AIP4 regulates adipocyte differentiation by targeting C/EBPa for ubiquitin-mediated proteasomal degradation

Adipogenesis, that is, the formation of terminally differentiated adipocytes is intricately regulated by transcription factors where CCAAT/enhancer binding protein alpha (C/EBPa) plays a key role. In the current study, we demonstrate that E3 ubiquitin ligase AIP4 negatively regulates C/EBPa protein stability leading to reduced adipogenesis. While AIP4 overexpression in 3T3-L1 cells preadipocytes inhibited lipid accumulation when treated with differentiation inducing media (MDI), AIP4 depletion was sufficient to partially promote lipid accumulation even in the absence of MDI. Mechanistically, overexpression of AIP4 inhibited protein levels of both ectopically expressed as well as endogenous C/EBPa while catalytically inactive AIP4 failed. On the contrary, AIP4 depletion profoundly enhanced endogenous C/EBPa protein levels. The observation that AIP4 levels decrease with concomitant increase in C/EBPa levels during adipocyte differentiation further indicated that AIP4 negatively regulates C/EBPa levels. We further show that AIP4 physically interacts with C/EBPa and ubiquitinates it leading to its proteasomal degradation. AIP4 promoted K48-linked ubiquitination of C/EBPa while catalytically inactive AIP4-C830A failed. Taken together, our data demonstrate that AIP4 inhibits adipogenesis by targeting C/EBPa for ubiquitin-mediated proteasome degradation.