Deubiquitylase Inhibition Reveals Liver X Receptor-independent Transcriptional Regulation of the E3 Ubiquitin Ligase IDOL and Lipoprotein Uptake

Abstract

Cholesterol metabolism is subject to complex transcriptional and nontranscriptional regulation. Herein, the role of ubiquitylation is emerging as an important post-translational modification that regulates cholesterol synthesis and uptake. Similar to other post-translational modifications, ubiquitylation is reversible in a process dependent on activity of deubiquitylating enzymes (DUBs). Yet whether these play a role in cholesterol metabolism is largely unknown. As a first step to test this possibility, we used pharmacological inhibition of cellular DUB activity. Short term (2 h) inhibition of DUBs resulted in accumulation of high molecular weight ubiquitylated proteins. This was accompanied by a dramatic decrease in abundance of the LDLR and attenuated LDL uptake into hepatic cells. Importantly, this occurred in the absence of changes in the mRNA levels of the LDLR or other SREBP2-regulated genes, in line with this phenotype being a post-transcriptional event. Mechanistically, we identify transcriptional induction of the E3 ubiquitin ligase IDOL in human and rodent cells as the underlying cause for ubiquitylation-dependent lysosomal degradation of the LDLR following DUB inhibition. In contrast to the established transcriptional regulation of IDOL by the sterol-responsive liver X receptor (LXR) transcription factors, induction of IDOL by DUB inhibition is LXR-independent and occurs in Lxr␣␤ ؊/؊ MEFs.Consistent with the role of DUBs in transcriptional regulation, we identified a 70-bp region in the proximal promoter of IDOL, distinct from that containing the LXR-responsive element, which mediates the response to DUB inhibition. In conclusion, we identify a sterol-independent mechanism to regulate IDOL expression and IDOL-mediated lipoprotein receptor degradation.Elevated levels of plasma LDL represent a major risk factor for development of atherosclerosis and cardiovascular disease (1). Because of its ability to promote LDL uptake into cells, the LDL receptor (LDLR) 2 is a major determinant of plasma LDL levels (2). The pivotal role of the LDLR in LDL metabolism is exemplified by the fact that LDLR mutations account for most incidences of familial hypercholesterolemia, a disease characterized by reduced hepatic LDL clearance, elevated plasma cholesterol levels, and accelerated cardiovascular disease (1, 3).The LDLR is subject to tight transcriptional and post-transcriptional regulation, which is largely governed by the intracellular levels of cholesterol (4). At the level of transcription, these pathways are regulated by two nuclear transcription factor families: SREBP1 and SREBP2 (5-7), and the liver X receptor ␣ and ␤ (LXRs) (8, 9). When cellular sterol levels decline, SREBPs are activated to induce genes required for de novo cholesterol biosynthesis, as well as the LDLR to increase uptake of LDL cholesterol (4). In contrast, when sterol levels rise, LXRs become activated by their endogenous ligands. These ligands include oxidized cholesterol derivatives (oxysterols) and intermediates of the cholesterol synthes...

LXR Regulation of Brain Cholesterol: From Development to Disease

“…The IDOL KO appeared to facilitate microglial clearance of Aβ, likely through permitting increased expression of LDLR that can act as a receptor for apoE, Aβ, or complexed apoE/Aβ species [30]. Recently a deubiquitylase, ubiquitin-specific protease 2 (USP2), has been reported to interact with IDOL and promote its deubiquitylation, which both stabilizes IDOL and decreases its ability to ubiquitylate the LDLR [31,32]. While USP2 expression did not appear to respond to LXR ligands, the participation of this protein indicates that the LXR-IDOL-LDLR pathway is part of a much larger and complex metabolic regulatory circuit that remains to be fully identified.…”

Pirh2 mediates the sensitivity of myeloma cells to bortezomib via canonical NF-κB signaling pathway

“…E3 ligases carry out the final step in the ubiquitination cascade and determine the substrate protein to be ubiquitylated before mediating the transfer of ubiquitin residues from an E2 enzyme to a lysine on the target. Thus, E3 ligases are of interest as drug targets because of their ability to regulate protein stability and functions (Liu et al, 2014 ), especially in oncogenesis (Jung et al, 2012 , Hsieh et al, 2013 , Severe et al, 2013 , Sharma and Nag, 2014 , Zhang et al, 2014 , Hao and Huang, 2015 , Yin et al, 2015 ), cancer progression (Lou and Wang, 2014 , Sun and Denko, 2014 , Goka and Lippman, 2015 ), metastasis (Wang et al, 2012 ), disease prognosis (Bielskiene et al, 2015 , Hou and Deng, 2015 ) and chemotherapy resistance (Nelson et al, 2016 , Petzold et al, 2016 ). A growing number of E3 ligases and their substrate proteins have emerged as crucial players in drug resistance, and new insights have been obtained into the roles of E3 ubiquitin ligases in bortezomib resistance (Malek et al, 2016 ).…”

IDOL gene variant is associated with hyperlipidemia in Han population in Xinjiang, China

“…Other possible downstream of IDOL includes VLDLR and ApoER2 11 . The upstream of IDOL is LXRs which have influence on extracellular cholesterol levels by combining with LXR element of IDOL 12 . Human genetic studies propose that IDOL may become a new therapeutic target for regulating plasma LDL-C levels.…”