Regulation of miRNA biogenesis as an integrated component of growth factor signaling

Abstract

Transcriptional control of microRNAs (miRNA) by cell signaling pathways, especially in the context of growth factor regulation, is a widely recognized phenomenon with broad-reaching implications. However, several recent studies indicate that not just transcription, but also processing of miRNAs is subject to regulation as part of an integrated physiological response to various stimuli and environmental changes. The canonical miRNA biogenesis pathway; sequential steps of nucleolytic cleavage by the RNase III enzymes Drosha and Dicer, are emerging regulatory hubs for the modulation of miRNA expression as part of both physiological and pathological responses. In this article we use well-characterized growth-factor signaling pathways such as transforming growth factor-β (TGF-β), Protein Kinase B (PKB, also known as Akt) and extracellular-signal-regulated kinase (ERK) to illustrate how basic cell signaling pathways modulate the activities of these components of the miRNA biogenesis pathway to achieve optimal miRNA expression patterns.

Figure 1. Basic components of the miRNA biogenesis pathway

miRNA begins as a long primary transcript (pri-miRNA) with a structure resembling mRNA. Pri-miRNAs contain a 5′ cap (m7-G) and a polyA tail at the 3′ end (AAAA). Pri-miRNA undergoes two sequential processing steps. First, the Drosha microprocessor complex which is composed of Drosha, DGCR8 and DEAD-Box RNA helicases (p68 or p72) cleaves pri-miRNA to generate precursor miRNA (pre-miRNA). Following cleavage by the Drosha microprocessor, the resulting pre-miRNA precursor maintains a stereotypical stem-loop structure. Following the export of pre-miRNA from the nucleus by exportin 5 (EXP5), Dicer processes the pre-miRNA into double stranded mature miRNA. Mature miRNA will be loaded into Argonaute proteins (Ago), which separates mature miRNA into two single stranded miRNAs. (B) Future miRNAs adopt a stereotypical stem-loop structure within the pri-miRNA sequence (shown here for miR-145). The mature sequence is encoded on one side of the loop and the degraded * strand on the other. More recent nomenclature has moved to calling each strand 5p and 3p according to which side of the strand they occupy. Here, the dotted lines indicate the site of future cleavage by Dicer and Drosha.

Figure 2. Integration of miRNA into cell growth factor signaling pathways

(A) Activation of receptor Smads by TGF-β family ligands leads to their translocation into the nucleus. In the nucleus, R-Smads bind, independent of the transcriptionally necessary co-Smad, to a conserved sequence in pri-miRNAs which they recruit to the Drosha microprocessor complex and facilitate pri- to pre-miRNA maturation. (B) PI3K-mediated activation of Akt induces the phosphorylation of KSRP. Phosphorylated KSRP is dissociated from mRNA and associates with targeted pri-miRNA sequence to recruit the Drosha microprocessor and facilitate pri- to pre-miRNA processing. (C) Activation of MAPK/ERK induces phophorylation of TRBP, which is subsequently recruited to Dicer and promotes the processing of miRNAs associated with cell proliferation.

Figure 3. Integration of biological signaling pathways at the sites of miRNA biogenesis

miRNA biogenesis pathways are emerging as critical components of gene regulatory mechanism controlled by growth factor signaling (and other) pathways. Activation or inactivation of the Drosha microprocessor by DNA binding proteins (Smads, p53, and ERα), RNA binding proteins (KSRP) or TBRP has been well documented. However, mechanisms of regulation of many other proteins in the miRNA biogenesis pathway by different signaling pathways remain to be uncovered.