Peroxisome proliferator-activated receptor g (PPARg), a member of the nuclear receptor superfamily of ligand inducible transcription factors, is viewed as the master regulator of adipocyte differentiation, function, and maintenance. Thiazolidinediones (TZDs) like rosiglitazone are synthetic PPARg ligands that display effective anti-diabetic activity, but their clinical use is limited because of serious undesired side effects. Detailed studies of PPARg action will not only expand our fundamental understanding of adipocyte biology but also reveal the possibilities and limitations of PPARg as an anti-diabetic drug target.

We identified the E3 ubiquitin ligase Rnf125 as a novel PPARg target gene. Both Rnf125 mRNA and protein expression significantly increased during 3T3-L1 differentiation and ChIP-seq experiments showed that PPARg binds at 3 potential enhancer sites in the Rnf125 locus together with its heterodimeric partner RxRa . Subsequent analyses showed that 2 of these enhancers are functional, supporting the classification of Rnf125 as a novel PPARg target gene. Interestingly, RNA-seq on purified adipocytes collected from subcutaneous samples of insulin sensitive and resistant human subjects reveals that RNF125 mRNA expression is significantly decreased in insulin resistant patients (GEO: GSE174475). Moreover, in patients with acquired obesity, RNF125 negatively correlated with adipocyte size and metabolic derangements, but positively with adiponectin and adipocyte number (Heinonen et al., 2014; Nature). To identify the role of Rnf125 in adipocyte biology we next aimed to identify substrates, which are expected to be degraded by the ubiquitin-proteasome system (UPS). Using different mass spectrometry-based methods, we identified multiple potential substrates including the metabolic enzyme Glutamine Synthetase (Glul). Interestingly, Glul converts glutamate to glutamine, which levels inversely correlate with fat mass and inflammation in human WAT (Petrus et al., 2020; Cell Metabolism).

Taken together, we identified Rnf125 as a novel PPARg target gene and show that the UPS may represent a level of spatiotemporal regulation in adipocyte biology and metabolism that has remained understudied so far.