The Wnt/APC destruction complex targets SREBP2 in a β-catenin-independent pathway to control cholesterol metabolism

ABSTRACT APC, the core scaffold of the Wnt destruction complex, targets the transcriptional co-activator β-catenin for proteolysis. There is no convincing evidence that APC directs degradation of other substrates. Using a reconstituted cytosolic extract-based system and complementary in vivo and cellular assays, we show that SREBP2, the master regulator of cholesterol biosynthesis, is a direct APC–AXIN1 substrate. APC-dependent SREBP2 degradation is conserved in Xenopus embryos, mouse colon, and human colorectal cancer cells and restricts SREBP2 target-gene expression, cholesterol synthesis, and tissue cholesterol levels. Mechanistically, APC and AXIN1 promote SREBP2 degradation via a conserved phosphodegron, which marks SREBP2 for ubiquitination by the E3 enzyme, FBXW7. Like β-catenin, SREBP2 is stabilized by extracellular Wnt ligands; unlike β-catenin, its regulation is independent of GSK3β and CK1α and requires the entire APC mutational cluster region (MCR), whereas β-catenin turnover can operate with only a partial MCR. These findings define a β-catenin-independent branch of Wnt signaling that couples APC to sterol metabolism, providing a mechanistic rationale to target the mevalonate/SREBP2 axis in APC-mutant colorectal cancer.