Tamoxifen, an estrogen receptor (ER) antagonist, is the mainstay treatment of breast cancer and the development of resistance represents a major obstacle for a cure. Although long non-coding RNAs such as HOTAIR have been implicated in breast tumorigenesis, their roles in chemotherapy resistance remain largely unknown. In this study, we report that HOTAIR (HOX antisense intergenic RNA) is upregulated in tamoxifen-resistant breast cancer tissues compared to their primary counterparts. Mechanistically, HOTAIR is a direct target of ER-mediated transcriptional repression and is thus restored upon the blockade of ER signaling, either by hormone deprivation or by tamoxifen treatment. Interestingly, this elevated HOTAIR increases ER protein level and thus enhances ER occupancy on the chromatin and potentiates its downstream gene regulation. HOTAIR overexpression is sufficient to activate the ER transcriptional program even under hormone-deprived conditions. Functionally, we found that HOTAIR overexpression increases breast cancer cell proliferation, whereas its depletion significantly impairs cell survival and abolishes tamoxifen-resistant cell growth. In conclusion, the long non-coding RNA HOTAIR is directly repressed by ER and its upregulation promotes ligand-independent ER activities and contributes to tamoxifen resistance.

Microtubule nucleation and organization by the centrosome require γ-tubulin, a protein that exists in a macromolecular complex called the γ-tubulin ring complex (γTuRC). We report characterization of CDK5RAP2, a novel centrosomal protein whose mutations have been linked to autosomal recessive primary microcephaly. In somatic cells, CDK5RAP2 localizes throughout the pericentriolar material in all stages of the cell cycle. When overexpressed, CDK5RAP2 assembled a subset of centrosomal proteins including γ-tubulin onto the centrosomes or under the microtubule-disrupting conditions into microtubule-nucleating clusters in the cytoplasm. CDK5RAP2 associates with the γTuRC via a short conserved sequence present in several related proteins found in a range of organisms from fungi to mammals. The binding of CDK5RAP2 is required for γTuRC attachment to the centrosome but not for γTuRC assembly. Perturbing CDK5RAP2 function delocalized γ-tubulin from the centrosomes and inhibited centrosomal microtubule nucleation, thus leading to disorganization of interphase microtubule arrays and formation of anastral mitotic spindles. Together, CDK5RAP2 is a pericentriolar structural component that functions in γTuRC attachment and therefore in the microtubule organizing function of the centrosome. Our findings suggest that centrosome malfunction due to the CDK5RAP2 mutations may underlie autosomal recessive primary microcephaly.

Understanding the mechanisms of androgen receptor (AR) activation in the milieu of low androgen is critical to effective treatment of castration-resistant prostate cancer (CRPC). Here, we report HOTAIR as an androgen-repressed lncRNA, and, as such, it is markedly upregulated following androgen deprivation therapies and in CRPC. We further demonstrate a distinct mode of lncRNA-mediated gene regulation, wherein HOTAIR binds to the AR protein to block its interaction with the E3 ubiquitin ligase MDM2, thereby preventing AR ubiquitination and protein degradation. Consequently, HOTAIR expression is sufficient to induce androgen-independent AR activation and drive the AR-mediated transcriptional program in the absence of androgen. Functionally, HOTAIR overexpression increases, whereas HOTAIR knockdown decreases, prostate cancer cell growth and invasion. Taken together, our results provide compelling evidence of lncRNAs as drivers of androgen-independent AR activity and CRPC progression, and they support the potential of lncRNAs as therapeutic targets.

BRCA1 promotes homologous recombination repair and antagonizes 53BP1-dependent nonhomologous end joining (NHEJ) pathway. However, the molecular basis of the competition between BRCA1 and 53BP1 pathways remains elusive. Here we report that RIF1 protein translocates to damage sites via ATM-dependent 53BP1 phosphorylation. Strikingly, loss of RIF1 rescues initial DNA end resection and checkpoint activation in BRCA1-depleted cells. Interestingly RIF1 accumulation at damage sites is antagonized by BRCA1 in S and G₂ phases. Conversely, the translocation of BRCA1 to damage sites is inhibited by RIF1 in G₁ phase. However, loss of RIF1 differs from that of 53BP1 deficiency, as it cannot fully rescue RAD51 foci formation, homologous recombination defect, and radio-hypersensitivity in BRCA1-deficient cells. This is likely because RIF1, but not 53BP1, also regulates the foci formation and chromatin loading of BLM (the Bloom syndrome helicase). Thus, RIF1 not only acts downstream of 53BP1 and counteracts BRCA1-mediated end resection but also has a secondary role in promoting BLM function in DNA repair.

The nucleus is a unique organelle that contains essential genetic materials in chromosome territories. The interchromatin space is composed of nuclear subcompartments, which are defined by several distinctive nuclear bodies believed to be factories of DNA or RNA processing and sites of transcriptional and/or posttranscriptional regulation. In this paper, we performed a genome-wide microscopy-based screening for proteins that form nuclear foci and characterized their localizations using markers of known nuclear bodies. In total, we identified 325 proteins localized to distinct nuclear bodies, including nucleoli (148), promyelocytic leukemia nuclear bodies (38), nuclear speckles (27), paraspeckles (24), Cajal bodies (17), Sam68 nuclear bodies (5), Polycomb bodies (2), and uncharacterized nuclear bodies (64). Functional validation revealed several proteins potentially involved in the assembly of Cajal bodies and paraspeckles. Together, these data establish the first atlas of human proteins in different nuclear bodies and provide key information for research on nuclear bodies.

DNA damage-induced proliferating cell nuclear antigen (PCNA) ubiquitination serves as the key event mediating post-replication repair. Post-replication repair involves either translesion synthesis (TLS) or damage avoidance via template switching. In this study, we have identified and characterized C1orf124 as a regulator of TLS. C1orf124 co-localizes and interacts with unmodified and mono-ubiquitinated PCNA at UV light-induced damage sites, which require the PIP box and UBZ domain of C1orf124. C1orf124 also binds to the AAA-ATPase valosin-containing protein via its SHP domain, and cellular resistance to UV radiation mediated by C1orf124 requires its interactions with valosin-containing protein and PCNA. Interestingly, C1orf124 binds to replicative DNA polymerase POLD3 and PDIP1 under normal conditions but preferentially associates with TLS polymerase η (POLH) upon UV damage. Depletion of C1orf124 compromises PCNA monoubiquitination, RAD18 chromatin association, and RAD18 localization to UV damage sites. Thus, C1orf124 acts at multiple steps in TLS, stabilizes RAD18 and ubiquitinated PCNA at damage sites, and facilitates the switch from replicative to TLS polymerase to bypass DNA lesion.