MiR-33 is a microRNA located within the intron of the SREBF2 gene, which regulates cholesterol synthesis. It modulates the expression of genes involved in cellular cholesterol transport, such as ABCA1 and ABCG1. In mouse and human cells, miR-33 inhibits ABCA1, reducing cholesterol efflux to apolipoprotein A1. In mouse macrophages, miR-33 also targets ABCG1, reducing cholesterol efflux to nascent HDL. Lentiviral delivery of miR-33 to mice represses ABCA1 expression in the liver, reducing circulating HDL levels. Conversely, silencing miR-33 increases hepatic ABCA1 and plasma HDL levels. Thus, miR-33 regulates both HDL biogenesis in the liver and cellular cholesterol efflux. Cholesterol is essential for cell membranes and metabolic pathways. SREBP transcription factors regulate cholesterol biosynthesis and uptake in mammalian cells. LXR receptors regulate genes involved in cholesterol response, including ABCA1 and ABCG1. MicroRNAs, such as miR-33, are posttranscriptional regulators of physiological processes. miR-33 is encoded within SREBF2 intron 16 and is highly conserved in mammals. miR-33 is down-regulated by cholesterol loading and up-regulated by cholesterol depletion. miR-33 levels are inversely correlated with cholesterol levels and ABCA1 expression and positively correlated with SREBF2 mRNA levels. miR-33 is also regulated in two mouse models of hypercholesterolemia. miR-33 targets the 3'UTR of ABCA1, NPC1, and mouse, but not human, ABCG1. miR-33 represses ABCA1 and ABCG1 protein in mouse hepatic cells, indicating its effects are not cell type-specific. Inhibition of endogenous miR-33 increases ABCA1 and ABCG1 expression in macrophages and hepatocytes. miR-33 represses ABCA1 and ABCG1 protein in mouse cells, but has a stronger effect on human NPC1. miR-33 more strongly represses hNPC1 than mNPC1 3'UTR activity. miR-33 modulates cholesterol efflux from macrophages, a critical step in reverse cholesterol transport. miR-33 overexpression reduces HDL levels, while miR-33 silencing increases HDL levels. miR-33 is widely expressed in different cell types and tissues, suggesting a global effect on cellular cholesterol homeostasis. miR-33 functions via a negative feedback loop triggered by cellular cholesterol content. miR-33 regulates cholesterol transport pathways that mobilize cholesterol from intracellular stores to HDL lipopMiR-33 is a microRNA located within the intron of the SREBF2 gene, which regulates cholesterol synthesis. It modulates the expression of genes involved in cellular cholesterol transport, such as ABCA1 and ABCG1. In mouse and human cells, miR-33 inhibits ABCA1, reducing cholesterol efflux to apolipoprotein A1. In mouse macrophages, miR-33 also targets ABCG1, reducing cholesterol efflux to nascent HDL. Lentiviral delivery of miR-33 to mice represses ABCA1 expression in the liver, reducing circulating HDL levels. Conversely, silencing miR-33 increases hepatic ABCA1 and plasma HDL levels. Thus, miR-33 regulates both HDL biogenesis in the liver and cellular cholesterol efflux. Cholesterol is essential for cell membranes and metabolic pathways. SREBP transcription factors regulate cholesterol biosynthesis and uptake in mammalian cells. LXR receptors regulate genes involved in cholesterol response, including ABCA1 and ABCG1. MicroRNAs, such as miR-33, are posttranscriptional regulators of physiological processes. miR-33 is encoded within SREBF2 intron 16 and is highly conserved in mammals. miR-33 is down-regulated by cholesterol loading and up-regulated by cholesterol depletion. miR-33 levels are inversely correlated with cholesterol levels and ABCA1 expression and positively correlated with SREBF2 mRNA levels. miR-33 is also regulated in two mouse models of hypercholesterolemia. miR-33 targets the 3'UTR of ABCA1, NPC1, and mouse, but not human, ABCG1. miR-33 represses ABCA1 and ABCG1 protein in mouse hepatic cells, indicating its effects are not cell type-specific. Inhibition of endogenous miR-33 increases ABCA1 and ABCG1 expression in macrophages and hepatocytes. miR-33 represses ABCA1 and ABCG1 protein in mouse cells, but has a stronger effect on human NPC1. miR-33 more strongly represses hNPC1 than mNPC1 3'UTR activity. miR-33 modulates cholesterol efflux from macrophages, a critical step in reverse cholesterol transport. miR-33 overexpression reduces HDL levels, while miR-33 silencing increases HDL levels. miR-33 is widely expressed in different cell types and tissues, suggesting a global effect on cellular cholesterol homeostasis. miR-33 functions via a negative feedback loop triggered by cellular cholesterol content. miR-33 regulates cholesterol transport pathways that mobilize cholesterol from intracellular stores to HDL lipop