCAP (SREBP cleavage activating protein) and prevents SREBP (sterol regulatory element-binding protein) Golgi proteolytic activation (1 to 13.55 μM in Rat hepatocytes CRL-1601) via a similar mechanism as oxysterols by inducing the association of SCAP with Insig-1 (insulin-induced gene-1), thereby causing SCAP-SREBPs complex ER retention. Unlike oxysterols, Betulin does not activate LXR to induce concomitant HMGCR (HMG-CoA reductase) degradation and SREBP-1 up-regulation. While both Betulin and HMGCR inhibitor Lovastatin (Cat. No. 438185) inhibit cellular cholesterol synthesis (by ~70% in CRL-1601 cultures with respective compound at 13.55 and 1 μM concentration), only Betulin suppresses cellular fatty acid synthesis (by ~55% at 13.55 μM in CRL-1601). Betulin is shown to exhibit comparable
in vivo efficacy as Lovastatin (both dosed at 30 mg/kg/day with chow) in ameliorate high fat/cholesterol diet-induced obesity (% fat/lean tissue ratio increase from non-fat diet mice = 167, 44, and 33 in mice consuming fat diet alone, with Lovastatin, with Betulin, respectively). However, only Betulin is demonstrated to lower fasting blood glucose and insulin levels and reduce fat cell size in white adipose tissue in high fat diet mice. Fatostatin (Cat. No. 341329) in comparison does not target SCAP via sterol-binding site, nor does it induce SCAP binding to Insig-1.
Anticancer Research
In addition, triterpenoids (betulin and its derivatives) isolated from HSSEactivated the signaling pathway regulated by p53 family genes, leading to the inhibition of BC cell viability or even the induction of apoptosis. And also, theseresearchers found that all the triterpenoids had no effect on normal breast cells.These findings provide an important basis for the use of those triterpenoids in thedevelopment of alternative therapies for breast cancer treatment (Hsu et al. 2015).
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