CAND1-dependent control of cullin 1-RING Ub ligases is essential for adipogenesis

Abstract

Cullin-RING ubiquitin (Ub) ligases (CRLs) are responsible for ubiquitinylation of approximately 20% of all proteins degraded by the Ub proteasome system (UPS). CRLs are regulated by the COP9 signalosome (CSN) and by Cullin-associated Nedd8-dissociated protein 1 (CAND1). The CSN is responsible for removal of Nedd8 from cullins inactivating CRLs. CAND1 modulates the assembly of F-box proteins into cullin 1-RING Ub ligases (CRL1s). We show that CAND1 preferentially blocks the integration of Skp2 into CRL1s. Suppression of CAND1 expression in HeLa cells leads to an increase of the Skp2 assembly into CRL1s and to significant reduction of the cyclin-dependent kinase (CDK) inhibitor p27. In contrary, CAND1 overexpression causes elevation of p27. The observed CAND1-dependent effects and CAND1 expression are independent of the CSN as demonstrated in CSN1 knockdown cells. Increase of p27 is a hallmark of preadipocyte differentiation to adipocytes (adipogenesis). We demonstrate that the accumulation of p27 is associated with an increase of CAND1 and a decrease of Skp2 during adipogenesis of human LiSa-2 preadipocytes. CAND1 knockdown reduces p27 and blocks adipogenesis. Due to the impact of CAND1 on Skp2 control, CAND1 could represent an important effector molecule in adipogenesis, but also in cancer development.