The Role of micro RNAs in Breast Cancer Metastasis: Preclinical Validation and Potential Therapeutic Targets

Abstract

Despite the approval of several molecular therapies in the last years, breast cancer-associated death ranks as the second highest in women. This is due to metastatic disease, which represents a challenge for treatment. A better understanding of the molecular mechanisms of metastasis is, therefore, of paramount importance. In this review we summarize the role of micro RNAs (miRs) involved in metastasis of breast cancer. We present an overview on metastasis-promoting, -suppressing and context-dependent miRs with both activities. We have categorized the corresponding miRs according to their target classes, interaction with stromal cells or exosomes. The pathways affected by individual miRs are outlined in regard to in vitro properties, activity in metastasis-related in vivo models and clinical significance. Current approaches that may be suitable for therapeutic inhibition or restauration of miR activity are outlined. Finally, we discuss the delivery bottlenecks which present as a major challenge in nucleic acid (miR)-based therapies.

Keywords: Colonization of distinct organs; epithelial-mesenchymal transition (EMT); exosomes; mesenchymal-epithelial transition (MET); metastasis-related in vivo models; migration and invasion; review; tumor cell/stromal cell interactions.

Figure 1. miRs promoting breast cancer metastasis by activation of ROCK signaling. miRs 10b, -126, -182, -548j and -1792 activate several small GTPases resulting in activation of ROCK signaling followed by modification of actin and migration and invasion of breast cancer cells. Cdc 42: Cell division cycle protein 42; HoxD10: homeobox domain 10; MIM: missing in metastasis; mTOR: mechanistic target of rapamycin; RhoA: ras homologue A; Rho C: ras homologue C; ROCK: Rho-associated kinase; STARD13: StAR-related lipid transfer domain protein 13.

Figure 2. Mode of action of selected BC-promoting miRs. The left-hand panel displays selcted miRs. The corresponding targets are shown in the middle panel and the physiological consequences of target inhibition are shown in the right-hand panel. LATS2: Large tumor suppressor, homology 2; PDCD4: programmed cell death 4; pEGFR: phosphorylated epidermal growth factor receptor; PTPN9: tyrosine-protein phosphatase non-receptor 9; PTPRF: receptor tyrosine phosphatase F.

Figure 3. miRs promoting BC- metastasis by interaction of tumor cells with stromal cells or by miR-based exosome transfer between tumor cells and stromal cells or vice versa. Exosomes are displayed as small circles. BMDC: Bone marrow-derived cell; CAF: cancer-associated fibroblast; EC: endothelial cell; FoxP2: forkhead box P2; MDSC: myeloid-derived suppressor cell; MEF-2C: monocyte enhancer factor 2C; MET NICHE: metastatic niche; MP: macrophage; MSC: mesenchymal/stromal stem cell; PTEN: phosphatase and tensin homolog; TC: tumor cell; ZO-1: zonula occludens-1.

Figure 4. miRs suppressing breast cancer metastasis by modulation of cytoskeletal components. Arrows indicate activation. Actin is displayed as the major downstream effector of miRs let-7b, miR-7 and miR-149. ARF: ADP-ribosylation factor GTPase activating factor; cdc 42: cell division cycle 42 homolog; GEF: guanine nucleotide exchange factor; GIT-1: G-protein coupled receptor interacting protein-1; GPCR: G-protein coupled receptor; PAK-1: p21-activated protein kinase; RAC-1: Rac-related C3 botulinum toxin substrate 1; Rap 1a, b: ras related protein 1a, b; RhoA: ras homologue A; ROCK: Rho-associated kinase; RTK: receptor tyrosine kinase.

Figure 5. Anti-metastatic function of miR-126 is mediated by inhibition of recruitment of stromal cells by breast cancer cells. miR-126 inhibits recruitment of MSC, INM and EC cells by tumor cells through inhibition of SDF-1 and CCL2 as well as inhibition of IGF-1/IGF-1R and MERTK signaling. CCL2: CC chemokine ligand 2; EC: endothelial cell; IGFBP2: insulin-like growth factor binding protein 2; IGF-1: insulin-like growth factor-1; IGFR-1: insulin-like growth factor receptor-1; INM: inflammatory monocyte; MERTK: c-MER tyrosine kinase; s-MERTK: soluble-MERTK; MSC: mesenchymal stem cell; PITPNC1: phosphoinositol transfer protein, cytoplasmic 1; SDF-1: stromal-derived factor-1; TC: tumor cell.

Figure 6. miR-200 promotes or inhibits metastasis of breast cancer cells in a context-dependent manner. Pro-metastatic actions of miR-200 can be mediated by exosomes or through inhibition of secretion of anti-metastatic proteins and inhibition of metastasis is based on modulation of actin interaction with WAVE3 and re-organisation of the ECM in CAFs. Exosomes and secretory vesicles are displayed as green or red small circles. CAF: Cancer-associated fibroblast; ECM: extracellular matrix; ER: endoplasmic reticulum; Fli-1: Friend leukemia integration-1; FN: fibronectin; IGFBP4: insulin-like growth factor binding protein 4; LOX: lysyl oxidase; sec 23A: sec 23 homolog A (S. cerevisiae); TC: tumor cell; TCF12: transcription factor 12; TINAGL: tubuli interstitial nephritis antigen-like 1; WASP: Wiskott Aldrich Protein; WAVE3: WASP verprolin homologue 3.

Figure 7. Pro- and anti-metastatic functions of miR-373/520. Pro-metastatic function of miR-373/520 in breast cancer cells is mediated by degradation of CD44 and activation of HIF-1α/Twist signaling. Anti-metastatic function of miR-373/520 is based on inhibition of TGFβ- and NFĸB signaling. ANGPTL4: Angiopoietin-like 4; CD44: cluster of differentiation 44; HIF-1α: hypoxia-inducible factor 1α; interleukin 6,8: interleukin 6 or 8; PAI-1: plasminogen activator inhibitor-1; PTHrP: parathyoid hormone-related protein; ROS: reactive oxygen species; Smad: contraction of Sma and Mad (mothers of decapentaplegic); TGFβ: transforming growth factor β; TGFBR1,2: transforming growth factor β receptor 1,2; TXNIP: thioredoxin-interacting protein.

Figure 8. Expression of selected microRNAs in breast cancer in comparison to matched normal tissues. A: Pro-metastatic miRs: miRs -105, -182, -21 and -9; B: Anti-metastatic miRs: miRs -126, -145, - 205 and -335. Steady-state RNA levels corresponding to miR-105, miR-181c and miR-210 based on 1,084 invasive breast cancer samples and 104 matched normal samples derived from cohorts of The Cancer Genome Atlas (TCGA) are shown. Expression was quantified by RNA sequencing and is shown as log2 of normalized read counts. The red lines indicate low versus higher expression. Expression data are shown as box plots. The line in the middle of the box represents the data median, the rectangles show the upper and lower 25% quartile and 50% of all data points are included in the rectangle. All other data points, except for outliers are located within the upper and lower whiskers.