| Sterol regulatory element–binding proteins (SREBPs) are basic helix-loop-helix–leucine zipper transcription factors (1,2). Inactive precursor forms of SREBPs are bound to endoplasmic reticulum (ER) membranes (1,2). When cells are starved for cholesterol, SREBPs move from the ER to the Golgi apparatus with the help of SREBP cleavage–activating protein (SCAP) (1,2). In the Golgi apparatus, precursor SREBPs are then cleaved by two proteases, Site-1 protease (S1P) and Site-2 protease (S2P) (1,2). The released N-terminal domains enter the nucleus and bind to sterol response elements in the promoters of a variety of genes responsible for the synthesis of cholesterol (1,2). SREBPs also activate the expression of genes involved in the synthesis of fatty acids and lipids (1,2). Among the isoforms of SREBPs, SREBP-1c activates all lipogenic genes in the liver (3). SREBP-1c has been implicated to contribute to the development of hepatic steatosis in the rodent model of insulin resistance and obesity (3). Recent studies have shown that AMPK interacts with and directly phosphorylates SREBP-1c and SREBP-2 (4). Phosphorylation of SREBP-1c at Ser372 by AMPK, which is stimulated by polyphenols and metformin, inhibits the proteolytic cleavage of SREBP-1c and therefore suppresses the expression of its target genes in the liver (4). This process leads to the reduction of lipid synthesis and accumulation in the liver (4). |
