MicroRNAs (miRs) are non-coding RNAs that inhibit expression of their targets in a sequence-specific manner and play crucial roles during oncogenesis. Here we show that miR-7 inhibits p21-activated kinase 1 (Pak1) expression, a widely upregulated signaling kinase in multiple human cancers including breast and gliomas, by targeting the 3′-UTR of Pak1 mRNA. We noticed an inverse correlation between the levels of endogenous miR-7 and Pak1 expression in human cancer cells. We discovered that endogenous miR-7 expression is positively regulated by a homeodomain transcription factor HoxD10, loss of which leads to an increased invasiveness. The HoxD10 directly interacts with the miR-7 chromatin. Accordingly, the levels of Pak1 protein are progressively upregulated while that of miR-7 and its upstream activator HoxD10 are progressively downregulated in a cellular model of breast cancer progression from low to highly invasive phenotypes. Furthermore, HoxD10 expression in highly invasive breast cancer cells resulted in an increased expression of miR-7 but a reduced Pak1 3′UTR-luciferase activity as well as reduced Pak1 protein. Finally, we show that miR-7 introduction inhibits the motility, invasiveness, anchorage-independent growth and tumorigenic potential of highly invasive breast cancer cells. Collectively, these findings establish for the first time that Pak1 is a target of miR-7 and that HoxD10 play a regulatory role in modifying the expression of miR-7, and consequently, functions of miR7 - Pak1 pathway in human cancer cells.

SUMMARY Chromatin dynamics play a central role in maintaining genome integrity, but how this is achieved remains largely unknown. Here, we report that microrchidia CW-type zinc finger 2 (MORC2), an uncharacterized protein with a derived PHD finger domain and a conserved GHKL-type ATPase module, is a physiological substrate of p21-activated kinase 1 (PAK1), an important integrator of extracellular signals and nuclear processes. Following DNA damage, MORC2 is phosphorylated on serine 739 in a PAK1 dependent manner, and phosphorylated MORC2 regulates its DNA-dependent ATPase activity to facilitate chromatin remodeling. Moreover, MORC2 associates with chromatin and promotes gamma-H2AX induction in a PAK1 phosphorylation-dependent manner. Consequently, cells expressing MORC2-S739A mutation displayed a reduction in DNA repair efficiency and were hypersensitive to DNA-damaging agent. These findings suggest that the PAK1-MORC2 axis is critical for orchestrating the interplay between chromatin dynamics and the maintenance of genomic integrity through sequentially integrating multiple essential enzymatic processes.

Breast cancer transcriptome acquires a myriad of regulation changes, and splicing is critical for the cell to “tailor-make” specific functional transcripts. We systematically revealed splicing signatures of the three most common types of breast tumors using RNA sequencing: TNBC, non-TNBC and HER2-positive breast cancer. We discovered subtype specific differentially spliced genes and splice isoforms not previously recognized in human transcriptome. Further, we showed that exon skip and intron retention are predominant splice events in breast cancer. In addition, we found that differential expression of primary transcripts and promoter switching are significantly deregulated in breast cancer compared to normal breast. We validated the presence of novel hybrid isoforms of critical molecules like CDK4, LARP1, ADD3, and PHLPP2. Our study provides the first comprehensive portrait of transcriptional and splicing signatures specific to breast cancer sub-types, as well as previously unknown transcripts that prompt the need for complete annotation of tissue and disease specific transcriptome.

Metastasis-associated protein 1 (MTA1), a component of the nucleosome remodeling and histone deacetylation (NuRD) complex, is widely upregulated in human cancers. However, the mechanism for regulating its protein stability remains unknown. Here we report that MTA1 is an ubiquitinated protein and targeted by the RING-finger E3 ubiquitin-protein ligase constitutive photomorphogenesis protein 1 (COP1) for degradation via the ubiquitin-proteasome pathway. Induced expression of wild-type COP1 but not its RING motif mutants promotes the ubiquitination and degradation of MTA1, indicating that the ligase activity is required for the COP1-mediated proteolysis of MTA1. Conversely, depletion of endogenous COP1 resulted in a marked decrease in MTA1 ubiquitination, accompanied by a pronounced accumulation of MTA1 protein. MTA1, in turn, destabilizes COP1 by promoting its autoubiquitination, thus creating a tight feedback loop that regulates both MTA1 and COP1 protein stability. Accordingly, disruption of the COP1-mediated proteolysis by ionizing radiation leads to MTA1 stabilization, accompanied by an increased coregulatory function of MTA1 on its target. Furthermore, we discovered that MTA1 is required for optimum DNA double-strand break repair after ionizing radiation. These findings provide novel insights into the regulation of MTA1 protein and reveal a novel function of MTA1 in DNA damage response.coregulator ͉ DNA repair ͉ ubiquitination R egulation of fundamental cellular processes demands dynamic coordinated participation of transcription factors and their coregulators at the target gene chromatin (1, 2), and deregulation of such processes plays a critical role in the development of malignant phenotypes. One emerging family of ubiquitously expressed chromatin modifiers is the metastasisassociated protein (MTA) family, which has an integral role in nucleosome remodeling and histone deacetylation (NuRD) complexes that modify DNA accessibility for cofactors (2, 3). MTA1, the founding member of the MTA family, is widely upregulated in human cancers and plays an important role in tumorigenesis and tumor aggressiveness, especially tumor invasion and metastasis (4-6). MTA1 functions not only as a transcriptional repressor of estrogen receptor ␣ (7), but also as a transcriptional activator on certain promoters, such as the breast cancer-amplified sequence 3 (BCAS3) promoter (8). In this context, MTA1 is acetylated at lysine 626 (K626) by histone acetyltransferase p300; such modification allows MTA1 to recruit RNA polymerase II (Pol II) on the BCAS3 enhancer region and confers its coactivator function upon BCAS3 (8). MTA1 is also a mechanistic mediator of c-Myc-regulated transformation as a downstream target of the oncogene c-Myc (9). Although a paramount role of MTA1 in cancer and coregulator biology, the mechanism for regulating its protein stability remains unknown.Constitutive photomorphogenic 1 (COP1; also known as RFWD2, RING finger and WD repeat domain protein 2), an evolutionarily conserved RING-finger ubiquitin-protei...