MicroRNAs and bone metastasis: how small RNAs regulate secondary tumor formation and progression in the skeleton

Introduction

Cancer metastasis occurs when cancer cells from a primary tumor spread to distant organs, with bone being one of the most common sites for breast, prostate, and lung cancers [ ]. Patients with secondary bone tumors usually show osteolytic lesions (bone destruction) when cancer cells derive from breast and lung cancers, while they show osteoblastic lesion (abnormal bone formation) when cells derive from prostate cancer. Although some bone metastasis can be asymptomatic, when symptoms are displayed, they are painful due to bone fractures, hypercalcemia, and spinal cord compression that drastically reduce patients' quality of life [ ]. For these reasons, it is crucial to minimize the incidence of skeletal metastatic disease by preventing the spread of cancer cells and/or impede their seeding and their consequential proliferation in bone. Currently, we know that the highly specialized bone niche together with the molecular characteristics of metastatic cancer cells are two important factors that contribute to secondary tumor formation and progression in the skeleton [ ]. In fact, cancer cells can “educate” the bone microenvironment both locally (e.g., secreting growth factors and cytokines) and remotely (e.g., secreting macrovesicles from primary tumor sites). Additionally, metastatic cancer cells can display specific osteomimetic features that make them more likely to migrate to bone [ ]. MicroRNAs (miRNAs) are regulatory small noncoding RNAs involved in many cellular processes, whose expression is often dysregulated in cancer. In fact, some miRNAs (called oncomiRs) can be upregulated in cancer and promote tumor progression, while some other miRNAs act as oncosuppressors, maintaining cell homeostasis and preventing their transformation into a malignant phenotype [ ]. In this chapter, we summarize key aspects of miRNA biogenesis and their functional roles in the pathogenesis of bone metastases in various cancers. Finally, we discuss how miRNAs may be used as prognostic biomarkers to predict bone relapse.

MicroRNAs: from biogenesis to biological functions

Despite the unquestioned importance of messenger RNAs (mRNAs) encoding for proteins, there is a huge class of RNAs with no transcriptional potential yet with a fundamental role in the regulation of gene expression within cells, which are known as noncoding RNAs (ncRNAs). Based on their length, shape, and mechanisms of action, ncRNAs are conventionally named long ncRNAs (lncRNAs)—comprising linear lncRNAs, circular RNAs (circRNAs), long ribosomal RNAs (rRNAs), long intergenic ncRNAs (lincRNAs)—and small RNAs (sncRNAs)—comprising microRNAs (miRNAs), short-interfering RNAs (siRNAs), PIWI-interacting RNAs (piRNAs), small rRNAs, transfer RNAs (tRNAs), small nucleolar RNAs (snoRNAs), small nuclear RNAs (snRNAs)—( Fig. 33.1 ) [ , ].

By definition, miRNAs are a heterogeneous and endogenous class of snRNAs, 21–25 nucleotides long, with biological functions highly conserved in animal and plant cells. As other RNAs, miRNAs are transcribed from a double-strand DNA template by the RNAse polymerase II (Pol-II) in a long primary transcript, which is then capped and polyadenylated. However, miRNA biogenesis requires a more complex system of enzymes to obtain the mature, biologically active structure of final miRNAs. In fact, the first transcribed product, also known as primary miRNA (pri-miRNA), need to undergo an enzymatic cleavage in the nucleus by the ribonuclease III Drosha/DGCR8 complex to produce a secondary structure in the form of short hairpin called precursor miRNA (pre-miRNA). Then, the pre-miRNA is exported in the cytoplasm by the Ran/GTP/Exportin-5 complex, where it is further processed by the endonuclease Dicer/TRBP to generate a miRNA duplex [ ]. From the miRNA duplex, two independent single-strand RNAs are generated (5′-3′ filament named miR-X-5p, 3′-5′ filament named miR-X-3p), and they can both potentially interact with an AGO protein and then with RISC to form an effector complex able to recognize a complementary sequence of the miRNA on a target RNA and mediate its degradation [ ] ( Fig. 33.2 ) . Although the most reported function of miRNAs is to repress its target expression by repressing its translation or favoring its degradation, some miRNAs associated with protein complexes (microRNPs) can upregulate the translation of their targets [ ]. The correct function of the miRNA biomachinery is an essential aspect to preserve physiological features of cells, whereas its dysregulation is associated with a pathological status, such as in cancer. In fact, the downregulation of Drosha and Dicer expression levels has been shown to be associated with specific subgroups of invasive breast cancer [ ], and with poor clinical outcome in patients with ovarian cancer [ ].

Of note, miRNAs have a multigene regulatory capacity by binding several different targets with a common binding site (usually in their 3′UTR), and a single mRNA can be targeted by multiple miRNAs making extremely complex the gene regulation operated by miRNAs at the posttranscriptional level. Indeed, a correct balancing between expression levels of miRNAs and their targets is fundamental to maintain important biological functions within cells. In cancer, several events, such as epigenetic defects and miRNA sponge (e.g., lncRNAs and circRNAs) dysregulations, lead to abnormal miRNA expression levels in cancer cells [ , ]. In this respect, oncomiRs are miRNAs overexpressed in a variety of cancer types known to promote tumorigenesis by inhibiting oncosuppressor genes, whereas oncosuppressor miRs are downregulated in cancer and they target oncogenes in physiological conditions [ ]. For these reasons, both oncomiRs and oncosuppressor miRs have been largely investigated during the last decades as fundamental players in the formation of primary tumors and progression to metastatic disease [ , ].