MicroRNAs: Tissue Expression and Role in 3T3-L1 Pre-Adipocyte Differentiation
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MicroRNAs, which are endogenous small RNAs around 22 nucleotides, play important roles in many physiological processes. We investigated whether microRNAs regulate 3T3-L1 pre-adipocyte differentiation. The expression of microRNAs during 3T3-L1 pre-adipocyte differentiation was detected by microarrays and confirmed by northern blot and quantitative real time PCR. Several microRNAs, including let-7, were up-regulated at the late stage of 3T3-L1 adipogenesis. let-7 expression specifically increased during the late stage of 3T3-L1 differentiation and ectopic introduction of let-7 in 3T3-L1 cells before hormonal induction inhibited 3T3-L1 adipogenesis. Both the mRNA and protein levels of HMGA2, a target of let-7, decreased with ectopic let-7 presence in 3T3-L1 cells. Also, HMGA2 protein level was inversely correlated to let-7 levels during 3T3-L1 adipogenesis. Knock-down of Hmga2 or E2f1 by siRNA inhibited 3T3-L1 pre-adipocyte differentiation. Our results suggest let-7 can stop clonal expansion of 3T3-L1 cells and bring them to final growth arrest and terminal differentiation by targeting Hmga2. In an effort to explore the role of microRNA, the expression of 111 microRNAs in 36 mouse tissues was detected by quantitative real time PCR. MicroRNAs either show universal expression in all tissues or specific expression in certain tissues, suggesting their roles in these tissues. Different isotypes of the same microRNAs or microRNAs transcribed from the same genomic location show similar expression pattern in mouse tissues. Hierarchical cluster analysis based on the expression of microRNA in tissues showed that tissues having similar physiologic functions or anatomic locations clustered together, suggesting the roles of microRNAs might be consistent with the functions of the tissues in which they are expressed. Comparison of the expression of microRNAs with that of nuclear receptors in mouse tissues showed positive correlations between a select number of nuclear receptors and microRNAs, but these relationships need to be verified by experimental data. In all, the expression profile of microRNAs in mouse tissues provides a useful tool for microRNA studies.