The SCFβ-TRCP E3 ubiquitin ligase complex targets Lipin1 for ubiquitination and degradation to promote hepatic lipogenesis.

The SCF β-TRCP E3 ubiquitin ligase complex plays pivotal roles in normal cellular physiology and in pathophysiological conditions. Identification of β-transducin repeat–containing protein (β-TRCP) substrates is therefore critical to understand SCF β-TRCP biology and function. We used a β-TRCP–phosphodegron motif–specific antibody in a β-TRCP substrate screen coupled with tandem mass spectrometry and identified multiple β-TRCP substrates. One of these substrates was Lipin1, an enzyme and suppressor of the family of sterol regulatory element–binding protein (SREBP) transcription factors, which activate genes encoding lipogenic factors. We showed that SCF β-TRCP specifically interacted with and promoted the polyubiquitination of Lipin1 in a manner that required phosphorylation of Lipin1 by mechanistic target of rapamycin 1 (mTORC1) and casein kinase I (CKI). β- TRCP depletion in HepG2 hepatocellular carcinoma cells resulted in increased Lipin1 protein abundance, suppression of SREBP-dependent gene expression, and attenuation of triglyceride synthesis. Moreover, β- TRCP1 knockout mice showed increased Lipin1 protein abundance and were protected from hepatic steatosis induced by a high-fat diet. Together, these data reveal a critical physiological function of β-TRCP in regulating hepatic lipid metabolic homeostasis in part through modulating Lipin1 stability.