Distinct functional roles of microRNA-23b and microRNA-26a in breast cancer pathogenesis

Abstract

Tumour formation and metastasis are distinct processes that arise from cumulative alterations of genomic and epigenetic regulation. Uncontrolled modulation of cell cycle-related genes is crucial to tumour growth and additional genetic modifications provide cancer cells with motile and invasive phenotypes, leading to metastatic dissemination. The cytoskeleton constitutes the structural support to cell motility, invasion and adhesion. Among the best-characterised cytoskeletal modulators are the p21-activated kinases (PAKs). In breast cancer (BC), the HER2 pathway controls the cytoskeletal dynamics and cell motility via PAK activation, through distinct downstream signaling mechanisms. MicroRNAs (miRNAs) are small, non-coding RNAs that modulate gene expression post-transcriptionally. MiRNAs dysregulation can contribute to tumorigenicity, cell motility and metastasis by affecting relevant signaling pathways. We identified PAK2 as target of both miR-23b and miR-26a, implicating a direct role for these miRNAs in cytoskeletal remodeling. Experimentally, expression of miR-23b and miR-26a in BC cells promotes focal adhesions and cell spreading on substrates, but miR-23b alone controls cell-cell junctions and lamellipodia formation. Despite sharing the same target, the two miRNAs show additional distinct functions. MiR-26a overexpression in BC leads to formation of aneuploid cells associated with higher tumorigenicity. On the other hand, miR-23b inhibition enhances BC cell migration, invasion and metastasis in vivo. Clinically, low miR-23b levels correlate with metastatic development in BC patients. Mechanistically, growth factor-mediated signal transductions activate the transcription factor AP-1 and we show that this transcriptionally reduces miR-23b expression thus releasing PAK2 from its translational inhibition. The distinct cellular phenotypes described by the two miRNAs indicate that their global functions depend upon all the genes they regulate. Using RNA-sequencing and luciferase reporter assays, we validated a subset of genes as direct targets of either the two miRNAs. These genes are crucial to distinct molecular pathways and contribute to elucidate the observed phenotypes induced by miR-23b and miR-26a modulation.