RNF34 Is a Cold-Regulated E3 Ubiquitin Ligase for PGC-1α and Modulates Brown Fat Cell Metabolism

The transcriptional coactivator PGC-1α is a master regulator of energy metabolism and adaptive thermogenesis in the brown fat cell. PGC-1α is a short-lived protein, and the molecular components that control PGC-1α turnover and their functional importance in energy metabolism are largely unknown. Here we performed a luciferase-based overexpression screen and identified a Ring-finger-containing protein, RNF34, as a specific E3 ubiquitin ligase for PGC-1α. RNF34 is a nuclear protein that interacts with and ubiquitinates PGC-1α to promote its turnover. Interestingly, RNF34 binds to the C-terminal half of PGC-1α and targets it for degradation independently of the previously identified N-terminal phosphodegron motif. In brown fat cells, knockdown of RNF34 increases the endogenous PGC-1α protein level, uncoupling protein 1 (UCP1) expression, and oxygen consumption, while the opposite effects are observed in brown fat cells ectopically expressing wild-type RNF34 but not in cells expressing the ligase activity-defective mutant. Moreover, cold exposure and β3-adrenergic receptor signaling, conditions that induce PGC-1α expression, suppress RNF34 expression in the brown fat cell, indicating a physiological relevance of this E3 ligase in thermogenesis. Our results reveal that RNF34 is a bona fide E3 ubiquitin ligase for PGC-1α and negatively regulates brown fat cell metabolism

FBXW7α attenuates inflammatory signalling by downregulating C/EBPδ and its target gene Tlr4.

Toll-like receptor 4 (Tlr4) has a pivotal role in innate immune responses, and the transcription factor CCAAT/enhancer binding protein delta (C/EBPδ, Cebpd) is a Tlr4-induced gene. Here we identify a positive feedback loop in which C/EBPδ activates Tlr4 gene expression in macrophages and tumour cells. In addition, we discovered a negative feedback loop whereby the tumour suppressor FBXW7α (FBW7, Cdc4), whose gene expression is inhibited by C/EBPδ, targets C/EBPδ for degradation when C/EBPδ is phosphorylated by GSK-3β. Consequently, FBXW7α suppresses Tlr4 expression and responses to the ligand lipopolysaccharide. FBXW7α depletion alone is sufficient to augment pro-inflammatory signalling in vivo. Moreover, as inflammatory pathways are known to modulate tumour biology, Cebpd null mammary tumours, which have reduced metastatic potential, show altered expression of inflammation-associated genes. Together, these findings reveal a role for C/EBPδ upstream of Tlr4 signalling and uncover a function for FBXW7α as an attenuator of inflammatory signalling