Isoxanthohumol stimulates ubiquitin-proteasome-dependent degradation of precursor forms of sterol regulatory element-binding proteins

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate a wide variety of genes involved in fatty acid and cholesterol synthesis. In the present study, we identified that isoxanthohumol (IXN) suppressed SREBP activity. Low concentrations of IXN (10 and 30 μM) reduced the amount of mature forms of SREBPs, while high concentration of IXN (100 μM) reduced both precursor and mature forms of SREBPs in Huh-7 cells. The IXN-mediated decrease in the precursor forms of SREBPs in Huh-7 cells was completely abolished by culturing cells under sterol-supplemented conditions and was partly abolished by treatment with a proteasome inhibitor, MG132, but not a lysosome inhibitor, NH4Cl. Moreover, IXN accelerated the ubiquitination of the precursor forms of SREBP-1a. These results suggest that IXN suppresses SREBP activity, at least in part, via ubiquitin-proteasome-dependent degradation of the precursor forms of SREBPs. Abbreviations: ACC1: acetyl-CoA carboxylase 1; DMEM: Dulbecco’s modified Eagle’s medium; ER: endoplasmic reticulum; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; 25-HC: 25-hydroxycholesterol; HMGCR: HMG-CoA reductase; HMGCS: HMG-CoA synthase; Insig: insulin-induced gene; IXN: isoxanthohumol; LPDS: lipoprotein-deficient serum; SCAP: SREBP cleavage-activating protein; SCD1: stearoyl-CoA desaturase; SREBPs: sterol regulatory element-binding proteins; XN: xanthohumol