The miR-106b-25 cluster mediates breast tumor initiation through activation of NOTCH1 via direct repression of NEDD4L.

Tumor-initiating cells (TIC) represent a subset of tumor cells with increased self-renewal capability. TICs display resistance to frontline cancer treatment and retain the ability to repopulate a tumor after therapy, leading to cancer relapse. NOTCH signaling has been identified as an important driver of the TIC population, yet mechanisms governing regulation of this pathway in cancer remain to be fully elucidated. Here we identify a novel mechanism of NOTCH regulation and TIC induction in breast cancer via the miR-106b-25 miRNA cluster. We show that the miR-106b-25 cluster upregulates NOTCH1 in multiple breast cancer cell lines, representing both estrogen receptor (ER+) and triple negative breast cancer (TNBC) through direct repression of the E3 ubiquitin ligase, NEDD4L. We further show that upregulation of NOTCH1 is necessary for TIC induction downstream of miR-106b-25 in both ER + and TNBC breast cancer cells, and that re-expression of NEDD4L is sufficient to reverse miR106b-25-mediated NOTCH1 upregulation and TIC induction. Importantly, we demonstrate a significant positive correlation between miR-106b-25 and NOTCH1 protein, yet a significant inverse correlation between miR-106b-25 and NEDD4L mRNA in human breast cancer, suggesting a critical role for the miR106b-25/NEDD4L/NOTCH1 axis in the disease. Further, we show for the first time that NEDD4L expression alone is significantly associated with a better relapse-free prognosis for breast cancer patients. These data expand our knowledge of the mechanisms underlying NOTCH activation and TIC induction in breast cancer, and may provide new avenues for the development of therapies targeting this resistant subset of tumor cells.

Non-dioxin-like polychlorinated biphenyls induce a release of arachidonic acid in liver epithelial cells: a partial role of cytosolic phospholipase A(2) and extracellular signal-regulated kinases 1/2 signalling.

Non-dioxin-like polychlorinated biphenyls (NDL-PCBs) have been shown to act as tumor promoters in liver; however, the exact mechanisms of their action are still only partially understood. One of the interesting effects of NDL-PCBs is the acute inhibition of gap junctional intercellular communication (GJIC), an effect, which has been often found to be associated with tumor promotion. As previous studies have suggested that NDL-PCB-induced disruption of lipid signalling pathways might correspond with GJIC inhibition, we investigated effects of PCBs on the release of arachidonic acid (AA) in the rat liver epithelial WB-F344 cell line, a well-established model of liver progenitor cells. We found that both 2,2',4,4'-tetrachlorobiphenyl (PCB 47) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), but not the dioxin-like, non-ortho-substituted, 3,3',4,4',5-pentachlorobiphenyl (PCB 126), induce a massive release of AA. The AA release, induced by PCB 153, was partially inhibited by extracellular signal-regulated kinases 1/2 (ERK1/2) signalling inhibitor, U0126, and by cytosolic phospholipase A(2) (cPLA(2)) inhibitor, AACOCF(3). Although PCB 153 induced both ERK1/2 and p38 activation, the specific p38 kinase inhibitor, SB203580, had no effect on AA release. Inhibitors of other phospholipases, including phosphatidylcholine-specific phospholipase C or phosphatidylinositol-specific phospholipase C, were also without effect. Taken together, our findings suggest that the AA release, induced by non-dioxin-like PCBs in liver progenitor cell line, is partially mediated by cytosolic PLA(2) and regulated by ERK1/2 kinases. Our results suggest that more attention should be paid to cell signalling pathways regulated by AA or eicosanoids after PCB exposure, which might be involved in their toxic effects.