Rrp1b, a new candidate susceptibility gene for breast cancer progression and metastasis

A novel candidate metastasis modifier, ribosomal RNA processing 1 homolog B (Rrp1b), was identified through two independent approaches. First, yeast two-hybrid, immunoprecipitation, and functional assays demonstrated a physical and functional interaction between Rrp1b and the previous identified metastasis modifier Sipa1. In parallel, using mouse and human metastasis gene expression data it was observed that extracellular matrix (ECM) genes are common components of metastasis predictive signatures, suggesting that ECM genes are either important markers or causal factors in metastasis. To investigate the relationship between ECM genes and poor prognosis in breast cancer, expression quantitative trait locus analysis of polyoma middle-T transgene-induced mammary tumor was performed. ECM gene expression was found to be consistently associated with Rrp1b expression. In vitro expression of Rrp1b significantly altered ECM gene expression, tumor growth, and dissemination in metastasis assays. Furthermore, a gene signature induced by ectopic expression of Rrp1b in tumor cells predicted survival in a human breast cancer gene expression dataset. Finally, constitutional polymorphism within RRP1B was found to be significantly associated with tumor progression in two independent breast cancer cohorts. These data suggest that RRP1B may be a novel susceptibility gene for breast cancer progression and metastasis.     

The MicroRNA-23b/27b/24 Cluster Promotes Breast Cancer Lung Metastasis by Targeting Metastasis-suppressive Gene Prosaposin

MicroRNAs (miRNAs) have been shown to function as key regulators of tumor progression and metastasis. Recent studies have indicated that the miRNAs comprising the miR-23b/27b/24 cluster might influence tumor metastasis, although the precise nature of this regulation remains unclear. Here, expression of the miR-23b/27b/24 cluster is found to correlate with metastatic potential in mouse and human breast cancer cell lines and is elevated in metastatic lung lesions in human breast cancer patients. Ectopic expression of the miRNAs in the weakly metastatic mouse 4TO7 mammary tumor cell line had no effect on proliferation or morphology of tumor cells in vitro but was found to increase lung metastasis in a mouse model of breast cancer metastasis. Furthermore, gene expression profiling analysis of miRNA overexpressing 4TO7 cells revealed the direct targeting of prosaposin (PSAP), which encodes a secreted protein found to be inversely correlated with metastatic progression in human breast cancer patients. Importantly, ectopic expression of PSAP was able to suppress the metastatic phenotype in highly metastatic 4T1 and MDA-MB-231 SCP28 cells, as well as in cells ectopically expressing miR-23b/27b/24. These findings support a metastasis-promoting function of the miR-23b/27b/24 cluster of miRNAs, which functions in part through the direct inhibition of PSAP.     

MicroRNAs: Oncogenes,tumor suppressors or master regulators of cancer heterogeneity?

The realization that microRNAs are intimately linked to cancer pathogenesis has spawned an explosion of research activity in recent years. Their presence is not merely predictive of tumor origin and behavior, they are causally linked to the emergence and development of cancer by acting as oncogenes or tumor suppressors. The understanding of the functional consequences of altered microRNA expression in cancer is progressing rapidly, even though the prediction of microRNA targets is still a hit and miss process. MicroRNAs may not act primarily by strongly reducing the expression of a few prominent cancer-regulatory genes, but by influencing the properties of the network of which these regulators are a central part. By coordinately regulating many genes, microRNAs are exquisitely suited to act as stabilizers of networks and to prevent extreme variations in phenotype due to intrinsic and extrinsic disturbances. Many advanced tumors show defects in microRNA expression and processing, which could increase phenotypic variability within tumors. This allows small subsets of cells with altered characteristics to emerge, which can have grave consequences as typically a small fraction of tumor cells is responsible for metastasis and treatment resistance, and ultimately treatment failure. Investigating microRNAs from the perspective of master regulators of network stability in cancer calls for new experimental approaches and may help to understand causes of cancer heterogeneity and disease progression.     

Metastasis Suppressor microRNA-335 Targets the Formin Family of Actin Nucleators

MiRNAs can have pleiotropic effects by targeting multiple genes belonging to diverse signalling networks. Alternatively, miRNAs can enhance the potency of their cellular effects by targeting multiple genes within the same genetic pathway. Previously, we and others have demonstrated that miR-335 is a potent suppressor of tumour cell migration, invasion and metastasis, in part by targeting several genes involved in these cellular processes, including ROCK1, MAPK1, LRG1, SP1 and SOX4. Here, we demonstrate that direct targeting of multiple members of the formin family of actin nucleators contributes to the inhibitory effects of miR-335 in neuroblastoma cells. We demonstrate that miR-335 regulates the expression of at least five formin family members and validate three family members, FMNL3, FMN2 and DAAM2, as direct targets of miR-335. The contribution of the formin family genes to cancer progression and metastasis has recently begun to emerge and here we demonstrate for the first time the ability of FMN2 and DAAM2 to regulate tumour cell migration and invasion, using siRNA-mediated inhibition of each of these formin genes. Finally, we demonstrate that the formin genes, in particular FMNL3, are responsible for the protrusion of actin-rich filopodia structures that contribute to the enhanced migratory and invasive potential associated with reduced expression of miR-335. Thus, direct targeting of the formin family contributes to the metastasis suppressing abilities of miR-335 by providing a direct regulatory link to the actin assembly machinery of the cell. We conclude that miR-335 is a master regulator of tumour cell migration and invasion by directly targeting a plethora of genes that effectively control cell migratory processes.