Carlos Fernandez-Hernando, PhD

Medicine (Cardiology) - Primary Mentor

Our research aims to identify and characterize novel mechanisms by which cholesterol and lipoprotein metabolism is regulated. To date, most lipid and lipoprotein research has focused on alterations of protein coding genes, whereas the functions of non-coding RNAs remain largely unknown. Particular efforts are focused on microRNAs (miRNAs), a novel class of small non-coding RNAs that mediate port-transcriptional gene silencing. Using mouse models and cell culture studies, we will elucidate the molecular basis of the miRNA functions in regulating lipid metabolism and explore the potential of miRNAs as therapetic targets. 

miRNAs have emerged as critical regulators of gene expression at the posttranscriptional level. miRNAs typically control the expression of their target genes by imperfect base pairing to the 3’ untranslated regions (3’UTR) of messenger RNAs (mRNAs) thereby inducing repression of the target mRNA. Bioinformatic predictions and experimental approaches indicate that a single miRNA may target more than a hundred mRNAs. Indeed, human miRNAs are predicted to control the activity of more than 60% of all protein-coding genes. This class of short (22 nucleotides) noncoding RNA molecules has been shown to participate in almost every cellular process investigated so far, and their dysregulation is observed in, and might underlie, different human pathologies including cancer, heart disease, and neurodegeneration. Very recently, we have demonstrated that miR-33, an intronic miRNA located within the SREBP-2 gene, plays important roles in the homeostatic regulation of cholesterol metabolism. miR-33 inhibits the expression of the ATP-binding cassette (ABC) transporter, ABCA1, thereby attenuating both cholesterol efflux to apoA1 and high-density lipoprotein (HDL) biogenesis. Conversely, silencing of miR-33 in vivo increased hepatic ABCA1 and plasma HDL. Because plasma HDL levels show a strong inverse correlation with atherosclerotic vascular disease, there has been intense interest in therapeutically targeting HDL and macrophage cholesterol efflux pathways. Our study suggests that antagonists of endogenous miR-33 may be a useful therapeutic strategy for enhancing ABCA1 expression and raising HDL levels in vivo. In addition, our recent preliminary data suggest that miR-33 also coordinates genes regulating fatty acid metabolism and insulin signaling. Therefore, we plan to continue investigating the potential relevance of miR-33 expression in metabolic syndrome. Moreover, we are working with other miRNAs involved in the regulation of cellular cholesterol homeostasis, and depending on the results, would pursue the most promising candidates in more detail.

A second major project is to characterize new genes involved in the regulation of cholesterol. A tightly controlled-but only partially characterized-network of cellular signaling and lipid transfer systems orchestrates the functional compartmentalization of cholesterol within and between tissues at the whole body level. Increased understanding of these processes and their integration at the organ systems level provides fundamental insights into the physiology of cholesterol metabolism. However several issues await further studies. For the most sterol transport processes, only a limited number of proteins that are involved have been identified and very little is known about cholesterol trafficking in many physiologically relevant cell types, such us hepatocytes, enterocytes or cells of the central nervous system. Future work will focus on determining the molecular mechanisms involved in the cholesterol metabolism in mammalian cells using functional genomic screens. Our current studies aim to identify new genes regulating low-density lipoprotein receptor activity and trafficking in human hepatic cell lines using a genome-wide RNA interference (RNAi) screens. Besides increasing our insights into the physiology of cholesterol trafficking, the information obtained should help to develop improved strategies for management of cholesterol-related pathologies.