Metabolic Disease

RESEARCH HIGHLIGHTS

METABOLIC DISEASE To assess the activity of betulin in vivo and demonstrate the therapeutic potential of SREBP inhibition, Birch bark compound combats mice on a Westernized diet (consist- ing of high fat and high cholesterol) were treated daily by gastric irrigation metabolic syndrome with either betulin or vehicle control. After 6 weeks, weight gain was lower in betulin-treated mice owing to increased energy expenditure, which suggests that betulin might prevent diet-induced obesity. In accordance with the in vitro studies, betulin decreased lipid levels in serum and tissues, and these effects were mediated through modulation of the SCAP–SREBP pathway. Furthermore, betulin improved glucose tolerance and insulin sensitiv- ity and lowered the elevated fasting blood glucose level seen in control mice. Moreover, the expression of genes involved in cholesterol and fatty acid synthesis was reduced, The sterol regulatory element complex, where it is cleaved and whereas white adipose tissue genes binding protein (SREBP) family of the mature, nuclear form of SREBP that are known to have anti-diabetic transcription factors controls cellular is released. Currently available inhibi- and anti-inflammatory effects were lipid homeostasis by regulating the tors of SREBP processing upregulated. Finally, in a mouse expression of genes involved in cannot be used clinically as they model of atherosclerosis, 14 weeks of the synthesis and uptake of choles- activate the liver X receptor (LXR), betulin treatment reduced the size and terol, fatty acids and triglycerides. which mediates cholesterol efflux improved the stability of plaques. Given that hyperlipidaemia is closely but also upregulates the SREBP1c Notably, in each of these in vivo associated with the development isoform, which leads to fatty acid studies, betulin was equally or more of metabolic diseases, modulating synthesis. Song and colleagues effective than lovastatin, which is a SREBP activity represents a promis- therefore aimed to identify a specific widely prescribed anti-hypercholes- ing therapeutic strategy. Here, Song SREBP inhibitor and investigate its terolaemic drug that acts through a and colleagues identify a specific potential in the treatment of meta- different mechanism and is associ- small-molecule inhibitor of SREBP bolic diseases. ated with adverse effects. processing, which may be a new First, using chemical screening, In summary, these findings lead in the development of novel they identified betulin as a potent support the concept of inhibiting therapies for type 2 diabetes and inhibitor of SREBP activity. Betulin the SREBP pathway to treat hyper- atherosclerosis. is a pentacyclic triterpene that is lipidaemia and associated metabolic The activity of SREBPs is regu- abundantly found in birch bark. In diseases, and this approach may offer lated by sterols. High levels of sterols in vitro mechanistic studies, betulin advantages over existing therapeutic stimulate the association between was shown to directly bind to SCAP strategies. SREBP cleavage activating protein and stimulate its association with Sarah Crunkhorn (SCAP) and insulin-induced gene 1 INSIG1 by blocking SREBP process-

(INSIG1) protein, which results in ing, downregulating genes involved in ORIGINAL RESEARCH PAPER Tang, J.‑J. et al. retention of SREBP in the endoplas- cholesterol and fatty acid biosynthesis, Inhibition of SREBP by a small molecule, betulin, mic reticulum. Low levels of sterols, and decreasing cellular lipid levels. improves hyperlipidemia and insulin resistance and reduces atherosclerotic plaques. Cell Metab. however, stimulate SCAP-mediated Importantly, betulin was specific for 13, 44–56 (2011) transport of SREBP to the Golgi SREBPs and had no effect on the LXR.