microRNAs and cholesterol metabolism

Abstract

Cholesterol metabolism is tightly regulated at the cellular level. In addition to classic transcriptional regulation of cholesterol metabolism (e.g. by SREBP and LXR), members of a class of non-coding RNAs termed microRNAs (miRNAs) have recently been identified to be potent post-transcriptional regulators of lipid metabolism genes, including cholesterol homeostasis. We and others have recently shown that miR-33 regulates cholesterol efflux and HDL biogenesis by downregulating the expression of the ABC transporters, ABCA1 and ABCG1. In addition to miR-33, miR-122 and miR-370 have been shown to play important roles in regulating cholesterol and fatty acid metabolism. These new data suggest important roles of microRNAs in the epigenetic regulation of cholesterol metabolism and have opened new avenues for the treatment of dyslipidemias.

Figure 1

Regulation of cellular cholesterol metabolism. Animal cells synthesize cholesterol from acetyl-CoA. In addition, cells obtain cholesterol from the circulation in the form of apolipoprotein B-containing lipoproteins, especially LDL. The circulating LDL particles carrying cholesterol and cholesterol esters are internalized through LDLr and transported to sorting endosomes. LDL particles are subsequently transported to late endosomes (LE) and lysosomes (LY), while LDL receptors are recycled to the plasma membrane. Free cholesterol egress from LE/LY in a process mediated by Niemann-Pick type C 1 and 2 proteins (NPC1 and NPC2). Under low intracellar cholesterol concentration, the SCAP-SREBP complex moves to the Golgi complex where the SREBP is processed to its nuclear form. The nuclear SREBP turns on genes involved in cholesterol biosynthesis (e.g. HMGCR) and cholesterol uptake (LDLr). Conversely, in response to cellular cholesterol excess, the oxysterols generated bind and activate the liver X receptors (LXRs), which heterodimerize with retinoid X receptors (RXR) and activate the expression of the ATP transporters (ABCA1 and ABCG1). ABCA1 and ABCG1 promote cholesterol efflux via apoA1 and HDL respectively and help to maintain intracellular cholesterol homeostasis.

Figure 2

miRNA biogenesis and function. miRNAs are transcribed in the nucleus into primary transcripts (pri-miRNAs). They are transcribed from independent miRNA genes, from polycistronic transcripts or from introns of protein-coding genes. Pri-miRNAs are then processed in two steps in the nucleus and cytoplasm, catalyzed by the RNase III type endonuclease Drosha and Dicer, respectively. These enzymes function in complexes with dsRNA-binding domains proteins, DGCR8 and TRBP for Drosha and Dicer, respectively. In the canonical pathway (illustrated here) Drosha-DGCR8 processes the transcript to a stem-loop-structured precursor (pre-miRNA). A subset of miRNAs, called miRtrons, also derived from introns, is processed into pre-miRNAs by the spliceosome and the debranching enzyme. Both canonical miRNAs and miRtrons are exported to the cytoplasm via Exportin 5, where they are further processed by Dicer-TRBP to yield ≈ 20-bp miRNA duplexes. The typical Dicer cleavage product features 5′ phosphate groups and two-nucleotide overhangs at the 3′ ends. One strand is selected to function as mature miRNA and loaded into the RISC, while the partner miRNA* strand is preferentially degraded. In contrast, the precursor of miR-451 is recognized directly by Ago2. The unusual structure of the precursor (short stem, miRNA sequence spans the loop) promotes binding and cleavage by Ago2 after the 30th nucleotide. Therefore, miR-451 is produced independently of Dicer. The miRNA is further matured by so far unknown mechanisms. The mature miRNA produced by these two mechanisms leads to translational repression or mRNA degradation. The key components of the RISC are components of the Argonaute family (Ago 1–4). Animal miRNAs usually show only partial complementarity to the target mRNA promoting translational repression (initiation and post initiation steps) or deadenylation coupled to exonucleolytic degradation of target mRNA. mRNAs repressed by deadenylation or at the translation-initiation step are moved to P-bodies for either degradation or storage.

Figure 3

miR-33 Reduces Cholesterol Transport And Efflux Under Low Sterol Conditions. When sterol conditions are low, SREBP-2 is cleaved in the Golgi and translocates to the nucleus, where it acts as a transcription factor to regulate genes containing a sterol response element (SRE), including SREBP-2 itself. miR-33a is located within intron 16 of SREBP-2, and is co-transcribed along with its host transcript. Once processed by the miRNA system, mature miR-33a can bind the 3′UTR of its target genes, such as ABCA1, ABCG1 and NPC1 repressing their expression. This results in reduced cholesterol efflux to apoA1 and HDL.