ARMC5 selectively degrades SCAP-free SREBF1 and is essential for fatty acid desaturation in adipocytes

Background: Armadillo repeat containing 5 (ARMC5)-CUL3 complex was recently identified as an E3 ubiquitin ligase of full-length SREBF. Although ARMC5 was involved in SREBF2 pathway in the adrenocortical cells and might participate in the development of primary bilateral macronodular adrenal hyperplasia (PBMAH), the role of ARMC5 in adipocytes has not been investigated. Methods and Results: Adipocyte-specific Armc5 knockout mice were generated. In the white adipose tissue (WAT) of these mice, all the stearoyl-CoA desaturase (Scd) were drastically downregulated under refed condition and high fat-high sucrose diet. Consistently, unsaturated fatty acids were decreased, while saturated fatty acids were increased in the WAT of these mice. Assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-Seq) peaks at the SREBF1-binding sites were markedly diminished around the Scd1 locus in the WAT of adipocyte-specific Armc5 knockout mice. Armc5-deficient 3T3-L1 adipocytes also exhibited downregulation of Scd. Mechanistically, disruption of Armc5 restored decreased full-length SREBF1 in the CHO cells deficient for Scap, an escort protein required for the transport of SREBF from the ER to the Golgi. In HEK293T cells, overexpression of Scap inhibited ARMC5-mediated degradation of full-length SREBF1, and overexpression of Armc5 increased nuclear SREBF1/full-length SREBF1 ratio in the presence of exogenous Scap in HEK293T cells. Conclusions: ARMC5 exerts selective removal of SCAP-free SREBF1 and stimulates SCAP-mediated SREBF1 processing, hence is essential for fatty acid desaturation in vivo.