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

ABSTRACT

The SCFß-TRCP E3 ubiquitin ligase complex plays pivotal roles in normal cellular physiology and in pathophysiological conditions. Identification of ß-transducin repeat-containing protein (ß-TRCP) substrates is therefore critical to understand SCFß-TRCP biology and function. We used a ß-TRCP-phosphodegron motif-specific antibody in a ß-TRCP substrate screen coupled with tandem mass spectrometry and identified multiple ß-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 SCFß-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). ß-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, ß-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 ß-TRCP in regulating hepatic lipid metabolic homeostasis in part through modulating Lipin1 stability.