MicroRNAs in body fluids--the mix of hormones and biomarkers

Abstract

Since the discovery of microRNAs (miRNAs), the study of these small noncoding RNAs has steadily increased and more than 10,000 papers have already been published. The great interest in miRNAs reflects their central role in gene-expression regulation and the implication of miRNA-specific aberrant expression in the pathogenesis of cancer, cardiac, immune-related and other diseases. Another avenue of current research is the study of circulating miRNAs in serum, plasma, and other body fluids--miRNAs may act not only within cells, but also at other sites within the body. The presence of miRNAs in body fluids may represent a gold mine of noninvasive biomarkers in cancer. Since deregulated miRNA expression is an early event in tumorigenesis, measuring circulating miRNA levels may also be useful for early cancer detection, which can contribute greatly to the success of treatment. In this Review, we discuss the role of fluid-expressed miRNAs as reliable cancer biomarkers and treatment-response predictors as well as potential new patient selection criteria for clinical trials. In addition, we explore the concept that miRNAs could function as hormones.

Figure 1

MiRNA biogenesis in the cell. MiRNAs are transcribed in the nucleus as pri-miRNA and then processed by Drosha into pre-miRNA. Pre-miRNA molecules are transported from the nucleus to the cytoplasm by exportin 5. Upon entering the cytoplasm, they are recognized by Dicer. Dicer modulates pre-miRNA and generates dsRNA, which are recognized by the RISC complex and converted into single-strand mature miRNA molecules. The RISC complex carries the mature miRNA molecule to complementary miRNA target sites within the mRNA molecule, where it affects gene expression by miRNA:mRNA sequence complementarity. A consequence of perfect complementarity between miRNA:mRNA molecules is mRNA cleavage and degradation. Imperfect alignment represses gene translation. Mutations in RNA processing are indicated with red lightning bolts. A frameshift mutation in exportin 5 caused premature codon termination and trapped pre-miRNAs in the nucleus. Other frameshift mutations in the RISC-loading complex subunit TARBP2 causes a loss of function of TARBP2, a secondary defect of Dicer activity and the loss of miRNA machinery regulation during tumorigenesis. Abbreviations: Ago2, Argonaute2; dsRNA, double-strand miRNA; miRNA, microRNA; POL II, RNA polymerase II; pre-miRNA, precursor miRNA; pri-miRNA, precursor primary miRNA; RISC, RNA-induced silencing complex.

Figure 2

A miRNA can function dually as both an oncogene and tumor-suppressor gene depending on the cancer type and cellular context. A duality of function in distinct types of cancer has been found for many miRNAs. An example is miR-125b, which has opposite roles (oncogene and tumor suppressor) in different cancer types or cell lines. As a tumor suppressor, miR-125b is downregulated in ovarian, thyroid, breast, and oral squamous-cell carcinomas, which promotes cell proliferation and cell-cycle progression.^, On the other hand, miR-125b is an oncogene in cancers such as prostate, thyroid, glioblastoma, and neuroblastoma. In neuroblastoma cells, miR-125b inhibits apoptosis in a p53-dependent manner, and promotes cell proliferation and invasion in prostate cancer cells.

Figure 3

Biogenesis and mechanism of action of circulating miRNAs. After being transcribed in the nucleus, pre-miRNA molecules can be processed further by Dicer in the cytoplasm. In addition, based on recent findings,^,– there are at least two ways that pre-miRNAs can be packaged and transported using exosomes and MVBs or other (not fully explored) pathways together with RNA-binding proteins. After fusion with the plasma membrane, MVBs release exosomes into the circulating compartments and bloodstream. Likewise, pre-miRNA inside the donor cell can be stably exported in conjunction with RNA-binding proteins, such as NPM1, and Ago2, or by HDL. Circulating miRNAs enter the bloodstream and are taken up by the recipient cells by endocytosis or, hypothetically, by binding to receptors present at the recipient cellular membrane capable of recognizing RNA-binding proteins. More studies are necessary to elucidate how miRNAs are loaded into exosomes and how they can be internalized by recipient cells. Exosomal miRNAs are processed by the same machinery used in miRNA biogenesis and thus have widespread consequences within the cell by inhibiting the expression of target protein-coding genes. For processing machinery see Figure 1. Abbreviations: MVBs, multivesicular bodies; NPM1, nucleophosmin 1; Ago2, Argonaute2; HDL, high-density lipoprotein.