  Recently, microRNAs (miRNAs) have emerged as central posttranscriptional
regulators of gene expression. miRNAs regulate many key biological processes,
including cell growth, death, development and differentiation. This discovery
is challenging the central dogma of molecular biology. Genes are working
together by forming cellular networks. It has become an emerging concept that
miRNAs could intertwine with cellular networks to exert their function. Thus,
it is essential to understand how miRNAs take part in cellular processes at a
systems-level. In this review, I will first introduce basic knowledge of miRNAs
and their relations to heart disaeses and cancer, highlight recently dicovered
functions such as filtering out gene expression noise by miRNAs. I will aslo
introduce basic concepts of cellular networks and interpret their biological
meaning in such a way that the network concepts are digested in a biological
context and are understandable for biologists. Finally, I will summarize the
most recent progress in understanding of miRNA biology at a systems-level: the
principles of miRNA regulation of the major cellular networks including
signaling, metabolic, protein interaction and gene regulatory networks. A
common miRNA regulatory principle is emerging: miRNAs preferentially regulated
the genes that have high regulation complexity. In addition, miRNAs
preferentially regulate positive regulatory motifs, highly connected scaffolds
and the most network downstream components of cellular signaling networks,
while miRNAs selectively regulate the genes which have specific network
structural features on metabolic networks.
