Bifunctional Role of Rev-erbα in Adipocyte Differentiation

Abstract

The nuclear receptor Rev-erb␣ is a potent transcriptional repressor that regulates circadian rhythm and metabolism. Here we demonstrate a dissociation between Rev-erb␣ mRNA and protein levels that profoundly influences adipocyte differentiation. During adipogenesis, Rev-erb␣ gene expression initially declines and subsequently increases. Remarkably, Rev-erb␣ protein levels are nearly the opposite, increasing early in adipogenesis and then markedly decreasing in adipocytes. The Rev-erb␣ protein is necessary for the early mitotic events that are required for adipogenesis. The subsequent reduction in Rev-erb␣ protein, due to increased degradation via the 26S proteasome, is also required for adipocyte differentiation because Rev-erb␣ represses the expression of PPAR␥2, the master transcriptional regulator of adipogenesis. Thus, opposite to what might be predicted from Rev-erb␣ gene expression, Rev-erb␣ protein levels must rise and then fall for adipocyte differentiation to occur.The conversion of fibroblastic preadipocytes to mature adipocytes involves the temporal and hierarchical coordination of intracellular signaling molecules and transcription factors (11,34). Differentiation of the widely used 3T3-L1 preadipocyte cell line requires extracellular signals, including cell contact, glucocorticoids, and serum-derived factors, as well as intracellular accumulation of cyclic AMP (39). Together, these initiators lead confluent preadipocytes to undergo two rounds of cell division that are required for adipogenesis (41). They also induce early transcription factors, notably, CCAAT enhancerbinding proteins (C/EBP) ␤ and ␦, to instigate a cascade of transcriptional events that ultimately results in withdrawal from the cell cycle in the process of differentiation into mature postmitotic adipocytes (8,47,49). C/EBP␤, in particular, induces nuclear receptor peroxisome proliferator-activated receptor ␥ (PPAR␥) (17,35,36,47), the master transcriptional regulator of adipogenesis whose high-level expression in adipocytes is both necessary and sufficient for adipocyte-specific gene expression and the adipocyte phenotypes (6,42,43). PPAR␥ and C/EBP␣ induce one another to maintain the differentiated adipocyte phenotype (9, 33). Other transcription factors, such as ADD1/SREBP and KLF5, also play important roles in adipogenesis (23, 30).Rev-erb␣ is an orphan nuclear receptor that has also been implicated in adipocyte differentiation (5,12,24). It is highly expressed in adipose tissue, and its gene expression is specifically induced in differentiating adipocytes (5, 14). Rev-erb␣ mRNA is transcribed from the opposite strand of the thyroid hormone receptor (TR) ␣ gene and is antisense to the TR␣2 splice product which encodes a non-thyroid hormone-binding TR variant (26, 29). The Rev-erb␣ protein lacks the classical nuclear receptor C-terminal activation domain but binds to nuclear receptor corepressor (N-CoR) and hence functions as a potent transcriptional repressor (18). Recently, Rev-erb␣ has been shown to function as a core negative ...

High Affinity Heme Binding to a Heme Regulatory Motif on the Nuclear Receptor Rev-erbβ Leads to Its Degradation and Indirectly Regulates Its Interaction with Nuclear Receptor Corepressor

“…The addition of MG-132, a proteasome inhibitor, completely reversed the effects of CHX on the wild-type protein (Fig. 15, F and G), demonstrating that the proteasome is responsible for degradation of Rev-erb␤, as suggested for Reverb␣ (85). Thus, heme binding inversely controls the concentration of Rev-erb␤ in the cell, likely by targeting the protein for proteasomal degradation.…”

“…Although peroxisome proliferator-activated receptor ␥ induces the high levels of Rev-erb␣ necessary for the early stages of preadipocyte differentiation (93), repression of peroxisome proliferator-activated receptor ␥2 and proteasomal degradation of Rev-erb␣ are required for later differentiation stages (85). Heme levels increase throughout adipogenesis at a rate closely matching that of Rev-erb␣ degradation.…”

A Time to Fast, a Time to Feast: The Crosstalk between Metabolism and the Circadian Clock

“…In cultured adipocytes, BMAL1 controls adipogenesis via the induction of mm^oγ, ^mO (c^_mQ), sterol regulatory element binding protein 1 (pob_mJN) and CCAAT/enhancer binding proteins (`Lb_më) (Shimba= et alK, 2005). Defects in adipocyte differentiation were also seen in the absence of REV-ERBα, an upstream regulator of _ã~äN transcription (Wang and Lazar, 2008). Additionally, REV-ERBα represses lipid metabolism by binding to an AGGTAC motif in the promoter region of ^éç`Jfff (Raspe et al, 2002).…”

“…Importantly, the two orphan nuclear receptors REV-ERBα (NR1D1) and RORα (RORA) together represent one of the transcriptional feedback loops that underlie circadian regulation at the cellular level (Preitner=et alK, 2002;Sato=et alK, 2004). As oÉîJÉêÄα is required for adipogenesis and oçêα is important for lipid homeostasis, these two NRs might directly link metabolism to circadian clock function (Lau et al, 2004;Wang and Lazar, 2008). Peroxisome proliferator-activated receptor α (PPARA) is another important NR, a sensor for fatty acids and is involved in the regulation of energy metabolism (Lefebvre= et alK, 2006).…”

Fin de l’orphelinat pour les récepteurs nucléaires REVERB

“…Leur expression est rythmique dans de nombreux tissus. Les REVERB ont été impliqués dans divers processus biologiques, leur expression est induite au cours de la différenciation adipocytaire [1,2] et diminuée au cours de la différenciation métabolisme lipidique et glucidique, la sécrétion hormonale (corticosté-rone, leptine, insuline) et le cycle veille-sommeil.…”