Adverse outcome pathway for pregnane X receptor-induced hypercholesterolemia

Abstract

Pharmaceuticals and environmental contaminants contribute to hypercholesterolemia. Several chemicals known to cause hypercholesterolemia, activate pregnane X receptor (PXR). PXR is a nuclear receptor, classically identified as a sensor of chemical environment and regulator of detoxification processes. Later, PXR activation has been shown to disrupt metabolic functions such as lipid metabolism and recent findings have shown PXR activation to promote hypercholesterolemia through multiple mechanisms. Hypercholesterolemia is a major causative risk factor for atherosclerosis and greatly promotes global health burden. Metabolic disruption by PXR activating chemicals leading to hypercholesterolemia represents a novel toxicity pathway of concern and requires further attention. Therefore, we constructed an adverse outcome pathway (AOP) by collecting the available knowledge considering the molecular mechanisms for PXR-mediated hypercholesterolemia. AOPs are tools of modern toxicology for systematizing mechanistic knowledge to assist health risk assessment of chemicals. AOPs are formalized and structured linear concepts describing a link between molecular initiating event (MIE) and adverse outcome (AO). MIE and AO are connected via key events (KE) through key event relationships (KER). We present a plausible route of how PXR activation (MIE) leads to hypercholesterolemia (AO) through direct regulation of cholesterol synthesis and via activation of sterol regulatory element binding protein 2-pathway.

Keywords: AOP; Cholesterol; Hypercholesterolemia; PCSK9; PXR; SREBP2.

Fig. 1

Graphic representation of the adverse outcome pathway from PXR activation to increased level of plasma LDL cholesterol. The molecular initiating event (MIE) is activation of pregnane X receptor (PXR), which leads to the first key event (KE), unstabilized structure of insulin-induced gene (INSIG1). Unstable structure of INSIG1 allows activation of sterol regulatory element binding protein 2 (SREBP2), the second KE in the pathway. Activation of SREBP2 increases the transcription of its target genes, leading to increased synthesis and activation of proprotein convertase subtilisin kexin type 9 (PCSK9) (KE3) and enzymes participating in cholesterol synthesis (KE4). PXR directly activates squalene epoxidase (SQLE) (KE5), rate-limiting enzyme of cholesterol synthesis. Increased serum level of PCSK9 leads to decreased amount of low-density lipoprotein (LDL) receptors, causing diminished uptake of LDL from circulation by liver. Simultaneously, activation of cholesterol synthesis enzymes enhances cholesterol synthesis. These mechanisms lead to increased level of plasma LDL cholesterol, the adverse outcome (AO) of the pathway. Arrows represent direct key event relationships (KERs) that link the KEs

Fig. 2

Graphical illustration of sterol regulatory element binding protein 2 (SREBP2) activation pathway. After the complex formed by SREBP2 and SREBP cleavage activating protein (SCAP) detaches from insulin-induced gene 1(INSIG1) the complex locates from endoplasmic reticulum (ER) to Golgi in the Coat Protein Complex II (COP2) coated vesicles. In Golgi, SREBP2 goes through a proteolytic processing by two cleavage enzymes known as membrane-bound transcription factor site-1-protease (S1P) and site-2-protease (S2P). Consequently, the active transcription factor (N-SREBP2) is released, and translocates to the nucleus where it binds to sterol regulating element (SRE) sequences and induces transcription of genes participating in cholesterol biosynthesis and uptake

Fig. 3

Mevalonate pathway of cholesterol synthesis. First, acetyl-CoA is converted to acetoacetyl-CoA by thiolase 2. Secondly, methylglutaryl-coenzyme A synthase (HMGCS) condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA (Brown and Sharpe 2016). Next, HMGCR reduces HMG-CoA to mevalonic acid. Mevalonic acid is phosphorylated to mevalonate-P by mevalonate kinase (MK) and mevalonate-P is further produced to mevalonate-PP by phosphomevalonate kinase (PMK). This is followed by catalyzation of mevalonate-PP to isopentenyl-PP by diphosphomevalonate decarboxylase. Next, dimethylallyl-PP is isomerized by isopentenyl-diphosphate Δ-isomerase (IDI1/IDI2). The isoprenoid-pyrophosphates are condensed to isoprenoid geranyl-PP by farnesyl diphosphate synthase (FPPS) or to farnesyl-PP by geranylgeranyl pyrophosphate synthase (GGPPS) and finally, squalene synthase (SQS) converts farnesyl-PP to squalene. Squalene is converted to (S)-2, 3-epoxysqualene by squalene epoxidase (SQLE) and (S)-2,3-epoxysqualene is further converted to lanosterol and finally to 7-dehydrodesmosterol or 7-dehydrocholesterol via Bloch or Kandutch-Russel (K-R) pathways, respectively